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B Questions about Quantum Gravity

  1. Oct 7, 2017 #1
    So my first question is :- Is there any quantum theory success to derive the principle of general relativity? and If this happen, Does this mean we succeeded in finding a quantum theory of gravity?.
    and My second question is :- Is quantized background-independet theories exists? and If so, is that mean there is must be modification that make us able to make a quantum theory of gravity?
  2. jcsd
  3. Oct 8, 2017 #2


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    The only answer I can give is that there is, as of now, no theory of quantum gravity.

    I suggest that you learn to do a forum search when you have questions that a moments thought will make you realize are very likely to have been asked by others. A forum search on "quantum gravity" for example, would have answered your first question rather quickly.
  4. Oct 8, 2017 #3
    Thanks, I'll do.
  5. Oct 8, 2017 #4

    king vitamin

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    Canonical quantum gravity (gravitons) contains GR in the low-energy limit, but it is not UV complete. String theory is a theory of quantum gravity which contains GR in the low-energy limit, and it may be UV complete, but string theories come with a ton of extra particle physics, and a particular theory containing our particle physics hasn't been found.

    Your second question is harder to answer. The full nonperturbative definition of string theory doesn't really exist right now. Loop quantum gravity claims to be background independent, but it cannot be used to derive all of GR (though you can get specific solutions from what I've heard).
  6. Oct 10, 2017 #5
    Gravitons are problematic since detecting one is extremely difficult due to the weakness of gravity. Such detection would settle the issue of their existence in our low energy lives. There are indeed some who doubt their very existence, but these spin-2 bosons have been used as a starting point to derive the equations of GR, and energy of gravitational waves. Are these simply a coincidence, or indirect evidence that gravitons are indeed an emergent propetry of effective field and'or emergent GR at low energy? If I understand it properly, the success of string theory hangs (in part) on the verification of supersymmetry, which is hanging in the balance at the moment (LHC has so far failed to find evidence of supersymmetry at energies where it might have been expected to begin appearing). Stay tuned.

    "Covariant Loop and Quantum Gravity" by Carlo Rovelli and Francesca Vidotto (Cambridge, 2015) is a reasonably up-to-date intro to loop quantum gravity. Chapter 12 therein addresses the graviton propagator and claims some success in deriving semiclassical linearized gravity, but they admit there is more work to do to clinch things.
  7. Oct 10, 2017 #6
    To solve this is the holy grail of theoretical physics.
  8. Oct 10, 2017 #7

    king vitamin

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    There's nothing in string theory which claims that SUSY must be detectable at LHC energies. Low-energy SUSY is mostly nice in addressing the hierarchy problem. There are many orders of magnitude between the LHC and the Planck scale.

    This is an unfortunate feature of most quantum gravity theories - humans might not ever be able to test it using our technology. But maybe we'll get lucky and quantum gravity will have some low-energy effects, or we will detect some QG remnants from the early universe.
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