  The Foldy-Wouthuysen transformation of the Dirac Hamiltonian is generally
taught as simply a mathematical trick that allows one to obtain a two-component
theory in the low-energy limit. It is not often emphasized that the transformed
representation is the only one in which one can take a meaningful *classical
limit*, in terms of particles and antiparticles. We briefly review the history
and physics of this transformation.

High Energy Physics - Phenomenology