NATURE|Vol 453|12 June 2008 NEWS & VIEWS

OBITUARY were a case in point. The first microwave- frequency equivalent of the laser, the maser, had been constructed by Charles Townes Willis E. Lamb Jr (1913–2008) and collaborators at Columbia in 1953. But Meticulous physicist and discoverer of the Lamb shift. like others working on the extension of the maser to higher, optical frequencies, Lamb was convinced that a closed cavity, such as Willis Lamb, who died on 15 May aged 94, larger radii (where the 2p state dominates). that in which light was amplified in a maser, received his highest recognition in 1955, Today, precise measurements of the Lamb would be needed to create a laser. It was left to when he was awarded one half of the Nobel shift have tested QED to an accuracy of better Gordon Gould to predict that an open cavity Prize in Physics for his discovery of the than one part in a million. could lase, and for Theodore Maiman to subtle quantum-mechanical shift in energy Lamb’s disposition made him entirely implement the idea in 1960. levels of the hydrogen atom that now bears suited to the difficult measurements of this Lamb’s part was nevertheless substantial. his name. But in truth, he might have been tiny shift. A perfectionist, his approach to His theory of laser action (W. E. Lamb Jr Phys. nominated twice more. He produced a physics embodied the principle that physical Rev. 134, A1429–A1450; 1964) predicted theory of the resonant absorption of γ-rays intuition is a necessary, but not sufficient, the ‘Lamb dip’ in the output intensity of a gas by solid-state atoms — the Mößbauer effect condition for success. A sound conclusion laser. This spectral feature is a consequence — 19 years before Rudolf Mößbauer himself demanded a solid calculation or experiment. of the Doppler effect, occurring when, published the observations that won him That perfectionism extended to Lamb’s owing to thermal motions of the gas, an a share of the 1961 prize. And in the early writing: he would spend considerable time atomic absorption line becomes smeared 1960s, Lamb developed one of the first, revising papers, or having his students revise out in frequency. In this case, two oppositely semi-classical theories of how directed light waves will lasers work. interact with atoms of the same Lamb was born on 12 July velocity if tuned to the centre 1913 in Los Angeles, and as an of the Doppler-broadened line, undergraduate at the University but with atoms of different of California, Berkeley, studied velocities when tuned off- BETTMANN/CORBIS chemistry. But he became resonance. The laser gain increasingly fascinated with thus saturates at a lower physics, and his presaging of intensity on resonance. This the Mößbauer effect was part of Lamb dip in lasers, and its a doctoral dissertation on the counterpart in absorptive electromagnetic properties of media, enabled the creation of nuclear systems, completed in incredibly stable devices that 1938 under the guidance of redefined the way we measure J. Robert Oppenheimer. time and distance. Unlike many of his generation The fact that Lamb’s 1964 of physicists, Lamb did not laser theory was semi-classical follow Oppenheimer into the was again entirely typical: wartime atom-bomb project. Lamb was fascinated by the Instead, he concentrated on his them, pondering long and hard over subtle extent to which nature was necessarily specialisms — microwaves and radar refinements such as the insertion of a comma. quantum-mechanical. In particular, he — at Columbia University in New York, That honed his students’ ability to write insisted that the original photoelectric performing the experiments that and led to unusually readable papers. But effect — the proof of whose quantum nature culminated in the observation of the Lamb his meticulousness was never overbearing, had garnered Albert Einstein his Nobel shift (W. E. Lamb Jr and R. C. Retherford and was always tempered by a wry, self- in 1921 — could be accounted for using a Phys. Rev. 72, 241–243; 1947). deprecating sense of humour. He was an classical electromagnetic field. He had little This shift is a tiny difference in energy outstanding supervisor, inspiring many patience for those who misused the term between two atomic orbitals in hydrogen, students and postdocs with sharp insight ‘photon’, particularly in situations where a denoted 2s and 2p, distinguished only by and sure guidance. I recall trying to convince classical electromagnetic wave would suffice, their angular momenta. Quantum theories him to take me on as a graduate student, even once saying that he should be given of the time predicted that these levels should presumptuously claiming expertise in his responsibility for issuing licences for its use. have identical energies. The discovery that field of interest. His response was that he was Tall and handsome, at first sight Willis they did not, demanded a fundamental always interested in expertise, as he had so Lamb seemed more Hollywood actor or theoretical rethink — one that was initiated little of his own. It was the start of a long and high-flying businessman than scientist. But almost immediately by Hans Bethe. fertile collaboration on laser theory. his dedication to physics was complete, and The Lamb shift thus became a In his later career, Lamb’s refusal to did not diminish in a 65-year career that cornerstone of the modern edifice of generalize conclusions purely on intuition encompassed Bell Labs (as a consultant on quantum electrodynamics (QED). This, the prevented him on occasion from publishing laser theory) and Stanford, Harvard, Oxford quantum field theory of the electromagnetic important work that didn’t meet his own high and Yale universities. He retired from the interaction, explains the shift as resulting standards. Perfection for him was a quality to faculty of the University of Arizona only in from energy fluctuations in the vacuum that be strived for, and perhaps not to be attained. 2002, at the age of 89. smear out the position of the electron in a He once told me that he regretted having Murray Sargent hydrogen atom. This process has a greater been so close to so many great discoveries Murray Sargent is at the Microsoft Corporation, effect on the Coulomb energy of the electron’s without realizing it. I replied that I wished I 1 Microsoft Way, Redmond, Washington 98052, binding to the central proton at smaller radii had that problem. USA. (where the 2s state is most likely) than at Lamb’s contributions to laser physics e-mail: [email protected]

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