A Note on the Electron’s Magnetic Moment

Sebens (2025) has proposed a semiclassical “precursor” to quantum electrodynamics (QED) in which the electron’s anomalous magnetic moment arises from the self-interaction of an extended charge distribution governed by the Dirac equation. The calculation reproduces Schwinger’s leading-order value only for suitably tuned, spatially extended wave-packets, and thus yields a state-dependent magnetic moment. This paper offers a systematic critique of that result. After reviewing the standard QED derivation—where the anomaly is fixed by gauge symmetry, the Ward–Takahashi identity, and renormalization—we show that the semiclassical model lacks the structural resources that guarantee universality. Drawing on a general distinction between phenomenological dependence and theoretical fundamentality, we argue that Sebens’s construction attains intuitive, mechanical appeal at the cost of explanatory depth: its high phenomenologicality cannot compensate for its low fundamentality. What Sebens treats as a puzzle for QED—how the theory “nails down” a single value of \(g-2\)—is instead a symptom of the precursor’s incompleteness. The episode illustrates a broader methodological point: in modern physics, structural principles, rather than classical pictures, underwrite genuine explanation.