qed

QED most commonly refers to Quantum Electrodynamics, the quantum field theory of electromagnetic interactions. It unites quantum mechanics with special relativity and treats the electromagnetic field as a quantized entity. Charged fermions interact by exchanging photons; the theory is based on a U(1) gauge symmetry and is described by a renormalizable Lagrangian that yields finite, testable predictions.

History and development: QED was developed in the 1940s by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga in equivalent formalisms. It resolved divergences that troubled earlier theories and produced precise results such as the Lamb shift and the electron’s anomalous magnetic moment, which agree with experiments to extraordinary accuracy.

Methods and predictions: The theory employs perturbation theory in the small coupling constant α ≈ 1/137. Feynman diagrams provide a practical way to organize contributions from interactions, including vacuum polarization and self-energy corrections. QED’s predictions match experimental measurements across a wide range of energies, enabling precise determinations of the fine-structure constant.

Status: QED is one of the most precisely tested theories in physics and a cornerstone of the Standard Model. It describes electromagnetic processes with exceptional accuracy but does not incorporate gravity or the weak and strong forces, which are described by the electroweak theory and quantum chromodynamics, respectively.

Other uses: Q.E.D., or quod erat demonstrandum, is a Latin phrase traditionally used at the end of mathematical proofs.