  Quantum gases of atoms and exciton-polaritons are nowadays a well established
theoretical and experimental tool for fundamental studies of quantum many-body
physics and suggest promising applications to quantum computing. Given their
technological complexity, it is of paramount interest to devise other systems
where such quantum many-body physics can be investigated at a lesser
technological expense. Here we examine a relatively well-known system of laser
light propagating through thermo-optical defocusing media: based on a
hydrodynamical description of light as a quantum fluid of interacting photons,
we investigate such systems as a valid, room temperature alternative to atomic
or exciton-polariton condensates for studies of many-body physics. First, we
show that by using a technique traditionally used in oceanography, it is
possible to perform a direct measurement of the single-particle part of the
dispersion relation of the elementary excitations on top of the photon fluid
and to detect its global flow. Then, using a pump-and-probe set-up, we
investigate the collective nature of low-wavevector sound modes of the fluid
and observe signatures of superfluid behaviour.
