Relativistic effects one particle

We continue to assume that any fields are rather weak, so that it is meaningful in the relativistic context to distinguish between the field and the molecule, and also assume that the energies involved are very much less than mc, where m is the particle mass and c the velocity of light. 

We start with the relativistic Lagrangian for a particle of rest mass m and charge q just as we started with the classical Lagrangian in Section 11.1. Instead of (11.1.10) we therefore use (Slater, 1960, Vol. 1, Appenda 4) 

Because this is a square root, we cannot immediately obtain an operator form, but by squaring and rearranging, we obtain 

Dirac suggested that for electrons (11.2.2b) should be factorized in the form 

Dirac also proposed that the single factor nearest the operand 

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