One of the important predictions of relativity theory is that particles with mass cant reach speed of light in vacuum and will always be slightly slower. I wanted to know more about the maximum speed which can be reached by particles with mass and looked for relevant experiments. But to my surprise, there was no difference experimentally confirmed. Within experimental error, particles with mass are reaching speed of light. This is especially valid for neutrino, which is particle with confirmed mass and with several experiments measuring the speed of neutrinos. And with each experiment, result of neutrinos reaching speed of light is confirmed with better and better precision. Latest experiment confirmed it at the level of 10 -19, so there is extremely small margin left for the predicted difference in speed. https://en.wikipedia.org/wiki/Measurements_of_neutrino_speed This cant be regarded as falsification of relativity theory, but it seems to me, that there is missing experimental proof or astronomical observation for one of the most important predictions of relativity theory. I wonder if somebody else here knows better about this topic.