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Speed's influence on space time

  1. Nov 6, 2007 #1
    HI,
    Is space time influencing by the speed or the acceleration ?
    Here the following problem :
    Marc and lisa are on earth. Marc takes his super space ship and (from the lisa's point of view) almost reaches the speed of light. Then marc remain still for a few minutes. He doesn't accelerate neither slow down. This is the moment which interests me now. afterwards, he returns to earth and have a romantic drink with lisa :!!)

    Well, when marc accelerated, lisa could see (with her super telescope) marc's rolex slowing down. After that, Marc remained for a moment at the same speed before to come back on earth. At this very moment, what difference did it make wether it is lisa or marc who was moving away from each other. As time is relative, there's no absolute time so why would marc's time would be spending slower than lisa's ? We could say that they are together moving from each other. So is space time "bending" only when acceleration occures or just whenever the speed from one thing to another is different ? In that case, why would it be marc's time which slows down and not lisa's.
    I hope that makes sence :S

    I'm very new to interest about physic, I really found what I was looking for in that forum.
    Thanks to all !
     
  2. jcsd
  3. Nov 6, 2007 #2
    As you said should be true, the "relative" part is that each person sees the other as traveling at a non-zero speed, and so each should see the other's time intervals as slowed. This is true while the two are traveling (i) relatively away or (ii) while traveling toward each other. BUT, to make a comparison of watches after separating, the relative observers must return. This involves an acceleration on Marc's part...we are now in the realm of non-inertial special relativity (no need for general relativity). For a discussion of all the ins and outs from here on, enjoy the relativity FAQ:
    [http://www2.corepower.com:8080/~relfaq/twin_paradox.html]
     
  4. Nov 6, 2007 #3

    HallsofIvy

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    At THAT moment, when both Marc and Lisa are in constant motion relative to one another, it doesn't make any difference at all! It is NOT the case that "Marc's time slows down and not Lisa's". Lisa would see Marc's clock (respiration, heartbeat, everything that could be used to measure time) as moving much slower than her's and Marc would observe Lisa's clock (etc.) as moving much slower than his.

    Now, when they got back together in the same frame of reference, Lisa, not having undergone any acceleration (which is NOT "relative") would see Marc as having aged less than her and Marc would observe the same.

     
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