Position Operators and Proper Time in Relativistic Quantum Mechanics

Covariant four-vector position operators $X^{\mu }$ are proposed, which form a natural operator generalization of the four-position in relativistic classical mechanics. These $X^{\mu }$ are defined by specifying commutation relations of the $X^{\mu }$ with the Poincaré generators $P^{\mu }$ and $M^{\mu \nu }$, and thereby extending the Poincaré algebra to a larger algebra whose representations are subsequently found. The $X^{\mu }$ are shown to be acceptable relativistic position operators within a proper-time dynamical framework. A single Hamiltonian is used for all spins, with a covariant proper-time description. The dynamics is capable of describing the time evolution of states which are not mass eigenstates. An automatic Foldy-Wouthuysen-type diagonalization is achieved for all spin representations, with spin and orbital angular momentum being separately conserved. The connection with the standard theory is made via the specific field equations. In making this connection to the standard theory of half-integral spins, the origin of Zitterbewegung and the nonseparate conservation of spin and angular momentum are clarified. The connection to Maxwell's equations provides an interesting statement of those equations and of gauge invariance. The unphysical representations of negative and imaginary mass and continuous spin are not present in this formalism. Other features are discussed.