In the Bohr model the energy levels of hydrogen are unique, yet observed spectral lines display a small splitting called fine structure. Using microwave resonance, Lamb measured the energy difference between the 2S1/2 and 2P1/2 levels with unprecedented accuracy and found that the gap (about 1058 MHz) did not match theoretical predictions. This discrepancy, later named the “Lamb shift,” was explained as a quantum electrodynamics (QED) effect where the electron interacts with virtual photons in the vacuum. Lamb’s results forced corrections to the Dirac equation and ushered in an era of particle physics and high-precision measurement. His spectroscopic techniques have since evolved into technologies such as atomic clocks and GPS that support everyday life.

The 2S1/2–2P1/2 Lamb shift in hydrogen can be expressed as the sum of the electron self-energy and vacuum-polarization terms originating from zero-point energy of the electromagnetic field. Lamb developed the microwave Lamb-Retherford resonance technique at a 20-cm wavelength and measured the transition frequency while controlling magnetic fields with high precision. The observed value agreed with the Dirac equation only when radiative corrections were included, thereby confirming the accuracy of QED calculations completed by Feynman, Schwinger and Tomonaga. The technique has been applied to other simple atomic systems such as Rydberg atoms, muonic hydrogen and antihydrogen, enabling stringent tests of energy levels. The resulting data help determine fundamental constants like the fine-structure constant and the proton-to-electron mass ratio, serving as benchmarks for the Standard Model. Coupled with frequency-comb technology and quantum information processing, high-precision spectroscopy now plays a key role in metrology. Lamb’s achievement thus bridges fundamental physics and cutting-edge measurement science.

From a metrological standpoint, Lamb exploited the metastability of the 2S state and built a Lamb–Retherford microwave resonance apparatus free of RF quenching. By homogenizing magnetic-field gradients and compensating Stark shifts he suppressed systematic errors to the MHz level and obtained 1057.77 ± 0.20 MHz. The value agreed with Schwinger’s one-loop self-energy calculation and allowed verification of higher-order terms such as the Uehling vacuum-polarization and Wichmann–Kroll corrections. The data were later used in determinations of α and as inputs to g-2 calculations, supporting the renormalizability of quantum field theory. Comparison with proton charge-radius extractions generated the “proton-radius puzzle,” an active topic seeking to reconcile spectroscopy and scattering measurements. Lamb’s methodology inspired frequency-comb stabilization, optical lattice clocks and EDM searches, providing the quantitative basis for precision experiments that probe physics beyond QED.