We usually heard that speed of light in vacuum $c$ remains same no matter how observer is moving?

I am wondering whether is it taken as a postulate or a proven phenomenon that $c$ is constant irrespective of observer's speed?

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    $\begingroup$ It is an empirical fact that the two-way speed of light (in an inertial frame) is the invariant speed $c$. Einstein's postulate that the one-way speed of light is $c$ turns out to be a convention which is to say that Einstein synchronization of clocks guarantees that the one-way speed of light is $c$ in all inertial reference frames. en.wikipedia.org/wiki/One-way_speed_of_light $\endgroup$ Commented Oct 29, 2014 at 0:03
  • $\begingroup$ @AlfredCentauri A great and oft overlooked, subtle point: definitely worth making permanent in a partial answer to add to the others! $\endgroup$ Commented Oct 29, 2014 at 2:26

3 Answers 3


I am wondering whether is it taken as a postulate or a proven phenomenon that c is constant irrespective of observer's speed?

Either one. Both.

Einstein took it as a postulate in his 1905 paper on special relativity. From it, he proved various things about space and time.

The frame-independence of $c$ is also experimentally supported. This is what the Michelson-Morley experiment showed (although it was not interpreted correctly until much later).

You can also take other postulates for special relativity, describing the symmetry properties of space and time. In this case the constancy of $c$ becomes a theorem rather than an axiom. From a modern point of view, this approach makes more sense than Einstein's 1905 axiomatization, which puts light in a special role and defines $c$ as the speed of light. Nowadays we know that light is just one of several fields, and $c$ is not the speed of light but rather a conversion factor between space and time units. The symmetry approach goes back to W.v.Ignatowsky, Phys. Zeits. 11 (1911) 972, and can be found in various other modern presentations, such as this one or my own.

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    $\begingroup$ Good answer. Could possibly add "with special relativity, which was founded on the concept of $c$ being constant, it became possible to make all kinds of predictions that have been proven experimentally. Thus the postulate is now well supported by experimental evidence - both direct, and indirect." $\endgroup$
    – Floris
    Commented Oct 28, 2014 at 23:42
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    $\begingroup$ Might be worth citing the "recent experiments" section of the MMX Wiki page: some of the cited $\Delta c/c$ values for the difference between speeds in the two arms are truly astounding. As not much of an experimentalist I find them mind boggling. $\endgroup$ Commented Oct 29, 2014 at 2:29
  • $\begingroup$ @WetSavannaAnimalakaRodVance: You mean this? en.wikipedia.org/wiki/… Wow, $\Delta c/c \lesssim 10^{-17}$ is indeed impressive! $\endgroup$
    – user4552
    Commented Oct 29, 2014 at 5:28
  • $\begingroup$ @BenCrowell That's the one. Mind you, I guess it's not too surprising in one sense given the amazing work being done to make gravitational wave interferometry work. The Einstein Telescope design study, for example, is worth browsing simply to get a feel for the breadth of scientific knowledge that is brought together for these projects. $\endgroup$ Commented Oct 29, 2014 at 6:16
  • $\begingroup$ @BenCrowell: "The frame-independence of c is also experimentally supported. This is what the Michelson-Morley experiment showed (although it was not interpreted correctly until much later)." I wonder what you mean by that. C is alway local, i.e. the particular photon cannot be seen by two different observers, i.e. in two different frames. Also, as far as I know, c is always a two-way measurement, which means it is always measured only in the frame from which it was sent. $\endgroup$ Commented Oct 29, 2014 at 8:18

By using orthogonal optical resonators, laboratory tests concerning verifying the isotropy of c have come a long way. As quoted from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.80.105011

"An analysis of data recorded over the course of one year sets a limit on an anisotropy of the speed of light of $\Delta c/c \sim 10^{-17}.$ This constitutes the most accurate laboratory test of the isotropy of c to date and allows to constrain parameters of a Lorentz violating extension of the standard model of particle physics down to a level of $ 10^{-17}.$."

This method therefore provides a more refined repeat of the Michelson–Morley experiment. It compares the resonance frequencies of two orthogonal optical resonators that are implemented in a single block of fused silica and are rotated continuously on a precision air bearing turntable. The experiment in turn provides an extremely accurate laboratory testing of the isotropy of c.

. . . . . . . . . . . . . . . . . . . . . . . enter image description here

However, this phenomenon that the measure of c is constant does confuse many people.

Imagine that we have a hollow tube that is 300,000 km's long. If we send a burst of light through the tube it will require 1 second to pass through the tube since the speed of light is 300,000 km/s.

. . . . . . . . . . . . . . . enter image description here $$(1)\quad L'=L\sqrt{1-v^2/c^2}\quad \quad(2)\quad t'=t\sqrt{1-v^2/c^2}\quad \quad \quad(3)\quad L(v/c^2)$$ However, as shown in the above diagram, if we accelerate the tube to a velocity of 260,000 km/s, the length will have contracted from 300,000 km's to 150,000 km's, and the clocks located on board the tube, 2A and 2B, will now be ticking at half speed when compared to any clock located on the Stationary Frame of Reference. These results are based upon SR equations (1) and (2). Based upon equation number (3), clock 2A is ahead of clock 2B by 0.866 of a second.

Once again we send a burst of light from the (S)ource to the (D)estination, and thus in turn the light passes through the tube from the rear end to the front end. To those located on the Stationary frame of reference, the speed of that burst of light across space is 300,000 km/s just as expected. Thus the light is traveling 40,000 km/s faster than the tube. It therefore takes approximately 3.73 sec. for the light to pass through the tube ( 150,000 km's / 40,000 km/s ). However, on board the tube the clocks are ticking at half speed, thus these clocks would only measure a time period of 1.866 sec ( 3.73 * 0.5 ).

Now imagine that the light reached the rear of the tube when clock 2A registered 0.00. When the light reaches the front of the tube, clock 2A will therefore register 1.866 of a second. But, clock 2B is lagging behind clock 2A by 0.866 of a second, and so when the light reaches the front of the tube, clock 2B will register 1.866 - 0.866 = 1 sec. Therefore, based upon the measurement instruments, it appears as though the light took one second to pass through the tube from the rear to the front. It appears as though nothing has changed from the tubes point of view.

Even if the light travels in the opposite direction that the tube is traveling, once again, due to the change of the measurement instruments, a 1 second time period is measured as being the time required for the light to pass through the tube. This occurs even though to an external observer, that is located in the Stationary Frame of Reference, it only took 0.268 of a second for the light to pass through the tube from front to rear. ( 0.268 sec. * 0.5 time dilation ) + ( +0.866 sec. clock offset ) = 1 sec.

Thus it is obvious that as the light passes through the tube, it does not change its speed of 300,000 km/s. However, to those on board the moving tube, despite being in motion, they too still measure the speed of the light as 300,000 km/s.

  • $\begingroup$ This experiment only tested the isotropy of the two-way speed of light, not of the one-way speed, correct? $\endgroup$ Commented Feb 17, 2022 at 10:33

It is a well-substantiated observed phenomenon. Science deals only with provisional truths, but this hypothesis has undergone (and passed) immense amounts of scrupulous experimentation and mathematical formulation.

In a Neo-Lorentzian interpretation, physics works differently in all reference frames except for one single, undetectable, privileged reference frame and everything else that occurs that does not comport with this reference frame is, for all intents and purposes, illusory. This view has been largely discredited as the contrary, Special Relativity interpretation, has been observationally supported, with no falsification thus far, and the isotropy of the universe (physics works at all reference frames irregardless of orientation or velocity) appears to be correct. Though the Neo-Lorentzian view is essentially unfalsifiable, even theories with predictive capability (which is essentially equal to that of SR) but with added components that are not required to explain the facts are more likely to be wrong.

  • $\begingroup$ +1, but I think the wording is a little bit woolly: you need to read very closely to see that you're saying SR and the idea of no privileged frame is confirmed. $\endgroup$ Commented Oct 29, 2014 at 2:32
  • $\begingroup$ I'm sorry, I'm not intending to assert that, rather that there has been no conversationally verified privileged reference frame, and the evidence is generally contradictory to it. Edit: I went ahead and reworded it to make my point more clear. $\endgroup$
    – Goodies
    Commented Oct 29, 2014 at 6:16

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