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Is spacetime an absolute reference frame?

  1. Mar 20, 2010 #1
    STUPID QUESTION ALERT!

    Is spacetime an absolute reference frame?
     
  2. jcsd
  3. Mar 20, 2010 #2

    JesseM

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    Re: Spacetime

    No, because the notions of being "at rest relative to spacetime" or "in motion relative to spacetime" have no physical meaning.
     
  4. Mar 20, 2010 #3
    Re: Spacetime

    So, is spacetime only used as a benchmark for accelerated motion?
     
  5. Mar 20, 2010 #4

    JesseM

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    Re: Spacetime

    What do you mean by "used as a benchmark"? I don't think it'd be meaningful to say you're accelerating "relative to spacetime" either, though acceleration is absolute (you know you're accelerating because you feel G-forces).
     
  6. Mar 20, 2010 #5
    Re: Spacetime

    For Newton's thought experiment about two rocks in deep space that will pull taut, with respect to what is it rotating? I'm trying to read Brian Greene's book "The Fabric of the Cosmos" but I'm getting very confused. Forgive me if this is a stupid question.
     
  7. Mar 20, 2010 #6

    JesseM

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    Re: Spacetime

    I think it would be misleading to say it's rotating relative to spacetime since spacetime doesn't have any landmarks that you can measure your motion relative to. I would probably say it's rotating relative to inertial frames of reference (coordinate systems), frames that can be identified because the equations for the laws of physics take a particular form when stated in terms of their coordinates. But maybe others would have different ways of describing what's going on here.
     
  8. Mar 20, 2010 #7
    Re: Spacetime

    Ok thanks for your responses and thanks for helping me!
     
  9. Mar 20, 2010 #8

    atyy

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    Re: Spacetime

    I agree that it's relative to an inertial frame. I'm not sure exactly what Greene says, but maybe there's an idea that the global inertial frames of special relativity reflect the symmetries of the flat metric - which can be colloquially identified with spacetime, and which in general relativity becomes curved and further identified with the gravitational field?
     
  10. Mar 21, 2010 #9
    Re: Spacetime

    Isn't the aether a concept used as an absolute reference frame? In fact, i'm sure Einstein said relativity was unthinkable without one.

    Today, we've replaced the word aether for the zero-point energy field, which acts like a quantum aether.
     
  11. Mar 21, 2010 #10

    JesseM

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    Re: Spacetime

    Einstein only said that spacetime was similar to an aether in the sense that it has physical properties of its own, but in relativity it is unlike the traditional notion of aether in that it doesn't have any rest frame of its own, so it is meaningless to talk about your velocity relative to spacetime (whereas it would make sense to talk about velocity relative to the aether). For example, see the last paragraph of http://www.tu-harburg.de/rzt/rzt/it/Ether.html [Broken] where he writes:
    The same is true of quantum fields, quantum field theory is Lorentz-symmetric so quantum fields look the same way in every frame, they don't have a rest frame.
     
    Last edited by a moderator: May 4, 2017
  12. Mar 23, 2010 #11
    Re: Spacetime

    That's quite interesting. Langevin's concept of aether was that it remained always at rest with regard to a particle or object in any frame. And maybe that was the later conclusion of Poincare and Lorentz. Therefore in following the motion of a particle translating between a laboratory frame and a frame moving in tandem with the particle, length and time contraction arises.

    http://en.wikisource.org/wiki/The_R...trons_to_Other_Branches_of_Science#cite_ref-1
     
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