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Question:

Consider large cloud of gas. Assume it is electrically neutral (but as always, matter is composed of smaller things which are actually charged). Is it possible that propagation of sound waves in this cloud leads to emission of electromagnetic radiation?

My random thoughts about it:

I am not sure what mechanism could lead to that. Maybe some local oscilating dipole moment could appear? This is not completely unplausible - I could imagine gas molecules in some tiny volume element polarizing a liitle bit every now and then when density in the volume element goes up. But this would require the gas to be highly non-ideal.

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    $\begingroup$ Sound waves is undergoing dissipation which leads to increasing temperature. Heat is nothing else than EM radiation in IR spectra. $\endgroup$ – HolgerFiedler May 29 '16 at 11:31
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    $\begingroup$ I don't think sonoluminescence would be an answer here, if the OP is looking for a gas phase phenomenon $\endgroup$ – Secret May 29 '16 at 11:45
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Apart from the heating due to sound absorption, as per the comment by HolgerFiedler, I don't think you will find a mechanism that can radiate due to polarization effects in the medium. Any EM radiation would be at the frequency of your acoustic waves. With the difference between the speed of sound and the speed of light, that would be very-long-wave radiation (30 km at 10 kHz). Your acoustic wave would have a wavelength of 10 cm, with half of each wave radiating in the opposite phase of the other half. You would never be able to radiate EM waves effectively that way.

If your cloud is a (net neutral) plasma, it could very well be that the acoustic wave generates charge-density fluctuations because the electrons and the ions don't move exactly synchronized. If you let the wave propagate over a half wavelength before reabsorbing it, it could radiate a bit. (Making the bigger than a half wavelength won't increase the radiated power because of the argument above.)

There is a different effect, sonolumininescence, which occurs if you focus ultrasonic waves in a liquid. At the focal point, a superheated gas bubble will occur, which can emit visibile light.

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