  As revealed by space-time probing, mechanics and field theory come out as
complementary descriptions for motions in space-time. In particular, quantum
fields exert a radiation pressure on scatterers which results in mechanical
effects that persist in vacuum. They include mean forces due to quantum field
fluctuations, like Casimir forces, but also fluctuations of these forces and
additional forces linked to motion. As in classical electron theory, a moving
scatterer is submitted to a radiation reaction force which modifies its
motional response to an applied force. We briefly survey the mechanical effects
of quantum field fluctuations and discuss the consequences for stability of
motion in vacuum and for position fluctuations.
