# Gravitational waves and quantum physics

1. Feb 2, 2010

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?

2. Feb 9, 2010

### FrankPlanck

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. Feb 9, 2010

### Matterwave

But what would the de broglie wavelength of spacetime be?

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. Feb 9, 2010

### FrankPlanck

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. Feb 9, 2010

I don't think your answer is right. The answer I've heard is that the mechanisms in the source and detector are both modelled well using classical relativistic physics. However that doesn't seem good enough to me.

6. Feb 9, 2010

### twofish-quant

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. Feb 9, 2010

### twofish-quant

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. Feb 9, 2010

Do you realise the theoretical frequency of a compact binary diverges? It is cut off at around innermost stable orbit though by coalescence though.

9. Feb 12, 2010

### FrankPlanck

Yes but how can we realize that quantum effects are not so important? Does a (or more) formula exist or something similar?

10. Feb 22, 2010

### Sourabh N

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. Feb 22, 2010

### Ich

As long as they are of small amplitude, you can treat them as a perturbation and keep the flat background with its PoincarĂ© symmetry.