  The causality issues concerning Hegerfeldt's paradox and the localization of
relativistic quantum systems are addressed through a proper-time formalism of
single-particle operators. The proposed description does not depend on
classical parameters connected to an external observer and results in a
single-particle formalism in which localization is described by explicitly
covariant four-vector operators associated with POVM measurements parametrized
by the system's proper-time. As a consequence, it is shown that physically
acceptable states are necessarily associated with the existence of a temporal
uncertainty and their proper-time evolution is not subject to the causality
violation predicted by Hegerfeldt.
