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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.

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closed as off-topic by Danu, Kostya, Sebastian Riese, Daniel Griscom, user36790 Jan 24 '16 at 2:20

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There are two things here that aren't quite right.

  • The speed of light is the same for all observers. The speed that light travels is invariant for any observers in any reference frames observing the same beam of light. I can move at 100 meters per second (as measured in one reference frame) in one direction, while you move 256 meters per second (as measured in the same reference frame) in a totally different direction. Both of us will still measure the same value of $c$.
  • There is no absolute reference frame. In other words, there is no privileged observer who can claim to be at rest. I can say that I'm at rest according to my reference frame, which is fine. You might appear to be moving away from me, but you can still say that you're at rest in your reference frame while I'm moving away from you. You can't measure how fast something is traveling unless you define some reference frame with respect to which it is moving.

So, to answer your question, the speed of light relative to the sphere will still be $c$.

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