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Will this method yield absolute synch?

  1. Feb 28, 2015 #1
    -simple procedure designed to absolutely synchronize clocks-

    Picture the face of a very large clock with a one hand.

    Let's place a small clock A at the 9 o'clk position of the
    large clock, and let's place another small clock B at the
    3 o'clk position.

    We use clock A to time the large clock's hand for one
    round trip. We then preset clock B to read half of this
    time, and we let B be started by the large clock's hand
    when it arrives after leaving B at its time zero.

    At this point, clocks A and B should be absolutely synch'd.
  2. jcsd
  3. Feb 28, 2015 #2


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    There is no such thing as absolute simultaneity in relativity, so no.

    The specific problem with your setup is that an observer in motion with respect to the clock won't (in general) agree that it will take the same amount of time to sweep out 9-3 as it will to sweep out 3-9.
  4. Feb 28, 2015 #3


    Staff: Mentor

    They also will not generally agree that the large hand is straight.
  5. Feb 28, 2015 #4


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    Gold Member

    Interesting, never thought of that... An effect of how rotation is viewed? Even if the clock is viewed "head on" (perpendicular to the direction of sight and motion)?
  6. Feb 28, 2015 #5


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    Gold Member

    Ah thanks - not that I quite grasp the effect yet, but at least I see how it could arise.. don't say more it's a fun little puzzle:)
  7. Feb 28, 2015 #6
    "combo reply"

    I realize that special relativity does not have absolute
    simultaneity, but this does not make it impossible.

    In my setup, it matters not what some outside observers
    may see; all that matters is the fact that the large hand's
    speed must be constant because the center shaft's speed
    is constant.

    And any observed hand curvature would not matter as
    long as there is no intrinsic curvature.

    If there is intrinsic curving, then I suppose it could be
    minimized by very slow hand movement.

    Of maybe it could be eliminated by replacing the large
    hand with a sweeping laser beam.

    Just some thoughts.
  8. Feb 28, 2015 #7


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    Staff: Mentor

    If SR is correct, then yes, it does make (or, rather, accurately describe) absolute simultaneity impossible.
  9. Feb 28, 2015 #8
    to russ:

    (What -- physically -- is there about SR that makes abs. sim. not possible?)
  10. Feb 28, 2015 #9


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    Science Advisor

    The Lorentz transforms don't preserve the angle between lines, so observers in motion won't in general agree that the central axis is rotating at a constant rate (or, more precisely, will say that the rate varies as a function of angle).

    As several posters have already noted, there is no absolute simultaneity in relativity. Any scheme purporting to show otherwise must be based on a misconception.
  11. Feb 28, 2015 #10


    Staff: Mentor

    Maybe not, but it does make it outside of the scope of this forum. Thread closed.

    Of course it does. "Absolute simultaneity" means that all observers agree on it. If outside observers don't agree on the simultaneity, then it isn't absolute.

    The shaft's speed being constant does not, by itself, imply that the large hand's speed is constant. That conclusion also requires the assumption that the shaft's shape remain constant in time, which is not true in all frames.

    The math and the experimental evidence simply do not support any of this.
    Last edited: Feb 28, 2015
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