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We know the the speed of light/c is the same to all observers. But i cant grasp something.

Observer is moving at $0.5c$ in relation to us. He doenst have any point of referece and everything around him is black.

  1. Can he find out he is moving at $0,5c$ in relation to someone?

  2. What gurantee our whole universe isnt moving at $0,5c$ in relation to another far away observer?

  3. Will he assume/is he right to say he is at $0c$? Can particles that dont interact to the universe, in their perpective, say they're at rest?

I can never be guranteed or assume myself to be at $0c$/rest, since there's a possibility someone far away can see me at $0,5c$?

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  • $\begingroup$ If he is finding out how he is moving "in relation to someone" then he does have a point of reference. There is no "absolute" notion of speed in relativity, "moving at 0,5c" only makes sense relative to some frame. Yes, c is the same in all frames, so it is possible to find out if something is moving at c "absolutely", but that does not apply to any other speed, 0.5c or 0c. And if something is moving at c it can not be an observer, see Does a photon in vacuum have a rest frame? Any observer in its own frame is moving at 0c. $\endgroup$ – Conifold Aug 10 '17 at 6:59
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1) Can he find out he is moving at 0,5c in relation to someone?

Of course he can find out he is moving 0.5c in relation to someone. When you are in your car, do you have any difficulty determining that you're moving at 50mph in relation to a nearby pedestrian? This is in priniciple no harder at .5c than it is at 50mph.

2) What gurantee our whole universe isnt moving at 0,5c in relation to another far away observer?

The guarantee is that there is no such faraway observer. If he's an observer, he's part of our universe.

3a) Will he assume/is he right to say he is at 0c0c?

I can't figure out who "he" is, but any non-accelerating observer is moving at 0c relative to himself, and is entitled to say so.

3b) Can particles that dont interact to the universe, in their perpective, say they're at rest?

All particles interact with the universe, because they are part of the universe.

Final unnumbered question: I can never be guranteed or assume myself to be at 0c/rest, since there's a possibility someone far away can see me at 0,5c?

You are at 0c relative to yourself and .5c relative to someone else. There's no contradiction here. All speeds are relative, and it makes no sense to ask what your speed is until you specify relative to what. Different specifications will then give different answers.

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Look into the principle of relativity. The laws of physics are the same in all inertial reference frames. So if the person moving with respect to us at constant 0.5c does any experiments on their spaceship (or wherever they are), their results will be the same as our experiments. There's no "absolute rest"; they can claim they're motionless, just as much as we can, since any experiments they do will give equivalent results to any we do.

Regarding the questions you asked: I'm not sure what you mean by "far away observer" or "particles that dont interact to the universe". But basically

1) Yes, but in his frame he's at rest.

2) Not well defined. What do you mean by "our whole universe", and "far away observer"? Any observer should be in our universe...

3) Yes.

Hopefully I was of some help, but you should try to study a bit more physics to clarify some concepts. Maybe start out with classical mechanics, or at least Galilean relativity. Then try to tackle special relativity.

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