How do sound waves in a Bose-Einstein condensate move so fast?

P. M. Stevenson
2003 Physical Review A. Atomic, Molecular, and Optical Physics  
Low-momentum excitations of a dilute Bose-Einstein condensate behave as phonons and move at a finite velocity v_s. Yet the atoms making up the phonon excitation each move very slowly; v_a = p/m --> 0. A simple "cartoon picture" is suggested to understand this phenomenon intuitively. It implies a relation v_s/v_a = N_ex, where N_ex is the number of excited atoms making up the phonon. This relation does indeed follow from the standard Bogoliubov theory.
doi:10.1103/physreva.68.055601 fatcat:x5fkvhjfhfhdnpkjnlgelj435q