Special relativity states that the speed of light is constant for all the references, as long as they are not accelerating. For example, the speed of light would be c for a train moving linearly with a constant speed and would also be c for an observer who's not moving at all (I took the earth as reference). I know that it was proved experimentally, but I don't completely understant it. Is it true because of space and time dilation? But what if we choose a photon as reference? How would the light itself perceive the movement of other objects, of the train and the observer for example? Wouldn't they be perceived as moving with the same speed?