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.