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

[lugita15]

No, I'm talking about Einstein's theory of Special Relativity and his argument in its favor. I am not saying anything differently than what he said.

Dr. Greg's post was not claiming that the one-way speed of light was measurable apart from a previously accepted timing convention or that it is intrinsic to nature.

The one-way speed of light is isotropic in all reference frames because of the way that a reference frame is defined according to Einstein's postulate, definitions and conventions. Apart from some type of postulate, definitions and conventions, it is impossible to discuss the meaning of time and therefore the meaning of speed.

I didn't say DrGreg claimed anything like that. His post was about how measuring the one-way speed of light with respect to slow transport synchronization constitutes experimental confirmation of SR. I (and I think Ohanian as well) agree wholeheartedly that you need a synchronization convention to measure the one-way speed of light. But which convention you choose affects whether certain experiments you perform are predictable in advance or provide useful and significant results.

I go back to what I said in post #23. These are the only relevant facts on this issue, all the rest is just interpretation; my preferred view is that if you have evidence of Postulates 1 and 2 combined, and you also have separate evidence of Postulate 1 alone, that suggests that you have some experimental reason to believe Postulate 2.

As I said, I think we're talking past each other.

 

[lugita15]

FYI, there's another way you can think about slow transport of clocks, which I think I may have gotten from Lieber's excellent book "The Einstein Theory of Relativity". Instead of transporting a clock from point A all the way to point B, you instead fill the line segment from A to B with lots of stationary clocks laid end to end. Each clock is just synchronized with its neighbors, so there's absolutely no motion required or distant signal exchanges. If you do this, then intuitively the first clock and the last clock should be in sync. According to Lieber, this gives you the same synchronization method as slow transport. I don't know how accurate her statement is, but if it's true it gives a lot of intuition to the slow transport method.

 

[PAllen]

FYI, there's another way you can think about slow transport of clocks, which I think I may have gotten from Lieber's excellent book "The Einstein Theory of Relativity". Instead of transporting a clock from point A all the way to point B, you instead fill the line segment from A to B with lots of stationary clocks laid end to end. Each clock is just synchronized with its neighbors, so there's absolutely no motion required or distant signal exchanges. If you do this, then intuitively the first clock and the last clock should be in sync. According to Lieber, this gives you the same synchronization method as slow transport. I don't know how accurate her statement is, but if it's true it gives a lot of intuition to the slow transport method.

I would think that amounts to piecewise Einstein convention, and is thus equivalent to clock transport only given other assumptions or experimental verification. The analog for slow clock transport would be a chain of people, and you hand the clock from person to person, as slow as you want.

Of course, I haven't seen their full discussion. I do remember this book from ages ago (think I took it out of the library once), but never owned it.

 

[ghwellsjr]

FYI, there's another way you can think about slow transport of clocks, which I think I may have gotten from Lieber's excellent book "The Einstein Theory of Relativity". Instead of transporting a clock from point A all the way to point B, you instead fill the line segment from A to B with lots of stationary clocks laid end to end. Each clock is just synchronized with its neighbors, so there's absolutely no motion required or distant signal exchanges. If you do this, then intuitively the first clock and the last clock should be in sync. According to Lieber, this gives you the same synchronization method as slow transport. I don't know how accurate her statement is, but if it's true it gives a lot of intuition to the slow transport method.

If there's no motion involved and if each pair of clocks is synchronized with its neighbor according to Einstein's convention, then, as Einstein pointed out in his 1905 paper, all clocks will be synchronized with each other. It's not the same as slow transport unless you actually slowly transport each clock.

 

[lugita15]

If there's no motion involved and if each pair of clocks is synchronized with its neighbor according to Einstein's convention, then, as Einstein pointed out in his 1905 paper, all clocks will be synchronized with each other. It's not the same as slow transport unless you actually slowly transport each clock.

Yes, I see now that it's just begging the question, because you need to know what method to use to synchronize the neighboring clocks. So ignore what I said.

 

[ghwellsjr]

I didn't say DrGreg claimed anything like that. His post was about how measuring the one-way speed of light with respect to slow transport synchronization constitutes experimental confirmation of SR. I (and I think Ohanian as well) agree wholeheartedly that you need a synchronization convention to measure the one-way speed of light. But which convention you choose affects whether certain experiments you perform are predictable in advance or provide useful and significant results.

I go back to what I said in post #23. These are the only relevant facts on this issue, all the rest is just interpretation; my preferred view is that if you have evidence of Postulates 1 and 2 combined, and you also have separate evidence of Postulate 1 alone, that suggests that you have some experimental reason to believe Postulate 2.

As I said, I think we're talking past each other.

OK, for the purpose of getting past the issue of whether synchronization by the slow transport of clocks is identical to Einstein's convention, I will stipulate on this thread that they are identical. I think this is largely the reason we have been talking past each other. So, please, no more trying to convince me on this issue.

Dr. Greg's post was not about "how measuring the one-way speed of light with respect to slow transport synchronization constitutes experimental confirmation of SR". It was, as he said in bold, about how "slow clock transport and Einstein synchronisation are equivalent", about which I have now stipulated. Furthermore, if you look at the wikipedia link at the bottom of his post, you will read these comments:

On the other hand, in special relativity both the one- and two-way speed of light is isotropic, and because only the two-way speed is accessible to experimental tests, Robertson's theory gives different experimental predictions as special relativity.

The value of e(v) depends only on the choice of clock synchronisation and cannot be determined by experiment.

...only the two-way speed is accessible to experimental tests...

However, it is possible to make such an ether/test-theory (independent of the chosen synchronization) experimentally equivalent to special relativity, by giving the effects of time dilation and length contraction the exact relativistic value. So Mansouri and Sexl spoke about the "remarkable result that a theory maintaining absolute simultaneity is equivalent to special relativity." They also noticed the similarity between this test theory and Lorentz ether theory of Hendrik Lorentz, Joseph Larmor and Henri Poincaré. Though Mansouri, Sexl, and the overwhelming majority of physicists, prefer special relativity over such an ether theory, because the latter "destroys the internal symmetry of a physical theory".

You have repeated again what you have said many times on this thread that the slow transport of clocks is experimental evidence for Einstein's second postulate. Now that I have stipulated that it is an identical synchronization method to Einstein's convention, I'd like you to focus on why neither one can be experimentally tested. I have already explained why in posts #35 and #52 which I quote here:

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.

A round-trip measurement of the value of the speed of light does not require synchronization because there is only one timing device used. It is impossible to track the progress of light away from us because we don't have anything faster than the speed of light to communicate back to us where it is at any given moment in time. We really need instantaneous communication to solve this problem. Without know where it is at any moment in time (or what time it is when it arrives at any location) means we cannot measure its speed. We know that moving a clock from where we are to some distant point and back again results in a loss of time compared to a clock that remains with us. But we cannot tell whether that loss of time occurred equally during both halves of the trip or whether it occurred more in one direction and less in the other. Furthermore, as we move the clock in one direction, it needs to advance in time (just like a stationary clock) but we cannot tell if it's advancing either faster or slower than the stationary clock and we cannot tell if it is advancing at the same rate when traveling in the two directions.

Please study these two posts and see if you understand what I am saying here. If you don't understand, please ask what the problem is so that I can add further clarification.

 

[harrylin]

Indeed, measuring the speed of light in two directions is just a purer way of measuring, as no assumption (or definition) needs to be made about the synchronization of a second clock.

And just a little remark:

[..] 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. [..]

I agree with Pallen on this. In fact, Einstein did not make such a distinction; the "LET" concept was invented later for the purpose of making such a distinction. He even denied to have known of Lorentz's 1904 paper and he also wrote a summary paper based on these two papers (what we now call "SR").

 

[Dale]

Mansouri & Sexl^[1] consider a "test theory" of relativity in which the transformation between two frames is postulated to be\begin{align}<br /> t &amp;= a(v)\,T + \epsilon(v)\,x\\<br /> x &amp;= b(v)\,(X - vT)<br /> \end{align}where a, b and ε are unknown functions to be determined by experiment. (Note: the first equation intentionally contains x, not X.) Special relativity is a special case of this test theory for a particular choice of these three functions. Experiments to test the validity of relativity can be performed from which the values of a(v), b(v) and ε(v) can be estimated. If the experimental values match the values predicted by SR, this is a confirmation of SR.

Mansouri and Sexl point out that the function ε(v) depends on the clock sync convention chosen, whereas a(v) and b(v) are both independent of sync convention. Under these assumptions, they go on to prove a result (pp.506–508) that slow clock transport and Einstein synchronisation are equivalent if and only if a(v) takes the value predicted by SR, viz<br /> a(v) = \sqrt{1 - v^2/c^2}<br />To avoid any misunderstanding, the term "slow clock transport" is defined to mean in the limit as the speed of clock transport tends to zero (as others have pointed out).

Thus, if you sync clocks by slow clock transport and then measure the one-way speed of light, if you get an answer of c regardless of direction, you have experimentally confirmed that a(v) takes the value predicted by SR.

That is not in dispute. And, if I am not mistaken, I believe that Doppler experiments fix the value of b as \sqrt{\frac{c+v}{c-v}}, but experiments do not fix the value of ε.

In the Mansouri and Sexl test theory the one way speed of light depends on a, b, and ε. Since a and b are fixed by experiment that leaves ε available as a free parameter to define a class of theories which are compatible with experiment.

If I did my math right the one way speed of light is given by c\frac{1-v\epsilon[v]}{1-c\epsilon[v]}. Thus, there is a class of theories which is compatible with experiment and in which the one way speed of light is not c. These theories are distinguished by different values of ε which, as you mentioned, is determined by the synchronization convention of the theory.

 

[PAllen]

I will summarize my point of view as follows (I am sure no consensus will be reached on this thread):

Both of the following are true statements, as of best current knowledge:

1) It is possible to perform non-tautological measurements of the one way speed of light (using slow clock transport; methods proposed in the papers in my post #32; a rotation method described (but not invented) by Ohanian).

2) No experiment can rule out logically valid interpretations of physical laws in which there is an unobservable absolute rest frame, and/or an unobservable anisotropic one way speed of light. (Isotropic two way speed of light, on the other hand, is a well established fact).

There are several defensible ways to respond to this state of affairs. One is to emphasize (2), and say no more than that the one way speed of light is unknowable. This is strongly justified by the philosophy of "don't say more than you can know". Another is to emphasize (1) and focus on measurable one way c as the useful element of physical interpretation. In which case, one states that measurable one way c is isotropic and constant in all frames. You may call this philosophy "avoid unobservable quantities in physical interpretations".

It is also worth noting the indisputable fact that if there were ever a measurement of anisotropic one way c in any inertial frame, SR would be refuted.

 

[lugita15]

I like everything PAllen said, especially the following:

It is also worth noting the indisputable fact that if there were ever a measurement of anisotropic one way c in any inertial frame, SR would be refuted.

To put it another way, the equivalence of Einstein synchronization and certain other methods, including slow clock transport, is a falsifiable prediction of special relativity.

 

[harrylin]

[..] 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.

Yes indeed. A synchronization convention (or any other convention) cannot itself be verified and therefore it isn't really part of a theory - it's just a tool to describe the predictions of a theory in a well defined way. In contrast, the predictions about the effects of clock transport (both slow and fast) are real physical predictions that can be verified.

 

[Dale]

1) It is possible to perform non-tautological measurements of the one way speed of light (using slow clock transport; methods proposed in the papers in my post #32; a rotation method described (but not invented) by Ohanian).

At best, you can say that you can measure the one-way speed of light wrt non-light synchronization conventions, such as slow-clock transport. But the assumption that slow clock transport gives synchronized clocks is as much an assumption as Einstein synchronization. The measurement in that case may not be tautological, but it certainly is still completely dependent on your synchronization convention.

Tautological or not, you cannot perform a measurement of the one-way speed of light independently of your synchronization convention.

 

[PAllen]

At best, you can say that you can measure the one-way speed of light wrt non-light synchronization conventions, such as slow-clock transport. But the assumption that slow clock transport gives synchronized clocks is as much an assumption as Einstein synchronization. The measurement in that case may not be tautological, but it certainly is still completely dependent on your synchronization convention.

I completely agree it is based on such a convention (and have said so in every post it was relevant). I have further noted you cannot treat:

- measuring agreement of slow transport and Einstein synchronization

- measuring one way speed of light with slow transport

as two separate experiments. They are the same experiment. You can choose either way to look at this single experiement.

 

[ghwellsjr]

I completely agree it is based on such a convention (and have said so in every post it was relevant). I have further noted you cannot treat:

- measuring agreement of slow transport and Einstein synchronization

- measuring one way speed of light with slow transport

as two separate experiments. They are the same experiment. You can choose either way to look at this single experiement.

Then do you completely agree that measuring the one way speed of light with clocks synchronized by slow transport is the same as measuring the one way speed of light with clocks synchronized by Einstein's convention?

 

[Dale]

I completely agree it is based on such a convention (and have said so in every post it was relevant). I have further noted you cannot treat:

- measuring agreement of slow transport and Einstein synchronization

- measuring one way speed of light with slow transport

as two separate experiments. They are the same experiment. You can choose either way to look at this single experiement.

OK, I think we are in agreement, or at least a close approximation thereof.

 

[PAllen]

Then do you completely agree that measuring the one way speed of light with clocks synchronized by slow transport is the same as measuring the one way speed of light with clocks synchronized by Einstein's convention?

Not quite. I agree SR and any equivalent theory/interpretation predicts they are the same. However, one is a tautology, the other is not, and if the universe worked differently than we think, could show us the error of our ways.

I agree with the statement: you cannot measure one way light speed without a synchronization or other purely conventional elements (as a result, the measurement tells you less than you might like).

I disagree with the statement that all measurements of one way light speed are tautologically true; or that it is impossible to measure one way lightspeed.

 

[ghwellsjr]

PAllen, thanks for your continued explanation. I think I'm finally getting what you are saying. Let me repeat it in my own words and you can tell me if I've got it right:

Einstein's synchronization convention is purely arbitrary and a tautology and if we had just that, then we really couldn't measure the one-way speed of light because we would be merely repeating back the time we arbitrarily set on the remote clock. In contrast, the slow transport of a clock is not arbitrary, it always yields the same time and so it allows us to experimentally determine the one-way speed of light. The fact that it is identical to Einstein's synchronization now puts the latter on a proven basis so that we can now say that Einstein's synchronization convention does indeed permit a legitimate meaurement of the one-way speed of light.

 

[PAllen]

PAllen, thanks for your continued explanation. I think I'm finally getting what you are saying. Let me repeat it in my own words and you can tell me if I've got it right:

Einstein's synchronization convention is purely arbitrary and a tautology and if we had just that, then we really couldn't measure the one-way speed of light because we would be merely repeating back the time we arbitrarily set on the remote clock. In contrast, the slow transport of a clock is not arbitrary, it always yields the same time and so it allows us to experimentally determine the one-way speed of light. The fact that it is identical to Einstein's synchronization now puts the latter on a proven basis so that we can now say that Einstein's synchronization convention does indeed permit a legitimate meaurement of the one-way speed of light.

Basically, but I would weaken this a little. There is a real experiment that can be performed, but as noted in my #73, you can consider it verification of agreement clock synch convention (as predicted by SR), or as a measurement of one way light speed (also as predicted by SR - that any reasonable measurement approach will yield c), but not both. That it is a different convention means there is real verification and possibility of falsification of SR; that it is still a convention limits the information it provides.

Finally, as we both know, there are experimentally equivalent theories (or interpretations) to SR that have anisotropic light speed, but predict that no measurement can discern this. Perversely, unless SR is wrong, no experiment can ever rule out such interpretations.

 

[lugita15]

Einstein's synchronization convention is purely arbitrary and a tautology and if we had just that, then we really couldn't measure the one-way speed of light because we would be merely repeating back the time we arbitrarily set on the remote clock.

The only thing I'd nitpick is that only statements can be tautologies, not procedures. So Einstein synchronization is not the tautology; the tautology is the statement that Einstein synchronization yields an isotropic one-way speed of light.

In contrast, the slow transport of a clock is not arbitrary, it always yields the same time and so it allows us to experimentally determine the one-way speed of light.

I would say that slow clock transport is just as arbitrary as Einstein synchronization, at least in the sense that you're free to use it or not, but personally I see it as more natural and intuitive. And I don't know what you mean by "it always yields the same time." But it is true that a measurement of the one-way speed of light using slow-transport synchronized clocks is a nontrivial experiment., in contrast to Einstein synchronized clocks.

The fact that it is identical to Einstein's synchronization now puts the latter on a proven basis so that we can now say that Einstein's synchronization convention does indeed permit a legitimate meaurement of the one-way speed of light.

I'm not sure what you mean by putting Einstein synchronization on a proven basis. As to your statement that "we can now say that Einstein's synchronization convention does indeed permit a legitimate meaurement of the one-way speed of light" - that's essentially my position, but I'd phrase it slightly less boldly:

(*)Since a method which happens to be equivalent (empirically equivalent in our universe, not logically equivalent) to Einstein synchronization allows for a nontrivial measurement of the one-way speed of light, Einstein's second postulate is arguably an empirically grounded fact about our universe.

I put the word "arguably" here, because I definitely agree with the point that LET is empirically indistinguishable from SR, and reasonable people can choose to focus on this point rather than what I said in (*). That's why I've said it's an issue of semantics or interpretation.

 

[ghwellsjr]

I'm confused. I want to take the concepts one at a time. First, you both used the word "tautology" in reference to Einstein's synchronization. My simple question is: If we just consider Einstein's 1905 paper which only allows for synchronizing a distant clock using light signals and not slow transport (or any other experiment), then are you saying that it is impossible to measure the one-way speed of light?

 

[PAllen]

I'm confused. I want to take the concepts one at a time. First, you both used the word "tautology" in reference to Einstein's synchronization. My simple question is: If we just consider Einstein's 1905 paper which only allows for synchronizing a distant clock using light signals and not slow transport (or any other experiment), then are you saying that it is impossible to measure the one-way speed of light?

If you only use light for synchronization, then turning around and using thus synchronized clocks to measure one way light speed is tautological (the answer is built into the synchronization). If you have an additional method of clock synch available, then you can perform a substantive experiment. You can use this method to measure one way light speed (and if you get the isotropic c in all directions, in all inertial frames, immediately infer that your alternate clock synch will always agree with Einstein sync); or you can simply compare clocks synchronized using the two methods, and if they agree in all cases in all inertial frames, infer that one way light speed would be measured as isotropic and constant in all inertial frames (using the alternate clock synch).

From here, look at my post #69 for limitations on the conclusions you can draw from all of this, and some different philosophical ways of characterizing the result.

 

[ghwellsjr]

If you only use light for synchronization, then turning around and using thus synchronized clocks to measure one way light speed is tautological (the answer is built into the synchronization). If you have an additional method of clock synch available, then you can perform a substantive.

Can I assume that your answer to my question is "yes"? Please answer this question with "Yes." or "Nope". No more than four letters, please.

 

[PAllen]

Can I assume that your answer to my question is "yes"? Please answer this question with "Yes." or "Nope". No more than four letters, please.

OK: Yes; it is impossible to measure one way light speed if light signals are your only method of clock synch. I could add, not so much impossible as tautological, as the answer is built into the synch convention, so is pre-determined.

 

[ghwellsjr]

Do you realize you answered yes and no?

 

[my_wan]

Yes, the description is accurate, but it also depends on your synchronization convention. So you are measuring what you assumed via your synchronization convention.

I have disagreed that the synchronization convention is important in terms of the empirical results in the unmentioned thread. You get equally as consistent empirical answer regardless of synchronization convention. Which does not invalidate differing synchronization conventions any more than a coordinate choice physically invalidates an alternate coordinate choice.

 

[my_wan]

OK: Yes; it is impossible to measure one way light speed if light signals are your only method of clock synch. I could add, not so much impossible as tautological, as the answer is built into the synch convention, so is pre-determined.

Though you are correct that it is a tautology the physical consequences remain consistent with any properly formulated synch convention.

 

[Dale]

I have disagreed

Yes, I know. Let's keep it in the other thread so as to not hijack this thread since the OP specifically didn't want our discussion here.

 

[PAllen]

Do you realize you answered yes and no?

I thought you wanted me to answer the following question:

"My simple question is: If we just consider Einstein's 1905 paper which only allows for synchronizing a distant clock using light signals and not slow transport (or any other experiment), then are you saying that it is impossible to measure the one-way speed of light?"

I answered "yes" (but you can go ahead and do it anyway if you don't care about circular reasoning).

 

[ghwellsjr]

I thought you wanted me to answer the following question:

"My simple question is: If we just consider Einstein's 1905 paper which only allows for synchronizing a distant clock using light signals and not slow transport (or any other experiment), then are you saying that it is impossible to measure the one-way speed of light?"

I answered "yes" (but you can go ahead and do it anyway you don't care about circular reasoning).

But if somebody did go ahead and do it anyway, by your answer, it would be appropriate to point out that they were not making a measurement but merely getting back the value they fed in and it would be appropriate to point out that it is impossible to make a measurement of the one-way speed of light using just the process described by Einstein in his 1905 paper, correct? ("Yes." or "Nope" will do just fine as an answer. No "if's", "and's" or "but's", please.)

 

[PAllen]

But if somebody did go ahead and do it anyway, by your answer, it would be appropriate to point out that they were not making a measurement but merely getting back the value they fed in and it would be appropriate to point out that it is impossible to make a measurement of the one-way speed of light using just the process described by Einstein in his 1905 paper, correct? ("Yes." or "Nope" will do just fine as an answer. No "if's", "and's" or "but's", please.)

Yes.