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Graviton and the equivalence principle

  1. Jan 10, 2005 #1

    StatusX

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    I posted this in general physics, but I didn't get an answer, so I'll try again here:

    Einstein's theory of general relativity was motivated by his equivalence principle, that no experiment can distinguish between an accelerated frame and a still frame under the influence of gravity. But if the gravitational force is mediated by a particle, the graviton, then couldn't they be distinguished? (if there are no gravitons, you are accelerating) So has this equivalence principle been abandoned, or is it still somehow true?
     
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  3. Jan 10, 2005 #2

    ohwilleke

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    I don't think it has to be abandoned. Also, keep in mind that many theories of quantum gravity have not only gravitons, but a conceptual underlying structure of space in which both time and space come in discrete units. There is real doubt about whether gravitons in Minkowski space (which is usually the default assumption in other parts of QM) can replicate GR.
     
  4. Jan 10, 2005 #3

    pervect

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    What makes you think that the frames could be distinguished if "the graviton" existed? You state this as a fact, but you give no details or method for distinguishing the frames.

    Consider the electromagnetic case. Two unlike charges attract each other because of their electric fields. It is possible (but not particularly inuitive) to explain this attraction in terms of *virtual* photons.

    So - first off, why don't you explain how you think the virtual photons between a pair of attracting charges could be/ should be detected (considering that they are virtual, and not real).

    Then, after you explain that :-), you can explain how you propose to detect virtual gravitions, when we can't even detect real gravitons. For that mater, we can't detect gravitational waves yet, though it wouldn't be too terribly surprising if some of the experiments to do this had positive results in the near future. Detecting the quantitization of such waves is a looooooonnnnggg way off, however.
     
  5. Jan 10, 2005 #4

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    I asked a simple question, I didn't state any "facts," so there's no need to go on the offensive. I am only beginning to study GR and QM, and it was just a simple apparent inconsistency that occurred to me. GR is based on the premise that accelerated frames and frames near a large mass are indistinguishable in principle. But gravitons are proposed as particles that transmit gravity. It seems these would be present in one of those cases but not the other, and so the frames wouldn't be indistinguishable. Now, maybe gravitons can never be observed except by the gravitational force they transmit, I don't know. But it still seems to violate his basic principle that the frames really aren't different at all, the symmetry that motivated the theory. But again, maybe I'm wrong, and that's why I'm asking for your help. I'm not a crackpot with the illusion I've stumbled upon something worth publishing, I'm just a curious student.
     
  6. Jan 11, 2005 #5

    pervect

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    Sorry, I've been a bit irriitable recently, (got a flu bug that won't let go), but really, I couldn't make out why you thought there was an apparent inconsistencency, and I'm still not sure I understand what you think the inconsistency is.

    I suspect that you're taking the idea that forces can be considered to be carried by virtual particles and interpreting the virtual particles as being a lot more "real" than they actually are.

    If you're really fond of the virtual particle theory and want to read more about it, there's a fairly advanced seciton in the sci.physics. faq that explains how attractive forces can be carried by virtual photons. (Which is a bit of a trick in and of itself if you sit down and think about it - a particle has to absorb a virtual photon and gain momentum towards the source of the virtual photon).

    the link for this is

    http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

    Dunno if it will answer your question or not, but it might be educational.

    If you're not particularly fond of virtual particles, (dunno if you've guessed, but I'm not particularly fond of them for explanatory purposes) you can probably understand forces just as well without them, by not using quantum mechanics where it is not needed, and thinking of the forces as being purely classical.

    This is an especially good course to follow for gravity, as we don't have a quantum theory of gravity, only a classical one.
     
  7. Jan 11, 2005 #6
    This may be the reason that Einstein initially doubted the existence of gravitational radiation. (Notice that the graviton is a hypothetical particle that could exist if the gravitational field quantized like the electromagnetic field. There is no evidence for this and classical gravitational radiation has so far only been indirectly observed)

    There are many versions of the equivalence principle, but to my knowledge all occur in flat space!!! Check out Rindler space. In the full theory of general relativity you can set up Riemann normal coordinates (local Lorentz or inertial frames). All along a timelike geodesic the metric has its flat space-time values and the christoffel symbols ("Newtonian" gravitational force) vanish. However the Curvature does not vanish along the geodesic in these freely falling coordinates. Near the timelike geodesic the metric makes slight departures from the flat space time values and the christoffel symbols are slightly non zero. Physicists approximate the region inside such a freely falling Einstein cage as completely flat. This is wrong, but not a bad approximation, if the curvature (tidal effect) is small. I think all forms of the equivalence principle require a uniform gravitational acceleration field. This approximates Galileo's Earth surface acceleration, wherein tidal effects are too small to measure. However such an approximation omits curvature and gravitational radiation which is bits of curvature obeying a wave equation. So the equivalence principle really has nothing to say about gravitational radiation, per se.
     
    Last edited: Jan 11, 2005
  8. Jan 12, 2005 #7
    Seems obvious to me that in non-inertial frames gravitons could be detected while in non-inertial frames gravitons would not be detected.

    Pete
     
  9. Jan 12, 2005 #8

    pervect

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    There's really a lot of similarity between this thread and the "accelerating charge" thread.

    There may be some minor differences as to when a source emits a gravitational waves (gravitions, if one insists on quantum language), and when it emits an eletromagnetic wave (photons, if one insists on quantum language) due to the fact that the former requires a quadropole moment.
     
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