Return to equilibrium

We study an atom with finitely many energy levels in contact with a heat ba th consisting of photons (blackbody radiation) at a temperature T > 0. The dynamics of this system is described by a Liouville operator, or thermal Ha miltonian, which is the sum of an atomic Liouville operator, of a Liouville operator describing the dynamics of a free, massless Bose field, and a loc al operator describing the interactions between the atom and the heat bath. We show that an arbitrary initial state that is normal with respect to the equilibrium state of the uncoupled system at temperature T converges to an equilibrium state of the coupled system at the same temperature, as time t ends to +infinity (return to equilibrium). (C) 2000 American Institute of P hysics. [S0022-2488(00)02406-3].