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2 clocks on a single satelite

  1. Jun 23, 2012 #1
    Hello here is small question:
    Lets say we have 2 clocks on spacecraft.
    Both are synchronized with Earth clock before start.

    Lets synchronize one of them with Earth clock again when they arrive to an orbit.

    Do relativity predict some faze diference for these two clocks now?
    If so how big?
  2. jcsd
  3. Jun 23, 2012 #2


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    Are you aware that there is no such thing as an Earth clock? Clocks on the surface of the Earth in different locations will tick at different rates, just like clocks in different orbits will tick at different rates. The differences are extremely small but can be measured with today's technology. That's why GPS was invented, just so that we could all synchronize not only the rates of our clocks, but the times on them.

    A practical way to perform your experiment would be to compare the tick rates of different precision clocks at different locations on the surface of Earth and in orbit to the tick rate provided by GPS. But the answers will depend on the specifics which you haven't provided.
  4. Jun 23, 2012 #3
    Yes, GR predicts a de-synchronization dependent on the length (and the path) of the rocket. The simplest case is the one where the rocket is moving radially away from the Earth.
  5. Jun 23, 2012 #4
    Tank You for the answer. I will try to clear my question with more details.
    Let at the begining all 3 clocks are at one point A on the Earth.
    All them goes the same.
    Then let say during some sort time (some minutes) 2 of them together go to an orbit.
    Let say 100 km up.
    Then with radio telescope from the same starting point A we additionally synchronize one of mentioned clocks in space.

    By saying synchronize I mean only phase adjustment, without effecting clocks rate.
    So both clocks on satelite will tick with the same rate anyway.
    But what about phase. Maybe synchronization itself will effect phase a little?

    If some very very little phase shift still occurs maybe you can estimate what order it can be at 100km orbit for example.

    We may even think that there is no gravitational time delay for simplicity.
    Just how synchronization of phase will effect (or will not) one of these clocks.
  6. Jun 23, 2012 #5


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    The difference between the two satellite clocks just comes from the rocket launch then - some parts from special relativity (the rocket moves relative to the ground) and some from general relativity (height difference to the surface).

    Note that for clocks orbiting the earth, there is no way to synchronize it with clocks on the ground in a meaningful, symmetric way. You have to define a reference system where you do this.
  7. Jun 24, 2012 #6
    Here is a little why I have asked this question:
    Lets say (just lets say for a moment) that speed of light is not constant.
    Lets say it is c-10km/s upward direction and c+10km/s downward direction (near the Earth).

    The question is what experiments may detect or reject this assumption?
    But please don't say just GPS.
    Most say it would be notable at GPS.
    But when I asking them to paint some simple draft/scheme with a few radio-wave (or light) emitters, receivers and clocks
    how it could look to test such asumtion nobody can do it.

    Thats is very strange. Maybe you can help to visualize simplest realistic experiment setup to test assumption I have mentioned.
    Maybe it would be interesting for other people also and would save a lot of my time.

    Thank you.
  8. Jun 24, 2012 #7
    There are many tests that falsify your assumption. See here.
  9. Jun 24, 2012 #8


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    You would have to define what you mean by a different speed of light first. In SI units, the speed of light is constant by definition (the length of a meter is the distance light travels in 1/299792458 s). In other words, a "different speed of light" would be equivalent to a different length of a ruler. Which can be measured.
  10. Jun 24, 2012 #9
    Tank you. Maybe you can recommend one or two the best most sutable the case I was asking?
    Really would take much time to study deeply them all.
    Sometime I also have read this article from interesting man from NASA http://arxiv.org/abs/0912.3818 . Therefore there are similar questions.
  11. Jun 24, 2012 #10
    I am not sure, maybe some hard body, like it was defined a long time before.
  12. Jun 24, 2012 #11
    The paper that you are citing is written by a known crackpot (Gezari).
    Read these:

    DeSitter, Physik. Zeitschr. 14, 429, (1913) http://www.datasync.com/~rsf1/desitter.htm. DeSitter, Physik. Zeitschr. 14, 1267, (1913) http://www.datasync.com/~rsf1/desitter.htm.
  13. Jun 24, 2012 #12
    From your link:
    "In the theory of Ritz light emitted by a source moving with velocity u is propagated through space in the direction of the motion of the source with the velocity c + u, c being the velocity of light emitted by a motionless source."

    Yes I know it and I agree with you about Ritz's theory. But this is not the case I was mentioned. Here all sources can be static and here are no assumption that speed of light depends on velocity of source.

    Here is assumption that gravity of the Earth makes speed of light different in all directions.
    But I don't need discussion about theory. By the way it even do not allow your forum rules.
    I just need simple setup for experiment to test it.
    Maybe somebody can paint it? Even with letters would be good enough.
    For example maybe somebody had done MMX with one leg up or maybe some other very different ideas?
  14. Jun 24, 2012 #13
    Here is what you asked:

    The papers by DeSitter explain exactly what you asked for.

    You mean that you make this assumption, right?

    Yes, the forum disallows discussing fringe ideas.
  15. Jun 25, 2012 #14
    I am sorry am not sure I understand how exactly these observations with stars contradict to mentioned assumption.
    I am not the first who was thought about it.
    If it is out of forum rules here is my email allvydas eta gmail point com
    Other experiments which most directly contradict to mentioned assumtion also welcom.
    In general I have not idea how someone can desing experiment setup without knowing a big set of alternative views.
  16. Jun 25, 2012 #15


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    I do not remember seeing this, but I would bet money that experiments similar to this have been performed.
    In addition, the increased precision of the measurements allow to measure the individual directions separately and other nice things.
  17. Jun 25, 2012 #16
    Would be extreemly interesting to see info about it.
    Just my guess: it likely could be free falling apparatus to avoid physical length contraction due gravity.
  18. Jun 25, 2012 #17
    Once again, this forum disallows fringe speculations.

    If that were true, GPS wouldn't work.
  19. Jun 25, 2012 #18
    Lets say you are right, but how to visualize the experiment setup.
    It would take a few seconds for you and me to paint MMX setup.

    But GPS... , it has too much unnecessary properties for calculation position in 3D space.
    Mentioned case I suppose is only linear.
    It means all necessary apparatus likely can be placed just along one line, vertically.
    (Let say we have 1km hight building. This way maybe would be easier to calculate and to understand.)
    But how?
    What minimal equipment it is necessary and how to run the experiment itself?
    This is pure technical question.
  20. Jun 25, 2012 #19
    Experiment proves I am right.
  21. Jun 26, 2012 #20
    Ok, but only if somebody will be able to draft some realistic scheme.
    If you think about GPS, just extract the most important pars from it and show how.
    I am not an expert for GPS, but generally I know how it works and I do not found any solution.

    The simplest picture would be much useful to be sure we talk about the same things,
    because my English is not good enough.
    But with big building maybe would be even easier for you, if it possible at all.

    I am not sure maybe I am asking not in good time, maybe most proper people are on holidays?
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