Can gravitational waves exhibit quantum superpositions?

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Discussion Overview

The discussion revolves around the potential for gravitational waves to exhibit quantum superpositions, exploring the implications for both quantum mechanics (QM) and general relativity (GR). Participants reference ongoing experiments, theoretical predictions, and the relationship between gravity and quantum mechanics.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants mention ongoing experiments in Vienna aimed at testing whether gravity can exhibit quantum superpositions, questioning the implications for the space-time continuum and Penrose's objective reduction theory.
  • Others assert that gravitational waves are classical and obey the superposition principle, seeking clarification on the specific experiments being referenced.
  • A participant notes that a similar experiment has been conducted without observing macroscopic superpositions, suggesting gravity behaves similarly to other interactions.
  • There is a discussion about the quantization of gravitational waves, with some expressing belief in their quantization and its potential to reveal differences between interpretations of quantum mechanics.
  • One participant emphasizes that gravitational waves are inherently superposed mathematically, raising questions about their relationship to black holes and the implications for QM's linearity.
  • Another participant challenges the notion that QM is strictly linear, arguing that interactions in QM can lead to non-linear equations of motion while maintaining linear evolution in Hilbert space.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of gravitational waves and their relationship to quantum mechanics, with no consensus reached on whether gravitational waves can exhibit quantum superpositions or how gravity interacts with quantum theory.

Contextual Notes

Some claims rely on specific interpretations of quantum mechanics and general relativity, and there are unresolved questions about the nature of gravitational waves and their quantization. The discussion includes references to ongoing research and theoretical frameworks that may not be universally accepted.

bremsstrahlung
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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?

Thanks!
 
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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.
 
Demystifier said:
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.

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.
 
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.
 
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.
 
bremsstrahlung said:
Have they made any progress on this?

Of course - check out:
http://arxiv.org/abs/1209.3511

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.

Thanks
Bill
 
Last edited:
haael said:
QM is awfully linear, while GR is not.
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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