Foldy-Wouthuysen transformation magnetic free-particle

One can gain some insight into the nature of the Dirac wave equation and the spin angular momentum of the electron by considering the Foldy Wouthuysen transformation for a free particle. In the absence of electric and magnetic interactions, the Dirac Hamiltonian is... 

The magnetic terms for the free-particle Foldy-Wouthuysen transformation are more complicated because the operator is not linear in the momentum. It is possible nevertheless to define the field-dependent transformation by the same procedure used in section 16.1. We arrive at a two-component field-dependent operator corresponding to Cp, which we will call p,... 

The complicated nature of the magnetic operators means that the transformed Hamiltonians based on the free-particle Foldy-Wouthuysen transformation are not particularly well suited to calculation of magnetic properties. An alternative is to use the transformed magnetic operator developed in section 13.7, which is also expanded in powers of 1/c, but in terms of even operators, which behave like electric field operators. 

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See also in sourсe #XX -- [ ]

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