General Theory of Elastic Waves in Crystals Based on Comparison with an Isotropic Medium

Abstract

Anisotropy literally means deviation from isotropy, and it is this deviation that distinguishes a crystal from an isotropic body. It is entirely possible to conceive of an anisotropic body that differs by an arbitrarily small amount from an isotropic one; moreover, such a medium can actually be produced simply by subjecting an isotropic body to some directional action, e. g., compression or extension, electric or magnetic fields, a uniform temperature gradient, etc. The induced anisotropy may be as small as may be desired if the action is suitably weak; all the various features, including the laws of propagation for electromagnetic waves, will differ only slightly from those for an isotropic medium, which facilitates res earch on them. On the other hand, the properties will be far from those of an isotropic body if the anisotropy is large. Experiment shows that natural crystals differ from isotropic media to various extents, i. e ., vary in anisotropy.