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Diamond
2004-Jan-26, 09:50 AM
...without Sam5 wrecking the thread by references to Einstein's papers which say the opposite of what Sam5 says, time dilation being an artifact of clock mechanism interference, arbitrary changes of subject from one fallacy to another, snide references to Einstein's "god-like status", Sam5's inability to do simple math or working out or endless trolling of one relativity thread after another?

I'd actually like to learn some Relativity Theory (since I'll be doing it in anger in a degree course I'm pursuing.

Kaptain K
2004-Jan-26, 09:57 AM
I second that! AFAIK, Sam5 only posts in this forum. Maybe if you post in BABBling, he won't find it. :roll:

Diamond
2004-Jan-26, 10:38 AM
I second that! AFAIK, Sam5 only posts in this forum. Maybe if you post in BABBling, he won't find it. :roll:

...or Against The Mainstream? :roll:

Kaptain K
2004-Jan-26, 10:47 AM
Sad isn't it, that you would have to post a Mainstream topic in Against the Mainstream to get away from someone who hijacks every Mainstream discussion of Relativity with stuff that is so far out of the mainstream that the question is not how to refute it, but where to start? :roll:

Diamond
2004-Jan-26, 10:51 AM
Sad isn't it, that you would have to post a Mainstream topic in Against the Mainstream to get away from someone who hijacks every Mainstream discussion of Relativity with stuff that is so far out of the mainstream that the question is not how to refute it, but where to start? :roll:

*psst* the speed of light is measured to be the same in all coordinate systems. Pass it on.... 8-[

Chip
2004-Jan-26, 11:01 AM
Technically speaking, as outlined in the descriptions, the "General Astronomy" board (which we're in now,) is for "discussing current events, nagging questions, and just plain old astronomy."

The "Against the Mainstream" board is described as the place "to discuss a theory that goes against the astronomical mainstream." The BA adds, "Have a beef with relativity, heliocentrism, the Big Bang? This is the place."

So Q&A and "nagging questions" about established theories (including special and general relativity,) belong in General Astronomy. (That won't prevent someone from jumping in with exceptions, off-the-wall ideas and wrong beliefs, but folks with a "beef with relativity" or any other established theory that applies to astronomy, really should present it and discuss it in "Against the Mainstream.")

I could only advise that if one had simple or complex questions about relativity for the board, go ahead and launch a thread here but start by saying: "If you disagree with relativity, start your own thread in "Against the Mainstream." Then simply post that one statement, (and link to the "Against the Mainstream" thread if it exists,) every time the flow is interrupted with hand-waving and distractions.
:wink:

Diamond
2004-Jan-26, 11:05 AM
Technically speaking, as outlined in the descriptions, the "General Astronomy" board (which we're in now,) is for "discussing current events, nagging questions, and just plain old astronomy."

The "Against the Mainstream" board is described as the place "to discuss a theory that goes against the astronomical mainstream." The BA adds, "Have a beef with relativity, heliocentrism, the Big Bang? This is the place."

So Q&A and "nagging questions" about established theories (including special and general relativity,) belong in General Astronomy. (That won't prevent someone from jumping in with exceptions, off-the-wall ideas and wrong beliefs, but folks with a "beef with relativity" or any other established theory that applies to astronomy, really should present it and discuss it in "Against the Mainstream.")

I could only advise that if one had simple or complex questions about relativity for the board, go ahead and launch a thread here but start by saying: "If you disagree with relativity, start your own thread in "Against the Mainstream." Then simply post that one statement, (and link to the "Against the Mainstream" thread if it exists,) every time the flow is interrupted with hand-waving and distractions.
:wink:

This all seems like hard work. I'll give it a go though....

milli360
2004-Jan-26, 12:19 PM
*psst* the speed of light is measured to be the same in all coordinate systems. Pass it on.... [
Only inertial CS--not so in the GR noninertial ones.

For instance, as an example, the speed of Pluto (a little over 4 lighthours away) in an earth-centered earth-fixed CS. Clearly, it would traverse more than one lightday in one day.

Diamond
2004-Jan-26, 01:25 PM
*psst* the speed of light is measured to be the same in all coordinate systems. Pass it on.... [
Only inertial CS--not so in the GR noninertial ones.

For instance, as an example, the speed of Pluto (a little over 4 lighthours away) in an earth-centered earth-fixed CS. Clearly, it would traverse more than one lightday in one day.

Which is why I mentioned the speed of light not the apparent speed of Pluto... [-X

milli360
2004-Jan-26, 02:08 PM
Which is why I mentioned the speed of light not the apparent speed of Pluto...
In that CS, the speed is actual, not apparent. That's why I pointed out that you should distinguish between "all coordinate systems" and "inertial coordinate systems."

Light, of course, is much faster than Pluto.

Diamond
2004-Jan-26, 02:20 PM
Which is why I mentioned the speed of light not the apparent speed of Pluto...
In that CS, the speed is actual, not apparent. That's why I pointed out that you should distinguish between "all coordinate systems" and "inertial coordinate systems."

Light, of course, is much faster than Pluto.

Are you saying that light from Pluto does not travel at c? Or that when I measure the speed of light here on Earth (a non-inertial coordinate system) and then measure it again out in deep space, I get different answers?

I do know about apparent superluminal speeds being resolved by looking at the geometry and the relativity of simultaneity. Are you referring to that?

Ian Goddard
2004-Jan-26, 02:22 PM
Is it possible to discuss Relativity Theory on this board without Sam5 wrecking the thread by references to Einstein's papers which say the opposite of what Sam5 says [...]
I asked Sam5 (http://www.badastronomy.com/phpBB/viewtopic.php?p=198359#198359) if he could be considerate enough to limit his against-the-mainstream views to the Against the Mainstream forum so that people who come to this forum for mainstream answers can get them. If they want his answers they can go to the other forum. A reasonable situation. His reply to challenge me to refute his views.

Diamond
2004-Jan-26, 02:25 PM
Is it possible to discuss Relativity Theory on this board without Sam5 wrecking the thread by references to Einstein's papers which say the opposite of what Sam5 says [...]
I asked Sam5 (http://www.badastronomy.com/phpBB/viewtopic.php?p=198359#198359) if he could be considerate enough to limit his against-the-mainstream views to the Against the Mainstream forum so that people who come to this forum for mainstream answers can get them. If they want his answers they can go to the other forum. A reasonable situation. His reply to challenge me to refute his views.

Don't tell me, he had the 1911 papers right in front of him. [-(

Edit: Do we spend our time refuting every crackpot theory, or do people have to explain themselves in a way that makes sense first?

milli360
2004-Jan-26, 02:35 PM
Which is why I mentioned the speed of light not the apparent speed of Pluto...
In that CS, the speed is actual, not apparent. That's why I pointed out that you should distinguish between "all coordinate systems" and "inertial coordinate systems."

Light, of course, is much faster than Pluto.

Are you saying that light from Pluto does not travel at c?
Is c the speed of light, or is it 300,000km/sec? In the first case, it's true by definition. In the second case, clearly, in that CS, Pluto is going much faster than 300,000km/sec--and light can travel much faster than Pluto.


Or that when I measure the speed of light here on Earth (a non-inertial coordinate system) and then measure it again out in deep space, I get different answers?
I'm talking about the noninertial coordinate system in which the Earth is fixed and non-rotating.


I do know about apparent superluminal speeds being resolved by looking at the geometry and the relativity of simultaneity. Are you referring to that?
No.

If they want his answers they can go to the other forum. A reasonable situation. His reply to challenge me to refute his views.
"His reply was to challenge me to refute his views."

Yes, he believes that his view is the mainstream view, and that we've got Einstein wrong.

He says he believes in GR, but that it is a correction to SR, and SR is wrong. I agree complelety with that. SR, like Newtonian mechanics, has been supplanted, and is only valid in limited application where the circumstances warrant its use an an approximation, sometimes a very good approximation.

What we disagree on, is where the "mistake" was made, in SR.

PS:

Edit: Do we spend our time refuting every crackpot theory, or do people have to explain themselves in a way that makes sense first?
That's a good question. James Oberg, who posts in the BABB occasionally (http://www.badastronomy.com/phpBB/viewtopic.php?p=131434#131434), has gone on record (Lessons of the 'Fake Moon Flight' Myth (http://www.csicop.org/si/2003-03/commentary.html), Skeptical Enquirer, May 2003) as advocating the latter. He says, "This is the way I see it: If many people who are exposed to the hoaxist arguments find them credible, it is neither the fault of the hoaxists or of their believers--it's the fault of the educators and explainers (NASA among them) who were responsible for providing adequate knowledge and workable reasoning skills."

Personally, I find that to be too forgiving, but I've always tried to take it to heart.

Diamond
2004-Jan-26, 03:10 PM
[quote=Diamond]Which is why I mentioned the speed of light not the apparent speed of Pluto...
In that CS, the speed is actual, not apparent. That's why I pointed out that you should distinguish between "all coordinate systems" and "inertial coordinate systems."

Light, of course, is much faster than Pluto.

Are you saying that light from Pluto does not travel at c?
Is c the speed of light, or is it 300,000km/sec? In the first case, it's true by definition. In the second case, clearly, in that CS, Pluto is going much faster than 300,000km/sec--and light can travel much faster than Pluto.

:-?

If I measure the speed of light in a non-inertial frame of reference, and one in an inertial frame of reference, do I get two different results?

milli360
2004-Jan-26, 03:17 PM
:-?

I thought that was a smiley at first. What is that?


If I measure the speed of light in a non-inertial frame of reference, and one in an inertial frame of reference, do I get two different results?
Probably?

Diamond
2004-Jan-26, 04:31 PM
:-?

I thought that was a smiley at first. What is that?


It's a "what is that?" emoticon



If I measure the speed of light in a non-inertial frame of reference, and one in an inertial frame of reference, do I get two different results?
Probably?

Is that a yes or a no. Is the speed of light frame dependent?

milli360
2004-Jan-26, 05:49 PM
Is that a yes or a no. Is the speed of light frame dependent?
Absolutely. In a non-inertial reference frame, it depends upon the metric coefficients. The reason I said probably, with a question mark, was because I supposed it might be possible to set up things so that the result was equality.

What do you suppose is meant by GR's claim that every coordinate system is allowed?

Taibak
2004-Jan-26, 07:47 PM
Is that a yes or a no. Is the speed of light frame dependent?
Absolutely. In a non-inertial reference frame, it depends upon the metric coefficients. The reason I said probably, with a question mark, was because I supposed it might be possible to set up things so that the result was equality.

What do you suppose is meant by GR's claim that every coordinate system is allowed?

Actually, could you explain this further, please? As I understand GR, one of the fundamental postulates is that the speed of light is constant even in non-inertial frames. Was this postulate simply a means to restrict the theory to reference frames that actually exist, or is GR more general than that, which, if I'm reading this right, seems to be implied?

milli360
2004-Jan-26, 08:24 PM
GR more general than that, which, if I'm reading this right, seems to be implied?
General Relativity, as its name implies, is more general than that. The laws of physics are the same, according to GR, no matter which coordinate system one uses--even noninertial ones. For convenience, we can find local inertial reference frames in which the theory of special relativity applies, but obviously special relativity (which does have the constancy of the speed of light as a postulate) will not hold up in a noninertial reference frame like ECEF (Earth-centered, Earth-fixed).

Normandy6644
2004-Jan-26, 10:04 PM
GR more general than that, which, if I'm reading this right, seems to be implied?
General Relativity, as its name implies, is more general than that. The laws of physics are the same, according to GR, no matter which coordinate system one uses--even noninertial ones. For convenience, we can find local inertial reference frames in which the theory of special relativity applies, but obviously special relativity (which does have the constancy of the speed of light as a postulate) will not hold up in a noninertial reference frame like ECEF (Earth-centered, Earth-fixed).

Yeah, to make it sound more complicated (but super cool! 8) ) GR shows that the metric is invariant under coordinate transformations. Even though that's only a small part of it, it is fundamental for the basis of GR.

russ_watters
2004-Jan-26, 10:13 PM
...without Sam5 wrecking the thread by references to Einstein's papers which say the opposite of what Sam5 says, time dilation being an artifact of clock mechanism interference, arbitrary changes of subject from one fallacy to another, snide references to Einstein's "god-like status", Sam5's inability to do simple math or working out or endless trolling of one relativity thread after another? 'fraid not. I'm up for some BA intervention, but until then - tough luck. :-?

Taibak
2004-Jan-27, 06:24 AM
GR more general than that, which, if I'm reading this right, seems to be implied?
General Relativity, as its name implies, is more general than that. The laws of physics are the same, according to GR, no matter which coordinate system one uses--even noninertial ones. For convenience, we can find local inertial reference frames in which the theory of special relativity applies, but obviously special relativity (which does have the constancy of the speed of light as a postulate) will not hold up in a noninertial reference frame like ECEF (Earth-centered, Earth-fixed).

Yeah, to make it sound more complicated (but super cool! 8) ) GR shows that the metric is invariant under coordinate transformations. Even though that's only a small part of it, it is fundamental for the basis of GR.

Gotcha. Now, not to belabor this, but I'm trying to make this clear in my head. I realise that GR reduces to SR when in an inertial frame and that the more general theory is needed whenever you have gravity or an acceleration. My question is where does this leave the speed of light? Does GR treat that as an invariant law of physics (i.e., if you perform a coordinate transformation and the speed of light changes you've screwed up the math) or does it hold true in reference frames where the speed of light is not c, *regardless of whether or not those frames exist in nature?* If I remember right it's the former, as c is built into the metric but for some reason I'm getting confused here.

Thanks though!

Taibak

Chip
2004-Jan-27, 06:44 AM
Here's a nice little summary of SR for anyone flying along the course of this thread who needs a quick reference:

http://en.wikipedia.org/wiki/Special_relativity

:wink:

Diamond
2004-Jan-27, 08:37 AM
Is that a yes or a no. Is the speed of light frame dependent?
Absolutely. In a non-inertial reference frame, it depends upon the metric coefficients. The reason I said probably, with a question mark, was because I supposed it might be possible to set up things so that the result was equality.

What do you suppose is meant by GR's claim that every coordinate system is allowed?

No that doesn't make sense to me at all.

Is the speed of light, when measured in a inertial laboratory and a non-inertial laboratory, measured to be the same or not?

Edit: Or do you mean according to the bookkeeper?

milli360
2004-Jan-27, 09:32 AM
No that doesn't make sense to me at all.

Is the speed of light, when measured in a inertial laboratory and a non-inertial laboratory, measured to be the same or not?
Here's a thought experiment. Consider the so-called twin paradox (which we know is not a paradox). Start the measurement of the speed of light at the same time that you start the twin experiment. Have the experiment arranged so that the twin that leaves arrives back at the same time that the light returns. When the light arrives, which twin's clock are you going to use?

Diamond
2004-Jan-27, 09:34 AM
No that doesn't make sense to me at all.

Is the speed of light, when measured in a inertial laboratory and a non-inertial laboratory, measured to be the same or not?
Here's a thought experiment. Consider the so-called twin paradox (which we know is not a paradox). Start the measurement of the speed of light at the same time that you start the twin experiment. Have the experiment arranged so that the twin that leaves arrives back at the same time that the light returns. When the light arrives, which twin's clock are you going to use?

Do you mean "according to the bookkeeper"?

milli360
2004-Jan-27, 09:59 AM
Do you mean "according to the bookkeeper"?
Who's the bookkeeper??

Diamond
2004-Jan-27, 10:22 AM
Do you mean "according to the bookkeeper"?
Who's the bookkeeper??

The one who keeps the books (http://www.amazon.com/exec/obidos/tg/detail/-/020138423X/qid=1075198913/sr=1-1/ref=sr_1_1/104-8818445-0418327?v=glance&s=books).

milli360
2004-Jan-27, 10:34 AM
Amazon.com (http://www.alcoholics-anonymous.org/)?

Diamond
2004-Jan-27, 11:02 AM
Amazon.com (http://www.alcoholics-anonymous.org/)?

I think we're missing the point here. If you measure the total length of the worldline of the light and the time taken for the light, you don't get c. But every observer on that worldline would locally measure the speed of the light to be c.

Did I miss something out? :-?

rwald
2004-Jan-27, 11:11 AM
Well, because the light will always go along at a 45 degree angle from the x axis and time is just measured along the t axis, it would appear that the "length" of the light's worldline is always going to be sqrt(2) * t. However, if I'm not mistaken, the Pythagorean Theorum that I just used to compute the "length" doesn't work in Minkowski space; rather, s^2 = t^2 - x^2. So the real "length" of the light's worldline (called the "invarient space-time interval," or just interval) is zero. As it is in all reference frames.

Diamond
2004-Jan-27, 11:20 AM
Well, because the light will always go along at a 45 degree angle from the x axis and time is just measured along the t axis, it would appear that the "length" of the light's worldline is always going to be sqrt(2) * t. However, if I'm not mistaken, the Pythagorean Theorum that I just used to compute the "length" doesn't work in Minkowski space; rather, s^2 = t^2 - x^2. So the real "length" of the light's worldline (called the "invarient space-time interval," or just interval) is zero. As it is in all reference frames.

Then it might be difficult to measure the speed of light wouldn't it? I did not say that the worldline of light was from the point of view of the light itself but from the point of view of an observer receiving the light.

rwald
2004-Jan-27, 12:08 PM
It's easy to measure the speed of light. You take a meter stick (or equivalent), have a light emitter on one end, and a photosensor on the other. You use a clock in your frame to measure how long it takes the light to cross the meter stick. That's the speed of light. The invarient space-time interval effectively tells you how "long" some space-time distance is in the reference frame of an object traveling along that distance. If the interval is positive, it tells you that the two events could have been observered by a person traveling the appropriated speed to have been in the same place and the interval apart in time. If the interval is negative, then some obsever thinks they happened at the same time and were the interval apart in space. In the reference frame of light, time doesn't exist. So of course its space-time interval is going to be zero. That means that in the light's own reference frame, it doesn't have velocity. But since you can never reach light's reference frame, that doesn't do you much good.