Abstract
We have shown that the covariant relativistic spin operator is equivalent to the spin operator commuting with the free Dirac Hamiltonian. This implies that the covariant relativistic spin operator is a good quantum observable. The covariant relativistic spin operator has a pure quantum contribution that does not exist in the classical covariant spin operator. Based on this equivalence, reduced spin states can be clearly defined. We have shown that depending on the relative motion of an observer, the change in the entropy of a reduced spin density matrix sweeps through the whole range.
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Choi, T. Relativistic spin operator and Lorentz transformation of the spin state of a massive Dirac particle. Journal of the Korean Physical Society 62, 1085–1092 (2013). https://doi.org/10.3938/jkps.62.1085
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DOI: https://doi.org/10.3938/jkps.62.1085