The theory of Relativity uses a constant c which has the approximate value of 3x108 ms-1 The measured speed of light in a vacuum vl has apparently the same value. What is the experimental evidence for this equivalence? As far as I know, the only way to measure c is to measure the speed of light. Of course, if c was slightly larger than vl, photons would have a small mass. Does this have consequences? It would make sense of the recent suggestion that neutrinos can travel faster than light.