Dissipation of quantum fields from particle creation

We discuss the nature and origin of the dissipation of quantum fields due to the back reaction of particle creation. We derive the effective action of a scalar $g{\phi }^3$ theory in the closed-time-path-integral formalism. From the real and causal equation of motion for the background field we deduce a dissipative function for this process and for the cosmological anisotropy damping problem studied earlier. This model illustrates that the appearance of dissipative behavior from the back reaction of particle creation in quantum fields is a general feature. It also suggests that the role of gravity in the display of dissipative behavior in semiclassical processes is not unique.