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brother_unknown
2009-May-25, 03:16 PM
The Andromeda galaxy is said to be about 2.5 million light years away. Does this mean it is 2.5 million light years away today? Or 2.5 million years ago?

alainprice
2009-May-25, 03:44 PM
It's supposedly 2.5 million light years away and will collide with the Milky Way in about 5 billion years.

I'd say it's still 2.5 MLY away, and closing in relatively slowly.

grant hutchison
2009-May-25, 03:48 PM
Both. :)
At an approach velocity of 300 km.s-1, the Andromeda Galaxy has moved closer to us by about 2500 light-years in the last 2.5 million years (it has moved a small fraction of its own diameter, in other words). Since its distance has changed by just 0.1%, that's not enough to make a difference to the rounded estimate of 2.5 million light-years.

Grant Hutchison

AndrewJ
2009-May-25, 06:35 PM
The Andromeda galaxy is said to be about 2.5 million light years away. Does this mean it is 2.5 million light years away today? Or 2.5 million years ago?

Consider a scenario where the difference in distance from us between then and now is more than negligible: a galaxy 2.5 billion lya receding from us at maybe 10% the speed of light. The photons we now receive from it were emitted 2.5 billion years ago and between their emission and our reception travelled 2.5 billion light years across ever-expanding space between photon and Earth. (However, the expansion of space was not as fast as the photon which eventually got to us). Because of the expansion the galaxy was closer to us than 2.5 billion lya 2.5 billion years ago but what we observe appears to have been 2.5 billion lya 2.5 billion years ago. To estimate the galaxy's actual distance from us now we must take the apparent distance of 2.5 billion lya (which factors in the expansion of space between photon and Earth) and add the expansion of space behind the photon, between photon and source galaxy, over the 2.5 billion years.

Well, I thought I understood this stuff before I started typing...

Cougar
2009-May-25, 06:40 PM
Well, I thought I understood this stuff before I started typing...

:lol:

Argos
2009-May-25, 06:54 PM
Well, I thought I understood this stuff before I started typing...

Well, sure you understand the idea. But actually, at this particular moment in the history of the universe, the expansion affects the distance between groups and supergroups of galaxies. As Grant pointed out, Andromeda [in the local group] is getting closer.

AndrewJ
2009-May-25, 09:22 PM
As Grant pointed out, Andromeda [in the local group] is getting closer.

I interpreted the first post to be about quoting distances to galaxies in general and that M31 was given as an example and was not the nub of the question. I thought that the answer "both" might not illuminate the heart of the matter. :)

grant hutchison
2009-May-25, 09:26 PM
I thought that the answer "both" might not illuminate the heart of the matter. :)A certain amount of illumination, directly relevant to the question posed, was provided after the word "both". :)

Grant Hutchison

slang
2009-May-25, 09:41 PM
Both. :)
At an approach velocity of 300 km.s-1, the Andromeda Galaxy has moved closer to us by about 2500 light-years in the last 2.5 million years (it has moved a small fraction of its own diameter, in other words). Since its distance has changed by just 0.1%, that's not enough to make a difference to the rounded estimate of 2.5 million light-years.

Wow.. Imagine an Australopithecus staring intensely at Andromeda (assuming they had the ability to do so), then locking him into some kind of SF stasis field (hi Larry), and opening up the field right now, 2.5 million years later, positioned such that he was looking at the exact same patch of sky. He'll blink.. and maybe notice a tiny bit of a skipping, but probably wondering if he did see it or not (forget stars. don't ruin it. :) ). Spacetime is BIG!

grant hutchison
2009-May-25, 09:49 PM
Well, I thought I understood this stuff before I started typing...There's a nice summary of what you're saying here (http://www.atlasoftheuniverse.com/redshift.html), with diagrams. :)
The distance at which a galaxy emitted photons is given by the Angular Diameter Distance; its distance from us now (in cosmic time) is given by the Comoving Distance. The relationship between those two is a function both of distance and the model used.

Grant Hutchison

brother_unknown
2009-May-25, 10:13 PM
"The quoted distances all refer to then." If z =1, a super-galaxy said to be at a distance r would actually be at a distance 2r today

grant hutchison
2009-May-25, 10:32 PM
"The quoted distances all refer to then." If z =1, a super-galaxy said to be at a distance r would actually be at a distance 2r todayThe relationship between Comoving Distance and Angular Diameter Distance is indeed about two to one for z=1, but (as the graph in my link (http://www.atlasoftheuniverse.com/redshift.html) shows) that ratio increases steadily with increasing values of z.

The most commonly quoted distance is unfortunately neither of the above, but Light Travel Time Distance, which tells you neither where the galaxy was when it emitted the light, nor where it is now: it tells you how long ago the galaxy emitted the light. The next most commonly quoted distance is the Comoving Distance, which refers to "now". In my experience, it's unusual to see the Angular Diameter Distance ("then") quoted, except in discussions like this one.

Grant Hutchison

Jeff Root
2009-May-25, 11:19 PM
The fact that the distance to Andromeda (and every other distant
astronmical object) is uncertain to a degree greater than the distance
it moves over a brief time interval like 2.5 millions years :) is important,
but going beyond that... What is the answer to the original question?
Does a distance in light-years refer to the distance of the object at the
time the light was emitted, the time the light was received, or what?

Grant's answer is correct: It is both. It is also neither. The distance
given is the distance the light has traveled, from the point in space and
time at which it was emitted, to the point in space and time at which it
is received.

-- Jeff, in Minneapolis

Ken G
2009-May-26, 03:40 AM
The distance
given is the distance the light has traveled, from the point in space and
time at which it was emitted, to the point in space and time at which it
is received.There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels. What there is, is two events-- the emission, and the absorption. How much distance, and how much time, that relate those events are purely given by the coordinates chosen, which presumably are in turn connected to the observables made by a given observer or set of observers. The sole universal constraint is the causal connection-- the absorption happens after (or essentially simultaneous) with the emission. The rest depends on the observers, but we can make certain assumptions to reduce the complexity of the "tower of Babel" of possible distances. We can assume the observers are "co-moving" with the prevailing mass flow of the universe, since most observers would be. Then we can sort those observers based on various distance measurements, like the cosmic time between emission and absorption (the "light travel-time distance"), or the angular diameter distance. The observers then collect on spheres that get the same answer to these various measurements. These spheres give meaning to the distances, but remember, it's all the observers. Absent of observers, distances have no meaning at all, and all causally connected pair of events can certainly be regarded as happening at the same place, separated by zero distance, just as any causally unconnected pair of events can be regarded as separated by zero time. And when those events are the emission and absorption of light, we have the singular opportunity to associate both zero time and zero distance between the events-- it is a "null geodesic". But we would generally not do so, we'd instead pick some observer and use coordinates that make sense to that observer.

It might seem like a nitpick, because we do talk about the distances to things, and how long ago they occured. But I still think it is important not to fall into the illusion that these are absolute statements of truth about the events themselves. It's all quite subordinate to the choice of observer-- generally, us. The answers should not have to state that every time, of course, but when statements like the above are made, I think it's time to set the record straight.

astromark
2009-May-26, 06:49 AM
Excellent and clear... Andromeda is 2.5 million L/Y away and closing. Its still 2.5 million L/Y away as any noticeable movement is as yet less than the error faction... Astronomy is the one subject where it is a good idea to stop and take in the very, Very big numbers at work here. In just 5 billion years at its current rate Andromeda will be in conflict with The Milky Way. and we do not need concern ourselves at all.

RussT
2009-May-26, 10:43 AM
There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels. What there is, is two events-- the emission, and the absorption. How much distance, and how much time, that relate those events are purely given by the coordinates chosen, which presumably are in turn connected to the observables made by a given observer or set of observers. The sole universal constraint is the causal connection-- the absorption happens after (or essentially simultaneous) with the emission. The rest depends on the observers, but we can make certain assumptions to reduce the complexity of the "tower of Babel" of possible distances. We can assume the observers are "co-moving" with the prevailing mass flow of the universe, since most observers would be. Then we can sort those observers based on various distance measurements, like the cosmic time between emission and absorption (the "light travel-time distance"), or the angular diameter distance. The observers then collect on spheres that get the same answer to these various measurements. These spheres give meaning to the distances, but remember, it's all the observers. Absent of observers, distances have no meaning at all, and all causally connected pair of events can certainly be regarded as happening at the same place, separated by zero distance, just as any causally unconnected pair of events can be regarded as separated by zero time. And when those events are the emission and absorption of light, we have the singular opportunity to associate both zero time and zero distance between the events-- it is a "null geodesic". But we would generally not do so, we'd instead pick some observer and use coordinates that make sense to that observer.

It might seem like a nitpick, because we do talk about the distances to things, and how long ago they occured. But I still think it is important not to fall into the illusion that these are absolute statements of truth about the events themselves. It's all quite subordinate to the choice of observer-- generally, us. The answers should not have to state that every time, of course, but when statements like the above are made, I think it's time to set the record straight.


the absorption happens after (or essentially simultaneous) with the emission.

Tim Thompson says differently...From...
http://www.bautforum.com/1311464-post72.html




Originally Posted by RussT
I am just trying to make sure that you/MS (Mainstream) does not have some 'special way' of 'showing' that we/science can see/detect photons, say from M31, that would show that we/science can see M31 as it is "Now", and where it is "Now", as seen from our 'Now' here in the Milky Way.



OK, is this what I am supposed to be answering? Haven't I already done that several times? How many different ways do you want to see the same answer? We detect photons in our "now", and that tells us what M31 looks like in our "now". Realizing that it took photons 2.5 million years to make the trip, and that our "now" is 2.5 million years after the "now" of M31 we see, we can then derive from physics how M31 would have changed over the intervening 2.5 million years. That way we can synthesize a picture if what M31 would look like if photons made the trip instantaneously, and that the two "now"s were simultaneous.My Bold/Red

Further...


Originally Posted by RussT
In other words, when we send a light beam to the moon, as in the lunar ranging project, it travels to the moon at "c" and Back at "c", for a ~2.5 second round trip, right.



Always.



Originally Posted by RussT
So, is light/photons ever traveling 'instantaneously' away from us towards any distant object?



Never.

astromark
2009-May-26, 11:11 AM
I think we all know that then and now are just confusions of location. Please tell me that you understand the fact that that stream of photons has gotten to your retina as quick as is could... There is no other way. No magic now button., and yes for those stars with enough lateral movement. They are not were they seem to be. The movement of galaxies might be rapid but, the enormous size and distance dictate that almost no movement is detectable in your lifetime. The difference you are looking for is almost undetectable.

slang
2009-May-26, 11:15 AM
the absorption happens after (or essentially simultaneous) with the emission.

Did you not read what Ken G wrote before that line? It is crucial context. Unless I'm mistaken, the seeming contradiction is because in what Tim and you wrote, you have chosen a coordinate system in which to say things about time and speed etc., while Ken G's statement follows from not choosing a coordinate system.

grant hutchison
2009-May-26, 02:31 PM
Unless I'm mistaken, the seeming contradiction is because in what Tim and you wrote, you have chosen a coordinate system in which to say things about time and speed etc., while Ken G's statement follows from not choosing a coordinate system.Ken will correct me if I'm wrong, but I believe the point is about our freedom to choose a coordinate system that suits us, within certain constrains.
If we wish the emission and absorption of a photon to be "essentially instantaneous", we just choose a frame in which those two events occur in essentially the same place: that is, we find an observer who moves with the photon at as close as we like to lightspeed. By that means, we can shave the separation of the two events as close as we like to zero, and make absorption and emission as close to instantaneous as we like.
However, we can never reverse the order of emission and absorption: we can never find a frame in which effect precedes cause. That's a universal constraint.

Tim Thompson and RussT were having a discussion in which the frame was stipulated quite clearly as being one close to local rest (in the absence of a special interpretation of the word "we"). Ken was talking about our ability to choose other frames.

Grant Hutchison

Ken G
2009-May-26, 02:32 PM
Did you not read what Ken G wrote before that line? It is crucial context. Unless I'm mistaken, the seeming contradiction is because in what Tim and you wrote, you have chosen a coordinate system in which to say things about time and speed etc., while Ken G's statement follows from not choosing a coordinate system.Right, Tim Thompson used the word "our" three times in that quote. Not in every single sentence, it's true, but at some point it has to be assumed to be clear that a particular coordinate choice has been made. Any statement taken out of context can sound like a claim on some absolute elapsed time, but there's no such thing, so one must look to the rest of the context.

Ken G
2009-May-26, 02:37 PM
Ken will correct me if I'm wrong, but I believe the point is about our freedom to choose a coordinate system that suits us, within certain constrains.Yes I think you and I and slang are saying the same thing here-- there are some absolute constraints on physically meaningful coordinatizations, but they are not nearly as stringent as our language all too easily falls into asserting. The language is to be forgiven when it is not attempting to make that particular point, but it bears keeping in mind lest we lose all contact with what is "actually real" for all, rather than for just one set of observers.

slang
2009-May-26, 03:48 PM
Yes I think you and I and slang are saying the same thing here [..]

I think so too (I intended to, anyway), but Grant says it more correctly. As usual. My "follows from" was a bit ambiguous.

tommac
2009-May-27, 04:33 PM
Both. :)
At an approach velocity of 300 km.s-1, the Andromeda Galaxy has moved closer to us by about 2500 light-years in the last 2.5 million years (it has moved a small fraction of its own diameter, in other words). Since its distance has changed by just 0.1%, that's not enough to make a difference to the rounded estimate of 2.5 million light-years.

Grant Hutchison

Wouldnt it accellerate as it got closer?

grant hutchison
2009-May-27, 04:52 PM
Wouldnt it accellerate as it got closer?Sure. By a small fraction of its current velocity, given the distances involved. Again, the change falls below the rounding error of the quoted velocity.

Grant Hutchison

RussT
2009-May-28, 10:41 AM
Ken will correct me if I'm wrong, but I believe the point is about our freedom to choose a coordinate system that suits us, within certain constrains.
If we wish the emission and absorption of a photon to be "essentially instantaneous", we just choose a frame in which those two events occur in essentially the same place: that is, we find an observer who moves with the photon at as close as we like to lightspeed. By that means, we can shave the separation of the two events as close as we like to zero, and make absorption and emission as close to instantaneous as we like.
However, we can never reverse the order of emission and absorption: we can never find a frame in which effect precedes cause. That's a universal constraint.

Tim Thompson and RussT were having a discussion in which the frame was stipulated quite clearly as being one close to local rest (in the absence of a special interpretation of the word "we"). Ken was talking about our ability to choose other frames.

Grant Hutchison

that is, we find an observer who moves with the photon at as close as we like to lightspeed.

And that lightspeed would be CONSTANT "c" @186,282.4 mps, right?

(and I do understand exactly what I am saying/asking here)

RussT
2009-May-28, 11:03 AM
Right, Tim Thompson used the word "our" three times in that quote. Not in every single sentence, it's true, but at some point it has to be assumed to be clear that a particular coordinate choice has been made. Any statement taken out of context can sound like a claim on some absolute elapsed time, but there's no such thing, so one must look to the rest of the context.

Any statement taken out of context can sound like a claim on some absolute elapsed time, but there's no such thing,

First all all...nothing taken out of context here!


Originally Posted by RussT
In other words, when we send a light beam to the moon, as in the lunar ranging project, it travels to the moon at "c" and Back at "c", for a ~2.5 second round trip, right.



Originally Posted by Tim Thompson
Always
Sounds like Constant "c" to me.

Second, Tim makes it very clear that we are looking at M31 as it was and where it was ~2,5 million years ago!

KenG, by using this...
If we wish the emission and absorption of a photon to be "essentially instantaneous", we just choose a frame in which those two events occur in essentially the same place: that is, we find an observer who moves with the photon at as close as we like to lightspeed. By that means, we can shave the separation of the two events as close as we like to zero, and make absorption and emission as close to instantaneous as we like.

Are you/Grant saying that we can scientifically see/detect photons from M31 where it is "Now"???

grant hutchison
2009-May-28, 01:38 PM
that is, we find an observer who moves with the photon at as close as we like to lightspeed.

And that lightspeed would be CONSTANT "c" @186,282.4 mps, right?

(and I do understand exactly what I am saying/asking here)In which case you know what the answer is going to be.
Yes, all observers will measure the speed of light to be c. But we (in that shared rest frame you and Tim were discussing) will see our chosen observer almost keeping pace with the photon. Special relativity at work, as usual.


Are you/Grant saying that we can scientifically see/detect photons from M31 where it is "Now"???Only by becoming that chosen observer for whom the emission and absorption are effectively simultaneous. That is, we need to stop being the "we" you and Tim were discussing, and become someone completely different: in this case, an observer who is next to M31 when it emits the photon.

Grant Hutchison

joema
2009-May-28, 01:56 PM
As already mentioned, Andromeda Galaxy is actually moving toward us due to gravitational effects. Although we picture it as "distant", it's actually very close in cosmological terms.

Typically you expect remote objects to be moving away. At great distances this has a major impact on various ways of stating that distance.

The link Grant posted (which I'll re-post) is one of the best descriptions of this: http://www.atlasoftheuniverse.com/redshift.html

A good thread discussing different distances at high redshift values:

http://www.bautforum.com/astronomy/3385-quasars-looking-back-time-early-universe.html#post52165

Space.com article on size of universe:

http://www.space.com/scienceastronomy/mystery_monday_040524.html

Javascript cosmological distance calculator: http://www.astro.ucla.edu/~wright/CosmoCalc.html

speedfreek
2009-May-28, 06:30 PM
Apart from the article (http://www.bautforum.com/against-mainstream/87814-can-expansion-universe-gr-illusion-2.html#post1492209) on the size of the universe (which is wrong!), all those links are good sources.

astromark
2009-May-28, 07:29 PM
It has not gone unnoticed that there is a constant trend by some to not accept the mainstream view. Just why this happens I do not understand, ( or am afraid to say).
Is this the old 'I know something no one else does...' syndrome.The only way we could see where M31 is right now would to be there right now. As that is a little beyond us we need to except what little we have learned as facts.

RussT
2009-May-29, 09:44 AM
In which case you know what the answer is going to be.
Yes, all observers will measure the speed of light to be c. But we (in that shared rest frame you and Tim were discussing) will see our chosen observer almost keeping pace with the photon. Special relativity at work, as usual.

Only by becoming that chosen observer for whom the emission and absorption are effectively simultaneous. That is, we need to stop being the "we" you and Tim were discussing, and become someone completely different: in this case, an observer who is next to M31 when it emits the photon.

Grant Hutchison

Geezz...if the chosen observer is right next to M31 where the photon is emitted and absorption is effectively simultaneous at his detector, THEN what the heck does this...almost keeping pace with the photon, even mean???

In which case you know what the answer is going to be.

Ah, but I asked you a very specific question...


Originally Posted by RussT
that is, we find an observer who moves with the photon at as close as we like to lightspeed.

And that lightspeed would be CONSTANT "c" @186,282.4 mps, right?
(and I do understand exactly what I am saying/asking here)

and you knew that I was asking IF THAT OBSERVER next to M31 was traveling at close to 186,282.4 mps Lightspeed, BUT if that were the case then it would take him ~2.5 million years too get here.

BUT, instead you gave a disingenuous response of...Yes, all observers will measure the speed of light to be c.

And then went to give your "BUT"...we (in that shared rest frame you and Tim were discussing) will see our chosen observer almost keeping pace with the photon. Special relativity at work, as usual.

But here you 'really meant' that when the observer is nearly keeping pace with the photon, that it is NOT the 186,282.4 that I asked you if he was traveling BUT, your Special relativity at work, where that observer is traveling at near Infinity!!!

All that boils down to...light travels at a Constant "c" of 186,282.4 and @ Infinity "Simultaneously"

grant hutchison
2009-May-29, 10:26 AM
Geezz...if the chosen observer is right next to M31 where the photon is emitted and absorption is effectively simultaneous at his detector, THEN what the heck does this...almost keeping pace with the photon, even mean???Just what it usually means.
Photon and observer leave M31 simultaneously. Photon arrives here a very short time in advance of the observer. The observer has therefore almost kept pace with the photon during its journey.
In that observer's frame, the photon travels a very short distance in a very short time, at velocity c. In "our" frame (local rest for you and Tim), the photon travels a long way in a long time, at velocity c.

It barely merits a question mark, let alone three question marks, two bold emphases and a set of capital letters.

Grant Hutchison

Ken G
2009-May-29, 03:19 PM
But here you 'really meant' that when the observer is nearly keeping pace with the photon, that it is NOT the 186,282.4 that I asked you if he was traveling BUT, your Special relativity at work, where that observer is traveling at near Infinity!!!Huh? I think neither Grant nor I have any idea what you are trying to say here. It sounds like you are saying you can get arbitrarily large speeds by mixing the time measured in one frame with the distance measured in another. That's certainly true, but is of no consequence, for mixing frames like that is not how physics works. All of Grant's answers, and mine, are straight from special relativity. It seems pretty clear you don't understand that theory very well, but I don't think we can be expected to explain it to you in its entirety. If you have specific questions about it, I'm sure either he or I would be happy to address them, but the key problem right now appears to be that you, and astromark, are still thinking in terms of absolute concepts of distance and time. Tim Thompson can speak for himself, but I wager he would not care to see his words being quoted in support of such a concept, as he is not under that misimpression. That is the first thing you must disabuse yourself of-- distance and time are just coordinates, and they only receive physical meaning when they correspond to measurements by a particular observer. That observer's measurements give them their physical meaning, but they only have that meaning for that observer. The role of the observer is paramount in relativity, I would start there if I were you.

grant hutchison
2009-May-29, 03:37 PM
Ah, but I asked you a very specific question...You probably shouldn't edit extensive additional material into posts retrospectively. People might miss it.
and you knew that I was asking IF THAT OBSERVER next to M31 was traveling at close to 186,282.4 mps Lightspeed, BUT if that were the case then it would take him ~2.5 million years too get here.As measured by "us" (you and Tim) that's correct. As measured by that observer, not so.
But here you 'really meant' that when the observer is nearly keeping pace with the photon, that it is NOT the 186,282.4 that I asked you if he was traveling BUT, your Special relativity at work, where that observer is traveling at near Infinity!!!No, I didn't mean anything quite so idiotic. I meant what I said, and I said it quite clearly.

All that boils down to...light travels at a Constant "c" of 186,282.4 and @ Infinity "Simultaneously"Well, your continuing incomprehension of special relativity is very much looking like your problem, rather than mine.
Do you have any more questions, or are you restricting yourself to statements?

Grant Hutchison

slang
2009-May-29, 03:42 PM
and you knew that I was asking IF THAT OBSERVER next to M31 was traveling at close to 186,282.4 mps Lightspeed, BUT if that were the case then it would take him ~2.5 million years too get here.

As measured by you, not by the observer. The point of adding an observer to a thought experiment like this is to figure out what the observer will observe. You don't let the observer observe, you observe the observer.

RussT
2009-May-30, 10:09 AM
Originally Posted by RussT
But here you 'really meant' that when the observer is nearly keeping pace with the photon, that it is NOT the 186,282.4 that I asked you if he was traveling BUT, your Special relativity at work, where that observer is traveling at near Infinity!!!



No, I didn't mean anything quite so idiotic. I meant what I said, and I said it quite clearly.

Then I guess Grey and Tim are both idiotic...



A more precise wording might be that if you travel from here to Alpha Centauri, moving at arbitrarily close to the speed of light, the trip will take roughly four years for outside observers, but will be arbitrarily short for the person travelling. Depending on how fast you go, the trip could take a year as measured by the traveller, or a day, or a second, or a nanosecond. Essentially no time as measured by an outside observer, if the traveller is moving quickly enough. And it still scales. If you're travelling fast enough that time dilation means you measure a nanosecond to go four light years, then it will take you a whole second to travel four billion light years.

And Tensor agreed...


Originally Posted by Grey
A more precise wording might be that if you travel from here to Alpha

Good explanation snipped.

then it will take you a whole second to travel four billion light years.



Your explanation is certainly more correct.

You see, IF you are NOT 'contracting space' near 0 distance when nearly going lightspeed, so you can say that emition and absorption are nearly simultaneous, then when you go to that observer next to M31, he will be traveling far faster than lightspeed 186,282.4 mps to make your time dilation work, as both Grey and Tensor show above.

The trouble is, there is absolutely no way "Scientifically" to know which of those is "Really" taking place. Why is that? The light we see/detect coming from any distant object, the moon, Alpha Cenrauti, M31 is coming to us @ a Constant 186,282.4 mps and any spaceship traveling with those photons, is traveling with them.

Please do not skip or ignore this.
Do this (KenG, Publius, any pro...jump in your brand new Enterprize Mark X, and take off from Earth (So you have a reference) at a constant (So there is no supposed curvature) 10% light speed. Traveling straight up vertically and for ease, eliminate escape velocity considerations.

1. At what velocity/speed will you be traveling?
2. After traveling for 60 seconds, how far will you have gone?

Repeat answering questions 1 and 2 using 1% light speed, and then try 50% and say 80% lightspeed.

grant hutchison
2009-May-30, 10:29 AM
Then I guess Grey and Tim are both idiotic...



And Tensor agreed...Nah, they know what they're talking about. They're talking about the same thing I'm talking about. (And I can't help but notice that you've snipped those quotes right out of context, suppressing the link back to the original text. Why would you do that?)
The trouble is, there is absolutely no way "Scientifically" to know which of those is "Really" taking place.This is wrong. We know exactly what's taking place: both things at once. Each observer has his or her own experience of time and distance, measured along his or her own worldline.
Please do not skip or ignore this.
Do this (KenG, Publius, any pro...jump in your brand new Enterprize Mark X, and take off from Earth (So you have a reference) at a constant (So there is no supposed curvature) 10% light speed. Traveling straight up vertically and for ease, eliminate escape velocity considerations.

1. At what velocity/speed will you be traveling?
2. After traveling for 60 seconds, how far will you have gone?

Repeat answering questions 1 and 2 using 1% light speed, and then try 50% and say 80% lightspeed.You've provided the answers to question 1 as part of the question (10%, 1%, 50% and 80% of light-speed, and we're probably safe to presume from context you're measuring relative to the "rest frame" of the Earth). Question 2 can't be answered with the information provided, since you're not telling us who measures the 60 seconds, and how the distance travelled is measured.

I know I've said this before, but it bears repeating. Why don't you go and read an introductory text on special relativity, and then get back to us? Watching you try to play off out-of-context and misunderstood quotes, one against the other, is unedifying for all concerned.

Grant Hutchison

grant hutchison
2009-May-30, 10:58 AM
Then I guess Grey and Tim are both idiotic...



And Tensor agreed...Nah, they know what they're talking about. They're talking about the same thing I'm talking about. (And I can't help but notice that you've snipped those quotes right out of context, suppressing the link back to the original text. Why would you do that?)Ah, here (http://www.bautforum.com/1058561-post68.html) is Grey in original context. Interesting that RussT's snip from that post removed the caveat:
As always, don't take a casual statement attempting to explain the general idea of a theory to someone as the theory itself. If you want to know how differently moving observers will measure time according to special relativity, you need to look at the math of special relativity. And if you want to address problems that you think exist within special relativity, you definitely need to be looking at the math itself.Grant Hutchison

Ken G
2009-May-30, 06:19 PM
I'm not quite sure what point you are trying to make RussT. All I see are quotes by Grant, Tim, Tensor, Grey, and myself, that are all fine and correct when you understand what they are saying, and some pretty bizarre interpretations to those quotes being given by you. I think that's just the difference between understanding special relativity, and not. All I could do is repeat: in relativity, you have to identify the observer before you make statements about distances and times, or else you are speaking nonsense.

RussT
2009-May-31, 10:20 AM
You've provided the answers to question 1 as part of the question (10%, 1%, 50% and 80% of light-speed, and we're probably safe to presume from context you're measuring relative to the "rest frame" of the Earth).

Great, let's see if that holds...

Let's send a light beam and your brand new Enterprize mark X @ constant 50% "c", from "Earths rest frame", straight toward Alpha Centauri at 4 light years away.

Then from your above statement, the light beam should get there in 4 years and Enterprize should get there in 8 years, right?

BUT, no that doesn't hold, does it?....that's the reason I asked the question that way. In SR, or SR>>GR (Where you have actual distances to compute using Light as a constant "c" at 186,282.4 mps) 50%, 80% 99% light speed, isn't even a 'speed/velocity', it is a comparison of time/space vs velocity with no distance to be able to determine speed. AND that is 0 to infinity which is why Grey and Tensor both agreed that @ 99.9999% with enough 9's you could get to 4 light years in a nano second, then you could travel 4 billion Lys in 1 second, AND at actual full "c" you get there "Instananeously". SO, the spaceship is obviously "In lights own Frame"
where 'somehow' distance is absolutely meaningless, as light/spaceship can travel to any distance A to B "instaneously/nearly instantaneously". So that spaceship is obviously traveling WAYYYYY faster than 186,282.4 mps.

Now, in the Contract Space scenario, because here you have virtually 0 distance, where emition and absorption are nearly "Simultaneous", all of a sudden you "Think" you can say that the spaceship is traveling at "c" of 186,282.4, with no impunity because of the shrunken distance.



This is wrong. We know exactly what's taking place: both things at once

SO, No, both things cannot be true at the same time!!!

BUT, LOL, That's not even the real point. It's really about comparing 0 to infinity and making up definitions of things that are happening (Supposed to be "REAL") at division by 0.

Light is Constant @ 186,282.4 mps in vacua by definition, which means that the light from M31 at ~2.5 million Lys away took 2.5 miilion years to reach HST. Which means we see/detect M31 where it was 2.5 million years ago. During the last 2.5 million years, M31 has, based on it's current speed in the local group, moved to where it is Now in space. That is what we KNOW as long as light is Constant at 186,282.4 mps Just as Tim said in his quotes I quoted!

RussT
2009-May-31, 11:04 AM
(And I can't help but notice that you've snipped those quotes right out of context, suppressing the link back to the original text. Why would you do that?)

Because I just wanted to cover the material points rather than getting into the normal "Get thee to a Library" type dismissals!!!

Once again the automatic acqusation of "I don't understand if I don't agree with SR considerations". You guy's Think you understand what impact the "Singularities" (and Yes this means the one in SR that is seldom even talked about anymore...why, because 'definitions' have been placed on the 0 and the infinity) but it has now become 100% obvious that you don't!



Ah, here is Grey in original context. Interesting that RussT's snip from that post removed the caveat:
Quote:
Originally Posted by Grey
As always, don't take a casual statement attempting to explain the general idea of a theory to someone as the theory itself. If you want to know how differently moving observers will measure time according to special relativity, you need to look at the math of special relativity. And if you want to address problems that you think exist within special relativity, you definitely need to be looking at the math itself.

Yes, it is continually used as "Authority", BUT as Tim and many others have said repeatedly..."The Maths is NOT the Theory"!!!

and here is another by Tensor



It's no such thing as a proof. It's simply your misconception about relativity. Let''s look at the equations:

g = 1/sqrt(1- v^2/c^2), where g is the gamma factor, v is the velocity in question and c is, of course the speed of light. If you put the speed a photon is moving into v you end up in the denominator, under the square root with this: 1-c^2/C^2, which reduces to 1-1. This leaves you with a zero in the denominator and division by zero is undefined. Which does not mean light travels instaneously or takes no time. It means the equations are undefined.





Originally Posted by RussT
And you just said so yourself!



Well since the equations at c are undefined, there's no way to define it as a frame of reference. You really need to quit taking simplified explanations as the actual theory. Simplified explanations, of a theory, is not the actual theory.




Originally Posted by RussT
Light getting here 'instantaneously', in its own frame, whether it is coming from a Quasar 13 Billion Ly's away or Alpha Centauri......IS Impossible.!!!



No one ever claimed it was, in a rigorous manner. Now, it has been suggested that in a simpler explanation that this is what happens as a way of explaining it to someone who doesnt' understand the math. Simply because the closer an object's speed gets to c, the slower the object's time moves, to an outside observer. But, again, the equations are undefined when anything moves at c. So, we can't really say what happens there. My Bolds

Over and over again it is said that the equations are "Undefined", and especially this one...Well since the equations at c are undefined, there's no way to define it as a frame of reference

You see, to get this...Simply because the closer an object's speed gets to c, the slower the object's time moves, to an outside observer

You have to say what is happening in the "Undefineable Frame".

And YET that is exactly what has been done, and you keep repeating...see the maths...and then...the maths are not the theory

grant hutchison
2009-May-31, 11:09 AM
Great, let's see if that holds...

Let's send a light beam and your brand new Enterprize mark X @ constant 50% "c", from "Earths rest frame", straight toward Alpha Centauri at 4 light years away.

Then from your above statement, the light beam should get there in 4 years and Enterprize should get there in 8 years, right?As measured in the rest frame of Earth and Alpha Centauri, yes.


BUT, no that doesn't hold, does it?....Yes, it does.


SO, No, both things cannot be true at the same time!!!Yes, they can. They demonstrably are. I suggest you live with it.


BUT, LOL, That's not even the real point.LOL, it's the fundamental point.


Light is Constant @ 186,282.4 mps in vacua by definition, which means that the light from M31 at ~2.5 million Lys away took 2.5 miilion years to reach HST. Which means we see/detect M31 where it was 2.5 million years ago. During the last 2.5 million years, M31 has, based on it's current speed in the local group, moved to where it is Now in space. That is what we KNOW as long as light is Constant at 186,282.4 mps Just as Tim said in his quotes I quoted!And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison

grant hutchison
2009-May-31, 11:16 AM
Over and over again it is said that the equations are "Undefined", and especially this one...Well since the equations at c are undefined, there's no way to define it as a frame of reference

You see, to get this...Simply because the closer an object's speed gets to c, the slower the object's time moves, to an outside observer

You have to say what is happening in the "Undefineable Frame".You don't, if the undefinable frame is unreachable. People have also already told you that.

You don't like Special Relativity: I get it. Letting us know that (again) is not what Q&A is for.

Grant Hutchison

captain swoop
2009-May-31, 11:25 AM
RussT
This is a thread in the Q & A forum. This thread is to ask questions and get answers on the Mainstream.
Your last post is pushing an ATM idea. If that's what you want to do start an ATM threead. DO not push your ideas in the Q & A forum.

RussT
2009-May-31, 11:30 AM
I'm not quite sure what point you are trying to make RussT. All I see are quotes by Grant, Tim, Tensor, Grey, and myself, that are all fine and correct when you understand what they are saying, and some pretty bizarre interpretations to those quotes being given by you. I think that's just the difference between understanding special relativity, and not. All I could do is repeat: in relativity, you have to identify the observer before you make statements about distances and times, or else you are speaking nonsense.

It's very late now...I'll type more tomorrow ie Viv Pope>>> Dobson-esk type treatments of SR>>>GR that you have seemed more and more prone to...you may or may not agree with that....:lol:

I guess the point is to try and get you guys to have more correct epiphanies like the obe you had here, which you did NOT need an 'outside observer' to realize...:)



Then I will ask you the same question as I asked Grant, but he did not wish to give an answer at this juncture. In SR, if you accelerate toward alpha Centauri quite suddenly to 0.8c, the distance to alpha Cen will 'really" just as suddenly fall to 0.6 of what it was before. If, in the next second, you change your mind about the journey and decelerate back to the speed you had before, alpha Cen will just as quickly return to its original distance. It will really do that, if you think SR-style


Or if you do see a difference, which seems more physically "real"? I'd say, physically speaking, the SR perspective is pretty close to patently absurd, and that's the reason it does not survive into GR

I realize you are speaking more of the path taken here, BUT in the straight SR version, as you indicated speeding up or slowing down to expand or contract space/distance OR going faster or slower going from point A to Point B, say Sacramento to LA at 200 mph vs 60 mph is basically the same thing.

grant hutchison
2009-May-31, 11:57 AM
Something of a tour-de-force of out-of-context quotation, I have to say, which I believe I must leave Ken to untangle. I wouldn't want to misrepresent his views on the reification (or otherwise) of SR coordinates, as contrasted with the effectiveness of SR in describing reality.

Grant Hutchison

Ken G
2009-May-31, 01:33 PM
I realize you are speaking more of the path taken here, BUT in the straight SR version, as you indicated speeding up or slowing down to expand or contract space/distance OR going faster or slower going from point A to Point B, say Sacramento to LA at 200 mph vs 60 mph is basically the same thing.You are confusing two seperate issues. In SR, a particular inertial observer can define a concept of "global distance" between events, because said observer has a global inertial reference frame. But even so, that distance is only physically real for that particular observer; it means nothing to other observers. My point, that you were quoting out of context (and against forum rules), was that in GR, an observer does not even have a unique global reference frame, only a local one.

But as I said, the main key here is, you can fix all your misstatements simply be recognizing that distance, time, and speed, are all subordinate to an inertial observer in SR, and have no unique global meaning at all in GR. In GR, speed is local, time is measured along a path by a clock taking that path, and distance is a conceptualization with multiple sensible but different definitions.

RussT
2009-Jun-01, 11:08 AM
Originally Posted by RussT
Light is Constant @ 186,282.4 mps in vacua by definition, which means that the light from M31 at ~2.5 million Lys away took 2.5 miilion years to reach HST. Which means we see/detect M31 where it was 2.5 million years ago. During the last 2.5 million years, M31 has, based on it's current speed in the local group, moved to where it is Now in space. That is what we KNOW as long as light is Constant at 186,282.4 mps Just as Tim said in his quotes I quoted!



And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Yes, in fact you did...


Originally Posted by RussT
Are you/Grant saying that we can scientifically see/detect photons from M31 where it is "Now"???



Orginally Posted By Grant
Only by becoming that chosen observer for whom the emission and absorption are effectively simultaneous. That is, we need to stop being the "we" you and Tim were discussing, and become someone completely different: in this case, an observer who is next to M31 when it emits the photon.

And as I have said, and Tim showed too...We cannot see photons or observers from where M31, or any distant object, is "Now".



And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

But, Now that you are agreeing with my orginal post, I want an apology from both you and KenG...

I didn't get involved with this until Jeff Posted this...



Originally Posted by Jeff Root
The distance
given is the distance the light has traveled, from the point in space and
time at which it was emitted, to the point in space and time at which it
is received.

Which KenG quoted in his Philosophy of science and reality kind of way....
http://www.bautforum.com/1495092-post14.html

Starting off with...


There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels.

Which, with your agreement that Tim was indeed correct has fully "Falsified" KenG's stance, as we can now readily see that with our HST's and ground based telescopes, as a single observer, with "c" as a constant of 186,282.4 mps there is definitely a distance light travels in a definite amount of time, which is all I was orginally saying.

Whether there is a secondary observer out there, moving at relavistic speeds is whole 'nother can of worms...;)

RussT
2009-Jun-01, 11:20 AM
Originally Posted by RussT
Great, let's see if that holds...

Let's send a light beam and your brand new Enterprize mark X @ constant 50% "c", from "Earths rest frame", straight toward Alpha Centauri at 4 light years away.

Then from your above statement, the light beam should get there in 4 years and Enterprize should get there in 8 years, right?



As measured in the rest frame of Earth and Alpha Centauri, yes.



Originally Posted by RussT
BUT, no that doesn't hold, does it?....



Yes, it does.

KenG, do you agree with Grant's assessment here?

Ken G
2009-Jun-01, 03:20 PM
And as I have said, and Tim showed too...We cannot see photons or observers from where M31, or any distant object, is "Now".Obviously. No part of anyone's argument here claimed that we could see that. The rest of us are clear on what is an observer-dependent statement.


Which, with your agreement that Tim was indeed correct has fully "Falsified" KenG's stance, as we can now readily see that with our HST's and ground based telescopes, as a single observer, with "c" as a constant of 186,282.4 mps there is definitely a distance light travels in a definite amount of time, which is all I was orginally saying.The issue is whether this claim is observer-dependent or generally true. Here you specify the HST, before you did not. That was the error from before, I just can't say it any more clearly.

Ken G
2009-Jun-01, 04:17 PM
KenG, do you agree with Grant's assessment here?Yes, the spaceship moving at 0.5 c in the Earth frame will require 8 Earth years to travel the 4 LY (in the Earth frame) to alpha Cen, and light will require 4 Earth years to do the same trip. Is that not what Grant said? What other possibilities are there, this follows from the definition of velocity, coupled with the appropriate attention to specifying a reference frame.

RussT
2009-Jun-02, 11:12 AM
Originally Posted by KenG
There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels.

So, KenG, do you now agree that this is a "Fatally Flawed" statement?

Jeff Root
2009-Jun-02, 11:43 AM
My opinion is and has been all along that although my reply to the original
question wasn't perfect, it was the appropriate answer, it was useful, and
it was understandable. Ken's objection was ridiculous. If there is no such
thing as the distance the light travels, and no such thing as the time the
light travels, then there is no such thing as distance and no such thing as
time, and no such thing as Ken.

When the distance of a galaxy is given in light-years, what is meant is the
distance from where and when its light was emitted, to where and when it
is observed.

-- Jeff, in Minneapolis

Ken G
2009-Jun-02, 01:20 PM
So, KenG, do you now agree that this is a "Fatally Flawed" statement?No, the statement is completely consistent with relativity, our best theory for understanding motion as seen by observers. Perhaps you just didn't understand it. What you were supposed to get from it was that thd concepts of distance and time have no unique meaning in the absence of specifying an observer. (Even then their meanings get slaughtered constantly, but that wasn't the issue here.) I regret I didn't spell out that last bit. Nor do any of the quotes your trotted out say otherwise, if understood in context.

Ken G
2009-Jun-02, 01:29 PM
My opinion is and has been all along that although my reply to the original
question wasn't perfect, it was the appropriate answer, it was useful, and
it was understandable. Your answer, which was

The distance given is the distance the light has traveled, from the point in space and time at which it was emitted, to the point in space and time at which it is received.

was actually quite problematic, to say the least. I realize that an OP is not always asking for a relativity tutorial, but this one was clearly a relativity-related question. What's more, the question had already been answered far more adequately and carefully by people who bothered to include the role of the observer and the latitude of choices about non-unique distance measures. Yet for some reason, you felt it was necessary to enter a sweeping statement about distances between "points", with no reference to any observer or coordinate choice, as if these were absolutely real or uniquely definable entities. That was my objection, your answer was quintessentially antithetical to the core notions of relativity, and I was aghast that the OPer might actually think one could explain relativity-related concepts in those terms. It is quite important to recognize that distance is an observer-defined quantity, so all answers about it should at the very least imply what observer is being referenced.


Ken's objection was ridiculous.That's the amazing thing about relativity: it really seems ridiculous, to anyone who doesn't understand it.


If there is no such thing as the distance the light travels, and no such thing as the time the light travels, then there is no such thing as distance and no such thing as time, and no such thing as Ken.Your logical syllogism falls into "straw man" fallacy, sadly. See my response to RussT above, it would seem you also missed the whole point of my remark. Perhaps I should have put "the distance the light travels" in scare quotes.


When the distance of a galaxy is given in light-years, what is meant is the
distance from where and when its light was emitted, to where and when it
is observed.Again you seem to think distance is an absolute and uniquely definable concept. Although it has been referenced many times on this forum already, perhaps you should have a look at Ned Wright's cosmology tutuorial, particularly http://www.astro.ucla.edu/~wright/cosmo_02.htm. There you will find there are many ways to measure distance for a particular observer, and that doesn't even count the infinitude of ways to define distance via a coordinate choice intended to convey measurements by different observers.

Jeff Root
2009-Jun-02, 10:42 PM
Your answer, which was

The distance given is the distance the light has traveled, from the
point in space and time at which it was emitted, to the point in
space and time at which it is received.
was actually quite problematic, to say the least. I realize that an OP
is not always asking for a relativity tutorial, but this one was clearly a
relativity-related question. What's more, the question had already
been answered far more adequately and carefully by people who
bothered to include the role of the observer and the latitude of choices
about non-unique distance measures.
No, the answers before mine were unclear. Not one of them refers
to the relativistic nature of the question that you brought up in reply
to me. They do refer to other measures of distance, but not to the
relativistic nature of distance. Grant's first post (#3) focused on
the large uncertainty in the distance. His post #12 focused on
alternative measures of distance. None addressed the relativity
aspect that you raised. None clearly answered the original question.



Yet for some reason, you felt it was necessary to enter a sweeping
statement about distances between "points", with no reference to
any observer or coordinate choice, as if these were absolutely real or
uniquely definable entities. That was my objection, your answer was
quintessentially antithetical to the core notions of relativity ...
You may reasonably infer from what I said that I might have had
an "absolute" reference frame in mind, but that inference would be
your interpretation. What I said is correct and not in conflict with
the relativistic description you are championing. It does not specify,
imply, or assume an absolute frame. But I agree that some people
might reasonably infer that it does, if they are of a mind to think
that way.

-- Jeff, in Minneapolis

Ken G
2009-Jun-03, 02:19 AM
You may reasonably infer from what I said that I might have had
an "absolute" reference frame in mind, but that inference would be
your interpretation. Yes, I did interpret your statement as having misleading content for those who are just learning relativity, and yes, that is my interpretation, as all language requires interpretation before it has meaning. If the OPer would not have been misled by your comments, then mine were unnecessary. Obviously, my money was on a different outcome. The problem is that the words that are easier to hear are often the wrong words for learning relativity.

AndrewJ
2009-Jun-03, 04:07 AM
The OPer's follow-up in post 11 confirms that the nub of the original question was whether quoted distances to galaxies refer to "then" or "now"; post 12 clarified. The discourse on page 2 of this thread regarding reference frames seems disproportionately complex.

Ken G
2009-Jun-03, 05:35 AM
It would indeed seem that, if you did not realize that the concepts of "then" and "now", applied accross millions of LY, are actually complex ideas. (Indeed, I tried to point out that those concepts have no unique or absolute meaning at all.) I suppose it all depends on if you, or the OPer, want to know the way reality actually works, or just the way you'd like to imagine it does.

RussT
2009-Jun-03, 10:26 AM
My opinion is and has been all along that although my reply to the original
question wasn't perfect, it was the appropriate answer, it was useful, and
it was understandable. Ken's objection was ridiculous. If there is no such
thing as the distance the light travels, and no such thing as the time the
light travels, then there is no such thing as distance and no such thing as
time, and no such thing as Ken.

When the distance of a galaxy is given in light-years, what is meant is the
distance from where and when its light was emitted, to where and when it
is observed.

-- Jeff, in Minneapolis

Ken's objection was ridiculous.

I absolutely concur! Especially when you consider the most likely "Observer" over there, in Andromeda, ~2.5 million light years away...The Scientist looking through his/her telescopes on or around their 3rd rock from their sun, in their "Rest Frame".

RussT
2009-Jun-03, 10:38 AM
It would indeed seem that, if you did not realize that the concepts of "then" and "now", applied accross millions of LY, are actually complex ideas. (Indeed, I tried to point out that those concepts have no unique or absolute meaning at all.) I suppose it all depends on if you, or the OPer, want to know the way reality actually works, or just the way you'd like to imagine it does.

The OP'er asked a relatively simple question about Andromeda (M31) in the local group with no expansion considerations and about the "Then" and "Now"
of M31 and our Milky Way.

And just like I showed in the message confirming Jeff's opinion, when you consider the "Most Likely" 'secondary observer' being The Scientist looking through his/her telescopes on or around their 3rd rock from their sun, in their "Rest Frame", then it is definitely you KenG that is confusing the OP'er!!!

And for those two observers...this...........Originally Posted by KenG
There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels. Most Definitely IS "Fatally Flawed"

SO, let's add some "Reality" to "Then" and "Now".

If a Super Nova (SN) exploded in Andromeda, we would not be able to see the 'first light' from that SN for ~2.5 million years After it exploded. Put the other way...When we see a SN in M31, we see it (First Light) as it happened ~2.5 million years ago.

antoniseb
2009-Jun-03, 11:54 AM
... SO, let's add some "Reality" to "Then" and "Now"...

RussT, I think with a little rereading, you'll see how the above statement might seem dismissive and offensive to people who posted some well thought-out statements about this topic. Please be respectful of the people who understand General Relativity. They have feelings just like you do.

Concerning the OP, the original question was something that makes intuitive sense on small scales (2 parts in 10,000 of the radius of the visible universe in this case), and starts to require an understanding of GR and the expanding universe when you discuss things on the scale of the entire visible universe.

Ken G
2009-Jun-03, 12:57 PM
I absolutely concur! Especially when you consider the most likely "Observer" over there, in Andromeda, ~2.5 million light years away...The Scientist looking through his/her telescopes on or around their 3rd rock from their sun, in their "Rest Frame".Here's the problem with your reasoning, and Jeff Root's as well. The OP involved two elements:
1) extreme distance
2) difference between distances "then" and distances "now"
To answer a question with these two elements, one encounters relativity. One may often use naive concepts of distance, like how far is it to Los Angeles, but then one does not encounter these unique elements of the OP. Conversely, when the unique elements of the OP are in play, then one may not use naive concepts of distance. In particular, one must specify a single observer, or stick to invariant concepts. Neither you nor Jeff Root did the latter, so we must assume you are doing the former.

Except, the observer is never specified. Perhaps it could be argued that is all right, because the implied observer is obvious. However, in your post here, you demonstrate quite cleary that the implied observer is not obvious. You seem to feel that the appropriate observer is one in the Andromeda galaxy! Jeff Root, on the other hand, appears to feel the appropriate observer is here on Earth. So to summarize, the problem with your naive approach is twofold:
1) the observer you are using is ambiguous or unknown
2) there is no recognition that there is even a need to specify an observer.

Now, you might think that these don't matter if both observers are stationary relative to each other, or approximately so. But then, the unique elements of the OP do not come into play at all. Indeed, that is the way grant hutchison answered the question-- for most levels of desired accuracy, there would be no difference between distance "then" and "now". So my comment was only to add that whenever there actually is a difference, one must use relativistically correct language, which neither you nor Jeff Root were doing. But you are welcome to see an understanding of relativity as ridiculous, naive concepts of distance are certainly useful in our daily lives and we can pretend they apply in other places that don't actually affect our lives, whenever genuine understanding is not our goal.

grant hutchison
2009-Jun-03, 01:54 PM
Originally Posted by RussT
Light is Constant @ 186,282.4 mps in vacua by definition, which means that the light from M31 at ~2.5 million Lys away took 2.5 miilion years to reach HST. Which means we see/detect M31 where it was 2.5 million years ago. During the last 2.5 million years, M31 has, based on it's current speed in the local group, moved to where it is Now in space. That is what we KNOW as long as light is Constant at 186,282.4 mps Just as Tim said in his quotes I quoted!
Originally Posted by Grant
And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.
Yes, in fact you did...
Originally Posted by RussT
Are you/Grant saying that we can scientifically see/detect photons from M31 where it is "Now"???
Orginally Posted By Grant
Only by becoming that chosen observer for whom the emission and absorption are effectively simultaneous. That is, we need to stop being the "we" you and Tim were discussing, and become someone completely different: in this case, an observer who is next to M31 when it emits the photon.Have you ever considered actually reading the content you quote?
That was a fine example of me not saying anything different. And saying it quite clearly, too.


But, Now that you are agreeing with my orginal post, I want an apology from both you and KenG...Since I don't agree with your original post, and have written nothing in which I agree with your original post, you'll perhaps understand why no such apology will be issued.

Grant Hutchison

RussT
2009-Jun-04, 08:28 AM
RussT, I think with a little rereading, you'll see how the above statement might seem dismissive and offensive to people who posted some well thought-out statements about this topic. Please be respectful of the people who understand General Relativity. They have feelings just like you do.

Concerning the OP, the original question was something that makes intuitive sense on small scales (2 parts in 10,000 of the radius of the visible universe in this case), and starts to require an understanding of GR and the expanding universe when you discuss things on the scale of the entire visible universe.

Exactly...So why I am the one that is being admonished when KenG is "Still" doing this...Here's the problem with your reasoning, and Jeff Root's as well. The OP involved two elements:
1) extreme distance
2) difference between distances "then" and distances "now"

And being disrepectful to both Jeff, myself and even Tim's orginal Correct statement???

antoniseb
2009-Jun-04, 08:50 AM
Exactly...So why I am the one that is being admonished when KenG is "Still" doing this...Here's the problem with your reasoning, and Jeff Root's as well. The OP involved two elements:
1) extreme distance
2) difference between distances "then" and distances "now"

And being disrepectful to both Jeff, myself and even Tim's orginal Correct statement???

I'm not reading it that way. I see Grant and Ken as being GR experts who've come to clarify some details. If you are complaining about having the answers improved, and your simplified answer not being taken as the final answer, then I think you're missing something about how the forum operates. If your complaint is some subtlety about the phrase "problem with your reasoning..." (as opposed to something like "improving on your simplified approach...") you might be being a bit thin-skinned here.

Let's drop this, and get back to the topic. No one got close to being warned here.

RussT
2009-Jun-04, 08:57 AM
Yes, in fact you did...
Quote:
Originally Posted by RussT
Are you/Grant saying that we can scientifically see/detect photons from M31 where it is "Now"???

Quote:
Orginally Posted By Grant
Only by becoming that chosen observer for whom the emission and absorption are effectively simultaneous. That is, we need to stop being the "we" you and Tim were discussing, and become someone completely different: in this case, an observer who is next to M31 when it emits the photon.


Have you ever considered actually reading the content you quote?
That was a fine example of me not saying anything different. And saying it quite clearly, too.

Here is how I read this Grant, and quite frankly, I don't know how anyone could interpret it any differently...

I ask you...Are you/Grant saying that we can scientifically see/detect photons from M31 where it is "Now"???

And you say....Only by becoming that chosen observer for whom the emission and absorption are effectively simultaneous.

I see that as saying Yes, we can see it's "Now", but "ONLY BY BECOMING THAT OBSERVER".

Why should I interpret that any differently?

And then you go on to say...That is, we need to stop being the "we" you and Tim were discussing, and become someone completely different: in this case, an observer who is next to M31 when it emits the photon.

And then both you and KenG keep saying that you never did say that we could see M31's "Now"

The 2500 ly distance difference from where it was 2.5 million years ago, to where it is today, is peculiar movement, which we figured out, by the exactly correct statement that I quoted from Tim Thompson in the first place.

Quote:
Originally Posted by Tim Thompson
OK, is this what I am supposed to be answering? Haven't I already done that several times? How many different ways do you want to see the same answer? We detect photons in our "now", and that tells us what M31 looks like in our "now". Realizing that it took photons 2.5 million years to make the trip, and that our "now" is 2.5 million years after the "now" of M31 we see, we can then derive from physics how M31 would have changed over the intervening 2.5 million years. That way we can synthesize a picture if what M31 would look like if photons made the trip instantaneously, and that the two "now"s were simultaneous.My Bold/Red

Now, How BIG should I make that little red "IF"

In the "Local arena" of space, the moon, sun, alpha Centauri, local galaxies out to where the first/closest "Voids" are, where space could be expanding, there is absolutely no need to find "a supposed alien in a spaceship Observer" who is traveling at "near "c"" Either 'next to the Milky way, traveling to M31 at near "c", nearly "Instantaneously", OR one 'next to M31' traveling toward the Milky way at near "c" so emition and absorption are nearly "Simultaneous"

I can't make it much clearer than that...;)

RussT
2009-Jun-04, 09:33 AM
Here's the problem with your reasoning, and Jeff Root's as well. The OP involved two elements:
1) extreme distance
2) difference between distances "then" and distances "now"

No, KenG, this is Your Problem...You keep restating the OP to suit yourself. See the other posts.

You keep agreeing that you are not seeing M31 in it's "Now" when this...find "a supposed alien in a spaceship Observer" who is traveling at "near "c"" Either 'next to the Milky way, traveling to M31 at near "c", nearly "Instantaneously", OR one 'next to M31' traveling toward the Milky way at near "c" so emition and absorption are nearly "Simultaneous"

Is done...so it is of no use what-so-ever!!!

In fact, if you are doing the Lorentz Contraction and not seeing M31 as it is and where it is "Now" and still seeing it as it was and where it was 2.5 million years ago, then what does it "Really Mean" to do the Lorentz contraction?

Jeff Root
2009-Jun-04, 10:54 AM
I have not put in the time and energy to follow Russ's arguments. I see
statements of his that, if I understand them correctly, I agree with, but
I am not at all sure I understand them, simply because they require too
much effort to sort out.

The original post asked a simple, straightforward question: When the
distance to the Andromeda galaxy is given as 2.5 million light-years,
does it mean the distance now or the distance 2.5 million years ago?
After several unclear answers, I gave a clear answer: "The distance
given is the distance the light has traveled, from the point in space
and time at which it was emitted, to the point in space and time at
which it is received." Ken replied to that by saying: "There's no such
thing as the distance the light travels. There isn't even such a thing
as the time the light travels." Very dramatic. And very useless.



Here's the problem with your reasoning, and Jeff Root's as well.
The OP involved two elements:
1) extreme distance
2) difference between distances "then" and distances "now"
To answer a question with these two elements, one encounters relativity.
Not necessarily. Not in this case. In this case, relativity is an add-on.
It has no direct bearing on the answer to the original question.

My superficial understanding of Russ's argument, from a quick read, is
that you, Ken, introduced an extreme, even absurd scenario to justify
bringing relativity into the discussion. Namely, the possibility that the
observer might be traveling from Andromeda to the Milky Way at a speed
that makes the trip last only a second or two in his frame.

In that case, Ken, where did the figure of 2.5 million light-years come from?

Yes, I generalized my answer to apply to the distance of any galaxy when
given in terms of light travel time, without spelling out the fact that it is
light travel time -- because that fact was already clear from the OP. And
because my answer applies to other, more distant galaxies as well as to
Andromeda. Just because more distant galaxies are involved in the cosmic
expansion doesn't mean that stated distances aren't the distance from the
place and time of light emission to the place and time of reception. They
are. And that is true even in your extreme case of an observer moving at
absurd speed from Andromeda to the Milky Way. The distance that would
be given in such a case would be whatever distance resulted from the
motion of the observer, even if it means the distance between the galaxies
is only one metre and the light travel time is only three nanoseconds.
The distance would still be the distance from the place and time of light
emission to the place and time of reception.

-- Jeff, in Minneapolis

grant hutchison
2009-Jun-04, 12:30 PM
Here is how I read this Grant, and quite frankly, I don't know how anyone could interpret it any differently...

I ask you...Are you/Grant saying that we can scientifically see/detect photons from M31 where it is "Now"???

And you say....Only by becoming that chosen observer for whom the emission and absorption are effectively simultaneous.

I see that as saying Yes, we can see it's "Now", but "ONLY BY BECOMING THAT OBSERVER".Only by becoming an observer that we are not now and cannot become, yes.


And then you go on to say...That is, we need to stop being the "we" you and Tim were discussing, and become someone completely different: in this case, an observer who is next to M31 when it emits the photon.Indeed. I then clarify in what way this observer diverges from anything we are capable of doing. We'd need to stop being members of current humanity, and become an observer who is next to M31 when it emits a photon. We wouldn't be "us" any more.

You ask, "Can I make my elbows meet inside in my head?" I answer, "Only if you break both your arms off and then drive your elbows vigorously through your temporal bones." By specifying actions that you are physically incapable of performing, and which would eliminate you promptly from conscious existence, I'm answering your question with a "No". You, in your current state of existence, can't make your elbows meet inside your head.
Only a remarkable individual would interpret my answer as an affirmation.

Grant Hutchison

Ken G
2009-Jun-04, 02:02 PM
The original post asked a simple, straightforward question: When the
distance to the Andromeda galaxy is given as 2.5 million light-years,
does it mean the distance now or the distance 2.5 million years ago?As I said, that "simple straightforward question" does involve the combination of extreme distance, and the concepts of "then" and "now". Hence, there are only two possible useful answers: the one Grant gave, if there is no important difference between "then" and "now" (as in the actual case of the OP), or mine if there is an important difference between those concepts (which would only appear in a genuinely relativistic situation). That simply covers the possibilities here.


Yes, I generalized my answer to apply to the distance of any galaxy when
given in terms of light travel time, without spelling out the fact that it is
light travel time -- because that fact was already clear from the OP. Again, if the OP was about light travel time, then it would have been about time, not distance, and there would be no difference at all between "distance then", and "distance now", the question would have been meaningless (apparently your interpretation). I assumed they wanted to learn something, not be confused by an answer that confuses distances with lookback times. If Ned Wright's admonishments about not confusing distances and times did not convince you of that folly, I'm sure I cannot say it better than he did.


And
because my answer applies to other, more distant galaxies as well as to
Andromeda.Please read Ned Wright's cosmology tutorial, and note that no words remotely like the ones you used will appear anywhere in it. There's a reason for that. What I'm trying to help you, and RussT, and the OPer, and anyone else who cares to pop in, to understand is that for distance to be meaningful on cosmological scales, two elements must be in place:
1) the distance measure being used has to be specified, and
2) the observer(s) using that measure must be specified
Together, these serve to determine a distance coordinatization, which is what is meant by attributing nunbers to distances. Absent them, there is no meaning at all to the term "distance", which is the point I was making. At this point, the only remaining question is, do you want to understand the meaning of distance on cosmological scales, or don't you, because it is eminently clear that you have some work to do if your current appreciation is "the distance between two points is the distance between two points", which is all you actually said.

RussT
2009-Jun-05, 09:53 AM
I have not put in the time and energy to follow Russ's arguments. I see
statements of his that, if I understand them correctly, I agree with, but
I am not at all sure I understand them, simply because they require too
much effort to sort out.

Jeff, we are actually in total agreement accept for one thing. Allow me to parse your statements here and I think you will agree.



The original post asked a simple, straightforward question: When the
distance to the Andromeda galaxy is given as 2.5 million light-years,
does it mean the distance now or the distance 2.5 million years ago?



After several unclear answers,

The only thing unclear about Grant's answer in Post # 3, was that he didn't show the speed of the Milky Way and the relation between M31's Co-moving with us.

That is the "Important Part" of figuring out M31's "Then" (Where it was ~2.5 million years ago when the light left M31) VS M31's "Now" (Where it has moved to over the last 2.5 million years based on it's speed)

Is it going to collide with us...That is the important part.



I gave a clear answer: "The distance
given is the distance the light has traveled, from the point in space
and time at which it was emitted, to the point in space and time at
which it is received."

Yes and as I have said numerous times, that is exactly correct.



Ken replied to that by saying: "There's no such
thing as the distance the light travels. There isn't even such a thing
as the time the light travels." Very dramatic. And very useless.

Exactly, which is why I said I got involved in this thread to begin with. Not only useless, but as I have claimed, and still do, "Fatally Flawed".


Originally Posted by Ken G
Here's the problem with your reasoning, and Jeff Root's as well.
The OP involved two elements:
1) extreme distance
2) difference between distances "then" and distances "now"
To answer a question with these two elements, one encounters relativity.



Not necessarily. Not in this case. In this case, relativity is an add-on.
It has no direct bearing on the answer to the original question.

Once again you and I are in total agreement. You will see below why.



In that case, Ken, where did the figure of 2.5 million light-years come from?

Exactly........why didn't kenG answer this one?



My superficial understanding of Russ's argument, from a quick read, is
that you, Ken, introduced an extreme, even absurd scenario to justify
bringing relativity into the discussion. Namely, the possibility that the
observer might be traveling from Andromeda to the Milky Way at a speed
that makes the trip last only a second or two in his frame.

Now, here it gets a little trickier.

My objection is two-fold. I already got warned once for the first objection about the frame of the traveling observer being in the same frame as light's own frame not even existing, so I won't go into that here.

But the second objection is exactly the same as yours.

Until you reach "Expanding Space"...IE past Voids between galaxy clusters...It has no direct bearing on the answer to the original question. and...Very dramatic. And very useless.
Your correct words and concept, Jeff.

Now, let me explain a few things...

If I would have come in this thread, by myself, to back up your correct 'take' on this, kenG and Grant probably would have used a 'lot more'...you don't understands, and other dismissive's.

Even with Tim Thompson's own words, showing the correct scenario, KenG has admonished us repeatedly, and is "Still" continuing to do so.

Where again, in his very last post he restates the OP bringing in the Expanding universe rather than local considerations.



As I said, that "simple straightforward question" does involve the combination of extreme distance, and the concepts of "then" and "now". Hence, there are only two possible useful answers: the one Grant gave, if there is no important difference between "then" and "now" (as in the actual case of the OP), or mine if there is an important difference between those concepts (which would only appear in a genuinely relativistic situation). That simply covers the possibilities here.

I have showed above why the "Then" and "Now" IS important, and why the "red" does not apply, just as we have shown.

Now, it has become obvious to me that Tim Thompson understood something here, that evidently KenG and Grant did/do not.

The Local distance ladder was developed by strictly using light as a Constant @ 182,2822.4 mps...;)

That's as clear as I can make it.

More on the "emition and absorption" being nearly "Simultaneous" when traveling at Near "c" in your favorite ATM thread.

slang
2009-Jun-05, 10:44 AM
Where again, in his very last post he restates the OP bringing in the Expanding universe rather than local considerations.

Was that very last post in a different thread?


More on the "emition and absorption" being nearly "Simultaneous" when traveling at Near "c" in your favorite ATM thread.

And there we have it. Relativity to be ATM. If a long chain of reasoning and arguing leads you to such a conclusion, should that not be a clear sign that something is wrong somewhere in the chain?

Ken G
2009-Jun-05, 07:04 PM
"The distance
given is the distance the light has traveled, from the point in space
and time at which it was emitted, to the point in space and time at
which it is received."

Yes and as I have said numerous times, that is exactly correct.
Let us examine, using nothing but unmistakably pure logic, what is "exactly correct" about this statement. First of all, there are two possibilities in the meaning of the words used:

possibiliy #1) The words "distance the light has traveled" means the same thing as just "distance". In this case, the statement is the same thing as "the distance given is the distance between the two points mentioned." Interesting, the distance is the distance. Or perhaps it is the two points described that hold the content of the statement? No, the two points are not specified in any way other than the point of emission and the point of reception. Is it mentioning both space and time that holds the content of the statement, as if there were something special about distances in spacetime that does not apply to space alone? Hmm, well there is the concept of proper time (related to proper distance), but it isn't proper distance being used here (more on that in a moment). So no, there is no added content in saying the points are in "space and time" either, the statement is nothing by "the distance between emission and absorption is the distance between emission and absorption." So in possibility #1, the "exactly correct" statement is also completely devoid of any content whatsoever.

possibility #2) the words "distance the light travels" means something different from just "distance". In this case, somehow the meaningful content of the remark must hinge on this difference, that somehow the fact that light is traveling this distance informs it with some special or useful meaning it would not have had if it had just been the word "distance". I wonder what new meaning is intended to be given to the distance because light traveled it? Here we must distinguish two new possibilities:

possibility #2a) The new meaning given to distance, owing to the fact that light traveled it, is the relativistic meaning of "proper distance". This would indeed be something special about the propagation of light, in regard to the proper distance it travels. Unfortunately for the above statement, light propagates along what are called "null geodesics", which means that the proper distance traveled by the light is always zero. That would not appear to be the point the above statement is trying to make, that the distance is zero, so we can assume it is not proper distance that is being referred to.

possiblity #2b) The new meaning given to distance, owing to the fact that light traveled it, is not the proper distance, but some other kind of special distance that is distinguished by the fact that light traveled it. Hmm, here my knowledge of relativity must be limited, for I know of no other such special distance other than proper distance. Are we in some ATM land here? No, the claim is made that the interpretation given is not ATM, and RussT feels he has some reason to agree with it without reference to any ATM concepts. So either we have here misconceptions about relativity, because said theory includes no such other special distance associated with light other than the one that comes out zero, or else possibility #2 is not the one intended. Sadly, in that case, possibility #1 is entirely inclusive of the remaining possibilities, and we already saw the content in that possibility.

So where does logic lead us? Inexorably to the conclusion that the quoted remark is null in content (for possibility #1), or false (possibility #2a), or ATM (possibility #2b). I really couldn't say which one Jeff Root intended, he'd be needed for that, but I can categorically state, based on the authority of pure logic, that these are the inclusive possibilities.


Exactly........why didn't kenG answer this one?There are so many misconceptions flying around in the posts I'm responding do that I had to kind of pick and choose which ones I would actually address specifically, but I'd be happy to address this one as well. The 2.5 million light years comes from a particular coordinatization of spacetime. It is the standard one, though by no means a unique one, and in the case of such a nearby distance with the uncertainty it has, there is no way to even distinguish the various possibilities that might be used. That was Grant's correct answer, it required no further embellishment if the OP was really only interested in such nearby galaxies. But in case the OPer was also curious about larger distances, say 2.5 billion light years, then the nature of the coordinatization is essential to understand. If lookback time is being used, then it is not a distance at all, it is a time given by convention in distance units (and related to a particular time coordinatization)-- a point made earlier in the thread, and entirely inconsistent with the quoted "exactly correct" statement above. If comoving-frame coordinates are being used, then it would be natural to say the distance is the "distance now", also in contradiction to your position, although even that statement requires clarification to avoid misconception. Or as was pointed out above also, if "distance then" is intended, the the angular-size distance is being used. What is all-too-often quoted is the lookback time, which is not a distance at all, so that right there would have been a far better, and more meaningful, answer than the one you quoted above.

This is perhaps a good place to clarify that distance measurements always require some interpretation, as they always stem from some other kind of measurement-- either a redshift, or a brightness, or an angular size, etc. So the actual meaning of the distance quoted requires understanding what was actually measured, and typically this is then plugged into some particular cosmological model to turn it into a coordinate in whatever coordinatization is being used (typically either lookback time, which is a time coordinate, or comoving-frame distance, which is a distance coordinate, and neither validate the quote above).


Even with Tim Thompson's own words, showing the correct scenario, KenG has admonished us repeatedly, and is "Still" continuing to do so.What you are ignoring here is your own misinterpretation of the meaning of Tim Thompson's words, which are obvious to anyone who does know relativity.

More on the "emition and absorption" being nearly "Simultaneous" when traveling at Near "c" in your favorite ATM thread.Again, to anyone who knows relativity, that near-simultaneity is mother's milk.

publius
2009-Jun-06, 12:52 AM
Ken,

A word about "proper distance". You know, I don't really know if that is defined hard and fast -- there may be some looseness in what is really meant by that.

Really, the integral of ds^2 should be called the "interval" or the norm, and not distance (although it is the generalization of "distance" into non-positive definite geometries). We should reserve "distance" for a space-like such interval, which will of course be coordinate dependent.

As I understand things, what is usually meant by proper distance, is this. We have some coordinates. We hold time constant, and integrate ds^2 between events thus simultaneous in those coordinates. In the typical FLRW style, for a spatially flat, 1, 1 manifold:

ds^2 = (c dt)^2 - a(t)*( dx^2)

A proper distance is then what you get when dt = 0, two spatial locations "now", D^2 = a(t)*dx^2 (and I forget if it's standard business to make a(t) squared or not to agree with the squares on everything else).

Now, that only means something in those coordinates, for obsevers sharing that "time-like congruence". To other observers, not sharing those spatial hyperslices, those events will occur at different times, and that "proper distance" is just some meaningless space-like interval between events outside of each other's light cones.

Light paths are simply null,

ds^2 = 0 --> dx/dt = c/a(t). And we can trivially see that the coordiantes of a ruler with constant "proper length", D, are simply "shrinking" with a(t), ie dx = dD/a(t), so dD/dt = c.

So trivially, almost "duh!", we must say, the light travel distance is simply ct, where t is the coordinate time interval since it was emitted (which is the proper time of co-movers).

To all lurkers, this is just the result of the defintion that that light travels a null path, the proper distance over the proper time of a stationary clock at the scene is simply c. Since all co-moving clocks, in these coordinates, are ticking at the same rate, which is merely a convention of choosing hyperslices with some nice properties, the proper clock rate doesn't vary with coordinate 'x' (go to coordinates where frame dragging is afoot, and this notion of stationary gets problematic -- beyond any stationary limit, it will be impossible to have a stationary clock).

So what does this mean. The "light travel distance" is simply a measure of the co-moving time of flight, scaled by 'c'. That is not the "distance" as defined above at the point of emission "when" it was emitted, nor is it a measure of the distance to the point of emission "now", when and now being simultaneity conventions defined by the coordinate hyperslices.

Bottom line is "distance" is a coordinate dependent thing -- it is not invariant and thus has no real meaning in GR. We can switch to some other coordiante system, say static coordinates (or as close as we can get in this dynamic universe), and all these "distances" will be something very different.



-Richard

publius
2009-Jun-06, 01:17 AM
Let me try to drive this business home about how trivial -- a tautology in fact -- this D = c T business is, where D = 'proper distance' and T = proper time.

Consider Schwarzschild:

ds^2 = K^2 * (c dt)^2 - dr^2/K^2, where K^2 = 1 - R/r, with R being the Schwarschild radius, and considering only the radial direction. What does the say about light, ds = 0?

dr/K = K * (c dt) --> [dr/K]/[K*dt] = c.

The proper distance over the proper time of an observer stationary in these coordinates of light is simply the constant c. In fact, this is the defintion of the local ruler and clock, not the other way around!

So really, the light travel "distance" is a tautology. In Schwarzschild, stationary clocks tick at different rates, so we've got a factor on the time in the denominator. In co-moving FLRW coordinates, that factor is not there, just one, so dD/dt = c.

In Schwarzschild, dD/dt = K*c, that is the "proper distance" per *coordinate time* is less than c for small r. In FLRW there is no distinction between coordinate time and proper time, which blurs this point.

So the light travel distance is just sort of silly. It's trivially ct, a measure of time

-Richard

Ken G
2009-Jun-06, 05:27 AM
As I understand things, what is usually meant by proper distance, is this. We have some coordinates. We hold time constant, and integrate ds^2 between events thus simultaneous in those coordinates.Yes, I believe the standard meaning of proper distance is the square root of the invariant "norm", as you call it, which is just like proper time except that it will be multiplied by c, and we would usually use whichever term comes out real (when one is real the other is imaginary, or for events connected by a light path, then both are zero). This was my objection to thinking that a "light travel distance" between events of emission and absorption of that light could mean anything physical, as the sole invariant there has to be zero.

In other words, the way I would use proper distance is, I agree that the concept applies when we have two events that are spacelike, rather than timelike, separated, so cannot be causally connected (except for the trivial case of the null geodesic separating an emission and absorption, where there is causal connection but the proper distance is zero so I would tend to prefer to use proper time, though it is also zero!). When we have two acausal events, we can look at the invariant interval separating them, and call it the square root of dx^2 - c^2 dt^2. If we set c=1, this is just choosing the sign that comes out real when we do the square root-- for casual events, we'd use dt^2 - dx^2 to get a real square root, and then we'd call it proper time.

I guess the key point for the thread is that the only distance that comes out nonzero between events that connect the emission and absorption of light is a coordinate distance, so is arbitrary and is unknown in the absence of stating what coordinates are chosen. Conversely, the sole distance that has a physically invariant meaning comes out zero in that situation.
Bottom line is "distance" is a coordinate dependent thing -- it is not invariant and thus has no real meaning in GR. We can switch to some other coordiante system, say static coordinates (or as close as we can get in this dynamic universe), and all these "distances" will be something very different.
Right, except the proper distance between events where light is emitted and absorbed, which will still be zero.

RussT
2009-Jun-06, 11:45 AM
Based on Publius's (Richard's) first repsonse to Keng starting with "Proper Distance" I am guessing that he has not been keeping up with how this thread has progressed and the real issues that are at hand here.

I also suspect that Jeff is totally confused and flaberghasted at how Lightspeed being defined as Constant "c" @ 186,282.4 miles per second and yet is being rendered as vitually meaningless when considering the Local neighborhood ie; the Moon (~1.25 Light Seconds), Sun (~93 million miles/~8.32 minutes), Alpha Centauri (~4lys), and out to the OP's orginal M31 which has been determined to be ~2.5 million light years in distance where light has traveled from @ 186,282.4 for the 2.5 million years it has taken to reach us from where it was when the light left M31. There is no supposed "Universe Expansion" out to at least that distance from us.
As viewed from our "Rest Frame", earths frame, where we see/detect the light/photons coming to our telescopes...when considering the moon, I even included this from Tim in my orginal Post...



Originally Posted by RussT
In other words, when we send a light beam to the moon, as in the lunar ranging project, it travels to the moon at "c" and Back at "c", for a ~2.5 second round trip, right.



Originally Posted by Tim Thompson
Always.

The moon is ~((238,857 miles) average distance from the earth

So, that is "Light Travel Time" twice (there and back) over a distance of (238,857 miles).

So from our point of view, that is "Proper Time" and "Proper distance" and is using the definition in Astronomy for lightspeed being Constant @ 182,282.4 in Vacua.
Same for Sol.

Which has been my point from the beginning.



I guess the key point for the thread is that the only distance that comes out nonzero between events that connect the emission and absorption of light is a coordinate distance

Do you mean a coordinate distance from Like the Earth to the Moon, OR the Earth to the Sun? You don't do you?


so is arbitrary and is unknown in the absence of stating what coordinates are chosen.

This tells me you don't!



Conversely, the sole distance that has a physically invariant meaning comes out zero in that situation.

Why do you, when trying to find the distance from here to there, "KEEP" insisting on choosing an observer (that alien in his spaceship), that is "right next to the earth" traveling at light speed for the 0, "Simultaneous" emission/absorption and near lightspeed, keep adding 9's but never reach "c", for the nearly "Simultaneous" emission/absorption?

Doing it that way, since spaceships cannot reach "c", you are trying to determine distances based on nano second differences, depending on how many 9's you add after you get up to about 7 of them...since like Grey said...If you're travelling fast enough that time dilation means you measure a nanosecond to go four light years, then it will take you a whole second to travel four billion light years

In fact, you not only "Insist" on using that "Observer" you are virtually saying that observer is the only observer that matters and that we mere mortals here on earth viewing the 'real' light with our telescopes are meaningless observers!

And not only that, you still make the same statement...Right, except the proper distance between events where light is emitted and absorbed, which will still be zero.[/B

After...


Bottom line is "distance" is a coordinate dependent thing -- it is not invariant and [B]thus has no real meaning in GR. My Bold

Shows you this.


"distance" is a coordinate dependent thing

Now, this coordinate dependant thing may work an expanding universe, BUT it is definitely not working for "c" as a Constant for the moon, sol, Alpha Centauri and evidently for the local group of galaxies where we are still using the Constant speed of light to determine distances.

slang
2009-Jun-06, 01:19 PM
RussT,

Are you aware that you are now using this quote from publius:


Bottom line is "distance" is a coordinate dependent thing -- it is not invariant and thus has no real meaning in GR. We can switch to some other coordiante system, say static coordinates (or as close as we can get in this dynamic universe), and all these "distances" will be something very different.

(your emphasis, context restored) to attempt to show that Ken G was wrong, while the first line from his first post was:


There's no such thing as the distance the light travels.

(emphasis mine) after which he explained why that was so in GR, and switching to another coordinate system to show it, and explaining how such choices are limited? You know, the context that you dropped before starting to attack a single line?

They say exactly the same!


Now, this coordinate dependant thing may work an expanding universe, BUT it is definitely not working for "c" as a Constant for the moon, sol, Alpha Centauri and evidently for the local group of galaxies where we are still using the Constant speed of light to determine distances.

That's curious, since "this coordinate dependant thing" (GR) was shown to be correct (Mercury, lensing) before we knew the universe was expanding. I suppose you also refuse to use GPS devices since GR supposedly doesn't work in Earth orbit? All these things concern distance and time. As does the OP. As Ken G and grant showed exhaustively.

FWIW, I do value these GR explanations. They certainly help my understanding, if not everyone's.

Ken G
2009-Jun-06, 02:19 PM
The moon is ~((238,857 miles) average distance from the earth.
So, that is "Light Travel Time" twice (there and back) over a distance of (238,857 miles).

You are still apparently unaware that both these statements have no absolute or unambiguous meaning, as stated, in any context where there is also an important difference between "distance then" and "distance now", i.e., in any situation where grant hutchison's original answer was not fully complete. I guess the distinction is too subtle for people who have not learned the other important concepts of relativity, but I wouldn't know how else to describe it than I already have.

So from our point of view, that is "Proper Time" and "Proper distance" and is using the definition in Astronomy for lightspeed being Constant @ 182,282.4 in Vacua. You see, this is why I can state that you do not know relativity. I will say it one final time: the only "proper time" or "proper distance" that connect the emission and absorption of light are zero. This is an invariant result, which means it is not coordinate dependent, and is true for all observers, including us. You seem to think that any time we use a concept of time or distance that depends on some coordinate system we have chosen, that we can call them proper times or proper distances, but this is incorrect. It's just plain wrong. The only events that are connected by coordinate distances that we could call a proper distance are events that we perceive as simultaneous, viz, events for which there is no difference between "distance then" and "distance now". Emission and absorption of light are not such events.

Do you mean a coordinate distance from Like the Earth to the Moon, OR the Earth to the Sun? Yes, I do mean coordinate distances like those observer-dependent distances. Note, however, that those distances do not have an important difference between "distance then" and "distance now", so Grant's answer would suffice for them.



This tells me you don't! Then you have misread it.


Why do you, when trying to find the distance from here to there, "KEEP" insisting on choosing an observer (that alien in his spaceship), that is "right next to the earth" traveling at light speed for the 0, "Simultaneous" emission/absorption and near lightspeed, keep adding 9's but never reach "c", for the nearly "Simultaneous" emission/absorption?Your question seems a bit rambling, what I am doing is using the concept of "proper distance" connecting the events in question. Your basic problem is you keep getting confused between what are observer-dependent quantities, and should be identified as such in the language we use, and those that are invariant quantities. This is not a minor issue, it is core concept of relativity.
In fact, you not only "Insist" on using that "Observer" you are virtually saying that observer is the only observer that matters and that we mere mortals here on earth viewing the 'real' light with our telescopes are meaningless observers!Nah, I just use the invariant concepts as they are defined. That's often a good thing to do in relativity.

Ken G
2009-Jun-06, 02:26 PM
FWIW, I do value these GR explanations. They certainly help my understanding, if not everyone's.Thank you slang, that is entirely the goal. "Winning the argument" is a pointless goal, as tightly as people hold to their misconceptions once they have a personal stake in them. There can be very little difference between debating evolution, and debating relativity, with those who refuse to accept it!

grav
2009-Jun-06, 11:23 PM
Okay, I have a question. Let's say we are receiving the light from a galaxy that is 2.5 million light years away. Let's say we are also stationary with that galaxy. So the light left the galaxy 2.5 million years ago and we are seeing it as it was 2.5 million years ago. Let's also say that it developed exactly the same way our galaxy did, so since we are stationary to the other galaxy, we can see how our galaxy was forming exactly 2.5 million years ago.

Now let's say we jump in a rocket and accelerate very quickly to .6 c toward the galaxy, barely losing any time or distance relative to our own galaxy. Length contraction at that relative speed is .8, so we will now see the galaxy as 2 million light years away. Depending upon the terminology, would we still then say the galaxy is 2.5 light years away, since we are still seeing the same light as before, so we are still seeing the light from 2.5 million years prior to the current form of our own galaxy, or do we say the light has travelled only 2 million light years now, and so was emitted only 2 million years ago, even though we can still see the other galaxy is 2.5 million years younger than our own while forming identically in the same frame as our own galaxy?

Also, if we figure for the simultaneity shift during our acceleration, the galaxy is pushed future-forward by an amount of L Z d v / (c^2 - v^2) = (.8) (.8) (2.5) (.6) / (1 - .6^2) = 1.5 million years, where Z is the time dilation and L is the length contraction. In that case, the light was emitted 1.5 million years in the future according to our clocks, which are still synchronized with the frame of our galaxy well enough for the quick acceleration, and is currently reaching us from 2.5 million years in the past according to the formation we observe. So since we are still seeing how the galaxy was forming 2.5 million years before our own, and the galaxy is now 1.5 million years more advanced than our own, shouldn't we now say the light has been travelling for 4 million years, so the galaxy is now 4 million light years away?

Ken G
2009-Jun-06, 11:55 PM
Okay, I have a question. Let's say we are receiving the light from a galaxy that is 2.5 million light years away. Let's say we are also stationary with that galaxy.[/qute]Then there will be no difference between "distance then" and "distance now", and the OP question is not applicable.

[quote] So the light left the galaxy 2.5 million years ago and we are seeing it as it was 2.5 million years ago.Assuming you mean in our reference frame and using special-relativity inspired coordinates because there's no reason not to, OK.


Depending upon the terminology, would we still then say the galaxy is 2.5 light years away, since we are still seeing the same light as before, so we are still seeing the light from 2.5 million years prior to the current form of our own galaxy, or do we say the light has travelled only 2 million light years now, and so was emitted only 2 million years ago, even though we can still see the other galaxy is 2.5 million years younger than our own while forming identically in the same frame as our own galaxy?Yes, that shows that it all depends on the coordinates. There is no absolute answer to your question, and it shows why special-relativity inspired coordinates only work unambiguously when observers do not change inertial reference frame. I hope certain other posters are paying attention.

RussT
2009-Jun-07, 01:56 AM
Okay, I have a question. Let's say we are receiving the light from a galaxy that is 2.5 million light years away. Let's say we are also stationary with that galaxy. So the light left the galaxy 2.5 million years ago and we are seeing it as it was 2.5 million years ago. Let's also say that it developed exactly the same way our galaxy did, so since we are stationary to the other galaxy, we can see how our galaxy was forming exactly 2.5 million years ago.

Now let's say we jump in a rocket and accelerate very quickly to .6 c toward the galaxy, barely losing any time or distance relative to our own galaxy. Length contraction at that relative speed is .8, so we will now see the galaxy as 2 million light years away. Depending upon the terminology, would we still then say the galaxy is 2.5 light years away, since we are still seeing the same light as before, so we are still seeing the light from 2.5 million years prior to the current form of our own galaxy, or do we say the light has travelled only 2 million light years now, and so was emitted only 2 million years ago, even though we can still see the other galaxy is 2.5 million years younger than our own while forming identically in the same frame as our own galaxy?

Also, if we figure for the simultaneity shift during our acceleration, the galaxy is pushed future-forward by an amount of L Z d v / (c^2 - v^2) = (.8) (.8) (2.5) (.6) / (1 - .6^2) = 1.5 million years, where Z is the time dilation and L is the length contraction. In that case, the light was emitted 1.5 million years in the future according to our clocks, which are still synchronized with the frame of our galaxy well enough for the quick acceleration, and is currently reaching us from 2.5 million years in the past according to the formation we observe. So since we are still seeing how the galaxy was forming 2.5 million years before our own, and the galaxy is now 1.5 million years more advanced than our own, shouldn't we now say the light has been travelling for 4 million years, so the galaxy is now 4 million light years away?

Grav....Do this same kind of examination, BUT use the Sun>>Earth distance and light travel time instead...;)

RussT
2009-Jun-07, 02:13 AM
I will say it one final time: the only "proper time" or "proper distance" that connect the emission and absorption of light are zero.

Great KenG...Please explain to the class, Starting with your "insisted upon" "Reference Frame" Ie: using the "Observer in the spaceship right next to the earth" Traveling AT "c" where emission and absorption are "Simulataneous", which by your definition is the "Proper Distance" to any known local body ie: the Moon, Sol, Alpha Centauri in space...

How you are able to/can calculate finding that distance "without knowing that distance in advance"?

Ken G
2009-Jun-07, 02:27 AM
I'd happily answer that question if I could understand it, but it just seems rambling and incoherent, so I do not know what you are asking. Can you frame a more specific question? It sounds like you are asking me to explain all of relativity to you. And by the way, it is not "my" definition of proper distance, it is the definition used in physics. If you really want to understand any of this, I suggest you start by looking up the Wiki entry on proper length, or any relativity textbook, as they'll all include the concept.

grav
2009-Jun-07, 03:40 AM
Grav....Do this same kind of examination, BUT use the Sun>>Earth distance and light travel time instead...;)Okay, well, let's see. Since we can't compare the formation of the sun to anything directly, we will place a clock on it. The motion of the Earth relative to the sun is extremely small compared to the relativistic speed I'm about to ascribe, so we can consider the clocks on the sun and Earth are synchronized for our purposes and any differences neglible. On Earth, we see the sun's clock is eight minutes behind our own and so the sun is 8 light-minutes away. We jump into a rocket and very quickly accelerate to a relative speed of .6 c toward it. The acceleration is so quick that we have barely moved in respect to the Earth and still see the sun's clock at 8 minutes behind our own since we are receiving the same light as before, but we now measure the sun at a distance of (.8) (8) = 6.4 light-minutes away. The time lag over that distance is (.8) (.8) (8) (.6) / (1 - (.6)^2) = (8) (.6) = 4.8 minutes future-forward, so if we see the sun's clock reading T = -8 minutes, then the sun emitted the light at T = -12.8 minutes according to our own clocks, and should have travelled 12.8 light-minutes to reach us. I'm not sure what to think about all of that yet.

grav
2009-Jun-07, 03:49 AM
The time lag over that distance is (.8) (.8) (8) (.6) / (1 - (.6)^2) = (8) (.6) = 4.8 minutes future-forward, so if we see the sun's clock reading T = -8 minutes, then the sun emitted the light at T = -12.8 minutes according to our own clocks, and should have travelled 12.8 light-minutes to reach us.Okay, that part doesn't matter because the light was already in transit when we accelerated, and it will become simultaneity shifted also, so it is now Doppler shifted and we will see the sun's clock tick much faster. Only the first part remains unresolved now.

grav
2009-Jun-07, 03:53 AM
Oh, oops. The first part is resolved also for the same reason. The lesser distance, once the Doppler shift is also accounted for, should work out correctly. So the only dilemma now is just a matter of seeing the same clock readings with a lesser distance. If we can tell the light's been Doppler shifted from its original frame, however, the relative distance can be worked out, or for the distance when stationary to the emitter.

RussT
2009-Jun-07, 10:10 AM
I'd happily answer that question if I could understand it, but it just seems rambling and incoherent, so I do not know what you are asking. Can you frame a more specific question?

Sure. My claim from the beginning has been very simple.

Lightspeed in SR>>GR is defined as Constant "c" @ 186,282.4 mps.

I am simply saying...
That when considering distances from the Earth to the Moon, Sol, Alpha Centauri, M31, SR>>GR did not, and cannot determine those "Real" known distances.

I am simply saying that lightspeed as a Constant "c" was used to determine those "Light Travel Time" "real" distances.

And, once again you 'Re-state' this with more inuendo of not understanding!!!


You are still apparently unaware that both these statements have no absolute or unambiguous meaning, as stated, in any context where there is also an important difference between "distance then" and "distance now", i.e., in any situation where grant hutchison's original answer was not fully complete. I guess the distinction is too subtle for people who have not learned the other important concepts of relativity, but I wouldn't know how else to describe it than I already have.



You are still apparently unaware that both these statements have no absolute or unambiguous meaning

They do have an absolute meaning when it comes to the the local neighborhood!!!

It's call Constant "c" @ 186,282.4 mps...That IS a time and distance measurement!



So from our point of view, that is "Proper Time" and "Proper distance" and is using the definition in Astronomy for lightspeed being Constant @ 182,282.4 in Vacua.



You see, this is why I can state that you do not know relativity. I will say it one final time: the only "proper time" or "proper distance" that connect the emission and absorption of light are zero. This is an invariant result, which means it is not coordinate dependent, and is true for all observers, including us. You seem to think that any time we use a concept of time or distance that depends on some coordinate system we have chosen, that we can call them proper times or proper distances, but this is incorrect. It's just plain wrong. The only events that are connected by coordinate distances that we could call a proper distance are events that we perceive as simultaneous, viz, events for which there is no difference between "distance then" and "distance now". Emission and absorption of light are not such events.

Okay, fine,...then I will call it "Real Time" and "Real Distance" because ...


Originally Posted by RussT
In other words, when we send a light beam to the moon, as in the lunar ranging project, it travels to the moon at "c" and Back at "c", for a ~2.5 second round trip, right.



Originally Posted by Tim Thompson
Always.

And because it takes light/photons ~8.32 minutes to travel the distance of ~93 million miles from the Sun to the Earth.

Now, you keep insisting on stating things where expansion of the universe is afoot, and I have told you umpteen times we are talking "Local"...so we could keep playing this restating game, where you can then keep insisting that SR>>GR somehow does/MUST correctly determine distances to the Moon and The Sun OR...

You can show us how this works using... I will say it one final time: the only "proper time" or "proper distance" that connect the emission and absorption of light are zero. This is an invariant result, which means it is not coordinate dependent, and is true for all observers, including us.

Show how Lightspeed at Constant "c" of 186,282.4 mps which is built into SR>>GR can or did determine the correct distance to the Moon and the Sun.

Ken G
2009-Jun-07, 02:55 PM
I am simply saying that lightspeed as a Constant "c" was used to determine those "Light Travel Time" "real" distances.The speed of light being a constant is a constraint between distance and time coordinates that can be used to separate coordinate systems that obey that constraint from those that don't. Coordinates that do obey the constraint have certain advantageous properties, like they behave locally in such a way as to be usable as measurable times and distances for some local observer. But none of that says anything about the "exactly correct" statement you quoted earlier, which referenced only distance, not a constraint between distance and time.


And, once again you 'Re-state' this with more inuendo of not understanding!!!Feh.


They do have an absolute meaning when it comes to the the local neighborhood!!!No, they don't, unless you specify the observer frame (and thereby drop the word "absolute"). But even if we adopt our own frame as implicit, the real point is that your answers only have a meaning in contexts where there is no important difference between "distance then" and "distance now", as I've said many times above. When there is no such important difference, Grant's answer suffices, and when there is, your answer, and Jeff Root's, are meaningless, and I showed exactly why with simple logic. All that remains is for you to take your pick between being superfluous to the OP question, or wrong, depending on whether "distance then" and "distance now" can be meaningfully distinguished.



Okay, fine,...then I will call it "Real Time" and "Real Distance" because ...Please feel free to actually supply definitions of the ATM concepts. I've never seen them in a physics book. Should that bother you?


Show how Lightspeed at Constant "c" of 186,282.4 mps which is built into SR>>GR can or did determine the correct distance to the Moon and the Sun.Those distances are coordinate distances, in a situation where the coordinates are obvious, and make no important distinction between "distance then" and "distance now". Which of these words are you not getting?

grav
2009-Jun-07, 10:43 PM
I haven't read all of this thread, so I'm not exactly sure what the debate is at this point, or who I might be agreeing with or disagreeing with here, but it seems to me that if we consider ourselves stationary and measure a galaxy at 2.5 million light-years away, then that is the distance "then" according to our own rulers and clocks, when the light was emitted at that distance according to our frame, assuming we haven't accelerated during that time. Even if the galaxy is travelling away from us at nearly light speed, it would only have succeeded in gaining less than twice the distance we currently measure in the time it took for the light to reach us, ignoring expansion.

Ken G
2009-Jun-07, 11:42 PM
I haven't read all of this thread, so I'm not exactly sure what the debate is at this point, or who I might be agreeing with or disagreeing with here, but it seems to me that if we consider ourselves stationary and measure a galaxy at 2.5 million light-years away, then that is the distance "then" according to our own rulers and clocks, when the light was emitted at that distance according to our frame, assuming we haven't accelerated during that time. Even if the galaxy is travelling away from us at nearly light speed, it would only have succeeded in gaining less than twice the distance we currently measure in the time it took for the light to reach us, ignoring expansion.If we ignore expansion, you are asking for a purely special relativity treatment. If you also specify what coordinates you are using (Einstein inertial coordinates in our own frame), then yes, there is no ambiguity in what distance is being talked about, because we have specified all the important things. Again note that coordinate distances are meaningless without there being a clear implication of
1) what coordinates are being used, and
2) that there is a need to specify coordinates.
We must ask ourselves if the statement that started all this, "The distance given is the distance the light has traveled, from the point in space and time at which it was emitted, to the point in space and time at which it is received." accomplishes those two requirements to achieve meaning.

RussT
2009-Jun-08, 02:11 AM
I haven't read all of this thread, so I'm not exactly sure what the debate is at this point, or who I might be agreeing with or disagreeing with here, but it seems to me that if we consider ourselves stationary and measure a galaxy at 2.5 million light-years away, then that is the distance "then" according to our own rulers and clocks, when the light was emitted at that distance according to our frame, assuming we haven't accelerated during that time. Even if the galaxy is travelling away from us at nearly light speed, it would only have succeeded in gaining less than twice the distance we currently measure in the time it took for the light to reach us, ignoring expansion.

This is exactly correct, Grav...If M31 were traveling directly away from our stationary position AT "c" then it would be 5 million lys away "Now".

And I am so sorry...I gave you the wrong assignment asking you to do the Sun>Earth examination...I thought you would be able to see something there with the "Future" part of your orginal M31 examination.

So, here is the correct one...Instead of saying that both M31 and The Milky Way were Born at the same time, for comparison....do this...

do the same examination of light travel time from M31 of 2.5 million years at the distance of 2.5 million lys, BUT use the "First Light" of a Super Nova exploding in M31 2.5 miilion years ago.

Show us what happens with that "Furure part"

NO Doppler or gravity wells!

grav
2009-Jun-08, 02:34 AM
do the same examination of light travel time from M31 of 2.5 million years at the distance of 2.5 million lys, BUT use the "First Light" of a Super Nova exploding in M31 2.5 miilion years ago.

Show us what happens with that "Furure part"

NO Doppler or gravity wells!It will be the same as the other post I had for the galaxy itself. The problem is, though, for what you stated at the end, if you want the future part, then there will be a simultaneity shift, which means we are now travelling at a relative speed toward the galaxy and are no longer stationary, so there will always be a Doppler shift as well.

I'm wondering if perhaps the best way to state a distance in a unified way would be in terms of the stationary distance. This could be done by finding the relative speed according to the Doppler shift observed and then calculating what the distance would be when stationary to the frame of the galaxy we are measuring.

Ken G
2009-Jun-08, 02:47 AM
I'm wondering if perhaps the best way to state a distance in a unified way would be in terms of the stationary distance. It may be a sensible convention, once you've also decided which distance measure you have in mind (of Ned Wright's list), but note it's still just a convention. The first step is to notice what is pure convention, and once you notice that, you can start looking at what conventions are the most sensible. That's why lookback time is often used-- it's also a conventional choice, and it is really a time not a distance, so that's why it should not be confused with "the distance", as slang appropriately placed the emphasis.

RussT
2009-Jun-08, 06:19 AM
It may be a sensible convention, once you've also decided which distance measure you have in mind (of Ned Wright's list), but note it's still just a convention. The first step is to notice what is pure convention, and once you notice that, you can start looking at what conventions are the most sensible. That's why lookback time is often used-- it's also a conventional choice, and it is really a time not a distance, so that's why it should not be confused with "the distance", as slang appropriately placed the emphasis.

"The Distance" to the moon from earth has been the same for a very long time! When the moon was much closer to the earth (For early man, the dinasaurs Etc), closer 'then' than 'now' and the Constant "c" was still a time and distance calculation.

The Sun to Earth, Alpha Centauri to Earth, and M31 to Earth have their unique "Then" and "Now"...they are ALL important!

You are continually trying to make the word "Important" mean something that it doesn't to fit 'focusing on lookback time' to get to an expanding universe scenario!

RussT
2009-Jun-08, 06:27 AM
It will be the same as the other post I had for the galaxy itself. The problem is, though, for what you stated at the end, if you want the future part, then there will be a simultaneity shift, which means we are now travelling at a relative speed toward the galaxy and are no longer stationary, so there will always be a Doppler shift as well.

Right, that is from the perspective of the alien in the ship...I just thought that using the scenario with the SN, that he would end up seeing the SN explode "Before" it actually could have...;)



I'm wondering if perhaps the best way to state a distance in a unified way would be in terms of the stationary distance. This could be done by finding the relative speed according to the Doppler shift observed and then calculating what the distance would be when stationary to the frame of the galaxy we are measuring.

This entire thread has been about "LocaL" distances and the "Light Flight Time".

Na...at rest, doppler doesn't start until photons begin 'stretching'. We haven't been including the slight "Gravitational considerations".

If it were that easy, they wouldn't have had to use Cephid Variables...;)

RussT
2009-Jun-08, 09:08 AM
Originally Posted by RussT
I am simply saying that lightspeed as a Constant "c" was used to determine those "Light Travel Time" "real" distances.



The speed of light being a constant is a constraint between distance and time coordinates that can be used to separate coordinate systems that obey that constraint from those that don't.

So, you agree that the Local neighborhood is one of those 'coordinate systems' that can be seperated out where Light Travel Time is 186,282.4 mps, where the distance from the sun to the Earth is ~93 million miles, for a travel time of ~8.32 minutes, and has been roughly the same for the last 4 biilion years.



Coordinates that do obey the constraint have certain advantageous properties, like they behave locally in such a way as to be usable as measurable times and distances for some local observer.[QUOTE]

Yeah, like all local observers, at rest, looking through their telescopes!

[QUOTE=KenG]
No, they don't, unless you specify the observer frame (and thereby drop the word "absolute").

Yes, they do!!! Because I have specified the observer frame, over and over and over..................and over again! and I don't have to drop the word absolute, because that just means Speed of Light as a Constant...absolutely 186,282.4 mps.



But even if we adopt our own frame as implicit, the real point is that your answers only have a meaning in contexts where there is no important difference between "distance then" and "distance now", as I've said many times above.

So, here's some absolute Logic for you and everyone following this!

Since 'our reference frame Exists', where we, at rest, looking through our telescopes know scientifically, that the Earth has been revolving around the sun for the last 4+ billion years at ~93 million miles distance (average) and using light travel time as a Constant @ 186,282.4 mps takes, as I said above, ~8.32 minutes to arrive at the Earth...

Means that there is definitely a 'distance that light travels' and that there is definitely a 'time that light travels'...

Which definitely makes this...


There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels.

Fatally False
and Both Jeff's and my statements are correct as they stand, with the acknowledgement, that there are other considerations for farther entities!

Regardless of whether there are more distant objects where there are...important distinction between "distance then" and "distance now".

Your, keying on the word "Important" is absolutely irrelevant to the Local neighborhood.

eburacum45
2009-Jun-08, 10:11 AM
The Andromeda galaxy is said to be about 2.5 million light years away. Does this mean it is 2.5 million light years away today? Or 2.5 million years ago?
I just remembered that this question can be considered part of a paradox, and that paradox has a name; the Andomeda Paradox, strangely enough. Also known as theRietdijk–Putnam argument
http://en.wikipedia.org/wiki/Rietdijk-Putnam_Argument

No doubt Ken and Grant haven't forgotten this.

You see, at the distance of the Andromeda Galaxy, the definition of 'today' depends on the speed of the observer; so much so that simply jumping in a sports car shifts the definition of 'today' into 'tomorrow' at the distance of the Andromeda Galaxy. Or if you don't want to get in a car, just move from high latitudes on the Earth to low latitudes, where the rotational speed is faster.

The key is- there is no single definition of 'today' at that distance; it is all frame dependent.

Ken G
2009-Jun-08, 12:50 PM
"The Distance" to the moon from earth has been the same for a very long time! When the moon was much closer to the earth (For early man, the dinasaurs Etc), closer 'then' than 'now' and the Constant "c" was still a time and distance calculation. But that is not the sense of "distance then" and "distance now" that was used in the OP, which connected the emission and absorption of light. Perhaps you realize that the light emitted by the Moon in the age of early man is not what we see when we look at the Moon. You are now talking about something completely different. Indeed, there is a nonzero proper distance between the Earth and the Moon in our frame, but it would not invoke the difference between "distance then" and "distance now" between the emission and absorption of a light pulse. That proper distance would not be responsive to the OP.


You are continually trying to make the word "Important" mean something that it doesn't to fit 'focusing on lookback time' to get to an expanding universe scenario!
Nah, I'm just trying to get you to use relativistically correct language that refers to a difference between the "distance then" when light was emitted, and "distance now" whan it is absorbed. That's what this thread has been about.

Jeff Root
2009-Jun-08, 12:52 PM
In connection with Russ's current argument with Ken, I will point out the
exact wording in both my first and second posts:


The fact that the distance to Andromeda (and every other distant
astronmical object) is uncertain to a degree greater than the distance
it moves over a brief time interval like 2.5 millions years :) is important,
but going beyond that... What is the answer to the original question?
Does a distance in light-years refer to the distance of the object at the
time the light was emitted, the time the light was received, or what?

Grant's answer is correct: It is both. It is also neither. The distance
given is the distance the light has traveled, from the point in space and
time at which it was emitted, to the point in space and time at which it
is received.


When the distance of a galaxy is given in light-years, what is meant is the
distance from where and when its light was emitted, to where and when it
is observed.
In both posts I explicitly said a distance "given". In this case the distance
given is the distance to the Andromeda galaxy, which was given as "about
2.5 million light years". That is a distance, not a time. It is a distance
supplied by a third party, not by me. When somebody says the distance
between the Andromeda galaxy is 2.5 million light-years, they mean that
light emitted from Andromeda 2.5 million years ago is observed now, and
the distance the light traveled in that time, by definition, is 2.5 million ly.
The place and time where the light was emitted to the place and time
where it was recieved is the given distance.

With regard to eburacum45's comment on simultaneity, I stated here
a year or two ago that I think simultaneity loses its meaning with distance.
The farther apart two events are in space, the less can be known about
their relationship in time. "Now" doesn't have any real meaning for events
which occur in widely-separated parts of the Universe.

-- Jeff, in Minneapolis

Ken G
2009-Jun-08, 12:58 PM
So, you agree that the Local neighborhood is one of those 'coordinate systems' that can be seperated out where Light Travel Time is 186,282.4 mps, where the distance from the sun to the Earth is ~93 million miles, for a travel time of ~8.32 minutes, and has been roughly the same for the last 4 biilion years.Obviously I agree with all that, that was never the part of what you were saying that was wrong. You can tell the parts that were wrong, because I quoted them and talked about them. It all comes down to failing to understand what a coordinate distance is, and when absolute (invariant) language can be used in regard to distances, not what the distance between an emission and absorption event happens to be in some coordinate system.


Yeah, like all local observers, at rest, looking through their telescopes!Again, the problem was with the way the quote in question pretended like it was referring to absolute concepts of "the distance between two points" rather than relativistically correct (coordinate-depenent) language. You might have noticed I was rather specific on that point, perhaps reviewing my posts might help with that. Apparently you have not understood a single word I said, so I guess there's not much chance you will start now.

Ken G
2009-Jun-08, 01:00 PM
The key is- there is no single definition of 'today' at that distance; it is all frame dependent.Yup, I forgot it was called that-- how ironic.

Ken G
2009-Jun-08, 01:12 PM
In both posts I explicitly said a distance "given". As opposed to one taken? What does this mean?

In this case the distance
given is the distance to the Andromeda galaxy, which was given as "about
2.5 million light years". That is a distance, not a time.Right, and that's exactly what is wrong about your answer, as Spaceman Spiff pointed out to you. I note you have not commented on the Ned Wright link, do you disagree with Dr. Wright, or did you just not read it?


It is a distance
supplied by a third party, not by me. It is well known that popular media incorrectly reports lookback times as if they were distances, that's what Ned Wright was talking about. It is also well known that there is a proper distance to Andromeda if we adopt our own reference frame, which would be an implicitly natural thing to do when talking about the Andromeda galaxy. But that proper distance does not connect the events of emission and absorption of the light emitted, which is what your statement referred to. So it's wrong either way: a lookback time is not a distance, and the proper distance of 2.5 milion LY does not connect the emission and absorption of the light we see, so would not distinguish "distance then" from "distance now". Perhaps that is what you were trying to say, had you used relativistically correct language.


When somebody says the distance
between the Andromeda galaxy is 2.5 million light-years, they mean that
light emitted from Andromeda 2.5 million years ago is observed now, and
the distance the light traveled in that time, by definition, is 2.5 million ly.Again, there is no such thing as "the distance the light traveled," it is highly ambiguous language. If we refer to proper distance the light travels, that distance is always zero in a frame approaching that of the light. Alternatively, if we are talking about the proper distance in our own frame, it is about 2.5 million LY-- but does not connect the events of emission and absorption of the light.


The place and time where the light was emitted to the place and time
where it was recieved is the given distance.Again, no, that is not the proper distance being referred to. There is a proper distance when the light is emitted, and a proper distance when it is absorbed, and if they are the same, the OP question is moot, and if they are different, then the above incorrect language becomes meaningless.

"Now" doesn't have any real meaning for events
which occur in widely-separated parts of the Universe.
It does not have any invariant meaning, that's true-- it has only a coordinate-dependent meaning. But your quote above makes the very error you are advising against in regard to simultaneity. You now need to extend the understanding you have about time to distance.

Jeff Root
2009-Jun-08, 01:13 PM
Ken's basic objection is that I referred to galactic distances as if they
were absolute distances, rather than relative, and that the cause of my
doing so was my failure to specify an observer. Note that my statements
were about distances "given" by other sources, such as a table of
galaxy distances in a reference book. The observer is specified by the
source of the distance being quoted. I repeat that Ken's objection was
and continues to be ridiculous, even if it is accurate relativity theory.

-- Jeff, in Minneapolis

Spaceman Spiff
2009-Jun-08, 03:00 PM
Right, and that's exactly what is wrong about your answer, as Spaceman Spiff pointed out to you. I note you have not commented on the Ned Wright link, do you disagree with Dr. Wright, or did you just not read it?
It is well known that popular media incorrectly reports lookback times as if they were distances, that's what Ned Wright was talking about.

For clarity, I think that Ken G was referring to this post of mine (http://www.bautforum.com/space-astronomy-questions-answers/88996-why-difference-observable-vs-farthest-galaxy.html#post1500745) (commenting on a statement made by Jeff Root) on a related thread called why the difference: observable vs. farthest galaxy (http://www.bautforum.com/space-astronomy-questions-answers/88996-why-difference-observable-vs-farthest-galaxy.html).

Ken G
2009-Jun-08, 03:09 PM
Oops, sorry for mixing threads.

Ken G
2009-Jun-08, 03:14 PM
Ken's basic objection is that I referred to galactic distances as if they
were absolute distances, rather than relative, and that the cause of my
doing so was my failure to specify an observer. That was part of the problem, but not the biggest part. You see, "galactic distances" should be thought of as proper distances in some "galactic" coordinate system. As proper distances, which publius and I pointed out above, they must connect events that are simultaneous in the galactic coordinates. That was the biggest problem with your statement "The distance given is the distance the light has traveled, from the point in space and time at which it was emitted, to the point in space and time at which it is received." Events connected by emission and absorption of a light pulse are not simultaneous in the galactic coordinates, and what's more the proper distance between the events you describe is zero. That's just a fact, I believe I may have mentioned that objection once or twice by now, yet you read right past if over and over.

I repeat that Ken's objection was
and continues to be ridiculous, even if it is accurate relativity theory.
Then you continue to fail to understand relativity, and the crucial role of invariants, and proper distances, in that theory. Once again, the only issue that remains here is, do you want to understand relativity or don't you?

Spaceman Spiff
2009-Jun-08, 03:22 PM
Ken's basic objection is that I referred to galactic distances as if they
were absolute distances, rather than relative, and that the cause of my
doing so was my failure to specify an observer. Note that my statements
were about distances "given" by other sources, such as a table of
galaxy distances in a reference book. The observer is specified by the
source of the distance being quoted. I repeat that Ken's objection was
and continues to be ridiculous, even if it is accurate relativity theory.

-- Jeff, in Minneapolis

:confused:
Jeff -- IMO the fundamental issue at hand here is that, as scientists or those interested in science, we want to understand precisely what we mean by a distance measurement (for a particular situation) so that we don't fool ourselves in the delusion that it is obvious and thus perhaps misapply our delusions and screw up our understanding of the world around us. This is not at all ridiculous, except maybe to the totally uninterested.

And you still haven't defined in any useful way what you mean by "given by other sources":
Note that my statements were about distances "given" by other sources, such as a table of galaxy distances in a reference book. The observer is specified by the source of the distance being quotedYou haven't said how any of this is important or relevant. And how were the distance measurements that appear in this famous reference book done? Geometric parallax? Proper motion parallax? Binary star geometry? Luminosity distance? Does it ever matter?

AndrewJ
2009-Jun-08, 09:08 PM
Folks, perhaps we might coin a short, rhetorical phrase for use in discussions of distance and light travel time to indicate acknowledgement that observer must be defined and there is no absolute measure of space or time.

Maybe: "...within relativistic reality" or "...remembering the usual realtivity caveats".

Relativity is crucial, particularly regarding anything that touches upon space-time or radiation, but sometimes (not necessarily re this OP) the nub of a question might be effectively met without it being fully elucidated.

:eek: [cowers, waiting to be shouted at.]

Ken G
2009-Jun-08, 09:52 PM
I think it's fine to speak loosely about distance, even without any relativistic caveats. It is only when answers are specifically being given about "distances then vs. distances now" (invoking simultaneity issues across large distances), or "points in space and time" (invoking the relativistic concept of spacetime), that manifestly correct relativistic language is called for. If such distinctions are not being made, then there's no need for relativistic language, and that's why Grant did not use it in his original answer, because none was needed for the OP question. I brought in the more relativistic language only to expose why certain other answers were inadequate and confusing, given the language they chose (which did sound like it was trying to invoke relativistic concepts about spacetime and coordinates). It's not a case in this thread where someone asked a nonrelativistic question and got a relativistic answer, it's that the nonrelativistic answer is "there's no important distinction between the elements of the OP", so there was reason to ask "what would be the answer if there had been an important distinction?"

Spaceman Spiff
2009-Jun-08, 10:16 PM
The relationship between Comoving Distance and Angular Diameter Distance is indeed about two to one for z=1, but (as the graph in my link (http://www.atlasoftheuniverse.com/redshift.html) shows) that ratio increases steadily with increasing values of z.

The most commonly quoted distance is unfortunately neither of the above, but Light Travel Time Distance, which tells you neither where the galaxy was when it emitted the light, nor where it is now: it tells you how long ago the galaxy emitted the light. The next most commonly quoted distance is the Comoving Distance, which refers to "now". In my experience, it's unusual to see the Angular Diameter Distance ("then") quoted, except in discussions like this one.

Grant Hutchison

I know this is a bit off on a tangent relative to the OP, but just to set the record straight -- in a spatially flat universe, the ratio of these two distances (Co-moving/Angular Diameter or then/now) is simply 1+z.

RussT
2009-Jun-09, 08:49 AM
Originally Posted by RussT
So, you agree that the Local neighborhood is one of those 'coordinate systems' that can be seperated out where Light Travel Time is 186,282.4 mps, where the distance from the sun to the Earth is ~93 million miles, for a travel time of ~8.32 minutes, and has been roughly the same for the last 4 biilion years.



Obviously I agree with all that, that was never the part of what you were saying that was wrong.

Then this statement...



There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels.

Has been "Fatally False" for the last 4+ billion years!!!


This is so blatently obvious, why don't/won't you just admit it???

Your/mainstreams treatment of "emission/absorption" being 0 does not and cannot apply here........period! And, it's the same for the Moon, Alpha Centauri, and yes even M31....In 'our frame' all the photons hitting our telescopes have been traveling ~2.5 million years from emission to absorption in the telescope...Period!

Now, sometime in the future, IF>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> we can, or we finally meet some Aliens that have, learned to travel at relavistic speeds, then you can say maybe this "MIGHT" be possible........BUT.......



Events connected by emission and absorption of a light pulse are not simultaneous in the galactic coordinates, and what's more the proper distance between the events you describe is zero. That's just a fact, By the way, it can't be zero! Spaceships can't travel at "c"...;)

Not for M31, there is no "Expansion".,..No 1+z.

And further more your...That's just a fact is trying to "Guarantee us" that there definitely is, without a doubt, "Always" an alien in a spaceship, over there, next to M31, or anywhere else, that "MUST" be traveling toward us at "c".

Please,,,,,,,,,,,,,,

RussT
2009-Jun-09, 09:43 AM
Indeed, there is a nonzero proper distance between the Earth and the Moon in our frame, but it would not invoke the difference between "distance then" and "distance now" between the emission and absorption of a light pulse

Indeed, there is a nonzero proper distance between the Earth and the Moon in our frame

And the Sun's....

So, when I said...for the moon and sun



Originally Posted by RussT
So from our point of view, that is "Proper Time" and "Proper distance" and is using the definition in Astronomy for lightspeed being Constant @ 182,282.4 in Vacua.

and then you admonished me...so I said



Okay, fine,...then I will call it "Real Time" and "Real Distance" because

and then you said...


Please feel free to actually supply definitions of the ATM concepts. I've never seen them in a physics book. Should that bother you?

I guess it looks like they should be in a physics book after all...;)



but it would not invoke the difference between "distance then" and "distance now" between the emission and absorption of a light pulse

And this just solidifies my claim, that the normal mainstream/your treatment of SR>>GR in the Local neighborhood cannot and does not work!

loglo
2009-Jun-09, 10:11 AM
RussT
I don't think anyone is denying that there is such a thing as a proper distance, just that it is not related to paths followed by massless particles, null geodesics. Part of the very definition of a space-like separation is that you can't connect it by a null geodesic. This is the same in any metric, FLRW or Schwarzchild or Kerr-Newman. It is part of the very result that preserves causality across reference frames and makes GR so useful.

I wish you would stop bashing GR. What's it ever done to you? :)

Ken G
2009-Jun-09, 04:12 PM
Has been "Fatally False" for the last 4+ billion years!!!
This is so blatently obvious, why don't/won't you just admit it???What is blatantly obvious is that you did not understand my statement. As slang pointed out quite awhile ago, the correct emphasis on my statement has to do with talking about the distance, as if it had some absolute or universal meaning. It does not, there's "no such thing". Even proper distance requires the specification of an observer, but as I also pointed out awhile ago, the statement that you are supporting would refer to a proper distance of zero, being the proper distance connecting events where a light pulse was emitted and absorbed. Like this one, addressing your other points would merely require repeating myself, so you can just look back to get those answers. The problem here is you are not understanding anything I say, it must be that one of us does not understand relativity at all.

Ken G
2009-Jun-09, 04:16 PM
I don't think anyone is denying that there is such a thing as a proper distance, just that it is not related to paths followed by massless particles, null geodesics. I would tend to say that proper distance can be defined for null geodesics, it just comes out zero, hence the "null". But it is a singular situation, perhaps best viewed as a kind of limit.


Part of the very definition of a space-like separation is that you can't connect it by a null geodesic. Yes, it is a kind of "boundary" value, a null geodesic is the boundary between where we talk about proper time, and where we can talk about proper distance (without the value being imaginary). So you can say neither concept works for this boundary, or you can say they become the same thing (zero) on the boundary, it's sort of the same either way.

Ken G
2009-Jun-09, 04:19 PM
Indeed, there is a nonzero proper distance between the Earth and the Moon in our frameThat is true, I assumed you were still talking about the distance in Jeff Root's quote that you said was "exactly correct". If you are changing the kind of distance you are talking about now, then yes, it could be a proper distance-- it's just not what the thread has been about, because it does not connect the "point in space and time" where the light was emitted, to the "point in space and time" where the light was absorbed. If you are going to change what you are talking about, then you need to indicate that in some way, or I might read over it.

RussT
2009-Jun-10, 01:09 AM
What is blatantly obvious is that you did not understand my statement.

I absolutely did NOT misunderstand anything about any of your statements!



As slang pointed out quite awhile ago, the correct emphasis on my statement has to do with talking about the distance, as if it had some absolute or universal meaning.

Both Jeff and myself have covered this extensively...........You keep "Assuming" and restating, that your take on the focus of this thread, is that once we get to the distance of M31, that we MUST change to the point of view of the Alien in the spaceship, next to M31 that is traveling at "c" toward us!!!

And you just skipped all of this...



[QUOTE=KenG]
Even proper distance requires the specification of an observer, but as I also pointed out awhile ago, the statement that you are supporting would refer to a proper distance of zero, being the proper distance connecting events where a light pulse was emitted and absorbed.



Coordinates that do obey the constraint have certain advantageous properties, like they behave locally in such a way as to be usable as measurable times and distances for some local observer.





Quote:
Originally Posted by KenG
No, they don't, unless you specify the observer frame (and thereby drop the word "absolute").

[QUOTE=RussT]
Yes, they do!!! Because I have specified the observer frame, over and over and over..................and over again! and I don't have to drop the word absolute, because that just means Speed of Light as a Constant...absolutely 186,282.4 mps. ETA: in our frame


Quote:
Originally Posted by KenG
But even if we adopt our own frame as implicit, the real point is that your answers only have a meaning in contexts where there is no important difference between "distance then" and "distance now", as I've said many times above.



So, here's some absolute Logic for you and everyone following this!

Since 'our reference frame Exists', where we, at rest, looking through our telescopes know scientifically, that the Earth has been revolving around the sun for the last 4+ billion years at ~93 million miles distance (average) and using light travel time as a Constant @ 186,282.4 mps takes, as I said above, ~8.32 minutes to arrive at the Earth...

Means that there is definitely a 'distance that light travels' and that there is definitely a 'time that light travels'...in our frame



Like this one, addressing your other points would merely require repeating myself, so you can just look back to get those answers. The problem here is you are not understanding anything I say, it must be that one of us does not understand relativity at all.

Jeff and I keep telling you we are talking "Local" from "Our Frame", and you keep restating, that once we reach M31 distance it MUST be universal.....that you MUST apply the mainstream?/your take, that we MUST take the point of view of the Alien in the spaceship, right next to M31, traveling toward us at "c" for the 0 "simultaneous" emission/absorption, and .99999999 keep adding 9's for the 'near simultaneous emission/absorption....the...I would tend to say that proper distance can be defined for null geodesics, it just comes out zero, hence the "null". But it is a singular situation, perhaps best viewed as a kind of limit. you are talking about here!

Can you say "Singularity".

you are wrong...........dead wrong!!!



That is true, I assumed you were still talking about the distance in Jeff Root's quote that you said was "exactly correct". If you are changing the kind of distance you are talking about now, then yes, it could be a proper distance-- it's just not what the thread has been about, because it does not connect the "point in space and time" where the light was emitted, to the "point in space and time" where the light was absorbed. If you are going to change what you are talking about, then you need to indicate that in some way, or I might read over it.[QUOTE]

Just like you did here!!!!!!!!!!!!!!!

I changed nothing about distances!!!
{QUOTE=KenG]
If you are changing the kind of distance you are talking about now, then yes, it could be a proper distance

Here you are refering to...


Indeed, there is a nonzero proper distance between the Earth and the Moon in our frame


And the Sun's....

So, when I said...for the moon and sun


Quote:
Originally Posted by RussT
So from our point of view, that is "Proper Time" and "Proper distance" and is using the definition in Astronomy for lightspeed being Constant @ 182,282.4 in Vacua.

and then you admonished me...so I said


Quote:
Originally Posted by RussT
Okay, fine,...then I will call it "Real Time" and "Real Distance" because

and then you said...

Quote:
Originally Posted by KenG
Please feel free to actually supply definitions of the ATM concepts. I've never seen them in a physics book. Should that bother you?
[QUOTE=RussT]
I guess it looks like they should be in a physics book after all...

I even held back the remark I really wanted to make....That should bother you/guys Wayyyyy more!!! Because you are admitting that it should be in the physics books that way for "Proper distances"distance for the Sun and the Moon, and every planet distance from the sun for light travel time and distance...+ Alpha Centauri etc!!!



it's just not what the thread has been about, because it does not connect the "point in space and time" where the light was emitted, to the "point in space and time" where the light was absorbed. If you are going to change what you are talking about, then you need to indicate that in some way, or I might read over it

KenG, you could be saying that there is an observer right next to the Sun, you know, that Alien in a spaceship, travel right at the earth, at "c", so that emission/absorption were 'simultaneous' at 0, and using the standard relativity procedure..........................Why are you and Grant Not doing that??? Because it makes no sense what-so-ever "In the Local Neighborhood.......Does it?

Here is the real mis-connect though...and it goes straight to the difference in Tim's correct understanding with his statement VS what you and Grant and so many others, not looking back through this thread to understand the real difference that is being showcased here...

This is rom my post #68...



Originally Posted by KenG
Here's the problem with your reasoning, and Jeff Root's as well. The OP involved two elements:
1) extreme distance
2) difference between distances "then" and distances "now"



No, KenG, this is Your Problem...You keep restating the OP to suit yourself. See the other posts.

You keep agreeing that you are not seeing M31 in it's "Now" when this...find "a supposed alien in a spaceship Observer" who is traveling at "near "c"" Either 'next to the Milky way, traveling to M31 at near "c", nearly "Instantaneously", OR one 'next to M31' traveling toward the Milky way at near "c" so emition and absorption are nearly "Simultaneous"

Is done...so it is of no use what-so-ever!!!

In fact, if you are doing the Lorentz Contraction and not seeing M31 as it is and where it is "Now" and still seeing it as it was and where it was 2.5 million years ago, then what does it "Really Mean" to do the Lorentz contraction?

When you, following the light path 'we see/detect' back to M31, in your 'alien in the spaceship frame' do either of the above... 'pulling M31 with you' to the Milky Way, to have the appearance of simultaneous emission/absorption, OR travel instantaneously form the Milky Way to M31, pulling the Milky way with you at "c" OR, just 'contract the space between the two galaxies to 0....

You are Still seeing M31 where it was 2.5 million years ago....so it really is meaningless...you are NOT making their "Now"'s simultaneous!!!

Tim Thompson understands this!!!

Now, If you think, that you can 'access an observer' that is where M31 really is "Now" verses where it was, 2.5 million years ago, when the light we see, in our frame left M31, and that that 'observer...from where it is actually is "Now" can then be said to be traveling to earth at "c" from right next to that Now position, to equal the 0 of emission/absorption as 'simultaneous'.........then some one really is crazy.........

Either way, we cannot get to M31's "now" and the relativity convention you are trying to use does NOT work.............At least for the Local Neighborhood!!!

Ken G
2009-Jun-10, 02:10 AM
You keep "Assuming" and restating, that your take on the focus of this thread, is that once we get to the distance of M31, that we MUST change to the point of view of the Alien in the spaceship, next to M31 that is traveling at "c" toward us!!!Actually, what I have said, and will say one more time, as it is the crux of the issue, is the following:

There are many statements that can be made in regard to distance, and some are even correct. The statement about distance that we have been discussing, which you termed "exactly correct", stated that the distance to a galaxy is the distance that connects the point in space and time where the light we are seeing was emitted, to the point in space and time where we are seeing it. As I've said a few times now, the only way that distance is an invariant quantity, that is, a quantity with a meaning that is the same for all observers, is if it is the proper distance between those points. That proper distance, between the events in the "exactly correct" statement, is zero. Any other distance that might be referred to is purely an arbitrary issue of coordinatization-- it could be anything at all, so means nothing in the absence of the defining instructions for how to unpack the convention being used (in special relativity, that convention would be the inertial reference frame being used, but in general relativity, even that is insufficient, you need the whole global coordinatization being used). Given the instructions on how the coordinates are built, any particular observation could be used to infer the appropriate distance. Alternatively, we could specify the observer and the technique being applied (angular diameter distance, brightness distance, etc.), and then the distance would carry the implication of that measurement type-- it would still need to be translated into a coordinate distance for the laws of dynamics to be used on it.

So if these details seem complicated, then you begin to understand why I said there was "no such thing" as the distance between those spacetime points, the only nonarbitrary version of that distance comes out zero.

Now, these statements are facts, at least within the astonishingly accurate theory of relativity. You don't have to like them, you certainly don't have to understand them, but neither case will make them untrue.



And you just skipped all of this...
Certainly. Until the above issue is recognized and resolved, there is no point in addressing all the other misconceptions.




Jeff and I keep telling you we are talking "Local" from "Our Frame", and you keep restating, that once we reach M31 distance it MUST be universal.....Actually, that is never what I said. From the start, I said the error in the "exactly correct" statement was its failure to
1) say what reference frame was being used, and
2) even recognize the need for a reference frame to give the statement meaning.
Yes, that's really what I said, you can find it many times in the thread. I don't feel I can be responsible for what you think I said, only for what the thread shows I said. But moving away from "who said what when", the above statement I started this post with is the crux of the matter. If you are still confused about that, you can certainly ask, but if you understand it now, then there is nothing further to discuss.

RussT
2009-Jun-10, 08:15 AM
From the start, I said the error in the "exactly correct" statement was its failure to

This is totally taken out of context!

I didn't even use that term "exactly correct" (and I was refering to his concept not his words!)until Post #78, and by then Jeff had already explained himself, and since then we have both...Jeff and I keep telling you we are talking "Local" from "Our Frame",

My observer and reference frame has been stated correctly since I posted Tim's statement in my first post.

I am not sure who you think you are fooling.

And this was your response to 'exactly correct'...



But in case the OPer was also curious about larger distances, say 2.5 billion light years, then the nature of the coordinatization is essential to understand. If lookback time is being used, then it is not a distance at all, it is a time given by convention in distance units (and related to a particular time coordinatization)-- a point made earlier in the thread, and entirely inconsistent with the quoted "exactly correct" statement above. If comoving-frame coordinates are being used, then it would be natural to say the distance is the "distance now", also in contradiction to your position, although even that statement requires clarification to avoid misconception. Or as was pointed out above also, if "distance then" is intended, the the angular-size distance is being used. What is all-too-often quoted is the lookback time, which is not a distance at all, so that right there would have been a far better, and more meaningful, answer than the one you quoted above.



There are many statements that can be made in regard to distance, and some are even correct.

Yes, like all the ones I have been talking about in the Local neighborhood from our rest frame, where there is a definite light travel time and distance that is a direct result of "c" being Constant!



The statement about distance that we have been discussing, which you termed "exactly correct", stated that the distance to a galaxy is the distance that connects the point in space and time where the light we are seeing was emitted, to the point in space and time where we are seeing it.

Why did you only include "Galaxy"? When you look back it is very obvious that I started locally with the moon, sun, alpha centauri, other stars in our Milky Way, and yes even out to M31, which by the definition of light being Constant at "c" from our rest frame is absolutely correct.



As I've said a few times now, the only way that distance is an invariant quantity, that is, a quantity with a meaning that is the same for all observers, is if it is the proper distance between those points. That proper distance, between the events in the "exactly correct" statement, is zero.

That doesn't work in the "Local Neighborhood", as I have shown.

Besides, it is NOT all observers, it is only for "the alien in a spaceship, traveling away from the earth at "c" "Instaneously" to any galaxy at any distance in the universe to infinity...and that is just the "Singularity" talking...



Any other distance that might be referred to is purely an arbitrary issue of coordinatization--

Arbitrary...yeah, like the solar system, stars in our galaxy, local group of galaxies! Not quite arbitrary!!!



defining instructions for how to unpack the convention being used (in special relativity, that convention would be the inertial reference frame being used, but in general relativity, even that is insufficient, you need the whole global coordinatization being used). Given the instructions on how the coordinates are built, any particular observation could be used to infer the appropriate distance.

Ah yes, Global considerations.



Alternatively, we could specify the observer and the technique being applied (angular diameter distance, brightness distance, etc.), and then the distance would carry the implication of that measurement type---- it would still need to be translated into a coordinate distance

Yes, the Local distance ladder with "c" as a Constant.



Now, these statements are facts

Not for the Local Neighborhood!!!


Now, you keep insisting on stating things where expansion of the universe is afoot, and I have told you umpteen times we are talking "Local"...so we could keep playing this restating game, where you can then keep insisting that SR>>GR somehow does/MUST correctly determine distances to the Moon and The Sun OR...

Show us how this works using... I will say it one final time: the only "proper time" or "proper distance" that connect the emission and absorption of light are zero. This is an invariant result, which means it is not coordinate dependent, and is true for all observers, including us.

Show how Lightspeed at Constant "c" of 186,282.4 mps which is built into SR>>GR can or did determine the correct distance to the Moon and the Sun.

But you can't show that, so all you do is keep repeating and repeating and then the old favorite... at least within the astonishingly accurate theory of relativity

And you just Ignored most of my last whole Post, so I will just let Tim Thompson's correct statement, along with my explanations of why it is correct speak for themselves!



Originally Posted by RussT
I am just trying to make sure that you/MS (Mainstream) does not have some 'special way' of 'showing' that we/science can see/detect photons, say from M31, that would show that we/science can see M31 as it is "Now", and where it is "Now", as seen from our 'Now' here in the Milky Way.



Originally Posted by Tim Thompson
OK, is this what I am supposed to be answering? Haven't I already done that several times? How many different ways do you want to see the same answer? We detect photons in our "now", and that tells us what M31 looks like in our "now". Realizing that it took photons 2.5 million years to make the trip, and that our "now" is 2.5 million years after the "now" of M31 we see, we can then derive from physics how M31 would have changed over the intervening 2.5 million years. That way we can synthesize a picture of what M31 would look like if photons made the trip instantaneously, and that the two "now"s were simultaneous.My Bold/Red

Further...



Originally Posted by RussT
In other words, when we send a light beam to the moon, as in the lunar ranging project, it travels to the moon at "c" and Back at "c", for a ~2.5 second round trip, right.



Originally Posted by Tim Thompson
Always.



Originally Posted by RussT
So, is light/photons ever traveling 'instantaneously' away from us towards any distant object?



Originally Posted by Tim Thompson
Never.

Jeff Root
2009-Jun-10, 08:37 AM
In the 1960's Mad Magazine had a spoof of TV commercials, in which the
products worked only when under the extreme conditions depicted in the
commercials. Floor wax only worked if a herd of elephants stampeded
over it. A guy had to carry around a barrel of water with an outboard
motor stuck in it, so that his wristwatch, strapped to the propeller,
would keep on ticking.

Ken's observer brought that spoof to mind.

-- Jeff, in Minneapolis

RussT
2009-Jun-10, 10:02 AM
In the 1960's Mad Magazine had a spoof of TV commercials, in which the
products worked only when under the extreme conditions depicted in the
commercials. Floor wax only worked if a herd of elephants stampeded
over it. A guy had to carry around a barrel of water with an outboard
motor stuck in it, so that his wristwatch, strapped to the propeller,
would keep on ticking.

Ken's observer brought that spoof to mind.

-- Jeff, in Minneapolis

Nice:)

He has done the same thing that he has done in this thread, as in the "Rotation-I just don't get it" thread, with his insistance on which observer in his opinion, get's to mean the most.

His Philosophy on what science can or cannot "Determine" is way over the top based on his relativity observer bias.

Your's and Grav's first concepts in that thread were right on, in terms of science has already correctly determined the rotation of so many entities that the universe cannot possibly be rotating in conjuction with even more than one, let alone all the rotation we actually know of. It is patently absurd to still think in terms of the universe rotating around the earth.

Ken G
2009-Jun-10, 01:35 PM
In the 1960's Mad Magazine had a spoof of TV commercials, in which the
products worked only when under the extreme conditions depicted in the
commercials. Floor wax only worked if a herd of elephants stampeded
over it. A guy had to carry around a barrel of water with an outboard
motor stuck in it, so that his wristwatch, strapped to the propeller,
would keep on ticking.

Ken's observer brought that spoof to mind.
How can my arguments, which appeal to relativity, compare to an argument that appeals to Mad Magazine? But if there's any part of you that would like to understand relativity, I suggest you look at all the other posts by the people who have proven they do, if you don't want to believe mine. The level of denial in your series of posts in this thread is pretty striking.

Ken G
2009-Jun-10, 01:59 PM
I didn't even use that term "exactly correct"
Oh really?


I gave a clear answer: "The distance
given is the distance the light has traveled, from the point in space
and time at which it was emitted, to the point in space and time at
which it is received."

Yes and as I have said numerous times, that is exactly correct.Apparently, not only did you say it, you said it "numerous times."
(and I was refering to his concept not his words!)So the words were wrong, but the concept was exactly correct, which you were able to infer from something other than the words. Mad Magazine spoofs are indeed entirely apropos.

slang
2009-Jun-10, 02:19 PM
Not for the Local Neighborhood!!!

Ditched your GPS devices yet?

RussT
2009-Jun-10, 11:28 PM
Originally Posted by RussT
I didn't even use that term "exactly correct"



Oh really?

GEZZZZZZZZ, and you take it out of contect again!!!!!!!!!!!!!!!!!!!

Here is what I wrote!!!

I didn't even use that term "exactly correct" (and I was refering to his concept not his words!)until Post #78, and by then Jeff had already explained himself, and since then we have both...Jeff and I keep telling you we are talking "Local" from "Our Frame",[/QUOTE]

I agreed with Jeff from the beginning, that it was pretty darned obvious, that he was referring to "Our Frame", but once it was stipulated, then it was 'exactly correct" Based on the speed of light being a Constant "c", that 'we' were seeing the light/photons from where they came from, ~2.5 million years ago...

What is sooooooooooooo frigg'in tough about that.....all you need to understand that is some "are you smarter than a fifth grader" maths!!!

RussT
2009-Jun-10, 11:50 PM
How can my arguments, which appeal to relativity, compare to an argument that appeals to Mad Magazine? But if there's any part of you that would like to understand relativity, I suggest you look at all the other posts by the people who have proven they do, if you don't want to believe mine. The level of denial in your series of posts in this thread is pretty striking.

The level of denial in your series of posts in this thread is pretty striking.

The denial problem in this thread KenG, has been entirely 100% on you, since you made your opening "Ridiculous" statement!



There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels.

I got you to agree that for the last 4+ billion years, that light/photons have constantly been arriving at earth, traveling the average 93 million miles in an average of ~8.32 minutes...

Which absolutely "Proves" your above statement WRONG, and yet........you still deny that your statement is wrong even though I asked you multiple times.

Ken G
2009-Jun-11, 02:07 AM
I didn't even use that term "exactly correct" (and I was refering to his concept not his words!)until Post #78, Goodness, this is indeed an interesting position-- the words became "exactly correct" after a series of posts, although they were exactly incorrect when they were uttered. Fascinating logic. And claiming that what you were actually saying was that the "exactly correct" words were actually wrong, but the concept was right, is a position that I'm surprised you can take with a straight face. If this is going to be your support for Jeff Root, I'm not sure if he wouldn't fare better with no support at all.

...and by then Jeff had already explained himself, and since then we have both...Jeff and I keep telling you we are talking "Local" from "Our Frame",You will note that my objection to the words was the very next post, so no, I was not overlooking some clarifying remarks. The words were wrong, the concept behind them was wrong, and much of this thread has been my explaining exactly why. I guess I'll just have to say it one more time:

Jeff Root's words were incorrect as written, as a distance between two events in spacetime, because there is no unique way to define that distance that does not come out zero (the proper distance), and neither you nor Jeff have ever shown a lick of understanding of that fact. Furthermore, if we wish to change the words and pretend they said something different, we can imagine that they referred to the "concept" of proper distance between us and Andromeda in our reference frame. In that case, the events in spacetime we are talking about are simply not connected by the emission and absorption of a pulse of light, because they would need to be chosen to be simultaneous in our reference frame.

You can read Jeff Root's "exactly correct" words any way you like and pretend they are not referring to the emission and absorption of the light we see, but they obviously are, so you, and he, are merely engaging in denial at this point. But there isn't much point in my simply repeating myself, I think everyone else has gotten the idea and lost interest.

Ken G
2009-Jun-11, 02:23 AM
I got you to agree that for the last 4+ billion years, that light/photons have constantly been arriving at earth, traveling the average 93 million miles in an average of ~8.32 minutes...Once again, those distances and times have the meaning of coordinate distances and times. As such, they
1) require instructions about the coordinates used
2) are as arbitrary as the possible coordinates, and
3) refer to a proper distance only if they connect simultaneous events on the Sun and the Earth, in the coordinates in use, so in particular they cannot connect the emission and subsequent absorption of photons.
These are the objections I lodged, from the very start, in reference to the "exactly correct" statement you quoted. So if you see those objections as correct, and indeed that you have been saying (1), (2), and (3) above all along, then the original statement is still quite incorrect. What's more, if, as you claim, you have been saying what I said above "all along", you will note that I said them first, so your position can hardly be stated as a refutation of mine.


Which absolutely "Proves" your above statement WRONG, and yet........you still deny that your statement is wrong even though I asked you multiple times.Since you have not understood anything I've said, and have demonstrated great difficulty with using correct relativistic language, I'm not sure you are in much of a position to "prove" anything here.

RussT
2009-Jun-11, 08:20 AM
What's more, if, as you claim, you have been saying what I said above "all along", you will note that I said them first, so your position can hardly be stated as a refutation of mine.

Wow, this is "REALLY" clever KenG...;)

astromark
2009-Jun-11, 10:49 AM
Good grief... Alfred E Newman would be proud... what is the issue here ?
If you stopped quipping at one another you might notice you seem to be saying the same thing. Just differently. Yes there is the odd discrepancy in understanding but we can all see that this universe does not do instant. Every thing takes some time. Dependant on your reference frame. It might not be much, like a plankt length or it could be billions of years. How can you argue so much about so little... you seem to have lost the OP complacently completely.

Spaceman Spiff
2009-Jun-11, 04:31 PM
Wow. This has been unbelievable. :(

Ken G was simply trying to provide a lesson in understanding what SR and GR have to say about distance measurements. Given the nature of the question on the OP, this was a reasonable excursion IMO. His physical explanations were sound, despite the ramblings to the contrary, above.

On the other hand, Russ T expended a lot of electrons TYPING REPLIES IN BOLD LETTERS AND SCREAMING OUT VARIOUS STATEMENTS OF LITTLE USEFUL CONTENT :whistle:, quoting Tim Thompson or Ken G again and again, providing very little physical insight -- and in a sneering tone of voice on top. But more importantly he either could not or would not acknowledge any understanding (or whether he had any) of the explanations concerning SR/GR laid out plainly by Ken G (e.g., "Oh yes, Ken G I see what you mean there, but I'd like to discuss a different aspect of this question (X,Y,Z...), and here are my thoughts on that...."). And to the extent that Ken G did not follow his arguments, then I'd say the problem lay at the feet of the proponent who spent more time trying to stick Ken G in the eye than to provide sound, clear arguments of scientific merit.

I honestly don't know where Ken G gets his patience. I lost mine early on in this thread, mainly because of the impolite and scientifically useless grief being served up to him. If somebody engaged in this means of argument at an APS or AAS meeting, his/her audience would dwindle rapidly to zero, and little would be accomplished other than to p--s off a whole bunch of scientists in your field. Make this a habit and you'd lose the respect of your colleagues.

If your goal is to engage in and be engaged in useful scientific discussions, and not to be left to stand on your soapbox with no one listening, the above is a prime example of what not to do.

Ken G
2009-Jun-11, 04:57 PM
I honestly don't know where Ken G gets his patience.I appreciate the support Spaceman, I think where I get the tenacity (some might say stubbornness) to "stick out" a thread like this is my genuine fascination with the subject matter. I really think this stuff is astonishing, and the more counterintuitive, the more astonishing I find it, so I don't mind making statements that tend a little toward the "provocative". It's just that for some people, "provocation" takes on a different character than simply the fascination of how our universe works. So I generally consider myself to be to blame for trying to get people to step outside the comfort zone of what they thought they knew. In any event, the process of going through a debate like this is actually a learning experience for me, both in terms of how to say things, but even, the content itself can evolve a bit, and benefits from the closer inspection. It is probably more for the lurkers, if there still are any, than for the proponents of the opposing view, because we can all find ourselves in a little fight between our desire to know the truth, and our desire to be right. But I think it's safe to say that this thread has gone on long enough that, anyone who can understand, probably does by now!

Spaceman Spiff
2009-Jun-11, 06:14 PM
I really think this stuff is astonishing, and the more counterintuitive, the more astonishing I find it, so I don't mind making statements that tend a little toward the "provocative". It's just that for some people, "provocation" takes on a different character than simply the fascination of how our universe works. So I generally consider myself to be to blame for trying to get people to step outside the comfort zone of what they thought they knew.

Your polite provocations to readers (myself included) to sharpen their understanding of physical processes to a fine edge and to eschew non-scientific baggage is most illuminating. :)

grant hutchison
2009-Jun-11, 06:21 PM
The relationship between Comoving Distance and Angular Diameter Distance is indeed about two to one for z=1, but (as the graph in my link (http://www.atlasoftheuniverse.com/redshift.html) shows) that ratio increases steadily with increasing values of z.

The most commonly quoted distance is unfortunately neither of the above, but Light Travel Time Distance, which tells you neither where the galaxy was when it emitted the light, nor where it is now: it tells you how long ago the galaxy emitted the light. The next most commonly quoted distance is the Comoving Distance, which refers to "now". In my experience, it's unusual to see the Angular Diameter Distance ("then") quoted, except in discussions like this one.

Grant HutchisonI know this is a bit off on a tangent relative to the OP, but just to set the record straight -- in a spatially flat universe, the ratio of these two distances (Co-moving/Angular Diameter or then/now) is simply 1+z.Did I say something misleading? If so I'm not seeing it.

Grant Hutchison

Ken G
2009-Jun-11, 06:33 PM
Did I say something misleading?I think Spaceman Spiff's reference to "setting the record straight" was merely intended to mean "to fill in the gaps so that the record is complete" moreso than "to correct misleading statements". The ambiguities in that particular expression are to blame for any miscommunication, I suspect.

grant hutchison
2009-Jun-11, 06:42 PM
I think Spaceman Spiff's reference to "setting the record straight" was merely intended to mean "to fill in the gaps so that the record is complete" ...Well, that was how I was choosing to interpret it, since I couldn't puzzle out any misleading content. :)
But given that this entire thread seems to have developed into a debate about misleading content, I thought it might be sensible to just ask. Perhaps my rather ideosyncratic choice of the word "relationship" rather than "ratio" leaves the interpretation of the phrase "two to one" open to misunderstanding.

Grant Hutchison

Spaceman Spiff
2009-Jun-11, 11:47 PM
I think Spaceman Spiff's reference to "setting the record straight" was merely intended to mean "to fill in the gaps so that the record is complete" moreso than "to correct misleading statements". The ambiguities in that particular expression are to blame for any miscommunication, I suspect.

Oops. :doh:Yes, my choice of words was less than optimal. It is indeed as Ken G inferred -- I was just stating the relationship explicitly so that the typical reader out there would know what to do in general (assuming spatially flat geometry). Sorry about that Grant.

RussT
2009-Jun-12, 09:28 AM
In any event, the process of going through a debate like this is actually a learning experience for me, both in terms of how to say things, but even, the content itself can evolve a bit, and benefits from the closer inspection.

"A bit" is a little understated, don't you think KenG? I would dare say that you/KenG and I (and maybe Grant) are the only ones who really understand what has happened in this thread!

You see, when you couch so much of your responses "in that correct relativity language", rather than plainly explaining what you really mean, even those on your side aren't really getting what you are laying down...ie:



Originally Posted by RussT
I got you to agree that for the last 4+ billion years, that light/photons have constantly been arriving at earth, traveling the average 93 million miles in an average of ~8.32 minutes...



Once again, those distances and times have the meaning of coordinate distances and times. As such, they
1) require instructions about the coordinates used
2) are as arbitrary as the possible coordinates, and
3) refer to a proper distance only if they connect simultaneous events on the Sun and the Earth, in the coordinates in use, so in particular they cannot connect the emission and subsequent absorption of photons.

Let's see, shall we...Wait...I was going to ask Spaceman Spiff to give his interpretation of what # 3 means, and then tell us what that means for Ken's stance throughout this thread...

But, I would rather ask you KenG a question first and see if I can get a straight forward and honest answer...by honest here, I don't mean anything about lying, but more about "Spin Control" type answers.

Ken, I understand how you could have started taking the stance you are with M31 and galaxies, so you have probably realized that (Your stance) for a while, BUT, since I have taken such a strong stance on what Constant "c", From Earth's Frame, means "Locally" and especially for our solar system...

Have you Just realized, during the course of this thread, what that stance really means...Locally???

Ken G
2009-Jun-12, 03:27 PM
"A bit" is a little understated, don't you think KenG? No, I think it is the appropriate word choice, and quite demonstrably so. Indeed, the beauty of a written discourse is that the record is quite clear on that very issue.

You see, when you couch so much of your responses "in that correct relativity language", rather than plainly explaining what you really mean, even those on your side aren't really getting what you are laying down...Unfortunately I feel constrained to try and use language in which the words, rather than just the imagined "concepts", are actually correct. It must be very liberating for you to throw off that yoke.

Let's see, shall we...Wait...I was going to ask Spaceman Spiff to give his interpretation of what # 3 means, and then tell us what that means for Ken's stance throughout this thread...It is somewhat mystifying to me why you think that point #3 above has not been a core point I have been making consistently and from the very start of this discourse. News flash: it has, and everyone else is clearly well aware of that.
Have you Just realized, during the course of this thread, what that stance really means...Locally???No, I have for over 20 years had a clear idea of what the speed of light means, locally. Indeed, I believe if you review my comments, you will find a very oft-repeated theme that the problematic elements of Jeff Root's remark that got this all started had entirely to do with extending local coordinates into a global coordinatization, and the arbitrariness involved when doing that. What's more, you will see where I, on numerous occasions, pointed out that local issues are not responsive to the OP question, as the latter asked about differences between "distance then" and "distance now" (a fundamentally nonlocal question). Finally, I was clear that in every situation where a purely local treatment is appropriate, then grant hutchison's initial answer is perfectly clear and needs no embellishment: there is no importance in the difference between distance then and distance now when a local coordinatization is all that is required, as for galaxies only a few million LY away. That is "what happened" in this thread, the rest was just trying to get you to see it. That by itself would certainly not be worth the effort, but the hope is that others reading the thread might gain some useful insights into this very subtle business that we are all still learning: relativity.

grant hutchison
2009-Jun-12, 05:39 PM
Oops. :doh:Yes, my choice of words was less than optimal. It is indeed as Ken G inferred -- I was just stating the relationship explicitly so that the typical reader out there would know what to do in general (assuming spatially flat geometry). Sorry about that Grant.No worries. I say misleading things sometimes. I actually prefer to be told when I've done that. Hence my question. :)

Grant Hutchison

RussT
2009-Jun-15, 09:38 AM
Here is exactly what has gone on in this thread.

And I am pretty darned sure that Jeff as well as many others...are not getting why we were ridiculed so severly for using a simple concept of light travel times and distances to the moon, sun, alpha centauri and yes, even M31!!!

In fact, in the beginning both KenG and Grant fully agreed...


Originally Posted by RussT
Light is Constant @ 186,282.4 mps in vacua by definition, which means that the light from M31 at ~2.5 million Lys away took 2.5 miilion years to reach HST. Which means we see/detect M31 where it was 2.5 million years ago. During the last 2.5 million years, M31 has, based on it's current speed in the local group, moved to where it is Now in space. That is what we KNOW as long as light is Constant at 186,282.4 mps Just as Tim said in his quotes I quoted!



And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison



Yes, the spaceship moving at 0.5 c in the Earth frame will require 8 Earth years to travel the 4 LY (in the Earth frame) to alpha Cen, and light will require 4 Earth years to do the same trip. Is that not what Grant said? What other possibilities are there, this follows from the definition of velocity, coupled with the appropriate attention to specifying a reference frame

Now, because Relativity has an "at Rest" observer, and Constant "c" in vacua, is part and parcel in the theory of Relativity, both Jeff and I fully expected that these kind of agreeing statements above, that we have all seen many many times, were agreeing based on "RELATIVITY"...as I would be willing to bet that MOST Astronomers/physicists think they are doing when they do the same!!! BUT with KenG's treatment of showing "Proper Distances/Time's"



Jeff Root's words were incorrect as written, as a distance between two events in spacetime, because there is no unique way to define that distance that does not come out zero (the proper distance), and neither you nor Jeff have ever shown a lick of understanding of that fact. Furthermore, if we wish to change the words and pretend they said something different, we can imagine that they referred to the "concept" of proper distance between us and Andromeda in our reference frame. In that case, the events in spacetime we are talking about are simply not connected by the emission and absorption of a pulse of light, because they would need to be chosen to be simultaneous in our reference frame.



He/KenG has shown, beyond a shadow of a doubt, that Relativity SR>>GR says absolutely Nothing (and I could go a step further here, but won't at this time) about light/photons travel time from point A to point B, for known local distances, at Constant "c", in our Local Neighborhood.



No, I have for over 20 years had a clear idea of what the speed of light means, locally.

So, tell us KenG, if you have known this for so long (and it seems that many of your collegues did not get the memo), why haven't you, or for that matter, any of your collegues, put the disclaimer on the two quotes, agreeing to light travel times and distances, that...

Those agreements to light travel times and distances are NOT part of the theory of Relativity, because those do not have "Proper Distance" of Zero???

Ken G
2009-Jun-15, 11:16 AM
So, tell us KenG, if you have known this for so long (and it seems that many of your collegues did not get the memo), why haven't you, or for that matter, any of your collegues, put the disclaimer on the two quotes, agreeing to light travel times and distances, that...

Those agreements to light travel times and distances are NOT part of the theory of Relativity, because those do not have "Proper Distance" of Zero???Um, because they do not connect events of a light pulse being emitted and then absorbed, whereas the problematic "exactly correct" statement above did? Didn't I say that a few times already?

RussT
2009-Jun-16, 08:48 AM
Originally Posted by KenG
Yes, the spaceship moving at 0.5 c in the Earth frame will require 8 Earth years to travel the 4 LY (in the Earth frame) to alpha Cen, and light will require 4 Earth years to do the same trip. Is that not what Grant said? What other possibilities are there, this follows from the definition of velocity, coupled with the appropriate attention to specifying a reference frame



Originally Posted by RussT
Light is Constant @ 186,282.4 mps in vacua by definition, which means that the light from M31 at ~2.5 million Lys away took 2.5 miilion years to reach HST. Which means we see/detect M31 where it was 2.5 million years ago. During the last 2.5 million years, M31 has, based on it's current speed in the local group, moved to where it is Now in space. That is what we KNOW as long as light is Constant at 186,282.4 mps Just as Tim said in his quotes I quoted!



Originally Posted by Grant
And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison

Gentlemen, Please explain to the class, what theory you are using to make these statements.

grant hutchison
2009-Jun-16, 09:15 AM
Gentlemen, Please explain to the class, what theory you are using to make these statements.Round and round, round and round ...
It's special relativity, with a specified rest frame.

Grant Hutchison

RussT
2009-Jun-16, 11:26 AM
Round and round, round and round ...
It's special relativity, with a specified rest frame.

Grant Hutchison

Nope, that is not possible!!!!

I have an SR experts 5 page dissertation on why that cannot be the case...

Starting with...



Originally Posted by KenG
There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels.

grant hutchison
2009-Jun-16, 12:38 PM
Nope, that is not possible!!!!

I have an SR experts 5 page dissertation on why that cannot be the case...

Starting with...
There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels.As I've said already, your disinclination to learn special relativity is not my problem.
There's no such thing as the distance light travels. There is a distance light travels, which varies from observer to observer. That's why it's called relativity.

Grant Hutchison

Ken G
2009-Jun-16, 07:00 PM
It's special relativity, with a specified rest frame.

RussT, I pointed out to you a long time ago that the problem you are having, and the problem in the Jeff Root quote, is all about failure to understand the role of a reference frame in making statements about distances (and times). It is not that every single quote you can pull from your posts is wrong in every situation (because in many situations the reference frame is obvious). I also pointed out that the sole versions of the concepts of distance and time that are not observer (and even measurement) dependent are proper distances and proper times, both of which come out zero for the events that the Jeff Root quote talked about. Jeff himself isn't even piping in to defend that statement any more, shouldn't that tell you something? The horse is dead, there's little point in our beating it any more, and even less in your efforts to revive it.

slang
2009-Jun-16, 10:21 PM
Round and round, round and round ...


Nope, that is not possible!!!!

followed by a quote from a post on page 1. Weapons grade irony.

Spaceman Spiff
2009-Jun-18, 02:10 AM
Hopeless....:(

RussT
2009-Jun-19, 01:08 AM
The Only thing that I have done this entire thread is...

Stick to the decidely correct mainstream axiom...that lightpseed is Constant at "c" of 186,282.4 mps...and the decidely correct mainstream understanding of the known distances to the Local entities IE: The Moon, The Sun, The star Alpha Centauiri, The galaxy M31...

SO, yes, there is a distance to everything in the Local arena, The distance to those know entiites, per this...



Originally Posted by Grant
And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison

From "Earths Frame" or any scientist, in their "Rest Frame" at Mercury, Venus, Earth, Mars out to all the planets in our solar system, and the scientist in her/his 'rest frame' on or around their third rock from the sun of alpha Centauiri, or the scientists in M31 'at rest' on their planet...

That the 'distance' is 'really' the distance that light travels in specified amounts of time, based on the distance.......hence the term "Light Year".

That's exactly what "we" (you and Tim and the rest of humanity) know.

Period.

Now, whatever is being done to make that "False"...................you can play all the word games you wish....but

Is the problem

Ken G
2009-Jun-19, 05:01 AM
The Only thing that I have done this entire thread is...

Stick to the decidely correct mainstream axiom...that lightpseed is Constant at "c" of 186,282.4 mps...and the decidely correct mainstream understanding of the known distances to the Local entities IE: The Moon, The Sun, The star Alpha Centauiri, The galaxy M31...
If you really think that's all you have done in this thread, to utter a standard boilerplate remark that adds nothing to anyone's understanding and does not even distinguish the concepts in the OP of "distance then" versus "distance now", then yes, you should be very confused about why so many people have had to tell you that you are wrong. This may help resolve your confusion: they (and I) were talking about all the other things you have done on this thread. I'll spare you the quotes-- they've all been quoted in the above already, just read them.

RussT
2009-Jun-19, 09:34 AM
you should be very confused about why so many people have had to tell you that you are wrong.

I am not confused or wrong!



does not even distinguish the concepts in the OP of "distance then" versus "distance now",

You didn't listen, when I told you, and everyone else, over and over and over again, "distance then" versus "distance now", was absolutely irrelevent, because we were talking about Local, before any expansion of the universe!

This question...


Originally Posted by KenG
Yes, the spaceship moving at 0.5 c in the Earth frame will require 8 Earth years to travel the 4 LY (in the Earth frame) to alpha Cen, and light will require 4 Earth years to do the same trip. Is that not what Grant said? What other possibilities are there, this follows from the definition of velocity, coupled with the appropriate attention to specifying a reference frame



Originally Posted by RussT
Light is Constant @ 186,282.4 mps in vacua by definition, which means that the light from M31 at ~2.5 million Lys away took 2.5 miilion years to reach HST. Which means we see/detect M31 where it was 2.5 million years ago. During the last 2.5 million years, M31 has, based on it's current speed in the local group, moved to where it is Now in space. That is what we KNOW as long as light is Constant at 186,282.4 mps Just as Tim said in his quotes I quoted!



Originally Posted by Grant
And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison



Gentlemen, Please explain to the class, what theory you are using to make these statements.

Took Jeff Root and the OP's distance then/now out of the equation!

And, Grant's response was...



Round and round, round and round ...
It's special relativity, with a specified rest frame.

Grant Hutchison

So, this...It's special relativity, with a specified rest frame.

Simply cannot be true

because, you just spent 5 pages telling us...

There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels. and both of the agreements above do have travel times of light/photons where distance is the distance specified.

There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels.

The Lunar Ranging Project has been proving that statement 100% False every day for decades!!!



adds nothing to anyone's understanding and does not even distinguish the concepts in the OP of "distance then" versus "distance now",

Yes, your thrust from the beginning, which became borderline 'bullying' as we have gone through this, all designed to avoid the very problematic Local SR>>GR, which by the way, I didn't fully appreciate (the problem, that is) until going through this thread.

Yes, I too learn alot each time these kind of threads occur.

This IS what is "Real"...



Originally Posted by RussT
I am just trying to make sure that you/MS (Mainstream) does not have some 'special way' of 'showing' that we/science can see/detect photons, say from M31, that would show that we/science can see M31 as it is "Now", and where it is "Now", as seen from our 'Now' here in the Milky Way.



Originally Posted by Tim Thompson
OK, is this what I am supposed to be answering? Haven't I already done that several times? How many different ways do you want to see the same answer? We detect photons in our "now", and that tells us what M31 looks like in our "now". Realizing that it took photons 2.5 million years to make the trip, and that our "now" is 2.5 million years after the "now" of M31 we see, we can then derive from physics how M31 would have changed over the intervening 2.5 million years. That way we can synthesize a picture of what M31 would look like if photons made the trip instantaneously, and that the two "now"s were simultaneous.My Bold/Red

Which includes the Earth frame from....The Moon, The Sun, Alpha Centauiri, and M31



Originally Posted by Grant
And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison

Now, instead of playing games using italics (to show superiority, over those ATM'ers that do that bolding, and coloring) on "the" and "a" let's look up the word "Know"

Jeff Root
2009-Jun-19, 10:39 AM
I still see nothing wrong with my first two replies in this thread. What I
said was a correct answer to the question asked. As far as I can tell,
although the argument has gone off in many other directions, Russ has
simply been trying to argue for the correctness of my simple answer.

I do not know how to argue for its correctness, because it is so simple
and clearly correct that no argument should be needed. Ken obviously
knows everything relevant to the question that Russ knows and that I
know. But none of his arguments contradict the correctness of my
answer.

When the distance of a galaxy is given in light-years, that distance is
the distance its light has traveled from it to the observer. The starting
point is the place and time the light was emitted, while the ending point
is the place and time the light was received. That is true nomatter what
happens before, after, or during the passage of the light. Even if the
space between the distant galaxy and the observer changes enormously
during the light's travel, the distance meant is still the distance that the
light traveled.

This thread began with the words:


The Andromeda galaxy is said to be about 2.5 million light years away.
If the distance to the Andromeda galaxy were said to depend on the
relative motion between Andromeda and the observer, I would have no
objection. But the original poster correctly stated that the distance is
given as approximately a specified distance in light-years. I correctly
responded that that means the distance from the place and time the
light was emitted to the place and time it was received.

-- Jeff, in Minneapolis

grant hutchison
2009-Jun-19, 11:09 AM
Now, instead of playing games using italics (to show superiority, over those ATM'ers that do that bolding, and coloring) ...And capitals. Don't forget capitals.

Grant Hutchison

Ken G
2009-Jun-19, 02:56 PM
You didn't listen, when I told you, and everyone else, over and over and over again, "distance then" versus "distance now", was absolutely irrelevent, because we were talking about Local, before any expansion of the universe!It is not just about expansion, you are also ignoring special relativity. But it's all right, you may continue to go with your own undertanding of distance, there's no reason you need to understand relativity. The OPer may want to understand it, or they may not, that is their choice as well.

Ken G
2009-Jun-19, 03:28 PM
I still see nothing wrong with my first two replies in this thread. What I
said was a correct answer to the question asked. Your first post included:

The distance
given is the distance the light has traveled, from the
point in space and
time at which it was emitted, to the point in space and time at which it
is received.

I'm sorry Jeff, but in relativity, that statement is simply false. If we are going to ignore relativity, fine, but I felt the OPer deserved to know the truth. I've told you precisely why it is false several times now, but I guess I'll just tell you again:
In relativity, there is no such thing as the unique or absolute distance between two events, in contradiction to the implication of your post. But there is one fairly natural way to coordinatize "the distance" between any two events, and it is called "proper distance" (let's not get into the differences between special and general relativity here, that really requires an expert, but imagine either one). To get proper distance, you choose a reference frame in which the two events are simultaneous, and connect the two events with a chain of stationary observers in that frame. Then the ruler distance for that integrated chain of observers is the proper distance. The problem is, if you first select the events, as in your statement, then you don't get to choose the reference frame, it is given-- and it is not our frame. For the events you describe, connected by emission and absorption of light, the frame in which those events are simultaneous is the frame of the light itself, and the proper distance is zero.

Now, if you intended for our own reference frame to be implied, then when you get a proper distance between us and some galaxy, you don't get to choose the events that are connected by that distance, in contradiction to your statement. In summary, if you choose the events you did, the proper concept of distance comes out zero, and if you want a proper distance that makes sense in our reference frame, you cannot be talking about the events you cite.

Einstein once said that we should make things as simple as possible, but no simpler. That is the problem with your statement, unless one is not doing relativity (but if one is not doing relativity, does anyone need your answer?).


I do not know how to argue for its correctness, because it is so simple
and clearly correct that no argument should be needed. That statement is strangely incongruent with the fact that it is badly incorrect. Perhaps your failure to be able to cite an argument to support it should have been some kind of clue there.


When the distance of a galaxy is given in light-years, that distance is
the distance its light has traveled from it to the observer. No, again, light does not travel that distance. That distance connects two events, one in that galaxy and one in ours, that light does not connect. However, in cases where there is no important difference between "distance then" and "distance now", the discrepancy is unimportant and we can safely ignore it-- as long as we know we are ignoring it. This is an entry point for you to expand on your relativity knowledge, if you choose to use it as such. If, instead, you choose to believe your statement is "clearly correct" no matter what, then you pass on that opportunity. Your choice.

Spaceman Spiff
2009-Jun-20, 02:01 AM
No, again, light does not travel that distance. That distance connects two events, one in that galaxy and one in ours, that light does not connect. However, in cases where there is no important difference between "distance then" and "distance now", the discrepancy is unimportant and we can safely ignore it-- as long as we know we are ignoring it. This is an entry point for you to expand on your relativity knowledge, if you choose to use it as such. If, instead, you choose to believe your statement is "clearly correct" no matter what, then you pass on that opportunity. Your choice.
(my emphasis)

This is it folks, wrapped up nicely in a nutshell. It's all you need to know regarding ~95% of this thread. Ken G has simply tried to explain this concept of 'distance' in the context of our most useful model(s) for understanding concepts concerning intervals of space, time, and space-time.

Last time I checked, distance and time intervals weren't invariants. I suppose one is free to insist that they are, in which case -- good luck to you. :whistle:

RussT
2009-Jun-21, 09:23 AM
And capitals. Don't forget capitals.

Grant Hutchison

Yes...We can't forget those...and I am a little amazed you left out the "Quotation Marks" and "Sizing"

HAPPY FATHERS DAY EVERYONE:lol:

RussT
2009-Jun-21, 10:32 AM
Russ has simply been trying to argue for the correctness of my simple answer.

I do not know how to argue for its correctness, because it is so simple
and clearly correct that no argument should be needed.

Yes, not only is it simple and in black and white, but they also agree that light/photons in Earths Frame coming from any entity in the Local neighborhood, do have a 'travel time'...and even uses the word "Exactly" just like I did for your statement that in 'earths frame' light/photons have an emission point from the distant object that travel to us over that distance in specified amounts of time based on that distance.



Originally Posted by Grant
And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison

I do not know how to argue for its correctness

That's the arguement right there....how can it be argued that something we "Know" to be true, is not true???

Is there any doubt that the light reaching us from the moon is traveling that distance at "c" in a specified amount of time.........Zero doubt....same for the Sun, etc. for the Local neighorhood.

So, there should be more than just you and I saying, what the heck is going on here.

The rest is just understanding how relativity is formatted.

And now that KenG has kept this up, other things have become obvious, that I didn't even realize before.



When the distance of a galaxy is given in light-years, that distance is
the distance its light has traveled from it to the observer. The starting
point is the place and time the light was emitted, while the ending point
is the place and time the light was received.

Just a suggestion here for the future.... when referring to this (Bold) 'observer'...just add the words....Earth frame or Earth Rest Frame observer.

And you want to be careful about using 'stationary', as the 'stationary observer' in SR is NOT equivalent to the "Earth Rest Frame" observer.

Happy Fathers day.........

RussT
2009-Jun-21, 11:10 AM
This is it folks, wrapped up nicely in a nutshell.

Sorry, but with all due respect....not quite.



Last time I checked, distance and time intervals weren't invariants

Sorry, this says from Earth's Frame they are...In The Local Neighborhood...


RussT said
This IS what is "Real"...

Quote:
Originally Posted by RussT
I am just trying to make sure that you/MS (Mainstream) does not have some 'special way' of 'showing' that we/science can see/detect photons, say from M31, that would show that we/science can see M31 as it is "Now", and where it is "Now", as seen from our 'Now' here in the Milky Way.

Quote:
Originally Posted by Tim Thompson
OK, is this what I am supposed to be answering? Haven't I already done that several times? How many different ways do you want to see the same answer? We detect photons in our "now", and that tells us what M31 looks like in our "now". Realizing that it took photons 2.5 million years to make the trip, and that our "now" is 2.5 million years after the "now" of M31 we see, we can then derive from physics how M31 would have changed over the intervening 2.5 million years. That way we can synthesize a picture of what M31 would look like if photons made the trip instantaneously, and that the two "now"s were simultaneous.My Bold/Red

RussT said
Which includes the Earth frame from....The Moon, The Sun, Alpha Centauiri, and M31


Quote:
Originally Posted by Grant
And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison

Now, that automatically brings in a problem, doesn't it?...

That means that agreeing to the above, which is 100% mainstream, is NOT based on "Relatvity", based on what KenG has insisted upon in this thread. Which I warned about earlier in this thread!

So, when I asked...


Gentlemen, Please explain to the class, what theory you are using to make these statements

And Grant Responded...



It's special relativity, with a specified rest frame.

That was a misrepresentation...

and then I said...



Nope, that is not possible!!!!

I have an SR experts 5 page dissertation on why that cannot be the case...

And Grant said...



There's no such thing as the distance light travels. There is a distance light travels, which varies from observer to observer. That's why it's called relativity

Which is a 'Gross Misrepresentation' of 'observers', because he is NOT including Earth Frame observers, he is only referring to SR "Aliens in spaceship" observers.

There is another one I could do, but this is enough for now.

Happy Fathers Day..............

grant hutchison
2009-Jun-21, 11:28 AM
That's the arguement right there....how can it be argued that something we "Know" to be true, is not true???
What we know to be true for us we also know to be untrue for other observers, in motion relative to us.
If I face north, east is on my right. If you face south, east is on your left. Dear heavens, something that is true for me does not apply to you!!!! The Universe has gone mad!!! (Not.)
Observers in special relativity are likewise obliged to rotate their personal coordinates relative to spacetime, although in a slightly more complicated way.


Just a suggestion here for the future.... when referring to this (Bold) 'observer'...just add the words....Earth frame or Earth Rest Frame observer.

And you want to be careful about using 'stationary', as the 'stationary observer' in SR is NOT equivalent to the "Earth Rest Frame" observerIndeed, a "stationary" observer is not equivalent to anything, until his rest frame has been specified.
The "rest frame of the Earth" also won't do the trick for objects that are being separated by the expansion of the Universe: our distant emitter of light does not share the rest frame of the Earth.

Grant Hutchison

grant hutchison
2009-Jun-21, 11:34 AM
There's no such thing as the distance light travels. There is a distance light travels, which varies from observer to observer. That's why it's called relativity.Which is a 'Gross Misrepresentation' of 'observers', because he is NOT including Earth Frame observers, he is only referring to SR "Aliens in spaceship" observers.If you could cut the insulting insinuations back to a dull roar, I'd be gratified.
All frames are addressed in that remark; I made no specification or exclusion at all. So the "Earth Frame observers" are in there, with their own measurements, just as the "Aliens in a spaceship" are there, with their own measurements. And they're all disagreeing with each other about distance and time, which are (as Spaceman Spiff remarks) not invariant properties of spacetime.

Grant Hutchison

speedfreek
2009-Jun-21, 12:49 PM
It seems to me that the problems started in this thread when RussT objected to the first post by Ken G. But Ken ended his post with this:


It might seem like a nitpick, because we do talk about the distances to things, and how long ago they occured. But I still think it is important not to fall into the illusion that these are absolute statements of truth about the events themselves. It's all quite subordinate to the choice of observer-- generally, us. The answers should not have to state that every time, of course, but when statements like the above are made, I think it's time to set the record straight.

It was a reply to Jeff's statement:


Grant's answer is correct: It is both. It is also neither. The distance given is the distance the light has traveled, from the point in space and time at which it was emitted, to the point in space and time at which it is received.

Which is essentially correct as long as you acknowledge the role of the observer, and that it is correct from the point of view of any observer. The thing that Ken is pointing out is that different observers will get different distances that the light travelled, but they all will be correct. It is not the distance that light travelled, it is one of many different distances that light travelled. Different observers will measure different distances between one event and the other. There is no absolute distance that the light travelled, it all depends on who you ask. Whose point of view can be considered to be the "correct" one? You might think that perhaps the frame of reference of the "light itself" might be the best frame in which to consider the problem, but then the answers for time and distance are both zero.

From our point of view, Andromeda was approximately 2.5 million light years away "then", when the light we see was emitted. From our point of view, it will be a little bit closer now, than it was when the light was emitted. If that is all you want to know then fine. Just as long as you realise that it is not an absolute measurement.

But from the point of view of someone floating in space, halfway between the two galaxies, the answers, their measurements of distance and time between these two events, would be slightly different from ours here on Earth. An observer in another galaxy, a few million light years away, would get slightly different answers too. Everyone gets different answers, some more different that others. In the end, the problem gets so bad that as we reach the larger cosmological scales, we have to use a generalised approximation, based on a series of theoretical observers that are at rest and under the least gravitational influence possible in their own frame of reference, but whose frame is moving with the expansion of the universe.

Where is Andromeda "now"? Well it all depends on what you are doing when you ask the question. As you are driving along a motorway in your car at 70 mph, an alien being in Andromeda is holding his newborn child. You take the exit, cross the overpass and go back on the motorway heading in the opposite direction and accelerate up to 70 mph again. Now that aliens child hasn't quite been born yet, from your frame of reference, but not, of course, from his.

Also, a photon emitted on the Moon might be detected on Earth a little over a second later, but there is a small but finite possibility that only half a second before we detected that photon, it was actually in Andromeda! How far did that photon travel? Can you say for sure?

(Note: In case anyone is wondering, this post is not directed at Jeff, who knows all this as well as Ken does. It is directed at RussT or anyone having trouble following the debate)

Jeff Root
2009-Jun-21, 12:59 PM
I still see nothing wrong with my first two replies in this thread.
What I said was a correct answer to the question asked.
Your first post included:


The distance given is the distance the light has traveled, from
the point in space and time at which it was emitted, to the point
in space and time at which it is received.
I'm sorry Jeff, but in relativity, that statement is simply false.
If we are going to ignore relativity, fine, but I felt the OPer
deserved to know the truth. I've told you precisely why it is
false several times now, but I guess I'll just tell you again:
In relativity, there is no such thing as the unique or absolute
distance between two events, in contradiction to the implication
of your post.
I never said that there is a "unique" or "absolute" distance.
If it was implied, that is because it was implied in the original
post. And it obviously was only implied there because it is
implied in the sources from which the original poster took the
specified distance, which is published ubiquitously:

The Big Bang, Joseph Silk, 1989, p. 39


With these calibrations we could see that the Magellenic Clouds
are very far away; indeed, we now know them to be separate companion
galaxies to the Milky Way, about 150,000 light-years distant but
mere dwarfs by comparison with our galaxy. Our nearest neighbor
of comparable size to the Milky Way is the Andromeda galaxy.
Barely visible to the naked eye as a fuzzy patch in the sky, the
Andromeda galaxy is some 2 million light-years distant.
Coming of Age in the Milky Way, Timothy Ferris, 1988, p. 166


Parallax is detectable only out to a few hundred light-years;
the Andromeda galaxy is over two million light-years away.
Voyage to the Great Attractor, Alan Dressler, 1994, p. 48


It takes only eight minutes for light from the Sun to reach the
Earth; the entire solar system is but a few light-hours across.
We now traverse this planetary realm with ease. Journeys on a
light beam to the nearest stars require a career change--it is
eight years to Sirius--but at 100,000 years, a pilgrimage to the
outer bounds of our galaxy is another matter altogether. Still,
one can at least imagine such a journey. A sizeable piece of our
galaxy could be crossed in the time that humans have recorded their
history, a tiny fraction of the nearly 5-billion-year lifetime of
planet Earth. We could even imagine sailing to the next galaxy,
Andromeda, some two million light-years away, about the length of
time the kin of Homo Sapiens have lived on this planet.
Discovering the Universe, William J. Kaufmann, III, 1987, p. 308


The Andromeda Galaxy (called M31 or NGC 224) This nearby galaxy
covers an area of the sky roughly five times as large as the full
moon. Under good observing conditions, the galaxy's bright central
bulge can be glimpsed in the constellation of Andromeda with the
naked eye. The distance to the galaxy is 2¼ million light-years.
p. 312


The distance to the Virgo cluster is about 50 million light-years,
too far away for Cepheid variables to be seen from our galaxy.
Instead, the distance to the Virgo cluster has been determined by
the apparent faintness of O and B supergiant stars, by the
brightness of globular clusters surrounding some of the galaxies,
and by the angular sizes of H II regions in some of the cluster's
spiral galaxies. The overall diameter of the Virgo cluster is
about 7 million light years.
Exploring the Cosmos - Second Edition, Louis Bergman, J.C. Evans,
1977, p. 289


The Andromeda galaxy, Messier 31, and its two elliptical companions.
They are a little over two million light-years away.
An Introduction to Astronomy - 7th Edition, Baker and Fredrick,
1968 (my textbook in the introductory astronomy course at the
U of M in 1971), p. 320


At a distance of 2.2 million light years from us, the diameter of
the Andromeda spiral is 180,000 light years, or more than twice the
diameter often assigned to the disk of our own galaxy.
Skyguide - A Field Guide to the Heavens, Mark R. Chartrand, 1990,
p. 64


Name Dist., million lt-yr
M31 2.1
SMC 0.2
M33 2.4
LMC 0.2
M81 6.5
M101 14.0
The Structure of the Early Universe, John D. Barrow and Joseph Silk,
Scientific American magazine, April 1980, p. 118


For example, galaxies 20 million light-years away (among the closest
to our own galaxy) have a red shift of .001 and galaxies 10 billion
light-years away (among the most distant) have a red shift of .75.
Not one source mentions that the distances given vary from observer
to observer or depend on relative motion or the structure of
spacetime -- even when discussing the structure of spacetime.

My reply answered the question asked in the original post without
going into relativity. Bringing up relativity is fine. You can
write about it all you like. But the fact that there is no such
thing as the unique or absolute distance between two events in
relativity does not make my answer false. The answer I gave is
not a correct answer to the questions you are answering, but it
is a correct answer to the question that was asked.



But there is one fairly natural way to coordinatize "the distance"
between any two events, and it is called "proper distance".
To get proper distance, you choose a reference frame in which the
two events are simultaneous, and connect the two events with a chain
of stationary observers in that frame. Then the ruler distance for
that integrated chain of observers is the proper distance.
Clearly that does not apply here. The distance of the Andromeda
galaxy has never been measured in such a manner, and cannot be.
Yet we do have a measured distance for the Andromeda galaxy, given
in the first sentence of the original post.

That distance was not measured using simultaneous events, because
light takes approximately two and a half million years to go from
Andromeda to Earth, and we have no recorded measurements from two
and a half million years ago. The best we can do is measure the
distance from where we see that Andromeda was two and a half
million years ago, to where we are now.

It is difficult enough to measure simultaneous events on Earth.
Measuring simultaneous events over cosmic distances is impossible.

When the distance of the Moon is measured using laser reflectors,
what is measured is the time from the start event, when light is
emitted from the laser, to the end event, when the reflected light
is received at the detector. The time difference tells us the
distance, if the speed of the light is constant. There is no way
to know directly that an event on the Moon is simultaneous with an
event on the Earth until both events have been observed. In that
case the simultaneity is calculted after the fact, based on the
assumed constancy of the speed of light. Once we have measured
the distance of the Moon, if we see a flash of light on the Moon
at 12:00:00.00, we can calculate that it was simultaneous with an
event on Earth at 11:59:58.72.

Once we have measured the distance of the Andromeda galaxy, if we
see a flash of light in the Andromeda galaxy now, we can calculate
that it must have been simultaneous with an event on Earth during
the Pliocene Epoch. However, we have no record of the relative
motions of Earth and the Andromeda galaxy during the time the light
was traveling, so the calculation is only a first approximation.
Simultaneity becomes less well defined as distance increases.

And, as you pointed out, obviously distances become less well
defined as the time difference between emission and reception of
the light increases.

Observations of Andromeda are restricted to light that was emitted
about two and a half million years ago.

Observations of Andromeda are restricted to light that is received
now.

We have no data on where Andromeda is now. We have no data on
where Earth was approximately 2.5 million years ago. So the
distance we measure is the distance between the place the light
we see originated, and the place the observer is located. We
have no choice.



The problem is, if you first select the events, as in your statement,
then you don't get to choose the reference frame, it is given--
and it is not our frame. For the events you describe, connected by
emission and absorption of light, the frame in which those events are
simultaneous is the frame of the light itself, and the proper distance
is zero.
I described no such thing. Nothing remotely like it.

I agree that I don't get to choose the reference frame. That is
given by the source of the measurements which the original poster
referred to. That is what I responded to. Nothing else. The
measurements of distance of the Andromeda galaxy comprise two
events: the place and time that light was emitted from the galaxy,
and the place and time that light was observed. Those events are
obviously not simultaneous, or the result of the measurements
would be "zero" rather than "about 2.5 million light-years".



Now, if you intended for our own reference frame to be implied, then
when you get a proper distance between us and some galaxy, you don't
get to choose the events that are connected by that distance, in
contradiction to your statement.
The fact that I don't get to choose the events does not contradict
anything I said. Nor does the fact that I don't get to choose the
reference frame.



In summary, if you choose the events you did, the proper concept of
distance comes out zero,
Which has nothing whatever to do with either the original question
or my answer.



and if you want a proper distance that makes sense in our reference
frame, you cannot be talking about the events you cite.
Of course I can. They are the events from which the distance is
measured. They are the only events available to us to measure.

-- Jeff, in Minneapolis

grav
2009-Jun-21, 01:10 PM
What is interesting is that two observers can be stationary in the same frame, and then one quickly accelerate toward the other with a burst of their engine, and then just after the burst takes place, the one that fired their engine would say they are now only half the distance to the other, say, due to Lorentz contraction while the one that remains stationary says the other is still at the same distance they were before. So the two observers are now travelling at the same speed relative to each other but each measures a different distance to the other. This is due to simultaneity effects, that neither can agree upon when the burst took place, but what is even more interesting is that both could actually agree upon that when in the stationary frame, just not in the new frame of the accelerated observer that now sees the other move future-forward, so says it would have happened in the past for the other observer according to the new frame of the accelerated observer with the lesser distance, even though it just barely happened according to the same frame, all of which turns out to be true.

Jeff Root
2009-Jun-21, 01:23 PM
speedfreek,

I don't know this as well as Ken does. Nowhere near it. Which makes
arguing with him a bit unnerving!

-- Jeff, in Minneapolis

speedfreek
2009-Jun-21, 01:32 PM
Heh, I know what you mean! :)

All I meant was that you already knew that time and distance aren't absolutes, you just didn't state it implicitly, as you didn't see the need to. I think Ken is simply trying to make sure nobody takes your statement out of context.

Ken G
2009-Jun-21, 02:27 PM
Which is essentially correct as long as you acknowledge the role of the observer, and that it is correct from the point of view of any observer. The thing that Ken is pointing out is that different observers will get different distances that the light travelled, but they all will be correct.Actually, there is an additional subtelty here. Had the only omission in Jeff's statement been failure to mention that we are the observers, I would have said nothing. The problem is, he also specified two events in spacetime. That was the crucial mistake, because he was speaking about a theoretical concept of a distance, not a measured one. The theoretical concept of distance is called proper distance, and the measured distances include many possibilities, two of which Grant mentioned right off (and they are different).

So we really have several points to get straight here. First, we have to decide if we are using a theoretical concept of distance, or a measured one. The theoretical concept of distance is called proper distance, that's just what distance means in the theory of relativity-- it's a "ruler" distance, where the ruler spans two events that are simultaneous in the ruler frame. No ruler spans the events in Jeff's quote.

Given that he therefore cannot be talking about a ruler (proper) distance, he must be talking about a measured distance. Note that it is actually necessary to use measured distances are proxies for proper distances, because we never have a ruler long enough. So, in that case, one chooses one's distance measure (and they are all different), and use inference and the laws of physics to decide how to map that to proper distance, appropriately. But of course, the proper distance never spans the events in Jeff's quote, they are not simultaneous in the Earth frame.

In summary: events, observer frame, meaning of distance: choose 2, not all 3 as Jeff continues to claim he was "exactly correct" in doing.

Statements that suggest there is a "simple" distance that connects events that are not simultaneous in the Earth frame are incorrect. It's not just that different observers get different answers, the statement is simply wrong, in any situation where simultaneity matters (distance "then" versus distance "now")-- in other words, in any situation that is responsive to the OP question where Grant's original answer does not completely cover it.

Ken G
2009-Jun-21, 03:07 PM
I never said that there is a "unique" or "absolute" distance.
If it was implied, that is because it was implied in the original
post. What you continue to miss is, the OP did not ask what the distance was, it cited what the distance was. So your list of references that talk about distances is completely irrelevant to the OP. The OP asked about the difference between distance then and distance now-- an issue that not one of the references in your list deals with in the least way. That is because the answer to the OP is either Grant's, or it is much more subtle (if one needs a level of accuracy that only relativity can supply). Your answer, on the other hand, is not useful in either case-- it either adds nothing to Grant's, or it is (relativistically) incorrect. It is nothing beyond the statement "the distance is the distance"-- any effort to add additional meaning to your words only adds false meaning. But I said that above, I clearly dissected all the possibilities present, and you just ignored it, returning constantly to statements that meaning nothing other than "the distance is the distance" and "I can't even say why". I already told you, if you can't say why it's true, you either aren't saying anything, or you are saying something wrong.


Not one source mentions that the distances given vary from observer
to observer or depend on relative motion or the structure of
spacetime -- even when discussing the structure of spacetime.
Which tells you what, that books don't always tell the full story? And this is a shock to you? What you should be asking yourself if, do any of those books make a distinction between distance then and distance now? The answer is, implicit assumptions are being made about what is happening to time, specifically, they are choosing 2 between observers, events, and meaning of distance, and they are just not telling you their choice because it is either the comoving-frame meaning, or it doesn't matter.


My reply answered the question asked in the original post without
going into relativity.But it referred to points "in space and time" as if it were possible to talk about such things without relativity. That's what was wrong about it. I'm glad we have found agreement at last-- one cannot discuss things like the differences between "distances then" and "distances now" when referring to light paths, without referencing relativity. It's make believe, if you do.

Spaceman Spiff
2009-Jun-21, 03:30 PM
In the end, the problem gets so bad that as we reach the larger cosmological scales, we have to use a generalised approximation, based on a series of theoretical observers that are at rest and under the least gravitational influence possible in their own frame of reference, but whose frame is moving with the expansion of the universe.

I hope you don't mind if I try rephrasing this a bit for the non-experts out there who might be able to picture in their heads observers floating effortlessly downstream in a large uniformly moving river...

"...based on a series of theoretical observers who are at rest within the Hubble flow. Because of its simplicity this is the most commonly adopted cosmological frame of reference, aka the co-moving frame. This is also the frame of reference by which 'cosmic' time is kept."
:)

Ken G
2009-Jun-21, 05:19 PM
Let me clarify one point that I think is causing much of the problem here. What I am talking about really only appears in general relativity, but although many people think of general relativity as meaning "what happens when gravity matters", I just think of it is as "what we can actually say about our physical reality without pretending we know things that we don't", whether or not there is any gravity. In other words, what general relativity does, in addition to allowing us to treat gravity, is it keeps track of the difference between what is something objective that we can infer about our universe, and what is pure convention, purely the imprint of our own choices on what we sometimes pretend is coming from somewhere outside of us. So general relativity is "the truth", as we now know it, and the rest is "make believe truth". So that's what I meant right from the start when I talked about there being "no such thing" as various notions of distance that get used all the time-- no such thing in the physical sense of what we can actually demonstrate as true, rather than what is an arbitrary choice of our conventional coordinates (which is different from a reference frame-- the former being global, the latter being purely local, and we don't need light for the latter, we have rulers.)

Spaceman Spiff
2009-Jun-21, 10:36 PM
...rather than what is an arbitrary choice of our conventional coordinates (which is different from a reference frame-- the former being global, the latter being purely local, and we don't need light for the latter, we have rulers.)

Ah, yes, that's right. Dang that imprecise language thingy :). So I'll try that (http://www.bautforum.com/space-astronomy-questions-answers/88698-distances-then-now-6.html#post1513661) again:

"...based on a series of theoretical observers who are at rest within the Hubble flow. Because of its simplicity this is the most commonly adopted cosmically global coordinates (or global coordinatization), often referred to as the co-moving frame. This is also the choice of coordinates by which 'cosmic' time is kept."
:)

Ken G
2009-Jun-22, 04:10 AM
Right. It's such a subtle, but I think important, difference between a "coordinate distance" and an "invariant distance". As an analogy, if you take the expression "all roads lead to Rome", then Rome is the invariant, and all the roads that lead there are all the possible coordinatizations that all make sense. The language we use to describe "the path taken", including imprecise concepts of distance, can be very different in different coordinatizations. So much so, that they really don't sound like the same reality at all-- but they are describing the same reality, because they connect all the same invariants to all the same measurable outcomes. They just connect them via different "roads". So talking about "the distance between two points in space and time" is like talking about "the road that leads to Rome"-- it doesn't really mean anything without the convention that specifies which road is being talked about, and then it is the convention that provides the meaning, not the allusion to a road.

RussT
2009-Jun-24, 10:05 AM
I have sorta been waiting for you to defend KenG's rebuttal...Jeff, your Post # 167 said nothing incorrect/false especially this...



My reply answered the question asked in the original post without
going into relativity. Bringing up relativity is fine. You can
write about it all you like. But the fact that there is no such
thing as the unique or absolute distance between two events in
relativity does not make my answer false. The answer I gave is
not a correct answer to the questions you are answering, but it
is a correct answer to the question that was asked.



Originally Posted by Ken G
But there is one fairly natural way to coordinatize "the distance"
between any two events, and it is called "proper distance".
To get proper distance, you choose a reference frame in which the
two events are simultaneous, and connect the two events with a chain
of stationary observers in that frame. Then the ruler distance for
that integrated chain of observers is the proper distance.



Clearly that does not apply here. The distance of the Andromeda
galaxy has never been measured in such a manner, and cannot be.Yet we do have a measured distance for the Andromeda galaxy, given
in the first sentence of the original post.

That distance was not measured using simultaneous events, because
light takes approximately two and a half million years to go from
Andromeda to Earth,My Bold

The bottom line is...0 distance as a "Proper Distance" in the Local neighborhood just simply does not apply!

In this next quote, the first part is just like Tim Thompson's, that Grant agreed was 'Exactly what You, I Tim and ALL of humanity "Knows".




From our point of view, Andromeda was approximately 2.5 million light years away "then", when the light we see was emitted. From our point of view, it will be a little bit closer now, than it was when the light was emitted. If that is all you want to know then fine. Just as long as you realise that it is not an absolute measurement.My Bold/which KenG has said also

So when they keep saying this.... If that is all you want to know then fine.

That is all there is to "Know" in the Local Neighborhood, as relates to distance...The Distance IS the Distance that light/photons travel in specified amounts of time, depending on the known distances, which relativity did NOT and CANNOT determine.

RussT
2009-Jun-24, 10:47 AM
If you could cut the insulting insinuations back to a dull roar, I'd be gratified.
All frames are addressed in that remark; I made no specification or exclusion at all. So the "Earth Frame observers" are in there, with their own measurements, just as the "Aliens in a spaceship" are there, with their own measurements. And they're all disagreeing with each other about distance and time, which are (as Spaceman Spiff remarks) not invariant properties of spacetime.

Grant Hutchison

Actually, I will apologize here Grant....sorry I wan't very clear at all with my generic statement...it's just that I had already explained about my observers...From "Earths Frame" or any scientist, in their "Rest Frame" at Mercury, Venus, Earth, Mars out to all the planets in our solar system, and the scientist in her/his 'rest frame' on or around their third rock from the sun of alpha Centauiri, or the scientists in M31 'at rest' on their planet...

That the 'distance' is 'really' the distance that light travels in specified amounts of time, based on the distance.......hence the term "Light Year".

None of which are disagreeing about any time/distances "In the Local Neighborhood!!!

So, what I meant was, that only your observers, the aliens in their spaceships are the ones that are confused about distances...

Let's see if we can determine how that 'Arguement' will play out...

"If" we become that 'observer' (IN fact jump in the seat right next to him/her)you/KenG/Relativity/mainstream says 'we' should...

Say that 'alien in a spaceship' traveling toward us, at "c" right next to our sun,
and let's say all the planets are lined up right behind us, like happens about every 175 years...

and let's say that ship you and the alien are on is going to Mars, Jupitor, Neptune, when you get there "Instantantly", did the 'space' between there and the sun contract to Zero? What happened to Mercury, Venus, the Earth?

How are "You and The Alien" going to convince anyone, that you actually just came from the Sun in the First Place, when you could have just as easily come from a galaxy that was 1000 billion^1000 billion light years away?

And If you landed at NASA, you may be able to convince someone that you actually did travel 'instantaneously', but, now that you are in Earth's Frame, how are you ever going to convince anyone that the "Proper Distance" the Sun is or should be, is Zero distance???

grant hutchison
2009-Jun-24, 11:06 AM
So, what I meant was, that only your observers, the aliens in their spaceships are the ones that are confused about distances...No, my observers include your observers. No-one is "confused" about distances. All measured distances are equally real, but all require the observer to state the conditions under which the distance was measured.

Grant Hutchison

RussT
2009-Jun-25, 10:59 AM
No, my observers include your observers. No-one is "confused" about distances. All measured distances are equally real, but all require the observer to state the conditions under which the distance was measured.

Grant Hutchison

No, my observers include your observers.

Uh, sorry, but that does not compute according to what KenG has been insisting upon this entire thread...He has insisted that the only observer that can be used is the one that makes "Proper Distance" come out to Zero!

But, even if we use your "Stationary observer" the 'Alien in the spaceship' can he perform a Lunar Ranging experiment? NO, because he cannot use light/time distance in the same frame.


All measured distances are equally real,

The "Real" distances have been determined, but not by using Relativity.

all require the observer to state the conditions under which the distance was measured

So, please show how "Your Observers" measured the distance to the moon, Jupitor, Sun, Alpha Centauri.

grant hutchison
2009-Jun-25, 11:43 AM
No, my observers include your observers.

Uh, sorry, but that does not compute according to what KenG has been insisting upon this entire thread...He has insisted that the only observer that can be used is the one that makes "Proper Distance" come out to Zero!Sigh. I'm not going to play your "out of context quote" game any longer. Read Ken's posts, which are detailed, and discuss the various ways in which distance can be treated, and the particular circumstances under which a proper distance of zero applies.


But, even if we use your "Stationary observer" the 'Alien in the spaceship' can he perform a Lunar Ranging experiment? NO, because he cannot use light/time distance in the same frame.
I recognize the individual words, but am unable to parse these two sentences. Perhaps you could rephrase?


All measured distances are equally real,

The "Real" distances have been determined, but not by using Relativity.Saying it doesn't make it true, so you're quite right to put the word "real" in scare quotes.


all require the observer to state the conditions under which the distance was measured

So, please show how "Your Observers" measured the distance to the moon, Jupitor, Sun, Alpha Centauri.You first.
There's no appropriate method that Earth-based observers use that can't, in principle, be used by any observer. Only the measurement results will differ.

Grant Hutchison

Ken G
2009-Jun-25, 01:48 PM
Clearly that does not apply here. The distance of the Andromeda
galaxy has never been measured in such a manner, and cannot be.Yet we do have a measured distance for the Andromeda galaxy, given
in the first sentence of the original post.

That distance was not measured using simultaneous events, because
light takes approximately two and a half million years to go from
Andromeda to Earth,
I saw this when RussT quoted it, for some reason I didn't see it before. It gives us a good entry point to explain what is confusing you. As I have been stressing, when we get a distance to Andromeda, we do indeed mean a proper distance (in effect, a ruler distance, for a verrrrry long ruler), so we do in fact want a distance between two simultaneous events, one in Andromeda, and one here (presumably at the time we are asking the question). However, you are correct that we have no direct access to that distance, so we must infer it. That doesn't mean it isn't the distance we mean, it just means we must choose a way to infer it! Enter all the "distance measures" that Grant and I have been talking about (including lunar ranging for objects as close as the Moon, RussT, which is actually a time measure). That is the basic reason why there are more than one (and they can give different answers in general, though their differences would never matter for Andromeda, as was also pointed out before you even entered the thread), it is because our goal is to infer something different from what we have access to with our measurements.

None of this makes correct your claim that the distance being measured is the distance between the spacetime events you cited, but it is an important point of clarification. If one is not doing relativity, all these subtle issues can be ignored, but then one is only making believe that one is answering a question about the difference between the "distance then" and the "distance now" for light propagation.

Jeff Root
2009-Jun-26, 06:06 AM
I never said that there is a "unique" or "absolute" distance.
If it was implied, that is because it was implied in the original
post.
What you continue to miss is, the OP did not ask what the distance
was, it cited what the distance was.
Oh, really. Let's look at what I said:


The distance given is the distance the light has traveled...


When the distance of a galaxy is given in light-years...


The original post asked a simple, straightforward question: When the
distance to the Andromeda galaxy is given as 2.5 million light-years,
does it mean the distance now or the distance 2.5 million years ago?
...
Yes, I generalized my answer to apply to the distance of any galaxy
when given in terms of light travel time...


In both posts I explicitly said a distance "given". In this case the distance
given is the distance to the Andromeda galaxy, which was given as "about
2.5 million light years". That is a distance, not a time. It is a distance
supplied by a third party, not by me. When somebody says the distance
between the Andromeda galaxy is 2.5 million light-years, they mean that
light emitted from Andromeda 2.5 million years ago is observed now, and
the distance the light traveled in that time, by definition, is 2.5 million ly.
The place and time where the light was emitted to the place and time
where it was recieved is the given distance.



In both posts I explicitly said a distance "given".
As opposed to one taken? What does this mean?
It means precisely what you are saying I continue to miss.

Obviously.


Note that my statements were about distances "given" by other sources,
such as a table of galaxy distances in a reference book. The observer
is specified by the source of the distance being quoted.
I mean, really, Ken, obviously.

You don't need to be a physics instructor to understand that. You don't
even need to be a fifth-grader. What I said from the very beginning was
clear, simple English that you could not possibly fail to understand.

You understood completely.



So your list of references that talk about distances is completely
irrelevant to the OP. The OP asked about the difference between distance
then and distance now-- an issue that not one of the references in your
list deals with in the least way.
Obviously. That is precisely what I said. Its relevance is in showing
that distances are given to distant galaxies without any reference to
relativistic effects. I cited two books on cosmology, a book on the
history of cosmology, three standard introductory astronomy textbooks
(two of which are fairly recent), a standard field guide to the night
sky, and a Scientific American article on cosmology. All written by
professional astronomers. Not one mentions that the distances given
vary from observer to observer or depend on relative motion or the
structure of spacetime. Those were not the only books and articles
I looked at. I could not find any book or article with distances of
other galaxies that did say anything about the distances depending
on relative motion or the structure of spacetime. Not one.

My list of references is irrelevant to the original question. It is
relevant to your objections to my answer to the original question.
You may need to be a fifth-grader to see this one, but certainly not
a physics instructor.

I've replied to the first three sentences of your June 21 reply to me.
More in my next post.

-- Jeff, in Minneapolis

Jeff Root
2009-Jun-26, 06:13 AM
The OP asked about the difference between distance then and distance
now-- an issue that not one of the references in your list deals with
in the least way. That is because the answer to the OP is either Grant's,
or it is much more subtle (if one needs a level of accuracy that only
relativity can supply). Your answer, on the other hand, is not useful
in either case-- it either adds nothing to Grant's, or it is
(relativistically) incorrect. It is nothing beyond the statement "the
distance is the distance"-- any effort to add additional meaning to
your words only adds false meaning. But I said that above, I clearly
dissected all the possibilities present, and you just ignored it,
returning constantly to statements that meaning nothing other than
"the distance is the distance" and "I can't even say why". I already
told you, if you can't say why it's true, you either aren't saying
anything, or you are saying something wrong.
I ignored that post because your main points were empty rhetorical
tricks as meaningless as you were accusing mine of being. It deserves
no reply that is permitted on BAUT. Nevertheless, I will now reply.





"The distance given is the distance the light has traveled, from the
point in space and time at which it was emitted, to the point in space
and time at which it is received."
Yes and as I have said numerous times, that is exactly correct.
Let us examine, using nothing but unmistakably pure logic, what is
"exactly correct" about this statement. First of all, there are two
possibilities in the meaning of the words used:

possibility #1) The words "distance the light has traveled" means the
same thing as just "distance". In this case, the statement is the same
thing as "the distance given is the distance between the two points
mentioned." Interesting, the distance is the distance.
The distance given on a road map is the distance along the road, as
opposed to the straight-line distance between the endpoints. The
distance given by time domain reflectometry is the distance along the
conductor between the point on the conductor at which the signal is
injected and a point at which the impedence or resistance changes, or
the conductor ends. As opposed to, for example, the straight-line
distance between those two points. The distance a radio signal is
said to have traveled from a transmitter orbiting Mars to a receiver
on Earth is the distance from the point in space and time at which it
was emitted to the point in space and time at which it was received.
As opposed to, for example, the distance between Earth and Mars at
the time the signal was sent or the distance between Earth and Mars
at the time it was received.

Very simple, very easy to understand. Fifth-grader stuff.



Or perhaps it is the two points described that hold the content of the
statement? No, the two points are not specified in any way other than
the point of emission and the point of reception. Is it mentioning both
space and time that holds the content of the statement, as if there were
something special about distances in spacetime that does not apply to
space alone? Hmm, well there is the concept of proper time (related to
proper distance), but it isn't proper distance being used here (more on
that in a moment). So no, there is no added content in saying the points
are in "space and time" either, the statement is nothing by "the distance
between emission and absorption is the distance between emission and
absorption." So in possibility #1, the "exactly correct" statement is
also completely devoid of any content whatsoever.
Are you as smart as a fifth-grader?



possibility #2) the words "distance the light travels" means something
different from just "distance". In this case, somehow the meaningful content
of the remark must hinge on this difference, that somehow the fact that light
is traveling this distance informs it with some special or useful meaning it
would not have had if it had just been the word "distance". I wonder what new
meaning is intended to be given to the distance because light traveled it?
Here we must distinguish two new possibilities:

possibility #2a) The new meaning given to distance, owing to the fact that
light traveled it, is the relativistic meaning of "proper distance". This
would indeed be something special about the propagation of light, in regard
to the proper distance it travels. Unfortunately for the above statement,
light propagates along what are called "null geodesics", which means that
the proper distance traveled by the light is always zero. That would not
appear to be the point the above statement is trying to make, that the
distance is zero, so we can assume it is not proper distance that is being
referred to.

possiblity #2b) The new meaning given to distance, owing to the fact that
light traveled it, is not the proper distance, but some other kind of special
distance that is distinguished by the fact that light traveled it. Hmm, here
my knowledge of relativity must be limited, for I know of no other such
special distance other than proper distance. Are we in some ATM land here?
No, the claim is made that the interpretation given is not ATM, and RussT
feels he has some reason to agree with it without reference to any ATM
concepts. So either we have here misconceptions about relativity, because
said theory includes no such other special distance associated with light
other than the one that comes out zero, or else possibility #2 is not the
one intended. Sadly, in that case, possibility #1 is entirely inclusive of
the remaining possibilities, and we already saw the content in that
possibility.
All of that is irrelevant.

Since it isn't possible to measure the distance of the Andromeda galaxy
instantaneously, we measure it using the only data we have, which come
from two locations in space and two locations in time: The distance we
measure is the distance from the point in space and time at which light
was emitted, to the point in space and time at which it is received, as
opposed to, for example, the distance between us and Andromeda at the
time the light was emitted, or the distance between us and Andromeda at
the time the light was received.



So where does logic lead us? Inexorably to the conclusion that the quoted
remark is null in content (for possibility #1), or false (possibility #2a),
or ATM (possibility #2b). I really couldn't say which one Jeff Root intended,
he'd be needed for that, but I can categorically state, based on the
authority of pure logic, that these are the inclusive possibilities.
There is nothing there fit to reply to, which is why I did not reply
to it when you posted it.



The 2.5 million light years comes from a particular coordinatization of
spacetime. It is the standard one,
So standard that it is the only one I could find in looking through
many books and articles.



though by no means a unique one, and in the case of such a nearby
distance with the uncertainty it has, there is no way to even distinguish
the various possibilities that might be used. That was Grant's correct
answer, it required no further embellishment if the OP was really only
interested in such nearby galaxies.
Grant's correct answer did not answer the question. Mine did.

Grant's correct answer remains correct only so long as the distance
to the Andromeda galaxy remains uncertain to a degree greater than
the effect our relative motion has on that distance. In the case of
the distance of Mars, for example, the effect of the relative motion
of Earth and Mars is far greater than the uncertainty in the distance,
so that the precise times of signal emission and reception can be
highly significant in measurement of the distance.

My answer is correct in all situations.



... But in case the OPer was also curious about larger distances, say
2.5 billion light years, then the nature of the coordinatization is
essential to understand. If lookback time is being used, then it is not
a distance at all, it is a time given by convention in distance units
(and related to a particular time coordinatization)-- a point made
earlier in the thread, and entirely inconsistent with the quoted
"exactly correct" statement above.
What was asked about and what I am discussing is distance, not time.
The simple relation between time and distance is used for convenience
of expression. Distance expressed in light-seconds is still distance.



If comoving-frame coordinates are being used, then it would be natural
to say the distance is the "distance now", also in contradiction to your
position, although even that statement requires clarification to avoid
misconception. Or as was pointed out above also, if "distance then" is
intended, the the angular-size distance is being used.
I am not using either co-moving frame coordinate distances or
angular-size distances. Both of those are too uncertain and too
model-dependent to give a meaningful distance for any particular
object.



What is all-too-often quoted is the lookback time, which is not a
distance at all, so that right there would have been a far better,
and more meaningful, answer than the one you quoted above.
I am talking about distance, not time.

I'll continue on your June 21 reply to me in my next post.

-- Jeff, in Minneapolis

Jeff Root
2009-Jun-26, 06:32 AM
Not one source mentions that the distances given vary from observer
to observer or depend on relative motion or the structure of
spacetime -- even when discussing the structure of spacetime.
Which tells you what, that books don't always tell the full story?
No, it tells me that those authors have better judgement than you
about when and where relativity theory is relevant.



And this is a shock to you?
The statements in all the books and articles I looked at are exactly
as I expected. So, no, there was nothing to shock me.



What you should be asking yourself is, do any of those books make a
distinction between distance then and distance now? The answer is,
implicit assumptions are being made about what is happening to time,
specifically, they are choosing 2 between observers, events, and
meaning of distance, and they are just not telling you their choice
because it is either the comoving-frame meaning, or it doesn't matter.
In all the cases I have referenced, it is not the co-moving frame
meaning. The authors had no choice of observer or events. Those
were chosen by our location in the Universe. Obviousy.




My reply answered the question asked in the original post without
going into relativity.
But it referred to points "in space and time" as if it were possible
to talk about such things without relativity.
It is possible to talk about points in space and time without relativity.
Relativity can tell us lots of interesting things, but it isn't essential
to understanding what is meant when it is said that the Andromeda galaxy
is about 2.5 million light-years away.



That's what was wrong about it. I'm glad we have found agreement at
last-- one cannot discuss things like the differences between "distances
then" and "distances now" when referring to light paths, without
referencing relativity. It's make believe, if you do.
Sometimes it is essential to understand the relationship between
distance and time in relativity when you want to know the difference
between "distances then" and "distances now". It isn't in this case.
In this case, it is an interesting sidelight.


The distance I walk through the lengths of several railroad cars in a
moving train is the distance from the point in space and time at which
I started walking through the train to the point in space and time at
which I completed that walk. That is true nomatter how fast or how
slow I walk, how fast or how slow the train is moving, or where the
observer is located. I can be the observer, a seated passenger can be
the observer, you, standing beside the track can be the observer, or
Beagle 2 on Mars can be the observer. The distance meant is always
the distance from the starting point in space and time to the ending
point in space and time.

-- Jeff, in Minneapolis

RussT
2009-Jun-26, 11:34 AM
I saw this when RussT quoted it, for some reason I didn't see it before. It gives us a good entry point to explain what is confusing you. As I have been stressing, when we get a distance to Andromeda, we do indeed mean a proper distance (in effect, a ruler distance, for a verrrrry long ruler), so we do in fact want a distance between two simultaneous events, one in Andromeda, and one here (presumably at the time we are asking the question). However, you are correct that we have no direct access to that distance, so we must infer it. That doesn't mean it isn't the distance we mean, it just means we must choose a way to infer it! Enter all the "distance measures" that Grant and I have been talking about (including lunar ranging for objects as close as the Moon, RussT, which is actually a time measure). That is the basic reason why there are more than one (and they can give different answers in general, though their differences would never matter for Andromeda, as was also pointed out before you even entered the thread), it is because our goal is to infer something different from what we have access to with our measurements.

None of this makes correct your claim that the distance being measured is the distance between the spacetime events you cited, but it is an important point of clarification. If one is not doing relativity, all these subtle issues can be ignored, but then one is only making believe that one is answering a question about the difference between the "distance then" and the "distance now" for light propagation.

Geezzzzzzzz Ken, you keep trying to fit a square peg in round hole:wall:



As I have been stressing, when we get a distance to Andromeda

Zero is not a distance! Which is why Relativity cannot determine the 2.5 million light years to M31. "Proper distance" as per your/Relativities requirement does NOT work in the Local Neighborhood.




However, you are correct that we have no direct access to that distance

There ya go! and 'inferring' zero distance to start off with, you/Relativity cannot then determine the distance to the Moon, Sun, Alpha Centauiri, M31, can you? So what good does it do to "Infer"?

[ETA: and check out the bolded one these that neither you nor Grant answered...

Let's see if we can determine how that 'Arguement' will play out...

"If" we become that 'observer' (IN fact jump in the seat right next to him/her)you/KenG/Relativity/mainstream says 'we' should...

Say that 'alien in a spaceship' traveling toward us, at "c" right next to our sun,
and let's say all the planets are lined up right behind us, like happens about every 175 years...

and let's say that ship you and the alien are on is going to Mars, Jupitor, Neptune, when you get there "Instantantly", did the 'space' between there and the sun contract to Zero? What happened to Mercury, Venus, the Earth?

How are "You and The Alien" going to convince anyone, that you actually just came from the Sun in the First Place, when you could have just as easily come from a galaxy that was 1000 billion^1000 billion light years away?

And If you landed at NASA, you may be able to convince someone that you actually did travel 'instantaneously', but, now that you are in Earth's Frame, how are you ever going to convince anyone that the "Proper Distance" the Sun is or should be, is Zero distance???]



so we do in fact want a distance between two simultaneous events, one in Andromeda, and one here (presumably at the time we are asking the question).

You already know that this cannot be and is not...at the time we are asking the question.

And now you will give some kind of philisophy of science answer, it is something we can't know....

BUT, since every single entity in our universe has the same 'proper distance' of zero, according to Relativity, that means that there is an 'Alien in a spaceship', right next to every entity in our Universe, traveling toward us at "c"...

Wow, what a Cosmic Coincidence that is...don't ya think? And when was the emission/absorption for all entities in the Universe 'Zero distance'???


RussT,[/B] which is actually a time measure).

Uh, NOOO...those would be the distances that Jeff and I have been talking about, and where the moon was 1.25 seconds before we recieve it photons is "Irrelavent". Same for the Sun Etc in the Local Neighborhood:hand:



including lunar ranging for objects as close as the Moon, RussT, which is actually a time measure

Come on KenG...you know perfectly well that this is a Time/Distance measurement from Earth's frame.

In fact it is pretty weird that you would even say it was a 'time' measurement, since light doesn't have travel time in SR.

Ken G
2009-Jun-26, 01:34 PM
It means precisely what you are saying I continue to miss.I'm not sure what you think piecing together fragments of quotes actually means. My aim all along has been to point out the error in a claim that there is a distance that connects two particular events you cited, and you have claimed all along there really is a meaningful distance between those to events that clarifies the issue of "distance then" versus "distance now". What I'm telling you is that the whole point of a proper distance, for some observer (even us), is to resolve the distinction about "distance then" and "distance now" by characterizing the distance between simultaneous events. That's what "distance" really means, it is the ruler distance, when no other type of distance is specified. The type you refe to, "the distance light travels", is make believe. If you use the frame of the light, that distance is zero, if you use our frame, distances connect simultaneous events,

I mean, really, Ken, obviously.What is obvious to me is that you refuse to "get" relativity, and what distance means in relativity, and why one has to be using relativity to distinguish meaningfully "distances then" versus "distances now".

You don't need to be a physics instructor to understand that. You don't even need to be a fifth-grader. What I said from the very beginning was clear, simple English that you could not possibly fail to understand.
And very suitable for a fifth-grade understanding, certainly.


Its relevance is in showing
that distances are given to distant galaxies without any reference to
relativistic effects. Yes-- but your error is that you think the distances mean something without relativistic effects. However, the sole meaning they have in that context is when there is no effort to make a distinction between distance then, and distance now. So the correct answer in that context is "there is no meaningful difference between distance then and distance now without bringing in relativity, and typically quoted distances don't do that". In other words, in that case, Grant's original answer was perfectly adequate, and your "clarifcation" added nothing that was actually correct. So why did you think that was a useful clarification?


I cited two books on cosmology, a book on the
history of cosmology, three standard introductory astronomy textbooks
(two of which are fairly recent), a standard field guide to the night
sky, and a Scientific American article on cosmology. All written by
professional astronomers. Not one mentions that the distances given
vary from observer to observer or depend on relative motion or the
structure of spacetime. And not one specifies that the distance connects the two events you mentioned! That's the part I objected to, nothing that was in those other quotes! Are you just choosing to ignore this fact? Those distances will all share one of these two possibilities:
1) there will be no distinction made between distance then and distance now, or
2) the distances will be comoving distances, which are the cosmological analog of proper distances in comoving frame coordinates
Please note that in neither case will your "clarificaiton" have any useful meaning. So much for all those textbooks you quoted, they merely serve to refute your claim!

Ken G
2009-Jun-26, 01:42 PM
In all the cases I have referenced, it is not the co-moving frame
meaning. Then you cherry-picked the fraction where there was no important distinction between then and now, possibility #1 above. The rest you didn't select will be possibility #2, unless they are professional publications, in which case they will probably cite one of the various different distance measures from which distance can be approximately inferred.

The authors had no choice of observer or events. Those
were chosen by our location in the Universe. Again, the problem is not just that you didn't specify our frame, it is that you cited two events that are not simultaneous in our frame. I said that a few times now.



It is possible to talk about points in space and time without relativity.Certainly, but what you cannot do without relativity is meaningfullly distinguish the distance when light is emitted from the distance when light is absorbed.

Sometimes it is essential to understand the relationship between
distance and time in relativity when you want to know the difference
between "distances then" and "distances now". It isn't in this case.
In this case, it is an interesting sidelight.Incorrect, in this case it is the OP question. Look it up.

The distance I walk through the lengths of several railroad cars in a
moving train is the distance from the point in space and time at which
I started walking through the train to the point in space and time at
which I completed that walk.One more time, that statement is only meaningful if you have made the choice to conflate the distance then and the distance now, so if you have chosen to try to dodge the OP question.


That is true nomatter how fast or how
slow I walk, how fast or how slow the train is moving, or where the
observer is located... The distance meant is always
the distance from the starting point in space and time to the ending
point in space and time.
And that is patently incorrect. I guess you just don't want to know relativity, that's your choice. All I could do at this point is repeat what I've said a half dozen times already.

Spaceman Spiff
2009-Jun-26, 05:16 PM
Jeff:

It really boils down to whether you agree or disagree with the snip of post 159 from Ken G (http://www.bautforum.com/space-astronomy-questions-answers/88698-distances-then-now-6.html#post1512480) (which I've already highlighted in post 160 (http://www.bautforum.com/space-astronomy-questions-answers/88698-distances-then-now-6.html#post1512990)), especially the point of emphasis (mine):


Originally Posted by Ken G http://www.bautforum.com/images/buttons/viewpost.gif (http://www.bautforum.com/space-astronomy-questions-answers/88698-distances-then-now-6.html#post1512480)
No, again, light does not travel that distance. That distance connects two events, one in that galaxy and one in ours, that light does not connect. However, in cases where there is no important difference between "distance then" and "distance now", the discrepancy is unimportant and we can safely ignore it-- as long as we know we are ignoring it. This is an entry point for you to expand on your relativity knowledge, if you choose to use it as such. If, instead, you choose to believe your statement is "clearly correct" no matter what, then you pass on that opportunity. Your choice.
If you do agree (and especially with the bold and underlined portions), then there really is no point of disagreement between you and Ken G (although you might want to brush up on your relativity). If not, then you either don't understand relativity and/or are making a veiled claim with regards to its lack of usefulness (IOW validity) as a scientific theory.

As Ken G said, your choice.

Ken G
2009-Jun-26, 09:29 PM
Thanks Spiff, that does focus clearly on the key issue. And note this issue goes beyond who said what when, it comes down to what concepts of distance truly are physically supportable notions, and what concepts are just a kind of make believe version that is not physically supportable in our actual universe (ergo that there is "no such thing"). If one thinks of general relativity as our means at getting at what are the fundamental physically supportable notions, because notions that are not supported by that theory are not distinguishable or demonstrable by any objective means and so are just a kind of pretense, then one reaches the conclusion that the only physically demonstrable concept of distance between two points in spacetime is the concept of proper distance, which requires that a coordinatization is in place where the two events are simultaneous, and then the distance will be the distance in that coordinatization. Then at least observers who do not perceive the events as simultaneous can convert to the conventional coordinates where they are, and get invariant results (i.e., physically real results) by making that conversion. If one does not require simultaneity, on the other hand, then the distance can be anything you like, and specifying who is doing the observing does not remove that ambiguity.

If, on the other hand, one feels that special relativity is a suitable description of what should count as a physically real description of distance, despite its limitations, then (and only then) can specifying the (inertial) observer count as sufficient for determining a coordinate system where there is a concept of the distance between events that are not simultaneous in those coordinates, you just take the component of distance and ignore what is happening with time, which sounds like what Jeff Root was advocating. But that would be like measuring the distance from New York to Los Angeles entirely in terms of the difference in latitude of the two, rather than the invariant distance that does not depend on how one chooses the latitude/longitude grid being used. That distance isn't terribly physically meaningful, but at least you could say what it is, in special relativity. But special relativity is already a kind of make believe version, accurate in some situations and not others, of the rules that we have so far found reality to obey on large scales: general relativity, where there is nothing special about mutually stationary observers.

Alternatively, you could invoke neither form of relativity, as Jeff Root claims is his choice, but then you are merely taking one more step into the realm of never-neverland, when discussing issues like the difference between "distance then" and "distance now" for events connected by a light path. It can only give useful results when the distinctions are irrelevant, it is akin to a guesstimation of the concept of distance, and should not be treated as any kind of precise description of a physically meaningful version of that concept. Framing the distance in those terms adds nothing to the concept that is actually correct.

Perhaps the best way to sum all this up is with a question for Jeff: if you think the distance between the points in space and time you mention are physically meaningful, then shouldn't there be some physical meaning to the ruler with that same length, albeit an impractically long ruler? If so, can you describe what is the physical connection of the ruler of the length you cite with us and the Andromeda galaxy?

Jeff Root
2009-Jun-26, 09:58 PM
Spiff,

I don't doubt the validity of relativity or its usefulness as a scientific theory.

I don't doubt that Ken's understanding of relativity is good, and much deeper
than my own.

Asking if I agree with the statement you bolded has the same functionality
as the question, "Have you stopped beating your wife?"

Rather than discussing it directly, let me ask you if you agree with the
statement I made at the end of my last post:


The distance I walk through the lengths of several railroad cars in a
moving train is the distance from the point in space and time at which
I started walking through the train to the point in space and time at
which I completed that walk. That is true nomatter how fast or how
slow I walk, how fast or how slow the train is moving, or where the
observer is located. I can be the observer, a seated passenger can be
the observer, you, standing beside the track can be the observer, or
Beagle 2 on Mars can be the observer. The distance meant is always
the distance from the starting point in space and time to the ending
point in space and time.
If you disagree, please tell me why.

-- Jeff, in Minneapolis

Ken G
2009-Jun-27, 01:08 AM
The distance I walk through the lengths of several railroad cars in a moving train is the distance from the point in space and time at which
I started walking through the train to the point in space and time at
which I completed that walk. Let's look at that example, and answer the following questions. What is the difference between "distance then" and "distance now" in that example? Why did you choose an example where there was no difference, and why is that example relevant to the OP? Why did you choose an example that does not involve light? What does it add to the OP question to talk about "the distance" between the starting point and ending point of that walk? The answers to these questions show that your current example is completely irrelevant to the OP and to the thread.

But we can make your example relevant very easily, so let me ask you: in your example, how far would you say you have walked if you could walk at 99% the speed of light? How about 99.99999%? Now what if you actually were light, as in the quote you made earlier about the distance "light travels" between its emission and absorption? What is the distance you walk then?

OK, so clearly you did not actually mean the distance you walked, you meant the distance someone on the train thinks you walked. See the difference? And that's only with special relativity, when you put in the "real" way things work, general relativity, there isn't even a clear meaning for what "the train" corresponds to. For example, if the train always extends from Earth to some distance galaxy, you are approximating "comoving frame coordinates", although even that not exactly-- but those are not the "reference frame of the observer", they are just an arbitrary coordinate system, and went unspecified in your remark (as did all coordinate systems).

RussT
2009-Jun-27, 01:30 AM
Let's look at that example, and answer the following questions. What is the difference between "distance then" and "distance now" in that example? Why did you choose an example where there was no difference, and why is that example relevant to the OP? Why did you choose an example that does not involve light? What does it add to the OP question to talk about "the distance" between the starting point and ending point of that walk? The answers to these questions show that your current example is completely irrelevant to the OP and to the thread.

But we can make your example relevant very easily, so let me ask you: in your example, how far would you say you have walked if you could walk at 99% the speed of light? How about 99.99999%? Now what if you actually were light, as in the quote you made earlier about the distance "light travels" between its emission and absorption? What is the distance you walk then?

OK, so clearly you did not actually mean the distance you walked, you meant the distance someone on the train thinks you walked. See the difference? And that's only with special relativity, when you put in the "real" way things work, general relativity, there isn't even a clear meaning for what "the train" corresponds to. For example, if the train always extends from Earth to some distance galaxy, you are approximating "comoving frame coordinates", although even that not exactly-- but those are not the "reference frame of the observer", they are just an arbitrary coordinate system, and went unspecified in your remark (as did all coordinate systems).

How was KenG able to edit this last post with NO edit record???

I just read the post with...OK, so clearly you did not actually mean the distance you walked, you meant the distance someone on the train thinks you walked. See the difference?

As the last recorded part of the post, and then a few minutes later...

this was added...

And that's only with special relativity, when you put in the "real" way things work, general relativity, there isn't even a clear meaning for what "the train" corresponds to. For example, if the train always extends from Earth to some distance galaxy, you are approximating "comoving frame coordinates", although even that not exactly-- but those are not the "reference frame of the observer", they are just an arbitrary coordinate system, and went unspecified in your remark (as did all coordinate systems)

RussT
2009-Jun-27, 01:45 AM
And that's only with special relativity, when you put in the "real" way things work, general relativity, there isn't even a clear meaning for what "the train" corresponds to. For example, if the train always extends from Earth to some distance galaxy, you are approximating "comoving frame coordinates", although even that not exactly-- but those are not the "reference frame of the observer", they are just an arbitrary coordinate system, and went unspecified in your remark (as did all coordinate


For example, if the train always extends from Earth to some distance galaxy,

And IF the train always extends to the Moon, Sun, Alpha Centauri, M31, then your co-moving frame coordinates are "Irrelevent"!

And in fact, choosing the observer that makes that Proper distance" Zero distance, extending that train 'always' to the emitter, makes absolutely no sense what-so-ever "In the Local Neighborhood".

01101001
2009-Jun-27, 01:46 AM
How was KenG able to edit this last post with NO edit record???

The software doesn't create a public notation about editing when it's done soon -- duration unknown to me -- after posting. Does that get in the way of a good game of Gotcha?

There is a record of editing (or the whole posts) kept, I believe, available to moderators, so your assumption may be wrong.

ShinAce
2009-Jun-27, 01:50 AM
Talking about how extended objects are treated in GR doesn't even relate to the question. Edited or not, it's an aside.

I'm pretty sure most forum software let's you edit cleanly within a minute or two; to fix typos.

Besides, that's a confusing example. What distance am I measuring? The distance you walked inside the train? The distance a railroad tie would have measured for your stroll? Pole-barn paradox anyone?

Jeff Root
2009-Jun-27, 01:51 AM
You can edit your post for twenty minutes before the footnote is added.

-- Jeff, in Minneapolis

RussT
2009-Jun-27, 01:58 AM
You can edit your post for twenty minutes before the footnote is added.

-- Jeff, in Minneapolis

I was pretty sure that I had seen numerous of my posts with edit tags right after my first edit, which is usually within seconds or minutes, because I always proof read after I have posted because I have lost so many posts before posting in the past.......But whatever....it's not that big a deal...though I had seen other examples of KenG's editing without tags that seemed a while later.

And, Okay....I just added...though I had seen other examples of KenG's editing without tags that seemed a while later. without edit tags...

And added this,.,..But back to the topic at hand...

RussT
2009-Jun-27, 02:10 AM
Talking about how extended objects are treated in GR doesn't even relate to the question. Edited or not, it's an aside.

I'm pretty sure most forum software let's you edit cleanly within a minute or two; to fix typos.

Besides, that's a confusing example. What distance am I measuring? The distance you walked inside the train? The distance a railroad tie would have measured for your stroll? Pole-barn paradox anyone?

Talking about how extended objects are treated in GR doesn't even relate to the question. Edited or not, it's an aside.

Besides, that's a confusing example

Yes, it is, and you are catching this in your 1st Post...:clap:

In Relativity, the Train 'contracts' with relavistic motion, it doesn't stretch!

In 'lights own frame' Relativity says that an observer can travel from A to B Instantaneously, from B to A Instantaneously, or 'space can contract to Zero' Instantaneously...and now KenG has added a fourth...the train/spaceship can now 'extend to infinity' instantaneously...now who's confused???

Jeff Root
2009-Jun-27, 02:49 AM
I didn't intend to skip replying to the earlier posts, but this looks like
something I should reply to immediately.




The distance I walk through the lengths of several railroad cars in a
moving train is the distance from the point in space and time at which
I started walking through the train to the point in space and time at
which I completed that walk.
Let's look at that example, and answer the following questions. What
is the difference between "distance then" and "distance now" in that
example?
That depends on the relative motions and where the observer is
located. I told you that the speeds could be anything and the
observer could be anywhere.



Why did you choose an example where there was no difference,
I didn't.



and why is that example relevant to the OP?
It says what it says. It is addressed to you, Ken, not to the original
poster or to the original post.



Why did you choose an example that does not involve light?
Because I can.



What does it add to the OP question to talk about "the distance"
between the starting point and ending point of that walk?
It is an analogous situation. Analogies can clarify situations which
are hard to understand by replacing them with situations which are
easier to understand. This situation is easier to understand because
it eliminates some extraneous elements of the original question that
you have been hung up on from the beginning.



The answers to these questions show that your current example
is completely irrelevant to the OP and to the thread.
Really? How?



But we can make your example relevant very easily, so let me
ask you: in your example, how far would you say you have walked
if you could walk at 99% the speed of light?
You need elephants to stampede across your kitchen floor in order
to make your floor wax work.

Would you please quit bringing up these ridiculous cases, Ken?
They have nothing to do with the original question, or the answer
to that question. They may be your pet subject, but they are not
relevant here.



How about 99.99999%? Now what if you actually were light,
as in the quote you made earlier about the distance "light travels"
between its emission and absorption? What is the distance you
walk then?
The distance I measure is zero. That has already been repeated
ad nauseum in this thread. Everyone gets it, Ken. Everyone had it
before the first tme you said it. Nobody disagrees. Nobody is arguing
against that fact. When you are light, everything measures zero.
Every distance measures zero, every time measures zero, every
dimension of every thing measures zero.



OK, so clearly you did not actually mean the distance you walked,
I meant what I said.

The distance I walked, according to me, or according to a passenger
sitting in a car, or according to another passenger running in the
opposite direction, or according to you standing beside the tracks,
or according to an observer on Mars. According to ANY observer,
the distance I walk through the moving train is the distance from the
point in space and time at which I started walking through the train
to the point in space and time at which I completed that walk.

That holds even in the extreme limiting case that I am massless and
moving at the speed of light.



you meant the distance someone on the train thinks you walked.
See the difference?
The difference between what and what else? Observers in different
states of motion with respect to me will measure different distances
that I walk, obviously. That can't be your point. That is a known.
So, what difference are you talking about?



And that's only with special relativity, when you put in the "real"
way things work, general relativity, there isn't even a clear meaning
for what "the train" corresponds to.
It can correspond to any object. It wouldn't make sense for it to be
a train of light waves, since I'm walking on it.



For example, if the train always extends from Earth to some distant
galaxy, you are approximating "comoving frame coordinates", although
even that not exactly-- but those are not the "reference frame of the
observer", they are just an arbitrary coordinate system, and went
unspecified in your remark (as did all coordinate systems).
Objects don't extend between widely-separated galaxies.

-- Jeff, in Minneapolis

Ken G
2009-Jun-27, 07:00 AM
That depends on the relative motions and where the observer is
located. I told you that the speeds could be anything and the
observer could be anywhere.Fine, pick any you want, and tell me what "distance then" and "distance now" means in that pointless example.

Because I can.Yes, you certainly can choose a pointless example and claim it is an "analogy", I do not dispute that fact, I merely dispute its usefulness.


Really? How?That will become clear when you actually answer any of the questions I raised.

Would you please quit bringing up these ridiculous cases, Ken?
They have nothing to do with the original question, or the answer
to that question. Incorrect, you brought up "the distance light travels" in the original quote that started all this. So you brought up light, not me. All I need is a simultaneity convention, any one will do.


The distance I measure is zero. That has already been repeated
ad nauseum in this thread. Everyone gets it, Ken.No, apparently you do not get it, if you think that your train example supports the idea that the distance to these galaxies is like the distance you walk on that train, which is obviously the question you put to Spaceman Spiff. So no, you still don't get it, or you would not have asked so irrelevant of a question.


The distance I walked, according to me, or according to a passenger
sitting in a car, or according to another passenger running in the
opposite direction, or according to you standing beside the tracks,
or according to an observer on Mars. OK, one final time: none of those distances are uniquely defined! They simply don't exist, they are not proper distances the way you described the scenario. There is no such thing as "the distance walked" in your scenario, it is all 100% purely up to the coordinatizaton chosen, which is analogous to the train chosen (there is, however, a meaningful "proper time" for that walk, so again you confuse a time with a distance). Ergo, it does not answer the OP, because the OP is actually asking "what is the coordinatization chosen", and your answer pretended there was no need to supply such a coordinatization. You simply pretended that there was a unique and meaningful thing called the distance between two non-simultaneous events in space and time, as seen from Earth, and there is no such thing. Again I'm just repeating.


That holds even in the extreme limiting case that I am massless and moving at the speed of light.Nope, it doesn't "hold" at all, until you specify a global coordinatization, which of course, is as arbitrary as the distance that you are talking about here. What is not arbitrary is the invariant distance, and you have never been talking about that.

Indeed, half the time you say your comments were not intended to be relativistically accurate, and the other half the time you say they apply for any observer and any speed. That's not a consistent position.



The difference between what and what else? Observers in different
states of motion with respect to me will measure different distances
that I walk, obviously.Oh, it's much worse than that. They can't measure the distance you walked at all, there is no ruler that extends between the event of your setting out and your arriving at the end of your walk, because the events are not simultaneous in any reference frame, and the ends of a ruler are simultaneous, in the frame of the ruler.


It can correspond to any object. That's the whole problem, it can indeed be any object that stretches between the events you describe, so it can also be any distance. The distance depends on what is happening to the train, which is the analog of how you have chosen to coordinatize distance.

You are probably imagining that the train is "stationary" with respect to the observer in question, and stays the "same length", but how could you tell which train did that? As I pointed out already, the only train that is somehow connected to an observer is a train that the observer thinks is stationary, but in general relativity (read, "in reality"), there is no unique way to specify such a train, it simply amounts to an arbitrary coordinate choice that is not unique to any observer.

Ken G
2009-Jun-27, 07:11 AM
Let me put the question to you a little differently. Forget general relativity, even though it is our best description of how reality works. You may stick to special relativity, and pick any inertial observer you like, receiving light from any object you like. Now do this: give me a situation where your interpretation of distance between the emission of the light and the absorption of the light is different from the concept of "distance then". You will fail, demonstrating that if one ignores what we really can know about distances, the "right" answer to the OP would have to be "it's distance then and not distance now", if your statement had been the right answer to the OP. That's just simple logic.

RussT
2009-Jun-27, 09:06 AM
Jeff:

It really boils down to whether you agree or disagree with the snip of post 159 from Ken G (http://www.bautforum.com/space-astronomy-questions-answers/88698-distances-then-now-6.html#post1512480) (which I've already highlighted in post 160 (http://www.bautforum.com/space-astronomy-questions-answers/88698-distances-then-now-6.html#post1512990)), especially the point of emphasis (mine):



Originally Posted by Ken G
No, again, light does not travel that distance. That distance connects two events, one in that galaxy and one in ours, that light does not connect. However, in cases where there is no important difference between "distance then" and "distance now", the discrepancy is unimportant and we can safely ignore it-- as long as we know we are ignoring it. This is an entry point for you to expand on your relativity knowledge, if you choose to use it as such. If, instead, you choose to believe your statement is "clearly correct" no matter what, then you pass on that opportunity. Your choice.


If you do agree (and especially with the bold and underlined portions), then there really is no point of disagreement between you and Ken G (although you might want to brush up on your relativity). If not, then you either don't understand relativity and/or are making a veiled claim with regards to its lack of usefulness (IOW validity) as a scientific theory.

As Ken G said, your choice.



No, again, light does not travel that distance. That distance connects two events, one in that galaxy and one in ours, that light does not connect.

And once again, I will simply say nonesense...If a Super Nova went off in M31 ~2.5 million years ago, we would just be receiving "First Light" Now, here in our Earth Frame!!!

There is no way around that period. Those photons did travel that distance in that anount of time based solely on the Constant Speed of light at 186,282.4 mps, and the proper distance for that could NEVER have been Zero.

It is quite obvious that something has been being ignored, and evidently you guys are convinced that it is the "Distance Then" VS the "Distance Now", BUT as I have shown all along,,,that ONLY applies Globally....it does NOT apply Locally...it is meaningless Locally!

Ken G
2009-Jun-27, 12:45 PM
There is no way around that period. Those photons did travel that distance in that anount of time based solely on the Constant Speed of light at 186,282.4 mps, and the proper distance for that could NEVER have been Zero.


The distance I measure is zero. That has already been repeated
ad nauseum in this thread. Everyone gets it, Ken. Everyone had it
before the first tme you said it. Nobody disagrees. Nobody is arguing
against that fact. Hmm, it appears you are either mistaken, or the words "everybody" and "nobody" don't mean what you think they mean.


It is quite obvious that something has been being ignored, and evidently you guys are convinced that it is the "Distance Then" VS the "Distance Now", BUT as I have shown all along,,,that ONLY applies Globally....it does NOT apply Locally...it is meaningless Locally!
What is the point of arguing with someone who doesn't even get that distance itself is a global concept, not a local one? It can certainly be approximated with a local extrapolation, but that's where the very first answer in this thread came from (by Grant), and if we are talking about rough approximations, no answer has improved on his.

Spaceman Spiff
2009-Jun-27, 02:42 PM
Spiff,

I don't doubt the validity of relativity or its usefulness as a scientific theory.

I don't doubt that Ken's understanding of relativity is good, and much deeper
than my own.

Asking if I agree with the statement you bolded has the same functionality
as the question, "Have you stopped beating your wife?"

-- Jeff, in Minneapolis

I have no idea how you connect my question to you, which is nothing more than an inquiry as to whether you acknowledge the usefulness of relativity and so recognize that we should always be aware of the approximations we decide to adopt for convenience, and the question you compare it to. :confused:

And as to your question to me, I do indeed disagree -- if you're claiming that there is an absolute answer to it. Why? Because space intervals and time intervals are not invariants. Now, I can decide that out of convenience I can get away with pretending that space and time intervals are invariants, and come up with an answer that most would be happy with. But that doesn't mean that being oblivious to the approximations I make somehow validates them. And if I remain blissfully happy in my ignorance, I might really screw up big time if I choose to apply my personal rules of space and time to a scenario in which they are poor approximations (or models) of the universe we inhabit.

Here again is the OP:


Distances: then or now?
The Andromeda galaxy is said to be about 2.5 million light years away. Does this mean it is 2.5 million light years away today? Or 2.5 million years ago?

01101001
2009-Jun-27, 02:57 PM
Any chance the two camps could summarize what they want the other side to admit, then have both camps swear they'll never be able to say that, and then this topic and the participants get a rest? The old agree to disagree?

Just trying to save you some effort.

Or, continue. It's fun to watch. From any distance!

Spaceman Spiff
2009-Jun-27, 03:11 PM
Any chance the two camps could summarize what they want the other side to admit, then have both camps swear they'll never be able to say that, and then this topic and the participants get a rest? The old agree to disagree?

Just trying to save you some effort.

Or, continue. It's fun to watch. From any distance!

I've already tried something along these lines, twice, and it changed nothing. :)

nokton
2009-Jun-27, 03:17 PM
Consider a scenario where the difference in distance from us between then and now is more than negligible: a galaxy 2.5 billion lya receding from us at maybe 10% the speed of light. The photons we now receive from it were emitted 2.5 billion years ago and between their emission and our reception travelled 2.5 billion light years across ever-expanding space between photon and Earth. (However, the expansion of space was not as fast as the photon which eventually got to us). Because of the expansion the galaxy was closer to us than 2.5 billion lya 2.5 billion years ago but what we observe appears to have been 2.5 billion lya 2.5 billion years ago. To estimate the galaxy's actual distance from us now we must take the apparent distance of 2.5 billion lya (which factors in the expansion of space between photon and Earth) and add the expansion of space behind the photon, between photon and source galaxy, over the 2.5 billion years.

Well, I thought I understood this stuff before I started typing...

You do, or you would have not the wit to write so. This site is about Phil,
and his thinking about bad science, and there is so much of it out there.
This site is about challenging that concept that scientists must be right,
and the current dogma in science is accepted as the truth.
Science and knowledge are a function of our still puny brains.
We still not understand quantum theory, or the power it offers us.
It is up to every student to challenge rote learning, and question everything,
only then can we have the open minds to secure our survival as a race.
Nokton.

Spaceman Spiff
2009-Jun-27, 03:37 PM
This site is about challenging that concept that scientists must be right,
and the current dogma in science is accepted as the truth.


This is getting off topic, but....
Who ever gave you the idea that scientists know that they must be right and that what science does and scientists do is create dogma? These are misconceptions. How is it, do you think, that we've made any progress in understanding the behavior of nature? Believing that one possesses absolute truth and establishing dogma to defend it are antithetical to the scientific process. Our models of nature (that's all science ever 'establishes') are open stand or fall in the illuminating light of the accumulation of data.

And there is a vast chasm between the admission of not knowing everything but that some ideas/models aren't at all useful in predicting the behavior of nature, and not knowing anything.

The moderator will probably ask that you present and defend your opinions on the nature of science and the purpose of the BA bulletin board in another thread.

Jeff Root
2009-Jun-27, 03:48 PM
Zero One,

I've thought about agreeing to disagree here. I think it would be easier
for me than for Ken, because I agree with Ken's physics, but he doesn't
agree with mine! I see him as stuck in a box, and he sees me as stuck
in another, very different box.

I hope this is fun to watch.

Spiff,

The problem for me in saying whether I agree with Ken's statement is
that I'm damned if I agree and damned if I disagree. That is how it is
like being asked if I have stopped beating my wife. (I'd really like to say
that she always beats me, but unfortunately we've never raced because
I've never been married.)

I think I'll find a way to answer the question eventually.

I have said more than once in this thread that I agree with relativity's
proposition that distances are not "absolute". If you interpret anything
I've said in this thread as implying absolute measure, you most likely
interpreted it badly.

-- Jeff, in Minneapolis

Jeff Root
2009-Jun-27, 04:25 PM
The distance I walk through the lengths of several railroad cars in a
moving train is the distance from the point in space and time at which
I started walking through the train to the point in space and time at
which I completed that walk. That is true nomatter how fast or how
slow I walk, how fast or how slow the train is moving, or where the
observer is located. I can be the observer, a seated passenger can be
the observer, you, standing beside the track can be the observer, or
Beagle 2 on Mars can be the observer. The distance meant is always
the distance from the starting point in space and time to the ending
point in space and time.
Spiff,

Given that this statement does not assume or imply any kind of
absolute or invariant notion of distance, do you still disagree with it?
If so, what do you disagree with?

Different observers in different states of motion relative to me may
of course get different measures for the distance I walk, depending
in part on the details of their measuring techniques.

-- Jeff, in Minneapolis

Ken G
2009-Jun-27, 04:35 PM
Any chance the two camps could summarize what they want the other side to admit, then have both camps swear they'll never be able to say that, and then this topic and the participants get a rest?

OK, this is my basic point. It seems to me this thread is an entry to discussing our current best understanding of how the concepts of "distance" and "time" interrelate, i.e., "is it distance then or distance now". Our current best understanding of distance and time is relativity, and so we can start with special relativity, but even that is not really our best understanding because gravity may play a crucial role in how space and time work (a la Mach). But it's useful to start with special relativity anyway, and I will share my own personal (non-expert) ideas about what the various flavors of relativity are trying to tell us:

In SR, we consider global arrays of mutually stationary inertial observers. Their rulers, and the Einstein simultaneity convention for time, generate our coordinate system. Then we need a concept of "proximity" between observer/events, which combines space and time and is called the "metric" of the spacetime. If the events are timelike separated, the invariant concept is proper time, which is the clock reading of an inertial observer (not in the stationary array) who is at both events. If the events are spacelike separated, the invariant concept is proper distance, which is the length of a rigid ruler that spans two events that are simultaneous in the ruler frame. Now, since we don't have an observer at both events, nor so long of a ruler, we actually end up needing indirect observations to infer these proper distances and proper times. And we just do the best we can, typically using light.

For example, we might do a "lunar ranging" experiment, where we bounce light off the Moon. Then we do a time measurement, not a distance measurement. However, in special relativity, the light will move at a known speed past that array of mutually stationary inertial observers, so a time measurement can be converted to a distance measurement. That conversion is consistent with the coordinate system, and can give distances between events that are not simultaneous in those coordinates, but the distance you get is actually the same as the proper "distance then", the proper distance between the source and the stationary receiving observer, at the moment that the light was emitted according to the stationary observer at the scene. So Jeff Root's statement in special relativity is claiming that the answer to the OP is "distance then", though he does not realize it.

Enter general relativity. Again, I do not think of GR as simply special relativity plus gravity, I think of it is as the "right" relativity, that corrects the misconceptions of special relativity (we get away with these misconceptions in many applications, but they are still not a correct representation of how reality behaves because they involve concepts that are not demonstrable, like the constancy of the coordinate speed of light when the light is somewhere you can't measure it). Now we find that there is nothing special about either inertial observers, or mutually stationary observers, as these both involve arbitrary conventions to even define. The conventions we would use to define these concepts amount to choosing a coordinate system, and the concept of distance amounts to specifying coordinates, like giving addresses to houses on your street. All the results can predict are the outcomes of experiments, but the "story" you tell, including the distances involved in the story, are completely arbitrary, because they depend on the arbitrary coordinatization.

Now there is certainly one standard coordinatization used in cosmological general relativity, which is comoving frame coordinates. There is also a list of types of distance measures, which can be used to try to infer from a dynamical model (not measure, because we have no direct access to the distance we are attempting to infer, as it involves simultaneous events that are widely separated) the "proper distance" in the comoving-frame coordinates. Other coordinates would of course require a different translation from the various distance measures to the proper distance. The value of proper distances is that they correspond to the metric, so they are a meaningful concept of "distance" that isn't obfuscated by mixing in some elements of time (because the events are simultaneous).

Notice that none of these ideas about distance support the contention that was made earlier that there is physical meaning to "the distance light travels" between its emission and its absorption. Also, even in the flawed special relativity version of reality, the distance "to the observer" between the emission and absorption of light is identical to the concept of "distance then", and is not distinguished from that concept by pretending the "distance between points in space and time" is somehow different from "distance then", as claimed by Jeff Root above, and supported as "exactly correct" by RussT.

PetersCreek
2009-Jun-27, 04:44 PM
You do, or you would have not the wit to write so. This site is about Phil,
and his thinking about bad science, and there is so much of it out there.
This site is about challenging that concept that scientists must be right,
and the current dogma in science is accepted as the truth.
Science and knowledge are a function of our still puny brains.
We still not understand quantum theory, or the power it offers us.
It is up to every student to challenge rote learning, and question everything,
only then can we have the open minds to secure our survival as a race.
Nokton.

This is getting off topic, but....


Yes, it is. Please take it to its own thread if you wish, before it develops further here. Thanks, folks.

Jeff Root
2009-Jun-27, 05:08 PM
There is no way around that period. Those photons did travel that
distance in that anount of time based solely on the Constant Speed
of light at 186,282.4 mps, and the proper distance for that could
NEVER have been Zero.


The distance I measure is zero. That has already been repeated
ad nauseum in this thread. Everyone gets it, Ken. Everyone had it
before the first time you said it. Nobody disagrees. Nobody is arguing
against that fact.
Hmm, it appears you are either mistaken, or the words "everybody"
and "nobody" don't mean what you think they mean.
Nope. You're wrong. Whether you are doing it conciously or
unconciously, I don't know, but you are leaving out essential bits
of information which resolve the conflicts you raise. In this case,
you left out the information that Russ's statement applies to an
ordinary observer composed of atoms, while my statement applies
to the case where I am light, as you requested in post #192.
I quoted that request immediately before the statement of mine
that you quote. My statement was a direct reply to your request
that I consider the situation where I am light. So that is what my
reply assumes.

An "observer" who somehow consists of light would see no distance,
no passage of time, no anything. An observer who is a human being
sees light travel a definite distance in a given time. Different
situations, different results. Fifth-grader stuff. It isn't for lack of
knowledge or intelligence that you aren't getting this, Ken.

-- Jeff, in Minneapolis

Ken G
2009-Jun-27, 06:20 PM
Whether you are doing it conciously or
unconciously, I don't know, but you are leaving out essential bits
of information which resolve the conflicts you raise. In this case,
you left out the information that Russ's statement applies to an
ordinary observer composed of atoms, while my statement applies
to the case where I am light, as you requested in post #192.Sorry Jeff, you are completely and demonstrably incorrect here. RussT's statement is perfectly clear, he is talking about the "path of photons" and he clearly claims "the proper distance for that could never be zero". You claim that "everyone" here knows that the proper distance between the emission and absorption of light is zero. Well sorry to tell you, but the "path of the photons" connects the emission and absorption of light. Ergo, those events do have a proper distance of zero, in obvious contradiction to the statement of RussT that I quoted. So no, not "everyone" knows that proper distance is zero, and at the moment it's not even clear that you do, despite your protestations to the contrary. You are completely mistaken that simply identifying us as the observers, or what we are made of, somehow makes the proper distance between those events not zero, you should look up the meaning of an "invariant", as this seems to be a continuing source of error for you.

George
2009-Jun-27, 09:43 PM
Whew.... I just read most of this and am perplexed as to just why it has gone on so far. A few years ago, when tempers were often less bridled, such a conflict would have gotten this thread closed by now. My compliments to both encampments, at least as far as courtesy goes.

I suspect that Spaceman Spiff introduced something that may have much to do with the more Newtonian camp view -- the Hubble flow. Though non-special, there seems to be a corelation between our local frame and spacetime itself, at least within our region in and around the Local Group, and from the perspective of almost any other being that might be on any other planet.

As Ken has noted, a very long ruler would stretch to about 2.5 million light years, assuming we extend it from our local frame here on Earth. Note that there is no imaginable way that we could accelerate, stand on our head, or doing anything else that would cause us to discover a ruler reading greater than 2.5 million light years. [Or am I wrong?] Thus there is a sense that the 2.5 million light year distance is more than just one from our frame of reference; all other observers on other planets within the Local Group would agree with our distance, more or less.

But there is nothing wrong with presenting the bigger picture.


Geezzzzzzzz Ken, you keep trying to fit a square peg in round hole:wall: But it is a round hole, and a very big one at that, much larger than the little square peg. Just because the case of our place in space presents a very large insert that has a little square hole within it does not mean there isn't more that is out there.

An analogy might be that we are on the first floor of a very tall skyscraper, with only a stairway, no elevator. The first floor is grand but those that have climbed to the higher floors can see things that those on the first can not. This isn't because of a lack of anyone's intelligence, of course, but because what is seen from the higher floors is simply different. Nevertheless, the "real world" happenings are commonly found on the first floor and it is the only floor that allows one to move out into the world (ie. Earth).

Regardless, what Ken and Grant, and others, have been presenting are very pertinent to the essence of the O.P. question as the view from higher floors are pertinent to science in the 21st century.

My point is that both camps have their merit, but to deny relativity its place and impact upon galactic distances and times measured seems unjustified.

grav
2009-Jun-27, 11:23 PM
As Ken has noted, a very long ruler would stretch to about 2.5 million light years, assuming we extend it from our local frame here on Earth. Note that there is no imaginable way that we could accelerate, stand on our head, or doing anything else that would cause us to discover a ruler reading greater than 2.5 million light years. [Or am I wrong?] Thus there is a sense that the 2.5 million light year distance is more than just one from our frame of reference; all other observers on other planets within the Local Group would agree with our distance, more or less.The distance will not be greater only if we consider ourselves stationary to the other galaxy and all other observers stationary to ourselves as well, more or less, and remain that way, or if measured immediately after we quickly accelerate. Otherwise, however, if there is any continued relative motion between ourselves and the other galaxy, either before or after acceleration takes place, then the distance will eventually become greater or less, of course, depending upon the direction of travel and when the measurement is taken according to a particular frame, so the question might then become something like "Is it 2.5 million light years then, or 2.5 million light years now?" Oh, wait... :)

Jeff Root
2009-Jun-28, 12:01 AM
Sorry Jeff, you are completely and demonstrably incorrect here.
RussT's statement is perfectly clear, he is talking about the
"path of photons" and he clearly claims "the proper distance for that
could never be zero".
What Russ says is often difficult to parse.

His claim is that 2.5 million years does not equal zero time, and
2.5 million light-years does not equal zero distance.

Based on what he said in post #78 and post #114, it is clear that he
is objecting to your use of light as the observer who is measuring the
distance. He is not saying that an observer traveling at c would
measure anything other than zero. Instead, he is saying that there
is no such observer, never has been such an observer, never will be
such an observer, and that your continually resorting to this fantasy
observer is utterly irrelevant to answering the original question, as
well as irrelevant to understanding what he and I are saying.

Russ,

Do you agree that an observer traveling at the speed of light would
measure all distances as zero?

Have you ever meant to claim in this thread that an observer traveling
at the speed of light would ever measure a distance other than zero?



You claim that "everyone" here knows that the proper distance
between the emission and absorption of light is zero.
No. I claim that everyone here knows that in the frame of the light,
the proper distance between emission and absorption is zero.

However, it is not possible for anyone to be in that frame.

In particular, distances of galaxies are never measured or reported
for such a frame.



Well sorry to tell you, but the "path of the photons" connects the
emission and absorption of light.
That's fine. I have no disagreement with that statement.



Ergo, those events do have a proper distance of zero, in obvious
contradiction to the statement of RussT that I quoted.
They may have a proper distance of zero in the frame of the light,
but they do not have a proper distance of zero in any other frame.
In particular, they do not have a proper distance of zero in any
frame in which anything is actually measured.

There is no contradiction with what Russ said.

I'm not sure I've deciphered everything Russ has said in this thread,
but what he has said that I have been able to parse, I agree with.



So no, not "everyone" knows that proper distance is zero, and at the
moment it's not even clear that you do, despite your protestations
to the contrary. You are completely mistaken that simply identifying
us as the observers, or what we are made of, somehow makes the
proper distance between those events not zero, you should look
up the meaning of an "invariant", as this seems to be a continuing
source of error for you.
Everyone here except perhaps you knows that the distance between
the Andromeda galaxy and us is not zero. Everyone here knows that
an observer moving between the Andromeda galaxy and us at the
speed of light will measure a distance of zero between emission and
absorption. Everyone here except perhaps you knows that there is
no such observer, and never has been.

-- Jeff, in Minneapolis

Ken G
2009-Jun-28, 12:50 AM
His claim is that 2.5 million years does not equal zero time, and
2.5 million light-years does not equal zero distance.No, his claim is what it is, not what you might want to pretend it is. What kind of argument is that? Your position now is "everyone on this thread understands what you are saying, and anyone who makes statements completely in contradiction to that should simply have their statements 'parsed' to say something other than what they said." This is a new one: "argument by indiscriminate parsing".


Based on what he said in post #78 and post #114, it is clear that he
is objecting to your use of light as the observer who is measuring the
distance. Let me say this one more time: it doesn't make the slightest difference who is measuring the distance, if you specify two events at the ends of a "path taken by light", and you say the "proper distance could never be zero", you are making a false remark and demonstrating that you do not understand my statements about that proper distance in fact being zero. Period. This i
s the beauty of facts, "parse" them anyway you want.


I claim that everyone here knows that in the frame of the light,
the proper distance between emission and absorption is zero. You are obsessed with the frame of the observer, but as I've told you, if you look up the concept of an "invariant", you find that the nice thing about them is that they are observer independent. What's more, proper distance is an invariant-- that is the whole point of it. Finally, being as science is based on whatever fraction of the shared experience of intelligent beings that is objective, science is focused entirely on the purpose of explaining the invariants. All else is sheer pretense, scientifically speaking.

They may have a proper distance of zero in the frame of the light,
but they do not have a proper distance of zero in any other frame.You see, you are incorrect that everyone in this thread understands that proper distance is zero. You do not understand it yourself! Since you won't take my word for it, you really need to google "invariant" and "proper distance", then get back to me. Until then, you are just wasting both our time.


In particular, they do not have a proper distance of zero in any
frame in which anything is actually measured.Wrong. Look up "null geodesic" while you are at it, and then reflect on the role of a metric in relativity.

Everyone here except perhaps you knows that the distance between
the Andromeda galaxy and us is not zero.Actually, I never said that distance was zero. I said the proper distance connects two simultaneous events, one in our galaxy and one in Andromeda, which contradicts both your statement and RussT's about proper distances crossed by light paths (which are null geodesics). I also said that we have no direct measurable access to that distance, so we must infer it the best we can using the data we do have access to, and any such inference comes with it an uncertainty based on what is not known about the relevant dynamics, i.e., the gravitational environment that actually determines the meaning of concepts like distance.

Everyone here knows that
an observer moving between the Andromeda galaxy and us at the
speed of light will measure a distance of zero between emission and
absorption.That is not what I want everyone to know-- for that, read my actual posts.

You see, I personally feel it is a common misconception, or at least a poor way to think about relativity, that moving at high speed somehow "contracts" the distance one travels, and light "contracts" the distance to zero. But there's no need to contract anything-- the point is, the distance travelled is really less for a fast moving observer, and it is really zero for light. So it's not that light is some kind of "weird observer", it is that light actually finds a shortcut. The distance between two galaxies depends on the path taken between them, and the path taken by light is a zero-distance path. It has nothing to do with who is doing the observing, that is an invariant of relativity. Now you understand why I say there is "no such thing" as "the distance" that any given observer "perceives" between two events-- the distance depends on the path that connects two events, but a path that connects them in such a way that they are simultaneous is the "proper distance" between them, if such a path exists (that's the ruler that spans the events).

George
2009-Jun-28, 12:54 AM
Otherwise, however, if there is any continued relative motion between ourselves and the other galaxy, either before or after acceleration takes place, then the distance will eventually become greater or less, of course, depending upon the direction of travel and when the measurement is taken according to a particular frame, so the question might then become something like "Is it 2.5 million light years then, or 2.5 million light years now?" Oh, wait... :) I am curious how any oberserver, relativistic or not, that is near the Earth could claim a much more distant Andromeda. Is not 2.5 million lyrs. the maximum distance any observer would claim? [I am ignoring peculiar motion, of course.]

I wonder whether or not we are approaching Andromeda or Andromeda is approaching us? It would be hard to determine just how much each is contributing to the gain in distance. Assuming we knew their masses, that would not be enough. It depends on whose reference frame is chosen, which is the point that many are making.

Spaceman Spiff
2009-Jun-28, 12:59 AM
Originally Posted by Jeff Root
The distance I walk through the lengths of several railroad cars in a
moving train is the distance from the point in space and time at which
I started walking through the train to the point in space and time at
which I completed that walk. That is true nomatter how fast or how
slow I walk, how fast or how slow the train is moving, or where the
observer is located. I can be the observer, a seated passenger can be the observer, you, standing beside the track can be the observer, or Beagle 2 on Mars can be the observer. The distance meant is always the distance from the starting point in space and time to the ending point in space and time.
Sorry, but if as your example seems to indicate that you really believe that an observer on the train and an observer anywhere else in the universe (even, for example, at rest relative to the tracks) actually measure the distance between the events you describe (or any two events), then you do not understand relativity -- plain and simple. To the contrary, these observers you mention can each measure a distance between the events in space and in time. The only invariant among intervals of space, time, and space-time is the last of these. But then, Ken G has already pointed out these things numerous times. You reply by claiming that you understand and accept relativity, but then claim that Ken G is missing the boat somehow, and then go on to give examples to illustrate your point -- many of which indicate that you don't understand the concepts.

There is no the distance, whether meant or anything else.

Jeff Root
2009-Jun-28, 01:33 AM
Spiff,

Your objections are to what you want me to have said. You haven't
disagreed with what I actually said.

-- Jeff, in Minneapolis

grav
2009-Jun-28, 01:58 AM
I am curious how any oberserver, relativistic or not, that is near the Earth could claim a much more distant Andromeda. Is not 2.5 million lyrs. the maximum distance any observer would claim? [I am ignoring peculiar motion, of course.]

I wonder whether or not we are approaching Andromeda or Andromeda is approaching us? It would be hard to determine just how much each is contributing to the gain in distance. Assuming we knew their masses, that would not be enough. It depends on whose reference frame is chosen, which is the point that many are making.Oh okay, I see what you are saying. So ignoring gravitational effects and acceleration, and just considering it inertially, if all observers coincide in the same place when making their measurements to Andromeda, although travelling at different relative speeds, I believe it would be the observer that is stationary to Andromeda that would measure the greatest distance, the one that observes no overall redshift or blueshift due to motion.

Spaceman Spiff
2009-Jun-28, 02:15 AM
Spiff,

Your objections are to what you want me to have said. You haven't
disagreed with what I actually said.

-- Jeff, in Minneapolis

I quote your own quote, tell you what you have written is a misconception (specifically, in bold (http://www.bautforum.com/space-astronomy-questions-answers/88698-distances-then-now-8.html#post1518246)), and your response is that I have disagreed with a statement that I have imagined?

You say you "understand" relativity, and then go on to say that of course "everyone" understands your argument. But then when prompted to explain that argument, you say things which indicate that you do not understand relativity. You can't have it both ways, but that appears to be 'the game' you've been playing with Ken G, whether intended or not.

Jeff Root
2009-Jun-28, 03:37 AM
Your objections are to what you want me to have said. You haven't
disagreed with what I actually said.
I quote your own quote, tell you what you have written is a misconception
(specifically, in bold (http://www.bautforum.com/space-astronomy-questions-answers/88698-distances-then-now-8.html#post1518246)), and your response is that I have disagreed with a
statement that I have imagined?
I'm afraid so, yes.



You say you "understand" relativity,
I just looked back through the 44 most recent posts in the thread,
and could not find any place where I said that I understand relativity.
I have said that I understand various things, I've said that Ken's
understanding of relativity is good, and much deeper than my own,
and I've (rashly) said that everyone here understands certain things,
some of those being aspects of relativity. I don't recall ever saying
that I understand relativity. Can you show me where I said that?



and then go on to say that of course "everyone" understands your
argument.
Oh, no. I certainly never said that everyone understands my argument.
I did say, for example, that "everyone here knows that in the frame of
the light, the proper distance between emission and absorption is zero."
From Ken's response to that assertion, I can see that I may have got
the physics wrong, in which case the statement as a whole is wrong,
too. Even if I got the physics right, the statement as a whole could
be wrong. But I have not yet analyzed Ken's response fully enough to
determine whether what he says about the physics actually conflicts
with what I said. I thought that what I said was equivalent to what
he has said numerous times in this thread, but if he disagrees-- which
he does-- then I need to figure out why he disagrees. I'll work on it.

In any case, I never said that everyone understands my argument.



But then when prompted to explain that argument, you say things
which indicate that you do not understand relativity. You can't have
it both ways,
My argument does not depend on relativity. Ken's does; mine doesn't.
So whether I understand relativity or not has no bearing on whether
everyone or anyone here understands my argument.

-- Jeff, in Minneapolis

mugaliens
2009-Jun-28, 04:02 AM
I wonder whether or not we are approaching Andromeda or Andromeda is approaching us?

Relative to what?


It would be hard to determine just how much each is contributing to the gain in distance.

Again, relative to what?

The key thing about relativity is that it's all relative. There is no standardized "ether" against which things moving in the cosmos are measured. Things move relative to other things, period. If there is a galaxy (Galaxy A) which is moving at .5 c relative to a cluster of galaxies (Cluster B) whose relative velocities to one another are less than .0001 c, one might assume that the .5 c galaxy is the speedy demon, but one would be wrong. It is just as relevant to say that Galaxy A is experiencing a half lightspeed flyby from Cluster B.


Assuming we knew their masses, that would not be enough. It depends on whose reference frame is chosen, which is the point that many are making.

Exactly! I knew you were going somewhere with this, George. My bad if I didn't see it coming.

George
2009-Jun-28, 04:19 AM
Oh okay, I see what you are saying. So ignoring gravitational effects and acceleration, and just considering it inertially, if all observers coincide in the same place when making their measurements to Andromeda, although travelling at different relative speeds, I believe it would be the observer that is stationary to Andromeda that would measure the greatest distance, the one that observes no overall redshift or blueshift due to motion. Yep. As Grant has shown much earlier, the peculiar motion (blueshift) is quite small and not of particular interest at this point.

What I am hoping to demonstrate is that the 2.5 million lightyears distance could be considered "the" distance because of the fact there can be no greater straight line distance. This may explain the hang-up for Jeff and Russ. Am I close, Jeff? [This Andromeda case is easy since the other ways to see it, as Grant demonstrated from his page one link, are not that important, but much more distant galaxies will require one to make a choice.]

Of course, the distance to Andromeda can be considered to be much less due to relativity, and that is an important point that Ken initiated.

George
2009-Jun-28, 04:20 AM
Exactly! I knew you were going somewhere with this, George. My bad if I didn't see it coming. :)

George
2009-Jun-28, 04:32 AM
BTW, my compliments on your latest avatar, Publius. It took me a second to recognize who you were lifting up. :clap:

Jeff Root
2009-Jun-28, 05:01 AM
George,

The idea you are pursuing, in which distance measurements made with
zero velocity relative to the place you are measuring to are considered
particularly meaningful, is something I've thought about, a few years ago,
but isn't what I'm arguing now. What I'm on about at present is simpler--
as I've already said too many times, its something most fifth-graders would
have no trouble with. It doesn't require any knowledge of relativity, though
an explanation of it could suggest the concept of at least Galilean relativity.

I don't understand this comment:


Of course, the distance to Andromeda can be considered to be much
less due to relativity ...
I can guess, but I really don't know what you are referring to. My best
guess is that you're just saying that observers in certain states of motion
relative to the Andromeda galaxy will see it as being closer than will other
observers who are at the same location, but in other states of motion.
If that's all you meant, then, okay, yes, of course. :)

-- Jeff, in Minneapolis

RussT
2009-Jun-28, 09:52 AM
The problem for me in saying whether I agree with Ken's statement is
that I'm damned if I agree and damned if I disagree.

I'll start here...

That is simply because there are two 'truths' being expoused in this thread.

The first was stipulated to very early on...



Originally Posted by RussT
Light is Constant @ 186,282.4 mps in vacua by definition, which means that the light from M31 at ~2.5 million Lys away took 2.5 miilion years to reach HST. Which means we see/detect M31 where it was 2.5 million years ago. During the last 2.5 million years, M31 has, based on it's current speed in the local group, moved to where it is Now in space. That is what we KNOW as long as light is Constant at 186,282.4 mps Just as Tim said in his quotes I quoted!



And he was right. I've never said anything different. That's exactly what "we" (you and Tim and the rest of humanity) know.

Grant Hutchison



Yes, the spaceship moving at 0.5 c in the Earth frame will require 8 Earth years to travel the 4 LY (in the Earth frame) to alpha Cen, and light will require 4 Earth years to do the same trip. Is that not what Grant said? What other possibilities are there, this follows from the definition of velocity, coupled with the appropriate attention to specifying a reference frame.

Here KenG and Grant and Tim Thompson all agreed that there IS a "light travel time", that is "Known" and that all of Humanity "Knows" it to be "True"!!!

Now, everyone that has replied to this thread, especially Spaceman Spiff, answer this question...in writing, as a simple response to this question...

The distance to M31 is known to be ~2.5 light Years from Earth, so IF a Super Nova exploded/imploded in M31 2.5 million years ago, how long would it have taken those photons to travel to the Earth, and when would we be detecting them, in Earth's frame???

Good...now compare your answer to this statement...



There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels

And Ken's correct...that statement is what Relativity dictates...

So, you wind up with two supposed 'Truths'...1. light has a specific travel time based on a well known ~ distance, and 2. light doesn't have a travel time or a distance.

Both Jeff and I have repeatedly shown, that those distances, to Local objects ie; the Moon, Sun, all our planets, alpha Centauri, and even M31 were NOT determined using SR/GR or SR>>GR...

So, based on your answers to the SN in M31, it doesn't take a rocket engineer, But it might take a fifth grader, to understand that light/photons do actually travel the distances shown...oh yeah...in Earth's Frame...can't forget that little tid bit, even though just mentioning light travel time should have been obvious to anyone who knows a little 'bout it.

Now, IF there are Aliens in spaceships out there, traveling at Relavistic speeds, and even all the way to "c", great, when they get here they will undoubtedly be able to teach us a thing or three...BUT all of these that no one has responded to do apply!!!

Let's see if we can determine how that 'Arguement' will play out...

"If" we become that 'observer' (IN fact jump in the seat right next to him/her)you/KenG/Relativity/mainstream and Grant says 'we' should...

Say that 'alien in a spaceship' traveling toward us, at "c" right next to our sun,
and let's say all the planets are lined up right behind us, like happens about every 175 years...

and let's say that ship you and the alien are on is going to Mars, Jupitor, Neptune, when you get there "Instantantly", did the 'space' between there and the sun contract to Zero? What happened to Mercury, Venus, the Earth?

How are "You and The Alien" going to convince anyone, that you actually just came from the Sun in the First Place, when you could have just as easily come from a galaxy that was 1000 billion^1000 billion light years away?

And If you landed at NASA, you may be able to convince someone that you actually did travel 'instantaneously', but, now that you are in Earth's Frame, how are you ever going to convince anyone that the "Proper Distance" the Sun is or should be, is Zero distance???]

RussT
2009-Jun-28, 10:35 AM
Whew.... I just read most of this and am perplexed as to just why it has gone on so far. A few years ago, when tempers were often less bridled, such a conflict would have gotten this thread closed by now. My compliments to both encampments, at least as far as courtesy goes.

I am going to respond to yours George, because it has some very key points and is well thought out.

And yes, this has been a wild ride, and if I were not very careful, this could have been closed...;)



Though non-special, there seems to be a corelation between our local frame and spacetime itself, at least within our region in and around the Local Group, and from the perspective of almost any other being that might be on any other planet.

I suspect that Spaceman Spiff introduced something that may have much to do with the more Newtonian camp view -- the Hubble flow.

I don't think the Hubble flow has anything what-so-ever to do with this discussion, as we are talking Local neighborhood of M31 inward to Earth.

But, yes, I covered...and from the perspective of almost any other being that might be on any other planet


From "Earths Frame" or any scientist, in their "Rest Frame" at Mercury, Venus, Earth, Mars out to all the planets in our solar system, and the scientist in her/his 'rest frame' on or around their third rock from the sun of alpha Centauiri, or the scientists in M31 'at rest' on their planet...

That the 'distance' is 'really' the distance that light travels in specified amounts of time, based on the distance.......hence the term "Light Year".



Thus there is a sense that the 2.5 million light year distance is more than just one from our frame of reference; all other observers on other planets within the Local Group would agree with our distance, more or less.

Well, from their Planet rest frame, yes, and we them.



But there is nothing wrong with presenting the bigger picture.

Yes, there is no doubt about this!!! And I have never said differently.

KenG keeps on saying things like...."If Jeff had said..."...so I will do the same here...

If kenG had said..."well that's all well and good for the Local stuff, BUT if you really want to know the difference between 'distance then VS 'distance now' as the expansion of the Universe comes into play, here is some important info"

Then we wouldn't be having this conversation, and now that he is stuck trying to defend...There's no such thing as the distance the light travels. There isn't even such a thing as the time the light travels

He is pulling every trick in the book and even making up new Relativity!!!

RussT
2009-Jun-28, 11:21 AM
Originally Posted by Ken G
Sorry Jeff, you are completely and demonstrably incorrect here.
RussT's statement is perfectly clear, he is talking about the
"path of photons" and he clearly claims "the proper distance for that
could never be zero".

That is exactly what I did claim, because Relativity did not and cannot determine distances in the Local neighborhood!
But why don't you respond to my posts instead of trying to work jeff and I against each other?



Based on what he said in post #78 and post #114, it is clear that he
is objecting to your use of light as the observer who is measuring the
distance. He is not saying that an observer traveling at c would
measure anything other than zero. Instead, he is saying that there
is no such observer, never has been such an observer, never will be
such an observer, and that your continually resorting to this fantasy
observer is utterly irrelevant to answering the original question, as
well as irrelevant to understanding what he and I are saying.

Those are both very good Posts and speak for themselves.

I even suggested that KenG could use an observer 'right next to the sun' travel at "c" toward us for his emission/absorption at Zero, and then show us how Relativity derives the 93 million miles...he passed on that.



Do you agree that an observer traveling at the speed of light would
measure all distances as zero?

I have agreed that that is what Relativity says, but that it is irrelevent in the Local neighborhood.


Have you ever meant to claim in this thread that an observer traveling
at the speed of light would ever measure a distance other than zero?

Now, that is a whole different kettle of worms...;) you will have to look at some way earlier posts, where I got warned to get a glimmer of that.



No. I claim that everyone here knows that in the frame of the light,the proper distance between emission and absorption is zero.

Knows that Relativity "Says"!!! It's a "Theory"...hence KenG's...the distance depends on the path that connects two events, but a path that connects them in such a way that they are simultaneous is the "proper distance" between them, if such a path exists in post # 219



However, it is not possible for anyone to be in that frame.

In particular, distances of galaxies are never measured or reported
for such a frame.

Bingo and Bingo




Originally Posted by Ken G
Ergo, those events do have a proper distance of zero, in obvious
contradiction to the statement of RussT that I quoted.



They may have a proper distance of zero in the frame of the light,
but they do not have a proper distance of zero in any other frame.
In particular, they do not have a proper distance of zero in any
frame in which anything is actually measured.

There is no contradiction with what Russ said.

I'm not sure I've deciphered everything Russ has said in this thread,
but what he has said that I have been able to parse, I agree with.

Again...Bingo.

hhEb09'1
2009-Jun-28, 01:16 PM
But why don't you respond to my posts instead of trying to work jeff and I against each other?If we focus on the topic instead of each other, this can be fun. As it is, I'm closing the thread.