Plar, there actually is quite a lot of proof that the speed of light remains constant from all reference points. On famous example would be the Michelson/Morley experiments where the speed of light was measured both parallel and perpendicular to the earth's motion. The experiment was initially designed to disprove the theory that light waves needed a medium to propagate through, but it did still contribute to later experiments in relativity. Among these theories were Einstein's theory of special relativity, as well as time dilation and length contraction.