Propagation of relativistic quantum particles in spacetime in quantum field theory

The paper deals with a recent systematic study of the propagation of relativistic quantum particles in spacetime. This study was a reaction to the overwhelming number of experiments dealing with the localization of not only massive but also of photons by detectors. The method of study is based on a configuration unitarity expansion of the vacuum-to-vacuum transition amplitudes as, massive and massless, particles propagate between emitters and detectors. Topics treated are the amplitudes of propagation from one time-space coordinate to another, limiting velocities of particles and their reconciliations with relativity, emergence of particles into cones in detection regions versus the direction of their momenta, stimulated emissions by external sources in spacetime, scattering theory in quantum field theory in configuration space, and finally a spacetime formulation of closed-time path for multi-particle states. (orig.)