"Zero sound" can refer to different concepts depending on the context. Here are a few interpretations: 1. **Acoustic Science**: In acoustics, "zero sound" may refer to a state where sound waves are absent. This can occur in a vacuum, where there are no molecules to carry sound waves, resulting in complete silence.
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Zero sound is a collisionless collective Fermi surface deformation sustained by Landau quasiparticle interactions.[ref]
Oscillations in a Fermi Liquid by Lev Davidovich Landau. Opening and section 1, pp.101–103
A useful analogy is a crowd whose members respond to their neighbors' density without repeatedly bumping into them: the restoring interaction can remain effective when collisions cannot relax the distribution within one cycle. Zero sound is collective; it is not a single Landau quasiparticle outrunning every other excitation.