The quantum field theory describing how light and matter interact, and a key part of the Standard Model.Quantum electrodynamics (QED) is the quantum field theory that describes how light and matter interact. It is the most precise and experimentally verified theory in physics, capable of predicting the properties of electrons and photons to an accuracy of one part in a trillion. QED is a cornerstone of the Standard Model of particle physics, unifying the electromagnetic force with quantum mechanics. It explains phenomena such as the Lamb shift (a tiny energy difference between two states of the hydrogen atom), the anomalous magnetic moment of the electron, and the scattering of light by light. QED also provides the framework for understanding how atoms emit and absorb photons, which is fundamental to spectroscopy, lasers, and all of modern optics. Its success has inspired the development of other quantum field theories, including quantum chromodynamics (QCD) for the strong force and the electroweak theory.
QED was developed in the late 1940s by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, who independently formulated the theory and shared the Nobel Prize in Physics in 1965. Their work built on earlier quantum mechanics and Dirac's relativistic equation for the electron, and it resolved the problem of infinite values that plagued earlier attempts at a quantum theory of electromagnetism.
QED treats particles like electrons and photons as excitations of underlying quantum fields. The interaction between them is mediated by the exchange of virtual photons—particles that exist only fleetingly as quantum fluctuations. The theory is formulated using Feynman diagrams, which represent the probability amplitudes for various interaction processes. Each diagram corresponds to a mathematical term in a perturbative expansion, and by summing an infinite number of these terms, physicists can calculate physical observables to extraordinary precision. A key feature of QED is renormalization, a mathematical procedure that handles the infinities that arise from self-interactions, allowing finite, measurable predictions to be extracted. The theory also predicts subtle effects like vacuum polarization, where virtual particle-antiparticle pairs screen the electric charge of an electron, and photon-photon scattering, which is forbidden in classical electrodynamics but allowed in QED.
QED is essential for the design of high-precision instruments such as atomic clocks, which rely on the accurate calculation of atomic energy levels, and for the development of quantum computing, where the manipulation of photons and electrons is governed by QED principles.
The prediction of the electron's anomalous magnetic moment using QED agrees with experiment to 10 significant figures, making it the most accurate prediction in all of science.
