In QED, scattering amplitudes are computed as a sum over Feynman diagrams. Each diagram contributes a term in the perturbation series in the coupling constant α ≈ 1/137. Fermion lines carry propagators i(p̸+m)/(p²−m²+iε), photon lines carry −igμν/(k²+iε), and each QED vertex contributes a factor −ieγμ.
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