How can the speed of light, c be the same to an observer who is moving in the same direction with 0.9c and to another observer who isn't moving?
If there is an observer who is following (moving in the same direction) a photon with 0.9c speed, how can that observer see the photon moving still at the same c speed? And if somebody is moving in the opposite direction with 0.9c speed, how can that observer see the same photon (that is moving in opposite direction) moving with the same constant speed c?
Extension: I understand that SR's time dilation makes to any observer c as a constant speed. But my question is: let's say there is something moving at a little bit slower speed, lets say an electron moving at 0.9c. Is this also true for the electron, so that any observer moving at any speed, any direction would see the electron moving at constant 0.9c?
If not (and that is what I am guessing) then I dont get it. Why is it true for EM waves propagating at speed c only? Anything moving even a little bit slower then c will not be seemed to any observer moving in any direction to be moving at the same fast constant speed? Why not?