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henriquefd
2010-Jan-15, 10:47 AM
Here we go:

Suppose a Spacecraft is going on a round trip to a planet 4,45 light years away. That means that the total distance is 8,9 light years.

I put the formulas on Excel to test Time Dilation on variable speeds(v)

v(c)......Lorentz...Distance(ly)....Earth time(y)......Ship time(y)
0,100c......0,995.......8,9.............89,00..... .....88,55
0,200c......0,980.......8,9.............44,50..... .....43,61
0,300c......0,954.......8,9.............29,60..... .....28,30
0,400c......0,917.......8,9.............22,25..... .....20,40
0,500c......0,866.......8,9.............17,80..... .....15,41
0,600c......0,800.......8,9.............14,83..... .....11,86
0,700c......0,714.......8,9.............12,71..... .....09,07
0,800c......0,600.......8,9.............11,12..... .....06,67
0,900c......0,436.......8,9.............09,88..... .....04,31
0,990c......0,141.......8,9.............08,98..... .....01,26
0,999c......0,045.......8,9.............08,90..... .....00,40

Earth Time formula = Distance / v
Ship Time formula = Earth Time * Lorentz

So, as you can see, as the Ship approaches lightspeed, the roundtrip gets close to become instantaneous.

I was under the impression that people inside the Ship would feel like 8,9 years had passed if the Ship travelled, at Lightspeed, through a distance of 8,9 light years. But that is not what those numbers are showing.

I believe there is something wrong with the Ship Time formula.

Jens
2010-Jan-15, 10:50 AM
I was under the impression that people inside the Ship would feel like 8,9 years had passed if the Ship travelled, at Lightspeed, through a distance of 8,9 light years. But that is not what those numbers are showing.


No, I think that if the ship traveled at light speed, the people in the ship would feel like no time had passed. If they accelerated to close to the speed of light, they would feel that little time had elapsed, as the chart shows.

henriquefd
2010-Jan-15, 10:54 AM
No, I think that if the ship traveled at light speed, the people in the ship would feel like no time had passed. If they accelerated to close to the speed of light, they would feel that little time had elapsed, as the chart shows.

But that means that the Distance is irrelevant and that it doesnt matter how far you go, if you are inside the Ship and if you travel at Lightspeed, the whole trip will only take a sneeze.

If that is so, why do we say the light from a star takes years to reach us, when in that case it would be almost instantaneous? Sure, from our perspective we could say it take years, the same way we can say it took years for the ship's roundtrip. But if that was so, shouldnt we be getting all the light of all the stars in the universe all at once in our sky?

Something is wrong.

grant hutchison
2010-Jan-15, 11:00 AM
I was under the impression that people inside the Ship would feel like 8,9 years had passed if the Ship travelled, at Lightspeed, through a distance of 8,9 light years.No, this is wrong. Your numbers look like they're telling you the right answer.
8.9 lightyears is what is measured by observers on Earth, who would indeed see the journey taking just a little more than 8.9 years if the ship travelled at very close to lightspeed.
But those aboard the ship see a length-contracted Universe, and measure the distance as much less than 8.9 lightyears. So they're not surprised to make the journey in much less than 8.9 years, as your figures show.

At unfeasibly extreme speeds, it would even be possible to travel millions of lightyears in just a few minutes of shipboard time, although the journey would still take millions of years as measured by those who stayed on Earth.


If that is so, why do we say the light from a star takes years to reach us, when in that case it would be almost instantaneous? Sure, from our perspective we could say it take years, the same way we can say it took years for the ship's roundtrip. But if that was so, shouldnt we be getting all the light of all the stars in the universe all at once in our sky?No, because it takes many years for the light to arrive in our frame of reference. Light takes a year to travel a lightyear, no matter who measures it. So it takes many years for light to reach us from a star that is many lightyears away. But for an (impossible) observer travelling with the photons, there is no distance between the star and our eyes: the journey is over in an instant. Relativity tells us that each point of view is internally consistent and equally valid.

Grant Hutchison

henriquefd
2010-Jan-15, 11:23 AM
I see...

So, instead of saying that the light takes 4,45 years to reach our eyes, shouldnt we be saying that we are the ones who have to wait 4,45 years for the light to instantly reach our eyes?

... my whole life I understood that with no problem and out of sudden I feel dumb again. *scratches head*

grant hutchison
2010-Jan-15, 11:29 AM
So, instead of saying that the light takes 4,45 years to reach our eyes, shouldnt we be saying that we are the ones who have to wait 4,45 years for the light to instantly reach our eyes?You could say that if you liked. But it wouldn't help very much. :)
It really does take the light 4.45 years to get here, in the reference frame we (more or less) share with the star emitting the light. That's a real value, with real implications. How long a photon thinks it takes to get here is irrelevant to us.

Grant Hutchison

henriquefd
2010-Jan-15, 11:38 AM
How about the aging of the ship crew? They would age as much time as it took them to reach their destination, right?

So, travelling at 0,999c to a distance of 8,9light years the ship would take 0,4 years to cover that distance and the ship crew would age 0,4 years also....right?

swampyankee
2010-Jan-15, 11:50 AM
How about the aging of the ship crew? They would age as much time as it took them to reach their destination, right?

So, travelling at 0,999c to a distance of 8,9light years the ship would take 0,4 years to cover that distance and the ship crew would age 0,4 years also....right?

Right. Biological processes, like aging, are affected by time dilation in exactly the same way as clocks. Were the crew of the space ship to send a live video feed to Earth it would look as if they were all moving very slowly.

henriquefd
2010-Jan-15, 12:02 PM
Ok. So, when the ship(travelling at 0,999c) arrives at a planet 8,9 light years away and sends a message back home(suppose there is a way to send the message at the speed of light) saying "WE DID IT!" that means that here on Earth we would only get that message 8,9 * 2 years after the ship left?

8,9 years for the ship to get to the planet and another 8,9 years for the message to arrive. So, to the ship crew, they got there in less than 5 months(0,4 years) while to us, here on Earth, we will only know that after 17,6 years! Is that it?

swampyankee
2010-Jan-15, 12:25 PM
Ok. So, when the ship(travelling at 0,999c) arrives at a planet 8,9 light years away and sends a message back home(suppose there is a way to send the message at the speed of light) saying "WE DID IT!" that means that here on Earth we would only get that message 8,9 * 2 years after the ship left?

8,9 years for the ship to get to the planet and another 8,9 years for the message to arrive. So, to the ship crew, they got there in less than 5 months(0,4 years) while to us, here on Earth, we will only know that after 17,6 years! Is that it?

More or less, yes. (well, 8.9*2 is 17.8, but I'm picky). As an aside, we routinely send messages at the speed of light, as all electromagnetic radiation, including radio, travels at the speed of light.

Presuming the spaceship's crew were able to keep track of their ship's speed, they would also know that their perceived time was different from the perceived time for observers on Earth: they would not be surprised.

henriquefd
2010-Jan-15, 12:29 PM
Ok. Thanks, I guess that answers all my questions. =)

01101001
2010-Jan-15, 02:24 PM
Something is wrong.

Yeah. Something is wrong. We don't have a stickied FAQ that just settles these very frequent issues about relativistic travel so people don't have to keep explaining it again and again. And again. Unless there are still some members who truly enjoy going all pedagogical on them.

It's not wrong that you asked, henriquefd. It's our collective fault for not anticipating your need for the answer and posting it in advance in a permanent prominent place.

Argos
2010-Jan-15, 02:31 PM
Something like 'time dilation FAQ'? Interesting idea.

Jeff Root
2010-Jan-15, 03:01 PM
There will always be new people along who will want to answer the old
questions. Maybe henriqedfd will want to answer this question when
someone asks it a year or two from now. The only reason I don't answer
it is that someone else always answers before I can.

-- Jeff, in Minneapolis

henriquefd
2010-Jan-15, 03:29 PM
I actually took the time to search the forum for threads about Time Dilation. I also searched the internet for explanations. I read the Twin paradox. I even got the formulas and built an Excel Sheet to try to see it better.

But sometimes the text is not enough. Sometimes we need to talk to someone else to understand. Because when you formulate a question, you do so antecipating an answer that could clear your doubts.

Maybe I am interpreting wrong what people are saying here, but it is leaving me the impression that some people here get upset over nothing. I'm just a dude asking a question and someone answered. Period. To me this thread could be closed since it has served its purpose. As I have stated. And right after people took the time, not to adress the OP, but to come after ME! Useless. Pointless.

Thanks very much for the kind sirs who took their time to teach me.

Maybe we will bump each other on another forum someday where I am the expert on the subject and you are the newbie. And your kindness shall be returned.

korjik
2010-Jan-15, 08:35 PM
I actually took the time to search the forum for threads about Time Dilation. I also searched the internet for explanations. I read the Twin paradox. I even got the formulas and built an Excel Sheet to try to see it better.

But sometimes the text is not enough. Sometimes we need to talk to someone else to understand. Because when you formulate a question, you do so antecipating an answer that could clear your doubts.

Maybe I am interpreting wrong what people are saying here, but it is leaving me the impression that some people here get upset over nothing. I'm just a dude asking a question and someone answered. Period. To me this thread could be closed since it has served its purpose. As I have stated. And right after people took the time, not to adress the OP, but to come after ME! Useless. Pointless.

Thanks very much for the kind sirs who took their time to teach me.

Maybe we will bump each other on another forum someday where I am the expert on the subject and you are the newbie. And your kindness shall be returned.

If we seem a little grumpy about your question, it is because every other person who has asked this same question has been completely unable to accept the answer. It ends up with a long pointless discussion, and alot of times the asker banned.

Henriquefd, you are by far the most reasonable person I have seen ask that question, and the first to understand it. If we seem grumpy, I apologize. You have been a breath of fresh air to all us cynical old physicists who lurk here to ambush people who want to dictate and not learn.

So as far as I am concerned, if you have more questions, ask away. It will be nice to teach to someone who is willing to learn.

:)

Ken G
2010-Jan-15, 08:38 PM
So, instead of saying that the light takes 4,45 years to reach our eyes, shouldnt we be saying that we are the ones who have to wait 4,45 years for the light to instantly reach our eyes?
The interesting thing about asking a question that has been asked many times is that often there remain subtleties in the answers that do not come out even after 100 times the question is asked and answered. This is one of those times. I think a useful thing to recognize about special relativity is that it provides a convention for saying how long the light took "in our frame of reference" (say, 4.45 years), but this is purely a convention. There really is "no such thing" as an absolute answer to that question, other than zero time. That's called the "proper time" the process takes, which is the time in the frame of the light, and that is the sole time that is authoritative about how long that process takes-- all other times are 100% purely a matter of language convention. So when we say "in our reference frame it takes 4.45 years", that is not conveying any physical meaning at all, it is merely a description of our own language about the light travel process, which mixes in the distance crossed (another arbitrary convention of how we set up space and time). I believe that your comment above comes in partial recognition of this fact, it all relates to the conveniences of language and not the laws of physics themselves (which are best expressed in invariant form and need no such conventions about "frames of reference").

George
2010-Jan-15, 08:55 PM
I believe that your comment above comes in partial recognition of this fact, it all relates to the conveniences of language and not the laws of physics themselves (which are best expressed in invariant form and need no such conventions about "frames of reference"). I wonder if there is a unique case against invariance for light, only because zero time, as it relates to motion, seems problematic, at best? Just a quirky thought.

stutefish
2010-Jan-15, 11:27 PM
One enjoyable side effect of time dilation is that it ends up being a plot point in science fiction stories. Joe Haldeman's The Forever War, for example: Starfaring soldiers are able to carry out in a single lifetime a war that lasts for generations back home.

PhillipJFry
2010-Jan-16, 03:08 AM
De-lurking here. I hope it's okay if I sneak in a question.

What would the passengers travelling close to c observe when they look at Earth? Would they see everything moving in a frantic pace, same as a movie on fast-forward?

Ken G
2010-Jan-16, 04:17 AM
I wonder if there is a unique case against invariance for light, only because zero time, as it relates to motion, seems problematic, at best? Just a quirky thought.
It is true that the "frame" of light is singular and could not be a real observer, but this presents no particular problem because we simply imagine a series of observers who are moving closer and closer to the speed of light (relative to some original observer). We can get the proper time elapsed, and the proper distance between the events, to be arbitrarily small without any singularities (though it is highly hypothetical of course, no such observers really exist). Such observers are only ruled out of "our reference frame" by the conventions of special relativity (which choose observers with no velocity relative to us to convey the meaning of "our reference frame" even though such observers have no special physical relationship to us that is not purely a convention of language). The larger point I mean to make is that a "frame of reference" should physically be regarded as something entirely local to the observer-- any effort to extend such a thing globally is sheer convention, and no law of physics cares what convention is in use for doing that as long as the conventions used respect the actual constraints applied by those laws.

Ken G
2010-Jan-16, 04:22 AM
What would the passengers travelling close to c observe when they look at Earth? Would they see everything moving in a frantic pace, same as a movie on fast-forward?It would depend on if they were moving away or toward Earth. In that case, what they see would involve two different things-- one is the time dilation on Earth (which seems to slow time on Earth for either direction of travel), and the other is the changing time-of-flight between successive frames of the "movie" (which seems to slow time even more if we are watching a movie from Earth while we are moving away from Earth, but seems to speed up time if we are moving toward Earth). Mixing the time dilation and the time-of-flight illusions is of dubious value in relativity, as it causes a lot of confusion when you mix these two wholly different things. Still, you can always ask the question, and it works out that things look very slowed when we are moving away, and speeded up on the return, and in the net more time elapses on Earth than for the passengers (as in the twin "paradox").

Jeff Root
2010-Jan-16, 06:01 AM
Mixing the time dilation and the time-of-flight illusions ...
I see no reason to call either of them "illusions". They are both real
geometric effects of relative motion.



... is of dubious value in relativity, as it causes a lot of confusion
when you mix these two wholly different things.
Since they always appear together, never just one or the other,
unmixing them takes more effort than leaving them mixed. We need
to always distinguish between the two, but we don't always need to
separate them, since they can't ever actually be separated.



Still, you can always ask the question, and it works out that things
look very slowed when we are moving away, and speeded up on the
return, and in the net more time elapses on Earth than for the
passengers (as in the twin "paradox").
Since the net effect is an overall speeding up, it seems backwards
to say it is "very" slowed when moving away, but just plain speeded
up on the return. If one wants the adverb, it is the speeding up.

-- Jeff, in Minneapolis

cjameshuff
2010-Jan-16, 07:30 AM
I see no reason to call either of them "illusions". They are both real geometric effects of relative motion.

The time-of-flight doppler effect is certainly an illusion. It occurs in Newtonian models and to different degrees in other waves moving at different speeds. It occurs even if you use Newtonian physics with an unambiguous rest frame and global absolute time, removing all possibility for it being a real effect. In contrast, time dilation is a real effect, not an illusory one...different observers really experience different amounts of time relative to each other that depend on relative motion.

Your claim that they always occur together is incorrect. The counterexample is simple: one object moving in a circular path around another can maintain a constant distance, with zero doppler effect, while having a high relative velocity and noticeable time dilation effects. They are two very distinct effects, one illusory and one real.

Jeff Root
2010-Jan-16, 08:56 AM
The time-of-flight doppler effect is certainly an illusion.
No it isn't. You can call it an "illusion" if you like, and you can think
of it as an "illusion" if you like, but doing so doesn't gain you anything.
It just makes the explanation slightly less straightforward, and
understanding slightly more difficult.



It occurs in Newtonian models and to different degrees in other waves
moving at different speeds.
Of course.



It occurs even if you use Newtonian physics with an unambiguous
rest frame and global absolute time,
Yes.



removing all possibility for it being a real effect.
Huh??? It always occurs, therefore it isn't real??? What???



In contrast, time dilation is a real effect, not an illusory one...
They are both real.



Your claim that they always occur together is incorrect. The
counterexample is simple: one object moving in a circular path
around another can maintain a constant distance, with zero
doppler effect, while having a high relative velocity and noticeable
time dilation effects.
You have to drag in accelerated circular motion to do it. This might
apply to a satellite in geostationary orbit communicating with a ground
station. I can't think of any other case where it would apply.

-- Jeff, in Minneapolis

cosmocrazy
2010-Jan-16, 12:13 PM
Yeah. Something is wrong. We don't have a stickied FAQ that just settles these very frequent issues about relativistic travel so people don't have to keep explaining it again and again. And again. Unless there are still some members who truly enjoy going all pedagogical on them.

It's not wrong that you asked, henriquefd. It's our collective fault for not anticipating your need for the answer and posting it in advance in a permanent prominent place.

This is fine, but then it becomes no different than wiki. Sometimes to understand an answer a person may require different ways of explanation. The beauty of asking a question on BAUT is that at some point someone will post an answer that clicks in the questioner's mind. This may take several different ways of giving the same answer.

I would not tire of answering the same question over and over if the person seeking is willing to accept and learn from the mainstream that they read or hear.

George
2010-Jan-16, 04:10 PM
It is true that the "frame" of light is singular and could not be a real observer, but this presents no particular problem because we simply imagine a series of observers who are moving closer and closer to the speed of light (relative to some original observer). Ah, that makes sense. I hadn't considered good-ole calculus....

Lim
v -> c
:)



I see no reason to call either of them "illusions". They are both real geometric effects of relative motion. If a spaceship takes both 8 years and 10 years to make the same trip, is that illusion or real, or something in between? It's really illusive either way. ;)

Ken G
2010-Jan-16, 05:38 PM
I see no reason to call either of them "illusions". They are both real
geometric effects of relative motion.And desert mirages are real geometric effects of light refraction in hot air. Nevertheless, we term it an "illusion" because the literal interpretation of what we see is not actually occuring, we are led to a false conclusion. Time of flight effects are quite similar-- if you watch a drive-in movie from a car that is racing at absurdly high speed away from the movie, it will look like the movie is happening in slow motion. Part of that will be because of time-of-flight effects, and that is an illusion-- if you fail to correct for your motion, you conclude something which is not in fact true. However, there is also a time dilation effect (which shows up only at the fastest speeds), and that appears even after you correct for your motion. There is no way to make the time dilation "go away", it is fundamental to the conventions of our global concept of time. Hence, we often say, and quite correctly, that time of flight effects are illusions, but time dilation is not. Indeed, understanding this key difference is often the first breakthrough in any student's understanding of relativity.



Since they always appear together, never just one or the other,
unmixing them takes more effort than leaving them mixed. No, it is quite common for early students of relativity to face extreme confusion, and inability to reach correct results, when these two effects are mixed in their minds. Unmixing them is often a very important step to understanding relativity. For example, Lorentz transformations say nothing at all about the time-of-flight issues, and they are the crux of relativistic problem solutions.


We need
to always distinguish between the two, but we don't always need to
separate them, since they can't ever actually be separated.
Odd that you would claim that-- Lorentz transformations separate them quite matter-of-factly. (And cjameshuff makes a good point about circular motion.)


Since the net effect is an overall speeding up, it seems backwards
to say it is "very" slowed when moving away, but just plain speeded
up on the return. If one wants the adverb, it is the speeding up.
Actually, I just edited out the "very" to reduce hype, but didn't catch it's other appearance. In fact, the factor by which the movie appears slowed on the way out is the same as the factor by which it appears sped up on the way back. That factor is sqrt[(1-v/c)/(1+v/c)], just as in the Doppler formula, and reversing the sign of v just inverts the factor. So in terms of factors, the adverb belongs in either place equally.

hhEb09'1
2010-Jan-16, 05:49 PM
Your claim that they always occur together is incorrect. The counterexample is simple: one object moving in a circular path around another can maintain a constant distance, with zero doppler effect, while having a high relative velocity and noticeable time dilation effects. They are two very distinct effects, one illusory and one real.There is an effect called the relativistic doppler, but we don't have to use that term.

I do detect a spillover from the language threads which argued about whether certain "languages" were "real" or not. It's a semantic difference that is unresolvable, unless we admit that there are two different concepts of "real" in play here.

Ken G
2010-Jan-16, 05:51 PM
You have to drag in accelerated circular motion to do it. This might
apply to a satellite in geostationary orbit communicating with a ground
station. I can't think of any other case where it would apply.
Think harder. Perfectly inertial motion that is instantaneously at a right angle to the direction to the observer exhibits, instantaneously, time dilation with no time-of-flight effects. This is sometimes called the "second order Doppler effect." Note also that the "illusion" of time-of-flight effects I was referring to had to do with watching a movie and using what you see to infer how fast time is elapsing in the movie theater from your own perspective. Regular Doppler shifts also involve time-of-flight effects, and they are not considered illusions because they indicate relative motion in a completely correct way. The false conclusion that I termed an illusion had to do with mixing time-of-flight and time dilation when one is inferring how fast time is elapsing at the source, compared to your own sense of global time (which is itself just a convention, but that's a whole other issue). Each of these points is an important step in understanding relativity.

Jeff Root
2010-Jan-16, 11:24 PM
If a spaceship takes both 8 years and 10 years to make the same trip,
is that illusion or real, or something in between?
The fact that the spaceship takes 8 years to make the trip is real
to some people, and the fact that it takes 10 years is real to some
other people. The idea that the spaceship takes two different times
to make the same trip is an illusion. The illusion could be caused by
one person having a faulty clock and not realizing it.

Something like this: http://www.freemars.org/jeff2/gold.htm

-- Jeff, in Minneapolis

George
2010-Jan-17, 12:39 AM
Something like this: http://www.freemars.org/jeff2/gold.htm (http://www.freemars.org/jeff2/gold.htm)
:) In a job interview, just out of college, an owner said that his shop was cutting one inch off of 11' x 9' steel plating, and welding that inch along the side to give them 10' x 10' plating, thus gaining the extra area. He looked serious and I wasn't sure whether he was testing me or was mentally affected by his age. :)

cjameshuff
2010-Jan-17, 12:58 AM
It's not a case of time being relative to the observer, with different observers making different measurements. Take an electric train trundling along some tracks here on Earth. It will have observable doppler effects in its electromagnetic radiation, in vibrations transmitted through the air, in vibrations transmitted along the rails, in vibrations transmitted through the ground, in electrical noise transmitted along the rails...each route of signal transmission having a different propagation velocity.

If these are modulated with an onboard clock, a single observer will observe that one clock to be ticking at a wide variety of rates. There can be no doubt about this being an illusion, it is clear that the clock is not ticking at all the various rates observed. In reality, the clock is ticking at one definite rate in the observer's reference frame, and all the various doppler effects are illusory. To measure the actual rate of the clock, the doppler effects resulting from varying time of flight must be accounted for, leaving the real time dilation.

I have no idea why you're bringing faulty clocks into this. Measurement error is a completely separate issue, and one that is irrelevant to this discussion.

Ken G
2010-Jan-17, 02:40 AM
If these are modulated with an onboard clock, a single observer will observe that one clock to be ticking at a wide variety of rates. There can be no doubt about this being an illusion, it is clear that the clock is not ticking at all the various rates observed. Yes, that's a good way to see that when we see/hear/feel about the rate that some distant clock is ticking can have illusory elements, which must be clearly separated from time dilation. Another way to say it is that even if there was no such thing as light, so no such thing as light time-of-flight effects, there would still be time dilation. The same is true if we had no way to observe light, if one objects to getting rid of it altogether-- there'd still be a twin "paradox" all the same.

StrangeBrew
2010-Jan-17, 04:42 AM
Ok, just to make sure I am not confused on this.

If a craft is traveling at the speed of light, and you are on that craft, you dont experince the same amount of time as say an observer not on that craft would. So traveling to A. Centauri (4.3 LY) would not take the 4.3 years that the outside observer would percive.

Also all your biological process's would experince time dilation the same, so you would not age at the same rate as the observer etc..

I just always assumed that even if you were on a ship capable of light speed it would take the the quoted amount of travel time to reach your destination. You learn something new every day.

I do have a question though and i dont mean to hijack the thread. How would you calculate the travel time for the individual traveling on the ship E.G. Days, hours, minutes?

cosmocrazy
2010-Jan-17, 11:32 AM
Ok, just to make sure I am not confused on this.

If a craft is travelling at the speed of light, and you are on that craft, you dont experience the same amount of time as say an observer not on that craft would. So travelling to A. Centuri (4.3 LY) would not take the 4.3 years that the outside observer would perceive.

Also all your biological process's would experience time dilation the same, so you would not age at the same rate as the observer etc..

I just always assumed that even if you were on a ship capable of light speed it would take the the quoted amount of travel time to reach your destination. You learn something new every day.

I do have a question though and i dont mean to hijack the thread. How would you calculate the travel time for the individual travelling on the ship E.G. Days, hours, minutes?

If you are on a craft thats capable of light speed then for you on that craft every journey you made would take what ever time & distance you measured to reach "C" once the craft reached C the journey would be over for you regardless of the initial journey's distance. At "C" time and distance becomes zero, a photon for example feels no time & distance travelled and as a result doesn't age either. This would be the same for anything travelling at C.
For any other observer they would still measure you to be travelling at the speed of light for what ever given distance the journey's start and end point was. So if that observer was here on Earth they would measure the journey to have taken the 4.3 years for you to get to Alfa Centuri. For you on board the craft you would measure the time and distance it took to accelerate to C then that would be it journey over. :)

Andrew D
2010-Jan-17, 02:17 PM
So if time of flight causes the "drive in" to play more slowly, and time dilation causes the time in the vehicle to pass more slowly, aren't the observers in the vehicle able to watch the film as if it were playing normally, because they experience it more slowly than the audience at the drive in?

Ken G
2010-Jan-17, 06:58 PM
So if time of flight causes the "drive in" to play more slowly, and time dilation causes the time in the vehicle to pass more slowly, aren't the observers in the vehicle able to watch the film as if it were playing normally, because they experience it more slowly than the audience at the drive in?No. First of all, the observers in the vehicle think time is going more slowly in the theater, not on their ship. So the time-of-flight effects augment the slowing down effect (if the ship is moving away).

cjameshuff
2010-Jan-17, 08:22 PM
No. First of all, the observers in the vehicle think time is going more slowly in the theater, not on their ship. So the time-of-flight effects augment the slowing down effect (if the ship is moving away).

To clarify: for closing motion, the Doppler and time dilation effects do partially cancel out, but not completely. The relativistic Doppler effect (combining the two) is still only zero at zero relative velocity. sqrt((1 - v/c)/(1 + v/c)) as opposed to c/(c + v).

Black line is simple Doppler effect, red is relativistic Doppler effect, x axis being velocity away from the observer in units of c (negative for closing velocity, motion assumed to be radial):
http://arklyffe.com/graph.html?x=-1.5,3&y=-0.25,5&xg=1,10,2&yg=1,0,2&fn0=1LE1AHAxF&fn1=sqrtEE1AJAxFLE1AHAxFF

If you're not moving on a line that goes through the theater, then non-relativistic doppler will drop to nothing with no change in time dilation as you pass by, and they would briefly cancel out before that point despite the non-zero relative velocity. You can't keep them canceled out without accelerating, however.

WayneFrancis
2010-Jan-18, 03:09 AM
I see...

So, instead of saying that the light takes 4,45 years to reach our eyes, shouldnt we be saying that we are the ones who have to wait 4,45 years for the light to instantly reach our eyes?

... my whole life I understood that with no problem and out of sudden I feel dumb again. *scratches head*

Correct, on your first part. You've worked out the right mechanics on your own. Very good.

One thing not to get hung up on is that distance has no meaning to a photon. IE in a photon's frame of reference distance is undefined and thus has no meaning. Since distance has no meaning time also has no meaning. If you could travel at the speed of light you could never get to your destination...ie you wouldn't know when to stop because there is no time relative to the rest of the universe, for you. But since we have mass and we can never get to that speed we'll have no problem with this.

clint
2010-Jan-18, 12:17 PM
If we seem a little grumpy about your question, it is because every other person who has asked this same question has been completely unable to accept the answer. It ends up with a long pointless discussion, and alot of times the asker banned.

Henriquefd, you are by far the most reasonable person I have seen ask that question, and the first to understand it. If we seem grumpy, I apologize. You have been a breath of fresh air to all us cynical old physicists who lurk here to ambush people who want to dictate and not learn.

So as far as I am concerned, if you have more questions, ask away. It will be nice to teach to someone who is willing to learn.

:)

To all those frustrated physicists:
even when reading this kind of thread for the 100th time, I always enjoy the competent answers and usually learn something new.

By the way, even when - in other occasions - the poster is 'unreasonable' and obstinate,
the answers often help my understanding of the subject all the same (actually, sometimes more than in a 'reasonable' discussion)

Kind regards (and thank you for your patience),

A frequent fence sitter :)

NorthernBoy
2010-Jan-18, 03:59 PM
I can't think of any other case where it would apply.

-- Jeff, in Minneapolis

But as you are aware that an argument from ignorance is really no argument at all, hopefully you'd never go so far as to post it on a website as a justification for anything...

WayneFrancis
2010-Jan-19, 02:15 AM
To all those frustrated physicists:
even when reading this kind of thread for the 100th time, I always enjoy the competent answers and usually learn something new.

By the way, even when - in other occasions - the poster is 'unreasonable' and obstinate,
the answers often help my understanding of the subject all the same (actually, sometimes more than in a 'reasonable' discussion)

Kind regards (and thank you for your patience),

A frequent fence sitter :)

Agreed, every explanation, unless just cut and pasted, tends to be slightly different and you never know what is going to make something click in someone's brain. There have been a few time here where I thought I understood something to find out that I was close but not completely right.

It is refreshing to see someone try to work out something see their work doesn't match their current understanding then have them see that their work is right and that they adjust their understanding.

forrest noble
2010-Jan-19, 03:14 AM
George, your posting #27


If a spaceship takes both 8 years and 10 years to make the same trip, is that illusion or real, or something in between? It's really illusive either way.

According to the chart at roughly a speed of .15c, time on the ship would pass at a rate of about 80% the rate as the time passes on Earth, 8 years for those in the ship and 10 years for those on Earth. Those on the ship would probably accept that their time is being dilated (slowed down). So for probably all concerned the "correct time" would be Earth standard time whereby the flight time lasted 10 years. Those in the ship would probably just be happy that they only aged 8 years.

Special Relativity however makes no such distinction as to one time frame being more valid or real than another.

RussT
2010-Jan-19, 12:12 PM
Correct, on your first part. You've worked out the right mechanics on your own. Very good.

One thing not to get hung up on is that distance has no meaning to a photon. IE in a photon's frame of reference distance is undefined and thus has no meaning. Since distance has no meaning time also has no meaning. If you could travel at the speed of light you could never get to your destination...ie you wouldn't know when to stop because there is no time relative to the rest of the universe, for you. But since we have mass and we can never get to that speed we'll have no problem with this.

in a photon's frame of reference distance is undefined and thus has no meaning.

I agree.

SO, if it is undefined, why would anyone think that a 'definition' of 0 is meaningful?

IE: it is a fact that SN 1987A is in the Large Magelanic Cloud (LMC), which is approximately 160,000 light years distant from Earth (And was also this approximate distance 160,000 years ago!)....so how did those photons, in their own frame, get from there to here in 0 time?

For the avoidance of doubt....these are "Questions"...not statements. IF Relativity is correctly modeling how the Universe(s) is working, it should be able to answer the tough questions, right?

Feel free to consult your son on this.

Ken G
2010-Jan-19, 02:15 PM
SO, if it is undefined, why would anyone think that a 'definition' of 0 is meaningful?
The relevant "definitions" there are simply that of how to measure time and distance, that's all. Given those, all observers, be they stationary, moving, accelerating, or standing on their head, can then take their measurements and make a simple mathematical construct that has the spirit of a kind of quantitative separation between the emission and absorption events for the light. Guess what they will all come up with? Zero. No other result will be the same for all of them, it is the invariant. We don't define it to be the invariant, we simply find that it is the invariant. It is the only number that is objectively determined, all the rest depend on frame conventions-- including the "distance to the supernova."

IE: it is a fact that SN 1987A is in the Large Magelanic Cloud (LMC), which is approximately 160,000 light years distant from Earth (And was also this approximate distance 160,000 years ago!)....so how did those photons, in their own frame, get from there to here in 0 time?How did anything happen? It just did.

George
2010-Jan-19, 03:11 PM
George, your posting #27:

According to the chart at roughly a speed of .15c, time on the ship would pass at a rate of about 80% the rate as the time passes on Earth, 8 years for those in the ship and 10 years for those on Earth. Those on the ship would probably accept that their time is being dilated (slowed down). So for probably all concerned the "correct time" would be Earth standard time whereby the flight time lasted 10 years. Those in the ship would probably just be happy that they only aged 8 years.

Special Relativity however makes no such distinction as to one time frame being more valid or real than another. Agreed, but to anyone who is unfamiliar with SR, it is a clear paradox; 8 years can not be 10 years. This is why the title "Twin Paradox" is so popular, though it is not actually a bonefied paradox. Variable time does not have that touchy feelyness that we all like, so experience works against us at first. This is why the right teaching approach requires some attention in order to prevent getting stuck in the muddy paradox puddle.

NorthernBoy
2010-Jan-19, 03:35 PM
Agreed, but to anyone who is unfamiliar with SR, it is a clear paradox; 8 years can not be 10 years. This is why the title "Twin Paradox" is so popular, though it is not actually a bonefied paradox.

Bonefied? Is that some kind of sexual reference?

Strange
2010-Jan-19, 04:33 PM
IE: it is a fact that SN 1987A is in the Large Magelanic Cloud (LMC), which is approximately 160,000 light years distant from Earth (And was also this approximate distance 160,000 years ago!)....so how did those photons, in their own frame, get from there to here in 0 time?

Because, in their own frame, the distance is zero as well?

forrest noble
2010-Jan-19, 09:24 PM
George,


Agreed, but to anyone who is unfamiliar with SR, it is a clear paradox; 8 years can not be 10 years. This is why the title "Twin Paradox" is so popular, though it is not actually a bonefied paradox. Variable time does not have that touchy feelyness that we all like, so experience works against us at first. This is why the right teaching approach requires some attention in order to prevent getting stuck in the muddy paradox puddle.

Agreed!! Muddy-puddle insights often mess up a lot of the understandings of science.

pzkpfw
2010-Jan-19, 10:06 PM
Because, in their own frame, the distance is zero as well?

This has struck me quite strongly.

It seems...

From the point of view of light, it takes zero time to "get" from anywhere to anywhere, because, as far as any light in the Universe is concerned the distance between any two points is zero.

That is, as far as light is concerned, the entire Universe may as well all be in one place.

(A singularity?)

IsaacKuo
2010-Jan-19, 10:30 PM
The distance is zero only in the direction of spatial compression. So, not the entire Universe is in one place, just the bit of the universe directly along the photon's path.

Really, it doesn't make much sense to talk about the frame of reference of a photon because you get into "divide by zero" or 0/0 weirdness. But you could look instead at a near-c frame of reference. So, a really fast cosmic ray would see the universe as very very flat along the direction of relative motion. The universe as a whole is still vast, but not along the compressed dimension.

forrest noble
2010-Jan-19, 11:07 PM
pzkpfw,


This has struck me quite strongly.
It seems...
From the point of view of light, it takes zero time to "get" from anywhere to anywhere, because, as far as any light in the Universe is concerned the distance between any two points is zero.

That is, as far as light is concerned, the entire Universe may as well all be in one place. (A singularity?)

Granted, it's an interesting idea but as you know (1) light seemingly could not have its own time frame in Special Relativity, and according to Special and General Relativity matter cannot travel at the speed of light. So that idea is seemingly a non-utilitarian perspective; (2) light (EM radiation) also disappears into the ZPF over time (fades out) so that there is a limit to its distance extension.

henriquefd
2010-Jan-19, 11:39 PM
Thank you all for your replies. I guess this would be easier for me if I was back in school. It is much harder to learn (and remember) so much at this stage of my life. 32 years old, working my butt everyday to try to get a promotion, getting married in a few months, buying a new apartment, travelling a lot... you know. life.

But I really wish to learn at least the basics of astronomy. It's a goal I have set for myself and I will do it! With your help, that is. =)

korjik
2010-Jan-20, 12:04 AM
This has struck me quite strongly.

It seems...

From the point of view of light, it takes zero time to "get" from anywhere to anywhere, because, as far as any light in the Universe is concerned the distance between any two points is zero.

That is, as far as light is concerned, the entire Universe may as well all be in one place.

(A singularity?)

Since a singularity is a place where an equation gets a divide by zero error, yes, it is a singularity.

forrest noble
2010-Jan-20, 12:23 AM
henriquefd,


Thank you all for your replies. I guess this would be easier for me if I was back in school. It is much harder to learn (and remember) so much at this stage of my life. 32 years old, working my butt everyday to try to get a promotion, getting married in a few months, buying a new apartment, traveling a lot... you know. life.

But I really wish to learn at least the basics of astronomy. It's a goal I have set for myself and I will do it! With your help, that is. =)


There's a lot of great stuff to read here on BAUT. Astronomy is often mixed with cosmology and related physics so you will probably learn a lot more than just astronomy. You also have to do research on the web if there are no links cited for what is being said, since there is often more than one perspective/ opinion of what is being discussed, and the poster may not have explained the subject properly.

Maybe a good starter would be to start with an elementary book on astronomy. With a high-school level or higher understanding of astronomy, you will be able to learn a lot more material contained in these postings.

WayneFrancis
2010-Jan-20, 06:32 AM
If you are on a craft thats capable of light speed then for you on that craft every journey you made would take what ever time & distance you measured to reach "C" once the craft reached C the journey would be over for you regardless of the initial journey's distance. At "C" time and distance becomes zero, a photon for example feels no time & distance travelled and as a result doesn't age either. This would be the same for anything travelling at C.
For any other observer they would still measure you to be travelling at the speed of light for what ever given distance the journey's start and end point was. So if that observer was here on Earth they would measure the journey to have taken the 4.3 years for you to get to Alfa Centuri. For you on board the craft you would measure the time and distance it took to accelerate to C then that would be it journey over. :)

Only one more problem with travelling at c...if you could get to c then their is the problem then you wouldn't know when to stop or where to stop since those 2 concepts are not valid at c. IE you'd overshoot your destination by infinity if you didn't crash into something and if you crashed into something...well actually you'd just plough through and destroy what ever you hit.

WayneFrancis
2010-Jan-20, 06:37 AM
Agreed, but to anyone who is unfamiliar with SR, it is a clear paradox; 8 years can not be 10 years. This is why the title "Twin Paradox" is so popular, though it is not actually a bonefied paradox. Variable time does not have that touchy feelyness that we all like, so experience works against us at first. This is why the right teaching approach requires some attention in order to prevent getting stuck in the muddy paradox puddle.

I keep telling my friend this. He gets hung up on the title "paradox" and insists that it modern science has a problem with it. He stubbornly ignores the answer and the explanation that it is not really a paradox if you know the answer.

WayneFrancis
2010-Jan-20, 06:41 AM
pzkpfw,



Granted, it's an interesting idea but as you know (1) light seemingly could not have its own time frame in Special Relativity, and according to Special and General Relativity matter cannot travel at the speed of light. So that idea is seemingly a non-utilitarian perspective; (2) light (EM radiation) also disappears into the ZPF over time (fades out) so that there is a limit to its distance extension.

Ummm what is this #2 you are talking about. This sounds like tired light and tired light is debunked given current observations.

forrest noble
2010-Jan-20, 07:07 AM
WayneFrancis,

my quote:


(2) light (EM radiation) also disappears into the ZPF over time (fades out) so that there is a limit to its distance extension.


Ummm what is this #2 you are talking about. This sounds like tired light and tired light is debunked given current observations.

No, nothing to do with tired light. Light fades in intensity according to the inverse square law until it is non-existent, like shining a small flashlight into the night sky from orbit. It wouldn't go very far outward into space until the light would no longer exist as a wave.

grant hutchison
2010-Jan-20, 07:46 AM
No, nothing to do with tired light. Light fades in intensity according to the inverse square law until it is non-existent, like shining a small flashlight into the night sky from orbit. It wouldn't go very far outward into space until the light would no longer exist as a wave.Just as well it's a particle of infinite range, then. :)

Grant Hutchison

Jeff Root
2010-Jan-20, 08:02 AM
forrest,

The light doesn't cease to exist. The photons get farther and farther apart.
If they aren't absorbed, the number of photons going out in all directions from
a flashbulb will be the same at ten billion light-years as at ten centimetres.

-- Jeff, in Minneapolis

RussT
2010-Jan-20, 09:51 AM
Only one more problem with travelling at c...if you could get to c then their is the problem then you wouldn't know when to stop or where to stop since those 2 concepts are not valid at c. IE you'd overshoot your destination by infinity if you didn't crash into something and if you crashed into something...well actually you'd just plough through and destroy what ever you hit.



Originally Posted by RussT
in a photon's frame of reference distance is undefined and thus has no meaning.

I agree.

SO, if it is undefined, why would anyone think that a 'definition' of 0 is meaningful?

IE: it is a fact that SN 1987A is in the Large Magelanic Cloud (LMC), which is approximately 160,000 light years distant from Earth (And was also this approximate distance 160,000 years ago!)....so how did those photons, in their own frame, get from there to here in 0 time?

For the avoidance of doubt....these are "Questions"...not statements. IF Relativity is correctly modeling how the Universe(s) is working, it should be able to answer the tough questions, right?

Feel free to consult your son on this.

IE you'd overshoot your destination by infinity

Okay...same question here...

in a photon's frame of reference distance is undefined and thus has no meaning.

IF, the time and distance "In the photons frame" are Undefined, why has there been an assigned 'speed' of either 0 OR Infinity to the photons in their own frame???

Photons by defintion are constant at 186,282.397mps in Vacua.

I mean those photons from SN 1987A really did take ~160,000 years to reach us, right?

Strange
2010-Jan-20, 10:54 AM
IF, the time and distance "In the photons frame" are Undefined, why has there been an assigned 'speed' of either 0 OR Infinity to the photons in their own frame???

I think that, as others have pointed out, you cannot formally (mathematically) refer to the photons' frame of reference (because you get into all that divide by zero stuff). But, by analogy with objects that are approaching the speed of light, you can use the zero time/distance thing as an informal explanation of what is happening. It may not be strictly correct, but it may help (some) people understand.

Note: I don't think anyone has assigned 'speed' of either 0 OR Infinity to the photons in their own frame, just time and/or distance. I suppose one should think of them still travelling at c in their own frame ... except that the math won't work because you will end up dividing by zero.

NorthernBoy
2010-Jan-20, 10:58 AM
No, nothing to do with tired light. Light fades in intensity according to the inverse square law until it is non-existent

You don't really believe that, I think, but if you genuinely do, here's an experiment that will show you that your assumption is wrong.

Get a laser pointer, and shine it on a piece of card 10cm away from where the light emerges, and then point it at a piece of card 100 metres away from whee the light emerges.

If your claim were true, the light would be one millionth as intense in the second case, or, essentially unobservable. In reality, for any decently collimated beam, you'll see that there is still quite a well defined point, and that it is still pretty bright.

You will therefore have proven, for yourself, that light intensity does not necessarily fall off as you thought.

Your "flashlight" is less collimated than a laser, but still does not follow an inverse square law, especially in the near field, and as others have pointed out, even sources which do simply have their photons spread more widely, they never just run out of breath and stop.

Jeff Root
2010-Jan-20, 12:52 PM
No speed is "assigned" to a photon in its own reference frame, because a
photon has no sensible reference frame. To a photon, every time is zero,
and every distance is zero. From a photon's point of view, nothing exists
and nothing happens. The result is that a photon has no point of view.

That was interesting for about five minutes when I first learned of it.

On the other hand, someone travelling at the speed of light would have
no more trouble stopping when he reaches his destination than a radio
signal has stopping when it reaches its destination. As long as he hits
his target, and the receiver is turned on and tuned in, he will be fine.

-- Jeff, in Minneapolis

Grey
2010-Jan-20, 04:46 PM
Only one more problem with travelling at c...if you could get to c then their is the problem then you wouldn't know when to stop or where to stop since those 2 concepts are not valid at c. IE you'd overshoot your destination by infinity if you didn't crash into something and if you crashed into something...well actually you'd just plough through and destroy what ever you hit.Amusingly, photons themselves always use exactly this method of not overshooting their target. That is, they are created by some process, and then travel at light speed until they run into something, and then they're absorbed. It's okay that a photon can't know when to stop, because it always just keeps going until it hits a wall. ;)



IF, the time and distance "In the photons frame" are Undefined, why has there been an assigned 'speed' of either 0 OR Infinity to the photons in their own frame???They aren't. As has been pointed out many times, the reference frame of a photon isn't really a valid reference frame in special relativity. No inertial observer can be travelling at such a speed relative to any other inertial observer, not even hypothetically, and the equations that tell you what distances and times such an observer would measure break down. We don't really assign any kind of speed to a photon "in it's own reference frame", because we never try to use the reference frame of a photon to work out the expected results of any experiment. That's largely because, if you try to do so, it won't work.

It is true that if you really want to imagine such a reference frame, you can take the limit as velocity approaches the speed of light, assume that it's meaningful to do that, and find amusing things like the photon travelling zero distance in zero time. But since you know in advance that it's not even theoretically possible for an observer to be using that reference frame, it really shouldn't surprise you that you get somewhat nonsensical results.



Photons by defintion are constant at 186,282.397mps in Vacua.Certainly. And that appears to hold for any valid reference frame.


I mean those photons from SN 1987A really did take ~160,000 years to reach us, right?Yes they did. Using our reference frame to measure the time, at least, and that's as reasonable a choice of reference frame as any. Someone moving with respect to us during that transit might choose a different (equally valid) reference frame, and would measure a different travel time. No real observer anywhere, no matter how they are moving with respect to any other observer, would ever measure a time of zero.

NorthernBoy
2010-Jan-20, 06:41 PM
On the other hand, someone travelling at the speed of light would have no more trouble stopping when he reaches his destination than a radio signal has stopping when it reaches its destination

A note to others who may be reading this, Jeff is again playing his game of either trying too hard to be clever with semantics (instead of just posting his point clearly) or of posting very ATM claims and then refusing to accept that they are ATM. The mainstream view is that a person cannot travel at c, no matter how much energy is supplied.

(Studying the form guide, I predict that we'll see a denial that he is proposing an ATM view here...)

Strange
2010-Jan-20, 06:49 PM
A note to others who may be reading this, Jeff is again playing his game of either trying too hard to be clever with semantics (instead of just posting his point clearly) or of posting very ATM claims and then refusing to accept that they are ATM. The mainstream view is that a person cannot travel at c, no matter how much energy is supplied.

I read his comment as just meaning that "someone" would have to be a photon.

Did you delete a previous comment about accelerating "mater" to almost the speed of light? I thought that was quite amusing; I wondered if she minded... (but maybe I imagined it)

NorthernBoy
2010-Jan-20, 06:57 PM
I read his comment as just meaning that "someone" would have to be a photon.

Yes, it's possible that he's playing word games when clarity would be a far better option. People are not photons, and there are plenty of lurkers who would be thrown by the suggestion that "people" can travel at c.

On the other hand, he's also insisted in the past that the magnetic moment of the electron is caused by its "internal motion", and that this view is mainstream. I am not going to spend time arguing with him, but will point out for the benefit of others when he slips in ATM views without admitting that that is what they are.

As for mater, we actually never got that advanced at the lab. Kittens was as big a subject as we fitted into the tubes, and a single crossing left a real mess at full speed.

Jeff Root
2010-Jan-20, 10:53 PM
On the other hand, someone travelling at the speed of light would
have no more trouble stopping when he reaches his destination than
a radio signal has stopping when it reaches its destination. As long
as he hits his target, and the receiver is turned on and tuned in, he
will be fine.
A note to others who may be reading this, Jeff is again playing his
game of either trying too hard to be clever with semantics (instead of
just posting his point clearly) or of posting very ATM claims and then
refusing to accept that they are ATM. The mainstream view is that a
person cannot travel at c, no matter how much energy is supplied.

(Studying the form guide, I predict that we'll see a denial that he is
proposing an ATM view here...)
That last bit, at least is correct. I was saying exactly the same as
Grey said in the post immediately following mine.

Since we all (including Russ) have read 3 zillion times that massive
objects cannot travel at c, it was totally unnecessary for me to
state it yet again. I took that as a given. So, given that fact, it
should have been obvious that the only way someone can travel at
the speed of light is by being light, or a signal composed of some
massless particles or waves. Made even more obvious by reference
to the receiver at the traveler's target.

[thumb on nose, twiddling fingers icon]

FYI-- mainly Ken: I'm working on replies to more serious concerns
raised in posts on page one of this thread. Replies like this one are
way easier and quicker!

-- Jeff, in Minneapolis

Ken G
2010-Jan-21, 04:16 AM
Note that another reason for us not to be concerned about singularities in the "frame of a photon" is that photons don't necessarily, or "really", travel at c either (no measurement of any photon speed is ever exact, so could never be said to be actually c, given the uncertainly principle and so forth, and what's more, no measurement can ever rule out the possibility that photons have a tiny but nonzero rest mass, and no photon propagates in a perfect vacuum, etc. etc.. People are forever confusing the idealizations of physical theories with the actual reality, and that's generally the issue whenever singularities are encountered). But none of this matters-- it suffices to pick any speed you like that is arbitrarily close to c, and you get the idea what is happening. There is never any need to discuss a reference frame at exactly c.

forrest noble
2010-Jan-21, 05:40 AM
Jeff,


The light doesn't cease to exist. The photons get farther and farther apart. If they aren't absorbed, the number of photons going out in all directions from a flashbulb will be the same at ten billion light-years as at ten centimetres.


There are a number of mainstream interpretations concerning the life of a photon. Some believe that the same photon is absorbed by a particle and at a later time emitted again. Others assert that photons are continuously created and upon absorption they would cease to exist (their energy being absorbed). According to this model a new photon would be created when an atom falls to a lower state concerning an electron's energy.

As a photon is moving at the speed of light it can be absorbed, refracted, or reflected. Depending open the density of the space it's going through for each given density medium "half-life" distance, half of the photons would remain unabsorbed. Our solar system has a higher particle/ atom density in its space than the galaxy, and the galaxy a higher particle density in its space than inter-galactic space. Most definitions in physics consider a photon's lifespan as "independently long," generally meaning they can exist in a straight path as long as they don't directly encounter matter. Other ZPF absorption possibilities exist, not tired light theory because only intensity/ luminosity would be effected, the frequency would not accordingly change.

http://www.ee.buffalo.edu/faculty/cartwright/java_applets/cavity/photon_lifetime/index.html

For these reasons my speculative guess would be for distances as far as the moon little of a small flashlight's light would remain unabsorbed, about 2 1/2 seconds. From here to lets say Mars' distance, I would expect all of the photons to be absorbed in the plane of the solar system. By using large directed transmitters amplifying the signals by satellites on both planets we can span the distance for communication, like we use for the Mar's rovers. By this amplification method we still have a signal left by the time they get to Mars from here, about 4 minutes, but not very strong. But in any event I don't think such a small flashlight signal could span a distance equivalent to the now observable universe. But I guess some theorists believe in the possibility as you suggest.

Jeff Root
2010-Jan-21, 06:43 AM
Forrest,

There is no doubt that as light travels through a transparent medium, the
light and medium interact with each other. Typically it is said that a photon
is momentarily absorbed and then re-emitted in the same direction that it
was travelling in. Or that a new photon is emitted in that direction. I have
used that language.

However, I have seen no evidence one way or another that it is possible
to distinguish between a single photon travelling from atom to atom, and
a series of different photons being emitted by successive atoms. As far
as I can tell, the two seemingly different scenarios actually describe the
same situation. It appears to be completely legitimate to say that a
single photon travels from atom to atom, being absorbed and re-emitted
along the way.

That is a transparent medium. If the medium is not perfectly transparent,
some photons will not be immediately re-emitted in the same direction.
They will, instead, hang up in the atom for a time, and the energy might
be re-emitted as photons of any energy in any direction. Typically what
happens is that the energy is thermalized. At ordinary room temperature,
visible light energy absorbed by a non-transparent atom is re-emitted in
mostly the far infrared part of the spectrum.

The scenario I described with a flashbulb specified that the light is not
absorbed. By that I meant it is not absorbed by any matter that is not
perfectly transparent. In that case, as many photons from the bulb's
flash would eventually be found ten billion light-years from the bulb as
there originally were just ten centimetres from the bulb.

In the real Universe, some light is absorbed by non-transparent matter.
We can see by looking in the sky that from a position in space, most
straight lines in all directions never hit anything that isn't transparent,
for as far as any telescope can see. That means light from early and
distant galaxies reaches us with little change. Same for the CMBR.

If I were to shine an ordinary flashlight at Mars on a clear day or night,
the wide beam means that most of the photons would miss Mars, flying
right past it. Most of the rest of the photons would hit the surface.
Some photons would be stopped by dust or clouds in Mars's atmosphere,
and some would be stopped by dust in Earth's atmosphere. Aside from
that, all the photons would make it all the way, even if they happen to
be absorbed and re-emitted a few times enroute.

The same applies to my flashlight beam travelling toward a distant galaxy.
If the beam travels through transparent space-- which space mostly is--
ten billion years from now, there will still be the same number of photons
in the beam. But the beam will be far, far wider than the galaxy. It would
be unlikely for a photon to hit any particular light detector that is aimed in
the direction of Earth during the time that the beam arrives. Most of the
photons will miss the galaxy completely, and those which hit something in
that distant galaxy will mostly hit things that are not light detectors.

-- Jeff, in Minneapolis

Jeff Root
2010-Jan-21, 06:50 AM
Forrest,

When you said "independently long" I think you meant "indefinitely long".

-- Jeff, in Minneapolis

RussT
2010-Jan-21, 07:15 AM
No speed is "assigned" to a photon in its own reference frame, because a
photon has no sensible reference frame. To a photon, every time is zero,
and every distance is zero. From a photon's point of view, nothing exists
and nothing happens. The result is that a photon has no point of view.

That was interesting for about five minutes when I first learned of it.

Jeff, I realize we/everybody has been over this umteen times, however...consider this...

I would submit that you 'are accepting' a "speed" for the photons in their own frame.

I know that you agree with me that M31 is ~2.5 million light years away, and was ~ that distance 2.5 million years ago, when say a SN went off in it that we would just be seeing first light for at our now, right here and now in our MW.

Same for SN 1987A in the LMC...160,000 light years away, which it also was ~160,000 years ago from 1987 when we first saw it's first light.

Those are pure facts...SO, for those photons to have any possibility of getting from there to here, in 0 time, then you would have to be accepting that their speed had to be "Infinitely Fast" correct?

Now, let's move to the "Earth Rest Frame Observer", who 'sees' those photons as taking 2.5 million light years and 160,000 light years for those respective photons from M31's SN and SN 1987A to reach us here as 'earth rest frame observers'.

Are Those "Earth Rest Frame Observers"/US, using SR/Relativity in any way what-so-ever, when agreeing to that???....IE.....they can/are 'see'(ing) distance and time of flight of those photons of 186,282.397mps. correct??? Also, our 'earth rest frame observers' do NOT have a second observer, on the other end to compare anything to...right? They just 'see' the light 'emitted' "Over There" and taking the time to come the distance to 'Over Here', right?

Now, let's look at the SR Observers...when you talk about 'The Limit' of traveling at "c"............all of a sudden you have 'switched' to talking about the limit of traveling at "c"....as the limit approaching "Infinity"...instead of the limit approaching 186,282.397mps. And, those SR/Relativity Observers cannot 'see' time and distance, even IF it is the Observer considered "at rest", because that 'at rest' observer could immediately be the one considered 'moving', right???

SO, whouldn't you/and everyone agree that the "AT Rest" 'earth rest frame observer' is NOT using SR/Relativity and that the SR/Relativity "At Rest" Observer, being able to be 'Swapped' for the 'observer B''s motion, on the other end of the observation are two totally different "At Rest Observers"???

Jeff Root
2010-Jan-21, 01:36 PM
I would submit that you 'are accepting' a "speed" for the photons in
their own frame.
I'll try to show that I am not.



I know that you agree with me that M31 is ~2.5 million light years away,
and was ~ that distance 2.5 million years ago, when say a SN went off in
it that we would just be seeing first light for at our now, right here and
now in our MW.
Yes, but sometimes it is vital to throw in some caveats when making
such statements. This will be one of those occasions.



Same for SN 1987A in the LMC...160,000 light years away, which it also
was ~160,000 years ago from 1987 when we first saw it's first light.

Those are pure facts...
No, not pure. They make certain assumptions. Reasonable assumptions,
but those assumptions mean that what you call "facts" are not always
facts at all.



SO, for those photons to have any possibility of getting from there to
here, in 0 time, then you would have to be accepting that their speed
had to be "Infinitely Fast" correct?
Yes, if you were talkng about the time measured by any real observer.
I know that you are not talking about the time of any real observer.
Instead, you are talking about the time as measured by a photon,
which is always zero, because it isn't able to measure anything.
Since the time as (putatively) measured by the photon is zero, the
speed is undefined.



Now, let's move to the "Earth Rest Frame Observer", who 'sees' those
photons as taking 2.5 million light years and 160,000 light years for
those respective photons from M31's SN and SN 1987A to reach us
here as 'earth rest frame observers'.

Are Those "Earth Rest Frame Observers"/US, using SR/Relativity in any
way what-so-ever, when agreeing to that???....IE.....they can/are
'see'(ing) distance and time of flight of those photons of 186,282.397mps.
As best I can think at the moment, the only way in which SR is
necessarily involved is in that it tells us the speed of light. Without
the SR premise of the constancy of the speed of light, we could not
assume that c is constant over the trip from M31 or the LMC.
Maxwell's equations don't requre that c be the same elsewhere as
we measure it in our laboratories. So your assertion that the light
travels from M31 and the LMC at 186,282.397 mps is a result of SR.



Also, our 'earth rest frame observers' do NOT have a second observer,
on the other end to compare anything to...right?
Right. Except in some peculiar circumstances, all of our observations
are made from "here".



They just 'see' the light 'emitted' "Over There" and taking the time to
come the distance to 'Over Here', right?
No, neither part of that is right. We don't see the light emitted. All
we see is the light when it arrives here. And we don't see the light
taking the time required to travel the distance. All we see is the light
when it gets here. That's it. Nothing more.

We have to measure distances to M31 and the LMC by examining the
luminosity and pulsation rates of Cepheid variable stars, and other
tricks, because they're too far away for their distances to be measured
by triangulation or other direct means. Our knowledge of all distances
beyond a hundred light-years or so is utterly dependant on theory.
In this case, primarily the theory describing the relationship between
luminosity and pulse rate of Cepheid variables.



Now, let's look at the SR Observers...when you talk about 'The Limit'
of traveling at "c"............all of a sudden you have 'switched' to talking
about the limit of traveling at "c"....as the limit approaching "Infinity"...
instead of the limit approaching 186,282.397mps.
No. I'll be the traveller. You be the observer here on Earth.

I get in my Super-Duper, Ultra-Mega Hyper Rocket, and accelerate
the bejesus out of my poor, flattened bottom. You see me going
faster and faster, approaching the speed of light as I fly away from
my launch point on the planet Krypton, and zoom past Earth. However
long you watch me, I am always approaching the speed of light more
closely. You also see my clock ticking more and more slowly, and my
beautiful spacecraft is getting more and more squabbish.

I take off from Krypton and accelerate so extravagantly that very
quickly I am passing other stars. In just a little bit, I am no longer
inside the Milky Way galaxy. (I assume that Krypton is in the Milky
Way. I know that others have differing opinions.) Before I can even
get lunch ready, I'm seeing galaxies whizzing past my porthole. But
I notice that all these galaxies are oddly shaped. They are squished
flat in the direction I'm travelling, just like I'm squished flat against
the rear bulkhead. I'm squished by the acceleration, but everything
outside is squished by my relative speed. I measure my speed to be
very close to the speed of light relative to the stars that I pass, but
the distances between those stars are all compressed down to a tiny
fraction of the distances I knew before I started my flight. I also
notice that everyone else's clocks are ticking more and more slowly.

I'm passing galaxies in seconds, now, but those galaxies are only a
few miles across! My speed relative to the stars in those galaxies is
still less than the speed of light. A light signal is being transmitted
from Krypton, alongside my ship, and I can see a small part of the
beam as it hits my ship's rear-facing light detector. The beam has
become very red-shifted, but the individual photons are still passing
me at the characteristic speed of light.



And, those SR/Relativity Observers cannot 'see' time and distance,
I assume that by "those SR/Relativity Observers" you mean anyone
or anything travelling at a relative speed approaching c, as I did
in my Ship, above, or light itself travelling right at c.



even IF it is the Observer considered "at rest", because that 'at rest'
observer could immediately be the one considered 'moving', right???
Aside from acceleration, there is no distinction between a moving
observer and an observer at rest. I should have written the Krypton
scenario with me being shot out of a cannon instead of flying in a
rocket, so as to avoid continuing acceleration. But editing it now is
too much work. Aside from the effects of acceleration, everything
is the same.



SO, whouldn't you/and everyone agree that the "AT Rest" 'earth rest
frame observer' is NOT using SR/Relativity and that the SR/Relativity
"At Rest" Observer, being able to be 'Swapped' for the 'observer B''s
motion, on the other end of the observation are two totally different
"At Rest Observers"???
I don't know what you think you mean by an "at rest observer".
There is no such thing as an observer at rest except relative to
something. An observer can be at rest relative to a particular very
distant star. If light from that star reaches him, he will see that its
light is not affected by relative motion, since there isn't any relative
motion.

The "at rest Earth rest frame observer" will see an object moving
relative to him at nearly the speed of light as squished in the direction
of its motion, and time dilated. The object, or "observer B" will see
the Earth moving relative to him at nearly the speed of light, squished
in the direction of its motion, and time dilated.

Let's say that you are one of the observers, and you will consider
yourself to be at rest because you will not be accelerating at all.
You will just sit still in empty space. Meanwhile, I've been building
a successor to my Kryptonian rocket: The Super-Duper Ultra-Mega
Hyper-Dyper Armageddinoutahere Cannon. I load the Earth into the
barrel of the cannon and fire it, so that the Earth flies past you at
so close to c that you can't count all the nines. You see it squished
in the direction of its motion past you, and all the clocks on Earth
ticking very slowly. The people on Earth see you fly past them at so
close to c that they can't count all the nines. They see you squished
in the direction of your motion past them, and they see your clock
ticking very slowly.

That was a lot of work, but still easier than replying to Ken.

-- Jeff, in Minneapolis

Strange
2010-Jan-21, 01:47 PM
SO, for those photons to have any possibility of getting from there to here, in 0 time, then you would have to be accepting that their speed had to be "Infinitely Fast" correct?

If you also consider the distance they have to travel to be 0, then they don't have to travel infinitely fast. In fact, you can't work out what speed they must travel at because it is 0/0. On the other hand, they can be travelling at c because c*0=0.

Jeff Root
2010-Jan-21, 02:00 PM
I notice that in that long post, I violated a suggestion I made in another
thread: I said things like "my speed" or "your speed". In the other thread,
I suggested that when talking with Russ, one should instead always say
something like "our relative speed".

-- Jeff, in Minneapolis

Ken G
2010-Jan-21, 07:32 PM
Yes, if you were talkng about the time measured by any real observer.
I know that you are not talking about the time of any real observer.
Instead, you are talking about the time as measured by a photon,
which is always zero, because it isn't able to measure anything.
Since the time as (putatively) measured by the photon is zero, the
speed is undefined.But note once again that all this "dividing zero by zero" stuff is a complete red herring. We can imagine a whole class of different observers, moving in various different ways, that all get different times and distances that the light from the supernova is traversing. That's the whole point of SR. It suffices that we can choose observers who get that the distance and time are arbitrarily close to zero-- it makes no difference at all that we cannot find one that is exactly zero. Anything that is much less than 160,000 years suffices to defeat what RussT is saying, so if we imagine an observer who leaves the dying star 1 second before it dies (in the frame of the unlucky ones left behind), and comes at constant speed to Earth at whatever speed is needed to pass by Earth the same time as the light from the supernova, then a simple calculation shows that the travelers will reckon the time (and the distance) that the light traveled to be 18 days (and 18 light days), not 160,000 years. End of story, no need for any dividing by zero to confuse matters. Those travelers will not be fried by the supernova light because of the Doppler shift, not because of the distance they are away from the blast site, the latter being only 18 light days. This is just how reality works-- there is nothing absolute about distance itself.

forrest noble
2010-Jan-21, 09:39 PM
Jeff,


When you said "independently long" I think you meant "indefinitely long".

Yeah, you're probably right. I tried to find the source the I originally "copied" it from but could not. Instead I did find two sources that said "indefinitely long." One or the other word makes no difference to me since I would have probably chosen to use the word "indeterminate," but that's probably just me with an ATM hat on.

Gomar
2010-Jan-22, 04:23 AM
Hold on, if you were to travel at or above the speed of light you wouldnt see anything. Why? Well, light from objects will never hit your eyes, thus you are blind. That means if you travel from Earth at 320,000/second and you wanted to look at it, the light from Earth cant reach you, thus to you the Earth disappears.
Just as flying faster than speed of sound you cant hear the sound of your airplanes engine.

RussT
2010-Jan-22, 12:06 PM
But note once again that all this "dividing zero by zero" stuff is a complete red herring. We can imagine a whole class of different observers, moving in various different ways, that all get different times and distances that the light from the supernova is traversing. That's the whole point of SR. It suffices that we can choose observers who get that the distance and time are arbitrarily close to zero-- it makes no difference at all that we cannot find one that is exactly zero. Anything that is much less than 160,000 years suffices to defeat what RussT is saying, so if we imagine an observer who leaves the dying star 1 second before it dies (in the frame of the unlucky ones left behind), and comes at constant speed to Earth at whatever speed is needed to pass by Earth the same time as the light from the supernova, then a simple calculation shows that the travelers will reckon the time (and the distance) that the light traveled to be 18 days (and 18 light days), not 160,000 years. End of story, no need for any dividing by zero to confuse matters. Those travelers will not be fried by the supernova light because of the Doppler shift, not because of the distance they are away from the blast site, the latter being only 18 light days. This is just how reality works-- there is nothing absolute about distance itself.

No, sorry, but it does NOT defeat what RussT is saying, because any of those ship observers is comparing their 'time and distance' against either "Infinitely fast" travel or an "Unreal" distance that is different than the distance where we have determined the light to be coming from with measurements other than SR/Relativity.

If a spaceship takes off from earth, going directly toward the Sun at say anything faster than 1/2 the speed of light, does the distance from the Earth to the Sun "Really Change"...if the ship is going .9999999999999c, what is the distance from the Earth to the Sun?

Strange
2010-Jan-22, 12:21 PM
If a spaceship takes off from earth, going directly toward the Sun at say anything faster than 1/2 the speed of light, does the distance from the Earth to the Sun "Really Change"...if the ship is going .9999999999999c, what is the distance from the Earth to the Sun?

The distance does "Really Change" from the PoV of those on the spaceship. The people left on Earth would not suddenly see the Sun filling the sky and suffer terrible sunburn (if that is what you are getting at).

RussT
2010-Jan-22, 12:28 PM
If you also consider the distance they have to travel to be 0, then they don't have to travel infinitely fast. In fact, you can't work out what speed they must travel at because it is 0/0. On the other hand, they can be travelling at c because c*0=0.

This is what happens when you/anyone tries to define anything when there is division by 0, which be definition means "Undefined".

If you also consider the distance they have to travel to be 0,

Why would you/anyone consider the distance to be 0 when it has already been established that those SN are at the distances that have already been determined by means other than SR/Relativity???

If the SN's in the LMC and M31 went Boom at the same time....say right now, our earth time...How long would we have to wait to see each of their first light?

Jeff Root
2010-Jan-22, 12:33 PM
Ken,

I agree with what you say in post #80, that the meaninglessness of
the photon's "point of view" is not essential to explain what happens
in relativity. I used it because it had been used in the thread by
others, and because I was replying to what Russ said about it.

As you say, it suffices to note that as the relative speed of two
observers increases, their measurements of each other approach zero
length in the direction of motion and zero time to traverse that length.

-- Jeff, in Minneapolis

RussT
2010-Jan-22, 12:45 PM
No, neither part of that is right. We don't see the light emitted. All
we see is the light when it arrives here. And we don't see the light
taking the time required to travel the distance. All we see is the light
when it gets here. That's it. Nothing more.

We have to measure distances to M31 and the LMC by examining the
luminosity and pulsation rates of Cepheid variable stars, and other
tricks, because they're too far away for their distances to be measured
by triangulation or other direct means. Our knowledge of all distances
beyond a hundred light-years or so is utterly dependant on theory.
In this case, primarily the theory describing the relationship between
luminosity and pulse rate of Cepheid variables.

You are correct here Jeff and this is actually a very good point, because it means that when we say that "Earth Rest Frame" observers see/measure the light from these farther objects, that it's really an "Observer Independent" observation, and we are just plugging in the Constant speed of light as 186,282.397mps to those known/assumed correct distances.

I would ask that you read my other post again, and try to see what I am saying about the observers in the ships, traveling really fast...."Switching" to comparing their speed/velocity to "Infinitely fast" (and being in light's own frame, even though they don't reach "Infinitely Fast") rather than just approaching 186,282.397mps.

RussT
2010-Jan-22, 12:50 PM
Ken,

I agree with what you say in post #80, that the meaninglessness of
the photon's "point of view" is not essential to explain what happens
in relativity. I used it because it had been used in the thread by
others, and because I was replying to what Russ said about it.

As you say, it suffices to note that as the relative speed of two
observers increases, their measurements of each other approach zero
length in the direction of motion and zero time to traverse that length.

-- Jeff, in Minneapolis

ONLY, if it is true that there is a frame where photons can travel to infinity in 0 time!!!!

Strange
2010-Jan-22, 02:09 PM
Why would you/anyone consider the distance to be 0 when it has already been established that those SN are at the distances that have already been determined by means other than SR/Relativity???

It sounds as if you accept time, but not space dilation?

Grey
2010-Jan-22, 02:32 PM
Why would you/anyone consider the distance to be 0 when it has already been established that those SN are at the distances that have already been determined by means other than SR/Relativity???Those are the distances from our frame of reference. Someone else moving differently from us would measure a different distance. In particular, someone moving at a substantial fraction of the speed of light along a line from us to the object in question would measure the distance as being smaller.


If the SN's in the LMC and M31 went Boom at the same time....say right now, our earth time...How long would we have to wait to see each of their first light?The same time according to who's reference frame? Simultaneity is as much dependent on motion as the rate of time and the measurement of distance.

Really, this shouldn't be that complicated. We usually make all of our measurements for any given experiment from a single reference frame, and they all work out. Someone who is moving differently will use a different reference frame for all of their measurements. They get different results for measured times and distances, but it still all works out. If for some reason* you want to know what someone else will measure for distances and times, and they are moving very fast relative to you, then you can do some relatively simple math and find out the answer. The only reason we don't use the simpler Galilean transformations to compare reference frames is that experiment has shown that they don't work, but that the Lorentz transformations do. It doesn't matter if it doesn't make sense to you (or anyone else) that measurements of distances and times should depend on how you're moving; that is nevertheless the way the world seems to work, and we don't get to change the laws of physics. We just get to try to work out what they are.

* Usually, we do this when some object or particle is moving at high speed relative to us, so that we can work out what's happening from "the particle's point of view".

Jeff Root
2010-Jan-22, 03:25 PM
No, sorry, but it does NOT defeat what RussT is saying, because any
of those ship observers is comparing their 'time and distance' against
either "Infinitely fast" travel or an "Unreal" distance that is different
than the distance where we have determined the light to be coming
from with measurements other than SR/Relativity.
What you say here suggests that you have no idea at all what is
going on, so I'll start from the beginning.

Let's say that I'm in my Super-Duper Hypership, no longer accelerating
(to my great relief), and cruising past Earth at nearly c. What that
means is that when I measure the relative speed between my ship
and the Earth, I measure it to be nearly c, and when you measure the
relative speed between my ship and the Earth, you also measure it to
be nearly c. Since my speed relative to pretty much everything in
the Solar System, and every star in the Milky Way galaxy, and every
nearby galaxy is also nearly c, I measure all of those things to be
shortened in the direction of our relative motion. On the other hand,
since you are pretty nearly at rest relative to everything in the Solar
System, every star in the Milky Way, and every nearby galaxy, it is just
my ship that you measure as shortened in the direction of our relative
motion. There are exceptions. My ship is not the only thing buzzing
around at super-high speeds, but for the most part, neither of us notice
those other high-speed things because they are either very small or
very far away, or both.

Let's suppose that the relative speed between my ship and the Solar
System is such that you see my ship cover the distance between Earth
and Jupiter in one hour. Close to c. I, however, see my ship cover
the distance in one minute. Since I measure the distance between
Earth and Jupiter as compressed compared to how I used to measure
it back when I was living on Earth, my measurement tells me also that
the relative speed between my ship and the Solar System is close to
but still less than c.



If a spaceship takes off from earth, going directly toward the Sun at say
anything faster than 1/2 the speed of light, does the distance from the
Earth to the Sun "Really Change"...if the ship is going .9999999999999c,
what is the distance from the Earth to the Sun?
The distance does not change for anyone who does not accelerate,
but the distances are different for observers moving at different speeds.

Your number, .9999999999999 c, contains more digits than are in the
value of the speed of light, making the calculation rather pointless, at
least from an engineering point of view. Allow me to substitute a value
of .9999 c, which is a value which could actually be measured.

If someone nearly motionless relative to the Solar System (such as us)
measures the distance, they find it to be about 150 billion metres.
They measure the time of flight for the ship to be about 500 seconds.

If someone moving relative to the Solar System at .9999 c on a line
from the Earth to the Sun measures the distance, they find it to be
about 8,400,000 metres, or 5.6 percent of the distance the observers
on Earth measure. They measure their time of flight between Earth
and Sun as about 21 seconds, or 4.2 percent of the time that the
observers on Earth measure.

I'll appreciate anyone pointing out errors in my results. I did round
the numbers at a couple of points in the calculation, such as the
figure of 500 seconds for the trip time as measured by Earthites.

-- Jeff, in Minneapolis

Jeff Root
2010-Jan-22, 03:56 PM
Ken,

I agree with what you say in post #80, that the meaninglessness of
the photon's "point of view" is not essential to explain what happens
in relativity. I used it because it had been used in the thread by
others, and because I was replying to what Russ said about it.

As you say, it suffices to note that as the relative speed of two
observers increases, their measurements of each other approach zero
length in the direction of motion and zero time to traverse that length.
ONLY, if it is true that there is a frame where photons can travel to
infinity in 0 time!!!!
No, the assertion Ken made, and my reformulation of it (which Ken
might disagree with) are independant of anything to do with photons.
It doesn't matter whether one can envision a reference frame that
moves with a photon, or whether anything could be measured in
such a frame. If a parade of spaceships are speeding past Earth
into the Sun at successively higher and higher speeds, their speeds,
according to your measurements from Earth, will approach c as a
limit, and the time of flight will approach 500 seconds as a limit.
The occupants will see the distance shrink to zero as a limit, and the
time of flight fall to zero as a limit. (By my way of looking at it, the
time of flight reduces more rapidly than the distance reduces.)

-- Jeff, in Minneapolis

Ken G
2010-Jan-24, 01:26 AM
No, sorry, but it does NOT defeat what RussT is saying, because any of those ship observers is comparing their 'time and distance' against either "Infinitely fast" travel or an "Unreal" distance that is different than the distance where we have determined the light to be coming from with measurements other than SR/Relativity.
It does defeat what you are saying, but there seems little hope that you will ever recognize it. None of what you are saying here is correct, not a shred.


If a spaceship takes off from earth, going directly toward the Sun at say anything faster than 1/2 the speed of light, does the distance from the Earth to the Sun "Really Change"...if the ship is going .9999999999999c, what is the distance from the Earth to the Sun?The problem is that distance is not a "real thing" in the first place, so asking if it "really changes" is a bogus question. The core lesson of relativity is that distance is a convention, it depends not only on your own motion but also on your conventions for saying what distance is at places other than your own local vicinity. Because it is pure convention, different observers get different answers for it, and they get different answers if they change their motion or even if they change their conventions without changing their motion. The only thing that is invariant among the observers and their conventions is the metric norm that connects the events along that path, and if the path is that of a photon, the only possible answer for that invariant norm is zero. Distance is something different, it has a very arbitrary flavor to it. You can say until you are blue in the face that the distance to the supernova is 160,000 light years, but it's pure convention, and no other physicist in the rest of the universe is beholden to agree with that distance.

north
2010-Jan-24, 05:45 AM
If a spaceship takes off from earth, going directly toward the Sun at say anything faster than 1/2 the speed of light, does the distance from the Earth to the Sun "Really Change"...if the ship is going .9999999999999c, what is the distance from the Earth to the Sun?

no , the distance does not change

distance is distance is distance , even for the speed of light


The problem is that distance is not a "real thing" in the first place, so asking if it "really changes" is a bogus question.

distance is not a " real thing " oh please !!!

north
2010-Jan-24, 05:57 AM
time dilation is about the perspective of observer(s) only

to a Universe without an observer , time dilation is irrelevant

Van Rijn
2010-Jan-24, 06:37 AM
time dilation is about the perspective of observer(s) only


to a Universe without an observer , time dilation is irrelevant

What do you think "observer" means in this context? I'm guessing you're equating "observer" with "person" or something along those lines? If so, that's incorrect: The physics of relativity is extremely relevant to how things work in this universe, whether people notice it or not.

RussT
2010-Jan-24, 12:57 PM
If someone moving relative to the Solar System at .9999 c on a line
from the Earth to the Sun measures the distance, they find it to be
about 8,400,000 metres, or 5.6 percent of the distance the observers
on Earth measure. They measure their time of flight between Earth
and Sun as about 21 seconds, or 4.2 percent of the time that the
observers on Earth measure.

And, Yet "Gravity" tells us the earth/sun system is still ~93 million miles/150 billion metres, and that photons traveling that 'locked in by gravity' distance will take ~8.32 minutes to travel that distance...me thinks that those guys in their ships have somehow been deluded if they think that they are cutting off distance by traveling "Really Fast".

Even IF they could travel to the Sun Instantaneously, they would get there in 0 time....BUT the Sun/Earth system would NOT be effected just because a 'tiny little spaceship' was going really really fast.

IF the distance doen't 'really change', then the maths used to try to show that it is changing is bogus.

Do you honestly think that the distance from the Sun to the earth would really change to nearly 0 if you flew at near light speed towards the Sun?



It doesn't matter whether one can envision a reference frame that
moves with a photon, or whether anything could be measured in
such a frame.

Yes, Yes, Yes.........it does most certainly matter...this is where KenG just tries to say that 'division by 0' doesn't matter...

This where you are getting your defintion of 0 Time at any distance to infinity for photons in their own frame....so...for "undefined" they 'have made a definition!!! that is what is Bogus....'not even wrong'

Even though Light has already been defined as Constant at 186,282.397mps in Vacua...SO, that gives light/photons motion of two constants simutaneously.

Then they go on to say....light in it's own frame is not a valid frame. then they use the spaceships moving 'in that frame', which once going 'really fast' is approaching a speed of 'infinitely fast, NOT approaching 186,282.397mps. Then they say they are NOT using the ships in 'lights own frame'....because it can never gets all the way to light speed.

SO, the "d" (Distance) is meaningless in SR as observers A and B can be "Any Distance to infinity" from one another...and "c" is meaningless in SR because once the ships are going fast enough, they are 'approaching infinitly fast' rather than approaching 186,282.397mps.

ugordan
2010-Jan-24, 01:43 PM
But that means that the Distance is irrelevant and that it doesnt matter how far you go, if you are inside the Ship and if you travel at Lightspeed, the whole trip will only take a sneeze.

Think about this: photons, travelling at the speed of light experience no passage of time. From their frame of reference, they can travel all across the universe instantly and (as the crazy Star Trek-ish notion says, be at all places in the universe at once (in their frame of reference, because they experience no time), but to us we see them travelling at a finite speed and take time (our time, not theirs!) to cover a distance. If a supernova exploded in a distant galaxy, billions of years ago and the light reached us just now, from the photon's point of view, the two events were instantaneous.

When your ship approaches the speed of light, it really is moving toward that point of view and your calculations are reasonable.

An example of this is high energy particle accelerators. They can produce unstable particles which have such short lifetimes that if you went by the classic distance-to-the-detector/speed (0.99xxxx c) it would result in a propagation time longer than their lifetime which would mean they would decay before reaching the detectors. However, because the particles experience such strong time dilation (measured in our frame of reference) due to high speeds, their internal clock is slowed down and they can cover the distance to the detector before disintegrating first. Their effective half-life is extended compared to their half-life at rest.
In their own frame of reference, they obviously perceive their time to run normally and since they cannot travel as fast or faster than light, they perceive the distance to the detector to shrink.

ugordan
2010-Jan-24, 01:51 PM
Even IF they could travel to the Sun Instantaneously, they would get there in 0 time....BUT the Sun/Earth system would NOT be effected just because a 'tiny little spaceship' was going really really fast.

Why is it easy to accept relative time changes with different relative speeds, but distances changing are bogus? Both are consequences of special relativity and they do say there is no absolute frame of reference in which to say: OK, this is the real distance from the Earth to the Sun.



Do you honestly think that the distance from the Sun to the earth would really change to nearly 0 if you flew at near light speed towards the Sun?

Yes, in fact the whole universe would pretty much compress itself into a 2d pancake from your point of view.

Ken G
2010-Jan-24, 02:34 PM
time dilation is about the perspective of observer(s) only

to a Universe without an observer , time dilation is irrelevant


The physics of relativity is extremely relevant to how things work in this universe, whether people notice it or not.

One can quibble. To be completely accurate, one must allow that "time dilation" is a concept, availing of language, invented by intelligent observers to make sense of and communicate their shared perceptions. Clearly, whatever the universe is doing that those intelligences label time dilation was going on before 1905. Nevertheless, without intelligent observers to notice that, there is not even a concept of time dilation. So in one sense, we may say that time dilation predates 1905, and in the other sense, it does not. Both of these senses are actually quite important for understanding what physics is, and indeed both of the above claims are equally true for all aspects of physics, they in no way single out relativity.

Ken G
2010-Jan-24, 02:43 PM
Yes, Yes, Yes.........it does most certainly matter...this is where KenG just tries to say that 'division by 0' doesn't matter...Actually, I didn't "try" to say it, I said it. I'll say it again: "division by zero doesn't matter." Not hard to say at all, no trying necessary.

SO, the "d" (Distance) is meaningless in SR as observers A and B can be "Any Distance to infinity" from one another...and "c" is meaningless in SR because once the ships are going fast enough, they are 'approaching infinitly fast' rather than approaching 186,282.397mps.Who said distance was meaningless? I said it was conventional and arbitrary. If you think that's the same thing as "meaningless", you know no more about the meaning of language than you know about the meaning of speed.

captain swoop
2010-Jan-24, 03:36 PM
RussT You know better than this. If you want to start arguing against the Mainstream answers given in the Q & A Forum you need to start a thread in the ATM Forum. I am giving you an Infraction point for this

Jeff Root
2010-Jan-24, 04:06 PM
If someone moving relative to the Solar System at .9999 c on a line
from the Earth to the Sun measures the distance, they find it to be
about 8,400,000 metres, or 5.6 percent of the distance the observers
on Earth measure. They measure their time of flight between Earth
and Sun as about 21 seconds, or 4.2 percent of the time that the
observers on Earth measure.
And, Yet "Gravity" tells us the earth/sun system is still ~93 million miles
/ 150 billion metres, and that photons traveling that 'locked in by gravity'
distance will take ~8.32 minutes to travel that distance...
This reference to gravity tells me that you fundamentally don't
understand what is going on. There are several different ways that
the distance between the Earth and Sun can be measured, but using
gravity isn't a particularly direct or clear way to do it. There are
simpler, more direct ways to make your point that the distance to
the Sun is what it is.

And on that point, I think I agree with you and I think I disagree
with Ken: The distance to the Sun is what it is. Ken does have a
valid point but he chooses to express it in about the most ridiculous
possible way. I disagree with what Ken says, but I don't disagree
with the physics on which his statements are based.



...me thinks that those guys in their ships have somehow been deluded
if they think that they are cutting off distance by traveling "Really Fast".
No. There are no guys in ships. That is a thought experiment which
I created. The guys do not exist, so they cannot be deluded. I made
up the thought experiment, so I might be the deluded person. But
I think that I can eventually convince you that I am not deluded. I am
applying the principles of physics which are the result of observation,
testing, careful reasoning, and checking by many, many people. The
principles have been very well tested and they are logically consistent.

Rather than me being deluded, what is happening here is that you are
rejecting that which sounds illogical. It sounds illogical to you, so you
say that it can't be right, and you haven't been able to get past that
mental block.

What you need to do to get past the mental block is follow the logic
carefully and without prejudice.



Even IF they could travel to the Sun Instantaneously, they would get
there in 0 time....BUT the Sun/Earth system would NOT be effected just
because a 'tiny little spaceship' was going really really fast.
You have expressed this objection numerous times in earlier threads.
It is a straw man. You are describing what physics says incorrectly,
then cutting down that incorrect description.

To the best of my knowledge, no-one has ever said that the Earth and
Sun are affected by someone travelling at high speed relative to them.

What has been said many times is that the distance depends on the
relative speed of the observer. If someone is approximately at rest
relative to the Earth and Sun, they will measure a distance between
them of about 150 billion metres. If someone else is moving along the
line between Earth and Sun at .5 c relative to the Solar System, he
will measure a slightly smaller distance. If someone else is moving at
.99 c, he will measure a much smaller distance.

It is analogous to this situation: You look up at the tip of the antenna
on the Empire state building, and see that it is at an angle of 30 degrees
up from the horizontal. Another person looks at the exact same point,
and sees that it is up 50 degrees from the horizontal. A third person
looks at it, and sees that it is up 70 degrees from the horizontal. Which
of them is right, and which are deluded?



IF the distance doen't 'really change', then the maths used to try to
show that it is changing is bogus.
The distance doesn't change. It is different for different observers.
Or to put it another way, it is different as seen from different reference
frames. If you accelerate in such a way as to change your reference
frame, then you will measure the distance differently from before.
Everyone who remained in the reference frame you had been in before
still measures the distance the same as you did before you accelerated.

It all works mathematically. It all fits together logically. If it seems
illogical to you, it is because you are not looking at it and seeing it for
what it is. You are seeing only what you want to see, and that is an
illogical mess.



Do you honestly think that the distance from the Sun to the earth
would really change to nearly 0 if you flew at near light speed towards
the Sun?
I know that if I somehow had the resources and the desire to fly very,
very, very far away from the Sun, and then accelerate back toward
the Sun for such a long time that my speed relative to the Earth and
Sun approached c, that the distance along my line of relative motion
between the Earth and Sun would approach zero, as I would measure it.
You would just see my ship fly past Earth at nearly c, and would not
detect any change in the distance between the Earth and Sun, because
there would be no change.




It doesn't matter whether one can envision a reference frame that
moves with a photon, or whether anything could be measured in
such a frame.
Yes, Yes, Yes.........it does most certainly matter...this is where KenG
just tries to say that 'division by 0' doesn't matter...

This where you are getting your defintion of 0 Time at any distance to
infinity for photons in their own frame....so...for "undefined" they 'have
made a definition!!! that is what is Bogus....'not even wrong'
Can you accept, Russ, that you don't have a clue?

Ken understands this very well. I understand it pretty well. You only
understand a little tiny bit of it. Not enough to do you much good.

The business about division by zero doesn't matter because, as Ken
pointed out to me a few posts back, it isn't necessary to bring it up
in the argument. We can leave the behavior of light out of the
argument completely, and just look at what would happen to massive
spacecraft that are moving at various relative speeds. We find that
the higher the relative speed, the smaller the distances measured,
such that as speeds approach c, distances approach zero as a limit.
No division by zero, no infinities, nothing undefined.



Even though Light has already been defined as Constant at
186,282.397mps in Vacua...SO, that gives light/photons motion of two
constants simutaneously.
One constant, c, the speed of light, is what all observers see.
No exceptions.

Theoretically, if a photon could see anything, it would see all distances
as zero and all times as zero.

Photons can't see anything.



Then they go on to say....light in it's own frame is not a valid frame.
It is not a valid frame because it is fundamentally impossible for any
observer to be in such a frame.



then they use the spaceships moving 'in that frame', which once going
'really fast' is approaching a speed of 'infinitely fast, NOT approaching
186,282.397mps.
That is just plain wrong. You need to pay closer attention to what
you have read probably dozens of times now. I just explained it to you
more than once. For whatever reason, it hasn't sunk in: An observer in
a spaceship which has a relative speed close to c has a relative speed
close to c. As measured by that observer or any observer. If I am in
a spaceship with a speed relative to the planet Krypton of nearly c,
every observer in any state of motion will say that my spaceship has a
speed relative to Krypton of nearly c.



Then they say they are NOT using the ships in 'lights own frame'....
because it can never gets all the way to light speed.
Who are you trying to convince? People who understand relativity?
Or other people who don't have a clue?



SO, the "d" (Distance) is meaningless in SR as observers A and B can
be "Any Distance to infinity" from one another...and "c" is meaningless
in SR because once the ships are going fast enough, they are
'approaching infinitly fast' rather than approaching 186,282.397mps.
As ugordan points out, you seem to accept time dilation but not
compression of space. The two go hand-in-hand. You don't have
one without the other. It works mathematically, and it matches
perfectly what is actually observed. It seems weird to you in part
because the effects are only apparent in things which are very small
or very far away, or both -- so you never see them. Other people,
who use the appropriate tools, do see them.

-- Jeff, in Minneapolis

Jeff Root
2010-Jan-24, 04:25 PM
To the best of my knowledge, no-one has ever said that the Earth and
Sun are affected by someone travelling at high speed relative to them.
I see that I was wrong about this. People have said the rest of the
Universe is affected. I think that is excessively sloppy use of language.
Sloppy is okay most of the time, but not in this specific case.

-- Jeff, in Minneapolis

RussT
2010-Jan-31, 10:04 AM
If it weren't for the warning above I would have continued...BUT I cannot let this go.

Earlier in this thread I dilineated between our "Earth Rest Frame Observers" being able to see/detect/ photons and calculate the Time and Distance to known local light sources ie; the moon at 286,000 miles and ~1.25 light seconds, the Sun at 93 million miles and ~ 8.32 light minutes etc...and that when you are agreeing to this that you are NOT using Relativity in any way what-so-ever.

And SR "Rest Frame" observers who are automatically attached to another observer "B" where their motions can be 'swapped' (Every single nano second by the way!!!)



Orginally Posted by RussT
SO, the "d" (Distance) is meaningless in SR as observers A and B can be "Any Distance to infinity" from one another...and "c" is meaningless in SR because once the ships are going fast enough, they are 'approaching infinitly fast' rather than approaching 186,282.397mps.



Who said distance was meaningless? I said it was conventional and arbitrary. If you think that's the same thing as "meaningless", you know no more about the meaning of language than you know about the meaning of speed.

SO, KenG, when you said...I said it was conventional and arbitrary, you are referring to us/earth rest frame observers, because they are the only ones that can do this (see time and distance together)!!!

When I said that distance was meaningless "IN SR" I was absolutely correct, as SR observers cannot see time and distance together at all...

Perhaps you would like to share with the class, in detail, how a "Light Year" in Relativity has no 'real' meaning!!!

And out of respect for the warning I will not go into what that means for the light second in SR!!!

Jeff Root
2010-Jan-31, 02:47 PM
Russ,

I want you to know that I disagreed that anything you said constituted an
infraction, and complained about it shortly after it was issued. I think BAUT
may be able to serve Vulcans like Nereid and Romulans like Ken without any
problem, but it has trouble dealing with humans.

Given the basics of special relativity, it is obvious why Ken says that
distances are conventional and arbitrary. I disagree, but my disagreement
is one of pedagogy (to hopefully not misuse a term of which Ken is fond)
rather than physics. Any measurement of distance is dependant on the
relative motion between the measurer and the thing being measured.
The simple fact is that nearly all measurements of distance are between
measurers and measurees which have such low relative speeds that the
speed makes no detectible difference to the measurements. That is the
basis of your point of view, and your objection to Ken's point of view.

Sometimes the speed does make a detectible difference, though, and you
need to understand that. Even when the speed does not make a detectible
difference, it always has an effect, and you need to understand that, too.

-- Jeff, in Minneapolis

Ken G
2010-Jan-31, 03:24 PM
Given the basics of special relativity, it is obvious why Ken says that
distances are conventional and arbitrary. I disagree, but my disagreement
is one of pedagogy (to hopefully not misuse a term of which Ken is fond)
rather than physics. Any measurement of distance is dependant on the
relative motion between the measurer and the thing being measured.It's actually even worse than that. The motion between them when? If you say at a moment simultaneous for the observer, then you have the problem that distance is not reciprocal-- an observer on the other object would have a different sense of simultaneity and would not agree on the distance that you get! And if you say you don't care about any observer but yourself, you still have the problem of your own motion changing-- if you get in your car and start driving, the distance to the supernova changes. The fundamental problem is that motion is a local phenomenon, and distance is a global one, so you will never be able to connect them in an unambiguous way-- you will always have to integrate local concepts of distance to get a global one, and how you do that will always be rather arbitrary. Distance is a coordinate, it's just like if I asked you the distance from New York to Los Angeles. You could give a number, but it wouldn't mean a thing until you also tell me if it's the straight-line distance, or as the crow flies, or the mapquest distance. Without the coordinate information, the data means nothing.



The simple fact is that nearly all measurements of distance are between
measurers and measurees which have such low relative speeds that the
speed makes no detectible difference to the measurements. Right, but that only means the distinctions usually don't matter. When do they matter? When one is talking about relativity, and distances on the astronomical scale. It's just like if I asked the distance from your house to your neighbor's, you wouldn't need to ask if I wanted the straight-line distance or the distance you would walk or the distance a crow would fly, they'd all be too close to matter. That doesn't mean there's no ambiguity, it means the ambiguity doesn't matter in that application, because the measurements are not precise enough to distinguish them. The same holds for nearby supernovae, but not for distant ones.
Sometimes the speed does make a detectible difference, though, and you
need to understand that. Even when the speed does not make a detectible
difference, it always has an effect, and you need to understand that, too.
Yes, that's what we're saying.

Ken G
2010-Jan-31, 03:27 PM
Perhaps you would like to share with the class, in detail, how a "Light Year" in Relativity has no 'real' meaning!My point is, it has no absolute meaning, without detailed information about the coordinate system being used to give it its meaning. Sometimes the coordinates are clear from context, other times the precision of the measurement doesn't distinguish the possibilities.

Strange
2010-Jan-31, 03:32 PM
Earlier in this thread I dilineated between our "Earth Rest Frame Observers" being able to see/detect/ photons and calculate the Time and Distance to known local light sources ie; the moon at 286,000 miles and ~1.25 light seconds, the Sun at 93 million miles and ~ 8.32 light minutes etc...and that when you are agreeing to this that you are NOT using Relativity in any way what-so-ever.

Relativity can only be ignored in this case because you are only considering a single frame of reference. If you were to measure the distance from Earth to the moon or sun from a spaceshift travelling at speed relative to the Earth, you would get different results (in miles, light minutes, light seconds or whatever other units you choose).

Ken G
2010-Jan-31, 05:10 PM
Incidentally, when we talk about distance, we should probably also distinguish the concept of a mathematical distance, involved in a coordinate system, and the physical distance, involved in making measurements. Physical distance is an even more complicated issue, as there are many ways of measuring the distance associated with some observation (say, of a distant galaxy), and in general they can all give different results. For example, you could look at the apparent brightness of the galaxy, or its angular size, or its redshift against the expansion of the universe, and they'd all give different distances, but different in ways that connect in a consistent way to the mathematical distance involved in coordinatizing that galaxy.

adamskiiix
2010-Jan-31, 06:19 PM
henriquefd,



There's a lot of great stuff to read here on BAUT. Astronomy is often mixed with cosmology and related physics so you will probably learn a lot more than just astronomy. You also have to do research on the web if there are no links cited for what is being said, since there is often more than one perspective/ opinion of what is being discussed, and the poster may not have explained the subject properly.

Maybe a good starter would be to start with an elementary book on astronomy. With a high-school level or higher understanding of astronomy, you will be able to learn a lot more material contained in these postings.


i think a good starter would be also creations of Arthur C.Clarke:)

RussT
2010-Feb-01, 02:22 AM
Russ,

I want you to know that I disagreed that anything you said constituted an
infraction, and complained about it shortly after it was issued.

Thank You for that support Jeff!



I think BAUT may be able to serve Vulcans like Nereid and Romulans like Ken without any problem, but it has trouble dealing with humans.

LOL...Then I must be coming off as a full on Klingon...:)

Actually I would classify Nereid as the Klingon (Maybe one of the two sisters/twins in numerous episodes...LOL), KenG as the Romulan, and ME as the Vulcan...;)

Why ME as the Vulcan? Simple...the Vulcans will have worked out "When" a SMBH becomes part of a galaxy/dwarf galaxies life...You know, the MOST important question in ALL of Cosmology...The chicken and the egg thingy...LOL

When you start from there, the Vulcans realize that ALL "Singularities" must be eliminated!

And now we are back to KenG's/mainstreams mandate that there be 0 distance and 0 time for emission/absorption of light

Ask yourself this...why is KenG/mainstream insisting that an 'observer in a spaceship' is traveling at "c" between any/every observer A and any/every light source at any and every distance?

In other words, according to SR/mainstream, every observer behind our line of sight to SN 1987A, at any and all distances to Infinity, also see's 0 distance in 0 time...how can that be?

Now, SN 1987A's light is isotropic, so that means ALL observers in the universe MUST see those photons as arriving in 0 time and at 0 distance....again, how can that be?

Now, I do not think that you believe that SN 1987A was ever at 0 distance during the 160,000 years it took those photons to reach us as "Earth Rest Frame Observers"...right?

SO, what have all those "Photons that kept getting here in 0 time', during that whole 160,000 years of continuous emission of SN light, been doing here, while we, as earth rest frame observers, were waiting for the "Real" photons to get here???

And finally, IF there were another galaxy, opposite our line of sight, on the other side of the LMC, 160,000 light years away...How could the photons from SN 1987A be 0 distance in 0 time from both???

Ken G
2010-Feb-01, 06:32 AM
And now we are back to KenG's/mainstreams mandate that there be 0 distance and 0 time for emission/absorption of light
Huh? I said there had to be 0 distance and 0 time? Where do you imagine I said that? Let's try what I really said: I said there is 0 proper distance and 0 proper time, and that many other distances and other times are also possible in various locally physical coordinatizations, subject to a simple algebraic relation between them (you've seen it, it's the null metric of SR). This means that neither the distance nor the time are absolute quantities, except if they are interpreted as proper distance and proper time, in which case you do get an invariant and it is zero.


Ask yourself this...why is KenG/mainstream insisting that an 'observer in a spaceship' is traveling at "c" between any/every observer A and any/every light source at any and every distance?My, I really don't recognize these things I'm insisting when they enter your head and come out your keyboard.


In other words, according to SR/mainstream, every observer behind our line of sight to SN 1987A, at any and all distances to Infinity, also see's 0 distance in 0 time...how can that be?Beats me. Incidentally, what are you talking about?

RussT
2010-Feb-01, 07:50 AM
Now, I do not think that you/anyone believe(s) that SN 1987A was ever at 0 distance during the 160,000 years it took those photons to reach us as "Earth Rest Frame Observers"...right?

SO, what have all those "Photons that kept getting here in 0 time', during that whole 160,000 years of continuous emission of SN light, been doing here, while we, as earth rest frame observers, were waiting for the "Real" photons to get here???

pzkpfw
2010-Feb-01, 09:07 AM
The Earth rest frame has a different experience than something not in that same rest frame. That's relativity (more or less).

RussT, going around and around in circles, refusing to accept the different realities of different "observers" - it is clear you are advocating an ATM point of view, not asking a question and accepting the mainstream answer.

It will stop now, please.

RussT
2010-Feb-01, 12:27 PM
This has struck me quite strongly.

It seems...

From the point of view of light, it takes zero time to "get" from anywhere to anywhere, because, as far as any light in the Universe is concerned the distance between any two points is zero.

That is, as far as light is concerned, the entire Universe may as well all be in one place.

(A singularity?)

pzkpfw, you even had this major concern yourself in Post #51, and kenG is not really answering the questions, he just keeps repeating 0 Proper time and distance.

So, I will go back to an answer he gave in Post #80 and then ask a question just based on that answer.



But note once again that all this "dividing zero by zero" stuff is a complete red herring. We can imagine a whole class of different observers, moving in various different ways, that all get different times and distances that the light from the supernova is traversing. That's the whole point of SR. It suffices that we can choose observers who get that the distance and time are arbitrarily close to zero-- it makes no difference at all that we cannot find one that is exactly zero. Anything that is much less than 160,000 years suffices to defeat what RussT is saying, so if we imagine an observer who leaves the dying star 1 second before it dies (in the frame of the unlucky ones left behind), and comes at constant speed to Earth at whatever speed is needed to pass by Earth the same time as the light from the supernova, then a simple calculation shows that the travelers will reckon the time (and the distance) that the light traveled to be 18 days (and 18 light days), not 160,000 years. End of story, no need for any dividing by zero to confuse matters. Those travelers will not be fried by the supernova light because of the Doppler shift, not because of the distance they are away from the blast site, the latter being only 18 light days. This is just how reality works-- there is nothing absolute about distance itself.



Now, I do not think that you/anyone believe(s) that SN 1987A was ever at 0 distance during the 160,000 years it took those photons to reach us as "Earth Rest Frame Observers"...right?

SO, what have all those "Photons that kept getting here in 0 time', during that whole 160,000 years of continuous emission of SN light, been doing here, while we, as earth rest frame observers, were waiting for the "Real" photons to get here???

SO kenG, how is that traveler going to convince us that we will not have to wait 160,000 years for those "real" photons to get here???

Ken G
2010-Feb-01, 03:04 PM
Now, I do not think that you/anyone believe(s) that SN 1987A was ever at 0 distance during the 160,000 years it took those photons to reach us as "Earth Rest Frame Observers"...right?That depends on which observers, whether they be real or hypothetical, you ask. The point is, there could be observers who use the standard SR coordinates and think the distance is arbitrarily small, though in reality there are probably not any such observers. We don't build our macroscopic physics around a requirement to know that observers are actually observing, we build it around what is happening that could have been observed (and even in quantum mechanics, if something is observed in one frame, we can freely imagine hypothetical observations in any other frame we like). I think you are hung up on the difference between real observers and hypothetical observers-- physics uses the latter concept routinely, and that's what I was also doing.


SO, what have all those "Photons that kept getting here in 0 time', during that whole 160,000 years of continuous emission of SN light, been doing here, while we, as earth rest frame observers, were waiting for the "Real" photons to get here???There's no distinction between the ones that took 0 time (in their own limit frame) and the real ones we see-- those are the same photons. You will never get your head around that as long as you imagine that time and distance are absolute.

Jeff Root
2010-Feb-01, 03:38 PM
Russ,

I very much dislike the way Ken explains this, but it is not as difficult to
understand as you are making it out to be.

I'd like you to answer the question I asked above: Can you accept that
you don't have a clue? I think it is essential that you do. You want to
understand this stuff, but you won't begin to understand it until you
accept that right now you don't understand it at all.

-- Jeff, in Minneapolis

Ken G
2010-Feb-01, 05:55 PM
I very much dislike the way Ken explains this, but it is not as difficult to
understand as you are making it out to be.Since you've also demonstrated difficulties understanding relativity in the past, I suggest there's a connection there that you are choosing to ignore. Distance, as you use the term (and is conventionally used), is nothing but a coordinate. The sole meaning of distance as something the universe "knows" in a way that is separate from arbitrary human semantic choices is proper distance. The way those arbitrary choices appear in special relativity is very often overlooked by most people: it appears in the arbitrary choice of associating other observers with no motion relative to you as somehow having more connection to your perspective than observers that have a motion with respect to you. That choice is pure philosophically motivated convention (it's the "special" in special relativity), there is zero empirically mandated physics in it.

pzkpfw
2010-Feb-01, 07:53 PM
pzkpfw, you even had this major concern yourself in Post #51,

That wasn't a "concern" at all and I was not arguing the mainstream physics, or disbelieving it; I was just thinking about the result or meaning of that aspect of physics. It has interesting implications.

That you categorise it as a concern is because you coloured my comment with your own interpretation; with what bugs you. It's what has resulted in several of your ATM threads and your questions (denials) in this thread.


Now, I do not think that you/anyone believe(s) that SN 1987A was ever at 0 distance during the 160,000 years it took those photons to reach us as "Earth Rest Frame Observers"...right?

The distance was not zero for us. It does not matter and is not contradictory that the distance was something else, from the point of view of another observer. Taken to the extreme, if a photon saw the distance as zero, that also is not contradictory.


SO, what have all those "Photons that kept getting here in 0 time', during that whole 160,000 years of continuous emission of SN light, been doing here, while we, as earth rest frame observers, were waiting for the "Real" photons to get here???

You imply that zero travel time means that they'd all get here at the same time. (And further that there's therefore an issue with what those photons "do" waiting around during the time we, from our point of view, take time to observe them all).

But you forget, they were not all produced at the same time.

RussT
2010-Feb-07, 12:13 PM
It's what has resulted in several of your ATM threads and your questions (denials) in this thread.

Yes and those Pro's were even confused and arguing amoungst themselves, while I stated that I had never been confused about the distance of SNe and the 'real' meaning of when we see/detect those photons. I'll pull some of those out if you like.




You imply that zero travel time means that they'd all get here at the same time.

No, sorry, but I did not even come close to implying that. In fact, right in the same quote that you bolded..., been doing here, while we...



Originally Posted by RussT
SO, what have all those "Photons that kept getting here in 0 time', during that whole 160,000 years of continuous emission of SN light, been doing here, while we, as earth rest frame observers, were waiting for the "Real" photons to get here???

I stated quite clearly... during that whole 160,000 years of continuous emission of SN light

But rather than responding to all the rest of the above, I'll just show one more thing KenG said and then ask the correct question.



There's no distinction between the ones that took 0 time (in their own limit frame) and the real ones we see-- those are the same photons.

IF, the 'real photons' are the ones we see/detect in our telescopes, and those photons can arrive at those telescopes instantly or in 0 time, from where the SN exploded, then why didn't we see/detect those photons from SN 1987A well before 1987?

ETA: I am referring to the "Photons in their own frame", which actually should be obvious since we are talking about them getting here either instantly or in 0 time!

Strange
2010-Feb-07, 12:59 PM
But rather than responding to all the rest of the above, I'll just show one more thing KenG said and then ask the correct question.


There's no distinction between the ones that took 0 time (in their own limit frame) and the real ones we see-- those are the same photons.

IF, the 'real photons' are the ones we see/detect in our telescopes, and those photons can arrive at those telescopes instantly or in 0 time, from where the SN exploded, then why didn't we see/detect those photons from SN 1987A well before 1987?

That is nowhere near the "correct question". (You left out the bit I bolded in KenG's comment)

Do really not see the difference between the time the photons took in their own frame and the time they took in our frame ?

RussT
2010-Feb-07, 01:08 PM
That is nowhere near the "correct question". (You left out the bit I bolded in KenG's comment)

Do really not see the difference between the time the photons took in their own frame and the time they took in our frame ?

Yes, I was just coming back on line on to clarify that I was referring to the "Photons in their own frame" as KenG said that those are the same photons we see/detect.

Strange
2010-Feb-07, 01:31 PM
Yes

Does that mean: Yes, you do see the difference? Or, yes you really don't?


I was just coming back on line on to clarify that I was referring to the "Photons in their own frame" as KenG said that those are the same photons we see/detect.

Of course they are the same photons. How could they not be?

Strange
2010-Feb-07, 01:36 PM
IF, the 'real photons' are the ones we see/detect in our telescopes, and those photons can arrive at those telescopes instantly or in 0 time, from where the SN exploded, then why didn't we see/detect those photons from SN 1987A well before 1987?

Because, they took 168,000 years to get here, in our frame of reference. What is so hard about that?

Hornblower
2010-Feb-07, 02:56 PM
Originally Posted by RussT
IF, the 'real photons' are the ones we see/detect in our telescopes, and those photons can arrive at those telescopes instantly or in 0 time, from where the SN exploded, then why didn't we see/detect those photons from SN 1987A well before 1987?

Because, they took 168,000 years to get here, in our frame of reference. What is so hard about that?

Let me add that the outcome of the thought-experiment exercise in the photon's frame of reference does not change anything about our real-world observations.

If the short answers in forums such as this are not good enough, then get thee to a university and ask the professors in the physics department for permission to audit their courses for a few years. If you still have questions, then come back and ask us for help.

Jeff Root
2010-Feb-07, 07:40 PM
I think that if Russ can meet the prerequisites, he should actually enroll
in a course on special relativity. He needs to be able to interact with
everyone else in the class, which isn't likely if he is only auditing it. He
needs a lab and either a good TA or a very smart lab partner.

-- Jeff, in Minneapolis

RussT
2010-Feb-08, 01:15 PM
Of course they are the same photons. How could they not be?

Quite simply because in the history of astronomy we have never seen/detected one single SN with instantaneous photons.

So that means that there are two seperate groups of photons: those that can travel at 186,282.397 mps (Constant "c") and...
the supposed/assumed ones (and it IS totally "Assumed" and NOT a Maths derived calc and it certainly is NOT an "observation") that can travel instantaneously anywhere in the Universe at any distance to infinity OR get from A to B in 0 time across any distance to infinity OR that the distance is supposed to be contracted to 0 between any points A to B across any distance to infinity.

SO, if a spaceship left SN 1987A and traveled fast enough to reduce the distance to say 7 light years from earth...then IF photons could travel that 7 light years in 0 time...........We would have seen that SN in 1980.

ETA: This would be true for all recorded SN...in other words, IF there were really instantaneous photons, we would see all SN earlier than we do at any distance greater than 0.

So, understanding that ALL of the SN ever recorded have ALL been from very different distances in the near Universe galaxies how can you think that a proper distance and time of 0 is even possible for even one of those let alone ALL of them?

RussT
2010-Feb-08, 01:22 PM
Let me add that the outcome of the thought-experiment exercise in the photon's frame of reference does not change anything about our real-world observations.

Sorry, But this simply not correct at all, although it is probably what Jeff thinks also.

Strange
2010-Feb-08, 01:39 PM
Quite simply because in the history of astronomy we have never seen/detected one single SN with instantaneous photons.

What is an "instantaneous photon"? No one is suggesting we would detect photons taking zero time in our frame of reference - i.e. as measured by us.


So that means that there are two seperate groups of photons: those that can travel at 186,282.397 mps (Constant "c") and...

Errr... no. :confused:

There are two separate sets of measurements. The photons travel at c whoever measures their speed. The distances and times involved differ depending on who does the measurement.

As has been repeatedly pointed out, there is just one group of photons. From their point of view (ignoring for moment the fact that photons can't have a point of view) the time taken is 0; from our point of view the time taken is ~168,000 years.

Is your objection to the very idea of the photon's frame of reference? This would be reasonable because, as has also been pointed out before, strictly speaking they don't have one. (And we can't ask them to measure things, 'cos they are just photons.) But it seems to me that it was you that kept dragging the discussion back to photons and zero travel time.

If, instead of photons, you had a bunch of astonauts on the USS Photon travelling at very nearly the speed of light, then the journey time from SN 1987A to earth as measured by them would be very nearly zero. Of course, as measured by observers on earth, their journey would still take something over 168,000 years.

And if they were traveling at very, very, very near the speed of light then the time would be very, very, very nearly zero - which is why we can talk about photons in the limit.

Simple, isn't it.

tusenfem
2010-Feb-08, 04:44 PM
Sorry, But this simply not correct at all, although it is probably what Jeff thinks also.


RussT, that is more than enough ATM here in Q&A.
You know better than to argue your ATM ideas here.
Infraction given.

Jeff Root
2010-Feb-08, 09:15 PM
Of course they are the same photons. How could they not be?
Quite simply because in the history of astronomy we have never
seen/detected one single SN with instantaneous photons.
You appear to be being deliberately obtuse, Russ. The photons were
emitted 160,000 years ago, as measured by our clocks. If those
photons could carry clocks with them, their clocks would still read
"zero" when they reached Earth. The photons reaching Earth now
are the same photons which were emitted 160,000 years ago by our
clocks, and zero time ago according to the imagined photon clocks.

We see that the photons take 160,000 of our years to get here.
The photons -- if they could see anything -- would see that they did
not take any time at all to get here, by their own clocks. You seem
to be pretending that you don't understand this, instead making silly
remarks about detecting instantaneous photons and photons loitering
around Earth waiting to be detected. Rather than make such silly
comments, you should put a little effort into trying to understand.



So that means that there are two seperate groups of photons: those
that can travel at 186,282.397 mps (Constant "c") and...
the supposed/assumed ones (and it IS totally "Assumed" and NOT a
Maths derived calc and it certainly is NOT an "observation") that can
travel instantaneously anywhere in the Universe at any distance to
infinity OR get from A to B in 0 time across any distance to infinity OR
that the distance is supposed to be contracted to 0 between any
points A to B across any distance to infinity.
You seem to have an inability to accurately paraphrase what someone
tells you, if you disagree with it.

This part of it seems pretty simple, to me. We see the distance from
the LMC to Earth as 160,000 light-years. We know that it takes light
160,000 years to travel from the LMC to Earth, by our clocks. Because
light travels at the speed of light, it "sees" the distance from the LMC
to Earth as zero, and the travel time from the LMC to Earth as zero.



SO, if a spaceship left SN 1987A and traveled fast enough to reduce the
distance to say 7 light years from earth...then IF photons could travel
that 7 light years in 0 time...........We would have seen that SN in 1980.
I'm going to change your parameter slightly because the one you provide
is unworkable.

If a spaceship had left SN 1987A at the moment it exploded, and headed
for Earth such that the trip would take seven years according to clocks
on the ship, then the ship would now still be enroute to Earth, but would
arrive shortly... within a few hundred years of our time after the light
from the supernova arrived in 1987. Before the year 3000 AD.



ETA: This would be true for all recorded SN...in other words, IF there
were really instantaneous photons, we would see all SN earlier than we
do at any distance greater than 0.
Photons really do travel any distance instantaneously in their own
reference frame, which is a reference frame that cannot ever be
experienced by any real observer. Photons cannot be observers.
But if they could be observers, they would always observe zero time
for their trips. That fact has no implications for when we see
supernovae or anything else.



So, understanding that ALL of the SN ever recorded have ALL been from
very different distances in the near Universe galaxies how can you think
that a proper distance and time of 0 is even possible for even one of
those let alone ALL of them?
The proper distance that a photon travels is always zero, because it
does not experience distance. The proper time that a photon spends
travelling is always zero, because a photon does not experience time.

-- Jeff, in Minneapolis

Jeff Root
2010-Feb-08, 09:49 PM
Let me add that the outcome of the thought-experiment exercise in
the photon's frame of reference does not change anything about our
real-world observations.
Sorry, But this simply not correct at all, although it is probably what
Jeff thinks also.
What a photon would observe, if it could observe anything, has no
effect on what a real-world observer sees. If you observe that the
tip of the antenna on the top of the Empire State building is at an
angle of 30 degrees above the horizontal, and I observe that the tip
of the antenna is at an angle of 40 degrees above the horizontal, my
observation has no effect on your observation, and your observation
has no effect on mine. This is analogous to the situation with speed
and distance: For the Empire State building, a difference in position
results in a difference in angle; for relativistic speeds, a difference in
speed results in different measurements of space and time.

-- Jeff, in Minneapolis