Quantum gravity

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.

n this interview, Stephon Alexander discusses current research into quantum gravity and possible extensions to string theory; work to merge quantum mechanics and general relativity; research into the connection between music and cognitive science; experience as a jazz musician; intersections of philosophy and physics; experience as president of the National Society of Black Physicists (NSBP); challenges and stigmas associated with being a Black academic; growing up in both rural Trinidad and the Bronx; undergraduate experience at Haverford; graduate work at Brown; guidance from Robert Brandenberger into the field of quantum gravity, applying particle physics to astrophysics and cosmology; thesis research on solitons and topological defects and its role in string cosmology and theory; decision to take postdoc at Imperial College London focusing on M-theory and integrating string theory with cosmic inflation; influence of Alan Guth; work on D-brane driven inflation; experience in the underground London music scene; decision to go to SLAC in Stanford and work under Michael Peskin; loop quantum gravity; time as faculty at Penn State; the role and responsibility of the Black academic; recruitment by Brown University; intellectual influence of David Finkelstein; the process of becoming president of NSBP. Toward the end of the interview, Alexander reflects on his books, The Jazz of Physics and Fear of a Black Universe; being an outsider in the field of physics; and revisits his current work on quantum gravity. He emphasizes the importance of in-person collaboration and improvisation. 

In this interview, David Zierler, Oral Historian for AIP, interviews Laurence Yaffe, chair of the department of physics at the University of Washington. Yaffe recounts his childhood in northern California and his early interests in science and the influence of his mother, who was a chemist. He discusses his undergraduate experience at Caltech, where he became absorbed in physics even as he continued in his major in chemistry. Yaffe explains his graduate offer from John Wheeler to pursue a Ph.D. in physics at Princeton. He describes the intellectual benefits of going back and forth between the Institute and the department, and he discusses his relationship with his graduate advisor, David Gross. Yaffe explains why he believes string theory should continue to be pursued, particularly in light of developments related to AdS/CFT duality. He describes his decision to return to Caltech for his postdoctoral research, and he recounts his considerations with competing faculty offers from Caltech and Princeton. Yaffe discusses his early faculty career at Princeton and his work on quark and lepton masses and the large-N limit of QCD or Yang-Mills theory. He describes the events leading to his decision to join the faculty at UW and his ongoing interests in QCD. Yaffe explains the evolution of quantum field theory over the course of his career, and he describes how advances in computers have revolutionized theory. He discusses some of the challenges inherent in the current state of the field, and he discusses his advisory work for the Department of Energy. At the end of the interview, Yaffe reflects on the overall and historic excellence of the department of physics at UW, and he explains why he will remain interested in quantum entanglement for the foreseeable future.

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 Juan Maldacena, Carl P. Feinberg Professor at the Institute for Advanced Study. Maldacena recounts his childhood in Buenos Aires, he discusses his undergraduate education at the University of Buenos Aires and his advanced work in physics at Instituto Balseiro where he had his initial exposure to string theory. He explains his decision to pursue a graduate degree at Princeton where he worked with Curt Callan and where he benefited from Ed Witten’s lectures on dualities in quantum field theory and in string theory. Maldacena describes his thesis research on conformal field theories with boundaries and the significance of Joe Polchinski’s discovery of D-branes, and he conveys the importance of his collaboration with Andy Strominger as a postdoctoral researcher at Rutgers. He describes his paper on AdS/CFT while at Harvard and he explains his work on non-gaussianities and his realization that string theory would be useful for cosmology. Maldacena explains his decision to leave the faculty at Harvard to join the Institute, and he describes his subsequent research on space-time and entanglement, the chaos of black holes and the likelihood that they are rapidly thermalizing systems. He explains the contributions of string theory research as offering physics a model for quantum gravity and for the quantum mechanics of spacetime itself, and he shares his perspective on broader debates about how many researchers should or should not be involved in string theory work. At the end of the interview, Maldacena describes his hope in the future to better understand the interiors of black holes.

Interview with Henry Tye, professor emeritus of physics at Cornell, and subsequently professor emeritus of physics at Hong Kong University of Science and Technology (HKUST), and currently, Researcher at the Jockey Club Institute for Advanced Study at HKUST. Tye provides a brief history of HKUST, and he offers his views on China’s long-term goals in high energy physics. He recounts his childhood in Hong Kong where his family fled from mainland China during the Communist revolution, and he explains the opportunities that led to his undergraduate admission to Caltech. Tye describes how discussions of the Vietnam War permeated his college experience, and he describes the influence of Gerry Neugebauer on his interest in physics but that cosmology was far from his considerations at that point. He discusses his decision to study at MIT, where Francis Low became his advisor, and how he worked closely with Gabriele Veneziano on the relationship between the Thirring model and bosonic string theory. Tye explains the excitement surrounding the “November Revolution” which was unfolding just as he arrived at the SLAC Theory Group in 1974. He describes the origins of his interests in cosmology, and the source of his collaboration with Alan Guth during his postdoctoral work at Cornell, where he pursued matter-antimatter asymmetry. Tye explains how this collaboration ultimately created the field of inflation and why this addresses fundamental cosmological problems associated with flatness and the horizon. He explains how and why the original theory of inflation was revised by Andrei Linde and Paul Steinhardt, among others, and why he developed a subsequent interest in cosmic superstrings and branes which he recognized would give a perfect model for inflation. Tye describes why he is optimistic that technological advances will make cosmic superstrings a testable proposition, and that collaborations including the Sloan Digital Sky Survey and LIGO/Virgo are positive steps in that direction. He bemoans the dearth of string theorists focused on phenomenological work and why he thinks string theory will solve the quantum gravity problem. Tye describes his decision to join the Cornell faculty, why his notions of a “string landscape” suggest philosophical implications, why the cosmic landscape is central for understanding the wavefunction of the universe, and why both the universe and all multiverses can begin from truly nothing. At the end of the interview, Tye discusses his recent interests on the cosmological constant problem, the KLT relation, and the observations and experiments that are most likely to push cosmology into new and exciting areas of discovery. 

Interview with William "Bill" Unruh, Professor of Physics and Astronomy at the University of British Columbia, and Hagler Fellow at the Institute for Quantum Science and Engineering at Texas A&M. He credits his mentor John Wheeler for the steady progress of interest and work in general relativity over the decades, and he reflects broadly on the original debates among the relativists and the founders of quantum mechanics. Unruh explains the inability to merge these foundations of physics as the source of his attempts to understand the black hole evaporation as found by Hawking. He recounts his upbringing in Manitoba as part of a Mennonite community and his early interests in Euclidean geometry, and he describes his undergraduate education at the University of Manitoba. Unruh explains his decision to pursue a PhD with Wheeler at Princeton on topology and general relativity, and scattering cross sections of black holes to scalar fields. He describes his postgraduate appointment at Birkbeck College where he worked with Roger Penrose and he narrates the origins of his collaboration with Stephen Fulling and Paul Davies. Unruh discusses his time at Berkeley and then at McMaster and he historicizes the point at which observations made black holes more "real," and he explains his first involvement with decoherence. He explains his involvement with LIGO from its origins and its quantum mechanical nature, and he narrates his reaction of amazement when gravitational waves were detected. Unruh describes the impact of his work in quantum mechanics on computation, and he explains some of the advances that have made observation more relevant to his recent research. At the end of the interview, Unruh describes his efforts to launch a Gravity Archive at UBC, he expresses his frustration with people who insist we do not know quantum mechanics, and he quotes Wheeler, quoting his favorite Grook to convey that he is having fun and wants to learn as much as he can, while he can.

Interview with John Ellis, Clerk Maxwell Professor of Theoretical Physics at King’s College London, and Visiting Scientist at CERN. Ellis discusses the g-2 experiment at Fermilab and where he sees current efforts geared toward understanding physics within the Standard Model, and pursuing new physics beyond it. He recounts his childhood in a small town north of London and his innate interest in physics before he understood that it was a proper field of study. Ellis discusses his education at Cambridge and the department’s strength in particle physics, general relativity, and cosmology, and he explains the relevance of the deep inelastic scattering research at SLAC for his thesis on approximate symmetries of hadrons. He describes the intellectual influence of Bruno Zumino and his decision to go to SLAC for his postdoctoral research to work on scale invariance. Ellis discusses his subsequent research at Caltech and he explains why he would have appreciated more the significance of asymptotic freedom had he better understood field theory at that point. He discusses his subsequent position at CERN and is collaboration with Mary Gaillard on semileptonic decays of charm. Ellis narrates the famous “penguin diagram” that he developed with Melissa Franklin and his interest in grand unification and how it differs from the so-called “theory of everything.” He describes the optimism in the 1980s that supersymmetry would be found and its possible utility in the search for dark matter. Ellis discusses his involvement with LEP and axion physics, and he reflects on the spirit of competition and collaboration between ATLAS and CMS in the run up to the Higgs discovery. He explains the new questions that became feasible as a result of the discovery and his interests in both gravitational waves and supernovae. Ellis describes the AION experiment, the important physics research currently in the works in China, and key recent developments in quantum gravity. At the end of the interview, Ellis conveys his belief in the importance of science communication, he minimizes the importance of the h-index as a measure of excellence, and in reflecting on his own career, he cautions against younger physicists becoming overly-specialized. 

Interview with Edward Witten, Charles Simonyi Professor in the School of Natural Sciences at the Institute for Advanced Study. Witten discusses his current interests in quantum information theory in gravity, and he recounts his childhood in Baltimore and the influence of his father Louis Witten, who is a physicist. He describes his undergraduate education at Brandeis, where he majored in history, a brief stint working for the McGovern campaign, and a false start in graduate school to study economics before landing at Princeton to study first applied mathematics and then theoretical particle physics with David Gross. He describes the significance of deep inelastic scattering in the emergence of QCD and his earliest exposure to the ideas that would develop into string theory. Witten describes his postdoctoral appointment at Harvard to work with Steve Weinberg, Sidney Coleman, Shelly Glashow, and Howard Georgi. He discusses t’ Hooft’s success at solving the U(1) problem and his early work in supersymmetry by the time he joined the faculty at Princeton. Witten narrates the string revolution of 1984 and the early optimism that string theory would be able to describe the real world. He describes his involvement in topological quantum field theories and he explains his decision to move to the Institute from Princeton. Witten discusses his work with Nati Seiberg on N=2 super Yang Mills in four dimensions, the origins of M-theory in the 1994 string revolution, and the impact of Juan Maldacena’s work on AdS/CFT. He describes his collaboration with Seiberg on noncommutative geometry, his interest in the Langlands program, and the role of axions in string theory. Witten conveys the sense of optimism when the LHC turned on and the significance of Khovanov homology and Morse theory. He explains the need to revisit perturbative superstring theory and the possibility that the g-2 muon anomaly experiment at Fermilab will lead to new physics. At the end of the interview, Witten reflects on how little has been seen at the LHC after the Higgs discovery, and he expresses hope that string/M-theory and quantum gravity make meaningful contact during his lifetime.

Interview with Lee Smolin, Founding and Senior Faculty Member at the Perimeter Institute with faculty appointments at the University of Toronto and the University of Waterloo. Smolin narrates the origins of the Perimeter Institute and he describes his unorthodox views on what exactly cosmology is. He describes loop quantum gravity and the notion of a “theory of everything” and why he has much love for string theory despite perceptions of the opposite. Smolin explains the utility and trappings of the Standard Model and he searches for deeper meaning in the origins and societal impact of the pandemic. He recounts his childhood in Cincinnati and his early appreciation for physics and the circumstances that led to his undergraduate education at Hampshire. Smolin explains his attraction in working with Sidney Coleman at Harvard, and why he saw a grand plan in his desire to learn quantum field theory. He describes meeting Abhay Ashtekar and his postdoctoral work at UC Santa Barbara and then at the Institute for Advanced Study. Smolin describes his formative relationship with Chandrasekhar at Chicago, his first faculty appointment at Yale, and his tenure at Syracuse where he found a strong group in relativity and quantum gravity. He explains his reasons for transferring to Penn State and his involvement in loop quantum gravity achieving a mature state amid a rapidly expanding “relativity community” throughout academic physics. He describes his time at Imperial College, where he developed a quantum gravity center with Chris Isham and he historicizes the technical developments that connected his theoretical work with observation. Smolin describes his book "The Life of the Cosmos" and his foray into thinking about biology and why he identifies as a self-conscious Leibnizian who tries to connect cosmology with the concept of a god and the centrality of astrobiology to these issues. At the end of the interview, Smolin explains why he continually returns to quantum gravity, and he conveys his interest in keeping philosophy at the forefront of his research agenda.