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

TL;DR

This paper explains how sound waves in a Bose-Einstein condensate propagate at high velocities despite individual atoms moving slowly, using a simple intuitive model supported by Bogoliubov theory.

Contribution

It introduces a straightforward 'cartoon picture' to understand the high phonon velocity and derives a relation between phonon speed and excited atoms.

Findings

The phonon velocity is much faster than individual atom velocities.

The relation v_s/v_a = N_ex is derived and supported by Bogoliubov theory.

A simple intuitive model explains the collective behavior of phonons.

Abstract

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.