[jprete]

I am not a physicist:

I've read that C is really the speed of causality, and that electromagnetic radiation is really a disturbance in the EM field that is propagating along a path. A photon is really information about a prior event, that caused the disturbance i.e. emitted the photon, propagating across the EM field at the rate C. Likewise gravity, the strong force, and the weak force are also propagated at the speed of causality. E.g. if the Sun suddenly doubled in mass for some reason, it wouldn't just take eight minutes for us to see the change in EM radiation, it would also take eight minutes for us to feel the gravitational change.

The whole bit about the physical laws being the same follows from that, and the assumption that there isn't any way to transmit forces across spacetime except through transmission of causality.