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I General Relativity - no universal frame, or no way to tell?

  1. Dec 5, 2016 #1
    I've heard that unlike Special Relativity, General Relativity claims that you can' say any reference frame is better than another frame, including frames in which Newton's Laws don't hold. I've seen debates about this specifically in the context of geo/helio-centrism. From what I understand, neither is more true than the other in the context of GR, but heliocentrism is still preferred due to its simplicity.


    What I want to know is: Does GR claim that there is no universal inertial frame, or that there's no way of telling if there is? Meaning, if the latter is true, then maybe one exists but we have no way of telling.

    This is probably a silly question, since I really don't know much about GR (I've learned SR though), so please correct me if I'm thinking about this incorrectly.
     
  2. jcsd
  3. Dec 5, 2016 #2

    Nugatory

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    Staff: Mentor

    That is not a disagreement between special and general relativity. They both agree that one frame is not better than another, and it doesn't matter whether Newton's laws hold in the frame (the frame is inertial) or not. They also both agree that it is nearly always easier to do your calculations in an inertial frame.

    They differ in that in special relativity it is possible to find global inertial frames, ones that cover all of spacetime, whereas in general relativity it is not. You can only have inertial frames when spacetime is flat, which is the case for all spacetime in SR so there'sno problem finding a global inertial spacetime. But GR is also applied to curved spacetimes, and there a frame can only be inertial if it covers a small enough region of spacetime that we can ignore the curvature and treat that small patch of spacetime as if it were flat.
     
    Last edited: Dec 5, 2016
  4. Dec 5, 2016 #3

    Dale

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    If there is no way of telling then it doesn't matter either way.
     
  5. Dec 5, 2016 #4
    1) There's no way of telling some things in quantum mechanics (eg. uncertainty principle), but people still argue about what reality is like. Or did you mean something else by that?
    2) Does GR really say that there's no way of telling? So if someone claims that they found a way to distinguish a universal frame from other frames, would that necessarily conflict with GR?
     
  6. Dec 5, 2016 #5

    Nugatory

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    Yes, and also with abundant experimental evidence. For example, gravitational lensing couldn't happen if there were a single inertial frame that covered all the paths that the various light beams are taking.

    (I do note that in the post I just quoted, you said "distinguish a universal frame", while in your first post you asked about "a universal inertial frame". The answer I just gave assumes that you're still talking about the existence of a universal inertial frame, something that cannot exist in curved space time so must necessarily conflict with GR).
     
  7. Dec 6, 2016 #6
    It says that there is no way to tell. And note that there are different definitions of "inertial frame"; I suspect that your definition differs from that of those who replied before me.
     
  8. Dec 6, 2016 #7
    Uncertainty principle does not say that "there's no way of telling some things".
     
  9. Dec 6, 2016 #8

    Dale

    Staff: Mentor

    You can ask anyone with at least two young children: the fact that people argue about something does not imply that the thing they are arguing about matters.

    Yes, such an experiment would indeed conflict with GR. However, since GR is known to be valid over a fairly broad domain of applicability, any theory that explained such an experiment would still have to reduce to GR in the appropriate limit.
     
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