Gravitational waves and quantum physics

In summary: Then you can apply the usual perturbation theory to get the quantum corrections. However, once the amplitude becomes significant, the perturbation theory breaks down and a full theory of quantum gravity is needed.In summary, gravitational waves are typically modeled in terms of general relativity because at low amplitudes, quantum effects are not significant and can be ignored. However, quantizing gravity waves becomes more difficult at higher amplitudes, requiring a full theory of quantum gravity.
  • #1
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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  • #2
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.
 
  • #3
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.
 
  • #4
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.
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
Do you realize the theoretical frequency of a compact binary diverges? It is cut off at around innermost stable orbit though by coalescence though.
 
  • #9
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.
 

1. What are gravitational waves?

Gravitational waves are ripples in the fabric of spacetime caused by the acceleration of massive objects. They were predicted by Albert Einstein's theory of general relativity and were recently detected for the first time in 2015.

2. How are gravitational waves different from electromagnetic waves?

Gravitational waves are fundamentally different from electromagnetic waves because they do not require a medium to propagate through. They can travel through a vacuum, while electromagnetic waves require a medium, such as air or water, to travel through.

3. What is the connection between gravitational waves and quantum physics?

Gravitational waves and quantum physics are both theories that describe different aspects of the universe. While quantum physics deals with the behavior of particles on a very small scale, gravitational waves deal with the behavior of spacetime and gravity on a larger scale. Currently, there is no unified theory that explains both quantum physics and gravity.

4. Can gravitational waves be used for communication?

No, gravitational waves cannot be used for communication because they are very weak and difficult to detect. They also have a very low frequency, making them unsuitable for carrying information. However, scientists are studying ways to use gravitational waves for other purposes, such as studying the universe and testing the theory of general relativity.

5. How are scientists able to detect gravitational waves?

Scientists use extremely sensitive instruments called interferometers to detect gravitational waves. These devices measure tiny changes in the length of two perpendicular arms caused by passing gravitational waves. The most advanced interferometers, like the Laser Interferometer Gravitational-Wave Observatory (LIGO), are able to detect these changes at a scale of one thousandth the diameter of a proton.

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