  When confined within an optical cavity, light can exert strong radiation
pressure forces. Combined with dynamical backaction, this enables important
processes such as laser cooling, and applications ranging from precision
sensors to quantum memories and interfaces. However, the magnitude of radiation
pressure forces is constrained by the energy mismatch between photons and
phonons. Here, we overcome this barrier using entropic forces arising from the
absorption of light. We show that entropic forces can exceed the radiation
pressure force by eight orders of magnitude, and demonstrate this using a
superfluid helium third-sound resonator. We develop a framework to engineer the
dynamical backaction from entropic forces, applying it to achieve phonon lasing
with a threshold three orders of magnitude lower than previous work. Our
results present a pathway to exploit entropic forces in quantum devices, and to
study nonlinear fluid phenomena such as turbulence and solitons.
