# What is the nature of sound from the quantum perspective?

1. Apr 23, 2013

### Double-Slit

So sound is a mechanical wave usually a propagation of atmospheric pressure in a medium in the classical interpretation.Im curious what happens in the quantum realm ? Also if this pressure force is giving kinetic energy to those atoms,but due to friction is also generates heat energy,and with heat energy affecting electrons in atoms they release their energy via by creating a photon,so in this sense we could think that sound creates light (low frequency light of course),can we make an analogy between the two ? But more im curious what is the nature of sound in the quantum scale (what is that wave made of ) the kinetic energy of the atoms or the atoms themselves called "wave" in classical mechanics,or else? (I know it's a silly question but why do we call it a wave anyway)

2. Apr 23, 2013

### DrewD

There is a lot going on here and I think the question may be a bit vague for one answer. Sound waves that you can hear don't really have a good quantum explanation. Technically they do if everything is emergent from quantum mechanics, but the energies involved make the consideration of the system at a quantum level an unnecessary hassle. However, there are a lot of interesting things to think about here.

1) Photo-acoustics http://en.wikipedia.org/wiki/Photoacoustic_imaging_in_biomedicine. One of my friends is working on this sort of thing. I knew that in theory it was possible, but didn't realize it was actually useful until about a year ago (when my friend told me about his work). This is sort of the opposite of what you were talking about, but related.

2)Phonon. Not really sound waves in the sense that we might think, but these are mechanical waves that are quantized. I mainly know about them from ion-trap qubits, but they come up in condensed matter physics as well.

3. Apr 24, 2013

### vanhees71

Sound is a macroscopic collective oscillation of a medium. In many-body quantum theory, which you most conveniently treat as quantum field theory (socalled "second quantization", which is, however a misnomer, because there is only one quantum theory, be it in its non-relativistic or relativistic form). Then macroscopic phenomena like sound waves, classical electromagnetic waves, etc. appear as appropriate mean-field parts in the many-body equations (Kadanoff-Baym equations). You can derive the macroscopic equations (Boltzmann, Fokker-Planck/Langevin, hydro, etc.) as approximations to the full quantum many-body dynamics.

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