Quantum Fields and Dissipation

Abstract

The description of thermal or non-equilibrium systems necessitates a quantum field theory which differs from the usual approach in two aspects: 1.The Hilbert space is doubled; 2.Stable quasi-particles do not exist in interacting systems. A mini-review of these two aspects is given from a practical viewpoint including two applications. For thermal states it is shown how infrared divergences occuring in perturbative quasi-particle theories are avoided, whereas for non-equilibrium states a memory effect is shown to arise in the thermalization.

Paper in ReVTeX, 12 pages. Figures and complete paper available via anonymous ftp at ftp://tpri6c.gsi.de/pub/phenning/h96neq, contribution to the Umezawa memorial volume of Physics Essays