  Since the discovery a century ago, spin describing the intrinsic angular
momentum of massive elementary particles has exposed its nature and significant
roles in wide ranges of (relativistic) quantum phenomena and practical
applications for future quantum technology. Emerging inconsistencies have also
disclosed its telltale incomplete description. Finding relativistic spins
(operators) of massive particles is a long-standing fundamental problem from
the beginning of relativistic quantum mechanics. Here we present the rigorous
derivation and the representation of spin operators from the spacetime
symmetry. The covariant parity operation, defined by the spin operators,
naturally leads to a fundamental equation equivalent to the covariant Dirac
equation, which manifests existent relativistic spins. Proper understanding
position operator in the Dirac theory on account of the spin operator through
total angular momentum predicts no Zitterbewegung as well as conserving orbital
and spin currents. The spin operators can be applicable for unraveling the
inconsistencies and for exploring unveiled physics of massive particles.
