Electron magnetic moment

Dirac's 1928 equation predicted the electron's moment to be exactly one Bohr magneton — g exactly 2. In 1947 Polykarp Kusch and Henry Foley measured atomic hyperfine splittings carefully enough to show it was about 0.1% too big. That excess, the anomalous magnetic moment, is the electron interacting with the vacuum around it: virtual photons and electron-positron pairs dressing the bare particle. Julian Schwinger computed the first correction, α/2π, and had it carved on his headstone.

The moment is negative because the electron's charge is: its magnetic moment points opposite its spin. Modern Penning-trap measurements pin g/2 to about one part in 10¹³, and the corresponding QED calculation — thousands of Feynman diagrams out to five loops — agrees. It is the most precisely tested prediction in all of physics, and the reason any disagreement here would be front-page news.