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Graviton, Blackhole and FTL.

  1. Apr 7, 2014 #1
    How fast do we expect the hypothetical graviton to travel?

    It seems that if the graviton were to mediate the gravitational force then it wouldn't be subject to the normal FTL rules otherwise blackholes would have no gravitational effect on the rest of the universe.

    What's the current understanding of this?
     
  2. jcsd
  3. Apr 7, 2014 #2

    Demystifier

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    First, a gravitional field does not need to travel at all. You may feel a static gravitational field around a black hole.

    Second, even if a gravitational field travels from the black hole, it may be created OUTSIDE of the black hole. In this way it can reach you without a FTL travel.

    How can a gravitational field be created outside if the matter is inside? That's because the static gravitational field also has energy (equivalent to a mass via E=mc^2), so gravitational field may be created by gravitational field itself.

    By contrast, electromagnetic field cannot be created by electromagnetic field itself. That's because electromagnetic field is created by charge, while electromagnetic field itself is not charged.
     
  4. Apr 7, 2014 #3
    So the static gravitational field effects would be mediated by the virtual particle which has no upper limit to its speed and acts instantaneously at an arbitrarily large distance. Dynamic gravitational field effects would be mediated by a real particle that orginiates on the horizon of the black hole. For an infalling observer it must originate at the apparent horizon rather than the absolute horizon, right?
     
    Last edited: Apr 7, 2014
  5. Apr 8, 2014 #4

    Demystifier

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    A static configuration does not involve something which moves with an infinite velocity. A static configuration involves something which moves with a zero velocity, i.e. does not move at all. That's what the word "static" means: not moving.

    Note also that I don't use the words "virtual particle". Tha's because virtual particles do not exist. They are merely a convenient mathematical tool useful for approximate calculations when interactions are not strong. Moreover, they are used mostly in quantum physics, while gravitational attraction by the black hole is better described with classical (not quantum) physics. So in the context of black holes, it is better to avoid the "virtual-particle" language.

    Furthermore, in the context of classical gravity, even the concepts such as "real particle" or "graviton" are not appropriate. It is better to speak of gravitational fields and gravitational waves.
     
    Last edited: Apr 8, 2014
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