Introduction

Previously, we investigated the statistical thermodynamics of ideal gases using a rather ad hoc combination of classical and quantum mechanics (see Sects. 7.6 and 7.7). In fact, we employed classical mechanics to deal with the translational degrees of freedom of the constituent particles, and quantum mechanics to deal with the non-translational degrees of freedom. Let us now discuss ideal gases from a purely quantum mechanical standpoint. It turns out that this approach is necessary to deal with either low temperature or high density gases. Furthermore, it also allows us to investigate completely non-classical ``gases,'' such as photons or the conduction electrons in a metal.