Lamb shift

The Lamb shift is the small difference between hydrogen’s 2S₁/₂ and 2P₁/₂ energy levels, which the Dirac equation predicts to be degenerate without radiative corrections.

Quantum electrodynamics: The quantum field theory describing interactions between electrically charged particles and the electromagnetic field. Radiative corrections in this theory lift the Dirac equation’s predicted degeneracy.
Willis Lamb: An American physicist who measured the hydrogen fine-structure levels that bear his name. His experiment established the anomalous separation between the 2S₁/₂ and 2P₁/₂ levels.
Hydrogen spectroscopy: The study of hydrogen’s quantized energy levels through its absorption and emission of radiation. Measurements of its spectral lines reveal the Lamb shift.
Quantum electrodynamics tests: Experiments that compare precise measurements with quantum electrodynamics’ quantitative predictions. The Lamb shift became an early, stringent benchmark for the theory.
Electron self-energy: A quantum correction to a particle’s energy arising from its interaction with its own field. This correction contributes to the electron’s changed energy in hydrogen’s 2S state.
Robert Retherford: An American physicist who collaborated with Willis Lamb on microwave spectroscopy of hydrogen. He helped perform the measurement that revealed the shift.
Hydrogen energy levels: The discrete allowed energies of an electron bound to a proton. The shift changes the relative energies of specific states, including 2S and 2P.
Anomalous magnetic dipole moment: The difference between a particle’s measured magnetic moment and its Dirac prediction. Like the Lamb shift, it tests radiative corrections in quantum electrodynamics.
Vacuum polarization: A quantum effect in which virtual particle pairs alter the electromagnetic field around a charge. It modifies the proton’s Coulomb field and contributes to the level difference.
Lamb–Retherford experiment: A 1947 microwave experiment that measured the separation of hydrogen’s 2S₁/₂ and 2P₁/₂ states. It supplied the experimental discrepancy that the Lamb shift names.

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