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Does time move faster while measured from space or from earth?

  1. Jul 22, 2012 #1
    If you were to have an atomic clock, would it registered that time is moving faster while earth-bound or while orbiting in a satellite?

    Also, most atomic clocks measure the amount cesium vibrates a second, which is 9,192,631,770. If time moved 'slower' would this number drop, or would it increase? My guess is increase but I just want to be sure.
  2. jcsd
  3. Jul 22, 2012 #2
    Well, first, the term's a bit misleading. But the stronger gravitational field would cause an orbiting observer to see a person on Earth's clock moving more slowly. (I imagine a person on Earth would see an orbiting observer's clock moving more quickly, but I have no evidence to back this.)

    What do you mean? An atomic clock, regardless of gravitational field, for any local observers, measures the same number of vibrations per second. Now, if the two are moving relative to each other, then SR kicks in, but neither's truly moving more slowly. When we finally get to GR, which is in a stronger gravitational field? The clock or the observer?
  4. Jul 22, 2012 #3

    Not true. Time does move at a slightly different pace in space then on earth. And I know the answere is known to scientists. GPS satellites and other perscise communications from earth to space need to take this 'time speed shift' into account or else everything would be off and GPS would not work in the manner intended. So the answere is somewhere, Im just hoping someone here knows it.
  5. Jul 22, 2012 #4
  6. Jul 23, 2012 #5
    True. However, note that I stated local observers. Nearby observers will still see cesium atoms have 9,192,631,770 vibrations/sec. Observers in stronger gravitational fields than the atoms should see more, weaker less.
  7. Jul 23, 2012 #6
    Define what you mean. Are you saying that observers on Earth will see observers in space have slower clocks? Far as I can tell, this is the exact opposite of what happens.
  8. Jul 23, 2012 #7
    I mean people in space will have a slower clock that registers a slightly less amount of vibrations per second then an atomic clock on the earths surface due to the presents of more gravity.
  9. Jul 23, 2012 #8


    Staff: Mentor

    A ground clock sees a space clock as gravitationally blue shifted. A space clock sees a ground clock as gravitationally red shifted. So, if anything, both agree that the space clock is faster.
  10. Jul 23, 2012 #9


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    Staff: Mentor

    If people in space observer their clock in space, it will always have 9,192,631,770 oscillations per second. This is the definition of a second! If they observe a proper clock on earth, they will see less oscillations there, per second on their own clock.

    With high orbits, general relativity dominates, and the effect is asymmetric - observers on earth will see that the space clock ticks more than 9,192,631,770 times per earth second.

    The GPS satellites operate with a frequency of 10.22999999543 MHz (in their own frame), and we on earth observe the signals with a frequency of 10.23 MHz - more ticks per second.
  11. Jul 23, 2012 #10


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    [what DaleSpam said]

    Note that it is not the gravitational field strength that matters, per se. It is the depth in the gravitational potential field. A weak field that extends over a long distance will have the same effect in this respect as a strong field that extends over a short distance.

    Note also that for orbitting clocks one needs to account for the relative velocity between a clock on the surface and one orbitting above. The orbitting clock will run slow because of its motion. [From an SR point of view, the orbitting clock is not at rest in an inertial frame]

    Whether the "GR" effect of the low clock running slow or the "SR" effect of the rapidly moving clock running slow will dominate depends on whether the craft is in a high fast orbit or low slow one.
  12. Jul 23, 2012 #11
    Thanks everyone this has been a big help
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