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Gravitons & spacetime-curvature-geometry

  1. Aug 20, 2006 #1
    i have a question about gravitons and spacetime-curvature theories of gravity,

    are gravitons supposed to curve spacetime, or do gravitons dispense with the idea of spacetime curvature, and travel through flat minkowski spacetime, much as photons do?

    what would be the difference phenomologically if you have curved spacetime on the one hand, and flat-spacetime with a particle field, gravitons, that interacted with all forms of matter and energy?

    if gravitons prove to be true, would there be a need to believe gravity curves space-time as distinct from a gravitional particle field that interacts with all forms of matter and energy?
     
  2. jcsd
  3. Aug 21, 2006 #2

    pervect

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    Gravitons are not used in the standard, curvature-based formalims of General relativity. General relativity as formulated in terms of curvature is purely a classical theory.

    The approach that I've seen used for "gravitons"

    http://xxx.lanl.gov/abs/astro-ph/0006423

    uses a non-standard defintion of distance to make space-time flat, rather than curved.

    Using non-standard rulers is tricky, but possible if one is careful to insure that one puts Lorentz invariance into the resulting theory "by hand".

    This hypothetical background "flat" space-time is then shown to be unobservable via standard measurements using standard SI rulers. Converting the theory from artifical "flat" coordinates to "physical" SI-based coordinates and distances results in the background metric changing from an unobservable, flat, static metric to an observable, curved, dynamical metric. (See the URL above).

    In short, one recovers the "curved" space-time of GR from a hypothetical "flat" space-time with gravitons.

    This formulation is equivalent to the standard formulation EXCEPT in terms of global topological issues. If we can ever demonstrate a non-trival global topology in the universe, the curvature formalism will be somewhere between a "must-have" and "highly desirable".

    However, usually the topology is trivial, and when that is the case the curvature formalism and the "graviton" formalism in the paper above are equivalent.

    However, as a perusal of the paper I cited above will show, it's not really trivial to develop the "flat-space" formulation, so one shouldn't think of it as being an "easier" route.

    Usually GR is presented in terms of the curvature formalism, and for most purposes it is easier to work with in that form. Combining GR with quantum mechanics in an "effective field theory" is one reason one might want to utilize the non-standard "gravitions in flat space" formulation.

    Note that the incompatibility of GR and quantum mechanics has been somewhat overplayed. We know that at small enough distances / high enough energies GR can't be right, but we can say much the same for theories such as QED as well. Meanwhile, as an effective field theory

    http://arxiv.org/abs/gr-qc/9512024

    gravity and quantum mechanics can live quite well together, if the energies are low.
     
  4. Aug 23, 2006 #3
    thanks,
    the ? was also asked here
    https://www.physicsforums.com/showthread.php?t=129458

    under "beyond standard model/gravitons piss me off"

     
  5. Aug 24, 2006 #4

    pervect

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    PF generally likes to keep threads together, without duplication, rather than scattered all over the place. Since the other thread has a lot of responses, and this thread has only mine, I would like to see it moved / appended to the existing thread in the "Beyond standard model" forum. (It will take a moderator with appropriate priveliges to do this).
     
  6. Aug 26, 2006 #5
    I understand how gravitons are part of the Standard Model.
    And with particle anti-particles part of the model I assume part of QM.
    So that even using it with or without SR it would be Non-Classical.

    But isn’t GR considered Non-Classical as well?

    Doesn’t classical require the use of Newtonian “absolute” space as in x,y,z.
    And Newtonian “absolute” time t.

    While GR requires at least 4 dimensions A,B,C,D that we cannot ‘see’ except that the 4 dimensional “space” can curve and warp to produce gravity we can experience only in the x,y,z space and t time also produced that we can observe.
    With x,y,z,t being produced in different ways at different places, inconstant with “classical” expectations, depending on the curves and warps in the unseen 4D space.
     
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