Nobel laureate Willis Eugene Lamb Jr died on 15 May 2008 in Tucson, Arizona. Willis was a towering figure in 20th-century physics and one of the last physicists to excel in both theory and experiment. Freeman Dyson, in his contribution to the 1977 Lamb Festschrift, wrote: “Dear Willis, You and Fermi were the only physicists I have known who stood in the top rank both as theorists and as experimenters.”
Willis was born in Los Angeles on 12 July 1913. The oldest of three boys, he had to cope with a severe eye condition all his life. As a teenager, he was a national-level chess champion and was torn between a career in chess or science. Fortunately, he chose science. As an undergraduate chemistry major at the University of California, Berkeley, he was advised by physical chemist Gilbert Lewis to go into experimental science. Lewis pointed out that if Willis ran out of ideas, he could always polish the glass in the lab. But Willis was not persuaded and decided to study theoretical physics under J. Robert Oppenheimer. Although he described his relationship with Oppenheimer as “strong,” it was not always easy. Willis recalled that once in the late 1930s, he was saying, “When the war comes….” Oppenheimer interrupted with, “There isn’t going to be a war, you d—fool!”
Willis’s thesis was on the electromagnetic properties of nuclei and on neutron capture. Although working with Oppenheimer was stimulating and rewarding, the two men never published together. They were intellectually orthogonal. Oppenheimer painted in broad strokes without involved calculations, while Willis reveled in detailed analysis and calculations.
As a graduate student, Willis showed an uncanny sense for choosing interesting problems and working through them carefully. He was the first to demonstrate recoilless emission of radiation from nuclei locked in a crystal lattice. Rudolf Mössbauer later discovered that effect experimentally, and Hans Frauenfelder dubbed it the Lamb–Mössbauer effect.
After finishing his PhD in 1938, Willis drove from Berkeley to New York with I. I. Rabi, who had just hired him as an instructor at Columbia University. But life at Columbia was less than ideal. A decade later Willis received tenure, but only as an assistant professor. According to Norman Ramsey, Rabi thought he was hiring the next Paul Dirac when in fact he was hiring a young Enrico Fermi.
During World War II, Willis worked on radar at Columbia. That expertise, together with his deep knowledge of quantum theory, put him in a good position to carry out his famous level-shift (that is, the Lamb shift) measurements in the hydrogen atom shortly after the war. In effect, he employed the atom as a laboratory to study the quantum properties of the vacuum. Thus he opened the field of modern quantum electrodynamics (QED). According to Dirac’s relativistic treatment of the hydrogen atom, the 2s and 2p levels should have the same energy. In fact, due to the virtual emission and reabsorption of photons, the 2s level is 1057.8 MHz above the 2p level, as measured in 1946 and 1947 by Willis and Robert Retherford.
The first theoretical calculation of the Lamb shift, by Hans Bethe, was based on a nonrelativistic treatment of the atom and yielded 1057.7 MHz. In 1949 Willis and Norman Kroll published the first fully relativistic QED calculation, which provided a much more satisfactory treatment of the problem.
The Lamb shift experiment was a turning point in modern physics. It stimulated people such as Julian Schwinger and Richard Feynman to develop relativistic QED and modern renormalization theory.
Willis described to his friends his first meeting with Dirac, in which Dirac congratulated him on his Lamb shift work. Willis responded, “I would rather have discovered the Dirac equation,” to which Dirac replied, “Such things were easier back then.”
I once asked Willis, “What did Oppenheimer say to you when you won the Nobel Prize?” His answer was, “He didn’t say anything. The first interaction I had with him after the prize was a chance encounter at Columbia in which he said, ‘Aren’t you sorry you didn’t come to Los Alamos?’ I replied, ‘No, because if I’d gone to Los Alamos, I wouldn’t have learned about hydrogen.’” Willis said he thought the reason Oppenheimer failed to write a congratulatory note was because Willis had not sent a supporting note when the House Committee on Un-American Activities was investigating Oppenheimer.
After spending his first golden era (1945–50) at Columbia, Willis moved to Stanford University. However, Willis was not totally happy there and decided to move on. He held a Morris Loeb Lectureship at Harvard University in 1953–54. In 1955, the year he won the Nobel Prize in Physics, he was lured to Oxford University, where he was appointed to the prestigious Wykeham Chair as professor of theoretical physics.
Willis enjoyed Oxford, and it was there that he developed his classic laser theory paper, published in Physical Review in 1964 and cited some 1500 times. However, his productivity was limited because he had no graduate students and was not able to do experiments.
In the early 1960s, when Yale University offered him the opportunity to get back into the laboratory, Willis accepted. His years at Yale constituted his second golden era. He had many students and produced superb papers in theoretical and experimental physics, especially on nonlinear effects in laser physics and laser spectroscopy.
For example, saturation nonlinearities in the atomic gain medium lead to a dip—known as the Lamb dip—in the power output of a helium–neon laser. The width of the dip is governed by lifetime broadening, which is much narrower than the Doppler broadening that characterizes the gain profile. That lifetime broadening is the basis for Lamb dip spectroscopy. Willis would quip that he was more famous for the Lamb dip than he was for the Lamb shift.
Another subtle aspect of nonlinear laser physics arises in the quantum theory of the laser, in which both the atoms and the light are treated quantum mechanically. The correct laser photon statistics can be calculated only when the saturation nonlinearities are included in the theory. Those nonlinearities play an important role in the analogy between the laser and the Bose–Einstein condensate.
Willis cared deeply for his students, as is illustrated in the following anecdote. On a visit to our New Mexico ranch, Willis, my 14-year-old son Steven Willis Scully, and I were having breakfast. Willis was not a morning person, so Steven tried to start a conversation by asking, “How’s the kids?” Willis replied, “I do not have any children, Steve. My graduate students are my children.”
Willis was an excellent adviser. For example, after we agreed on a problem, I would work it as many ways as I could, often generating pages of calculations. Then Willis would spend long hours going over the results and hammering out the physics.
In the early 1970s, the Optical Sciences Center at the University of Arizona was growing rapidly, and we convinced Willis to join the center in 1973. There, Willis worked mostly with postdocs and faculty but had no graduate students. His main interests were in fundamental aspects of quantum and statistical physics, computational physics, and some aspects of the philosophy of physics. He was active in research well into his nineties.
Willis is remembered and celebrated by everyone who knew him and by the large community of physicists whose science was advanced by his contributions.
