Gravitational waves and quantum physics

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SUMMARY

The discussion centers on the modeling of gravitational waves primarily through general relativity (GR) despite their minuscule amplitudes, which are several orders of magnitude smaller than a proton. Participants argue that quantum mechanics (QM) does not currently provide a framework for describing gravitational waves, as the effects of quantum phenomena are negligible at low frequencies typical of these waves. The conversation highlights the challenges of integrating QM with GR due to the non-linear nature of GR and the complexities involved in linear perturbation theory. Theoretical exploration of quantizing gravitational waves at weak limits is deemed possible but not practically interesting, as detectable quantum effects are unlikely.

PREREQUISITES
  • Understanding of General Relativity (GR)
  • Familiarity with Quantum Mechanics (QM)
  • Knowledge of linear perturbation theory
  • Concept of de Broglie wavelength
NEXT STEPS
  • Research the implications of the Poincaré symmetry in quantum theories
  • Explore the current theories of quantum gravity
  • Study the mathematical complexities of linear perturbation theory in GR
  • Investigate the experimental methods for detecting gravitational waves
USEFUL FOR

Physicists, astrophysicists, and students interested in the intersection of quantum mechanics and general relativity, particularly those exploring gravitational wave phenomena.

madness
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If gravitational waves have such tiny amplitudes (typically several orders of magnitude smaller than a proton), then why do we model them entirely in terms of general relativity? Aren't quantum effects important at this scale?
 
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I'm not an expert, but I think you should use de broglie wavelength (or something similar) to estimate quantum effects, therefore amplitude is not so important, wavelength is.
 
But what would the de broglie wavelength of spacetime be? o.O

I don't think there is a current way for QM to describe gravity waves (I'm not so familiar with the forefront of quantum gravity). I think gravity waves are exclusively in the realm of GR, for now.
 
Matterwave said:
But what would the de broglie wavelength of spacetime be? o.O
something similar? :)
I don't know, maybe the de broglie wavelength of a graviton?
I'm not saying this is the answer, but this is a nice problem, I would like to know the solution.
 
I don't think your answer is right. The answer I've heard is that the mechanisms in the source and detector are both modeled well using classical relativistic physics. However that doesn't seem good enough to me.
 
madness said:
If gravitational waves have such tiny amplitudes (typically several orders of magnitude smaller than a proton), then why do we model them entirely in terms of general relativity? Aren't quantum effects important at this scale?

Quantum effects depend on the wavelength and not the amplitude of something. So if you have low amplitudes you don't need quantum. It's if you have high frequency that you do, and the gravity waves that we are looking for have pretty low frequency.
 
Matterwave said:
I don't think there is a current way for QM to describe gravity waves (I'm not so familiar with the forefront of quantum gravity). I think gravity waves are exclusively in the realm of GR, for now.

I don't think it would be that hard to come up with a QM theory of gravity waves at the weak limit. The problem with mixing QM and gravity is that GR is very non-linear so that when you use linear perturbation theory then the math gets very, very, very messy, and you don't come up with any usable answers.

If you have a situation in which the gravity waves were weak, then I think it wouldn't be difficult to come up with a way of quantizing gravity waves. The trouble is that it's not interesting theoretically, and it's also not interesting experimentally, because what you'll find is that the you wouldn't really be able to detect any quantum effects.
 
Do you realize the theoretical frequency of a compact binary diverges? It is cut off at around innermost stable orbit though by coalescence though.
 
twofish-quant said:
I don't think it would be that hard to come up with a QM theory of gravity waves at the weak limit. The problem with mixing QM and gravity is that GR is very non-linear so that when you use linear perturbation theory then the math gets very, very, very messy, and you don't come up with any usable answers.

If you have a situation in which the gravity waves were weak, then I think it wouldn't be difficult to come up with a way of quantizing gravity waves. The trouble is that it's not interesting theoretically, and it's also not interesting experimentally, because what you'll find is that the you wouldn't really be able to detect any quantum effects.

Yes but how can we realize that quantum effects are not so important? Does a (or more) formula exist or something similar?
 
  • #10
I'm not sure if it would be 'easy' to quantize gravity waves. After all, they are (very simple) solutions of einstein equations and have general covariance whereas quantum theory has the poincare invariance. Hamiltonian in GR is identically zero, contrary to formulation of quantum theories which are largely based on the evolution of hamiltonian in time.

PS : I'm new to these things too, please correct/improvise if I'm wrong/unclear.
 
  • #11
I'm not sure if it would be 'easy' to quantize gravity waves.
As long as they are of small amplitude, you can treat them as a perturbation and keep the flat background with its Poincaré symmetry.
 

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