Optical excitations in a nonideal bose gas

Optical excitations in a Bose gas are demonstrated to be sensitive to many- body effects. At low temperature the momentum relaxation is provided by mom entum exchange collisions, rather than by elastic collisions. A collective excitation mode forms, which in a nondegenerate gas is manifest in a collis ion shift and dramatic narrowing of spectral lines. In the Bose-Einstein co ndensate state, each spectral line splits into two components. The doubling of the optical excitations results from the physics analogous to that of t he second sound. We present a theory of the line doubling, and calculate op tical absorption spectrum for a uniform density system.