  In terms of a photon wave function corresponding to the (1, 0)+(0, 1)
representation of the Lorentz group, the radiation and Coulomb fields within a
source-free region can be described unitedly by a Lorentz-covariant Dirac-like
equation. In our formalism, the relation between the positive- and
negative-energy solutions of the Dirac-like equation corresponds to the duality
between the electric and magnetic fields, rather than to the usual
particle-antiparticle symmetry. The zitterbewegung (ZB) of photons is studied
via the momentum vector of the electromagnetic field, which shows that only in
the presence of virtual longitudinal and scalar photons, the ZB motion of
photons can occur, and its vector property is described by the polarization
vectors of the electromagnetic field.
