In einsteins theory of how time is dilated at relativistic speeds he assumes that light moves at the same speed in all reference frames... how is that possible? if we assume this then we come to the conclusion that time slows down relative to a stationary observer at very high speeds... all of this makes perfect sense but only if you make the assumption that light travels at the same speed relative to an observer. How can a photon move at a certain speed but also be moving at a different speed at the same time? it is very hard to get my head around... i can see how it works but time being relative to velocity and the speed of light being the same in all references seem to depend on each other... if one is true then the other is too... but what if we dont assume that light moves the same speed in all reference frames? time would be the same in all reference frames... why do we assume light moves the same speed in all reference frames?