# How to measure the one way speed of light.

by yuiop
Tags: light, measure, speed
P: 1,115
 Quote by harrylin I found that paper but have reason to suspect that it's not peer reviewed and it's certainly not free of errors - already its main message is wrong. However, the "Sagnac correction" of GPS is effectively the same as the one-way speed of light relative to a moving object (in modern jargon, their "closing speed" c-v).
As things stand you've got me confused, hopefully just a case of unspoken qualifications. In #31 "And yes, truly measuring the one way speed of light is indeed tantamount to measuring absolute velocity.", which I took as saying it was an impossibility. but then in #50 "The relative speed (also called "closing speed") of radio waves and a GPS receiver at rest on the Earth is c-v. This is merely an example that a nominal difference of one-way speeds can be measured..." - Gift's paper was I thought arguing just that. Let's bypass all the technical intricacies of how GPS works and consider physics in some nominally inertial lab frame. A basic consequence of a real nonreciprocal one-way c : from the fundamental relation c = lambda*f, if f is fixed any nonreciprocal c automatically means a nonreciprocal lambda in that frame. This regardless of whether the light source is within that or another frame. Do you agree with this? And contrary to the consensus viewpoint, lambda variation is at least in principle readily measurable (see below).
 - yes it sounds to me a bit like an experiment with cables that was done some years ago in Belgium and from which a positive effect was claimed... let's see the FAQ: http://www.phys.ncku.edu.tw/mirrors/...istent_with_SR ["Amateurs look for patterns, professionals look at error bars" - hmm, that's way oversimplified - but that's another topic!]
Thanks for that link - more details on one-way tests than the Baez site.
 Ah I found it, something like that was done once by de Witte (regretfully the experiment isn't directly discussed there). Anyway, in theory such experiments will only eventually find effects from the non-inertial motion of the Earth.
Not so - see below.
 The Silvertooth experiment there also looked for effects on phases, and was also discredited on theoretical grounds (the alternative theory could not explain the claimed results either). If I correctly recall (for I have now no time to redo the analysis), waves (from one side as well as from two sides) interfere the same when in motion as in rest - perhaps someone else can eventually correct me and elaborate.
Found the 1986 paper by Silvertooth - seems OK in principle. Interested though in any actual detailed critique you may know of. A very similar style of differential phase type test was performed by C Navia et al, referenced in the above link. Interesting that the only criticism leveled there was one of sloppy technique - but nothing about fundamental theoretical inadequacy.
 Originally Posted by Q-reeus: grav-universe - my understanding is SR and LET are indistinguishable in any operational sense. "They are if all frames are completely symmetrical, which so far they appear to be. LET covers a broader scope than SR, however. SR only works if all frames are symmetrical, while LET does not necessarily require it."
That was from #42. I would now have to agree with grav-universe on that point: SR by the 2nd postulate forbids one-way c variation, whereas LET implicitly requires it. My assumption was LT's automatically compensate in both theories. That's only true for two-way measurements and 'normal physics' which depends on invariant two-way c. Hence one-way tests are crucial to distinguish between SR and LET - and differential phase type measurements are perfectly capable in principle of doing that - as per c = lambda*f discussed above. How so? Well my proposed twin fibers setup for one. A nonreciprocal lambda equally stretches/compresses lambda along both lines - and the associated node locations that for sure can be measured; just a question of required sensitivity. Another even simpler example, not very sensitive but 'for sure' in principle - a single match terminated line containing one or more sections of a very low Q almost '1/4 lambda' transmission cavity. Won't go into the details here, but not hard to show that any non-reciprocity in c and thus lambda will result in a corresponding reflection phase and amplitude variation. Once we see past the false dilemma of 'clock sync' and realize phase methods completely bypass this issue, 'impossibility of one-way c measurement' now looks pretty stupid imho. I've done a 180 degree flip on outlook here. Desperately resisting being sucked into a SR/LET theoretic dispute BH!
PF Gold
P: 4,743
 Quote by Q-reeus I would now have to agree with grav-universe on that point: SR by the 2nd postulate forbids one-way c variation, whereas LET implicitly requires it. My assumption was LT's automatically compensate in both theories. That's only true for two-way measurements and 'normal physics' which depends on invariant two-way c. Hence one-way tests are crucial to distinguish between SR and LET - and differential phase type measurements are perfectly capable in principle of doing that - as per c = lambda*f discussed above. How so? Well my proposed twin fibers setup for one. A nonreciprocal lambda equally stretches/compresses lambda along both lines - and the associated node locations that for sure can be measured; just a question of required sensitivity. Another even simpler example, not very sensitive but 'for sure' in principle - a single match terminated line containing one or more sections of a very low Q almost '1/4 lambda' transmission cavity. Won't go into the details here, but not hard to show that any non-reciprocity in c and thus lambda will result in a corresponding reflection phase and amplitude variation. Once we see past the false dilemma of 'clock sync' and realize phase methods completely bypass this issue, 'impossibility of one-way c measurement' now looks pretty stupid imho. I've done a 180 degree flip on outlook here. Desperately resisting being sucked into a SR/LET theoretic dispute BH!
The only difference between LET and SR is a philosophical one; LET claims that nature operates on a single unidentifiable absolute ether rest frame in which we are almost always moving with respect to, while SR claims that every inertial observer can consider himself to be at rest in, what amounts to, the absolute ether rest frame. There can be no test that can distinguish between the two.
P: 1,115
 Quote by ghwellsjr The only difference between LET and SR is a philosophical one; LET claims that nature operates on a single unidentifiable absolute ether rest frame in which we are almost always moving with respect to, while SR claims that every inertial observer can consider himself to be at rest in, what amounts to, the absolute ether rest frame. There can be no test that can distinguish between the two.
That's what I've thought till now. As for LET reference frame, 'absolute' need not mean that as such - I'm rather partial to an increasing view that CMBR provides a reference guage for a 'locally absolute' rest frame. Think of the analogous 2D balloon model - any point locally stationary on the skin can serve as a local rest frame, though no globally 'absolute rest frame' is possible. But really I think it gets down to whether non-reciprocal c is in principle detectable - currently I would say yes - via properly designed one-way differential phase measurements. If you know of a reference that can shoot that idea down, please post it!
P: 3,187
 Quote by Q-reeus As things stand you've got me confused, hopefully just a case of unspoken qualifications. In #31 "And yes, truly measuring the one way speed of light is indeed tantamount to measuring absolute velocity.", which I took as saying it was an impossibility. but then in #50 "The relative speed (also called "closing speed") of radio waves and a GPS receiver at rest on the Earth is c-v. This is merely an example that a nominal difference of one-way speeds can be measured..." - Gift's paper was I thought arguing just that. Let's bypass all the technical intricacies of how GPS works and consider physics in some nominally inertial lab frame. A basic consequence of a real nonreciprocal one-way c : from the fundamental relation c = lambda*f, if f is fixed any nonreciprocal c automatically means a nonreciprocal lambda in that frame. This regardless of whether the light source is within that or another frame. Do you agree with this? And contrary to the consensus viewpoint, lambda variation is at least in principle readily measurable (see below). Thanks for that link - more details on one-way tests than the Baez site. Not so - see below. Found the 1986 paper by Silvertooth - seems OK in principle. Interested though in any actual detailed critique you may know of. A very similar style of differential phase type test was performed by C Navia et al, referenced in the above link. Interesting that the only criticism leveled there was one of sloppy technique - but nothing about fundamental theoretical inadequacy.
Aargh - I spent one hour on replying (basically it was all "no" + long explanations) but lost it all because I didn't make a backup and this site threw me out... too bad, won't have time anymore and should be sleeping now. If you still have any question one or two days from now, please ask again!
 Emeritus Sci Advisor PF Gold P: 29,238 Er.. not sure if anyone brought up this link already (I didn't read through all 4 pages of this discussion), but in case it hasn't, one might want to look at this review: http://arxiv.org/abs/1011.1318 Zz.
P: 1,115
 Quote by ZapperZ Er.. not sure if anyone brought up this link already (I didn't read through all 4 pages of this discussion), but in case it hasn't, one might want to look at this review: http://arxiv.org/abs/1011.1318 Zz.
Yes it was listed early on in the thread, but I never got to read all of it myself. They claim clock synch is no barrier to making one-way tests, and quote Clifford Will to that effect. On the other hand these reviewers deny it can truly be done; ie. 'one-way' is always de facto 'two-way': http://math.ucr.edu/home/baez/physic...#one-way_tests http://www.phys.ncku.edu.tw/mirrors/...istent_with_SR
P: 1,115
 Quote by harrylin Aargh - I spent one hour on replying (basically it was all "no" + long explanations) but lost it all because I didn't make a backup and this site threw me out... too bad, won't have time anymore and should be sleeping now. If you still have any question one or two days from now, please ask again!
Amazing coincidence - precisely same thing happened to me (Microsoft automatic update = unwelcome reboot!!) Ditto way overdue for sleep here too.

EDIT: In #55 "Another even simpler example, not very sensitive but 'for sure' in principle - a single match terminated line containing one or more sections of a very low Q almost '1/4 lambda' transmission cavity. Won't go into the details here, but not hard to show that any non-reciprocity in c and thus lambda will result in a corresponding reflection phase and amplitude variation." That was just outright wrong and is withdrawn unreservedly. Reflection always implies a two-way measurement!
P: 3,187
 Quote by Q-reeus Yes it was listed early on in the thread, but I never got to read all of it myself. They claim clock synch is no barrier to making one-way tests, and quote Clifford Will to that effect. On the other hand these reviewers deny it can truly be done; ie. 'one-way' is always de facto 'two-way': http://math.ucr.edu/home/baez/physic...#one-way_tests http://www.phys.ncku.edu.tw/mirrors/...istent_with_SR
Both are somewhat correct: if a one-way experiment can break the PoR, then in principle a suitably designed two-way experiment should be capable of the same. However, different set-ups are more suited to test other aspects of the theory. In particular the repeat of the Marinov one-way experiment will be interesting.
P: 3,187
 Quote by Q-reeus Amazing coincidence - precisely same thing happened to me (Microsoft automatic update = unwelcome reboot!!) Ditto way overdue for sleep here too. EDIT: In #55 "Another even simpler example, not very sensitive but 'for sure' in principle - a single match terminated line containing one or more sections of a very low Q almost '1/4 lambda' transmission cavity. Won't go into the details here, but not hard to show that any non-reciprocity in c and thus lambda will result in a corresponding reflection phase and amplitude variation." That was just outright wrong and is withdrawn unreservedly. Reflection always implies a two-way measurement!
OK. Just a short precision about my unclear explanation: with "nominal c-v" I meant that that measurement is merely a result of the observer's reference and synchronization -> you can transform it away to v=0 in which case you obtain isotropic light speed (see dictionary.com, "nominal" ).

Cheers,
Harald
P: 1,115
 Quote by harrylin OK. Just a short precision about my unclear explanation: with "nominal c-v" I meant that that measurement is merely a result of the observer's reference and synchronization -> you can transform it away to v=0 in which case you obtain isotropic light speed (see dictionary.com, "nominal" ). Cheers, Harald
Just knew it had to be something like that; glad we see eye to eye on that one! Meanwhile I've had an epiphany event of sorts.
ghwellsjr wrote in #56:
"The only difference between LET and SR is a philosophical one; LET claims that nature operates on a single unidentifiable absolute ether rest frame in which we are almost always moving with respect to, while SR claims that every inertial observer can consider himself to be at rest in, what amounts to, the absolute ether rest frame. There can be no test that can distinguish between the two."
Yep, now agree.
harrylin wrote in #62:
"Both are somewhat correct: if a one-way experiment can break the PoR, then in principle a suitably designed two-way experiment should be capable of the same. However, different set-ups are more suited to test other aspects of the theory. In particular the repeat of the Marinov one-way experiment will be interesting."
Yes and yes and yes.

Focusing on means to measure a notional one-way c, hadn't stop to think what a non-null result would fully imply. In the proposed twin fiber arrangement of #53, if null balancing was foregone, as the interference pattern shifted with orientation, the coupling probe will act as a variable scatterer/reflector - changing the overall energy flow and thus the physics. Consequently we can draw the conclusion a finite one-way c effect is automatically incompatible with the basic postulate of SR/LET - physical equivalence of all inertial reference frames. Way too much observational support for that to be in question, at anything above Planck scale physics anyway. So another 180 degree turn and it's back full circle. No point in hunting for a non-event. I'm exhausted - catch you all much later!

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