  Fermi gases with magnetically tunable interactions provide a clean and
controllable laboratory system for modeling interparticle interactions between
fermions in nature. The s-wave scattering length, which is dominant a low
temperature, is made to diverge by tuning near a collisional (Feshbach)
resonance. In this regime, two-component Fermi gases are stable and strongly
interacting, enabling tests of nonperturbative many-body theories in a variety
of disciplines, from high temperature superconductors to neutron matter and
quark-gluon plasmas. We have developed model-independent methods for measuring
the entropy and energy of this model system, providing a benchmark for
calculations of the thermodynamics. Our experiments on the expansion of
rotating strongly interacting Fermi gases in the normal fluid regime reveal
extremely low viscosity hydrodynamics. Combining the thermodynamic and
hydrodynamic measurements enables an estimate of the ratio of the shear
viscosity to the entropy density. A strongly interacting Fermi gas in the
normal fluid regime is found to be a nearly perfect fluid, where the ratio of
the viscosity to the entropy density is close to a universal minimum that has
been conjectured by string theory methods.
