The relativistic quantum mechanics with Lorentz-invariant evolution parameter and indefinite mass is a very elegant theory. But it cannot be derived by quantizing the usual classical relativity in which there is the mass-shell constraint. In this paper the classical theory is modified so that it remains Lorentz invariant, but the constraint disappears; mass is no longer fixed—it is an arbitrary constant of motion. The quantization of this unconstrained theory gives the relativistic quantum mechanics in which wave functions are localized and (...) normalized in spacetime. Though many authors have published good works in support for such a localization in time, the latter has been generally considered as problematic. Here I show that wave packets restricted to a finite region of spacetime are not a nuisance, but just the contrary. They have the physical interpretation in the fact that an observer perceives a world line event by event, as his experience of “now” proceeds in spacetime. Quantum mechanically this means that at a certain value of the evolution parameter τ the event is most probably to occur within the spacetime region around {ie1005-1} occupied by the wave packet; at later value of τ the position {ie1005-2}—and hence the time coordinate t—of the wave packet is changed. This is closely related to the interpretation of quantum mechanics in general. (shrink)