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My title might not be precise but I didn't know how to decribe it better.

I've been reading about the special and general theories of relativity for a short while and I started wondering. First, every movement is only relative to some other object/observer, there is no absolute movement/speed, correct? Assuming so, if an object moves at almost the speed of light relative to me, do I also move at almost the speed of light relative to it? If so, am I affected by all the "weird" effects that follow moving at such speeds (increased mass, different time flow, whatnot...)?

From everyday observation I'd have to say not but in that case what defines "moving at almost the speed of light"? Does it have to be relative to something (and what)? If not, is there any absolute measure of speed?

I know I might be talking nonsense but I'm new to these concepts and I'm trying to understand.

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    $\begingroup$ All velocities below the speed of light being relative is the point of relativity. What's your exact question about that? $\endgroup$ – ACuriousMind May 4 '15 at 19:14
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The key point here is the "weird" change in the values you measure depends on what frame you observe them in relative to the object. There is no experiment you can conduct to determine an absolute value for your speed. Speed only makes sense relative to an object. So if a near light speed object relative to us was heading towards us, from its perspective they would measure very different values for the length of objects and time for events to occur etc, and likewise we would measure these values for it as different than in its rest frame.

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  • $\begingroup$ What about mass? Is it also relative or is it absolute? Doesn't it increase with speed? $\endgroup$ – NPS May 4 '15 at 19:21

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