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The Formulation of a Hamiltonian Model of the Interaction of a Particle Field with an Electromagnetic Field 

The system being considered is two quantized field and their interaction. The Hamiltonian H is then composed of three parts; i.e,,
Since the objects are fields their Hamiltonians are the integrals over all space of their Hamiltonian densities, which in this case are same as their energy densities. In the case of the electromagnetic field the energy density at a point in space is given by
where E is the electric field intensity at that point in space and B is the magnetic field intensity at the same point.
Thus
For a particle in a potential V the Hamiltonian operator is
where m is the mass in the particle field.
If φ is the wave function for the particle field and φ* is its conjugate then the Hamiltonian for the particle field is
The interaction of the EM field and the particle field is best described in terms of the vector potential of the EM field. The vector potential A is such that
This is known as the Coulomb gauge for the EM field.
The Hamiltonian operator for the interaction is then given by
and hence
where e is the charge of the particle field, i is the square root of negative one and c is the speed of light.
The interaction term is derived from replacing (h/i) in the Hamiltonian operator with (h/i)−(e/c)A
which gives the Hamiltonian as
When the term (h/i)−(e/c)A² is expanded the terms involving A become part of the expression
for the interaction Hamiltonian
and the others become part of the field particle Hamiltonian.
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