Using functional techniques (the closed-time-path formalism) we derive a systematic way of computing the dynamical operator for the fluctuations of the mean value around a stable configuration. The equations are real and causal and contain some dissipative terms. We examine the effects predicted by them for a $\lambda {\Phi }^4$ theory in the one- and two-loop approximations. We estimate the time needed for the damping of the oscillations around a zero and a nonzero mean value (the dominant processes are different in these two cases). We discuss some conceptual issues about the nature of this dissipative process.

• Received 20 November 1989

DOI:https://doi.org/10.1103/PhysRevD.42.529