# What experiment shows that the universal speed limit is the same as light?

I figure the answer is somewhere in the sticky FAQ above but I’m having trouble finding it. I’m hoping y’all can help.

I understand the Michelson–Morley experiment shows that the speed of light is the same in all directions and that that is a postulate in SR. But just like every other experiment, it has its accuracy limits. I figure there must be some other experiment that shows they’re exactly the same or it might be that the universal speed limit is slightly faster than light speed. I don’t think the universal speed limit is less than light speed because that truly would mess up the theory of SR. So I guess I’m looking for the upper limit. To assume they’re exactly the same seems unnecessary. Such an assumption is not required by the theory of SR. All the formulas would stay the same except “C” would be slightly faster than light speed. In other words light speed would not be a constant but the universal speed limit would be that constant used in SR.

atyy
The two technical statements that are equivalent to your question are:
1) local Lorentz invariance holds: asserts there is a universal speed limit,
2) the photon is massless (which in terms of classical light waves means that all wavelengths of light travel at the same speed): asserts that light travels at the universal speed limit.

Experimental limits on local Lorentz invariance are reviewed in http://arxiv.org/abs/gr-qc/0502097 and http://pirsa.org/11100056/.

Experimental limits on photon mass are reviewed in http://arxiv.org/abs/0809.1003.

Re: What experiment shows that the universal speed limit is the same as light?
Think for a second how an experiment could demonstrate something like that. The answer is you can't. It is like proving a negative.

However if you accelerate a particle in an accelerator you will observe it can approach but never reach c.

Atyy and Passionflower, Thank you both. This gives me a lot to think about. Atyy, I'll work on slogging through those ARXIV links. I've never been good at that. And thanks for the video!