In non-relativistic Quantum Mechanics, the static Electric field is represented by a scalar potential, magnetic fields by the vector potential, and the radiation field also through the vector potential. It will be convenient to keep this separation between the large static atomic Electric field and the radiation fields, however, the equations we have contain the four-vector with all the fields mixed. When we quantize the field, all E and B fields as well as electromagnetic waves will be made up of photons. It is useful to be able to separate the E fields due to fixed charges from the EM radiation from moving charges. This separation is not Lorentz invariant, but it is still useful.
Enrico Fermi showed, in 1930, that together with give rise to Coulomb interactions between particles, whereas gives rise to the EM radiation from moving charges. With this separation, we can maintain the form of our non-relativistic Hamiltonian,
In a region in which there are no source terms,
If , then we cannot eliminate , since and we are only allowed to make gauge transformations for which . In this case we must separate the vector potential into the transverse and longitudinal parts, with
We will now study the radiation field in a region with no sources so that . We will use the equations
Jim Branson 2013-04-22