One of us (A.J.K.) has found the position operator for the Lorentz-invariant localization of scalar mesons, electrons, neutrinos, and photons. It was found uniquely except for an unknown constant for the electrons. With a set of postulates we find uniquely the spin angular-momentum tensor for the electrons and the value of that constant. We also find some general common properties for all the particles considered.
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Kálnay et al. (1974) studied this question.
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