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Freely Falling Inertial As Seen By Distant Inertial Observer

  1. Mar 15, 2013 #1
    Apologies if this is in a FAQ somewhere.

    A is out in deep space.

    B is falling toward the planet.

    Does A need both SR & GR to calculate B's time/space dilation as determine by A's IRF?
  2. jcsd
  3. Mar 15, 2013 #2


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    You need GR anytime gravity is involved which it is in this case.
  4. Mar 15, 2013 #3


    Staff: Mentor

    What you are calling "A's IRF" is not actually a global inertial frame. Locally, near A, it will look like an inertial frame; but it will *not* look like an inertial frame closer to the planet. There is no such thing as a global inertial frame when gravity is present. So you will need GR, as ghwellsjr says.
  5. Mar 15, 2013 #4
    Thanks. so basically it doesn't even make sense to speak of an "SR piece" since the spacetime is not flat there.
  6. Mar 15, 2013 #5


    Staff: Mentor

    Correct. Far enough away from the planet, in the vicinity of A, you can still use SR as a good approximation since the spacetime is approximately flat there. But that won't allow you to model B's motion since B does not remain far away from the planet.
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