Willis Lamb Jr., who shared the 1955 Nobel Prize in Physics for the discovery of a slight and subtle discrepancy in the quantum theory describing how electrons behave in the hydrogen atom, died on Thursday in Tucson, Ariz. He was 94.
The cause was complications of a gallstone disorder, according to the University of Arizona, where Mr. Lamb was an emeritus professor of physics and optical sciences.
Although the discrepancy, which became known as the Lamb shift, in the hydrogen atom was slight, it was one of the first direct experimental signs that empty space is not empty. Instead, empty space roils with “virtual particles” that pop into and out of existence too quickly to be detected. The Lamb shift results from the virtual particles bumping into an electron orbiting in the hydrogen atom and altering its orbit slightly.
The discovery of the Lamb shift led to a rethinking of quantum mechanics and the development of quantum electrodynamics, which incorporated the virtual particles into the modern theory of electricity and magnetism.
Mr. Lamb’s research crossed many subjects in theoretical physics, including lasers, the scattering of neutrons off crystals and how to make the most precise measurements of objects or processes, given the intrinsic uncertainties of quantum mechanics.
His laser work, for instance, predicted another effect that bears his name, the Lamb-Bennett dip. The dip describes how the intensity of a laser drops under certain circumstances. It turned out that a colleague, William Bennett, had already observed that effect experimentally. The Lamb-Bennett dip has been used to set laser frequencies with great precision.
“Lamb would take the process apart in his mind,” said William Wing, a professor of physics and optical sciences at Arizona. “Often he would discover new effects by careful thinking at a very deep level.”
Mr. Lamb received a bachelor’s degree in chemistry from the University of California, Berkeley, in 1934 and a doctorate in theoretical physics, also from Berkeley, in 1938.
He then became an instructor and, later, a professor at Columbia. Mr. Lamb shifted universities several times in his career, to Stanford in 1951, to Oxford in 1956, to Yale in 1962 and to the University of Arizona in 1974. He retired in 2002.
It was in November 1955 that an early morning call from Stockholm announced that Mr. Lamb had won the Nobel. He went back to bed and slept two more hours.
He shared the prize, and the accompanying $36,720, with Polykarp Kusch, who discovered other effects of the virtual particles in a different experiment.
