= Zero sound
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<Zero sound> is a collisionless collective <Fermi surface> deformation sustained by <Landau quasiparticle> interactions.<Oscillations in a Fermi Liquid by Lev Davidovich Landau>[Opening and section 1, pp.101–103]{ref}
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.
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