There's this unfortunate tendency to stick the words ``Casimir effect'' everywhere (not to mention the term ``dynamical''...).

Sonoluminescence is the emission of electromagnetic waves by sound waves-it's a classical effect and doesn't depend on the ability to resolve either the photons or the phonons, whose superpositions the corresponding waves are.

The Casimir effect regards the quantum fluctuations of the intensity of some field about its average value in the vacuum, in the presence of boundaries. This doesn't have anything to do with sonoluminescence beyond pertaining to the shot noise of the photons and/or the phonons involved in sonoluminescence, when these can be resolved, in the presence of boundaries.

If the boundaries are moving-the boundary conditions change with time-the appearance of the corresponding shot noise is called the ``dynamical Casimir effect''.

However if you change the boundary conditions of Maxwell's equations in a medium, you will, of course, produce fluctuations of the intensity of the electromagnetic field. These don't have anything to do with the Casimir effect-though they are relevant for sonoluminescence.

Dear Stam Nicolis, thank you for the insightful answer! Actually, I'm more curious about the origin of the sonoluminescence. The standing theory on sonoluminescence as reviewed by Brenner [Single-bubble sonoluminescence, 2002] says that Bremsstrahlung and plasma recombination are mainly responsible for the photon emission.

And, I have a follow-up question: If one could suppress bremsstrahlung and plasma recombination, and there is still a weak signal, can this only signal, i.e. the "noise of the photons" be generated by the dynamical Casimir effect?

Since the wall of the cavitation bubble moves at high speed during collapse, it provides moving boundary conditions. The photons and anti-photons (or other pairs of particles and anti-particles) generated by the quantum fluctuation in the cavitation might not find their partners, and subsequently can't annihilate each other, because of the fast-moving walls. The virtual-turned-real photons may be the source of this noise?

Bremsstrahlung and plasma recombination are both classical effects-so I don't see why it's necessary to talk of quantum fluctuations at all-nor of single photon effects.

Time-dependent boundary conditions can be taken into account at that level, also.

Once more: the fact that it may be possible to measure the fluctuations, due to shot noise of photons and/or phonons, doesn't man that it is these fluctuations that describe sonoluminescence; they would describe the fluctuations about it!

Thanks for the quick response!

Of course, Bremsstrahlung and plasma recombination are classical effects. However, when these classical effects are removed from the experiments, what remains is only the quantum effects.

Actually, sonoluminescence is not the interesting part here, the fluctuation of sonoluminescence is. This is to say, the noise and only the noise is the interesting part. Because the cavitation bubble may provide a way to study dynamical Casimir effect. Again, this is not my study, but the study of Schwinger and Eberlein. I'm just interested in the current research state on this topic :)

But, it seems like [Wilson, 2011] has already observed the dynamical Casimir effect in another type of experiment. So, the cavitation bubbe experiment might not be interesting...