  Kinematically forbidden processes may be allowed in the presence of external
gravitational fields. These ca be taken into account by introducing generalized
particle momenta. The corresponding transition probabilities can then be
calculated to all orders in the metric deviation from the field-free
expressions by simply replacing the particle momenta with their generalized
counterparts. The procedure applies to particles of any spin and to any
gravitational fields. transition probabilities, emission power, and spectra
are, to leading order, linear in the metric deviation. It is also shown how a
small dissipation term in the particle wave equations can trigger a strong
backreaction that introduces resonances in the radiative process and deeply
affects the resulting gravitational background.
