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Thread: Relative motion and special relativity in space?

  1. #1 Relative motion and special relativity in space? 
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    Could there be a solar system somewhere that orbits the galaxy opposite our orbit, or one in another galaxy perhaps where our motion away from it and it's away from us make a relative motion greater than the speed of light?

    Now if that could happen then according to special relativity it would appear to reduce in volume increase in mass and slow down time. i think? So from our view wouldn't it appear to be a black hole?

    Or should I re-read my physics book?


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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by sirheathen View Post
    Could there be a solar system somewhere that orbits the galaxy opposite our orbit, or one in another galaxy perhaps where our motion away from it and it's away from us make a relative motion greater than the speed of light?
    Because of the expansion of the universe, the speed at which things are receding is proportional to distance, therefore there are (and always have been) galaxies which are moving away from us faster than the speed of light.

    However, nothing locally (e.g. in our galaxy or galactic cluster) is moving at anything near the speed of light, relative to us.

    Now if that could happen then according to special relativity it would appear to reduce in volume increase in mass and slow down time. i think?
    You don't need speeds greater than (or even near) the speed of light for those effects to become apparent. Even GPS receivers have to take these effects into account because of the speed of the satellites.

    So from our view wouldn't it appear to be a black hole?
    The existence of a black is dependent on the rest mass of an object.
    If you go too fast, do you become a black hole?

    Or should I re-read my physics book?
    Maybe...


    ei incumbit probatio qui dicit, non qui negat
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    Thank you for the reply Strange. I started that train of thought after reading Stephen Hawking's a Brief History of Time, and watching a lecture from Neil Degrasse Tyson, both of whom are way smarter than me. Sort of expected that my thinking was a bit off. However I didn't think it would become a black hole but rather take on the appearance of one. And I know you can't see a black hole but we can detect it's gravity.

    Thanks again. tc.
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  5. #4  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by sirheathen View Post
    However I didn't think it would become a black hole but rather take on the appearance of one. And I know you can't see a black hole but we can detect it's gravity.
    The gravitational signature of an object is ( through Einstein's field equations ) ultimately the result of a construct called the energy-momentum tensor. A tensor is a mathematical object which is special in the sense that it is the same for all observers; hence, if one observer determines the existence of an event horizon ( which is the defining characteristic of a black hole ), then all other observers necessarily agree. Conversely, if one observer perceives a "normal" object ( no event horizon ), then no other observer can possibly determine the same object to be a black hole, regardless of their states of relative motion.

    The upshot is simple - you can't turn something into a black hole just be putting yourself into relative motion to it, by virtue of the fact that the relevant quantities involved are covariant in nature.
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