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

by goodabouthood
Tags: directions, light, measure, speed
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 P: 127 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?
Mentor
P: 17,545
 Quote by goodabouthood 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.
P: 132
 Quote by goodabouthood 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.

P: 5,632
Why do you need to measure the speed of light in two directions?

Check out the convention for synchronizing clocks here:

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

and this link at the bottom of the article:

http://en.wikipedia.org/wiki/One-way_speed_of_light

 ....Experiments that attempted to probe the one-way speed of light have been proposed, but none has succeeded in doing so......Thus the measured value of the average one-way speed is dependent on the method used to synchronize the start and finish clocks. This is a matter of convention.
 PF Gold P: 4,792 There's no better explanation than the one Einstein provided in his famous 1905 paper introducing Special Relativity: http://www.fourmilab.ch/etexts/einstein/specrel/www/ Just read the first few sections.
P: 15,319
 Quote by goodabouthood ...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.
 Sci Advisor PF Gold P: 5,091 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.
PF Gold
P: 4,792
 Quote by PAllen 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?
P: 1,583
 Quote by ghwellsjr 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.
Sci Advisor
PF Gold
P: 5,091
 Quote by ghwellsjr 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)'.
PF Gold
P: 4,792
 Quote by PAllen 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?
Sci Advisor
PF Gold
P: 5,091
 Quote by ghwellsjr 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.
 Quote by ghwellsjr 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.
 Quote by ghwellsjr 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.
 Quote by ghwellsjr 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.
PF Gold
P: 4,792
 Quote by PAllen I apologize if I wasn't clear enough.
Thank you, I appreciate your additional clarification. However, I also have additional comments.
 Quote by PAllen 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.
 Quote by PAllen 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.
 Quote by PAllen 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.
 Quote by PAllen 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.
 Quote by PAllen 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.
Sci Advisor
PF Gold
P: 5,091
 Quote by ghwellsjr 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.
 Quote by ghwellsjr 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?
 Quote by ghwellsjr 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.
 Quote by ghwellsjr 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.
 P: 1,583 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=4Du...gbs_navlinks_s The discussion is around page 95.
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PF Gold
P: 5,091
 Quote by lugita15 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=4Du...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.
PF Gold
P: 472
 Quote by lugita15 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.

 Quote by lugita15 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 travelling clock and the destination clock. This difference will be the one predicted by SR: the travelling 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?
Mentor
P: 17,545
 Quote by PAllen 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.

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