How do sound waves in a Bose-Einstein condensate move so fast?
Abstract
Low-momentum excitations of a dilute Bose-Einstein condensate behave as phonons and move at a finite velocity vs. Yet the atoms making up the phonon excitation each move very slowly; va = p/m --> 0. A simple "cartoon picture" is suggested to understand this phenomenon intuitively. It implies a relation vs/va = Nex, where Nex is the number of excited atoms making up the phonon. This relation does indeed follow from the standard Bogoliubov theory.
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