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Trivial question about gravity affecting itself

  1. Jul 17, 2008 #1
    I was in discussion about black holes with a friend, and I seem to have found a big misconception. I apologize for asking such a trivial question, but I'm not certain even as to how to precisely ask it. I've waited a long time to learn about GR as it's a bit intimidating to me.

    Me: A static electromagnetic (I realize that if the field is static it's either one or the other) field will not affect the propagation of photons, similarly the propagation of gravity is unaffected by gravity."

    Him: This part is wrong.

    Gravity curves space-time itself. Gravitons, just like photons, travel along geodesics of the space-time. All massless particles do. This means that the graviton will follow the same trajectory as light. That means, it will be affected by gravity.

    I'm not familiar enough with theories including gravitons to make any statements concerning them, but it is it not true that changes in gravity propagate at c regardless of its own effects? Would his statements mean that gravity is capable of trapping itself?

    I guess my problems arise from my assumption that gravity can't be viewed as a force, but only as warp of spacetime. If gravity waves, quantized or not, are not allowed to leave a black hole, how would it have a measurable angular momentum?
    Last edited: Jul 17, 2008
  2. jcsd
  3. Jul 17, 2008 #2
    You can think of it this way; Maxwell's equations which govern electromagnetism are linear partial differential equations which means two solutions can be added together without interacting with each other.

    Einstein's equations are nonlinear partial differential equations so do not satisfy such a superposition principle; the theory involves self-interactions.
  4. Jul 17, 2008 #3
    So how do gravitons escape from a black hole?
  5. Jul 17, 2008 #4
    Perhaps a better question than my original would be this:

    If gravity waves are not able to escape a black hole, how is the information that a black hole has angular momentum or has increased in mass mediated to spacetime outside the event horizon? I can visualize a simple widening of a pit or widening of a sphere of arrows pointing toward the singularity, but would this not be a gravity wave?

    I realize intuition becomes less useful as physics becomes more complex. I'm learning the math very soon.

    "So how do gravitons escape from a black hole?"

    For this, I would guess (a very naughty word in physics), that many theories that include gravitons use virtual gravitons.
    Last edited: Jul 17, 2008
  6. Jul 17, 2008 #5
    Yeah, but even virtual photons are virtually affected by gravitational forces.

    I think you bring up a very interesting question. The gravitational force of the black hole is so strong that the geodesic at the event horizon becomes circular. Crudely speaking, we say "light cannot escape." The light keeps moving at 'c', but orbits the black hole, rather than away from it.

    Gravitons (virtual or not) should have to follow a geodesic as well. Therefore they should take the exact same path that light does.

    Clearly that's not the case, since black holes definitely exhibit gravitational effects outside their event horizon. So there's something that either I'm missing or no one understands.
  7. Jul 17, 2008 #6
    Certainly yes. But you can ask the same thing about photons which should tell you about the charge of a BH.

    The advantage of discussing about photons is that you avoid difficulties with gravitons :wink:

    Virtual particles do not follow geodesics.

    How does the gravity get out of the black hole?
  8. Jul 17, 2008 #7
    Is anyone sure 'gravitons' exist? The cause of gravity is far from being understood and I think someone came up with the 'graviton' concept and gave it mystical properties. It has been mentioned so often people are beginning to believe they are real.
  9. Jul 18, 2008 #8
    "Is anyone sure 'gravitons' exist? The cause of gravity is far from being understood and I think someone came up with the 'graviton' concept and gave it mystical properties."

    I don't think they've been observed, and I don't think they're part of the Standard Model or GR. I don't even think gravity waves have been directly observed, but binary pulsar systems give good evidence for them. Quantum gravity (if there is such a thing) is not well understood, I think, but GR seems to do great otherwise.

    I have a further question that will probably alleviate my difficulties:

    To an observer far from the event horizon, will a black hole gain mass as objects fall into it?
    If I'm correct, an object takes longer and longer (from the perspective of the observer) to fall past the event horizon as it gets closer to an outside observed. At the event horizon, the object would be frozen, but since nothing can escape once past that, there will be no other information aside from a change in spacetime to convey the information that it's past?
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