Second Sound

Abstract

We already indicated in the introduction to this book that the processes on the atomic level of description are connected with the motion of molecules, which is usually limited to a small part of the configuration space. For instance, in the case of crystalline solids, the molecules constituting the lattice vibrate around their equilibrium positions and their maximum displacement from these positions, being of the order of magnitude of 10^-11m (0.1 Å), is small — even in comparison with the lattice constants. In some cases, such as the free electrons in metals or the particles of ratified gases, these displacements may be larger but they are still much smaller than the characteristic dimensions limiting the applicability of macroscopic continuous models (long wave approximation!).