  A new formulation of relativistic quantum mechanics is presented and applied
to a free, massive, and spin zero elementary particle in the Minkowski
spacetime. The reformulation requires that time and space, as well as the
timelike and spacelike intervals, are treated equally, which makes the new
theory fully symmetric and consistent with the Special Theory of Relativity.
The theory correctly reproduces the classical action of a relativistic particle
in the path integral formalism, and allows for the introduction of a new
quantity called vector-mass, whose physical implications for nonlocality, the
uncertainty principle, and quantum vacuum are described and discussed.
