Quantum Electrodynamics (QED) establishes that the electromagnetic potential $A_\mu$ constitutes a physical field representing photons in relativistic quantum theory, where gauge freedom allows for arbitrary scalar transformations without altering observable electric and magnetic fields. The core principle utilizes the Lorenz gauge condition ($\partial^\mu A_\mu = 0$) to reduce Maxwell's equations to the wave equation $\square^2 A_\mu = J_\mu$, which yields massless solutions implying that photons possess zero rest mass. Consequently, photon polarization is constrained such that only two independent components exist after fixing residual gauge freedom, defining the intrinsic degrees of freedom for electromagnetic radiation in relativistic physics.
Quantum Electrodynamics (QED) establishes that the electromagnetic potential $A_\mu$ constitutes a physical field representing photons in relativistic quantum theory, where gauge freedom allows for a…