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Who is in motion?

  1. Sep 3, 2009 #1
    Imagine space contains nothing but two people, and the distance between them is reducing at some constant rate.

    Do we have the ability to tell which person is in motion towards the other, or if they are both in motion at half speed?
  2. jcsd
  3. Sep 3, 2009 #2


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    The question is meaningless in relativity, all motion is defined relative to your choice of inertial reference frame, and the laws of physics work the same way in all inertial frames so there's no physical basis for preferring any of them.
  4. Sep 3, 2009 #3
    To add to the previous response: We therefore say things like "the observers are moving relative to each other at such-and-such speed" or that "they are in relative motion".
  5. Sep 3, 2009 #4
    This is like asking-> james looks bigger paul. paul looks smaller than james.
    But in reality is it that James is bigger than paul or paul is smaller than james?
  6. Sep 3, 2009 #5


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    It's not really like that, since James bigger than Paul and Paul smaller than James are equivalent, whereas relativity says that James can have a greater velocity than Paul in one frame, but Paul can have a greater velocity than James in another frame.
  7. Sep 4, 2009 #6
    JesseM, no Analogy can be equivalent in all respect to the main system.
    I was just analoging motion. I was just trying to show -- who was in real motion was as irrelevant as who was actually smaller or bigger.
    My analogy doesn't (and i didn't claim it does) take velocities into account.
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