Philip Anderson, the theoretical physicist whose ideas reshaped condensed matter physics and stretched to the forefront of other fields, died yesterday in Princeton, New Jersey. He was 96. Anderson had spent the past 45 years at Princeton University, which confirmed his death in a statement.
Feisty and cantankerous, Anderson made contributions that rival those of famed American theorist Richard Feynman, who died in 1988, says Michael Norman, a theorist at Argonne National Laboratory: “Phil was a true giant of physics, one of the greatest ever.”
Anderson established himself in the 1950s by showing how disorder in the arrangement of atoms in a crystal could trap otherwise free-flowing electrons in a definite spot, a quantum effect called Anderson localization, for which he shared the 1977 Nobel Prize in Physics. The phenomenon is much deeper than it may sound, as it requires the quantum wave of the electron to overlap and interfere with itself to keep it from spreading.
Around the same time, Anderson deciphered materials known as antiferromagnets, which are a weird riff on more common magnetic materials called ferromagnets. In a ferromagnet such as iron, all the atoms act like little magnets and they all point in the same direction to magnetize the entire material. In an antiferromagnet, such as chromium, neighboring atoms point in opposite directions to form an up-down-up-down pattern.
At the time, that pattern vexed physicists. That was because, on very general quantum principles, they could think of no interaction among the magnetic atoms that would have the right symmetry to produce that pattern. However, Anderson employed a concept call spontaneous symmetry breaking to argue that point was irrelevant. He showed that a material could have a lowest energy ground state that featured the pattern even if the interactions did not explicitly encode it. In essence, the symmetry of the interaction is broken by the ground state.
In the early 1960s, Anderson used the concept of spontaneous symmetry breaking to explain why a superconductor—a material that will carry electricity without resistance if cooled sufficiently close to absolute zero—expels a magnetic field. In doing so, he showed that a photon would become massive inside a superconductor. Just 1 year later, British theorist Peter Higgs fleshed out that idea in a bit of theory that ultimately has become particle theorists’ explanation of how all fundamental particles get their mass from interactions with the vacuum. (Yes, the theory posits that the vacuum is in some very abstract way like the inside of superconductor.) Thus, Anderson came within just a few steps of inventing the Higgs mechanism and the particle that goes with it, the Higgs boson, says Piers Coleman, a theorist at Rutgers University, New Brunswick.
Later, Anderson claimed to have solved another mystery: high-temperature superconductors. In the late 1980s experimenters discovered a class of complex materials that contains copper and oxygen and can superconduct at temperatures far above those predicted by the conventional theory of superconductivity. Anderson quickly proposed his own theory, called the resonating valence bond theory, which he claimed explained the phenomenon. However, others found the idea unconvincing—one prominent theorist dubbed it “rather vague bullshit”—and the puzzle of high-temperature superconductivity remains unresolved to this day.
Although Anderson’s efforts stretched over many fields, they shared a common conceptual foundation, Coleman says. In the mid-1900s, many physicists employed an extreme reductionist approach that assumed a problem was solved once a system’s most fundamental constituent had been identified and their interactions characterized, a tack exemplified in particle physics. In contrast, Anderson expounded the concept of emergence, which stated that as any system grew larger, new phenomena—such as antiferromagnetism and superconductivity—could emerge that could not be predicted from the fundamental interactions. “You have to see him as having made these tremendous scientific contributions, but also having this philosophical point of view that was tremendously powerful,” Coleman says.
Over his long career, Anderson earned a reputation for being combative and, at times, making scientific disputes personal. “He was not afraid of a fight, even when he was wrong,” Norman says. That approach likely grew out of Anderson’s years at the famed Bell Labs, where Anderson worked from 1949 to 1984 and where a culture of brutal honesty and combativeness reigned. Norman recalls a particularly sharp barb Anderson threw one evening. “We went to dinner and somebody made the mistake of asking Phil what he thought of his theory,” Norman says. “Phil just looked at him and said, ‘Not much.’”
But Anderson was also kind to his students and collaborators, says Coleman, who was Anderson’s graduate student from 1980–84. “He was extremely sweet with his students and pushed very hard for them.”