# Speed of light constant?

Is there any chance that the speed of light in a vacuum isn't constant in all reference frames, implying that Relativity theory is generally false? Do some Physicists believe that it is?

If no, why not or how can we be so sure?

jtbell
Mentor
Any part of physics is subject to revision if new experimental evidence comes along that requires it.

The constancy of the speed of light has been tested a lot, along with the related concept of "Lorentz invariance." So far, experiments support them with high precision. See section three of the following FAQ:

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

To be pedantic, the speed of light is constant in all reference frames by definition. Instead, it makes sense to ask whether the length of a meter is constant in all reference frames.

bcrowell
Staff Emeritus
Gold Member
As jtbell correctly notes, the most direct way to interpret this question is to think of it as a question about the current state of the art in testing Lorentz invariance.

There are other possibilities.

Loop quantum gravity predicts that the vacuum is dispersive, so the speed of light isn't constant even in a single frame: http://arxiv.org/abs/0908.1832

The photon could turn out to have a nonvanishing rest mass:
R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters , 1998, 80, 1826-1829, http://silver.neep.wisc.edu/~lakes/mu.html
If this is the case, then there are no implications for the foundations of relativity, but gauge theory would be in trouble.

There are other scenarios motivated by quantum gravity, e.g., doubly-special relativity: http://en.wikipedia.org/wiki/Doubly-special_relativity If doubly-special relativity is right, then there is no violation of Lorentz invariance, but there is a higher symmetry as well.

You can also search for Lorentz violation in the gravity sector rather than the photon sector: http://www.lightandmatter.com/html_books/genrel/ch02/ch02.html#Section2.4 [Broken] (subsection 2.4.3)

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Another possibility worth exploring is whether the speed of light is frequency dependent - varying with the frequency of the photon. In Jackson's Electrodynamics textbook he describes the experiments which have so far been done. At lower frequencies the experimental deviation is very, very small. But at very high frequencies the possible deviation is on the order of one part in a thousand. That doesn't necessarily mean there actually is a consistent deviation because the margin of error may be far greater based the measurement technique. So further study seems warranted.

bcrowell
Staff Emeritus