Talk:One-way speed of light/Archive 2

one way speed of light
Can you measure one way speed of light? I think you can : Let’s have 2 points A and B and point C exactly in the middle. We can indirectly synchronize the clocks at A and B by sending the signals to C and calculating back the time on A and B clocks so the signals from A and B would arrive at C at the same time. Let's assume temporarily that uni- directional speed of light is the same in both directions and equals c. We can now synchronize the times at A and B with C knowing that at the moment signals were send towards C (assuming the signals arrived at C at the same time) the time at A and B should be same as time at C minus travel time of the photon from A (or B) to C. Now we can send the signal from A and B towards C at the same time as indicated on clocks A and B. If the speed of light is indeed same in both directions, both signals will arrive at C at exactly the same time. If there were any differences, we could easily calculate speed of light in either direction 220.239.121.106 (talk) 12:58, 15 May 2014 (UTC)


 * Please note that to add content here, we need wp:reliable sources. I have reverted your addition to the article. - DVdm (talk) 07:17, 18 June 2014 (UTC)

Laue, Timiryazev
In popular and educational books on the theory of relativity, it is frequently claimed that Michelson and Morley experimentally proved the fallacy of the ethereal concept. This is an untrue statement. The outcome of the Michelson-Morley experiment demonstrated the impossibility of detecting ether using an experiment of this type, but not the absence of ether. One must not lose sight of the fact that the results of the Michelson and Morley experiment did not prevent the notions about the ether from being retained for a good two decades after they obtained their negative measurement result. Michelson himself and Lorentz shared these notions. Indeed, FitzGerald and Lorentz explained this result by way of the longitudinal shortening of objects moving through the ether, i.e., they explained it within the framework of the ethereal world view. And so it was that in the waning years of his life, in 1952, Einstein wrote in the article “Relativity and the Problem of Space”: Concerning the experiment of Michelson and Morley, H. A. Lorentz showed that the result obtained at least does not contradict the theory of an ether at rest” In this regard, the remark of a proponent and popularizer of the theory of relativity, M. von Laue, should also be clear, who wrote: “…it was experimentally impossible to make a choice between this theory (the Lorentz theory) and Einstein’s theory of relativity, and if the Lorentz theory nonetheless took a back seat – even though it still has proponents among physicists – this then undoubtedly occurred due to reasons of a philosophical nature”. Commenting on this statement of the German physicist M. von Laue, Soviet physicist A. Timiryazev wrote: “This argument does not have compulsory force for a physicist: for him, philosophy not confirmed by experience is an empty sound"--Olgmtv (talk) 22:26, 28 January 2015 (UTC)
 * Olgmtv, the article includes the sentence, 'Even though this theory is experimentally indistinguishable from special relativity, Lorentz' theory is no longer used for reasons of philosophical preference and because of the development of general relativity'. This seems to express what you say above.  Martin Hogbin (talk) 09:40, 29 January 2015 (UTC)

Reichenbach, Grünbaum, and Edwards
It is not clear from the cited sources that the work of Reichenbach and Grünbaum predates that of Edwards. The earliest work of Reichenbach certainly does but it is not clear exactly what that is and whether it includes the, so called, Reichenbach-Grünbaum notation. Martin Hogbin (talk) 21:11, 5 October 2014 (UTC)

It seems to me that these theories, in which light has a different one-way speed from its two-way speed, are all simple variations of the aether theory presented by Lorentz in 1904 and should therefore be attributed to him. In Lorentz' theory there is an arbitrary and experimentally indeterminable vector parameter representing velocity through the aether

Einstein was well aware of the arbitrariness of the synchronisation used in SR and made this quite clear in his work in 1905.

Poincare probably fits in somewhere.

Reichenbach certainly did propose a theory with an arbitrary parameter in 1958 and Grunbaum in 1960

Edwards described his theory in 1963.

Reichenbach, Grunbaum, and Edwards are all mentioned in a reliable secondary source, Zhang's book, but for whatever reason, he refers to the theory as Edwards' theory. I do not think that the names of Reichenbach, Grunbaum, or Edwards are in common usage with respect to this theory.

In WP,we should either name the theory after its first proposer, Lorentz, or after the name used in a good secondary source, 'Edwards'. Martin Hogbin (talk) 08:37, 12 October 2014 (UTC)


 * As there have been no opposing comments I may reword this to refer to Edwards synchronisation. Martin Hogbin (talk) 10:01, 29 January 2015 (UTC)

Theories not equivalent to special relativity
This section currently contains a sub-section on preferred reference frames, which is mainly about the Lorentz Ether Theory, a theory which is equivalent to SR. This seems somewhat anamalous to me. Any suggestions? Martin Hogbin (talk) 10:00, 29 January 2015 (UTC)

Old, but very simple and good book, worth to be mentioned in further reading: Leslie Marder, Time and The Space – Traveller, University of Pennsylvania Press, 1971 --Olgmtv (talk) 22:54, 29 January 2015 (UTC)

What does this mean?
We currently have:

'Some authors such as Mansouri and Sexl (1977) as well as Will (1992) argued that this problem doesn't affect measurements of the isotropy of the one-way speed of light, for instance, due to direction dependent changes relative to a "preferred" (aether) frame Σ. They based their analysis on a specific interpretation of the RMS test theory in relation to experiments in which light follows a unidirectional path and to slow clock-transport experiments. Will agreed that it is impossible to measure the one-way speed between two clocks using a time-of-flight method without synchronization scheme, though he argued: "...a test of the isotropy of the speed between the same two clocks as the orientation of the propagation path varies relative to Σ should not depend on how they were synchronized...". He added that aether theories can only be made consistent with relativity by introducing ad-hoc hypotheses. In more recent papers (2005, 2006) Will referred to those experiments as measuring the "isotropy of light speed using one-way propagation". '

It is not clear what this means, in particular it is not clear whether we are saying current scientific consensus says that isotropy of the one-way speed of light can be experimentally measured or not. I think it is misleading and would like to remove it. Martin Hogbin (talk) 18:06, 1 March 2015 (UTC)

Theories not equivalent to special relativity
Martin, what do you mean "anomalous?". Generally, it is not clear in this chapter, why Lorentz ether theory is equivalent, and aether theory and preferred frame theory "is not equivalent"? Isn't it all the same? What the term equivalent means? What is that "equivalence"? They all are "mathematically equivalent". I still believe, readers do not understand that. It is clear, that moving in ether body contracts in ether according to Lorentz law, but why the body at rest does contract, too? How and why symmetry happens in ether, though there physically are "body at rest" and body "in motion"? It is not Lorentz transfromations themselves make them equal. It is symmetry of relativistic effects makes them mathematically equal --Olgmtv (talk) 17:30, 17 March 2015 (UTC)
 * I do not see "anomalous?" on the page. What are you referring to?


 * 'Aether theory' refers to a simple aether as envisaged before Lorentz. Martin Hogbin (talk) 17:39, 17 March 2015 (UTC)

This section currently contains a sub-section on preferred reference frames, which is mainly about the Lorentz Ether Theory, a theory which is equivalent to SR. This seems somewhat anamalous to me. Any suggestions? Martin Hogbin (talk) 10:00, 29 January 2015 (UTC)--Olgmtv (talk) 17:47, 17 March 2015 (UTC)
 * The sub-section that you refer to is not mainly about LET it about test theories and the 'simple' aether theory, both of which are not equivalent to SR.


 * There is mention of LET and the correct situation is made quite clear. LET has a preferred frame in principle only, experimantally this must be undetectable and so not preferred in any measurable sense.


 * What wording do you object to? Martin Hogbin (talk) 19:23, 17 March 2015 (UTC)

Hmmm... I don't think I object yet. I just try to understand what the definition "equivalent" means in each exact case. Olgmtv 20:16, 17 March 2015 (UTC) What is the non-equivalence of test theories and preferred frame against SR? This is about definition Olgmtv (talk) 20:19, 17 March 2015 (UTC) No. Everything is clear enough in the article.Olgmtv (talk) 21:34, 19 March 2015 (UTC)

Non-standard clock synchronization
The article claims: ''„Experiments that attempted to directly probe the one-way speed of light independent of synchronization have been proposed, but none has succeeded in doing so. Those experiments directly establish that synchronization with slow clock-transport is equivalent to Einstein synchronization, which is an important feature of special relativity. Though those experiments don't directly establish the isotropy of the one-way speed of light, because it was shown that slow clock-transport, the laws of motion, and the way inertial reference frames are defined, already involve the assumption of isotropic one-way speeds and thus are conventional as well. In general, it was shown that these experiments are consistent with anisotropic one-way light speed as long as the two-way light speed is isotropic" ''

However, there is quite interesting clocks synchronization method which (seems) stands apart, since it doesn‘t use light to synchronize spatially separated clocks. The idea is quite simple. Imagine a long rod (shaft) in uniform rotation. Initial torque is applied and after the elastic force vanishes torsional „twist“, the shaft will eventulally rotate uniformly with constant angular velocity. The method was first proposed by R.W. Wood as the Double Fizeau Toothed Wheel. Digital clocks or clock faces can be attached to the ends of the rod. In the (SR) laboratory reference frame this method gives perfect synchronization. We can measure one – way speed of light and the outcome should be C. It should be noted, that from the reference frame in relative motion the rod will appear as „twisted“. In the LET, from the reference frame in relative motion the rod will appear as „twisted“ too, but this twist will be purely „visible“ effect due to „improper“ clock synchronization by Einstein [1], [2] It is not clear, whether this synchronization is „absolute“ in the LET framework. Some ether theorists postulated so – called „twist“ of the shaft as a compensating factor, which would lead to null – result. The article by H. Ives provides such analysis. The analysis is open to question, though. This stress - free torsion looks quite cryptic and there are arguments that call into question its existence. If this twist is not Lorentz invariant, this experiment in principle can distinquish between SR and LET.

S. Marinov [3] and Md Farid Ahmed [4] used this synchronozation in their one-way measurement experiments. It is not very clear how their devices would ensure the required accuracy.

Is this synchronization conventional, despite of the outcome of the experiment?

Some citats:

Herbert E. Ives - Theory of Double Fizeau Toothed Wheel (1939) „In a discussion of the theory of relativity R.W. Wood has considered a modification of Fizeau‘s method for determinig the velocity of light, in which two toothed wheel are mounted at the ends of a long axle. And light is sent in one direction only.“

Reza Mansouri - Some Dynamical Aspects of a Test Theory of Special Relativity (1979) „Another synchronization procedure which has been discussed very often in the literature is the so-called "shaft-synchronization". Consider a long, straight shaft of circular cross section mounted on frictionless bearings at rest in an inertial frame of reference. A narrow, straight yellow line is painted on the shaft. Counting devices A and B are placed at two different positions along the shaft (on a straight line parallel to the axis of the shaft). Each device records the passage of the yellow line and displays the number of passages on a dial. The shaft is initially at rest with A and B dials set at zero. A torque is applied for a short time, setting the shaft into rotation. Eventually all torsional vibrations damp out and the shaft is rotating uniformly with a constant angular velocity. Now the numbers on the dials at A and B can be translated into clock readings„

S.J. Prokovnik - The Empty Ghosts of Michelson and Morley A Critique of the Marinov Coupled-Mirrors Experiment

„All null-effect experiments to date are taken as justification for not questioning the significance of Einstein's light and relativity principles; however, the results of these experiments are also elegantly explicable in terms of a fundamental reference frame and the complex of anisotropy effects due to motion in this fiame. Hence Marinov's experiment would stand apart from all the evidence to date and its associated theoretical basis if his claims are proved valid.“

References [1] Michel Janssen - A COMPARISON BETWEEN LORENTZ’S ETHER THEORY AND SPECIAL RELATIVITY IN THE LIGHT OF THE EXPERIMENTS OF TROUTON AND NOBLE, Chapter 3.5.5, p. 95

[2] http://www.theoryrelativity.com/EN/all-articles/14-how-to-measure-the-one-way-speed-of-light.html

[3] Marinov S., Found. Phys. Vol. 8(1/2), 137-156, 1978.

[4] Farid Ahmed Md., B. Quin, S. Pagiatakis and A.D. Staufer, “Results of a One-Way Experiment to Test the Isotropy of the Speed of Light,”  http://arxiv.org/abs/1310.1171

[5] F. E. Hackett, Phil. Mag. 44, 740 (1922). [6] E. Feenberg, Found. Phys. 4, 121 (1974). [7] H. Arzeliez, Relativistic Kinematics, Pergamon Press, New York (1966). [8] W. A. Rodriguez, On experiments to detect possible failures of relativity theory (1985) Olgmtv (talk) 09:01, 27 July 2015 (UTC)


 * The above argument contains nothing new and is based on a misunderstanding of LET The rod will only be 'really' straight and not twisted in the aether frame.  Unfortunately it is not possible, in LET or similar theories, to determine which is the aether frame.


 * The argument is also counter-productive for those who prefer LET because LET predicts exactly the same experimental results as SR. SR has been extensively experimentally verified, thus arguing that there are experiments that could detect a difference between SR and LET is essentially arguing that LET is wrong. Martin Hogbin (talk) 17:52, 27 July 2015 (UTC)

The both theories do not create physical phenomena but in different ways explain null – result of ether-drift experiments. SR claims, that there is no absolute motion and we can consider a laboratory frame to be at rest. LET (in particular) claims that null-results are explained by retardation of moving through the ether clocks and contraction of moving through the ether rods. According to LET those factors make detecting ether drift impossible.

Imagine I would like to conduct such experiment, and I tend to believe in SR. In the SR framework every synchronization is fine, for example using rotating shaft, mechanical clocks coupled with chain, or simply throwing clock in different directions, since in SR there is no absolute motion and our laboratory reference frame is “at rest”. That means all those synchronizations are ok and are equal to Einstein convention, and the outcome of experiment should be C. If it is not C, SR has a problem. Please note that SR is not a cause of physical phenomena’s, but theirs explanation.

However, if there is just a drop of brains in my head, before investing money into the device, I have to check whether LET can predict an outcome of experiment under the terms of this theory.

LET (in particular) claims, that null-results of ether-drift experiments (in particular interferometriс, but not this one) are explained by dilation of moving through the ether clocks and contracting of moving through the ether rods. That means, that mechanical clocks coupled with chain will give null-result. The same will be with clocks, thrown into different directions. The clock which moves “back” runs faster, and clock which moves forward runs “slower”. That discrepancy would lead to null – result of the experiment, even if one-way speed of light is not C.

I don’t know, how LET explains any compensatory factors, that would lead to dyssinchronization of synchronized by rotating rod clocks. If simultaneity is not absolute, would you like to say, that would lead to physical twist and even physical destruction of a rod?

''“The rod will only be 'really' straight and not twisted in the aether frame. Unfortunately it is not possible, in LET or similar theories, to determine which is the aether frame.”''

This statement implies that in LET except of the ether frame, there are some kinds of other frames and in those frames (obviously in motion through ether) the rod twists? In LET there is the preferred ether frame in which other material bodies and associated with them reference frames move relatively to each other. Reference frames in motion cannot detect their motion due to clock retardation and contraction of rods, as Lorentz explains. Do you want to say, that if a rotating shaft was at rest and after some acceleration in ether (if one exists) it would not only contact but would twist either? That’s exactly what Ives says, but, just read his article; I can send it to you. Will it break due to twisting, as its speed increases?

Yes, in Lorentz theory, the same way like in the SR, from a reference frame in motion rotating rod shall appear as twisted. This twist would be purely “visible, mathematical” in LET, since moving observer has its own local time and synchronizes clocks in his own reference frame by Einstein. That leads to full reciprocity of Lorentz transformations and mathematical equivalence of SR and LET and “visible” twist of the rod.

That (probably?) doesn’t mean, that moving rod physically twists. Do you think it does? According to what known law? Again, you say that this experiment will give exactly C, because:

Bidirectional speed of light is measured? No…

Light is isotropic? But this is only assumption, which exactly we would like to verify…

Synchronization is not “synchronous” due to? Twist? Maybe… Why?Olgmtv (talk) 21:06, 27 July 2015 (UTC)

I read the article by Peter Ohrstrom. DVdm gave me a link. He (ir seems) claims, that due to Lorentz transformations the rod is relatively twisted observing from different reference systems. That's clear but there is no answer in which exactly. His arguments in favor of conventionality of synchronization is doubtful.Olgmtv (talk) 21:20, 27 July 2015 (UTC)

Will vs Zhang
We have, 'Some authors such Mansouri and Sexl as well as Will argued that this problem doesn't affect measurements of the isotropy of the one-way speed of light'.

I believe that this point is overstated and that Zhang later showed Will's argument to be incorrect (and that Will acknowledged that). Does any opinion or sources that may help to resolve this? Martin Hogbin (talk) 17:48, 30 January 2016 (UTC)
 * As there is still no response, I will change the text. Martin Hogbin (talk) 22:23, 31 January 2016 (UTC)

Fringe results
A new user's edits  are about variable light speed and Selleri transformations, with unpublished sources like arxiv, ResearchGate. I have undone the edits per wp:FRINGE, wp:UNDUE, and wp:Secondary sources. Comments welcome. - DVdm (talk) 16:06, 26 May 2020 (UTC)
 * DVdm's revert looks appropriate to me. Aoki and Mitsui (2007) do not claim to have performed a one-way speed-of-light measurement, and even Greaves et al. (2009), who claimed to have done so, said that Aoki and Mitsui's measurement was two-way. So, the statement in the text you removed is flatly incorrect. XOR&#39;easter (talk) 19:31, 26 May 2020 (UTC)
 * My doubts were first raised by the last sentence of the abstract of this Researchgate source.
 * Could you have a look at the wp:DUEness of this edit by same user in article Variable speed of light as well? As a subsection of the section "Various other VSL occurrences", this does not really look like an example of it. Looks rather very fringy to me, but I can't assess the quality of the cited source, or even access it. Tia. - DVdm (talk) 20:10, 26 May 2020 (UTC)
 * Yeah, that last sentence is a red flag. As for the other, Progress in Physics is not a reliable source. XOR&#39;easter (talk) 21:59, 26 May 2020 (UTC)
 * That is good to know. Thx. - DVdm (talk) 22:25, 26 May 2020 (UTC)
 * I wonder how an error can be corrected if only “fringe sources” are known to explain the nature of the error. Specifically, my own writings which are only deemed fringe because I am a retired math instructor not a physicist. The error I have in mind is the statement that one-way speed of light cannot be measured without synchronizing clocks. My essay “A Scheme for Measuring Anisotropy of One‑Way Light Speed. (Part I)” is on my website http://physicsfixes.elementfx.com/index20.html DavidBryanWallace (talk) 01:38, 27 October 2020 (UTC)
 * I do agree that Dr. Gift erred in his Researchgate article and that Progress in Physics is generally pretty fringy. DavidBryanWallace (talk) 02:09, 27 October 2020 (UTC)

Suggest that this article be updated to include mention/discussion of the CalTech femtosecond camera.
The article presents the view that all measurements of the one way speed of light require two spatially separated clocks requiring synchronization. That may have been true of 20th century technology. But the Caltech femotsecond camera appears to provide a measurement of the one speed of light using one clock and not requiring clock synchronization. There is the film of a one way laser pulse traversing a distance and also a superluminal phase measurement of a one way pulse directed at an angle to the camera which implies epsilon = 1/2.

Theophilus71 (talk) 17:33, 20 February 2022 (UTC)