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In 1947, Willis Eugene Lamb Jr. and Robert Curtis Retherford dissociated hydrogen in a tungsten oven, crossed its atomic jet with electrons for collisional excitation, and detected surviving $2S$ atoms by electron ejection from metal. An electrometer and galvanometer read the current. They interrupted fixed-frequency microwaves while sweeping the static magnetic field; a flip coil calibrated the field. Their Zeeman splitting comparison placed $2S_{1/2}$ about $1000$ $\mathrm{MHz}$ above $2P_{1/2}$; the plotted $1000$ $\mathrm{MHz}$ displacement was explicitly not a best fit.^[ref]

The 1955 lecture's apparatus cross-section identifies movable slits, an electron bombarder, a radiofrequency transmission line with beam slots, a d.c. quenching electrode, metal target, electron collector, and magnet poles. A $2S\to 2P$ transition followed by rapid spontaneous emission leaves a ground-state atom unable to eject detector electrons. The scan holds radiofrequency and amplitude fixed. Natural linewidth and unresolved hyperfine structure broaden resonances, whose analysis yields zero-field separations.^[ref]

Static electric field quenching supplied a control. The 1947 report identifies natural linewidth, hyperfine structure, excessive microwave intensity, and field inhomogeneity as width contributions, and flags imperfect magnetic calibration. It does not specify an absolute microwave-frequency calibration procedure.^[ref]

The surname is Robert Curtis Retherford's: “Retherford” and “Rutherford” name different people. The historical Lamb-Retherford experiment should retain the former spelling.^[ref]

The Lamb shift is a radiative displacement of atomic energy levels beyond the values from the Dirac equation in a Coulomb potential. In hydrogen, the $2S_{1/2}$ and $2P_{1/2}$ levels are degenerate for a point Coulomb potential in the Dirac equation, yet the $2S_{1/2}$ level lies higher by roughly a gigahertz. This is distinct from the fine structure splitting between different $J$ levels and from nuclear-spin hyperfine structure.

Electron self-energy in the quantized electromagnetic field is the dominant contribution to the familiar hydrogen splitting; vacuum polarization contributes too. Renormalization separates the physical electron mass from its divergent free-electron self-energy. A useful leading-log scale for a hydrogenic $nS$ level isnot an exact numerical formula. The enhanced sensitivity of S states follows from their nonzero wavefunction at the nucleus; leading short-distance corrections act like contact perturbations. A precision prediction also includes finite nuclear size and recoil, and must consistently separate those effects from the radiative contribution.^[ref]

The original Lamb-Retherford experiment used a beam of hydrogen atoms, electron-impact excitation into metastable $2S$, and microwave-induced transfer into short-lived $2P$ states. Those atoms decayed before reaching the detector. Metastable atoms surviving to a metal target ejected electrons, so a drop in the measured current signalled a resonance. Varying the microwave excitation and magnetic field established the displacement from the Dirac equation expectation. Microwave quenching converted a small energy difference into a measurable loss of a long-lived population, avoiding optical Doppler widths that obscure the splitting.^[ref]

In the ideal point-nucleus Dirac-Coulomb problem, hydrogen levels depend on $n$ and $j$, so $2S_{1/2}$ and $2P_{1/2}$ are degenerate despite their different orbital angular momenta. The Lamb shift removes this degeneracy: the $2S_{1/2}$ level lies about $h\times 1.06\mathrm{GHz}$ above $2P_{1/2}$, an energy of roughly $4.4\mu \mathrm{eV}$. This is distinct from the much larger $2P_{3/2}-2P_{1/2}$ fine-structure splitting.

quantum electrodynamics changes the electron's interaction with the Coulomb field. electron self-energy and vacuum polarization give state-dependent radiative corrections; a complete prediction also keeps specified recoil and nuclear-size contributions. An $S$ wave samples the nucleus differently from a $P$ wave, so equal $n,j$ no longer ensures equal energy. A leading-log estimate for an $nS$ radiative contribution scales as $\alpha (Z\alpha {)}^4{m}_e{c}^2\ln [(Z\alpha {)}^{-2}]/n^3$, with a numerical prefactor and further state-dependent terms. This is a scale estimate, not a replacement for renormalized bound-state QED. The measurable shift is a difference relative to a specified reference Hamiltonian, rather than an unregulated absolute vacuum energy.^[ref]

The original microwave experiment used a tungsten oven to dissociate hydrogen, an electron beam to populate metastable $2S$ atoms, and electron ejection from a metal target to measure surviving metastable atoms. Microwaves transferred population to rapidly decaying $2P$, reducing detector current. At a fixed microwave frequency, interruption of the radiation and a scan of static magnetic field revealed Zeeman-tuned resonances. Comparing resonance locations with the Dirac-level prediction showed the $S$ level shifted upward. Field calibration and homogeneity, excessive microwave intensity, unresolved hyperfine structure and stray-electric-field quenching limit the measurement; the signal is not direct absorption by a large equilibrium excited population.^[ref]

An experimental atomic physicist who, with Willis Eugene Lamb Jr., measured the Lamb shift in 1947. Their microwave method distinguished hydrogen levels that the simplest relativistic theory placed at the same energy. The experiment made a small spectral discrepancy into a precise test, helping motivate improved calculations in quantum electrodynamics.^[ref]

Willis Lamb and Robert Curtis Retherford measured a small separation between hydrogen energy levels that the simplest relativistic quantum mechanics placed at equal energy. Their 1947 experiment used microwave transitions involving metastable atoms, rather than trying to resolve the separation in an ordinary optical line. The Lamb shift required improved quantum electrodynamics, including the interaction of an electron with the quantized electromagnetic field. A small spectral discrepancy thereby became a quantitative test of the theory.^[ref]

A physicist.^[ref][ref]