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“Dirac's equation not only accounted for the spin of the electron and its observed magnetic moment, but also correctly explained the fine structure of the hydrogen atom. If the derivation of the Sommerfeld-like formula for the spectrum of the hydrogen atom was one of the striking successes of the Dirac equation, some of its other features were very troublesome.” — Silvan S. Schweber
Dirac's equation not only accounted for the spin of the electron and its observed magnetic moment, but also correctly explained the fine structure of the hydrogen atom. If the derivation of the Sommerfeld-like formula for the spectrum of the hydrogen atom was one of the striking successes of the Dirac equation, some of its other features were very troublesome.