  An effective model for the spacetime foam is constructed in terms of nonlocal
interactions in a classical background. In the weak-coupling approximation, the
evolution of the low-energy density matrix is determined by a master equation
that predicts loss of quantum coherence. Moreover, spacetime foam can be
described by a quantum thermal field that, apart from inducing loss of
coherence, gives rise to effects such as gravitational Lamb and Stark shifts as
well as quantum damping in the evolution of the low-energy observables.
