= Quantum electrodynamics
{title2=$\mathrm{QED}$}
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= QED
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<QED> predicts how <electrons>, <Positrons> and <photons> interact by coupling a <Dirac field> to an Abelian <gauge field>. For a field of signed <electric charge> $q_e$, the interaction is $\mathcal L_{\rm int}=-q_e\bar\psi\gamma^\mu\psi A_\mu$; an <electron> has $q_e=-e$, $e>0$. Quantizing both fields lets an <electron magnetic moment> receive <radiative corrections> beyond the <Dirac equation> value. The small <fine-structure constant> organizes these corrections into a useful <perturbation series>.
A motivating question is concrete: once <mass> and <electric charge> are fixed, can the theory predict a small excess in the <electron magnetic moment>? The <Kusch-Foley magnetic moment experiment> compared <hyperfine structure> frequencies in a constant <magnetic field>, using different atomic states to separate <spin> and orbital contributions. Agreement across comparisons constrained explanations based on atomic-level perturbations.<The Magnetic Moment of the Electron by Polykarp Kusch and Henry Michael Foley (1948)>[Abstract: states, frequency ratios and perturbation control]{ref} The <Lamb-Retherford experiment> posed a complementary <energy level> problem. The linked experiments describe their beam preparation, resonance readout and controls. Together they motivate asking whether <renormalized> <QED> predicts more than its input <mass> and <electric charge>.
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