# Evidence for the absolute speed of light

I'm wrapping my mind over Einstein's Special Relativity. He postulates an abolute speed of light which concludes (in my perspective) to several illogical paradoxes. Is there any evidence, that the speed of light is absolute and not relative to the speed of the emitter, which sends out the light?

Example of skepticism:

http://www.complexrelativity.com/

Every round trip measurement of the speed of light has been on a moving platform. Therefore, there hasn't ever been two identical round trip measurement of the speed of light. Nobody can dispute the statement.
The reason is simple. While both points for the round trip are fixed relative to each other, they are moving relative to the center of the earth and to the sun and to the galaxy and so on. This paper has two parts. First, I show that the speed of light is relative and additive to the speed of earth. Second, I show how the issue is addressed in the theory or relativity.

• @Rafael you might want to discuss `several logical paradoxons` (from your perspective) on Physics.SE Jun 7, 2012 at 10:53
• @RafaelT - I think you would be better served on physics than skeptics. It doesn't seem that you are skeptical of it so much as you do not understand it making that a more appropriate place to ask the question.
Jun 7, 2012 at 17:25
• I have cleaned up this comment section with an axe. Please let's avoid the "should we question basic science" angle (go here to do so). Let's also keep the comments on the question and not the answer. Jun 8, 2012 at 10:30
• Considering the speed of light changes any time it changes medium, I think it's self-evident that the speed of light isn't "absolute". `c` is the speed of light in a vacuum. Jul 16, 2014 at 19:17
• Special relativity was formulated to explain how the speed of light can be absolute and still make sense of physics. We discovered the speed of light did not change no matter how fast you are travelling more than 10 years before someone came up with a theory that fixed physics. Jun 26, 2018 at 6:35

obeying the usual laws of Newtonian mechanics, and we expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v).

If you look at the section entitled Refutations of emission theory, you'll see some of the most concrete examples of how this was shown not to hold true for light.

One convincing argument comes just from looking up at the night sky:

In 1910 Daniel Frost Comstock [1] and in 1913 Willem de Sitter [2] wrote that for the case of a double-star system seen edge-on, light from the approaching star might be expected to travel faster than light from its receding companion, and overtake it. If the distance was great enough for an approaching star's "fast" signal to catch up with and overtake the "slow" light that it had emitted earlier when it was receding, then the image of the star system should appear completely scrambled.

de Sitter's double star argument

Experiments (similar to Michelson--Morley which was already mentioned) can measure photons from the same source travelling both in the same direction of earth's travel, and in the opposite direction of earth's travel. In the world of classical mechanics, one would expect the one launched in the direction of earth's travel to get a fixed distance away from the source faster, but experiments repeatedly show that both travel the same distance in the same time.

Such experiments include that of Sadeh (1963) [3] who used a time-of-flight technique to measure velocity differences of photons traveling in opposite direction, which were produced by positron annihilation. Another experiment was conducted by Alväger et al. (1963), who compared the time of flight of gamma rays from moving and resting sources. Both experiments found no difference, in accordance with relativity.

These are the simplest arguments that can be conjured up without requiring you to dig too deeply into things like interferometry and quantum electrodynamics - but these too are easily observable in the classical world and are completely incompatible with any theory that postulates a speed of light that is affected by the speed of the source.

[1] Daniel Frost Comstock. A Neglected Type of Relativity. Physical Review, Feb 1910, 30 (2): 267

[2] W. de Sitter. An Astronomical Proof for the Constancy of the Speed of Light. Physik. Zeitschr. 14, 429, (1913)

[3] D. Sadeh. Experimental Evidence for the Constancy of the Velocity of Gamma Rays, Using Annihilation in Flight. Phys. Rev. Lett. 10, 271–273 (1963)

Well there were experiments that confirmed it, some in a straightforward way. Most notable was Michelson--Morley experiment.

When Einstein said that speed of light is absolute he really simplified view of the universe. Basically to state the opposite we must say that there is some `magical` base reference and that all measurements of speed of light are done with respect of the speed of the emitter relative to that base reference. One of ways to test it would be to check if there is any change in measured speed of light when we measure light speed in the same direction the earth travels and opposite direction earth travels around sun. Experiments (Michelson Morley and other) repeatedly shown that we can't measure change of measured light speed in this manner (and we know that our equipment would be capable to detect that difference). This leaves us with two options: either light speed is absolute or Earth is this magical base reference that all measurements of light speed are taken with reference to.

Moreover despite being 'illogical' relativity theory is used in commodity equipment --- for example GPS wouldn't work without relativity. Basically GPS rely on a very accurate time measurements --- and these need a correction based on gravity strength on GPS satellite orbit.

• Can you give a brief summary of the Michelson-Morley experiment? Jun 7, 2012 at 13:48
• In my understanding. Michelson-Morley disproved the idea of an aether - an absolute reference. That's a separate concept to confirming the theory of relativity, which it predates. Jun 7, 2012 at 14:22
• @RafaelT Basically disproving aether and proving relativity is the same. Because aether was the medium in which light traveled and hence --- that measurements of light speed were relative to, since there is no aether light speed is absolute.
– jb.
Jun 7, 2012 at 15:23
• @Oddthinking In doing so it showed that the universe actually obeys the symmetries of Maxwell's equations, not the apparent symmetries of Newtonian mechanics. Relativity follows. If you have all of the mathematical insight, the willingness to take the results seriously and the sheer balls to publish. Indeed, Einstein actually circulated the idea to prominent physicists before getting up the courage to publish as such. Jun 8, 2012 at 2:12
• @jb., Michelson-Morley wasn't measuring the "forward" versus "backward" speed, it was measuring the "parallel" versus "across" speed. Additionally, they were doing some clever optical tricks to measure the difference in speeds, rather than the absolute speeds (differences tend to be measurable to much greater precision than absolute values).
– Mark
Jun 26, 2018 at 2:02

There is plenty of evidence for the speed of light being not relative to the speed of the emitter.

Here's a decentish overview.

The rules of special relatively form a central part of particle physics and predictions to do with design and construction of Particle accelerators (for decades of research).

In these particles are accelerated close to speed of light but don't exceed it for a variety of experiments. Their behaviour at high speeds can be observed, and fit with Special Relativity's predictions.