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Would gravity waves exhibit quantum properties?

  1. Apr 29, 2014 #1
    I was watching Fay Dowker's "Spacetime and the quantum: united by history" on Youtube, and a question occurred to me. She has the unique ability to combine enough detail with a popularized version to accurately portray some of the big, current issues facing physics. The question is:

    Would gravity waves exhibit quantum properties?

    Given that experiments have not detected gravity waves directly, I am guessing that at this point the question would have to be answered theoretically. I wouldn't even know how those quantum properties would manifest themselves or what an experiment to test such a thing would look like. Has anyone done any research or exploration on this question?
  2. jcsd
  3. Apr 29, 2014 #2
    Yes, in the same way that electromagnetic waves exhibit quantum properties. Electromagnetic waves actually come in discrete packets of energy called photons. Similarly gravitational waves would come in discrete packets of energy called gravitons. However, while we have high hopes of detecting gravitational waves in the next few years, it would be amazingly, incredibly difficult to detect a single graviton. The problem is that single gravitons just pass through everything with very very high probability--like neutrinos but much worse. So we have no hope of directly detecting gravitons in the foreseeable future.
  4. Apr 30, 2014 #3
    If I had to guess, I would agree, but that does not account for much. Is the positing of gravitons a pretty strong theory with predictive power or will we have to wait until a solid theory of quantum gravity is arrived at before we can confirm the graviton? In other words, is that one of the goals of a theory of quantum gravity, understanding how the graviton works?
  5. Apr 30, 2014 #4


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    Gravitational waves can be observed via classical physics. For example, the loss of gravitational energy from a pair of neutron stars in close orbit agrees with the gravitational wave theory derived from General Relativity.

    The LIGO (Laser interferometric gravitational observatory) is described in detail here:

    And recently "primordial" gravitational waves were detected by means of within the cosmic background radiation:

    Much analysis and many papers, and more experiments! will be done before the dust settles on this result.
  6. Apr 30, 2014 #5


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    Its a mistake to think QM and GR are incompatible:

    And that predicts gravity waves have quantum properties exactly as QED does for light.

  7. Apr 30, 2014 #6


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    Yes and yes.

    As UltrafastPED pointed out, the BICEP2 Collaboration has recently announced Detection of B-mode Polarization at Degree Angular Scales, which indicates gravitational waves in the early universe, aka "primordial gravitational waves".


    This is also the first direct evidence of Alan Guth's inflationary hypothesis, and the first time we have been able to detect anything beyond the "time of last scattering" (380,000 years after the Big Bang).


    Quite amazing...

    BICEP2 Press Conference - March 17, 2014

    The MP4 video can be downloaded here (552 MB), and the paper is here.
  8. Apr 30, 2014 #7
    The behavior of gravitons at low energies is pretty much determined by what we already know about quantum mechanics and general relativity. But we need a quantum theory of gravity if we want to describe the behavior of gravitons at high energies, where "high" means "near the Planck scale."
  9. May 3, 2014 #8


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