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Gravitational time-dilation

  1. Apr 12, 2012 #1
    Ive heard that; when astronaunts look down at civilization, they see everything as if it was going in slow motion. This makes sense becouse of gravitational time-dilation. But i wonder: if we could see up at the astronauts - and lets say they had a big civilization up there wich we could see just as clear as they can see ours - would we see things up there going faster or slower?

    I know that when it comes to timedilation from moving fast, then both observers observe the same thing! (time goes slower in the train for the man standing on the passing field, but time goes slower on the field for the man sitting in the train)

    *I wonder if this is the case for gravitational time dilation also;)
     
  2. jcsd
  3. Apr 12, 2012 #2
    Its not. Inertial Time Dilation is symmetric (both observers see the other's time moving slower), but gravitational time dilation is not (one observer will see the other's time moving slower, that observer will see the other's time moving faster).
     
  4. Apr 13, 2012 #3
    Thanks;)
     
  5. Apr 13, 2012 #4
    If you can say that "Inertial Time Dilation is symmetric" Can you then say that: Gravitational Time Dilation is asymmetric?

    Or is there perhaps some other word that neatly explains the difference?
     
  6. Apr 13, 2012 #5

    mfb

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    You can do this.
    In other words, you can say "gravitational time dilation is absolute", as it does not depend on the frame of the observer, while "time dilation from relative velocity is relative", as it depends on the observer.

    Oh, and keep in mind that
    is a really small effect. You can measure it with atomic clocks, and I think one year in space gives several microseconds of gravitational time dilation. In addition, no astronaut was above the low earth orbit for more than a few days (Apollo missions). In low earth orbit, the velocity-related time dilation dominates.
     
  7. Apr 13, 2012 #6

    PAllen

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    This is a little imprecise. Gravitational time dilation is asymmetric for static observers (I assume you know this). Static observers are special, non-inertial observers, and can only be unambiguously defined in special solutions of the EFE.

    For two arbitrary observers, the ability to separate gravitational and kinematic time dilation in any well defined manner depends on the existence of colocated static frames - which is possible only in very restricted solutions of EFE.
     
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