Quantum Theory of Relaxation
Consider a system which is not closed but in continuous contact with its surroundings, exchanging energy, polarization and so forth. If initially the system is in a nonequilibrium state then—under certain conditions which will be specified below—it will at some later time go over into an equilibrium state determined by external conditions such as temperature. This gradual evolution into an equilibrium state is called a relaxation process. In the present chapter we will consider some methods for studying processes of this kind.