Relativistic spin operator and Lorentz transformation of spin state of a massive Dirac particle

TL;DR

This paper demonstrates that the covariant relativistic spin operator for a massive Dirac particle is a valid quantum observable and explores how spin states transform under Lorentz boosts, affecting entropy.

Contribution

It establishes the equivalence of the covariant relativistic spin operator with a commuting operator and analyzes the Lorentz transformation of spin states.

Findings

Covariant relativistic spin operator is a valid quantum observable.

Spin entropy varies with observer’s relative motion.

Pure quantum contribution exists in the covariant relativistic spin operator.

Abstract

We have shown 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 the pure quantum contribution which does not exist in the classical covariant spin operator. Based on this equivalence reduced spin states can be claerly defined. We have shown the change in the entropy of a reduced spin density matrix sweeps through the whole range according to the relative motion of an observer.