Einstein in 1905 [General Relativity in 1915]
Relative motion. But the speed of light is the same.
1. The laws of nature are the same for all inertial reference frames
2. The speed of light is the same for everyone
Why can’t you go faster than light?
Do things happen at the same time? It depends.
If we are going to give up ‘common sense’, it had better fit the observations.