Let's say there are two objects in space - Station and huge non-transparent sphere. Sphere starts to move from station 5000 km/h. Inside the sphere, beam of light is produced in direction the sphere moves(so light would move on its diameter).
What will be the speed of light relative to the sphere?(when measured in one point of sphere and catched in second point)?
Will it be = $c$? If yes, then relative to station it would be = $c$ + 5000 km/h.
Or it will be = $c$ - 5000 km/h - absolute speed of station, as absolute speed can't exceed $c$, so it would be possible to measure absolute speed of the station.