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Can gravitational waves exhibit quantum superpositions?

  1. Dec 8, 2014 #1
    I have read that the team at Vienna are conducting experiments to test whether even gravity can exhibit quantum superpositions or not. Is anyone tracking their late developments on this? What do the physics community think is going to happen when even gravitational waves are superposed. Will the space-time continuum collapse like Penrose's objective reduction theory predicts or will it strictly follow the rules of quantum mechanics even here? Will QM win again?

  2. jcsd
  3. Dec 8, 2014 #2


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    Gravitational waves are essentially classical (in the same sense in which light is classical) and certainly obey the superposition principle. I don't know what exactly the experiment you talk about is supposed to measure, so a link with more details would be very useful.
  4. Dec 8, 2014 #3
    Physicists Eye Quantum-Gravity Interface

    “I don’t see why these concepts of quantum theory that have proven to be right for the case of light should fail for the case of gravity,” Aspelmeyer said.

    Have they made any progress on this? Its important because people like Penrose believe that gravity is going to behave differently and he is with Einstein on this.
  5. Dec 8, 2014 #4


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    Last edited: Dec 8, 2014
  6. Dec 8, 2014 #5


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    Gravity can certainly be used in quantum-mechanical setups. They made neutrons "bounce" above a surface due to gravity and found quantized energy states, in the same way a particle in other potential wells has them.
    Those keywords lead to many articles about it, here is one from CERN.

    A QM-superposition of gravitational fields is something different, but I would not expect a deviation from quantum mechanics.
  7. Dec 9, 2014 #6
    Gravitational waves are superposed "by default". Mathematically, the oscillating gravitational wave is a superposition of localized point-like deformations of spacetime, as you can always do a reverse Fourier transform on it. I wonder if the "position" states of g-waves do not correspond to black holes, but I will not make any statement here.

    Are gravitational waves quantized? I believe so, because why not. But that would mean that QM will finally enter the non-linear realm. This fact has a potential to finally make various QM interpretations distinguishable.

    QM is awfully linear, while GR is not. This linearity is silently assumed in many discussions. For instance, the MWI holds that individual worlds are not interacting, thus the underlying QM is completely linear. If it was not linear, the state of "other" worlds would affect the state of the "current" (observer's) world.

    Bringing the nonlinearity of GR into QM will finally expose the differences between QM interpretations.
  8. Dec 9, 2014 #7


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    Of course - check out:

    Basically gravity is like other interactions such as EM that breakdown beyond a certain cut-off. For EM we know what lies beyond that cut-off - the electroweak theory. For gravity - well research is ongoing - but its not true to say gravity and QM do not mix.

    And yes - according to the effective field theory of gravity gravitational waves are described by Quantum Field theory the same way EM waves are described by QFT.

    Last edited: Dec 10, 2014
  9. Dec 10, 2014 #8


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    That's a highly misleading claim. Any interaction (except harmonic oscillator) in QM leads to a non-linear Heisenberg equation of motion for the position operator. And yet, the evolution of the state in the Hilbert space is linear.

    Likewise, in quantum gravity the evolution of the state is linear, despite the fact that metric satisfies a non-linear equation.
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