  We analyze free elementary particles with rest mass $m$ and total energy $E <
m c^2$ in the Rindler wedge, outside Reissner-Nordstrom black holes and in the
spacetime of relativistic (and non-relativistic) stars, and use
Unruh-DeWitt-like detectors to calculate the associated particle detection rate
in each case. The (mean) particle position is identified with the spatial
average of the excitation probability of the detectors, which are supposed to
cover the whole space. Our results are shown to be in harmony with General
Relativity classical predictions. Eventually we reconcile our conclusions with
Earth-based experiments which are in good agreement with $E \geq m c^2$.
