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

[ghwellsjr]

Ohanian is making a big mistake by thinking that the one-way speed of light is really constant independent of a synchronization convention.

I thought that was what was being questioned here; Is the one way speed of EM c, and is it constant. And that the agreement is yes it is.

The only way to know the value of c is to make measurements. Depending on how these are done, synchronizing maybe needed. All of that seems independent of what is being measured (and what is measured in one FoR is independent of what others measure).

So I guess my question is why is it a big mistake to assume that the one way speed of c is constant?

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 reread the last paragraph of my post #35. I just noticed that it had a truncated ending which I just repaired.

 

[Dale]

This is precisely why we need to compare SR against theories which make different experimental predictions.

Why would we do that? Such theories have been experimentally falsified on other grounds. What they might say about the one-way speed of light is not interesting to me.

 

[ghwellsjr]

I assume that you're just making the standard definition of an inertial reference frame in terms of homogeneity and isotropy of space and time.

I meant what Einstein explained in his 1905 paper. My only point was that if he had merely stated his postulate without further explanation, no one would have any idea what he was proposing. Even with his explanation, many people don't get it. I encourage those who are still confused over this issue to read that paper again, especially the first part.

Can't you imagine a universe in which measurements using slow clock transport did not yield the speed of light being constant?

I don't have to imagine such a universe, this one already has that characteristic.

The definition of slow transport synchronization involves taking the limit as the speed of the transported clock goes to zero, as I specified in post #25.
Again, you need to take the limit as the speed goes to zero.

In the limit, as the speed goes to zero, it takes infinite time and never gets there. For any non-zero speed, no matter how small, we know how to calculate the "error" and it is never zero. But please don't get sidetracked on this minor point. I only raised it with you to refute Ohanian's charge that Einstein forgot about the slow transport of clocks. Even if you do what PAllen suggested by transporting many clocks at different speeds and extrapolating to the "correct" synchronized time, that synchronized time is just an arbitrary definition of time and from which you can derive an arbitrary definition of the one-way speed of light, but it is still nothing more than an arbitrary definition, not an independent measurement of any reality in nature.

I think it's a metatheoretical issue, not a theoretical issue. SR says the one-way speed of light is c. The question of whether this is an arbitrary feature of SR or something that has basis in physical reality is a question about SR, not a question within SR.

I could equally say: LET says the one-way speed of light is c only in a single absolute frame in which the ether is at rest. The question of whether this is an arbitrary feature of LET or something that has basis in physical reality is a question about LET, not a question within LET.

 

[Dale]

Without assuming SR apriori, you can measure the one way speed of light using slow transport.

Only if you assume that a slowly transported clock remains synchronized.

 

[DrGreg]

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.


Reference
[1] Mansouri, R and Sexl, R U (1977), "A Test Theory of Special Relativity: I. Simultaneity and Clock Synchronization", General Relativity and Gravitation 8 (7), pp.497–513, Bibcode: 1977GReGr...8..497M, DOI: 10.1007/BF00762634


Further reading
Test theories of special relativity, Wikipedia

 

[lugita15]

In the limit, as the speed goes to zero, it takes infinite time and never gets there.

If the speed were actually zero, of course, then the transported clock would never get there. But (using the language of my post #25) as the speed of the transported clock C goes to 0, the time it takes for C to reach B gets larger and larger, and the difference between clocks B and C once C arrives approaches a constant. If this constant is zero, then we say that clocks A and B are slow-transport synchronized.

Even if you do what PAllen suggested by transporting many clocks at different speeds and extrapolating to the "correct" synchronized time, that synchronized time is just an arbitrary definition of time and from which you can derive an arbitrary definition of the one-way speed of light, but it is still nothing more than an arbitrary definition, not an independent measurement of any reality in nature.

The definition of slow-transport synchronization is precisely what PAllen suggested: we transport clocks at different nonzero speeds, and we take the limit as the speed goes to zero. You can call this an arbitrary definition if you like, but in my mind it seems rather natural. If you had never heard of SR, you would first assume that there's no such thing as time dilation, and if you heard that moving clocks tick slowly you would assume that really slow-moving clocks don't tick that much more slowly then clocks in your rest frame.

Anyway, the key point is that even if slow clock transport is an arbitrary synchronization convention, the question of what the one-way speed of light will be measured to be relative to that convention is not a trivial matter knowable in advance (in stark contrast to Einstein synchronization). It is a significant experimental fact that relative to slow transport, the one-way speed of light is constant. (Sorry for repeating myself.)

 

[lugita15]

DrGreg, that was exactly the kind of experiment I had in mind. ghwellsjr, how would you interpret the result of M&S? I would say that since we already have such voluminous evidence of the principle of relativity, and since this experiment is evidence of SR, the natural conclusion to reach is that this kind of experiment is evidence of the second postulate. I think I would especially say this if I were living in Newton's time. But as I said, this is an question of interpretation, not physics.

 

[ghwellsjr]

DrGreg, that was exactly the kind of experiment I had in mind. ghwellsjr, how would you interpret the result of M&S? I would say that since we already have such voluminous evidence of the principle of relativity, and since this experiment is evidence of SR, the natural conclusion to reach is that this kind of experiment is evidence of the second postulate. I think I would especially say this if I were living in Newton's time. But as I said, this is an question of interpretation, not physics.

Einstein already described what would happen with the slow transport of clocks, as I have pointed out. He also described what would happen with the fast transport of clocks, I might add. The fact that all of Einstein's predictions have been verified experimentally supports the fact that SR comports with reality. But every one of these experiments that supports SR also supports LET. SR postulates that the speed of light is c in any reference frame. LET postulates that the speed of light is c only in the ether frame. There is no experiment that can decide for us which of these two opposing theories is "correct" at the expense of the other one. Identifying the reality of the one-way speed of light would do just that but it can't be done. To think that Einstein's convention of synchronizing clocks being consistent with his prediction of the timing on slowly moving clocks proves that either or both of these together results in an independent method to determine how light propagates is to miss Einstein's argument.

I can see why you defend Ohanian in his attack on Einstein in the book excerpt you referenced. Please don't make the mistake of thinking that I am offering ideas that are contrary to Einstein's. I just plead with you to read Einstein's paper or any of his other writings on the issue of one-way speed of light. He never argued that his theory was proved to be correct over LET, just that since the ether state was unmeasurable (meaning the one-way speed of light was unmeasurable) there was nothing to be gained by clinging to LET.

 

[lugita15]

Einstein already described what would happen with the slow transport of clocks, as I have pointed out. He also described what would happen with the fast transport of clocks, I might add. The fact that all of Einstein's predictions have been verified experimentally supports the fact that SR comports with reality. But every one of these experiments that supports SR also supports LET. SR postulates that the speed of light is c in any reference frame. LET postulates that the speed of light is c only in the ether frame. There is no experiment that can decide for us which of these two opposing theories is "correct" at the expense of the other one. Identifying the reality of the one-way speed of light would do just that but it can't be done. To think that Einstein's convention of synchronizing clocks being consistent with his prediction of the timing on slowly moving clocks proves that either or both of these together results in an independent method to determine how light propagates is to miss Einstein's argument.

I can see why you defend Ohanian in his attack on Einstein in the book excerpt you referenced. Please don't make the mistake of thinking that I am offering ideas that are contrary to Einstein's. I just plead with you to read Einstein's paper or any of his other writings on the issue of one-way speed of light. He never argued that his theory was proved to be correct over LET, just that since the ether state was unmeasurable (meaning the one-way speed of light was unmeasurable) there was nothing to be gained by clinging to LET.

ghwellsjr, I feel like we're talking past each other; I'm interested in theories other than LET, and you're talking about nothing but LET. Without a doubt, if someone believes in what is today known as the Lorentz Ether Theory, then absolutely no experiment can convince them to switch to SR. (That may not be true of Lorentz's historical theory, however; he believed that the electron had physical stresses which could lead to detectable electromagnetic effects.) In other words, if you believe a priori that lengths contract and time slows down for objects moving with respect to the ether, I can't convince you that the one-way speed of light is isotropic in all reference frames. But this is precisely the reason that LET is not an interesting comparison to SR. We can only find out the empirical validity of the various statements in a theory if we think along the lines DrGreg outlined.

If, however, you initially believed in Newtonian mechanics, an experiment measuring the one-way speed of light with respect to slow clock transport synchronization might be good reason to hop on to the special relativity bandwagon, in a way that the same experiment with respect to Einstein synchronization could never do.

 

[ghwellsjr]

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.

 

[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.

 

[Dale]

My simple question is: If we just consider Einstein's 1905 paper ...

Please answer this question with "Yes." or "Nope". No more than four letters, please.

("Yes." or "Nope" will do just fine as an answer. No "if's", "and's" or "but's", please.)

I don't think that this rhetorical approach is productive. By limiting the question and the response in the way you are demanding, you will get the answer you want but it will be to a question that was never of interest nor in dispute.

 

[ghwellsjr]

Yes.

Thank you.

Now I want to ask you about an experiment. But I want to put this in a context prior to Einstein's 1905 paper. I want to put this even prior to MMX. I want to put this at the time when Maxwell realized that light was a wave in the electromagnetic field his equations described and he believed it would be possible to detect the Earth's motion through this field by measuring the one-way speed of light.

His only problem was that technology was not available for him to perform the type of experiment that we can perform today but let's imagine that it was. So let's suppose that he took two accurate and stable atomic clocks that were synchronized at one location and slowly moved one of them some distance away and the distance was measured using a rigid calibrated ruler. (Let's stipulate that there was no error in his distance measurement.) Now let's also say that he constructs a tube or pipe that he evacuates with a perfect vacuum and he puts a light source at one end that can log the time from the atomic clock located next to it when the light is turned on and a light detector at the other end that can log the time from the other atomic clock located next to it when the light is detected.

So now he does his experiment and he divides the difference between the two logged times into the measured distance. I believe he will get c as the answer and I believe there is no controversy about this, correct?

But let's also assume that this answer would have surprised Maxwell and so he repeats the experiment at different times of the day and at different seasons of the year. Let's say the experiment was so easy to do that other people repeat the same experiment. They do it in every conceivable location, at the bottom of the deepest valley, at the top of the highest mountain, at the poles, at the equator, even at the bottom of the deepest ocean. They repeat the measurement with the apparatus pointed in all different directions of the compass. Everybody always gets the same value for c, correct? Everybody agrees that this is what would have happened, correct?

So then they put the apparatus on the longest flatbed railway car and repeat the measurement at different constant speeds. I'm assuming that their apparatus has no errors and that the accuracy is good enough that they had every reason to believe that they could measure any motion through the field for the speeds they were traveling. Everybody agrees they still always measure exactly c, correct?

So my question for you is: Is there any reason to believe that the development of science would have progressed any differently than it did as a result of MMX which was a two-way measurement instead of a one-way measurement?

 

[PAllen]

So my question for you is: Is there any reason to believe that the development of science would have progressed any differently than it did as a result of MMX which was a two-way measurement instead of a one-way measurement?

No one would say you can't measure the one way speed of light

More seriously, to say an experiment is meaningful means that it matters how it comes out. If the experiment came out showing anisotropy, physics would be very different. Thus the experiment has content.

[edit: and to contrast with a non-meaningful experiment: between two unsynchronized clocks at a measured distance, measure a two way speed of light (using one of them and a mirror). Then, synch them with Einstein's convention, then 'measure' the one way speed to see if it is different. People would think you were deranged.]

 

[ghwellsjr]

OK, now suppose that the size and delicacy of the atomic clocks prevented a fast transport, do you think that anyone would have figured out that they would have gotten a different experimental result if they did transport the clock rapidly? Please explain your answer. If you were there, what argument would you use to make this prediction?

 

[PAllen]

OK, now suppose that the size and delicacy of the atomic clocks prevented a fast transport, do you think that anyone would have figured out that they would have gotten a different experimental result if they did transport the clock rapidly? Please explain your answer. If you were there, what argument would you use to make this prediction?

They would immediately see the tension between Galilean relativity and the experimental light results. Other experiments would show electric and magnetic fields didn't distinguish inertial frames. Eventually someone would figure out the Lorentz transform, showing it preserved the form of Maxwell's equations and explained the light speed measurements. Then predictions for time dilation would follow, and would be sought as soon as technology allowed.

 

[ghwellsjr]

Good. But if you really believe the order of events as you just described them, just based on the Lorentz transform, they already had the experimental evidence for time dilation, even without waiting for new technology. They would have concluded that the atomic clocks themselves were subject to time dilation as they moved at different speeds during the course of a day depending on their arbitrarily chosen Frame of Reference. And part of that explanation of the light speed measurements would show them that even transporting a clock at slow speed would result in its time at the new location being different than the one that remained behind and was dependent on the chosen Frame of Reference.

But they would also conclude that the length of the pipe was changing during the course of a day and that would exactly compensate for the difference in the time between the two clocks resulting in them always measuring the one-way speed of light being the same constant value c.

 

[PAllen]

Good. But if you really believe the order of events as you just described them, just based on the Lorentz transform, they already had the experimental evidence for time dilation, even without waiting for new technology. They would have concluded that the atomic clocks themselves were subject to time dilation as they moved at different speeds during the course of a day depending on their arbitrarily chosen Frame of Reference. And part of that explanation of the light speed measurements would show them that even transporting a clock at slow speed would result in its time at the new location being different than the one that remained behind and was dependent on the chosen Frame of Reference.

But they would also conclude that the length of the pipe was changing during the course of a day and that would exactly compensate for the difference in the time between the two clocks resulting in them always measuring the one-way speed of light being the same constant value c.

Sure, but that would all be inferences from experimental observation. The whole point of theory is to explain observations. An observation explained does not become a non-observation.

Anyway, I think a lot has been well discussed in this thread. I don't see anything new being added since around post #69-#75. If you have some new insight on this, I will respond further, otherwise I probably won't.

 

[ghwellsjr]

But I was saying the same thing back in post #66 which contained quotes from #35 and #52 which got ignored then just like now. Your idea that it is possible to measure the one-way speed of light using the slow transport of clocks is at odds with virtually all the literature on this subject including those in wikipedia and this one, for example:

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#one-way_tests

 

[PAllen]

But I was saying the same thing back in post #66 which contained quotes from #35 and #52 which got ignored then just like now. Your idea that it is possible to measure the one-way speed of light using the slow transport of clocks is at odds with virtually all the literature on this subject including those in wikipedia and this one, for example:

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#one-way_tests

You don't seem to understand my position, no matter how many ways I explain it. The following first sentence from your link:

"Note that while these experiments clearly use a one-way light path and find isotropy, they are inherently unable to rule out a large class of theories in which the one-way speed of light is anisotropic."

is something I have said six ways from sunday (including, with great emphasis, in #69). But the logical completion of this statement is that a large class of possible theories are ruled out as well. An experiment that rules out possible theories is a real measurement with content. (As opposed to measuring one way lightspeed after Einstein synch, which has no content). Further, as an interpretational bias, given a choice between an interpretation that has unobservable anisotropic ligthspeed , and an interpretation that takes observable ligthspeed as the only quantity of theoretical merit, I prefer the latter (still fully accepting that the former cannot be ruled out).

I believe I have said exactly this at least 6 times. I am confident my view is actually not in conflict with expert opinion, only with overly narrow interpretation of limited quotes of expert opinion.

[edit: Right from #69:
"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).
"]

 

[ghwellsjr]

I used to think that the rationale for the slow transport of clocks was based purely on the final result being the same as Einstein's synchronization but that the process was really different. It took some time for me to understand that it was the process that was also identical. Since you believe that Einstein's convention does not allow for the measurement of the one-way speed of light but yet you do believe that slow clock transport does allow for the measurement of the one-way speed of light, it only shows that you do not yet understand that the process is the same for both of them. Don't you have any curiosity as to why some of us keep emphasizing this point? It has nothing to do with alternate theories. Please try to think about this in another way.

I will ask you a question: why do you believe that when you move a clock at a slow speed, the time on that clock has not shifted in some unknown way? We know that if you rapidly move a clock from A to B and back to A, the time on it will be different than the time on a clock that remained at A. So we know that moving a clock can affect the rate of its ticking.

Now it's not like we have two clocks that aren't ticking and they both display noon and we move one around and when it gets back to the first clock it still displays noon on it. These clocks are constantly changing their times. How can you say that just because they track when together, and they track after slowly taking one of them on a round trip, that they continue to track when they are separated? How do you know that as you move one of them from A to B, it runs slower than the stationary one and so has a different time on it when it gets to B and then when you bring it back it runs faster so that it now has the same time on it when you compare it to the first clock?

Until you can prove that this isn't happening, then you have no justification that the slow transportation of clocks results in them having the same time on them at the remote location. And if you can't prove that, then you can't prove that your measurement of the one-way speed of light is actually measuring what you claim to be measuring even if you get a constant value of c.