Electron g-factor

Dirac's 1928 equation predicted g = 2 exactly, a triumph over the classical expectation of 1. Then in 1947 Kusch and Foley measured a value about 0.1 % larger, and Schwinger explained the excess within a year: the electron is dressed in virtual photons, and the leading correction is α/2π = 0.001 161. That anomaly, a = (|g| − 2)/2, launched quantum electrodynamics.

Today g_e is measured with a single electron in a Penning trap to 13 significant figures and computed from QED to five loops — thousands of Feynman diagrams — and the two agree. It is the sharpest confrontation between theory and experiment humans have ever staged, and it doubles as the best independent determination of α. The minus sign is real: the electron's magnetic moment points opposite its spin because its charge is negative, and many texts quietly quote |g_e| = 2.002 319 instead.