Broken Relative Symmetry and the Dynamics of the A1 Phase of 3He

Second sound in the A₁ phase of superfluid ³He has a velocity three orders of magnitude larger than in the A phase and is at the same time a spin wave. A magnetic field inhomogeneity leads, depending on the geometry, either to a pressure or temperature difference. These results, among others, contradict predictions by previous authors and are obtained from the hydrodynamic theory derived in this Letter by employing the concept of broken relative symmetry.