The core principle establishes that for interacting charged fermions described by Dirac theory, electromagnetic interactions introduce a potential term ($V$) derived from the coupling between the particle current and the external field four-potential ($A_\mu$). Through perturbative analysis, this interaction yields transition amplitudes expressed as integrals over invariant currents ($j^\mu = \bar{\psi}\gamma^\mu\psi$), demonstrating that scattering processes for charged particles are fundamentally governed by their contravariant current densities and the propagator factor inversely proportional to momentum transfer squared. This concept situates Dirac field theory within quantum electrodynamics, formally defining the mechanism where free particle dynamics ($p_\mu \to p_\mu + eA_\mu$) transition into interaction regimes characterized by conserved four-currents rather than scalar potentials found in non-relativistic Schrödinger descriptions.
The core principle establishes that for interacting charged fermions described by Dirac theory, electromagnetic interactions introduce a potential term ($V$) derived from the coupling between the par…