A key test of the theory of critical phenomena using acoustics in liquid helium

Abstract

A significant success of modern theoretical physics has been in the study of critical phenomena, where a system displays singular properties while undergoing a transition between different phases of matter. The modern theory (recognized by a Wolf Prize and a Nobel Prize) involves the notions of length scale invariance, power law singularities, the renormalization group etc. A significant experimental test of the theory has been the study of the transition between normal liquid and superfluid behavior of liquid helium-4 (recognized with a London Prize). The singular properties which characterize a critical transition typically involve a divergence in a susceptibility, such as a heat capacity, compressibility, or other property which may couple to a local configuration of atoms or molecules. Since these latter properties may be probed with sound waves, acoustics can be a useful probe of critical behavior. In the case of superfluid helium there are five fundamentally different modes of sound propagation, and this acoustic abundance contributed to the success of the critical phenomenon theory test. This talk will review the theory of critical phenomena, describe the different modes of sound propagation in superfluid helium, and summarize the results of the key test of the critical phenomenon theory.