Schwinger's α/2π

Plate 55 speaks for itself: “Small extra spin.”

The Physics

Dirac's theory gives the electron g = 2. Quantum electrodynamics adds a small correction; Schwinger's first-order term is α/2π.

a = (g − 2)/2 ≈ α/2π

• a: anomalous magnetic moment, dimensionless
• g: g-factor, dimensionless
• α: fine-structure constant ≈ 1/137.036, dimensionless

Dirac g = 2; measured: slightly more. Schwinger 1948: a = α/2π ≈ 0.0011614.

Honest caveat: α/2π is the leading term; the measured a_e = 0.00115965218… agrees once higher-order QED terms are included.

Sources

• J. Schwinger, Phys. Rev. 73, 416 (1948).
• X. Fan et al., Phys. Rev. Lett. 130, 071801 (2023).

On the poster

Plate 055, “Schwinger's α/2π”: the pixel-art plate with its slogan “Small extra spin.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.