kvantumelektrodinamika

Kvantumelektrodinamika (QED) is the quantum field theory that describes how light interacts with matter. It combines quantum mechanics with special relativity and the quantization of the electromagnetic field. In QED, the electromagnetic interaction is mediated by photons, and charged particles such as electrons and positrons exchange photons during interactions. The theory is formulated as a gauge theory with U(1) symmetry; its Lagrangian includes the electromagnetic field and the Dirac field for fermions, with interactions governed by the electric charge e. Observables are calculated using perturbation theory in the small fine-structure constant α ≈ 1/137, organized in Feynman diagrams.

QED was developed in the 1940s–1950s by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, who showed that the theory could be renormalized to yield finite predictions. Early successes included explaining the Lamb shift and the anomalous magnetic moment of the electron beyond the Dirac value. Over decades, precision measurements of phenomena such as electron and muon g-2, Compton scattering, and light-by-light scattering have confirmed QED to extraordinary accuracy.

Key features include renormalizability, gauge invariance, vacuum polarization, and the running of the electromagnetic coupling. The theory's predictions agree with experimental results to parts in 10^12 or better in many cases. As the quantum field theory of electromagnetism within the Standard Model, QED provides a paradigmatic framework for calculating electromagnetic processes in atomic, particle, and condensed-matter physics.