Particles, mass spectra and interference in relativistic quantum theory

The asymptotic properties of world lines, in the case of classical relativistic dynamics, and of Stueckelberg type wave functions, in the case of quantum relativistic dynamics, characteristic of particle-like behavior, are defined and studied. The structure of the mass spectrum for systems with internal degrees of freedom (e.g. the two-body problem) is shown to be related to the spectrum of the reduced problem. Interference experiments are proposed to affirm the interpretation and applicability of Stueckelberg type wave functions for actual physical phenomena