Broken Symmetry and Nonequilibrium Superfluid 3He Circular Birefringence of Propagating Transverse Currents

The superfluid phases of 3He provide a unique physical system to study the dynamical effects of spontaneous symmetry breaking in condensed matter. The theory of superfluid 3He is grounded in what are arguably two of the most successful theories of strongly interacting matter, Landau’s Fermi-liquid theory and the BCS pairing theory of superconductivity. These two theories were placed into a common theoretical framework in the late 60’s. I discuss applications of this theory to nonequilibrium dynamics of superfluid 3He. In 1957 Landau predicted that liquid 3He would support propagating shear waves at low temperatures, i.e. a transverse sound mode. Such waves have recently been observed at low temperatures in the superfluid B-phase of liquid 3He. These observations provide a beautiful example of the effect of spontaneous symmetry breaking in condensed matter. I discuss the theory of transverse wave propagation in 3He and the recent detection of these waves by magneto-acoustic rotation of the polarization in a magnetic field.