Why do you need to measure the speed of light in two directions?

In summary: I think what PAllen is trying to say is that there are a variety of universes we can imagine, some in which slow clock transport synchronization and Einstein synchronization yield different results, and others in which they yield the same result.
  • #1
goodabouthood
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I have another thread going right now but I don't want you to refer to that thread. I frankly don't understand what is going on in that thread so please answer my question here.

Why do you need to measure the speed of light in both directions for an accurate reading?

If I am in an inertial frame, and the light goes from mirror A to B in one light second, do I have an accurate description of the speed of light?

Am I totally missing the point here?
 
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  • #2
goodabouthood said:
If I am in an inertial frame, and the light goes from mirror A to B in one light second, do I have an accurate description of the speed of light?
Yes, the description is accurate, but it also depends on your synchronization convention. So you are measuring what you assumed via your synchronization convention.
 
  • #3
goodabouthood said:
I have another thread going right now but I don't want you to refer to that thread. I frankly don't understand what is going on in that thread so please answer my question here.

Why do you need to measure the speed of light in both directions for an accurate reading?

If I am in an inertial frame, and the light goes from mirror A to B in one light second, do I have an accurate description of the speed of light?

Am I totally missing the point here?

If you can be sure that the clocks at A and B are synchronised, then you can measure the speed of light on the one-way journey from A to B. The problem is how to synchronise the clocks: you cannot send signals from A to B faster than the speed of light. In fact, to synchronise your clocks you will need to use knowledge of relativity that already assumes the speed of light: the whole thing becomes circular.

You can get rid of the synchronisation problem by shining a beam from A, reflecting it at B and measuring the time the light takes on the journey ABA. In this case you only need one clock.
 
  • #4
  • #6
goodabouthood said:
...the light goes from mirror A to B in one light second...
Just for your own edification, a light second is a distance, not a duration of time.

You said the mirrors are 186,262 miles apart, but nothing about how long light takes to traverse them.
 
  • #7
I will add an additional perspective to this. If you synchronize clocks over distance using EM signals, then one way light speed measurement fails to constitute and independent measurement. As others noted, it will come out c by construction. Further, in SR, other plausible synchronization methods (e.g. slow clock transport) are equivalent to light synchronization, thus providing no new information.

However, against a pool of theories that includes those not consistent with SR, slow clock transport is an independent synchronization method (and in one way speed of light measurement is possible), and the finding that SR's predictions are correct, counts, in my mind, as and independent confirmation of SR. Unfortunately, within SR, it still fails to independently establish one way constancy of speed of light.
 
  • #8
PAllen said:
However, against a pool of theories that includes those not consistent with SR, slow clock transport is an independent synchronization method (and in one way speed of light measurement is possible), and the finding that SR's predictions are correct, counts, in my mind, as and independent confirmation of SR.
Theories inconsistent with SR are confirmation of SR? How does that work?
 
  • #9
ghwellsjr said:
Theories inconsistent with SR are confirmation of SR? How does that work?
I think what PAllen is trying to say is that there are a variety of universes we can imagine, some in which slow clock transport synchronization and Einstein synchronization yield different results, and others in which they yield the same result. In our universe, it is an experimental fact that they yield the same result. This is a significant reason to believe in SR.
 
  • #10
ghwellsjr said:
Theories inconsistent with SR are confirmation of SR? How does that work?

Did you read what I wrote? Among a pool that includes those not consistent with SR (by obvious implication also including SR), then a one way light light speed measurement using slow clock transport that measures c independent of inertial states of motion, is a confirmation of SR. Rejected is any theory that allows measurement of a different one way speed. In particular, time dilation for slow clock transport in different frames of reference must follow exactly the law derived under a constancy of one way c assumption (whether or not light 'really' moves at c in all directions)'.
 
  • #11
PAllen said:
Did you read what I wrote? Among a pool that includes those not consistent with SR (by obvious implication also including SR), then a one way light light speed measurement using slow clock transport that measures c independent of inertial states of motion, is a confirmation of SR. Rejected is any theory that allows measurement of a different one way speed. In particular, time dilation for slow clock transport in different frames of reference must follow exactly the law derived under a constancy of one way c assumption (whether or not light 'really' moves at c in all directions)'.
Yes, I read it and read it again and again. I tried to figure out what in the world you were trying to say and how that would add to the OP's question in which he already stated that he doesn't understand what's going on in his other thread and he doesn't want us to refer to that other thread.

In particular, I read that you are claiming that there are other theories in which the "one way speed of light measurement is possible" (contrary to what I and others and Einstein say that it's impossible to measure the one-way speed of light, that is, we cannot know how long it takes light to propagate from one point to another).

So I have two questions:

What theory is there (that comports with reality) and claims the "one way speed of light measurement is possible"?

How is this going to help the OP?
 
  • #12
ghwellsjr said:
Yes, I read it and read it again and again. I tried to figure out what in the world you were trying to say and how that would add to the OP's question in which he already stated that he doesn't understand what's going on in his other thread and he doesn't want us to refer to that other thread.
I apologize if I wasn't clear enough. lugita15 got exactly what I meant. I am referring to a common concept in theory verification - to verify the predictions of one theory, you want a different theory that is, in some sense(s) plausible, that makes different predictions.
ghwellsjr said:
In particular, I read that you are claiming that there are other theories in which the "one way speed of light measurement is possible" (contrary to what I and others and Einstein say that it's impossible to measure the one-way speed of light, that is, we cannot know how long it takes light to propagate from one point to another).
But the reason we cannot know is because SR is true. It is only because of SR's time dilation law that slow clock transport behaves the same as light synchronization.
ghwellsjr said:
So I have two questions:

What theory is there (that comports with reality) and claims the "one way speed of light measurement is possible"?
Obviously, not a theory that comports with reality. However, two obvious examples of 'candidate' theories for which slow clock transport would have no correlation with light synchronization, and for which a measurement of one way speed of light different than c is possible are: (1) Newton's corpuscular theory of light (including Galilean relativity) (2) Galilean relativity plus Aether theories of light other than Lorentz's.
ghwellsjr said:
How is this going to help the OP?

It will help by explaining that it is actually useful to do one way light speed measurements. If SR were wrong, different plausible synchronization methods need not be equivalent; this could show up as different one way speed measurements for synchronization methods that don't directly use EM signals.

Within SR, it is useful to show that slow clock transport must measure the same one way speed of light in any inertial frame due to the formula for time dilation.
 
  • #13
PAllen said:
I apologize if I wasn't clear enough.
Thank you, I appreciate your additional clarification. However, I also have additional comments.
PAllen said:
lugita15 got exactly what I meant. I am referring to a common concept in theory verification - to verify the predictions of one theory, you want a different theory that is, in some sense(s) plausible, that makes different predictions.
I always thought you verified the predictions of a theory by doing experiments.
PAllen said:
But the reason we cannot know is because SR is true. It is only because of SR's time dilation law that slow clock transport behaves the same as light synchronization.
No, the reason why we cannot know how long it takes light to propagate from one point to another is because of the truth of the principle of relativity, not because of any theory. The principle of relativity is only the first postulate of both SR and LET. They both define how light propagates differently but neither one can make the exclusive claim to be true. The principle of relativity assures us that any experiment we do in one state of inertial motion will yield the same result in another state of inertial motion. That by itself, without any additional theory about time dilation, will guarantee the same measurements will be made.
PAllen said:
Obviously, not a theory that comports with reality. However, two obvious examples of 'candidate' theories for which slow clock transport would have no correlation with light synchronization, and for which a measurement of one way speed of light different than c is possible are: (1) Newton's corpuscular theory of light (including Galilean relativity) (2) Galilean relativity plus Aether theories of light other than Lorentz's.
If your point is to show how prior to doing experiments, scientists adhering to these incorrect theories thought they could measure different one-way speeds of light for different states of inertial motion, then I can see some merit. But I don't think that is what you are saying.
PAllen said:
It will help by explaining that it is actually useful to do one way light speed measurements. If SR were wrong, different plausible synchronization methods need not be equivalent; this could show up as different one way speed measurements for synchronization methods that don't directly use EM signals.
No, it would help to explain that any attempt to measure the one-way speed of light is exactly and identically an attempt to measure the rest state of the ether or to measure a non-zero ether wind. It would help to point out that all such measurements always yield a null result, no matter how they are performed. Any experiment you perform to measure the one-way speed of light will always yield a constant value of c. Two observers in relative motion measuring the same one-way light beam will each measure the same value of c which doesn't make sense. That is the reason we need a theory to explain this incomprehensible result. Prior to Einstein, that theory was LET which explained everything perfectly and allowed for the one-way speed of light to be c only in the ether frame but which explained why an observer in motion to the ether would still measure the one-way beam to be c, because his rulers and clocks were fooling him into thinking he was making a valid measurement. Einstein, on the other hand, took a different approach and came up with a different theory. But the only way to prove one theory true and the other theory false is to be able to measure the one-way speed of light which is impossible to do.
PAllen said:
Within SR, it is useful to show that slow clock transport must measure the same one way speed of light in any inertial frame due to the formula for time dilation.
Time dilation, and length contraction, and the relativity of simultaneity--they're all needed to explain why any measurement of the one-way speed of light yields c. But it is also useful to show a similar explanation within LET and the two theories treat the one-way speed of light differently and there is no measurement that can help us decide which one is true.
 
  • #14
ghwellsjr said:
I always thought you verified the predictions of a theory by doing experiments.
You really claim to have no idea about the use of test theories to provide alternate predictions (that you check to see if they are rejected)? In SR, GR, QM, I read experimental papers all the time that are couched in terms of: if the universe worked like conceivable model/theory X instead of the currently accepted model, you might measure some specific deviation from what is expected by currently accepted theory. A recent example in QM were test theories that allowed for new types of 3 slit interference effects, compared to standard QM.
ghwellsjr said:
No, the reason why we cannot know how long it takes light to propagate from one point to another is because of the truth of the principle of relativity, not because of any theory. The principle of relativity is only the first postulate of both SR and LET. They both define how light propagates differently but neither one can make the exclusive claim to be true. The principle of relativity assures us that any experiment we do in one state of inertial motion will yield the same result in another state of inertial motion. That by itself, without any additional theory about time dilation, will guarantee the same measurements will be made.
You seem to insist that the universe of conceivable theories is SR and LET. For me, mostly, I don't event distinguish these given that they are mathematically equivalent.

Galilean relativity has the same relativity postulate as SR/LET - physics cannot distinguish rest, nor distinguish among different inertial frames. What it lacks is an additional postulate concerning light. Galilean relativity combined with a corpuscular theory of light allows one way speed of light measurements. It would predict that such measurements are frame dependent, just like the speed of bullets is frame dependent. If this were actually measured, we would now be saying the (Galilean) principle of relativity is confirmed. We would still have the view that absolute motion is undetectable.

As for time dilation, both SR and LET are identical in having this phenomenon, and in all predictions about it. They only differ in explanation - is it caused by moving through aether or not?
ghwellsjr said:
No, it would help to explain that any attempt to measure the one-way speed of light is exactly and identically an attempt to measure the rest state of the ether or to measure a non-zero ether wind.
No, these are far from the only conceivable possibilities.
ghwellsjr said:
It would help to point out that all such measurements always yield a null result, no matter how they are performed. Any experiment you perform to measure the one-way speed of light will always yield a constant value of c. Two observers in relative motion measuring the same one-way light beam will each measure the same value of c which doesn't make sense. That is the reason we need a theory to explain this incomprehensible result. Prior to Einstein, that theory was LET which explained everything perfectly and allowed for the one-way speed of light to be c only in the ether frame but which explained why an observer in motion to the ether would still measure the one-way beam to be c, because his rulers and clocks were fooling him into thinking he was making a valid measurement. Einstein, on the other hand, took a different approach and came up with a different theory. But the only way to prove one theory true and the other theory false is to be able to measure the one-way speed of light which is impossible to do.
No experiment can distinguish these two theories. Don't know why you insist these are the only two conceivable theories. Another relevant example comes to mind - so called 'doubly special relativity'. These made predictions different from SR. I believe that even most original proponents of such now agree that new experiments and observations have ruled them out. This is a perfect example of the role test theories (whether they are constructed as such, or seriously believed by proponents). DSR suggested new observations and analyses should be done that had not previously been done. Ruling out DSR further increases our confidence in SR.
 
  • #16
lugita15 said:
Ohanian does a good job of arguing why slow clock transport allows for genuine experimental confirmation of the second postulate:
http://books.google.com/books?id=4DunN-eD3VIC&source=gbs_navlinks_s
The discussion is around page 95.

But if you think about the argument given, it follows that:

- if the Titanic were at rest, slow clock transport would agree with Einstein synchronization. If in motion, it could disagree. If this occurred, that would violate the principle of relativity (you could detect a preferred frame).

What I think is the case is:

If you assume the principle of relativity and the constancy of the two way speed of light, and that relative motion of emitter and receiver has no effect on light speed (one way or two way, if conceivably different), then it follows that any difference between slow clock transport and Einstein synchronization would violate these principles. I believe it is necessary to have all of these assumptions to force the equivalence (or other equivalent sets; but they are less interesting, because equivalent sets typically include isotropy, which is victory by definition, directly).

Where I differ from gwellsjr, is that for testing purposes, you may want to ask what range of theories could I confirm with some experiment, and these would include theories that don't satisfy all the above assumptions. For universe of theories including SR/LET plus others, one way light speed measurement with slow transport would be direct measurement of one way speed of light.

Within theories sharing the assumptions above, one way light speed measurement using slow clock transport would constitute a robust test of the validity of these principles acting together. It would test much more than a measurement of constancy of two way speed of light.
 
  • #17
lugita15 said:
I think what PAllen is trying to say is that there are a variety of universes we can imagine, some in which slow clock transport synchronization and Einstein synchronization yield different results, and others in which they yield the same result. In our universe, it is an experimental fact that they yield the same result.

Sorry to interrupt the discussion asking for a small clarification: when you say that SCT yields the same result as Einstein convention, I suppose you mean that it *would* yield that equivalent result in the *limit*, as v of the transported clock approached zero.

lugita15 said:
This is a significant reason to believe in SR.

Here I suppose you mean that the validation of SR comes from the opposite, i.e. from the two methods NOT yielding the same result: in practice, as the limit of v = 0 cannot be reached if we still want transport, the transported clock will have some velocity, no matter how small it is. If so, there will be a difference between the readings of the traveling clock and the destination clock. This difference will be the one predicted by SR: the traveling clock will lag behind by the time dilation factor (the v in this factor having been measured with clocks synced through the Einstein convention). Right?
 
  • #18
PAllen said:
Further, in SR, other plausible synchronization methods (e.g. slow clock transport) are equivalent to light synchronization, thus providing no new information.
Any theory which is physically possible would show that slow clock transport is equivalent to light synchronization since that is physically testable. And since light synchronization is "incorrect" according to some of those theories then slow clock synchronization would be incorrect by the same amount. So such theories would agree on the experimental outcome, but not on the one way speed of light.
 
  • #19
Saw said:
Sorry to interrupt the discussion asking for a small clarification: when you say that SCT yields the same result as Einstein convention, I suppose you mean that it *would* yield that equivalent result in the *limit*, as v of the transported clock approached zero.
Yes, the word "slow" in slow transport means the limit as the speed goes to zero.
 
  • #20
PAllen said:
You really claim to have no idea about the use of test theories to provide alternate predictions (that you check to see if they are rejected)?
Yes.
PAllen said:
In SR, GR, QM, I read experimental papers all the time that are couched in terms of: if the universe worked like conceivable model/theory X instead of the currently accepted model, you might measure some specific deviation from what is expected by currently accepted theory. A recent example in QM were test theories that allowed for new types of 3 slit interference effects, compared to standard QM.
But the stated purpose of this forum is help people learn the Theory of Relativity, not argue its validity, and I'm trying to do the former, not the latter.
PAllen said:
You seem to insist that the universe of conceivable theories is SR and LET. For me, mostly, I don't event distinguish these given that they are mathematically equivalent.
At the time Einstein wrote his 1905 paper, that universe did not include SR but it did include LET. His paper was written in that context. If you don't distinguish between SR and LET, then you're missing the whole point of the second postulate which is what distinguishes SR from LET.
PAllen said:
Galilean relativity has the same relativity postulate as SR/LET - physics cannot distinguish rest, nor distinguish among different inertial frames. What it lacks is an additional postulate concerning light. Galilean relativity combined with a corpuscular theory of light allows one way speed of light measurements. It would predict that such measurements are frame dependent, just like the speed of bullets is frame dependent. If this were actually measured, we would now be saying the (Galilean) principle of relativity is confirmed. We would still have the view that absolute motion is undetectable.
The OP asked why we can't measure the one-way speed of light and you again state that there is a theory that allows for this (except it doesn't work).
PAllen said:
As for time dilation, both SR and LET are identical in having this phenomenon, and in all predictions about it. They only differ in explanation - is it caused by moving through aether or not?

No, these are far from the only conceivable possibilities.

No experiment can distinguish these two theories. Don't know why you insist these are the only two conceivable theories.
And since one of them claims that light propagates at c only in a single ether frame and the other claims that light propagates at c in any frame, this is your best statement that it is impossible to measure the one-way speed of light.
PAllen said:
Another relevant example comes to mind - so called 'doubly special relativity'. These made predictions different from SR. I believe that even most original proponents of such now agree that new experiments and observations have ruled them out. This is a perfect example of the role test theories (whether they are constructed as such, or seriously believed by proponents). DSR suggested new observations and analyses should be done that had not previously been done. Ruling out DSR further increases our confidence in SR.
If you had affirmed that it is impossible to measure the one-way speed of light as evidenced by all these failed attempts to find a way around this, then it would have been in line with the OP's question and his specific request not to go into arguments that confuse him, but the way you have been doing this has only casts doubt on the very issue he is asking about.
 
  • #21
DaleSpam said:
Any theory which is physically possible would show that slow clock transport is equivalent to light synchronization since that is physically testable. And since light synchronization is "incorrect" according to some of those theories then slow clock synchronization would be incorrect by the same amount. So such theories would agree on the experimental outcome, but not on the one way speed of light.

Theories that make no different predictions are what I call interpretations rather than distinct theories. In making a measure of the one way speed of light, you are testing and rejecting the conceivable theories that allow it to measured as different. Similarly, in comparing different clock synch techniques, you are confirming conceivable theories that predict they are the same, and rejecting those that predict differences.
 
  • #22
PAllen said:
Theories that make no different predictions are what I call interpretations rather than distinct theories.
That is fine, I tend to agree with this. But then you cannot make the isotropy of the one way speed of light part of the theory since some of the different interpretations don't require it. I think that is a good idea, although it may not be the best pedagogical approach.
 
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  • #23
Can we all agree on the following:
1. Relative to the Einstein synchronization, Einstein's second postulate regarding the one-way speed of light is a tautology.
2. Relative to the slow clock transport synchronization, Einstein's second postulate is an experimentally testable fact.
3. The equivalence of Einstein synchronization and slow clock transport synchronization is an experimentally testable fact.
4. The equivalence of the two methods is also a theoretical consequence of SR.
Beyond these four facts, I think the question of whether we can measure the one-way speed of light is largely a semantic or interpretational issue.
 
  • #24
lugita15 said:
Ohanian does a good job of arguing why slow clock transport allows for genuine experimental confirmation of the second postulate:
http://books.google.com/books?id=4DunN-eD3VIC&source=gbs_navlinks_s
The discussion is around page 95.
And here is the start of his discussion on page 95:
And this was Einstein's big mistake: He forgot that besides synchronization with light signals there are other synchronization procedures--such as synchronization with transported clocks--by means of which it is possible to detect his trick and expose it as fraudulent. Synchronization by light signals does not permit us to check whether the one-way speed of light is really constant. But synchronization by other procedures permits us to check whether the one-way speed of light is really constant.
But Einstein did not forget about the slow transport of clocks and he specifically addressed it in his 1905 paper in section 4 entitled: Physical Meaning of the Equations Obtained in Respect to Moving Rigid Bodies and Moving Clocks.

If you look down near the end of that section you will read:
"If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by ½tv2/c2 (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B."​
His formula for the error in the transported clock, ½tv2/c2, is not exact, it is an approximation that applies only to slow moving clocks. This popular idea that slow transport of clocks is equivalent to Einstein's synchronization convention is wrong. No matter how slowly the clock is moved, it will be out of synch and Einstein tells us by how much. Don't let the v squared function fool you as if the error becomes rapidly vanishingly small as v approaches zero. If we replace v/c with β and use compatible units with d being the distance, the expression reduces to:

½dβ

So for any given distance, the error is proportional to the speed. And the time to move the clock is inversely proportional to the speed.

In any case, this patent clerk was well aware of the slow transport of clocks and rejected it as an equivalent method of synchronization.
 
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  • #25
ghwellsjr said:
This popular idea that slow transport of clocks is equivalent to Einstein's synchronization convention is wrong. No matter how slowly the clock is moved, it will be out of synch and Einstein tells us by how much. Don't let the v squared function fool you as if the error becomes rapidly vanishingly small as v approaches zero. If we replace v/c with β and use compatible units with d being the distance, the expression reduces to:

½dβ

So for any given distance, the error is proportional to the speed. And the time to move the clock is inversely proportional to the speed.
But if the error is proportional to the speed, then clearly in the limit as the speed goes the zero the error will also go to zero, which is what matters. We can spell out the slow transport synchronization method more precisely as follows: Consider two clocks A and B located at different positions, and let C be a clock which moves from A to B and which is initially synchronized with A. Then we say that A and B are slow-transport synchronized if the limit of the error between clocks B and C, as the speed of clock C goes to zero, is equal to zero.
 
  • #26
ghwellsjr said:
If you look down near the end of that section you will read:
"If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by ½tv2/c2 (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B."​
His formula for the error in the transported clock, ½tv2/c2, is not exact, it is an approximation that applies only to slow moving clocks. This popular idea that slow transport of clocks is equivalent to Einstein's synchronization convention is wrong. No matter how slowly the clock is moved, it will be out of synch and Einstein tells us by how much. Don't let the v squared function fool you as if the error becomes rapidly vanishingly small as v approaches zero. If we replace v/c with β and use compatible units with d being the distance, the expression reduces to:

½dβ

Disagree with your arithmetic here. I see:

1/2 d β / c

Further, you could do the experiment at a number of speeds and extrapolate to zero.
 
  • #27
lugita15 said:
Can we all agree on the following:
1. Relative to the Einstein synchronization, Einstein's second postulate regarding the one-way speed of light is a tautology.
2. Relative to the slow clock transport synchronization, Einstein's second postulate is an experimentally testable fact.
3. The equivalence of Einstein synchronization and slow clock transport synchronization is an experimentally testable fact.
4. The equivalence of the two methods is also a theoretical consequence of SR.
Beyond these four facts, I think the question of whether we can measure the one-way speed of light is largely a semantic or interpretational issue.

Works for me.
 
  • #28
DaleSpam said:
That is fine, I tend to agree with this. But then you cannot make the isotropy of the one way speed of light part of the theory since some of the different interpretations don't require it. I think that is a good idea, although it may not be the best pedagogical approach.

You can still make isotropy of lightspeed part of an interpretation, even if it is not independently verifiable within a given theory. Motivating this would be other phenomena which suggest isotropy is a fundamental aspect of universal laws.
 
  • #29
lugita15 said:
Can we all agree on the following:
1. Relative to the Einstein synchronization, Einstein's second postulate regarding the one-way speed of light is a tautology.
2. Relative to the slow clock transport synchronization, Einstein's second postulate is an experimentally testable fact.
3. The equivalence of Einstein synchronization and slow clock transport synchronization is an experimentally testable fact.
4. The equivalence of the two methods is also a theoretical consequence of SR.
Beyond these four facts, I think the question of whether we can measure the one-way speed of light is largely a semantic or interpretational issue.

Very well said!

Especially the first point.
 
  • #30
Here is another approach to arriving at SR, in which measurement of one way speed of light (e.g. by slow clock transport) is fundamental.

You start by accepting the principle of relativity as fundamental, as it was from Newton/Galileo until the 1800s, when it came into some doubt. Then ask, let us measure how light fits into the framework of relativity. In this framework, it is absolutely meaningful to measure the one way speed of light. You find it is constant, and isotropic. You scratch your head, and think about how to keep relativity and this new finding. You arrive at SR, or something mathematically equivalent to it.
 
  • #31
PAllen said:
You can still make isotropy of lightspeed part of an interpretation, even if it is not independently verifiable within a given theory. Motivating this would be other phenomena which suggest isotropy is a fundamental aspect of universal laws.
Sure. You can make it part of an interpretation without even bothering with explanation or motivation.
 
  • #32
Regarding that surprising surprise by ghwellsjr on the use of broader test theories (or theory classes) to test a given theory, here are a few recent papers specifically discussing one-way light speed measurements in relation to test theory classes that include SR as a subset, with the aim to either confirm or reject SR. In my view, discussing test philosophy is in no way disputing a theory. On the contrary, in my view, you must conceive of violation of a theory in order to test it.

http://arxiv.org/abs/1010.2057

http://arxiv.org/abs/1011.1318

http://arxiv.org/abs/1103.6086
 
  • #33
PAllen said:
Disagree with your arithmetic here. I see:

1/2 d β / c

Further, you could do the experiment at a number of speeds and extrapolate to zero.

I assume this wasn't really a mistake, but the comment about units means 'assuming c=1'. And the point being that, for a given distance, halving the speed only halves the error. True.

Not very relevant for two reasons:

1) For typical scenarios, the error is completely ignorable. E.g. for a proposed one way light measurement over 1 km, moving clock at 1 km/hr, you get an error of about 1.5*10^-15.

2) Further, as I and others mentioned, you could start with, e.g. 4 clocks synchronized at one place, move 3 at different speeds to a destination, and extrapolate the differences to zero speed.

This is why essentially all authors, whatever their other views on these matters, state that is a prediction or requirement of SR that slow clock transport will match Einstein light synchronization. In general, if you have a theory that says two different procedures must be equivalent, it is something you want to test.
 
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  • #34
Hmm, using units where the speed of light is 1 and where the distance to be transported is also 1 then I get that the time on the light synchronized clock when the transported clock reaches the destination is:
[itex]\Delta t=1/v[/itex]

and the time on the transported clock when it reaches the destination is:
[itex]\sqrt{\Delta \tau^2}=\sqrt{\Delta t^2-\Delta x^2-\Delta y^2-\Delta z^2}=\sqrt{1/v^2-1} = 1/v - v/2 +O(v^3)[/itex]

So the error is:
[itex]\Delta \tau - \Delta t = -v/2 + O(v^3)[/itex]

Which does indeed go to 0 as v goes to 0, but only linearly in v.
 
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  • #35
Please go back and read my post #24. I was responding to Ohanian's accusation that Einstein was not aware of the slow transport of clocks and made a big mistake and was guilty of fraud and trickery. He also emphasized that the slow transport of clocks allows us to determine that the one-way speed of light is really constant whereas Einstein's convention does not.

In any case, it is only in the context of Einstein's arbitrary synchronization convention that you can calculate the "error" in a slowly transported clock. Einstein didn't call it an error and I shouldn't have called it an error. It's a difference in the times on two clocks where one is synchronized by Einstein's convention displaying coordinate time and the other one is moved displaying proper time. It's no different than an analysis of what happens to moving clocks (according to SR) such as in the twin paradox which is what Einstein then lead his discussion into in his 1905 paper. Ohanian is making a big mistake by thinking that the one-way speed of light is really constant independent of a synchronization convention.

But the real issue is, does a moving clock lose exactly the same amount of time when you move it from point A to point B as it does when you move it back from point B to point A? If you analyze it according to SR in a frame where A and B are stationary, then the answer is yes (because it is defined to be such). But if you transform to a different frame which is moving in the direction from A to B, the answer is no. This is because there is a different time dilation as the clock is moving in the two different directions. There is of course a fixed amount of time dilation while the clock is stationary at A or B but when it moves from A to B the time dilation increases and when it moves back from B to A the time dilation decreases. The net difference in time compared to a clock that remained at A is the same no matter what frame is used but that difference is made up of two unequal times corresponding to the trips in each direction. This difference is frame dependent.
 
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<h2>1. Why is it important to measure the speed of light in two directions?</h2><p>The speed of light is considered to be a fundamental constant in physics and has a significant impact on our understanding of the universe. By measuring the speed of light in two different directions, we can confirm that it is the same in all directions, which supports the principle of isotropy. This also allows us to test various theories and models, such as Einstein's theory of relativity.</p><h2>2. How is the speed of light measured in two directions?</h2><p>The most common method used to measure the speed of light in two directions is the Michelson-Morley experiment. This involves splitting a beam of light into two perpendicular paths and then recombining them to measure the interference pattern. By comparing the time it takes for the light to travel in each direction, we can calculate the speed of light in both directions.</p><h2>3. What is the significance of measuring the speed of light in two different mediums?</h2><p>The speed of light is known to be different in different mediums, such as air, water, or glass. By measuring the speed of light in two different mediums, we can determine the refractive index of each medium and understand how light behaves as it travels through different materials. This is crucial for various applications, including optics and telecommunications.</p><h2>4. Can the speed of light in two directions ever be different?</h2><p>According to the principles of relativity, the speed of light is constant and the same in all directions, regardless of the observer's frame of reference. Therefore, the speed of light in two directions should always be the same. However, there have been some controversial experiments that suggest a slight difference in the speed of light in different directions, but these results are not widely accepted in the scientific community.</p><h2>5. How does measuring the speed of light in two directions contribute to our understanding of the universe?</h2><p>The speed of light plays a crucial role in our understanding of the universe, as it is the fastest speed at which information can travel. By measuring the speed of light in two directions, we can confirm the principles of relativity and better understand the nature of space and time. It also allows us to make more accurate calculations and predictions about the behavior of light and other objects in the universe.</p>

1. Why is it important to measure the speed of light in two directions?

The speed of light is considered to be a fundamental constant in physics and has a significant impact on our understanding of the universe. By measuring the speed of light in two different directions, we can confirm that it is the same in all directions, which supports the principle of isotropy. This also allows us to test various theories and models, such as Einstein's theory of relativity.

2. How is the speed of light measured in two directions?

The most common method used to measure the speed of light in two directions is the Michelson-Morley experiment. This involves splitting a beam of light into two perpendicular paths and then recombining them to measure the interference pattern. By comparing the time it takes for the light to travel in each direction, we can calculate the speed of light in both directions.

3. What is the significance of measuring the speed of light in two different mediums?

The speed of light is known to be different in different mediums, such as air, water, or glass. By measuring the speed of light in two different mediums, we can determine the refractive index of each medium and understand how light behaves as it travels through different materials. This is crucial for various applications, including optics and telecommunications.

4. Can the speed of light in two directions ever be different?

According to the principles of relativity, the speed of light is constant and the same in all directions, regardless of the observer's frame of reference. Therefore, the speed of light in two directions should always be the same. However, there have been some controversial experiments that suggest a slight difference in the speed of light in different directions, but these results are not widely accepted in the scientific community.

5. How does measuring the speed of light in two directions contribute to our understanding of the universe?

The speed of light plays a crucial role in our understanding of the universe, as it is the fastest speed at which information can travel. By measuring the speed of light in two directions, we can confirm the principles of relativity and better understand the nature of space and time. It also allows us to make more accurate calculations and predictions about the behavior of light and other objects in the universe.

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