Murray Gell-Mann (1929─2019)

Murray Gell-Mann, one of the greatest physicists of the 20th century, died on 24 May in Santa Fe, New Mexico. He was 89. Endowed with a photographic memory, Gell-Mann was an intellectual powerhouse who coupled insatiable curiosity with a Humboldtian urge to explain everything on his horizon. He came close to doing just that in elementary particle physics, a field he led for more than a quarter of a century, a feat unequaled in modern times. Gell-Mann was born in New York City on 15 September 1929. As a prodigy, he grew up out of sync with his peers, and was much affected by his father's remark that his birth, a month before the great stock market crash, came at an inconvenient time. At 15 years old, he began his freshman year at Yale University. Although he preferred archeology, linguistics, and biology, he heeded his father's suggestion and graduated with a B.S. in physics in 1948. He then joined the Massachusetts Institute of Technology's graduate school as physicist Victor Weisskopf's assistant and earned his Ph.D. in physics in just 3 years. Gell-Mann spent a year at the Institute for Advanced Study in Princeton, New Jersey, and a short time as an instructor at the University of Chicago. In 1955, he became a professor at the California Institute of Technology (Caltech), where he remained until his retirement from particle physics. Gell-Mann then embarked on a second career. Returning to his childhood interests in linguistics, ecology, and archeology, he cofounded the Santa Fe Institute, a research center devoted to studying correlations and deciphering principles behind apparently independent complex systems. At the time Gell-Mann graduated, the structure of matter seemed understood: The periodic table described matter as combinations of atoms, consisting of heavy nuclei of electrically neutral neutrons and positively charged protons, surrounded by clouds of negative electrons. Nuclear forces treat neutrons and protons equally, resulting in the isospin symmetry. However, new particles were being discovered, first in cosmic rays, and then by particle accelerators of ever-increasing energy. All were unstable, but some decayed very fast, and others stayed around much longer. Observing that “nature is most easily described by a sequence of approximations,” Gell-Mann found that when weak and electromagnetic interactions were “turned off,” simplicity emerges from chaos: A judicious isospin assignment to the new particles is enough to forbid some rapid decays mediated by the (strong) nuclear force. Gell-Mann then identified a universal connection between the electric charges and the isospin values of the new particles, by introducing a new attribute he coined “strangeness.” In 1961, Gell-Mann showed that all strongly interacting particles fit into representations of the Lie group SU(3). The proton and neutron, together with six of the new particles, form an octet representation. ![][1] PHOTO: SCIENCE HISTORY IMAGES/ALAMY STOCK PHOTO With style and panache, Gell-Mann named his scheme “The Eightfold Way,” in reference to the Buddha's teachings. Of the other unstable particles, some assemble in an octet, and the rest seem to fit in a decuplet: Consistency with the Eightfold Way requires a tenth particle. An experiment at Brookhaven National Laboratory soon discovered the tenth particle at the exact mass that Gell-Mann had predicted. For these contributions, Gell-Mann was awarded the 1969 Nobel Prize in Physics. Gell-Mann then further categorized the particles, inspired by the mathematics of group theory: The octet and decuplet are compound representations, built out of products of triplet and antitriplet representations of the group SU(3). He identified the triplet representation as one state (u) with fractional charge two-thirds and two states (d,s) with charge minus one-third. He called them quarks. A proton and neutron were each a bound state of three quarks. He noted that the currents made out of quarks had the same algebraic properties as the V-A currents, in which weak interactions appear as the product of the difference of vectors and axial vectors currents, written in terms of the observed particles. Quarks acquired reality with extra attributes that Gell-Mann called color, and he named the theory that glues them together “quantum chromodynamics.” Several of the conceptual advances were not Gell-Mann's alone (George Zweig posited the same constituents, which he called “aces”), but no one contributed more to our modern view of the fundamental interactions. The scope of Gell-Mann's contributions did not stop there. With his colleague Richard Feynman, he wrote the seminal V-A theory. With Francis Low, he invented the renormalization group that expresses the scale dependence of couplings in quantum field theory. With Maurice Levy, he introduced the nonlinear sigma model for the effective description of strong interactions. With Abraham Pais, in a paper he called “the hardest I ever wrote,” he analyzed the neutral kaon duo as an entangled quantum mechanical system. All of these papers found relevance in later years, in particular by inspiring Bruno Pontecorvo to introduce quantum oscillations in neutrino physics. Murray was a great partner in one-on-one conversations, although in groups some found him overbearing. He and I often worked together. As I scribbled furiously on the blackboard, I would occasionally stop, suddenly aware of a mistake. “Oh good,” Murray would say from his desk, “for a moment I thought you were faster than me!” We often worked at the Aspen Center for Physics, which Murray's intellectual charisma had established as a premier place to do physics. Murray's perfect recall was amazing. When shown a picture of the Lascaux Paleolithic cave, which he had visited 51 years earlier, Murray immediately declared “It is not Lascaux! There is only one human drawing at Lascaux, and that is not the one.” Of course, he was right. Murray's greatest legacy was to show the central role of symmetry in the universe of the very small, hinting of simplicity at its origin. Later in life, he applied his belief that the natural world is determined by organizational principles to linguistics and ecology at the Santa Fe Institute. At my last visit to Murray, I mentioned his enormous footprints in establishing the standard model of particle physics and fostering new symmetry-based approaches to fundamental physics. He replied, with a twinkle in his eyes, “Bigfoot.” [1]: /embed/graphic-1.gif