  Quantum fluids of light are realized in semiconductor microcavities by
exciton-polaritons, solid-state quasi-particles with a light mass and sizeable
interactions. Here, we use the microscopic analogue of oceanographic techniques
to measure the excitation spectrum of a thermalised polariton condensate.
Increasing the fluid density, we demonstrate the transition from a
free-particle parabolic dispersion to a linear, sound-like Goldstone mode
characteristic of superfluids at equilibrium. Notably, we show that excitations
are created with a definite direction with respect to the condensate, analogous
to how a sea breeze develops surface waves aligned with the wind. These results
reveal the effect of asymmetric pumping on the collective excitations of a
condensate. Furthermore, we measure the critical sound speed for polariton
superfluids close to equilibrium.
