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Courtesy: Daniel Freedman
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Interview of Daniel Freedman by David Zierler on May 26, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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In this interview, David Zierler, Oral Historian for AIP, interviews Daniel Z. Freedman, Professor Emeritus of Applied Mathematics and Physics at MIT and long-term visiting professor at Stanford. Freedman explains his understanding of the term’s mathematical physics and physical mathematics, and he bemoans the broad decoupling of experiment and theory in physics. He recounts his upbringing in West Hartford, Connecticut, and he describes his undergraduate education at Wesleyan. Freedman describes his early attachment to theory and his graduate work at the University of Wisconsin, where he worked under the direction of Ray Sawyer on Regge poles. He discusses his postdoctoral research as a NATO fellow in Europe at CERN and Imperial College London, and he conveys the sense of excitement at the time about the weak and strong interactions. Freedman describes his appointment at UC Berkeley before joining the Institute for Advanced Study, and he explains the opportunity that led to his faculty job at Stony Brook. He reflects on his interactions with Yang and he narrates the origins of supersymmetry, and shortly after, the origins of supergravity. Freedman explains what is “super” in supergravity, supersymmetry, and super-space, and he describes why the reality of supersymmetry must be true even if we lack the tools to see it. He explains his decision to move to MIT, and he connects the arc from the 1984 string revolution to the discovery of AdS/CFT in 1997. Freedman describes winning the Dirac medal and subsequently the Breakthrough Prize, which he understood as confirmation in the community about the importance of supergravity. At the end of the interview, Freedman connects his work to larger questions in cosmology and astrophysics, he expresses surprise by the increasing centrality of mathematics to physics, he explains his early work on neutrino scattering and why after 40 years, his original intuition has been vindicated.
Okay, this is David Zierler, Oral Historian for the American Institute of Physics. It is May 26, 2021. I am delighted to be here with Professor Daniel Freedman. Dan, it's great to see you. Thank you so much for joining me today.
I appreciate the opportunity to speak with you.
To start, would you please tell me your current title and institutional affiliation?
That's a little complicated. I'm retired from MIT, so I'm called Professor Emeritus of Applied Mathematics and Physics. At Stanford, I am a long-term visiting professor.
Now, at MIT, was that a dual appointment? Or was it a specific mathematical physics appointment?
I was first appointed to the applied math group within the math department at MIT. I was somewhat more mathematical than I became in later years. I've always had good relations with the Center for Theoretical Physics at MIT, after several years the physics department offered me a formal joint professorship.
A nomenclature question. There's mathematical physics, and there's physical mathematics. Do those have any distinctions for you, or is that just nomenclature?
No, they do have distinction for me. Mathematical physics usually implies very rigorous mathematics, interpreting physics equations in mathematical terms. That activity has really expanded very rapidly over the last twenty years. Physical mathematics was what many people did in the applied math group at MIT. They applied mathematics to actual phenomenon, like fluid motion. Which is physics. Everything is physics, when you come right down to it.
A very in the moment question, it's been a lot of fun asking eminent physicists in real time what they take so far from the muon anomaly experiment at Fermilab and the possibility that this might be new physics. What's your take?
I'm very cautious. We've seen a lot of preliminary evidence of new effects come and go. I hope for the best, I hope it is something new, but I'm cautious.
What would you be looking for that would sway you to be less cautious and more excited.
Confirmation in another experiment. Improved accuracy. I've not read the details of the accuracy claimed. This is a situation where the theoretical calculations involve approximation, and the data has the usual type of experimental error. So, it's a bit hard to judge when one is going to be confident.
A broader question than that, in the subfields that are most important to you currently, are theorists providing the guidance to the experimentalists or the other way around?
Neither is true, unfortunately. Experiment and fundamental theory have largely decoupled. In elementary particle physics, we've had the luxury of building higher and higher accelerators as the years go by, and the governments and agencies have seen fit to fund those accelerators. But the LHC has seemed to reach a difficult point where it's going to be very hard to justify going beyond because one doesn't know at all what one's going to find. And the expense of building a high energy machine is just incredible to contemplate. So, the kind of experiments which would provide guidance for the people in my profession, elementary particle theorists, are just not likely to be done in my lifetime. I hope in your lifetime, they might be.
What does that mean for the long-term fate of supersymmetry?
I don't think it will be confirmed in my lifetime. That may not be a long-term question. I'm eighty-two years old. But I hope eventually, it will. I think it's a beautiful subject. Nature somehow, at some level, at some energy scale, has to know about supersymmetry. And if it knows about supersymmetry, it will know about supergravity. That's my view and my hope.
Just a snapshot in time, what are you working on these days? What's compelling to you?
I'm working on an application of string theory, but largely in the limit of string theory in which supergravity has a lot to say. It's called the AdS/CFT correspondence. It's now about twenty-five years old. I've written lots of papers on that. I'm collaborating with younger physicists at Princeton. That has kept me busy for the entire year of the pandemic. It was very, very good to have had a focus during that period of time.
I wanted to ask you about that, if remote work and social isolation, in some ways, has conferred benefits to you.
It would've been much better to have been able to go to my Stanford office and talk with people. I was able to go there one day only, when nobody else was in the building. But with technology, with Zoom, intellectual activity has continued during the pandemic. Research by theoretical physicists has not declined, judging by the number of papers that are submitted daily to our archive. Some benefits will continue. If I can give a colloquium at MIT from my desk in Palo Alto, California, why fly across the country? Why ask the funding agencies to pay the expense of that travel?
Let's take it all the way back to the beginning. Let's start, first, with your parents. Tell me a little bit about them and where they're from.
Well, Annabel was my mother; Edward was my father. He was born in Hartford, Connecticut. My mother was born in Minsk, in today’s Belarus. She came to the United States at the age of three or four. We were a lower middle-class family, a Jewish family, not terribly religious, but observant. We lived in West Hartford, Connecticut, which was a nice suburb of the city of Hartford. We lived in a decent house, but we were the poorest on the block. We were the last to get a TV set. Our neighbors were kind enough to invite my sister and me to watch the key shows. That's my background. Through it all, my parents valued education for my sister and me.
What was your father's profession?
He had no education beyond high school. He was a retail merchant for many years. He owned a liquor store. In the early years of World War II, he owned three liquor stores at the same time, but he had to close two of them because finding employees was difficult in the war years. For many years he worked from eight o'clock in the morning to eleven o'clock at night six days a week. I basically saw him only on Sunday, when liquor stores, and most other stores, were closed.
What kind of European sensibilities did your mom have from the old country?
Very little because she was too young. At that time, foreign birth was a stigma. Jewish immigrants were referred to as “greenhorns.” For most of her life my mother claimed that she was born in the U.S. It was only when she was sixty-five years old and couldn't find her birth certificate that she admitted to us that she wasn't born in this country. To collect Social Security, she presented an arrival certificate from Ellis Island.
Was anti-Semitism something that your family had to contend with at all?
Only very, very, mildly. West Hartford had many Jewish families and there were three or four synagogues in our community. I felt accepted. I've rarely personally met with any anti-Semitism.
What were some of the key Jewish cultural markers that you remember from your childhood?
We observed the holidays. I did go to Hebrew school, and a Bar Mitzvah was obligatory. But when my mother became the president of the Parents’ Association of the Hebrew school, I rebelled a lot and didn't pay much attention. I'm a little bit of a rebel. Not the most rebellious person you've ever interviewed, I'm sure. But I have that sort of streak.
As a really young boy, what memories do you have of World War II or the United States being in the War?
I have a vague memory of air raid alerts, when every house had to use dark shades to eliminate stray light. During one alert the doorbell rang. It was the air raid warden who complained that our house was not completely dark.
When did you start to get interested in science? Was it from a young age?
Not markedly. Believe it or not, I got a D in eighth grade science. The teacher was very boring, so I sat in the back row, reading model airplane magazines. That alarmed my parents enormously, and they arranged to take me out of the public-school system and enroll me in Loomis School, which was a high-quality prep school on the east coast, and that's where I spent my four years of high school. My interest in science and math developed gradually there and solidified early at college. In 1956, I went off to college at Wesleyan University in Connecticut, which is quite well- known as a small liberal arts college with excellent teaching. I had some vague ideas of being a doctor and going pre-med. But I noticed in the course catalogue that there was a special joint program in math and physics, in which the required mathematics, largely calculus, was coordinated with the physics lectures. And I said, "Gee, why not try this?" You had to interview, you had to be selected to partake in this program. I was accepted and joined a class of about fifteen other freshmen. That course really turned me on. We had a wonderful, clear professor. His name was Tom Green. What turned me on was the realization that you could solve Newton's laws of motion and compare the result to experiment. That’s what physics was about. Theory being confirmed by experiment. And at that point, my career choice was made.
That was the pleasure that you were bemoaning earlier in terms of the decoupling of experiment and theory that's happening currently in the field.
When you came of age, you're saying, though, they were much more connected.
They were much more connected. I came of age just after Sputnik came along, and sciences were very, very well-supported by the government. So, the timing of my birth and education was favorable.
I've once heard it said that if you were to ask Enrico Fermi if he was an experimentalist or a theorist, he would've looked at you funny. Does that resonate with you, in terms of when you started to think about physics for your own career?
I was decidedly a theorist, at the ninety-nine percent level. Much later I spent six months developing a medical instrument, which I may tell you about, but I'm really a theorist. As I said, however, I believed for many years that theory and experiment should be closely linked. But then came supersymmetry and supergravity, and I realized that the link was far off. Although I decried the decoupling of experiment and theory, I realized it was somewhat inevitable, and I accepted it. And my community now is mostly theoretical physicists.
As an undergraduate at Wesleyan, what were some of the most exciting things happening in physics, at least insofar as your professors were telling it?
As I remember, little was said about the research frontier. I was content with the steady development of my own knowledge through my undergraduate courses. That process was intellectually satisfying and sufficiently challenging for me.
What were your favorite courses? What captured your imagination more than anything else?
Quantum mechanics was my favorite course. That was fantastic. Electromagnetism was my least favorite, somehow. I understood Maxwell's equations in the vacuum. But the textbook treatment of electromagnetic fields in materials was confusing, it didn’t seem to be derived from fundamentals. Whereas in quantum mechanics, you could focus on the hydrogen atom, you could focus on hyperfine structure. Deep and far-reaching truths came from simple systems.
Did you take any courses in general relativity as an undergraduate?
No, and I didn't take them as a graduate student either.
Is that partly because, at that point, GR was considered a bit of a backwater?
It was not a backwater, but it was a specialization, not part of mainstream education in most graduate schools.
What kind of advice did you get from professors or other mentors about programs to apply to for graduate school or advisors to work with?
I don't recall any advice. To tell you the truth, I had some problems getting into graduate school. I did very well as a freshman in that special course. But in the first part of my sophomore year, I had “sophomore slump.” I just couldn't focus on my studies, and I did rather poorly. Although I bounced back from it, and the next two and a half years were stellar, it caused me some problems. I didn't get into Harvard or Princeton. I got into Wisconsin. And that's where I went to graduate school.
How did you develop the relationship with Ray Sawyer?
Well, he taught graduate quantum mechanics my first year at Wisconsin. He wasn't the best lecturer in the world, but the ideas were somehow coming through as very deep. And that forced me to go home, and read the books, and become more solidly knowledgeable about those ideas. Let me tell you my personal model for education in physics. The textbook should offer one viewpoint and the lectures another, but the student should go home and work in the dark of the night to synthesize his own viewpoint.
What was the intellectual process of putting your doctoral thesis research together?
Well, as I said, I was attracted to Ray Sawyer, and I asked him if he would be my advisor. And I guess I stood out in my early classes at Wisconsin, and he said, "Sure." He was working on a general subject area called Regge poles, which was then very much in vogue. I will just say that it involves treating angular momentum as a complex variable. Miraculously, this can relate low energy resonant peaks in one scattering amplitude to the high energy behavior of a different amplitude. Regge theory had some genuine successes in the 1960’s, but limitations soon surfaced. It plays a more limited role in modern theory.
Who else was on your thesis committee besides Ray
Frankly, I don't remember. You're talking 1964.
As you were writing your thesis, to go back to this theme of the interplay between theory and experiment, what were some of the experiments happening that were relevant to your research?
Results from accelerators such as the Bevatron in Berkeley, California and the AGS at Brookhaven Lab on Long Island were most relevant. They were uncovering features of the strong interactions such as the resonances I mentioned. A resonance can be interpreted as a new particle which is very short-lived.
What did you see as your contributions at this stage in your career?
I didn’t think in those terms. I trusted the advice of my advisor, and then followed my own nose in the choice of further problems to work on. I wasn't so concerned with the big picture.
When you defended, what opportunities were available to you next? Postdocs, faculty positions? What were you considering?
Both, really. I was offered an instructorship at Princeton, which involved both teaching and research. But I was twenty-five years old. I'd never been out of the United States, and I wanted to go to Europe. So, I got what was called a NATO post-doctoral fellowship. It was a two- year fellowship. I went to Europe and spent the summer of 1964, first at a summer school in Italy then in Geneva at CERN. In the fall I moved to London to work at Imperial College, which I selected as my host institution.
What was going on at CERN while you were there?
There was excitement about both the weak and strong interactions, but mostly the latter. Experiments were beginning to find a spectrum of unstable particles. Their masses and spins (i.e. angular momenta) seemed to be related as Regge pole theory suggested. There was a very active environment at CERN, and I met lots of people who became friends, and some friendships have lasted.
Do you have a specific memory of first coming across the term Standard Model?
I don't have a specific memory, but it probably came into broad use in the 1970’s.
At what point did you feel, term or not, that there was some cohesive theory that would put all of these things together?
Well, what really cemented it for me was the discovery of the weak neutral current in 1973. This was predicted by the Weinberg-Salam model, which unified the weak and strong interactions. The guiding principle was gauge symmetries, as in non-Abelian Yang-Mills theory. Unification really clicked with me, as it did for most theorists. Unification with the strong interaction was soon proposed in 1974. I realized that the next big step in that program was to unify gravity with the rest of particle physics. And I'd never taken general relativity! By then, I was a young faculty member at Stony Brook. I decided to spend the summer of 1973 at Brookhaven Lab, which is close to Stony Brook. My summer goal was to study Steve Weinberg's book on gravitation. I learned enough to master the mechanics of tensor analysis. That was very good, a very useful thing to have done. No research papers were written that summer. But I put my knowledge of GR to good use later.
I'm curious about your time at Berkeley in the mid-1960s. By the time you got there in 1965, had the campus protests already started?
The big year was 1964. That was the year of the Free Speech Movement, Mario Savio and such people. But things were very active in 1965 when I arrived. The Vietnam War was raging. And I did engage in a lot of anti-war activity. A group of us wrote letters to the editor of the New York Times, which were published. It was an exciting time.
Given that you were political, did you think specifically about the ways that scientists and physicists in general were contributing to the U.S. military?
I definitely did. There was this shadowy group called Project JASON, which seemed to involve some key theoretical physicists. They met each summer and designed bigger and better bombs. At least that was our impression. Of course, what they actually did was secret.
Did you interact with Geoff Chew during your Berkeley years?
I did. Geoff Chew was a very influential professor. At that time, he was a strong advocate of Regge pole theory. My first paper from Berkeley, written together with a young graduate student in Geoff’s group, solved a knotty mathematical problem in the application of Regge theory. I want to come back to that a little later.
Tell me about your time at the Institute for Advanced Study (IAS). That must've felt very prestigious.
It felt wonderful. It really was wonderful. I arrived at the IAS in the fall of 1967. Robert Oppenheimer had been the director of the IAS, but he had died that past winter. He suffered from throat cancer for many months and was ensconced in the director's house, which was across the field from the main buildings at the Institute. After I arrived, I met another post- doc who had been there the year before, and he told me the following story. He was very anxious to talk with Oppenheimer, so he called the secretary at the director's house several times and was told that Oppenheimer was not well enough to see him. Finally, one morning, he got a call from the secretary, "Dr. Kuriyan, you can come over this afternoon. Professor Oppenheimer will see you." So, he came, and he walked into the room, and Oppenheimer was in an armchair reading a paper. He said, “This is an interesting paper, you should read it.” And it turned out to be my paper! A few events like that mark my career. That's one of them.
What was that paper?
It was a follow-up to the first paper I wrote at Berkeley on Regge pole theory. And it's the reason I got an appointment at the Institute of Advanced Study.
Oppenheimer was interested in Regge poles?
Apparently enough to read the paper.
Who were some of the other luminaries in physics at the Institute at that time?
Let’s just talk about the School of Physics. Freeman Dyson was the senior person since Lee and Yang had left a few years earlier. The Institute had recently appointed Steve Adler and Roger Dashen to memberships. They had both received PhD degrees in 1964 and had done outstanding early work. They were effectively the leaders of the particle theory group. The rest of the group were postdocs. It was a very cohesive community, and we all lived and worked well together.
In the late 1960s, were you aware of the work of Green, Schwarz, and Veneziano, the very earliest iterations of string theory? Was that on your radar?
I was certainly aware of Veneziano’s famous formula, and I wrote a paper with some extensions. So, I was interested, although it didn't profoundly affect my views.
When did you decide to go on the job market for faculty appointments?
I had a year at Imperial College in London, two years at Berkeley, and one year at the Institute by that time, so I decided to apply for a faculty position. I received an offer from the University of Chicago and another from Stony Brook University in New York, where there was a new institute headed by Frank Yang.
Was that the attraction for you?
That was an attraction for me. The fact that my family and my wife’s’ were both on the East Coast and easy to reach was an attraction. Chicago seemed to be an interesting opportunity, but I chose Stony Brook.
Did you get to work with Yang closely?
I got to interact with him closely, I never did research with him.
What was he like?
I think he was and is a wonderful man. I really and truly do. My wife and I were invited to join his family on a ski trip in Stowe, Vermont one winter. We did that. It was a wonderful weekend. And he was in touch with his group. But he had very, very fixed ideas about physics, and they didn't include supersymmetry. And after 1973, my life was supersymmetric. We might go into that in a few minutes. I can tell you an anecdote from a conference I attended. I don’t remember the exact date, but it was after the discovery of supergravity. The last session of the conference was a panel discussion with Yang and Steve Weinberg, and other famous theorists. People asked questions from the audience. Someone asked Yang if he considered supersymmetry to be an important theoretical development. Yang answered in a somewhat indirect way. He said he thought that fundamental new ideas in physics should be associated with fundamental concepts in mathematics. Therefore, he asked his friend, S.S. Chern, who was a prominent geometer at Berkeley, whether he thought anti-commuting numbers, which were needed for supersymmetry, were fundamental or not. Chern had answered in the negative. Yang promptly sat down. I went back to Stony Brook and said to my supergravity collaborator, Peter van Nieuwenhuizen, "What are we doing here if our boss doesn't think our work is valuable?" So, Yang, as a physicist, had his own special views. As an administrator, he supported us completely, and I respected that enormously
Between mentioning Chern and being at Stony Brook, I wonder if you interacted at all with Jim Simons?
Jim Simons threw the best parties at the whole university.
I'm not surprised to hear that.
We were in the same building as the mathematicians, and there was a lot of interaction. Jim figured in prominently.
In the 1970s, before we get to supersymmetry, there were so many exciting things that were happening, at Harvard, at SLAC. What was most relevant for your research interests at that point? What were you paying the most attention to?
I knew I had to learn quantum field theory better. I had a sabbatical leave at MIT in 1970-71. Sidney Coleman was giving his famous course at Harvard. So twice a week, I'd walk or take the subway to his lectures at Harvard, and I learned a lot. I needed to know much more about non-Abelian gauge theories. One of my colleagues at Stony Brook, Benjamin Lee, had written a very accessible pedagogical introduction to Yang-Mills theory, and I studied that. Ben Lee was unfortunately killed in a traffic accident sometime after that. He was a good friend.
When did you first meet Sergio Ferrara and Peter van Nieuwenhuizen?
Peter came and gave a talk at the Institute for Advanced Study when I was there on a short visit. It must have been 1974. I thought Peter gave a terrific talk. Sometime later at Stony Brook, our group met to discuss young people we might invite for seminars. I suggested that we should get to know Peter van Nieuwenhuizen. So, we invited him to give a talk at Stony Brook, and eventually that turned into a faculty offer for him. Faculty at Yang's Institute had very special privileges. We had half the teaching load of the rest of the physics department. Because of that, I was able to accept an invitation to go to École Normale Supérieure in Paris in the fall of 1975 and spend a couple of months there. The ENS staff found a lovely apartment for my wife Miriam and our daughter Julie who was three years old. It was a wonderful visit, both for cultural experiences and for physics, but let’s get to the latter. Wess and Zumino's first paper on global supersymmetry was written in the fall of 1973. It is probably the single paper which has had the most influence on my career. There's one other one, namely Juan Maldacena's first paper on the AdS/CFT correspondence. I told you, I was enamored of the idea of unification. Supersymmetry unified particles of different spin in one mathematical symmetry framework. That was a totally new idea and turned me on immediately. During the next two years at Stony Brook, I wrote two papers on global SUSY with Bernard de Wit, (then a postdoc who has recently retired after an outstanding career in Holland). The approach to SUSY most popular at that time was a formulation called superspace, where extra dimensions were adjoined to spacetime. The new dimensions were not described by ordinary numbers such as 1, 2, 3, and 5, but rather by anti-commuting numbers. It was as if (2 x 3) = -(3 x 2), and 2 x 2 =0. This is a challenging idea for layman, but it was very important to that branch of physics. Nevertheless, I always thought that it obscured the physics of supersymmetry. Global supersymmetry did not include gravity, and that was an outstanding problem. Just before leaving for Paris, I had heard a lecture by Zumino. He was describing his work with Julius Wess attempting to formulate supergravity in superspace. They were making some progress, but it looked very, very murky. My attitude was that Wess and Zumino were very smart people, I had tremendous respect for them, and they were eventually going to solve this problem. But whatever their solution was, the physical elements should just be the gravitational field describing the spin 2 graviton plus its supersymmetric partner, a field describing a spin 3/2 particle which came to be called the gravitino. The classical field theory of the graviton, namely general relativity, is well understood. But previous work on spin 3/2 was problematic and mysterious. A free field wave equation for the gravitino was developed by Julian Schwinger and a more obscure collaborator, William Rarita, in 1941. It was simple enough, but it only worked for a free field theory in which particles just propagate freely through space without interaction. When interactions were introduced, the equations became mathematically and physically inconsistent. For example, particles traveled faster than light, which contradicts relativity theory. In Paris I started reviewing the early literature on spin 3/2 fields trying to come to grips with these difficulties. Sergio Ferrara was then a postdoc at CERN, who had written some important papers on global SUSY. He came to École Normale to collaborate with another young Italian physicist. Our offices were back-to-back, his collaborator’s and mine, and the partitions were very thin. Italians tend to talk very loudly, but the background noise in a foreign language didn't really bother me. There was also a certain amount of cigar smoke, which Ferrara was famous for. Somehow, that didn't bother me either. At lunch, the group would get together. I told Sergio that I was studying spin 2 and spin 3/2 with the hope of bringing them together. He suggested I try an approach similar to what I had done a year earlier with Bernard de Wit. We had formulated super Yang-Mills theory in terms of its ordinary fields, the gluon and the gluino, as opposed to the effort in superspace, which characterized most of the current work. I thought that was a very good idea, and I sat down and tried to do it. After about two or three days, I managed to find the first approximation to a theory in which gravitons and gravitinos interact consistently. Even as an approximation, it was the most beautiful calculation I had ever done. Anyway, that's a long answer to your question of how I first met Sergio Ferrara. Can I go on to describe the subsequent steps to a complete theory of supergravity.
Before we get there, I wonder if you can explain the prefix. There's supersymmetry, there's supergravity, there's superspace. Why the word super? What does it convey?
“Super” was the prefix used by Wess and Zumino to indicate the unification of particles with different spin. It is well-known that there are two classes of particles in nature, particles with integer spin, like the photon with spin-1 and the graviton with spin-2, and particles with half-integer spin. The electron, the neutrino, the quark, and the proton all have spin 1/2, and, prior to supergravity, this was the only consistent half-integer value. Supersymmetry allowed these two classes of particles to be unified. And I've already told you what I think about unification.
That answers my question. Let’s get back to the completion of supergravity.
When I got back to Stony Brook in January 1976, I thought I could finish the job in the next two weeks, but I couldn't do it. The gravitino field, has sixteen components. It’s coupling to gravity requires another field with sixteen components called the vierbein. This was much too difficult to handle. So, I asked Peter van Nieuwenhuizen to join me. We worked for two and a half months, and we were getting nowhere. We tried lots of things, and they didn't work. We decided that I would give an internal seminar to our group at Stony Brook to tell them where things stood. I said, "We tried this, we tried that, and nothing worked." They were deeply unimpressed. Yet somehow, that seminar crystallized things. Peter and I sat down for a strategy session. We decided to formulate a general and systematic way to modify the approximation I had brought home from Paris and then check step-by-step that supersymmetry was maintained. By late March, the systematic procedure succeeded, and the theory was complete! Those 3 months were a very exciting period of my life!
What was the reaction when you announced these findings?
The reaction was positive but tempered. One reason was that many elementary particle theorists knew very little about general relativity, as was the true for me until a few years earlier. It takes time and motivation to learn that stuff.
What further developments did you expect?
I thought there would be an explosion of activity in the construction of models of the electro-weak and strong interactions, coupled to gravity and supergravity. Eventually there were many, many models, but it took several years of work in the theory community.
What features do those models have?
First, every known elementary particle must have a SUSY partner, whose spin differs by 1/2 unit. Exact SUSY would require that the two partners have the same mass and electric charge. This was obviously false; the electron does not have a spin 0 or spin 1 partner. Mechanisms were soon found to break the supersymmetry and make super-partners heavy, while maintaining the consistency of the equations.
Did you think, in those early years, that experimental verification of supersymmetry and supergravity was within reach?
I certainly hoped for confirmation, but as you know, it has so far failed to come. Super-partners with mass up to two Tev (almost twenty times the mass of the Higgs boson) should have been seen at LHC outside Geneva, but none were found. What I like to say is that Nature is hiding its secrets.
I wonder if you can talk about the value or not of intuition in physics. In other words, a general sense that, "This must be true, even if we don't have the tools currently to demonstrate it."
I confess that I do have a sense that SUSY and SG are “inevitable.” That comes from the importance of symmetry principles and especially gauge symmetries in fundamental physics. The spin one-gauge principle governs the weak, electromagnetic, and strong forces, and the spin two-gauge principle governs gravity. With supergravity, a spin 3/2-gauge principle was introduced. There are rigorous theorems that these three-gauge principles are the only ones that allow consistent interactions. So, I feel that since Nature makes use of the first two, it must also know about supergravity.
Did you see supergravity as being central to unifying the forces?
Well, at the very least it brought gravity and particle physics closer together. I don’t want to speculate about the form that further unification might take. It may involve superstring theory, it may involve theoretical ideas not yet discovered, or it may simply be unknowable.
Tell me about your visiting professorship at Caltech. What were some of the motivations to going to Pasadena for a year?
Murray Gell-Mann got very excited about supergravity. He was a fan, and he invited me to spend a year there. I didn't really work with Murray, although I talked with him extensively. I also talked a lot with John Schwarz. String theory, of course, became closely related to supergravity, and John is a very eminent string theorist. So, it was good to go there.
Tell me about the connections between supergravity and string theory. What was John Schwarz interested in, with regard to your work?
The short answer to your first question is that supergravity is the low energy limit of superstring theory. A solution of string theory describes an infinite number of elementary particles of many different masses. The finite set of particles of lowest mass are well described by a supergravity theory. John Schwarz was willing to put aside, temporarily, his commitment to string theory and work on supergravity because he saw that it might be promising. That summer, 1977, we both went to the Aspen Center for Physics together, and we wrote a paper involving the coupling of supergravity to the super-Yang-Mills system which was well known as a theory with global SUSY.
What was so exciting about this? What was the development?
The theory found at Stony Brook in 1976 is called pure N=1 supergravity. It is the field theory of the spin (2,3/2) “gauge multiplet” of particles. It was a natural step to couple it to the spin (1,1/2) “Yang-Mills multiplet.” That was what John and I did. It turned out that Peter van Nieuwenhuizen went to Europe that summer and (with Ferrara and others) also worked out the super-Yang-Mills coupling. The coupled theory is an example of a matter-coupled supergravity theory.
Now, did you come back to Stony Brook knowing that you were headed off to MIT? Or that happened later?
Professionally, I was very happy at Stony Brook. We were near our families. There were nice beaches. With two other faculty members, we shared a canoe. I used to canoe through the salt marches on the north shore. Life was good there, and I had no particular reasons to think beyond Stony Brook.
But MIT recruited you. They wanted you to join.
One thing that changed the situation was that Miriam went to law school. She started at USC the year we were in Pasadena and continued at NYU. That was a three-year process, bringing us to 1979. That was a natural time to think about moving. There was one senior physicist in the applied math group at MIT, namely Hung Cheng. He was a good friend of C.N. Yang and was spending his sabbatical at Stony Brook. I told him that if MIT were interested, I would be interested. He went back and told this to his group. I was eventually invited to give a talk at MIT, which turned into an offer. While thinking about MIT, I was contacted by the University of California at Santa Barbara, where the ITP, now called the KITP, was just getting underway. They wondered whether I might be interested in going out there. I said that I would consider it and agreed to visit. During the visit, they said that they would make a verbal commitment to an offer but could not make a formal offer until January which was several months away. I said, “Okay" and thought that I could wait. But time went by, and I became unhappy with the uncertainty. I just got to the point where I had to make a decision. So, I called MIT and said, "I accept your position."
In the early 1980s, at the very earliest planning stages for what would become the SSC, were you following these developments? Did you recognize the possibility that at those energies, supersymmetry might be seen?
Yes, I had high hopes for it. Unfortunately, hope slowly dimmed.
Did you think at the time that the energies, from a theoretical perspective, that the SSC was planning were the appropriate energies for which supersymmetry would be seen?
I hoped so. The planned energy of the SSC, had it been built, would be higher than the LHC energy. You asked about theoretical perspective so let me mention the idea of “naturalness,” which captivated a large fraction of the supersymmetry community for several years. It is not easy to define it, partly because is a desired qualitative property of a theory rather than a property that can be precisely derived. Let’s just say that the standard model without supersymmetry is not natural, but most supersymmetric versions are. In those models the mass scale of the super-partners should not be much higher than that of the Higgs boson at 125 Gev. Many, many papers were written about naturalness. Although I felt that it was not based on hard scientific methods, I accepted it cautiously, since I hoped that super-partners would be found. The negative evidence from the LHC shows that naturalness is a failed idea.
Tell me about your reactions to the so-called second-string revolution of 1984.
Like most theorists I recognized that the paper of Green and Schwarz was very important. Their calculations were actually done in the 10-dimensional SUSY/SG field theory that is the low energy limit of superstring theory. Nevertheless, I never saw the need to think about extended objects such as strings. I also have to admit that I found the formalism incredibly difficult despite the fact that I have written one or two papers in collaboration with string theorists. So, the second-string theory revolution largely passed me by. That changed in late 1997 when Maldacena wrote his paper proposing the AdS/CFT correspondence and follow-up papers of Witten and Gubser, Klebanov, and Polyakov indicated how to apply it. It became clear that string theory offered a lot! Not experimentally, but for amazing new structural relations between theories that superficially look very different.
Let's talk about AdS/CFT. What did you recognize immediately as the major impact of what Maldacena had done?
It showed that the physics of quantum field theories in d spacetime dimensions with gravity neglected is precisely related to the physics of theories in d+1 (or higher) dimensions in which gravity is essential. The arguments in Maldacena’s paper require string theory and its branes, but most precise calculations involve the limit of string theory in which gravity or supergravity is valid. It is truly amazing that theories in different spacetime dimensions can be physically equivalent.
That sounds mysterious. Can you explain it?
I will try. There are solutions of ten-dimensional superstring theory that include a five-dimensional spacetime with negative curvature called anti-de Sitter space (AdS). Gravity is essential to produce the curved spacetime and there are other fields. When you solve a wave equation on AdS spacetime you have to specify both initial conditions at time t=0 and boundary conditions at large spatial distance. This mathematical boundary of the AdS bulk becomes a physical boundary in AdS/CFT, a 4-dimensional spacetime where the non-gravitational theory lives. Waves propagating in the bulk leave an imprint at the boundary from which observable quantities in the boundary field theory can be calculated.
Dan, that is a lot for readers to follow. Let me ask what you saw as your role in the development of AdS/CFT?
I was very lucky. The AdS spacetime appeared naturally in some supergravity theories and I had studied and written papers on it in the 1980’s. Further, although five-dimensional supergravity was pretty complicated, I was in a good position to understand it. Finally, Edward Witten’s paper provided a clear procedure to calculate boundary observables from bulk solutions. With my MIT colleague Samir Mathur and my excellent student Leonardo Rastelli and later with Eric D’Hoker from UCLA, we developed new methods for bulk calculations to be compared with the boundary field theory which was a four-dimensional Yang-Mills theory, usually with global SUSY.
Given that AdS/CFT is still so relevant today, are you surprised by that? Or did you see just how rich this development would be?
I saw it as rich, but its actual richness has exceeded my expectations. It has brought different branches of theoretical physics together, gravity and particle physics and even condensed matter. That is wonderful to contemplate.
In what ways did the role of computers become relevant in your research?
In my experience the key development is the software package Mathematica. People can use it for computations which would otherwise take many pages of paper (with many errors likely). Its inventor Stephen Wolfram was at Cal Tech during my year there. As he reminded me in some recent email, I tried to sell him my used car when I left Pasadena. Good thing he didn't buy it. We wouldn't have been friends. The younger generation of physicists can do spectacular things with Mathematica, which couldn't possibly be done with a pencil and paper. That has had an enormous impact on theoretical physics. Luckily, I have had collaborators who can do this.
Tell me what it was like when you won the Dirac Medal in 1993.
That was one of the years I spent at CERN, I was thrilled, of course. My wife and I took trains from Geneva to Venice, and then Trieste. It was wonderful. I gave a talk entitled “Some Beautiful Equations of Mathematical Physics.” I wore a t-shirt with Maxwell's equations written on it. On our return we celebrated by staying in a hotel on the Grand Canal in Venice. Although there is not yet a t-shirt available with the supergravity field equations, there is a frieze in the building of the Simons Center for Physics and Geometry at Stony Brook. This has many important equations of mathematics and physics engraved in stone including the basic equation of supergravity.
I wonder how you might compare that with the honor of winning the Dannie Heineman Prize.
Both were meaningful to me. Supergravity was a bigger sport in Europe than in the U.S., so the Dirac Medal indicated its acceptance in Europe. The Heineman prize brought official recognition in the American community.
Finally, there was the Breakthrough Prize in Fundamental Physics two years ago. What was your first reaction?
My Stanford colleague Andre Linde called with the good news. Frankly, my reaction was very emotional. I put down the phone and cried. I wanted to tell my parents, but of course that was impossible. It was a spectacular honor for me. It meant that the leaders of my profession considered that supergravity is a major discovery.
Do you see one of the challenges with the Nobel Committee that your work eludes experimental verification in a way that, say, asymptotic freedom did not?
Oh, absolutely. I have no quarrel with the Nobel Committee, which almost exclusively holds the standard that theory has to be confirmed with experiment before theorists are honored.
But obviously, the Breakthrough Prize does not hold by that standard.
I wonder if you could reflect on that, what that might mean, especially in light of the fact that the Breakthrough Prize is much more modern in the way that it recognizes teams and Big Science.
Yuri Milner has said that he views the Breakthrough Prize as a modern alternative to the Nobel Prize. For better or worse, Big Science is an important feature of twenty-first century physics.
In what ways do you see your research contributing to larger questions in astrophysics or cosmology?
Inflationary cosmology has several branches which differ in how the fields which generate inflation arise. Andrei Linde and Renata Kallosh at Stanford have developed models in which those fields arise within supergravity theories. They are quite committed to that framework. It is my (admittedly limited) understanding that gravitinos play a significant role in the evolution of the universe soon after inflation stops. It is conceivable that cosmology might provide a pathway toward experimental confirmation of the ideas of supergravity.
When the Higgs was discovered, what did you expect would happen next?
I was convinced that the Standard Model was right, and the Higgs would be found at some mass. So, I was not deeply surprised by its discovery. I was happy to see the Standard Model confirmed, but I was more concerned with going beyond the Standard Model. Supersymmetry was the prime direction I was interested in, but I would have been happy with any new direction, provided that the experimental evidence was solid. That is where the LHC has failed so far. It's not their fault. It is the fact that Nature is hiding its secrets.
In what ways have advances in black hole research been relevant for your work in supergravity?
I was thrilled to watch the LIGO detector actually find clear evidence for the collision and merger of two black holes. That is a major experimental discovery, but it doesn't really affect the science that I think about.
Let’s get back to what captivated your imagination early on. In what ways has your understanding of quantum mechanics changed and matured over the course of your career?
The big step in my education was to proceed from quantum mechanics to quantum field theory. In the former you deal with a finite number of quantum variables, such as the x,y,z coordinates of the electron in the hydrogen atom. In quantum field theory there are an infinite number of quantum variables, for example, the electric field at each point in space is quantized. That educational process took a number of years and I am still trying to learn.
A question I always like to ask, the foundational disagreement between Einstein and Bohr. From your vantage point in 2021, what is settled and what remains provisional?
That was always an excessively philosophical issue for me. There was so much experimental evidence in favor of quantum mechanics, that I didn't worry about its foundations. Of course, I was committed to fundamental physics, but always in terms of questions that can be explored and hopefully settled by calculation. My attitude was that a properly chosen calculation could have important implications beyond its narrow initial scope.
I would like to ask about your decision to retire and what led you to Stanford.
At MIT, January was a month without teaching duties, and that is the coldest month in Boston. My mother-in-law lived in a senior home here in Palo Alto. So, Miriam and I would come here to spend the month of January. Stanford was always welcoming. Our two children had finished college (and medical school for one of them) on the east coast. Independently they both came to the Bay Area, so that was an important attraction. So, when my relationship with the people at Stanford became stronger, and they were willing to entertain the idea that I would be a long-term visitor, that was very attractive to me.
In all of your collaborations, papers, research endeavors, what has surprised you the most in physics?
The fact that the level of mathematics needed for fundamental theory has increased markedly over the years. Young theorists, especially string theorists, have to know subjects such as topology, complex geometry, algebraic geometry and more.
I understand that a result in your early work on neutrino scattering was confirmed experimentally more than forty years later. Tell us about that. When did you first get involved?
That was back in 1974, soon after the weak neutral current was first discovered. People were developing field theory models to describe the experimental situation. I was visiting Fermilab, where some of the neutral current experiments were done. A theorist named J.J. Sakurai came from UCLA to give a talk. His model assumed that the weak neutral current was proportional to the baryon number current. Baryon number counts the number of protons plus neutrons in an atomic nucleus. I left that seminar and began to think as follows. If the weak neutral current is coupled to baryons, there should be enhanced effects when neutrinos are scattered from a heavy nucleus. In the low energy limit, you should see the whole nucleus recoiling from the scattering. If the nucleus has mass number A, where A is the number of protons plus the number of neutrons, there would be an enhancement in the neutrino cross section by a factor of A-squared. That can be a huge number; for example, iron has baryon number A=57, and iodine A=127, So legendarily tiny neutrino cross sections might become measurable. So, I calculated the cross section. It was the paper that took me the shortest length of time to research and write, less than two weeks, as I recall. Forty-three years later, the effect was confirmed. It took forty-three years, because the experiment has to detect the nucleus that recoils from the collision. That recoil energy is very, very tiny and very, very difficult to measure. It took many years of detector development, to reach the sensitivity needed. The first experiment was done at Oak Ridge National Lab in 1977. It used a sodium iodide detector. Astrophysics have speculated that coherent neutrino scattering is important in Type II supernova explosions where scattering from the dense iron core of a heavy star is relevant.
You called this suggestion an act of hubris. What did you mean by that?
It seemed to me that it was too daring to propose it when the experimental situation appeared to be so difficult.
And yet, you were vindicated.
It took forty-three years, but I was vindicated.
To return to our earlier discussion of super-partners, I wonder if that gives you optimism, that sometimes decades of patience is required for the experiments to catch up to the theory.
That's a lesson which could be drawn, but forty years is a large fraction of a human lifetime. Is it realistic to be patient over that period? I don't know.
As our interview comes to a close, is there anything that you would like to add to topics we have already discussed.
Yes. Our discussion of supergravity focused on N=1 models and the problems of super-partners. But supergravity is a much broader subject. There are theories for all spacetime dimensions from D=2 to D=11. The case D=11 is related to M-theory. In 4 dimensions there are “extended” supergravity theories with one graviton plus N gravitini, from N=1 to N=8. Elegant ideas are involved in these theories. There is a large community of theorists who have worked on them. The INSPIRE archive lists over 14,000 papers with the word supergravity in the title or the abstract.
What I hear in your remarks is an adherence to supersymmetry despite its immediate experimental prospects. Is that a belief or is it something more?
It's a belief which stems from confidence in the powerful symmetry which underlies the subject. Some human beings indulge in beliefs which have no basis whatsoever. Some of those beliefs are destroying our society at the moment. My belief in a credible and interesting physical theory isn't going to hurt anybody.
It's been a great pleasure spending this time with you. I will give you the last word.
Let me end on a different note. To work closely with many bright young physicists has been incredibly rewarding throughout my career. It is a joy to see how their minds work when they crack problems that come along in our research. I have learned a lot from them, and they have made major contributions to our joint projects.