quantumelectrodynamics

Quantum electrodynamics (QED) is the relativistic quantum field theory that describes how light interacts with matter. It treats the electromagnetic field and charged fermions, principally electrons and positrons, as quantum fields whose quanta are photons and Dirac particles. As a gauge theory based on U(1) symmetry, QED gives the electromagnetic interaction as the exchange of photons.

The theory is formulated through the QED Lagrangian, L = -1/4 F_mu nu F^mu nu + ψ̄(i γ^μ D_μ - m) ψ, with D_μ = ∂_μ + i e A_μ. The electron field ψ interacts with the photon field A_μ via the covariant derivative, and the electromagnetic coupling e is related to the fine-structure constant α ≈ 1/137. The framework is renormalizable and gauge invariant.

During the 1940s, Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga developed the practical formulation of QED, unifying quantum mechanics, special relativity and electromagnetism. Their approaches led to a calculational framework based on perturbation theory and Feynman diagrams. The trio shared the Nobel Prize in Physics in 1965 for this work.

QED provides highly accurate predictions for processes involving photons and charged leptons. Key successes include the electron and muon anomalous magnetic moments (g-2), the Lamb shift in hydrogen, vacuum polarization effects, and deep inelastic scattering cross-sections. Experimental measurements agree with QED calculations to parts per billion in some cases, making it one of the most precisely tested theories in physics.

It is a renormalizable, Abelian gauge theory with a running coupling constant; the fine-structure constant increases slowly with energy. Renormalization absorbs infinities into redefined parameters, yielding finite, predictive results across a wide range of energies. QED forms the electromagnetic sector of the Standard Model and provides the foundation for much of atomic and particle physics.