  We measure the free decay of a spatially periodic density profile in a normal
fluid strongly interacting Fermi gas, which is confined in a box potential.
This spatial profile is initially created in thermal equilibrium by a
perturbing potential. After the perturbation is abruptly extinguished, the
dominant spatial Fourier component exhibits an exponentially decaying
(thermally diffusive) mode and a decaying oscillatory (first sound) mode,
enabling independent measurement of the thermal conductivity and the shear
viscosity directly from the time-dependent evolution.
