  We derive a general quantum master equation for the dynamics of a scalar
bosonic particle interacting with a weak, stochastic and classical external
gravitational field. The dynamics predicts decoherence in position, momentum
and energy. We show how our master equation reproduces the results present in
the literature by taking appropriate limits, thus explaining the apparent
contradiction in their dynamical description. Our result is relevant in light
of the increasing interest in the low energy quantum-gravity regime.
