  Single-vertex Feynman diagrams represent the dominant contribution to
physical processes, but are frequently forbidden kinematically. This is changed
when the particles involved propagate in a gravitational background and acquire
an effective mass. Procedures are introduced that allow the calculation of
lowest order diagrams, their corresponding transition probabilities, emission
powers and spectra to all orders in the metric deviation, for particles of any
spin propagating in gravitational fields described by any metric. Physical
properties of the "space-time medium" are also discussed. It is shown in
particular that a small dissipation term in the particle wave equations can
trigger a strong back-reaction that introduces resonances in the radiative
process and affects the resulting gravitational background.
