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Graviton and GR

  1. Oct 24, 2003 #1


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    Does Loop Quantum Gravity support the existence of graviton?
    I can't imagine how is it possible that graviton can exist if it is to agree with GR, since GR says that gravity is merely a curvature of spacetime and is isn't a force.

    GR => SR => time dilation & length contraction =>> how does graviton be able to explain time dilation and stuff like that? Am I missing something here?
  2. jcsd
  3. Oct 24, 2003 #2
    Loop quantum gravity will not have the graviton as a fundamental particle, but like any theory of quantum gravity, it (if it is viable) should have a limit in which spacetime behaves approximately like a fixed background filled with gravitons. That's what the early weave state constructions, and lately the Fock space representation and "shadow state" research is aimed toward.

    The leading terms of a perturbation expansion of GR gives rise to linearized gravity, which upon quantization leads to the graviton. The higher order terms will correct the graviton-graviton interaction to become successively closer to the physics of full GR. Of course, non-renormalizability means that you can play this game only so far, but renormalization group considerations require that the linearized gravity picture approximation to full GR become, upon quantization, a graviton approximation to full QG.

    Do you understand how you can have time dilation and length contraction in classical linearized gravity?
  4. Oct 26, 2003 #3


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    No, I don't. Can you explain?
  5. Oct 27, 2003 #4
    In linearized gravity, you interpret the gravitational field as just a tensor field on flat spacetime. Nevertheless, you can recover position-dependent (i.e., gravitational) time dilation; in the linearized approximation, we calculate it as ordinary velocity-dependent SR time dilation, but because the motions of bodies in this flat spacetime are influenced by the gravitational field, so are their relative velocities, and you end up observing the same dilation you would get in a curved spacetime.

    The details of this are worked out in section 3.6 of Ohanian and Ruffini, and sections 5.2 of the Feynman Lectures on Gravitation.
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