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Fast Gravity Slow W+ W- Z0

  1. Jul 17, 2009 #1
    If a black hole has a radius past which light cannot escape. How does the gravity get out?

    If the force carrier of gravity, the graviton, travels at the speed of light it too would be trapped.

    Assuming there are black holes and they are gravitating bodies, then the force carrier of gravity must exceed the speed of light.

    Is this possible? Expansion faster than light is already part of Guth's early inflationary model.

    Two opposing views might be that gravity is faster than light or that space-time once curved by a gravitating body remains curved even after the body has disappeared into a black hole.

    Which of these ideas fits the galactic rotation velocity curve better?
     
    Last edited: Jul 17, 2009
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  3. Jul 17, 2009 #2

    Jonathan Scott

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    In standard GR, nothing "gets out" of a black hole or exceeds the speed of light. The gravitational field is determined by the shape of space-time. The field at a given distance from a black hole is determined at the time something falls past that point, and remains unchanged afterwards. The field at a distance from an object is only determined by the mass and angular momentum of the object, and not by whether it is a black hole.

    In other words, this is the option which is correct (except that it should be "space-time" rather than space"):

    "Space once curved by a gravitating body remains curved even after the body has disappeared into a black hole."

    Gravitons are effectively an alternative QM way of looking at gravity, instead of general relativity, and at present I am not aware of any consistent relativistic theory of gravitons, so it is difficult to say anything about them.

    GR does predict gravitational waves, which relate to rapid changes in the field, and propagate at the local speed of light.

    Experiments are consistent with gravitational changes propagating at the speed of light, as predicted by GR, and with other very accurate predictions of GR in regions dominated by single central massive sources, such as in the solar system.

    Any idea involving some form of influence propagating faster than light is not compatible with special relativity and causality; it implies that either there are preferred frames or that causality can be violated. (However, any deterministic form of QM requires such an influence).

    GR on its own (without dark matter and dark energy) does not appear to be consistent with cosmological observations either on the galactic scale or higher. I'm not personally sure that dark anything is the correct fix; I think it more likely that GR isn't right on larger scales.
     
  4. Jul 17, 2009 #3
    Jonathan,

    Thank you for your response.

    How does GR fail to fit the galactic rotation curve data?
     
    Last edited: Jul 17, 2009
  5. Jul 18, 2009 #4

    Chronos

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    The bottom line, IMO, is that gravity is not intermediated by an EM like 'particle'. Or in other words, Einstein was right.
     
  6. Jul 18, 2009 #5

    Jonathan Scott

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    On galactic scales, GR basically reduces to Newtonian theory plus some minor relativistic considerations. However, it is well-known that Newtonian theory based on the visible distribution of luminous objects does not explain the rotation curves for most galaxies.

    I'm sure you should be able to find plenty about that by searching the web, for example looking up "galaxy rotation curve" in Wikipedia, which looks like a reasonable starting point.

    The main alternatives seem to be "dark matter" or some change to GR, as in the empirical MOND model or Moffat's MOG. There have also been attempts to investigate the possibility that the geometrical distribution of stars in structures such as spiral arms may give rise to additional forces which are not being taken into account in the models which assume a more uniform mass distribution. However, I don't think any of these ideas have been particularly successful.
     
  7. Feb 14, 2011 #6
    Well, I know NOTHING about this stuff, but if a gravition was supposedly the force carrying particle of gravity (which we have no real proof for as of yet) then it IS gravity. It's not affected by gravity. It would be fundamentally different from a photon or something, and would not be sucked back in by it's own gravity. You can't have the space-time trampoline analogy and also have the particle billiard ball analogy. You're having your cake and eating it too!
     
  8. Feb 15, 2011 #7

    Vanadium 50

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    That is not correct.

    IF so, that's probably a good reason to ask questions rather than make pronouncements. In ay event, this thread is 2 years old.
     
  9. Feb 25, 2011 #8
    To consider the existence of gravitons, as in quantum physics, is to completely disregard General relativity (im not saying GR is perfect but..). We know objects react instantly (or very fast anyway) to gravity and that to have gravitons means that they must travel faster than light.
    So why are quantum physicists theorising and looking for gravitons. Or do they acknowlege that, if they exist, they must be faster than light? Yeah its an old topic .. I couldnt find a more relevant section.
     
  10. Feb 26, 2011 #9
    Actually, the effect of gravity travels at exactly light speed, so in theory a graviton would have to too.
     
  11. Feb 26, 2011 #10

    Janus

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    GR actually predicts that the effects of gravity travel at the speed of light. Gravitons would allow us to bring gravity into the fold along with the other forces, so it is an appealing notion to science.
     
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