S-matrix theory

In this interview, Oscar Wallace (Wally) Greenberg recalls his experiences growing up in New Jersey as the child of Jewish immigrants from Eastern Europe and his accelerated education at Rutgers University and Princeton University, where his advisor was Arthur Wightman. He discusses his dissertation called “The Asymptotic Condition in Quantum Field Theory,” postdocs at Brandeis with S. S. Schweber and at MIT with Francis Low, and early work on high-energy limits and the general structure of quantum field theory. He reflects on his landmark proposal that quarks have a three-valued charge, later called color, as well as the delayed acceptance of the idea, his prediction of later measurements of the excited states of baryons, and his propensity not to promote his contributions. Greenberg also discusses his acceptance of a position at the University of Maryland, where he would spend most of his career, as well as visiting appointments elsewhere, and he offers anecdotes about his interactions with J. Robert Oppenheimer and Albert Einstein at the Institute for Advanced Study. The interview concludes with discussions of what remains unknown in particle physics and of cosmology as a “laboratory” with particle energies not available on Earth. A technical addendum to the interview lists 24 of Greenberg’s key contributions to physics.

Interview with David Gross, Chancellor’s Chair Professor of Physics at University of California in Santa Barbara and a permanent member of the Kavli Institute of Theoretical Physics (KITP). Gross begins by describing his childhood in Arlington, Virginia and his family’s later move to Israel. This led to his decision to enroll at the Hebrew University of Jerusalem for his undergraduate studies in physics and mathematics. Gross recalls his acceptance at Berkeley for his graduate studies, where Geoffrey Chew became his advisor. He explains his early interests in strong interactions, quantum field theory, and S-matrix theory. Gross then describes taking a fellowship at Harvard after completing his PhD, where he recalls his early involvement in string theory. He speaks about his subsequent move to join the faculty at Princeton, as well as his introduction to Frank Wilczek, one of his first graduate students with whom he later shared the Nobel Prize. Gross takes us through the discovery of asymptotic freedom, the development of quantum chromodynamics, and the impact these had on the Standard Model. He discusses his decision to leave Princeton for UCSB, where he focused on growing the KITP and securing funding. Gross describes how his research interests have shifted over the years across topics such as confinement, quantum gravity, and more recently back to string theory. Toward the end of the interview, Gross speaks about his work to develop institutes similar to KITP in other countries, as well as his term as President of the American Physical Society in 2019.

Interview with Toichiro Kinoshita, a Japanese-born physicist who is best known for pioneering the value of muon g-2, the anomalous magnetic moment of the muon. Kinoshita describes his education—Daiichi High School, Tokyo University—how he avoided military service during World War II, and meeting and marrying his wife, Masako Matsuoka. He describes his introduction to quantum electrodynamics and renormalization through papers by Dyson and Feynman. His early research also involved work on the C-meson theory developed by Sakata. After the war, Kinoshita came to the United States to the Institute for Advanced Study, then as a postdoc at Columbia in 1954. In 1955 Kinoshita moved to Cornell. He became particularly interested in making calculations to test the theory of quantum electrodynamics. He describes his introduction to computers at Princeton, using von Neumann’s computer. The interview covers how he became interested in calculating g-2 at CERN in 1966, and his subsequent efforts, the first being the sixth order calculation, where the light-by-light diagram enters for the first time. He describes his efforts doing the eighth order calculation, and his collaboration with Makiko Nio, as well as his calculations of the tenth order. Physicists whom he describes more than briefly include Kodaira, Tomonaga, Nambu, and Nio. Near the end, Kinoshita describes the importance of g-2 experiments, and his recent work. 

Interview with Robert M. Wald, Charles H. Swift Distinguished Service Professor of Physics at the University of Chicago, where he also has appointments with the Kadanoff Center and the Kavli Institute for Cosmological Physics. Wald recounts his childhood in New York, he describes the tragedy of losing his parents in an airplane crash when he very young, and he explains the ongoing legacy of his father Abraham Wald who was a prominent professor of statistics at Columbia. He describes his high school education at Stuyvesant and his decision to pursue a physics degree at Columbia, where he became close with Alan Sachs, who supervised him at Nevis Laboratory. Wald explains his decision to focus on general relativity for graduate school and his interest in working with John Wheeler at Princeton. He describes the excitement surrounding recent advances in approaching astrophysics through relativity, the significance of the discovery of pulsars and the field of black hole uniqueness, and he discusses his postdoctoral research with Charles Misner at the University of Maryland. Wald describes the impact of Saul Teukolsky’s discovery of a variable Weyl tensor component that satisfied a decoupled equation, and he explains the circumstances leading to his faculty position at Chicago, where he was motivated to work with Bob Geroch. He reflects on the experience writing Space, Time, and Gravity, the advances in black hole collapse research, and he explains why he felt the field needed another textbook which motivated him to write General Relativity. Wald discusses his work on the Hawking Effect and his long-term interest in quantum field theory, and he explains the influence of Chandrasekhar on his research. He describes his contributions to the LIGO collaboration, and he explains what is significant about the Event Horizon Telescope’s ability to capture an image of a black hole. Wald explains the state of gravitational radiation research and the accelerating universe, he prognosticates on what advances might allow for a unification of gravity and the Standard Model, and he explains why dark energy is apparently a cosmological constant. At the end of the interview, Wald discusses his recent work on the gravitational memory effect and, looking to the future, he explains his interest to continue working to understand the S-matrix in quantum electrodynamics.

Interview with Michael Green, Lucasian Professor Emeritus at Cambridge University and visiting professor at Queen Mary University. He recounts his childhood in London as the child of secular Jewish parents who immigrated to London just before World War II. Green discusses his early interests in physics and the opportunities that led to his enrollment at Cambridge, and he conveys Geoff Chew’s influence with his ideas on S-matrix and bootstrap theory, which informed his thesis research on hadronic interactions. He narrates the founding ideas that led to string theory and how the work on dual models became transformed into string theory. Green describes his postdoctoral work at the Institute for Advanced Study and his interactions with Veneziano. He explains his decision to return to Cambridge and the importance of the CERN theory group for his research, and he narrates the origins of his collaborations with John Schwarz. Green connects string theory to the ideas that led to supergravity, and he explains why he does not like the term “revolution” in relation to advances in string theory to explain what was happening between 1981-1984. He explains the meaning of the pronoun “super” in relation to string theory, and he conveys his disappointment that supersymmetry has yet to be observed. Green describes the importance of AdS/CFT and his contributions to the origins of D-branes with Joe Polchinski. He discusses his increasing reliance on computers for understanding aspects of AdS/CFT correspondence. Green reflects on winning the Breakthrough Prize, and the supposed aspirational recognition on working to unify the forces which are not yet unified, and he discusses the generational de-coupling of string theory education from particle physics. He provides sociological perspective in response to the impatience that certain physicists have expressed regarding string theory. At the end of the interview, Green ponders the future relationship between string theory and quantum computing, and he describes the field as an intellectual adventure which makes it difficult to predict the significance of these changes.

Interview with Albert Schwarz, Distinguished Professor of Mathematics Emeritus at UC Davis. Schwarz discusses his current interests in pursuing a geometric approach to quantum theory, and he recounts his family origins in Russia and Eastern Europe and their travails under Stalin’s oppression. He describes his early interests in math and his education at the Ivanovo Pedagogical Institute under the guidance of Professor Efremovich, who guided him in the new field of geometric group theory. Schwarz discusses his graduate research at Moscow University, where he focused on the homology of the space of closed curves and on the topology of the space of Fredholm maps during his postgraduate work. He explains the impact of Polyakov’s and t’Hooft’s work on magnetic monopoles and gauge fields in the 1970s, and he describes his contributions to instanton research. Schwarz recounts his earliest exposure to string theory and his subsequent work on supergravity, and he explains the opportunities and considerations that allowed him to emigrate to the United States. He discusses his initial contacts with Ed Witten and his appointment at the Institute for Advanced Study and his job offer at Davis. Schwarz explains his interest in Batalin-Vilkovisky formalism and his appreciation of the value in relating non-commutative geometry to string theory and M-theory. He describes why a geometric approach to quantum theory de-emphasizes the differences between classical and quantum mechanics. At the end of the interview, Schwarz reflects on some of the life lessons he learned from the difficulties of his youth, how his background gives him a uniquely Russian approach to math and physics, and he explains a duality in string theory where it does not currently explain reality but that ultimately, the “right” physics will arise from it.

In this interview, David Zierler, Oral Historian for AIP, interviews Leonard Susskind, Felix Bloch Professor of Theoretical Physics at Stanford University.  Susskind recounts his childhood in the Bronx, and describes his good fortune of being the product of public education in New York from elementary school through CCNY for college. He discusses his discovery that he had a talent for physics, and the difficulties he faced convincing his father that that he would pursue this path and not join in the family plumbing business. Susskind explains the formative advice given to him by professor Harold Rothbart, and the influence of Jesse Douglas and Harry Soodak on his intellectual development as a theorist. He recounts his experience in graduate school at Cornell, where he worked under the direction of Hans Bethe calculating the ground state of infinite nuclear matter. He describes his studies under Richard Feynman and how he admired Feynman’s ability to cut through problems.  Susskind discusses his teaching career at the Belfer Graduate School of Science at Yeshiva University, and he provides an intellectual history for the origins of string theory starting with Geoffrey Chew and the S-matrix of hadronic collisions, culminated in the Veneziano amplitude by Gabriele Veneziano, and he describes his contributions from there, for which he is popularly knows as one of the “fathers” of string theory.  He describes joining the faculty at Stanford, he discusses the advances made by Stephen Hawking, and he asserts that our understanding of the origins of the universe remain at the primitive stage. Susskind explains why he is devoted to explaining physics concepts to broad-based audiences, and he explains what he sees as the most critical threats posed by the Trump administration.  In part II, the interview returns to Susskind’s early years, and he recounts his father’s support for civil rights, and how this influenced his own politics in the 1960s. He describes his goals in his debate with Lee Smolin and engages in some of the spiritual and metaphysical implications that can arise from studying the universe.  At the end of the interview, Susskind reviews, over the course of his career, the ways string theory has, and has not, contributed to efforts to unify all theories of physics, and he affirms that he more closely aligns with Einstein’s approach not to tolerate a clash of physics principles, over that of Niels Bohr.

In this interview, David Zierler, Oral Historian for AIP, interviews John Schwarz, Harold Brown Professor of Theoretical Physics, Emeritus, at Caltech. He describes his family background as a childhood of European emigres, both of whom were scientists, and who escaped Nazi persecution at the beginning of World War II. Schwarz recounts his childhood in Rochester and then on Long Island, and he describes his undergraduate experience at Harvard, where he studied mathematics. Schwarz explains how his interests in the “real world” drew him to physics, which he pursued in graduate school at Berkeley and where he worked with Geoffrey Chew on pursuing a theory of the strong nuclear force. He explains Chew’s conclusion that quantum field theory was not relevant toward developing a theory on the strong nuclear force, and he proposed, alternatively, the S-matrix, which in turn was overtaken by the Yang-Mills gauge theory known as quantum chromodynamics. Schwarz explains how Veneziano’s Eular beta function grew out of the S-matrix program, which extended into a new theory called the dual resonance model, which came to be known as string theory because the model was understood as a kind of quantum theory of one-dimensional objects called strings. Schwarz recounts his contributions to these developments during his time at Princeton, where he collaborated with David Gross, André Neveu, and Joël Scherk. He discusses the significance of Claud Lovelace’s work at CERN, where he found that singularities could be made into poles, and he explains how the second string theory came about in 1971 which required ten spacetime dimensions. Schwarz explains why string theory was not part of the work Glashow and Georgi were doing to unify the three forces of electromagnetism, weak interactions, and strong interactions within a larger gauge symmetry. He describes Feynman’s reluctance in accepting QCD but why, in the end, it proved to be the superior way to explain the strong nuclear force. Schwarz describes his decision to join the faculty of Caltech with the encouragement of Gell-Mann, and he explains the ongoing value of string theory even with QCD firmly established, because it gives gauge theory interactions. He recounts the “second revolution” of string theory in 1984 and his work with Michael Green, and he describes the initial optimism that supersymmetry would be discovered with the advent of the LHC. Schwartz describes Ed Witten’s rising stature in the field, and he shares his views on why thousands of people remain captivated by string theory today. He provides a response to the common criticism that string theory is untestable, and he explains the significance of Juan Maldacena’s discovery of the connection between string theory and conformally invariant field theories. At the end of the interview, Schwarz reviews what among the original questions in string theory he feels have been answered, and which remain subjects of inquiry, including his interest in new approaches to quantum gravity.