quantum electrodynamics
/ KWON-tum ee-lek-troh-dy-NAM-iks /
Quantum electrodynamics, QED, is the quantum theory of light and matter, of how electrons, positrons and photons interact. It is the oldest and most thoroughly tested part of the Standard Model, and by any reasonable measure the most accurate theory in all of science. Everything electromagnetic that you experience, from the stiffness of a table to the color of a sunset, is QED in action. Its central picture is deceptively simple: charged particles interact by emitting and absorbing photons, and every electromagnetic process is a sum over all the ways that exchange can happen.
QED is the abelian gauge theory of the symmetry group U(1), with the photon as its single massless gauge boson and the electron charge (or equivalently the fine-structure constant alpha, about 1/137) as its coupling. Amplitudes are computed as a perturbation series in alpha, with each term drawn as a Feynman diagram: the more vertices, the higher the power of alpha and the smaller the contribution, so a few diagrams already give staggering accuracy. The infinities that appear in loop diagrams are tamed by renormalization, reabsorbing them into the measured values of mass and charge, a procedure worked out by Feynman, Schwinger and Tomonaga, who shared the 1965 Nobel Prize.
The showcase test is the electron's anomalous magnetic moment, the tiny deviation of its magnetism from the naive Dirac value. QED predicts it, and experiment confirms it, to about twelve significant figures, as if you measured the distance from New York to Los Angeles and agreed to the width of a human hair. QED is also the template on which the whole Standard Model is built: quantum chromodynamics and the electroweak theory are its more elaborate, non-abelian descendants. A caveat: those Feynman diagrams are bookkeeping for terms in a series, not literal photographs of an electron's trajectory.
The electron's magnetic moment is measured as g/2 = 1.001 159 652 180..., and QED, summing thousands of Feynman diagrams, reproduces it to a precision better than one part in a trillion, the most precise confrontation of theory and experiment ever achieved.
QED's prediction of the electron g-factor is the benchmark by which agreement between theory and experiment is measured.
The perturbation series of QED is believed to be asymptotic, not convergent; it gives fantastic results for the first many terms but would eventually diverge if summed to all orders, a reminder that even our best theory is an approximation scheme.