  The effects, upon the Klein--Gordon field, of nonconformal stochastic metric
fluctuations, are analyzed. It will be shown that these fluctuations allow us
to consider an effective mass, i.e., the mass detected in a laboratory is not
the parameter appearing in the Klein--Gordon equation, but a function of this
parameter and of the fluctuations of the metric. In other words, in analogy to
the case of a nonrelativistic electron in interaction with a quantized
electromagnetic field, we may speak of a bare mass, where the observed mass
shows a dependence upon the stochastic terms included in the metric.
Afterwards, we prove, resorting to the influence functional, that the
energy--momentum tensor of the Klein--Gordon field inherites this stochastic
behavior, and that this feature provokes decoherence upon a particle immersed
in the region where this tensor is present.

General Relativity and Quantum Cosmology