Acoustic waves provide a powerful tool for studying the structure of matter. For example, the speed, attenuation and dispersion of acoustic waves can give useful information on molecular forces and the microscopic mechanisms of absorption and scattering of acoustic energy. In solids, both compression and shear waves occur—longitudinal and transverse sound, respectively. But normal liquids do not support shear forces and consequently transverse waves do not propagate in liquids, with one notable exception. In 1957 Landau predicted that the quantum-liquid phase of helium-3 might support transverse sound waves at sufficiently low temperatures, the restoring forces for shear waves being supplied by the collective quantum behaviour of the particles in the fluid. Such shear waves will involve displacements of the fluid transverse to the direction of propagation, and so define a polarization direction similar to that of electromagnetic waves. Here we confirm experimentally the existence of transverse sound waves in superfluid ~3He-B by observing the rotation of the polarization of these waves in the presence of a magnetic field. This phenomenon is the acoustic analogue of the magneto-optic Faraday effect, whereby the polarization direction of an electromagnetic wave is rotated by a magnetic field applied along the propagation direction.
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