Quantum mechanical spin angular momentum density, unlike its orbital counterpart,is independent of the choice of origin. A similar classical local angular momentum density maybe defined as the field whose curl is equal to twice the momentum density. Integration by partsshows that this spin density yields the same total angular momentum and kinetic energy asobtained using classical orbital angular momentum. We apply the definition of spin density toa description of elastic waves. Using a simple wave interpretation of Dirac bispinors, we showthat Dirac’s equation of evolution for spin density is a special case of our more general equation.Operators for elastic wave energy, momentum, and angular momentum are equivalent to thoseof relativistic quantum mechanics.
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