Philip W. Anderson (1923–2020)

Philip Anderson, groundbreaking physicist, died on 29 March at the age of 96. During his prolific career, Anderson launched many major branches of condensed matter physics. He excelled in extracting deep, foundational principles from raw experimental data and designed models that elegantly captured the essence of seemingly intractable quantum phenomena. Anderson was born on 13 December 1923 in Indianapolis, Indiana. After obtaining his Ph.D. in physics from Harvard University in 1949 under the guidance of John H. Van Vleck, Anderson joined Bell Labs (where P.A.L. met him). In 1975, he joined the faculty at Princeton University (where he met N.P.O.) and remained there until his retirement in 1996. Anderson had an uncanny ability to sniff out the deep questions raised by experimental data. In the late 1950s, experiments on silicon had found that the diffusion of a spin wave packet became anomalously slow at a temperature of 4 K. Anderson theorized that this “localization” (which he initially called “cisport,” a wordplay on “transport”) resulted from coherent interference of multiple reflections of the wave packet in a disordered crystalline environment. This 1958 prediction, dubbed “Anderson localization,” was cited in his 1977 Nobel Prize. In the 1980s, Anderson localization blossomed into a major experimental industry, and it has since been applied to optics, astronomy, ultracold atoms, and tumor detection in mammography. Anderson's work in the 1950s also laid the foundation for the modern theory of magnetism by explaining how electron spins form local magnetic moments. This is an example of broken symmetry, where a system has less symmetry than suggested by basic physical laws. In a 1972 article in Science titled “More is different,” Anderson expanded on this concept, emphasizing that knowing the laws of physics at the microscopic level is not sufficient for understanding nature on a macroscopic scale. Anderson resolved another fundamental problem soon after physicists John Bardeen, Leon Cooper, and John Robert Schrieffer published the theory of superconductivity in 1957. The symmetry breaking that was identified was expected to lead to a zero-mass mode, which was not seen in a superconductor. Anderson resolved this puzzle by pointing out that, in a superconductor, the wave function is coupled to the electromagnetic field, which merges with the zero-mass mode to become the familiar plasma oscillations. The photon has acquired a mass inside the superconductor. Anderson recognized that this mechanism solved the important problem of unwanted massless particles in theories hotly pursued at the time to unify quantum electrodynamics and weak interactions. Physicist Peter Higgs and others carried out the fully relativistic formulation, but Higgs gave Anderson credit in his paper and Nobel lecture. The Anderson-Higgs mechanism is now a cornerstone of particle and condensed matter physics. Shortly after the discovery in 1986 of high-temperature (high- T c) superconductivity in cuprates, Anderson proposed a radical theory. He suggested that the parent state of the high- T c superconductor is a magnetic insulator driven by strong electron repulsion. Removing some of the electrons by chemical substitution allows the vacancies (“holes”) to move freely and conduct current. Anderson's identification of the parent state is now universally accepted. To explain why the mobile holes pair up to realize superconductivity, Anderson invoked the physics of spin liquids, which he pioneered in 1973. In a spin liquid, spins fail to attain the antiferromagnetic state because of conflicting lattice constraints and quantum fluctuations. Anderson called this the resonating valence bond (RVB) state. He proposed that when carriers are introduced into this state, it becomes a superconductor. These revolutionary ideas encountered considerable resistance. Today, although Anderson's specific mechanism remains controversial, many of the ideas in his 1987 paper, such as superconductivity arising from strong repulsion, have gained wide acceptance. The RVB state is the archetypal example of a quantum spin liquid, currently a topic of intense interest. Another idea of great importance is that the excitations of the spin liquid behave as electrons that have lost their charge but retain their spin. This early example of the notion of fractionalization is supported by exactly soluble models as well as by recent experiments. In time, Anderson's spin liquid RVB theory may well be remembered as his most profound and prescient. Despite being a self-professed curmudgeon, Anderson was compassionate and amazingly loyal to friends and colleagues. Former students who had hit a rough patch often moved back to Princeton to work with him until they regained their footing. After learning that a collaborator had suffered a stroke, Anderson flew to stay with him for a week. He had a puckish sense of humor. During dinners that he and his wife, Joyce, regularly hosted, Anderson, offering coffee, would ask each guest “caffeine or non?” He would then prepare a single pot mixed with the correct proportions of each. To our amusement, he and Joyce took great pleasure in belting out songs by Tom Lehrer (a friend from college). Both held strong antiestablishment convictions. At the height of the Vietnam War, Anderson was once detained by security for posting “Stop the Bombing” pamphlets at Bell Labs. He was a vocal opponent of the Strategic Defense Initiative. Well into his 90s, Anderson retained a vibrant curiosity. He wanted to know who Dr. Dre was (a frequent clue in the New York Times crossword puzzle) and wanted to hear “Desolation Row” after Bob Dylan's Nobel Prize win. Anderson's expansive intellect, his passion, and the special insights he brought to everything he touched will be sorely missed. His passing marks the loss of the last of the intellectual giants who shaped the field of quantum matter.