A Quantum Physical System in a Quantum Environment — The Gauge Theory of Molecular Physics

Abstract

In Chap. 3 we considered a complicated physical system which consisted of a quantum physical system (spinning particle with magnetic moment) and a classical physical system (external, time-dependent magnetic field); the latter we called the environment. Now we want to consider a complicated physical system which is entirely quantum mechanical. To understand it we divide it into simpler parts which are again quantum systems. A natural division for a large class of quantum systems is the separation into a fast moving part and a slow moving part. The slowly moving part usually consists of the collective motion of the quantum system as a whole. The paradigm of this procedure is the Born-Oppenheimer method in molecular physics. The swiftly moving electron or electrons corresponds to the quantum system of Chap. 3 and the slowly moving nuclei, which determine the collective motion of the molecule as a whole, take the place of the quantum mechanical environment.