Hi, Can someone tell me the reason why it is impossible for the photon to have a rest-mass? Without referring to the theories which are actually build on the fact that the photon is massless. Is there any possibility that the speed of light is not about light and that there is some fundamental critical speed (let's call it c)? Why can't we consider that maybe the photon has a very small mass and that the speed of the photon is infinitely close to this critical speed? For example we could consider that the energy difference between "red" and "blue" photons comes from the speed-difference of this photons. But the speed difference is so infinitely small that we have an impression that the speed of light doesn't depend on it's frequency. I really can't figure out why we can't assume the possibility of a "heavy" photon. Many people say that if the photon has a non-zero mass it will become short range. But I'm really not convinced by this kind of explanations because they are based on the theories which are build on the "massless photon" assumption. For me, photon will travel infinitely until it hits something. Thank you all in advance.