Do sound waves in a gas consist of phonons? What about a glass? Or other non-cry

Question

Do sound waves in a gas consist of phonons?

What about a glass? Or other non-crystalline materials such as quasicrystals?

How does the lack of translational symmetry affect the quantization of the displacement field?

All the answers so far have treated this question at a much more elementary level than I was expecting. I am already quite familiar with the properties of phonons in crystals. Therefore, do not explain the well-known derivations of the dispersion relation and second quantization of phonons in crystal lattices in your answer (and especially don't get them wrong!).

Explanation / Answer

Phonons were named after photons and have the same functionality quantum mechanically. They are both waves and particles. Nobody disputes the wave nature of sound in general. In non ordered materials there is no way that a sound wave will give all its energy to an atom, or a cluster of atoms, as an example. The reason is because all these atoms are in an incoherent state and no pure quantum mechanical state function can be defined. An atom can have a pure state function but the wavelengths of sound are way larger than the wavelengths that an atom can absorb.

In ordered materials like crystals this can happen because phases are defined so that there can be large dimensional coherent scattering: the order allows a quantum mechanical state function for the crystal to be defined, which can interact with a sound wave so that the whole energy of that sound is absorbed by the crystal, thus a phonon.

Edit: In the comments to the question it becomes clear that there is a confusion on the use of the term "phonon". I am using the definition in wikipedia.

Edit2: Copying from Carl's comment in the question, I would add that the quantization is the familiar E=h?=??, and that this applies to fluids as well. But without a translational symmetry, this cannot display quantization but instead can take any value of ?.

If we expand the definition of a phonon to a continuous spectrum, it seems that the answer to the question above finally is yes, yes, yes. Though I guess that in disordered media the particle nature is not manifest. Actually this is also true about photons in ambient light, as an example. Maybe somebody should expand the Wikipedia article.

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