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B Synchronized Clocks

  1. Jan 31, 2017 #1
    Will the synchronized clocks placed in an inertial frame remain synchronized forever?
  2. jcsd
  3. Jan 31, 2017 #2


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    Yes. Two clocks each at rest and remaining at rest relative to the same inertial frame after having been synchronized once will remain so indefinitely.

    Note that you do not place things in a frame of reference. You set them at rest relative to such a frame.
  4. Feb 1, 2017 #3
    To an observer moving relative to the frame, will they appear to lose their sync?
  5. Feb 1, 2017 #4
    To an observer moving relative to the frame, will they appear to lose their sync?
  6. Feb 1, 2017 #5


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    They will never be in sync, viewed from a frame where they are not at rest (assuming they are synced in their rest frame).

    Edit: be careful with "appear". A clock at rest right next to you showing 12:00:00 is in sync with one a light second away that appears to you to show 11:59:59. Relativity is what's left after you correct for that.
  7. Feb 1, 2017 #6


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    This would be correct if you'd left out the "single inertial frame". It is true that there is no absolute sense in which two clocks are synchronised. Adding in the bit about the inertial frame is just confusion. An inertial frame is a choice of clock synchronisation convention - so "with a choice of clock synchronisation convention there is no absolute synchronisation" is tautological at best.

    Conventionally, you assume light speed is the same in both directions. Then one clock sets itself to zero and emits a light pulse. The other clock receives the light pulse at a time it calls T and returns the pulse. The first clock receives this pulse at time 2T and returns the pulse. The second clock receives this pulse at 3T. This is all it needs to work out the value of T and deduce when it should have zeroed itself. Assuming the clocks are at rest with respect to one another and working properly they will now be in sync. Both can easily confirm that the other always appears to lag by time T.

    This is the Einstein synchronisation convention, and is what is meant by "synchronised" in special relativity unless otherwise stated, as far as I am aware. Other conventions are, of course, possible.

    That clock synchronisation cannot be absolute does not mean that it is impossible to understand. It just means that synchronisation must be a matter of conventional choice.
  8. Feb 1, 2017 #7
    Yes, two (ideal) synchronized clocks at rest in the same inertial frame will remain synchronized forever.
    There is nothing to "lose", to a moving observer they were never in sync. The two clocks can be observed in sync and not in sync depending on the inertial frame chosen for the observation, it's just a matter of perspective. Yet, that doesn't make it "apparent" either, it just means that synchronization is relative.
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