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.