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General Relativity and a Force Mediating Graviton?

  1. Aug 24, 2010 #1
    This may be a naive question as I am only still working on my undergrad in physics and while I am quite familiar with special relativity it will be another year or two before I formally study general relativity. On top of that my knowledge of the modern work to explain gravity alongside quantum mechanics is qualitative at best.

    Either way, this is what I was wondering. From what I understand a fundamental assertion of general relativity is that gravity is not a force in itself but rather the appearance of a force due to changes in relative motion as a result of space-time distortion caused by some large mass. Does that seem correct or am I way off?

    So assuming I am not wrong, there seems to be an obvious contradiction in the Standard Model of expecting gravity to be mediated by a particle since this is precisely what general relativity claimed it was not. With this realization the whole notion of searching for a graviton seems completely silly.

    ***

    So assuming that I am not completely wrong in my interpretation here, I can only see a couple of reasonable justifications for postulating the graviton:

    1) Einstein has gone the way of Newton. Physicists have decided that while general relativity works very well it does not in fact describe an actual reality and a different model is needed to more correctly explain the underlying mechanism.

    2) The graviton is not actually expected to mediate gravitation directly, but rather it is the component of mass which distorts space-time (this statement may be particularly naive of me since I honestly have no clue how exactly mass bends space-time in general relativity). I suppose what I am saying is that it would be something that gives things mass, so to say. But from what I have read about the graviton this does not seem to be what is expected of it.

    3) No good reason. Excitement over the successes of particle physics and quantum mechanics have moved the focus towards and quantum/particle description of gravity just because it would tie everything up in a nice convenient little package.

    ***

    Numbers 1 and 3 kind of say the same thing, particularly because #1 would be a rather hasty move in my opinion since so little has been accomplished in replacing general relativity. It may not describe an actual reality but doesn't it seem reckless to assume that before you have any better explanation?

    So if I am wrong about anything, please let me know.

    Maybe this is another one of those embarrassing gray areas where no one agrees, in which case just give me your two cents!
     
  2. jcsd
  3. Aug 24, 2010 #2

    bcrowell

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    When people say that gravity is not a force in GR, it's sort of a shorthand for suggesting the most useful way of thinking about the theory. It's not an empirically testable statement like "opposite charges repel" or "the range of the nuclear force is about 1 fm." Newtonian gravity can also be described in geometrical terms, but that isn't a testable statement about Newtonian gravity. Newton found it useful to think about gravity as a force within his mathematical description of gravity. If you start doing a lot of calculations in the geometrical representation of Newtonian gravity, you'll naturally start thinking in that language, where gravity isn't a force. Both mathematical techniques give the same answers, and both are correct when applied within the circumstances where Newtonian gravity is a good approximation (speeds much less than c, etc.). GR can also be treated non-geometrically; if you started doing a lot of calculations within such a system, you would find it more convenient to start thinking of gravity as a force.

    We currently have observations that are successfully explained by GR, and observations that are successfully described by quantum mechanics. We don't have any observations that require both theories at once to explain them, and we probably never will have such observations with any foreseeable technology. So in that sense, GR is a very healthy theory.

    We do know that if you try to combine the principles of GR (equivalence principle, no prior geometry, ...) with the principles of quantum mechanics (probabilities always between 0 and 1, ...), you seem to get contradictions, but nobody knows how to resolve that. For instance, it's possible that the principles of GR are all just fine, but one of the principles of quantum mechanics is only approximately true. For instance, when I took a class from Lee Smolin, who works on quantum gravity, one of the ideas he said he was interested in (this was back in the 90's) was that unitarity might be violated (loosely, that probabilities wouldn't have to be between 0 and 1).
     
  4. Aug 24, 2010 #3
    It sounds like our knowledge of GR may be equally limited so take this with reservation.
    From what I have gathered it seems to be agreed that the geometry does not change instantaneously wrt changes in the mass that it is associated with.
    SO if hypothetically the mass were to disappear it would take a time equivalent to the distance/c for the resultant change in geometry to effect any point in that geometry.
    This seems to suggest something propagating from the mass , whether it is particles or waves or whatever is your favorite concept. IMO
     
  5. Aug 24, 2010 #4
    You ARE way off, perhaps, except for the first phrase, but so is science...not to worry....Einstein was "way off" on many occasions, too.....nobody understands what any force really is....but as Crowell explains above, we have convenient ways of describing observed behaviors and thinking about those observations. Some are REALLY weird, like quantum mechanics.

    Gravity has nothing to do with changes in relative motion, except that uniform acceleration is just like gravity via a correspondence (similarity) principle that Einstein used to gain insight into gravity as he began general relativity. He saw the deep connection between gravity and accelerated motion and used his understanding of the latter to gain insights into the former...

    ....

    while there are plenty of unknowns, one of them is how to even get gravity within the framework of the standard model....gravity is excluded so far (because we don't understand how to include it) and "grand unification" would bring it together (with strong,weak, and electromagnetic forces) within the model.

    So far there are ways to mathematically generalize the other three forces in a single entity and from them derive those three...gravity eludes that "unification" so far....I find a convenient way to think about this as from the bang (big or otherwise) there was and very high energy and unstable environment...it quickly decayed and in doing so out popped most and maybe all of what we now observe as distinct entities..space,materr, force, even time.

    Crowell posts
    Two examples where those " contradictions" abound, and in fact its even worse than that because our mathematics breaks down (divergences, infinities,etc) completely, are singularities at the big bang and at the center of black holes. That's a major reason black holes hold such interest in science: we might be able to experimentally detect some clues about our world so far not understood.
     
    Last edited: Aug 24, 2010
  6. Aug 24, 2010 #5

    jtbell

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    If gravitons really exist, that is, if there is someday a generally-accepted quantum theory of gravity (which we don't have now), then they will probably be related to general relativity similarly to the way that photons and quantum electrodynamics (QED) are related to classical electrodynamics.

    In QED, the classical electric and magnetic fields do not exist, at least not as fundamental entities. All electromagnetic interactions are fundamentally mediated by real and virtual photons. Nevertheless, as you move from the "microworld" to the "macroworld," you can define the classical electric and magnetic fields as "emerging" from the quantum description.

    Similarly, I would expect that in a "final" quantum theory of gravity, the macroscopic equations of general relativity could somehow be made to "emerge" from the microscopic description in terms of gravitons.
     
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