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Gravitons and Black Holes

  1. Oct 22, 2008 #1
    Assume we all accept that gravitational influence moves at the speed of light, just as EM radiation does. Light cannot escape from a black hole because the escape velocity from a BH exceeds the speed of light. If gravitons are a proposed particle-interaction based solution to explain gravity, how can gravitons escape from a BH if light cannot? ie, it seems that gravitons, as particles, cannot be a correct explanation of the operative mechanism of gravity.

  2. jcsd
  3. Oct 22, 2008 #2
    Good question.... you stumbled across one of the most basic and important questions in all of physics...but, perhaps, a less than "stellar" conclusion:

    OR.....think, man,think!!!! what are alternative explanations????

    Why do you suppose photons (electromagnetic radiation) are described by Maxwell's equations and gravitational waves (gravitons) described via Tensors in General Relativity?

    One of the most significant theoretical hurdles faced by physicsts today is how to unify (combine) gravity with the other forces. They are DIFFERENT! So they behave differently!

    If they behaved identically, would they not have identical mathematical formulations?? Yes!!

    Another theoretical analogy that may help with a bit of insight:
    It is possible that gravity leaks into additional dimensions postulated in string theory...hence it appears "weak" as we observe it in traditional four dimensional spacetime....the other three forces do NOT leak....they are open strings stuck on branes while gravitons are postulated as closed strings (loops) free to roam around among many universes. Gravitons also have unique spin....2. No other entity has that spin (I don't think.) So again, gravity is unique and different.

    Keep up the questions..good questions are the start of good solutions!!!
  4. Oct 22, 2008 #3

    Jonathan Scott

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    Gravitational forces don't work by action at a distance; they are caused by interactions with the gravitational field in local space-time, or in GR terms by the local shape of space-time. The field around a black hole at a given distance is essentially the same as it was when the concentration of mass first shrank inside that radius.

    Also, the way in which a mass moves is determined by the shape of space around it, so it isn't necessary to "communicate" with the black hole itself in order to change its course, for example if it passes near another mass.

    As far as I know, the case where gravitons would be involved is when a field changes as a result of interactions, in which case energy flows to adjust the field, and in some cases that energy may propagate right out of the system as gravitational waves.

    (However, despite all that, I'm very sceptical about both black holes and gravitons!)
  5. Oct 22, 2008 #4
    Thanks Naty - I do TRY to think :-)
    Perhaps gravitational waves are not due to particle interaction, but rather perturbations in the gravitational field. I dont think Einstein believed gravity was due to particle interaction because that would indicate that gravity is a force conveyed by the graviton particles - instead he believed that gravity was not a force, but rather the shape of spacetime.

    i understadnd this begs the question of "how can the shape of spacetime be conveyed to passing particles/bodies?". Hence our continuing confusion on exactly what a "field" is, and how it operates.

    But I tried to make my original question as simple as possible to avoid the mathematical and quantum interpretations - I posed it based solely on the idea that ANYTHING could escape from a BH, no matter what its properties are - if it cannot exceed the escape velocity of the BH, it should not be able to convey ANY type of information to the outside universe. Given the fact that gravitational influences DO indeed eminate from a BH, I was suggesting that gravity is indeed NOT a force conveyed by ANY type of particle, and MUST be a reflection of a distorted field (again, whatever a "Field" might be).

    again, thanks for your comments - I am, like us all, still learning (or trying to learn...)
  6. Oct 22, 2008 #5
    Yes, Einstein developed a continuous (wave/field) theory for his work...hence it seems to encounter problems at tiny spaces, high matter density, etc, down at the quantum level.
    I guess these are complimentary descriptions, perhaps because we don't have a universal description to fit all situations..."waves" "perturbations" "particles" are different descriptions for an underlying commonality....maybe akin to "M" thoery for the other string theories..

    I posted an analogous question here recently (How does light slow in the presence of a gravitational field?). And I now believe it to be a matter of perspective, that is, dependent on frame of reference alone...This perhaps can be explained based on the Einstein tensor details in GR but I don't have that knowledge at this point.

    Note that a field interpretation might not require "escape" as would a particle interpretation. Also note that quantum tunneling creates Hawking Radiation...so some "stuff" does get out. Note also that the interior entropy(information) is displayed on the horizon surface. Quantum descriptions and mathematical interpretations may be the only ones we have.

    In any case, I believe I know enough to say that curving of space takes some energy..a gravitational influence is not "something for nothing" not "a free ride" so energy somehow is provided by the blackhole to maintain the gravitational field potential....although I guess it could be an indirect interaction through the event horizon. Yet particle creation/annihilation just outside the event horizon and quantum tunneling through it might be part of that.
  7. Oct 22, 2008 #6


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    Phrase it differently: perturbations in the gravitational field should be quantized, just as perturbations in the electromagentig field are. The former quanta are called gravitons, the latter photons. Both are "real particles" in a quantum mechanical sense - where you should be cautious with classical analogues.
    Neither a static gravitational field nor a static electromagnetic field are thought to emit real gravitons or photons (except you force them to, as discussed in this thread). No need for gravitons to escape the black hole. This means that any perturbation that might happen inside (whatever that means) cannot be seen from the outside.
    The static gravitational "force" does not have to be emitted, as it is static. It is spacetime curvature. If they ever find a proper quantum mechanical treatment, the field might act via "virtual particles". Those do whatever they like to do, including crossing the event horizon from the inside. They are called "virtual" for a reason.
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