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Thread: Circumnavigating the globe and losing time.

  1. #1 Circumnavigating the globe and losing time. 
    Forum Radioactive Isotope
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    If we have 2 frames of Reference (A and B ) moving wrt to each other,

    an observer in A will see clocks in B moving slower.

    Also an observer in B will see clocks in A moving slower.

    If A and B meet up I would expect the difference in the clocks to cancel out (don't jump on me -I am not claiming to be right! -).

    However I have been told this does not happen when an astronaut has circled the earth for a year or so and then returns to the site of his departure (his clock will be "behind" one that had not left the earth).

    Is this on account of the effects of GR rather than Special Relativity -or is it on account of both? (I realize that for the astronaut to return to earth he must undergo "acceleration" )


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    The effect has little to do with general relativity. The main point is that the one leaving has to keep changing inertial frames.

    If both of them go in high speed circular orbits in opposite directions, the effects cancel out.


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    Forum Radioactive Isotope
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    thanks

    Isn't the one on the ground moving relative to the one circling the earth?

    Doesn't his (the man on the ground) clock also appear (to the astronaut) to have slowed relative to that of the astronaut ?

    Do the astronaut and the man on the ground have different motions relative to each other? Is it not symmetrical ?

    Is this particular relationship of motion different from that between a train and the embankment ,for example? (as I have seen frequently used as an example of relative motion)
    Last edited by geordief; January 18th, 2014 at 06:07 PM.
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    Quote Originally Posted by geordief View Post
    thanks

    Isn't the one on the ground moving relative to the one circling the earth?

    Doesn't his (the man on the ground) clock also appear (to the astronaut) to have slowed relative to that of the astronaut ?
    No. The astronaut is not in an inertial frame but travels in a circle. Circular motion is accelerated motion and the rules for accelerated motion are different. Now the ground clock also moves in a circle, but a smaller one at a slower speed.

    Do the astronaut and the man on the ground have different motions relative to each other? Is it not symmetrical ?

    Is this particular relationship of motion different from that between a train and the embankment ,for example? (as I have seen frequently used as an example of relative motion)
    The difference is that with the train and embankment, we don't have any circular motion to worry about. We assume that the tracks are perfectly straight.
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    Forum Masters Degree Implicate Order's Avatar
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    Quote Originally Posted by geordief View Post
    If we have 2 frames of Reference (A and B ) moving wrt to each other,

    an observer in A will see clocks in B moving slower.

    Also an observer in B will see clocks in A moving slower.
    You are correct in your interpretation for inertial motion conditions (constant velocity with no forces acting), but as Janus has pointed out there is accelerated motion going on as the plane orbits the earth due to the circular motion and consequently the symmetry between two different frames of reference under inertial motion is broken and the clock of the orbiting plane will tick at a different rate (slower). Furthermore , the altitude of the plane (it's location with respect to the planes position and motion in the graviational field) also affects relative motion in general relativity where the clock ticks slower compared to a clock positioned on the earth. :-))

    In my mind I attribute these all to time dilation effects on clocks. The system is assumed to be stable in that spacetime is not altering it's shape over the course of the experiment (from other large masses being involved) hence you do not need to consider tidal effects (spatial effects on spacetime geometry)....assuming of course that the plane has very low mass when compared to the earth so it's motion does not give rise to an altered spatial geometry.

    Under this interpretation I assume therefore we can deal with this in a hypothetical flat spacetime arena using special relativity with incremental adjustments to the frames of reference, but I would need an expert to confirm whether this is the case or not but it appears to tbe the case. :-))
    Last edited by Implicate Order; January 18th, 2014 at 09:54 PM.
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