  Light in a nonlinear cavity is expected to flow without friction -- like a
superfluid -- under certain conditions. Until now, part-light part-matter
(i.e., polariton) superfluids have been observed either at liquid helium
temperatures in steady state, or at room temperature for sub-picosecond
timescales. Here we report signatures of superfluid cavity photons (not
polaritons) for the first time. When launching a photon fluid against a defect,
we observe a suppression of backscattering above a critical intensity and below
a critical velocity. Room-temperature and steady-state photon superfluidity
emerges thanks to the strong thermo-optical nonlinearity of our oil-filled
cavity. Numerical simulations qualitatively reproduce our experimental
observations, and reveal how a viscous photon fluid reorganizes into a
superfluid within the thermal relaxation time of the oil. Our results establish
thermo-optical nonlinear cavities as platforms for probing photon superfluidity
at room temperature, and offer perspectives for exploring superfluidity in
arbitrary potential landscapes using structured mirrors.
