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Interview of Steven Kivelson by David Zierler on July 10, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Interview with Steven Kivelson, Prabhu Goel Family Professor of Physics at Stanford University. Kivelson recounts his childhood in Los Angeles as the son of academic scientists, and he describes his transition from career ambitions in the law toward physics. He discusses his undergraduate experience at Harvard, and he describes his lack of appreciation of the stature of many of the physics professors, such as his advisor Paul Martin, whom he knew first as a friend of his parents. Kivelson explains his decision to continue at Harvard for his graduate degree, and he discusses how he developed his interest in amorphous semiconductors under the guidance of Dan Gellat. He recounts his postdoctoral work at UC Santa Barbara, where he worked with Bob Schrieffer on the physics of conducting polymers. Kivelson discusses his first faculty position at Stony Brook, and he discusses the excellent group of graduate students he advised during his tenure there. He discusses some of the broader research questions in condensed matter of the time, including the significance of macroscopic quantum tunneling, invented by Tony Leggett. Kivelson explains his reasons for moving to UCLA, and he discusses Ray Orbach’s efforts to make recruitment a priority there. He discusses his long interest in fractionalization with regard to conducting polymers to be generalized to spin liquids, and his move to Stanford, which attracted him in part because of the condensed matter experimental group. At the end of the interview, Kivelson discusses his current research interests in exploring well-controlled solutions of paradigmatic models of strongly correlated electron systems, and he explains why the concept of a grand unified theory of physics is not a scientific but rather a religious proposition.
This is David Zierler, oral historian for the American Institute of Physics. It is July 10th, 2020. It is my great pleasure to be here with Professor Steven Kivelson. Steve, thank you so much for joining me today.
Okay. So, to start, would you tell me your title and institutional affiliation?
I'm the Prabhu Goel Family Professor of Physics at Stanford University.
And, if I can put you on the spot, can you tell me a little bit about Prabhu Goel?
Yeah. So, at Stanford, endowed professorships are really something that's useful for Stanford. It means that my salary is paid by an endowment that was given by a donor rather than by Stanford. That and I got an actual chair (laughter).
What I'm asking is, is Goel—
I met the- is Goel what?
Is Goel connected to condensed matter physics in any way?
No, but I did meet the Goel family. They made this very generous donation to Stanford. Stanford made me the Prabhu Goel Family Professor, and there was a dinner to introduce us to each other. And they were very generous. I was happy to do this. But I must say I wasn't looking forward to it. But then I met the Prabhu Goel family and just was blown away by them. They were so interesting and so accomplished in so many ways. Prabhu Goel is the patriarch of the family. I think he was an IBM executive as well as being involved in R & D. He's also done quite a bit with hotels in India, but also with very significant educational philanthropies in India. His children all went to Stanford and they're very grateful to their Stanford education, so this was their way of giving back. The children are all extraordinarily accomplished and interesting and devoted to sharing their good fortune with other people.
So, it was an astonishingly positive experience. I've kept up to some extent with them. You know, everybody gets busy. But they're really remarkable people. I'll tell you the thing that I remember that they initiated- it is an educational philanthropy in India which funds academically gifted underprivileged students going to college, and it pays all their expenses, including any tuitions and expenses for living and for books and so on. And there's no official quid pro quo. The money is just given to them as a grant. But it's suggested to them that, if and when they become economically stable, that they might want to give back to the foundation so that it can continue with its work with other students. And, apparently, the foundation has become self-sustaining just on optional donations by the people who've benefitted from it. I can't remember the numbers, but it's supported some really large number of people who otherwise would not have been able to have a college education.
Wow, that's wonderful!
Yeah, that was really great.
Well, let's take it right back to the beginning. I don't think it's possible for you to have been born into a more scientific family, so I want to hear a little bit about your parents. Tell me about, first of all, where your parents are from.
My parents both grew up in Manhattan, actually a few blocks from each other, but they didn't know each other. They both attended Harvard as undergraduates, where they met and fell in love and married. And they stayed on there for graduate school.
And where did you grow up?
I was born in Boston, but we moved to Los Angeles when I was 1-1/2.
I grew up in Los Angeles.
And growing up, did your parents involve you in their careers, both scientifically and academically in terms of, did you grow up knowing what it was like to be a science professor from your parents?
I guess so. Certainly dinner was a very serious business in our home. First place, dinners were always beautiful. They were beautifully set. It was a multicourse meal. At the time, this seemed like a normal thing. That's what families do. In retrospect, how my parents found time for such elegant living while pursuing academic careers is really astonishing to me, but they did that. Understand that by "serious affair," I don't mean grim; I mean we engaged in discussion which was always fascinating at dinner. While many of the topics were intellectual, we rarely discussed science. It was largely history and politics and philosophy. They also entertained rather regularly, and a large fraction of the people who came over were academic colleagues. And we certainly enjoyed—that's my sister and I enjoyed being part of this adult discussion from very early ages. In that sense, it is very much the case that their careers shaped me. Also, I certainly saw how much my parents liked what they did, how positive their interactions were with their academic colleagues. However, I never was aware of being groomed for anything or that they had any particular expectations of me in this regard. Just - intellectual pursuits were fun (laughter).
Growing up, Steve, did you have an appreciation for what a pathbreaker your mom was? I mean, it's not normal to have a mom who's this unbelievably significant person in physics as a woman in her time.
Did you appreciate that when you were a kid?
Not at all. I mean, I did understand that most of my friends' mothers were at home and were homemakers and my mom was different in that sense. But, no, it just seemed normal.
Yeah, yeah. Did you go to public schools throughout?
I did, yeah, yeah.
And how was your education in high school in math and science, was it a strong program?
I don't know how to really judge. I certainly never had a math course that I liked. However, I was lucky in high school; I had quite an extraordinary physics teacher.
And that may well have made a difference. I know at least three prominent practicing physicists who were students of his, and this is in just some urban high school.
Do you remember his name?
His name was Bill Layton. And, actually, I was later a colleague of his at UCLA. He was, some years later, hired to run a master's in science education program at UCLA. So, when I was on the faculty there, I had the opportunity to be a colleague of my high school physics teacher.
So really, it was as much a teacher as your mom as a source of inspiration for you to go into physics?
It's hard for me to tell. When I went to college, I definitely was not going to go into physics. I was not going to go into science. I wanted to be a constitutional lawyer. More specifically, I wanted to be Thurgood Marshall (laughter).
(Laughter) A lawyer!
But I wanted to argue civil rights cases in the Supreme Court. That's what I really wanted to do. And I also- despite this very positive impression I had of my parents and of scientists from interaction with my parents- I thought that going into science would be an immoral thing to do. You have to remember, this was at the height of the Vietnam War.
And, other than this physics teacher, my best teachers in high school were my English teachers and my history teachers, and many of them were very hostile to the science-military-industrial complex.
So, Steve, is this to say, when you were thinking about science, you were thinking about atomic bombs and agent orange and napalm?
And the electronic battlefield, yep.
Yeah. Even though you had such close affinity for your parents and what they were doing—
—which had nothing to do with that?
Admittedly, there's a bit of a cognitive dissonance there. How I could know that my parents and their colleagues were such good and ethical and devoted people and yet have this negative ideology about science, I don't know, when you're young, there are some connections that haven't been made (laughter).
So, you're on your way to Cambridge thinking you're going to become the next Thurgood Marshall, that's the plan?
Did you apply anywhere else or was that it for you, Harvard?
No. It was such a different time. I applied to Berkeley. Since I was in the top ten percent of high school graduates, I was essentially guaranteed to get into Berkeley.
So, my gosh, I was not worried in the slightest. It was so different than now.
I decided to go to Harvard partly because I guess my parents had filled me with stories about how much fun they had there. Also, my parents had gone to Harvard for a year when I was in sixth grade on sabbatical, and I actually had a miserable time there, but I had all sorts of positive memories of it (laughter).
And, particularly from your mom, did you have a sense of the luminaries that she worked with during her time there?
Not a clue. So, there were a number of luminaries there who I knew well as friends of my parents.
For instance, Paul Martin was a very, very close friend of my parents.
But again, as I said, there are a lot of cognitive dissonances in my youth (laughter). And Paul Martin was this friend of my parents. He wasn't this illustrious Harvard professor (laughter).
Right, right. Now, when you got to campus, your sense of the Vietnam War was pretty well developed. By the time you got there, had the antiwar movement pretty well died down or it was still going strong by the time you were there?
It was one of the first really transformative things. The first year I was there, there were still active protests, I got to participate in the takeover of some administration building, which was really fun. We got tear-gassed, which was sort of fun. There were police marching through Harvard Square. The part that wasn't fun was that I got a draft number and my draft number was 120-something, which meant that there was a very good chance that I would be drafted, and that was quite alarming. But that was the last year. They stopped the draft my year, so only the first few were taken, and then the draft was eliminated. They went to the Volunteer Army. And, soon after that, while there were still protests on campus, their intensity and focus dropped off considerably. And, yeah, even at the time I realized that what this meant: I had been under the impression that we were protesting the War based on principles and, instead, it turned out, quite legitimately, we were protesting that we might die and, in all fairness, that we might be forced to kill.
There was still a lot of campus activity, but after my first year, the intensity of the political activity on campus diminished considerably.
And, Steve, in terms of your own academic interests, when did you sort of switch gears?
Well, I majored in physics. I majored in physics because it had fewer requirements than any other major and because I knew I liked it.
Even though you wanted to pursue a legal career at first?
Right. It had fewer requirements than any other major, so it allowed me to pursue my interests. I was interested in taking the physics classes anyway. I ended up, I think, probably taking as many history classes as physics classes as an undergraduate. My sophomore year, I guess, I took a course on constitutional law that was taught by a professor in the law school. It was a fantastic course. It was really one of the best courses I've ever had. But he told us that there had been riots at Walpole Maximum Security Prison, which is the federal maximum security prison for New England, and that part of the arrangement that got the prisoners to step down was that there would be civilian observers in the prison twenty-four-hours a day to prevent retaliation by the guards. And he said, "This is an opportunity for you to both do something good and at the same time to get to know something about the legal system." So, for the next maybe two years, I went there something like once a month or once every other month and would spend 10 hours locked in there as a civilian observer. And, well, there were two things that became clear to me. I got to know the prisoners, some of them. You know, the prisoners were bored, and we were something new, so I would spend all night talking to guys there. And they were really interesting and bright. And this place was just inhuman.
And I thought to myself, if I put somebody in prison, how could I live with myself thinking about what I had just done to them? And, on the other hand, most of these guys had killed somebody and had some sort of impulse control problems. And even they told me that if they got out, they'd likely end up killing somebody again. And I thought to myself, if I got somebody out of prison and they killed somebody, how could I live with myself? And somehow physics seemed quite a bit simpler than that (laughter).
Steve, who were some of the professors as an undergraduate that you became close with?
Close with? I don't think I became close with any of them.
Was that not the culture of the department for an undergraduate?
No. Now, certainly there were professors who inspired me. Indeed, the other thing that made me go into physics was, as an undergraduate, I took the graduate quantum mechanics class from Sidney Coleman.
And I was unprepared for it. I got a- I don't know, a B the first semester, and I've never worked so hard for a B in my life. Then, by the end of the second semester, I did well. I worked my butt off for this course. It was thrilling. It is another identifiable influence that convinced me that maybe physics was what I wanted to do with my life.
It was an amazing course. The other courses were good, but that one was amazing.
And that was probably formative for your graduate work, as well?
Yes. Yeah, yeah. I have to say, contrasting teaching styles to nowadays, Coleman was not nurturing in his approach to the students (laughter). He was inspiring. Very different things.
Yeah. One of the things he said was, "If you get back your exam and you compare notes with one of your colleagues and if you find that I've given them a higher grade for something that looks similar to what you've done, please feel free to bring in both exams and I'll lower your friend's grade” (laughter).
That's great. Steve, by the end of your undergraduate career, did you know that you wanted to focus on condensed matter or that came only later on?
No, that came later on.
And what was the terminology at that point? Had it transitioned from solid state—
No. It was called solid-state physics.
Yeah. I don't know. It's hard to know. So, yeah, Paul Martin was my undergraduate advisor, because he was a friend of the family, and he made it very clear to me that I shouldn't go on in physics. He made two arguments: One was that there were no jobs in physics, which was—
At that time, for sure.
—absolutely true at that time.
The second was that I wasn't one of the top physics students, and that doubly made it an unpromising avenue. He was, of course, completely right about this as well. I mean, this was good advice, but I didn't know what a job meant. This was sort of interesting. I wasn't thinking about what was going to happen after graduate school. I rarely thought about what was happening after next week (laughter). So, his advice, while I completely believed that he was giving it honestly and probably correctly, had no impact on me whatsoever (laughter).
What made you not listen to that advice?
No. As I said, I just didn't think about the future. This was a very different time, in this respect, I don't think I was out of step with my generation. The idea of thinking about a job, I mean, why did you think about jobs? This was interesting, you did it.
Yeah. This is not how students think today, of course.
No. I understand. It's very different. And I think, in some ways, healthy. I mean, maybe part of it was based on the thought that probably we were going to die in a nuclear war anyway so why think about the future? (laughter)
(Laughter) Did you apply to other programs or it was just automatic for you to stay on at Harvard?
No, I did. In fact, everybody told me that I should go somewhere else. And I applied to and got to Cornell, which would possibly have produced a beautiful symmetry in that Ken Wilson was, of course, there.
My father was a student of E. Bright Wilson, who was an extremely distinguished physical chemist at Harvard, and who happened to have been Ken Wilson's father (laughter).
But my then girlfriend was still an undergraduate at Harvard and that trumped other considerations (laughter).
Did you take any time off or you went straight through?
No, I went straight through.
And, to get back to the question of when you developed the specialty in condensed matter, or solid-state at the time, when did that happen?
I don't know. Sometime in my first year of graduate school. I remember one incident, whether this was all that important or not I can’t honestly say, one tends to simplify history and memory.
But I remember one incident where I was hanging out with a friend and she asked what was I studying? I said, "Well, I'm taking a solid-state physics course. We've been studying the properties of silicon." And she said, "So what does it look like?" And I said, "Well, it's shiny." But, you see, I'd never seen silicon. I'd never read anything about what silicon looked like, but I knew that its band gap is smaller than the energy of visible light so that it should behave like a metal. And then, the thought that, based on theoretical reasoning, I could answer a question as concrete as, what does it look like, that seemed really cool to me (laughter).
Yep. And when did you connect with- help me pronounce his name- is it, Gellat?
Gellat. When did you connect with him?
I don't remember. He was an assistant professor. He was in Henry Ehrenreich's group. I sort of started hanging out with Henry Ehrenreich's group, and Dan was super nice and super bright, but I don't remember actually how I ended up working with him.
Mm-hmm. Was working with Bert Halperin, was that a possibility?
Bert came after I was already there.
And, yeah, I guess it would've been a possibility. As I said, I hadn't been serious about physics as an undergraduate. I didn't really understand what a singularity Bert was.
And working with Dan was- Dan was such a wonderful person, so it was very pleasant. I never thought about working with- I guess I must've already been working with Dan when Bert arrived. I'm not sure. But it never occurred to me that I should even think about it (laughter).
Yeah. Or even consider what a different experience it is to work with a senior person versus an assistant professor?
Yeah, yeah. In retrospect, I think I couldn't have chosen a better course. I was not very sophisticated. I was not very knowledgeable. If I had been in amongst the best students who decided to work with Bert, I probably would have been discouraged.
And, as a corrective to your naivety, did it occur to you to ask your mom for advice? Did you not want to do that? Did she not want to do that? I mean, talk about having an ace in your back pocket.
I never did.
It's not that I was hiding things from my parents. I've always been really close with my parents. Our relationship has always been really interesting and open. It's just, again, it was a different time. When you went off to college, you were on your own.
It wouldn't have occurred to me to ask my parents' advice on these things. It wouldn't have occurred to any of my friends to ask their parents' advice. We were now off at college or graduate school. I mean, we're not babies anymore. I mean, we were babies, but we didn't know that (laughter).
Graduate school, what was the split between coursework and lab work for you?
Well, I was a theorist, so I didn't do lab work. I don't remember. I didn't take a lot of courses. The course that I definitely learned the most from in graduate school was one I didn’t strictly “take.” I was the TA for Bert Halperin's first course. He taught the graduate solid-state physics course, and I was his TA. And, I don't know, something like thirty or forty people signed up for it. Within about two or three weeks it was down to four students. He had come from Bell Labs. He didn't appreciate how the course was intellectually a work of art. It was beautiful. But it was very demoralizing for the students. And you know how, as a TA, you usually can count on some student or other doing the problem set right so, in grading them, you can first crib from whatever student happens to do the problems right. I think that, during the course of that semester, not a single student got a single problem on a problem set right. So, I worked my butt off in that class and learned an awful lot (laughter). That class also stands- it's not a class I took, it's a class I taught. But it stands out as one of the major educational milestones in my history.
And, Steve, how did you go about developing your dissertation. Did Gellat hand you a problem that was related to his research?
No. I worked on something that he hadn't worked on at all. In fact, nobody in the Ehrenreich group had. The good news about my thesis work is it really was all mine. That's more or less all one can say that's good about it (laughter). I might've been better having received some guidance.
And what did you work on? What was your dissertation?
I worked on localized states in amorphous semiconductors.
And when you say you developed this on your own, how did you come to this topic?
Well, I don't remember, but the book that affected me and the model I was following was a book on disordered systems by Sir Nevill Mott. I fell in love with the way he did physics. And he makes all these physical arguments and sort of pulls numbers out of a hat and uses intuition on this and intuition on that. I didn't really understand that A) that was how Nevill Mott did physics; that wasn't necessarily a good model for how everybody should do physics. He had some particular intuitive genius that you couldn't bottle. And, B) he made lots of egregious errors, as well.
But, anyway, I modeled my approach to physics on Mott. I developed a lot of good habits. I thought about things on my own. I spent a lot of time studying the experimental literature and talking to people in the experimental groups there. Dan was always extremely helpful and supportive with my work. I had a wonderful education, from the point of view of learning how to do physics and learning how to be self-reliant, again, I couldn't have imagined a better experience. However, I could've imagined being more aware of the ways that physical arguments can lead you astray and that they need to be backed up by more rigorous analysis.
So, you've already sufficiently denigrated your own dissertation, but if you would permit yourself, what do you see as some of your contributions with it?
With my dissertation?
I don't think my dissertation made a significant contribution to physics. I mean, it's fine. Certainly, it prepared me for later work in good ways, but my thesis is not particularly outstanding in any way.
And how did your postdoc at Santa Barbara come about?
Okay. I think I only received two postdoc offers. I don't remember how many places I applied to. I received a postdoc offer from Chicago and I received a postdoc offer from Bob Schrieffer. My suspicion is, I did talk some with Bert and I think I made a good impression on him so, I think that I may well owe some of the opportunities I had to a good word Bert put in for me, but I don't know that directly.
I think Henry Ehrenreich also thought well of me, and so it may well be that his letter also carried quite a bit of weight. Also, though, Bob Schrieffer had gone through a bad period, which I didn't know anything about, just prior to my postdoc. So, it may be that it wasn't—for people in the know, it wasn't as attractive a postdoc as they didn't know how attractive a postdoc position it would be. At any rate, Bob was still at Penn and I actually did not want to go far away because my still then girlfriend, same girlfriend as when I was choosing graduate schools, was then a graduate student at Columbia. When I applied, the fact that Philadelphia was close to New York was also an attractive thing. And I think that may have been what decided me to go there rather than Chicago. Between the time I applied and the time I took up my postdoc, two things happened. One is my girlfriend fired me and the other is that Bob Schrieffer decided to move to Santa Barbara.
So, I went to Penn for three months and then moved with him to Santa Barbara. And every aspect about that was fantastic. Working with Bob was transformative for me and being at Santa Barbara at the beginning of the ITP couldn't have been a better spot for a postdoctoral position.
Yeah, right. And I wanted to ask; you know that I talked to Doug Scalapino; I've also talked to Jim Hartle, and they talk about how exciting it was ten years prior to come to Santa Barbara, which was really in growth mode and they were really looking to establish a powerhouse for physics. So, I'm curious, from your vantagepoint ten years later, how complete did you see that process? Was it still in growth mode or had those founding fathers really- had they achieved what they were looking to by the time you arrived?
That's not really a question I can comment on. As a junior person, you don't think about institutional issues. The ITP was quite a thing major institution that they attracted there. Certainly, I felt that I had come to the center of the physics universe. Everybody was coming through. It was new, it was thrilling, it was the best people in the world coming there for programs. And so, I knew I was at the center of the universe. I guess the only other place that I would've thought was comparably central would've been maybe Bell Labs or maybe IBM Yorktown Heights. But I think, as far as I could tell, ITP was the center of the universe.
So, it sounds like mission accomplished really in ten short years.
What were you looking to accomplish there? Was this an opportunity for you to continue on with your dissertation topic or to take on new projects entirely?
No. The fact that I had studied localized states and disordered systems allowed me to bring something to the circle of ideas that were developing around Schrieffer that nobody else had, and to be able to do work that was the work that got me my first recognition. But, no, I was learning new things. I switched topics to the things that Bob Schrieffer was interested in. And then, also, the other thing that happened there was that during my first year there was a workshop at Santa Barbara on common problems in field theory and statistical mechanics, which brought together all sorts of people who were applying insights from the renormalization group to problems that spanned all of theoretical physics. And that was something of a revelation to the physics community in general. But, for me, it just blew my mind. It totally changed my view of what physics was and what condensed matter physics was. And so, yeah, all sorts of things changed about how I saw physics during those years.
And what were some of the things that you accomplished in terms of your own research output?
I worked with Bob Schrieffer on the physics of conducting polymers, on solitons in polyacetylene. The work that I did on my own was to try to understand how the measurable transport properties of polyacetylene reflected this beautiful soliton physics that Su-Schrieffer-Heeger had developed. That was work that I did. Of course, I had guidance from people around me, but that was done on my own. It's not among the works of mine that I love the most. I was still, I think, naïve in some ways. But it was original, it had quite a bit of impact, and it was of topical importance at the time. Then, the other thing was that I did make some contributions to more fundamental problems at the time. These were not things that were or even now have generally been that recognized, but there is a body of work I did on proving that fractional charge is a sharp quantum observable.
And that is work that I'm still very proud of. It was fundamental. There were all sorts of people, people like Victor Weisskopf and other heavy hitters who argued that fractional charge could never be a sharp quantum observable. I wrote two papers, one in collaboration with Bob Schrieffer, and the other on my own, constructing a rather precise proof that in these one-dimensional contexts, fractional charge is indeed a sharp quantum observable. And that was work that I- even the work with Bob that I initiated. I mean, I got lots of guidance from him. Bob is one of the towering influences in my life.
He was just as good a mentor as- both with Dan and with Bob, I've had as good mentors as I could've possibly hoped for.
Although, it sounds like, in terms of creating your identity as someone with the capacity to make real contributions, your time at Santa Barbara was in some ways more formative?
It was. But I think that that was because I was immature, that I needed the time, the slow-growth time that I had at Harvard. I guess that if I had tried to become a full-fledged physicist at that time, I would've failed.
This is exhibit A for what a postdoc is supposed to accomplish, of course.
Right. And then, also, the other thing that I should stress is that part of what made Santa Barbara so formative for me was the other postdocs there from whom I learned enormously, many of whom were much more sophisticated and knowledgeable and brilliant by far than I am. Certainly, Sudip Chakravarty, and Eduardo Fradkin, Stephen Shenker, these people really shaped my view of physics in a very profound way.
Yeah. How was the job market in the early 1980s?
Right. It got better, so the year I was hired. I think there were something like four or five faculty positions in the U.S. in condensed matter theory. Jorge Hirsch was clearly the top candidate at the time, and was offered every position, and the rest of us waited until he had decided where to go. And then I got offered the position at Stony Brook. I think I owe that in large measure to the intervention of Sudip Chakravarty, who had been a friend of mine as a postdoc and then had gone there previously as a junior faculty member.
Again, I don't remember fretting about it all that much. That's a small number of positions. There were certainly a much larger number of people who were eminently qualified for these positions. It was far from obvious that I would get one of them.
Although, that seems like it would be a reason to fret about something like this.
No. I understand. That's what I'm saying. There was every reason to fret. It is just that thinking about the future was not part of my makeup (laughter).
Well, so far, so good. It's served you well so far (laughter).
Yeah. Planning has never been one of my strong suits.
Stony Brook, of course, is a very different kind of environment than where you went to school at Harvard. I wonder, in what ways did you have to adjust to the very different kind of undergraduate student body that you would be interacting there than you would at a place like Harvard?
I did teach some undergraduate classes. I mostly remember my interactions with graduate students there. I can't tell you what fraction of my teaching was undergraduate and what was graduate, but the interactions that made the impression on me were with the graduate students. And I think, at that time, the graduate students at Stony Brook were as good as anywhere in the world. One of the things was that, for reasons about which I'm not completely clear, Stony Brook was able to accept a lot of foreign students.
Anyway, we had just off-scale students. The fact that we had off-scale students from China was, presumably, in large measure due to C.N. Yang's presence.
But we also had incredible European students and Indian students. And they came partly for Yang and partly for Peter van Nieuwenhuizen and partly, I don't know.
Well, what about Brookhaven? Brookhaven must've also been an attraction?
For the students? Maybe. I don't know. I would imagine for most of the foreign students they didn't really understand that Stony Brook was near Brookhaven.
And the students that I dealt with were mostly theory students. So, I think that to some good approximation they came really because of the presence of two extremely prominent people, Yang and van Nieuwenhuizen. Maybe for Gerry Brown, as well. He also had a very large reputation in Europe. In any case, we had phenomenal students. And, of course, I had amazing students working with me during that time, as well.
Who were some of the standout graduate students that you had at Stony Brook?
Well, Shoucheng Zhang. He came because of Yang. He worked with Peter van Nieuwenhuizen, but in his last year of his graduate work, he switched to work with me on solid-state physics, so I was his solid-state physics graduate advisor and I unfairly claim him as my student. And he was usually agreeable on that (laughter). Shivaji Sondhi, who's now a professor at Princeton. Jainendra Jain, who worked with Phil Allen but did the last year or so of his graduate work with me, and I started him on the quantum Hall effect. So, although he wasn't my student, he's a student I worked with. He's now at Penn State and he's a Buckley prizewinner. Okay. Well, that's three.
There are others who worked with other people. Some of Sudip's students, Rajiv Singh, who's now a professor at UC Davis. Peter Kopietz, who's now a professor in Germany somewhere. I can't remember exactly where. There were just a lot of good students around.
And, in terms of your own research, what were you working on during the Stony Brook years?
I continued working on conducting polymers for a while. It was still an interesting topic. It was what I was known for. But I also started working on two other things that I've continued to work on for the rest of my life. One is the fractional quantum Hall effect, and the other is on quantum phase transitions in superconductors, especially, superconductor-to-metal and superconductor-to-insulator transitions.
And, more broadly, what were some of the major questions in superconductivity during that time?
There was a topic called macroscopic quantum tunneling. It was a topic that was sort of invented by Tony Leggett, borne out of his interest in whether there were fundamental aspects of quantum mechanics of macroscopic systems that hadn't ever been tested. I think his original idea in looking at this was that we were going to do experiments that were going to contradict quantum mechanics. It rapidly became clear that the ideal sorts of systems to do explore these issues in were superconducting devices where you could try to control the degree of dissipation as much as possible. And Sudip Chakravarty, my colleague at Stony Brook – in my opinion he did one of the absolutely seminal contributions to this field showing that, as a function of the strength of the dissipation, there could be a phase transition in some small superconducting devices from a regime in which it behaved quantum mechanically to one in which it behaved as if classical. Or put another way, there was a transition from a regime in which the system could be in a quantum coherent superposition of two macroscopically distinguishable states, to a regime where, even just following the laws of quantum mechanics, it would behave classically. There was also Jim Lukens at Stony Brook who was doing experiments on small superconducting devices to look for these effects.
This is a field that eventually became the study of qubits with all of the excitement associated with the subject now. But we were interested in the problem for fundamental reasons. Quantum computing was not a rage at the time. The problem seemed important just because these were systems where you could hope to look at quantum effects at a macroscopic scale. It was through my interactions primarily with Sudip that I got fascinated in this problem. I collaborated with Sudip on a number of works on dissipative quantum mechanics and on quantum transitions in superconducting devices because this looked like a very fundamental problem. In a sense, we were way ahead of the curve in looking at superconducting qubits, even though we didn't know to call them that. We quickly transitioned to looking at associated so quantum phase transitions. Superconducting systems are suitable for this since they can be tuned through such transitions. There were exciting developments by Allen Goldman's group and by Bob Dynes's group. There was a lot of experimental data on quantum phase transitions and superconductors, and, correspondingly, a lot of intellectual theoretical foment, and I was involved with that.
Steve, I'm curious, also, from a theoretical vantagepoint, what were some of the advances in technology that were relevant for your research?
Honestly, I didn't spend a lot of time thinking about this, but, of course, all of these macroscopic quantum devices were directly a result of advances in microfabrication, as well as advances in materials, material purity, and so on. These whole fields were made possible by technological advances.
Right, right. And which drove which? Where the theoretical advances driving the technology or vice versa, or was it really a yin-yang kind of thing?
I'm really making you think (laughter).
Mostly, I'm going to admit, again, that I didn't spend a lot of time thinking about broad issues like this. From my perspective, I was fascinated by the theoretical developments, but I think that's probably naïve. There was a big effort to make superconducting computers. Not quantum computers, but computers whose logical elements would be largely built about superconductors. This was a very popular effort. IBM deeply invested in it, as did the DOE. Around the time I got interested in this subject, both IBM and the US government decided that it wasn't promising and dropped funding for it. None-the-less, I think that a lot of the technology that made possible the fundamental inquiries that I was interested in were actually the result of this technology-driven research. But, by the time I was interested in it, it was a period where everybody had decided that these areas were not technologically promising. And so, even as theoretical interest was ramping up for these things, funding for experimental research in these areas was diminishing.
Mm-hmm. How did the opportunity at UCLA come about?
Were you looking to leave Stony Brook? Were you recruited?
I was not looking to leave Stony Brook. I was very happy at Stony Brook. On the other hand, my wife hated Stony Brook (laughter).
Did she hate Stony Brook, or did she hate Long Island?
She hated Long Island (laughter).
(Laughter) Where's your wife from?
She mostly grew up in Palo Alto.
Okay. You can't blame her.
She loved New York City, but one of the things I learned when I moved to Stony Brook is that Long Island is long (laughter).
(Laughter) And they say they're from New York, but they're from Long Island. Very different.
(Laughter) Right. So, what led to this? Certainly Sudip was very unhappy at Stony Brook. He may have made his unhappiness known, I don't know. My wife also knows everybody in the world, so she may have broadcasted that I was recruitable. I certainly was not thinking about moving. I was very happy at Stony Brook.
And what was Sudip's issue with Stony Brook? What was going on there?
Well, I shouldn't probably talk on his behalf. Certainly, one thing was that there was a- just one second. [Unrelated conversation] There was a little bit of a two-tier system of citizenship. There was an Institute for Theoretical Physics at Stony Brook, which was high-energy physics and formal statistical mechanics. However, Sudip and I were not members of that. And that was both symbolic, but also members of the institute had half the teaching load of the rest of the people in the department. And I think Sudip, quite rightly, felt that he was sufficiently distinguished that he should be a member of this institute and was rather put out that he was being treated in this way as a second-class citizen. As I said, I tend to be fairly oblivious to things (laughter). So, probably if I had thought about it, I would have been equally outraged, but it just didn't occur to me.
And was the culture among faculty where these were not things that could've been hashed out and resolved?
In fact, they eventually were. In reaction to Sudip and me being recruited by other places, we were eventually offered positions in the institute. As I said, for me, it was a sufficiently minor issue that I didn't think about it. I was friendly with people at Stony Brook. I felt intellectually supported by people at Stony Brook. I was delighted with the students I was getting at Stony Brook, the postdocs I was getting at Stony Brook.
You were happy to just stay there?
Yeah. But, well, except for the fact that my wife was unhappy. That's, of course-you can't be happy if your wife isn't happy.
Happy wife, happy life. That's true.
So, then, how did UCLA come about? Was it just an opening and-
Oh, I don't know. Somehow it must've been understood that Sudip and I were movable, because we both got offers from UCLA and from Minnesota. And I got an offer from USC, as well, all at the same time. So, exactly how these things happened I don't know. At UCLA, I think in large measure, there were a number of things that led to this. One very big thing was that Ray Orbach was provost.
And Ray Orbach decided that what was important for UCLA was to recruit really top people. To do this, he realized he was going to have to concentrate the funds that he had available as provost to recruiting people who would enhance the scholarly reputation of UCLA. That's obviously a difficult decision to make because the university has lots of priorities and there are lots of demands on the provost's resources. But that was what he decided. So, UCLA embarked on a very aggressive hiring program of mid-career scholars. Ray, I think, managed to totally bankrupt the College of Letters and Sciences and go deeply into the red, which I think is something that should be admired. For example, at the same time that Sudip and I moved to UCLA, just in physics they also hired Roberto Peccei and Claudio Pellegrini and maybe other people that don't come to mind directly. But it was definitely part of an aggressive program. And it wasn't just physics. It was across the College of Letters and Sciences to focus UCLA's resources on this. Again, this is me reconstructing what happened after the fact. I got an offer- LA was home. The opportunity to move close to my parents and to move to Los Angeles, which I viewed as the nicest place in the world to live, was something that was extremely attractive to me.
I'll ask another institutional question, and you're a full professor at Stony Brook at this point, so you can't hide behind being a junior scholar to answer this one, and that is: At some point UCLA, and, as you said, the way that it was building aggressively, particularly in physics, it was looking to establish itself as a counter pole to Berkeley, right?
Did you feel that? Were you part of that wave? Was that sort of complete? Was that at the beginning when you arrived at UCLA?
Yeah. Unfortunately, this aggressive hiring didn't last long. It continued for a while after I arrived there, and we managed to make a number of other hires, mostly junior hires, but very fine people that we competed with the top institutions in the world to get. At some point, that commitment lessened. Both UCLA had some financial problems, but also, after Ray left, the priorities of the university changed somewhat. And there are things to be said in favor of this change, I am sure. Universities are called upon to do different things, so I'm not criticizing UCLA for this, but, nonetheless, there was definitely a change. Early in my tenure at UCLAA there was a period when, if you could think of something that you thought would really make a difference to the scholarly standing of UCLA, you could count on getting support from the administration. Later, the situation changed so that you felt you were lucky if you got support for any sort of initiative of this sort.
Did you bring graduate students with you from Stony Brook?
I did, yeah. I brought two students with me from Stony Brook.
And, when you got to UCLA, did you take on new graduate students right away?
No. It was a little while before I took on students.
Mm-hmm. And, in terms of part of this aggressive hiring, were you the condensed matter theory guy? Was that the niche you were filling?
No. So, both Sudip and I were hired at the same time.
Okay. So, did you see that kind of as a package deal?
Yeah, yeah. I mean, we were made separate offers but certainly the fact that Sudip was going to be there was very attractive to me. And I think conversely. We've been friends for a long time. We've been very effective collaborators with each other. We have very different personalities. We have somewhat different strengths and weaknesses as physicists, but we've been very successful at collaborating with each other. And that was certainly significant, although I probably would've gone to UCLA anyway because of personal reasons. But the fact that Sudip was going there really made it an easy decision.
Right, right. In what ways, Steve, was this an opportunity to take on new research projects and in what ways were you looking to continue with what you were doing at Stony Brook?
Yeah. My intellectual course mostly was independent of where I was. Being at Stony Brook, it was very natural to go into these quantum transitions and superconductors because then I could work with Sudip on that, so that's an example of my being affected by what was going on there. But, at UCLA, I think my course of study was not particularly determined by where I was. At that stage I had contacts everywhere, so.
Yeah. And so, what were you working on when you got to UCLA?
I did the last of my work on conducting polymers at UCLA. I worked a lot on the quantum Hall effects. The most successful of my quantum Hall work was done at UCLA. And I started working on high-temperature superconductivity because that was- well, I started that at Stony Brook actually. I should have mentioned that. I started that in 1987, so that was at Stony Brook. That became my primary obsession while I was at UCLA for some period. I also worked with Sudip on superconductivity in doped buckyballs. Actually, my work became much more spread out because I had more senior collaborators from around the world, so I was working on more different things. But high-temperature superconductivity was certainly very much the dominant thing I worked on while I was at UCLA.
And, obviously, this is a field that sustained your attention for quite some time. So, how had the field changed over the years, and in what ways were you contributing to those changes?
I've got a good story about that. In winter of 1987, I was at- I think it was still called ITP. I was in Santa Barbara.
Before Kavli, you mean?
Before Kavli, yeah. And Phil Anderson came through from a meeting in India where high-temperature superconductivity had been discussed, and he gave a seminar at ITP. And it was exciting, it was mystical, but it had pictures in it that invoked some of the deep physics of conducting polymers, which nobody else had understood.
And I immediately saw that some of these ideas of fractionalization that we understood well in conducting polymers could be generalized to spin liquids there. And I got very excited. I came home that night and I told my wife, "I know what I'm going to be working on for the next year." That was thirty-three years ago (laughter). Still working on it.
So, Steve, that's great. Explain what's been so compelling about it over the years.
That's complicated and there's a multifaceted answer. One thing is that there are really striking, robust, reproducible phenomena that are new and unlike things that we were used to before, starting with high-temperature superconductivity. Unlike many of the things. For instance, think of the fractional quantum Hall effect; it's spectacular, it's wonderful. It's one of the great developments of physics. But it's a little bit effete. You have to get the most carefully engineered devices. You have to go to super-low temperatures and high magnetic fields, and measure with exquisite care. High-temperature superconductivity by contrast, kids can make high-temperature superconductors with their Mr. Superconductivity kits in their garages. With a little bit of liquid nitrogen, you can do an experiment on magnetic levitation. It's not something effete; it's something that you can see with your eyes as exciting and new. And that's the most dramatic and the most robust feature. But there's lots of new phenomena that are not just of narrow interest. And so that's one of the things that makes the field thrilling.
Tied in with that is that the cuprate high temperature superconductors are certainly the highly correlated electronic system on which there is the most empirical knowledge. The most experimental effort has gone into it, improving the materials, improving the experiments. Well, maybe not as much effort as has gone into silicon or gallium arsenide, which are of enormous technological importance. But concerning anything other than a few semiconductors, I think there's no family of materials that's been studied with anywhere near this intensity. So, the number of facts we have, the level of certainty and precision that we have for what are the phenomena, are unprecedented. Which means that if you're interested in fundamental problems in correlated electron systems, you probably should be interested in how it plays out in the high-temperature superconductors.
Now, a lot of exciting ideas emerged in those early days, based on the conjecture that we are seeing something that reflects new and topologically ordered phases of matter, the behaviors of spin liquids and fractionalized excitations. This circle of ideas, as an intellectual branch of condensed matter physics, has continued to thrive and grow. And we have very profoundly better theoretical understanding of what those words mean. However, in terms of providing clues to what's actually going on in the cuprate high-temperature superconductors, it's not at all clear that those initial ideas that excited me and sucked me into the field are actually relevant. The things that I think are most relevant are things that, as theoretical constructs, are more conventional and more traditional. Still, exploring new regimes and new areas- I mean, there's plenty that's new, but it's not quite as conceptually fresh as some of the most radical ideas we explored in those early days. None-the-less, those new ideas instead have had intellectual children in somewhat other areas. They remain vibrant ideas, just not so much in the context of high-temperature superconductivity.
Consequently, what I worked on under the rubric of high-temperature superconductivity has evolved quite a lot over those thirty-three years. I'm working on rather different things than were the focus of my interest at the beginning.
When did you start to get interest in quantum Hall systems?
There's also a good story here. In the beginning, when the integer quantum Hall effect was discovered, it was the most exciting development in solid-state physics - as the field was still called, then. So, of course, I studied it. I studied Laughlin's paper on quantization. I studied Bert Halperin's paper on how that relates to edge states. I understood this stuff well. I gave informal seminars explaining what was going on. Now, it was natural, because I'd worked on fractionalization, that I was particularly interested in the fractional quantum Hall effect. I don’t recall precisely how this next step came about - possibly Schrieffer received a paper by Laughlin to referee, and he instead told them they should send it to me. In any case, I was the referee on Laughlin's first paper on the fractional quantum Hall effect. Already in this first paper on the fractional quantum Hall effect, he knew he was trying to find fractionalized excitations, but he used a rather straightforward analogy with fractionalization that occurs in one dimension with the work on conducting polymers that Schrieffer and Su had done, and which I knew quite a bit about.
Now recall that I was a postdoc at the time, while Laughlin was, although still a rather junior person, quite famous because he had already had this home run of explaining the precise quantization of the integer quantum Hall effect. Consequently, I spent a really long time refereeing that paper, and it was exciting, and it obviously had some ideas that were very promising, but there were some technical ways it was wrong. After I had spent a long time on it, I wrote a report. The reason I can be open about this is that somehow, much later, somebody told Laughlin that I had been the referee on this paper, so I'm not breaking a confidence here. At any rate, I wrote a review explaining what was wrong with the first paper, and Laughlin withdrew that paper and then, of course, soon after that produced his just totally brilliant, out of the blue solution of the fraction quantum Hall effect. But, at the very least, I saved him from that being his second paper on the subject. In my fantasies, I hope that some of the criticisms that I gave were helpful to him in getting to this second solution, but that you would have to ask him about. (laughter). At any rate, that's when I started really thinking hard about the fractional quantum Hall effect. Soon after that, Schrieffer was involved in a famous paper by Arovas, Wilczek, and Schrieffer in which they were the first to recognize that the quasiparticles in the fractional quantum Hall effect have fractional statistics. And this was just right up my line. So, after those two papers, I was working very hard on the fractional quantum Hall effect for some time.
And how long did you stay on quantum Hall topics; as long as superconductivity?
Well, I still am thinking about it and working on it, so, yes.
Maybe longer (laughter).
And we got off the chronology a little bit on the topical issues. Let's talk about the transition to Stanford. Same kind of question. What were the circumstances leading to that switch?
Right. Understand that I was completely happy at UCLA.
It's a pattern, I see (laughter).
Maybe you're going to be happy wherever you are if you have the right tools around you.
That's the generous conclusion. I like that conclusion. On the other hand, honestly, it's never been tested because really objectively the positions I've been in have all been pretty spectacularly good.
It's hard to complain about any of them.
Yeah. And were you impressed, I mean, you spoke so glowingly of Stony Brook. Just in terms of the physics department at UCLA and where it was headed, the dynamism of it, were you also happy in that regard, as well, just in terms of your perch there?
I was. I have to say that- I had a few very good students at UCLA, but the density of spectacular students I had at Stony Brook was higher. So, in terms of the students I had access to, that was actually a step down.
That's very interesting. Is it the foreign students; is that the issue, you think? Was that the pivotal role?
But it also may have been the role of having Yang there. We're talking about small numbers.
And Yang didn’t take students, so the fact that the students were coming because of Yang was a mistake on their part. But, nonetheless, I think it shouldn't be underestimated how big Yang was, and to a lesser but nontrivial extent there was Jerry Brown and Peter van Nieuwenhuizen. All of these people had extremely high reputations internationally. And I think foreign students in particular looking at the United States are going to be extraordinarily influenced by where they know there is this distinguished scholar. There were, from time to time, students at UCLA who were as good as students anywhere. Just the density was lower.
Yeah. So, one of the motivating factors-
And, on the other hand- what?
One of your motivating factors was you had more to offer as a mentor that you knew, in terms of your accomplishments at Stony Brook, than what you were given to work with at UCLA?
Yeah. That was actually one of the arguments in favor of going to Stanford.
You mentioned your wife is from Palo Alto. I assume that might've been—
That's the main thing. It was our tradition to come up to San Francisco every Christmas and other times and we would hang out with Pam's family, but we would also hang out with colleagues at Berkeley and Stanford. My wife is very strong-willed, and I think that she communicated that she would look well on us moving to Stanford (laughter).
(Laughter) Steve, did you take advantage while you were in LA to spend time with your parents who were close by?
The first year- no, eight months- we lived with my parents.
(Laughter) Oh, that's close.
And we lived less than a mile from my parents. Many days my kids went to an elementary school on UCLA campus, so there were some days when five of us commuted into UCLA campus in one car together.
Oh, that's so nice.
I saw my parents on campus probably daily. We had dinner with them a few times a week. It's hard to complain about Palo Alto, Menlo Park as a place to live, but, outside of the university, I actually would prefer to be living in Los Angeles.
And it was a hard decision for me to move.
Yeah. You're an LA guy, it sounds like.
I'm an LA guy. On the other hand, in my area of condensed-matter physics, I think Stanford is the best place in the world.
And where did this leave Sudip?
Yeah. Sudip and I continue to be close friends, and we talk with each other regularly, but he felt fairly abandoned, I think. I feel bad about that.
But you've maintained a collaboration?
Yeah. We haven't collaborated explicitly on a paper for some years, but we talk about physics regularly. We send each other our papers and get critiques on them. He's one of the physicists I consult with most often.
Mm-hmm. And you said that, as a move to Stanford in terms of condensed matter, that that made a lot of sense.
Why so? What was going on at Stanford when you got there?
Well, look, first place, they have this condensed matter experimental group—what used to be called solid-state physics and now I guess we would call hard condensed-matter physics group, that's both larger, more diverse, and more outstanding than anything anywhere else.
And this is theory and experimental?
No. In theory, Stanford is good, but there are a number of places that are at least as good. We're actually relatively small in theory, so I think that if you were to rank places strictly on condensed-matter theory, we would be one of the top places. But I would not remotely say we were the top place. But, in condensed-matter experiment, I would be prepared to defend this position.
Yeah. Right. And so, in terms of that being a draw strictly on the academics, when you were thinking about your next move from UCLA, thinking about condensed matter, were you also thinking widely, like an Illinois or a Cornell, or was it really Palo Alto was just like—that just makes so much sense on the home front and on the academic front?
Yeah. No, I didn't think about other places. I don't know whether I would've had opportunities at other places, but I didn't explore it.
I wasn't planning to leave. I thought I was living in the best place in the world to live. Especially in Northern California, to say that about Los Angeles is heresy (laughter).
Sure (laughter). Not just heresy, it's crazy. That doesn't even make any sense.
When you got to Stanford, did you sort of glom onto this group right away?
Yeah. There were a couple of things that appealed to me although whether I would've come here if it weren't for the fact that my wife was keen on it, that's a separate question.
Having gotten the offer from UCLA, the opportunity to move home, there's no doubt I would've moved from Stony Brook to UCLA.
From UCLA to Stanford, if my wife hadn't been keen on it, it's much less clear to me.
But the things that were motivating for me are three big things. One is having such brilliant graduate students and postdocs to be able to work with directly.
How did you know that that was true, though? I mean, did you just know by reputation that you had stronger graduate students to work with?
Yeah. Yeah. I wasn't sure that that was going to be true, but it was. Again, as I said, some of my students at UCLA were as good as students anywhere.
But just the density of really good students that I get at Stanford, it's a problem also. I'm now supervising way more students than I have intellectual bandwidth for. And I don't know how to do this. One manages to make problems out of anything. But, in terms of the number of off-scale students, it's just astonishing. This recent move to ban foreign students may change all that.
Yeah. Hopefully not.
Then, more generally, because of the large condensed-matter group here, both in theory and experiment, I saw an opportunity to participate in shaping the way the next generation of condensed matter physicists view the topic and frame the problems. And so, I spend a lot of time on the courses I teach. My office is open so lots of graduate students in the various condensed matter experimental programs come to discuss their work with me. That was appealing to me; to have an impact on the field through intellectual leadership of this larger group of brilliant young people. And then, it is extremely stimulating having these experimental colleagues coming to me with new discoveries that none of us know what to think about, and sitting around and brainstorming with them about what the hell this could mean, that's really thrilling. In short, the things that I thought would be good about coming here are, indeed, good. And UCLA, in the meantime, has suffered. It's suffered from an administration that is less committed to the scholarly research aspect of the university's mission. And that's reflected- UCLA has been losing good people at a rate that's exceeded the rate at which they've hired really good new people.
Yeah. Well, I guess Ray Orbach couldn't be running things forever, right?
No. No. And, in fact, he, in some sense, committed administrative suicide by over-spending. That's something that doesn't get you promoted. It does accomplish something great for the university, but for your personal career, that was a very brave and selfless thing for him to do.
Who have been some of your standout graduate students at Stanford in the early years, at least?
When I first came to Stanford, I got two new graduate students. One is Hong Yao who is now a professor of physics at Tsinghua University. He's actually been visiting here this year. And the other is Erez Berg, who's currently a professor at Weizmann Institute. He was briefly hired away by University of Chicago, but decided to move back to Weizmann, so he's there. These guys are just off scale. You don't encounter people with this sort of level of intellect (laughter).
And both of them think I helped mentoring them. I may well be wrong about this but I'll take credit for it anyway (laughter).
(Laughter) Steve, was your work on polymers mostly during your Stanford years or were you doing that prior?
On polymers, no. Polymers was mostly prior. Conducting polymers was what I worked on primarily as a postdoc. It was a large fraction of what I did at Stony Brook. It petered out in the early years at UCLA.
And what about glasses and super-cooled liquids?
Glasses and super-cooled liquids was a topic that my father was an expert on.
So, since we had coffee with each other most days and talked about science, I got involved in that. And I continued to dabble on it. I'm not an expert, but I collaborate with Gilles Tarjus, who was my father's closest collaborator. And the three of us collaborated together while my father was alive. But after my father died, Gilles and I have stayed in touch. He comes and visits regularly. I try to come and visit him regularly. And we talk about- we've collaborated on a number of things, but I like to think about glasses. I think it's one of the deepest and most important problems in condensed matter physics. Unfortunately, right now it's a problem I don't have any new ideas on. So, I'm not working on it now, not because I think it's solved and not because I think it's uninteresting, but just because I don't have any really good ideas. But, as you've noticed, I have a long attention span. I may not have the largest amount of intellectual wattage of people in my field, but I think I'm as stubborn as anybody (laughter).
What have been some of the major questions around this topic of studying how electrons interact with each other and how that translates to how solids behave?
I'm not exactly sure what the question is, but I can give you an answer, nonetheless. I wrote an article with my daughter. My daughter is- well, she's just graduated from college. She's about to start a master's program in CS, but she's had a longstanding interest and expertise in philosophy. And we talk a lot. When I tell her what I'm doing and why, she's usually fairly scornful of my explanations and rips my logic apart fairly effectively. And so, she and I spent a long time discussing what it is that you're trying to do when you're trying to understand a complex system. Ultimately, we wrote an article on this called Understanding Complexity. It's published in Nature Physics, but I can send you the link or you can look it up.
But I think that that is the- although I didn't completely understand what your question was, I think that article is my answer to it.
Okay. No. It just seems that that's sort of fundamental to your overall research agenda, if you're looking at how solids are behaving and you're looking at it from the particles.
Right, right. Let me explain to you why it's a very complicated question for me. There are two extreme views of what you're doing in condensed matter theory.
One is that you're interested in emergent phenomena where you can make, in line with the usual notions of the scientific method, you can make precise, quantitative predictions which are either right or wrong. If the number is wrong, you're wrong. Now, for complicated systems, since we can't hope to do quantitative calculations, there's only very specific questions that you could possibly answer this way. A large fraction of this is universal phenomena at critical points, like critical exponents.
If I could solve the Ising model exactly, I would be able to get exact values of the critical exponents that you could compare in the lab to measurements of critical exponents in any of a large number of systems that are in the universality class of the Ising model. And if I calculate the exponent and you measure something that's different from that, then either my calculation is wrong or your experiment is wrong, but it's really—we understand precisely what we're doing. We're after asymptotically exact results. Another example is the Josephson relation. I tell you that if you put a voltage across a Josephson junction, you'll see oscillations of the current at a frequency that's exactly equal to 2e times the voltage divided by Plank’s constant. And this has been tested to one part in 10 to the 17th accuracy. If you see any deviations from that, then probably your experiment's wrong. But, if not, there's something fundamentally wrong with the theory of superconductivity.
Such things exist. They're very beautiful. You understand what the criteria to judge theory is, which is the traditional criteria of the scientific method: either you're right or you're wrong, full stop. There is another branch of solid-state physics that says, look, our job is to understand the properties of materials. Critical points are extremely special points where the properties of materials are very anomalous, but mostly what we would like to do is to understand the properties of materials; why copper conducts and what the value of its conductivity is and why its structure is what it is and so on. And these are problems that you can't hope to solve exactly.
So, the other extreme of approaches is, okay, I understand that, but I'm going to do the best damn job I can. I'm going to start with Schrödinger's equation, I'm going to do an ab initio calculation. Yes, there are all sorts of uncontrolled approximations, but I'll do LDA, I'll fix up LDA with DMFT. I'll do whatever I can to get as quantitative a connection as possible between Schrödinger's equation and the properties of materials. When I went to graduate school, this was the primary focus of solid-state physics. It's the band theory of solids. An astonishing amount of progress has been made on this. It doesn’t much appeal to me because it doesn't have any of the mathematical beauty and sophistication and control of the universal phenomena approach. And I don't really understand well why certain calculation should work and others shouldn't. I mean, I accept that they do, but it doesn't appeal to me intellectually. So, I want to use mathematics the way one would if one were studying universal critical phenomena, but I want to study more robust phenomena in condensed matter, so I'm working in a middle ground where I'm neither prepared to start with the microscopically realistic Schrödinger's equation, I'm always going to be starting with some simplified effective model problem, but from there I am trying to do mathematically well-controlled solutions of stripped-down models, and then comparing the results of those qualitatively to experiments in real materials rather than quantitatively. And so, exactly what questions I'm trying to answer and exactly what qualitative comparison means is a little bit vague.
I think it's valid approach. I think, on the one hand, it's too cute, it's too effete to only answer the questions that admit to asymptotically exact solution. And, on the other hand, I think it's too optimistic to think that one can really start in some controlled way with Schrödinger's equation and get an understanding of complex emergent phenomena. So, I'm trying to follow a middle course that has some of the merits of both extreme courses, but, of course, it also has some of the demerits of both extreme courses. And so, it's a little hard for me to give you a very precise articulation of exactly what's the structure of what I'm doing.
Right. Steve, besides these two long-range interests, what else are you working on currently?
A large part of my effort for some time has been to try to obtain, in whatever limits I can, well-controlled solutions of paradigmatic models of strongly correlated electron systems. For instance, the Hubbard model or the Holstein model, these are the simplest models of electron-electron interactions or of electron phonon interactions. They're very stripped-down models. They're not realistic models, representations of the local quantum chemistry of any material. But there's a lot of evidence that they have much of the essence. Many of the same phases seem to occur, many of the same scales seem to emerge. And these are models that are simple enough, they're interacting quantum problems so a priori they're not solvable, but with the years we find more and more limiting cases where we can solve them in the limit where this thing is small or this thing is big or where we can find particular versions of the model that are amenable to numerical methods. I spend a lot of my time trying to explore these limits, these are abstracted problems. They're problems, if you want, in mathematical physics. But while what I have in mind is to learn about all sorts of phenomena in real materials, solving these models is an intellectual block that's independent of its relation to any particular phenomenon. To what extent can we really control the solution of these paradigmatic models that actually underlie a lot of our qualitative discourse but is often based on still uncontrolled solutions?
Steve, now that we're at the point of talking about your work in the current state, I want to ask you a few sort of broader questions. What ongoing work is needed in superconductivity from a theoretical perspective? Where does the field go from here?
That's hard to answer without being self-serving. Obviously, the right thing is to support the sort of research that I do (laughter).
(Laughter) I'm asking more on the basis of what is not known; what still needs to be figured out?
Yeah. So, look, for the most part, the big leaps in understanding in condensed matter physics follow from new discoveries that are made somewhat serendipitously in condensed matter experiment. There were a lot of ideas around about the mechanism of high-temperature superconductivity and how it related to the rather unusual magnetism that was seen in the cuprates. There were a lot of people who thought, and I have all along, that this is central—the relation between the magnetism and the superconductivity is central to the problem. We still have to work out maybe some of the details, but that this is the central feature of the problem. But there were lots of people who pointed out that there were lots of other ways that one could think about this problem. It wasn't so clear. With the discovery of the iron-based superconductors, which has a very similar interplay between high-temperature superconductivity and magnetism, to my mind, that broad question was settled.
And most people who work on the field agree with that. It's not universally accepted but, for the most part, the discovery of the iron-based superconductors and how magnetism and superconductivity interplay there, there are lots of differences between the iron-based superconductors and the cuprates. And they are not the same problem by any stretch of the imagination. That's part of what makes this argument so strong, that there are so many things that are different between the two, but this interrelation between unconventional high-temperature superconductivity and rather strong local moment magnetism is common to both. That makes the case that this is central to the physics extremely powerful. And even though it's a discovery of another material and another set of phenomena, it really has a big impact on how we think about the cuprate high-temperature superconductors.
Now, I know you're coming at this from a theoretical perspective, but I wonder if you can talk a little bit about what advances in superconductivity might be really relevant for our day-to-day lives?
I better move inside now. My battery is about to go dead.
Somebody was hanging on your every word.
No. She just got here. She's living in San Francisco. We're socially distancing, which is extremely difficult.
Having your daughter here and not being able to embrace her is just really painful.
Sure. It's a weird time. It's a weird time.
It's a weird time. What was I saying?
So, I mean, the excitement around like room temperature—
Oh, yeah. You said, how will it affect our lives?
I mean, so, for example, there's excitement around room temperature superconductivity, right?
What are we going to see in superconductivity that's going to make our lives easier, better, faster, so on and so forth?
Yeah. [Unrelated conversation]. Here, let me introduce- this is Sophia Kivelson.
Hello. Nice to meet you.
Nice to meet you, too. I'm going to go back to work. Have fun.
Okay. All right. So, let me first give you my prepared speech on this. This comes up. I think we have very strong historical evidence that advances in understanding of fundamental physics and, in particular, of the physics of materials does have profound positive practical importance in the long run for people and society. Thus, I'm sure that as we get better understanding of these materials and discover new materials, it will have important practical impact. There are some already that I can point to, but I predict that it'll be much stronger than this. That's the first statement. The second statement is that when physicists, and, in particular, theoretical physicists, tell you about what's going to be of practical importance, they're either misguided, lying, or both (laughter).
Okay. So those two statements are the only two statements in this field that I really believe I understand. Look, superconductivity is something that you can imagine all sorts of very important practical applications. It is less clear which aspects of superconductivity are important, whether room temperature is important or whether superconductors that are malleable and easy to make into wires is more important, or whether it is made out of cheap materials or if it is easily compatible with silicon or—there's all sorts of real-world practical things that are not particularly physics that have a big effect on what aspects of the fundamental materials physics are actually of any use or not. And that's why, in this particular context, I'm particularly reticent on this.
But there's various things you can imagine. One thing that people imagine is superconducting transmission lines so you would reduce power loss over long-distance transmission. That's almost certainly a stupid idea. We could reduce power losses by just making bigger wires. The current level of power loss on long-distance transmission lines is determined by economic considerations, not by physics.
And, unless we happen to find a room-temperature superconductor that's also made out of really cheap materials, there's no way it's going to be used for long-distance power transmission. Now, maybe people have talked about using it for power transmissions in cities where real estate is the important thing and so power density might be important and it might not matter how expensive the transmission lines are because, however expensive they are, they're cheap compared to real estate. Okay. That sounds reasonable to me. I don't know if it's true or not, but I could imagine that being true. You could certainly imagine that if you got to a stage where you could use superconducting interconnects in computers that that would be extremely beneficial because you would reduce the amount of heat that the computers give off. That would both reduce the power consumption of computers which, I gather is a fairly significant contributor to our total power usage. So, that's not a trivial thing, to reduce its power consumption. But, probably more importantly you can miniaturize much more, because I think that one of the limiting things in how dense you can make logic elements in computers is that you have to dissipate the heat. And so, if you reduce the amount of heat you generate, you probably can make much smaller, faster, denser computers. I may be totally wrong about this. I'm bullshitting at this stage. But because—
It could be headed that way?
It could be headed that way?
It could be- because superconductors have such spectacular, obviously, in some intuitive sense, useful properties, I'm confident that as we get better super conductors they will be used. There are some places they're already used. They're used in MRI machines. You couldn't have MRI machines without superconductors. They're apparently used in the generators for really big windmills because, if you made the coils out of anything else, they would be too heavy. So, that's an existence proof that they'll be useful in some significant ways. Obviously, neither of those are transformative technologies, but they're also not totally trivial technologies.
Right. I want to ask this- it's so funny, if you remember the question that I posed to Doug where he said you've got to talk to Steve about this. So, I'll really put you on the spot with this. Very broadly, physicists are working towards a unified theory, right? Everybody is working towards an understanding of how the universe works. What is the role of condensed matter physics in cosmological questions? In other words, how do you connect these earthly matters that you're involved in with the cosmological studies about how the universe really works?
What are the connections there that might help unify all of these ongoing disparate research fields?
Okay. I have two answers to that. The first is that the idea that there is a grand unified theory of everything is a religious proposition.
It may or may not be true. Okay. But since we don't have such a description, there's no scientific reason to either believe that it does exist, or it doesn't exist. I think, on the whole, condensed matter physicists intuitively feel it doesn't exist. We're very impressed by emergence, which implies that, at one scale, you can describe properties of systems in which the input from higher scales is compressed into a few numbers, but you don't really need to know anything about the physics at those much shorter scales. If I'm trying to understand the physics of baseball, I really don't need to know anything about solid-state physics or atoms or quantum mechanics. All I need to know is the mass and elastic constants and surface roughness of the baseball.
And, really, nothing else about what happens at shorter length scales and higher energies makes any impact at all. I can have a perfectly good understanding of all of the emergent phenomena of baseballs without knowing what occurs in a more fundamental scale, at a higher-energy scale. Then we go and we do atomic physics, and atomic physics we can do perfectly well and make brilliant quantitative predictions without knowing anything about nuclear physics other than the charge and the mass of the nuclei. Knowledge about nuclear physics is really of no use and no relevance. There's no signature of it whatsoever that you could possibly see when you're doing atomic physics. And, when you do nuclear physics, you can do a perfectly good job of that without knowing anything about quarks and gluons. And, when you do quarks and gluons, you don't need to know anything about quantum gravity and quantum black holes. And we don't see any reason why it shouldn't be turtles all the way down (laughter). You know the—
I think that if you push most condensed matter physicists, they would say, yeah, maybe string theory is fine. It might give us some more decades of understanding, but it's just the effective of low-energy theory of some higher-energy theory which is, in turn, the effective low-energy theory of some higher-energy theory. And there's no particular reason to think that there is “the fundamental theory of everything.” That's a religious proposition also.
Your answer is emphasizing the relative lack of utility of cosmological questions on condensed-matter theory. I'm asking more—
—the other way.
That wasn't what I was answering. What I was saying was, for us, at each level of size, scale, energy scale, time scale, there is a description. That description is fine. That description seems fundamental. You really don't need to know anything about the "more fundamental scale." It has no impact. It's only when you go and look deeper, you do higher energy experiments or shorter length-scale experiments, that you discover, nope, I need a new, more fundamental theory. So, then you develop that fundamental theory, and that does a beautiful job of explaining all the phenomena that you want. But, if you then probe at still shorter length scales or higher energies, you discover you need a new theory. So, the fact that you have a theory that seems to work. Suppose string theory is shown to work. That doesn't to me prove that it's a fundamental theory. It means that if I went to scales down at the Planck scale, maybe I would discover that string theory starts breaking down and I would need a yet more fundamental theory.
So, at each stage, we're thinking we're discovering “the fundamental theory.” It explains all the phenomena we know. And then, when we go to a lower scale, we discover, nope, to go to a lower scale you need an even more. I mean, fundamental is a word that we tend to bristle at because it says it's more important, but let's use it in the conventional sense. There's a more fundamental theory at higher and higher energy scales, but why should that stop with string theory?
Why shouldn't string theory be the emergent properties of something that occurs at much higher energies.
So, in your assumptions, bristling against the idea that there is a grand unified theory, it suggests, obviously—
No, no, no. That's not what I'm bristling at. That's a religious proposition—
I'm saying, as a scientist—
It's no better and no worse than my proposition that it doesn't exist.
The thing that I bristle at is this word "fundamental," because I think that studying emergent phenomena, even if you know the "fundamental theory," is just as fundamental.
Sure, sure. Right, right, right. That there's an assumption that there's a hierarchy there that you're saying it's the wrong way of looking at it.
Right. These are both addressing fundamental questions. They're different fundamental questions, but they're both fundamental. And one of the beautiful things is that, in terms of not the particular problems, but in terms of theoretical physics, there's been a long history of very, very successful exchanges of ideas between the two. Ideas that prove to be successful in condensed-matter physics then get repurposed to have some new handle on fundamental physics and vice versa.
And, so putting aside the grand unified theory, that indicates that there is a comfort or an acceptance that, obviously, there are indefinite divisions within physics, that not everything needs to sort of fit together to explain everything, right?
So where are those divisions, the fundamental divisions? Where are those divisions where you're comfortable saying, you know, this is in this jar and this is in this jar, and if they never mix, that's fine with me?
Right. I listed for you some. For looking at the macroscopic properties of fluids, you can use hydrodynamics, and you don't need to know anything about quantum mechanics, and you don't need to know anything about atomic physics. There's a very big separation in scales between the atomic scale, say 1 angstrom, and the scale at which you're using hydrodynamics, say 1 mm. Because there's this big separation in scales, it's unambiguous that hydrodynamics stands on its own. Now, critical phenomena is actually an example of a problem where it's hard to draw a line, because there you have microscopic length scales, the correlation lengths at which some microscopic behavior occurs, that diverge as you approach the critical point. And below that scale, in critical systems, you get scale invariance, which means you have phenomena occurring at all scales. It means that anything you look at will be scale dependent. So, critical phenomena are an example of a system where- on the one hand, it's the most emergent of phenomena, because you've got universality. And, on the other hand, it's the one in which this notion of physics at different scales being decoupled from each other works the worst.
So, again, atomic physics and nuclear physics, there's three decades separating an angstrom and a few femto-meters or whatever the diameter of the nucleus is—there are three orders of magnitude or four orders of magnitude difference in scale and not much happens in that scale. So, if you're on the scale of atoms, you really don't have to worry to any degree about what happens in the nucleus. But then there are other cases like the critical case or like glasses- glasses are a horrible problem. That's part of why it's so hard. You have timescales of relaxations that go from the lifetime of a graduate student to the lifetime of an atomic vibration. And there's things happening at all time scales in between. And so, it's very hard to know how to dice it up.
Who are some of the younger scholars, the next generation in your field who are doing some of the most exciting work now?
Ah, so you mean I'm one of the old guys? (laughter)
(Laughter) I didn't say that. You did talk about the impact of the Vietnam War on your thinking about how the world works, so, you know.
I know (laughter). I'm just joking. Oh, boy. There are lots and lots of people. Let's give a definition of younger generation. So, I don't know, say fifty and below, is that younger? What, forty and below? What do you want?
People who have already made a name for themselves in their field and they're now really doing exciting work.
Well, there are lots of people. Certainly, Subir Sachdev, Senthil Todadri, well, Shivaji Sondhi, who I mentioned, Erez Berg and Hong Yao. We have an assistant professor named Vedika Khemani who's setting the world on fire.
Steve, a through-line here clearly is that this is a very international kind of group.
Yeah. Actually, almost everybody I mentioned was Indian, weren't they?
Well, no, not my students. But all of the others who weren't my students were all Indian. And not Indian American, but Indian imports.
Yeah. Xiaogang Wen, maybe Jörg Schmalian.
It's exciting. It's a lot of people doing a lot of exciting stuff.
Oh, yeah. The field is full of brilliant people. On the other hand, if we cut off our access to the international physics community, it's not that there aren't any Americans who are going into the field, but we sure benefit from being part of an international community.
Right, right. Well, Steve, for my last question, allow me to recover on implying that you were one of the old guys. I'll give you my benchmark, particularly because you were at UCLA. I had the pleasure of interviewing Robert Finkelstein, who's now 104 years old, and he's active and he's working on improving the standard model. So that's the benchmark for an old guy, I think, right?
Yeah. He's a great guy. Yeah.
That's it right there. And, obviously, by the standards of physicists- physicists never retire, right? So, by any measure, it's easy to say that minimally you're at the middle of your career. There's plenty more for you to do. So, on that basis, what are you looking to accomplish personally? You've answered about where the field is headed, your ideas about its relations to physics in general, but there's only so much time, there are only so much resources, right? And with all of the graduate students who easily can take up all of your time not allowing you to do your own work, in those precious moments where you can actually continue to do your own work, what are you most interested in accomplishing yourself?
Right. I have a heterodox answer to that, which is that I try not to think about that.
And let me explain why.
Well, it's a theme. You haven't thought about anything in terms of your career. That's sort of the whole- that's a constant.
Yes, but on this one it's conscious. One of the things that I've noticed is that a lot of really brilliant people become much less productive as they get older.
And part of it is that they want to be working on really important things. And I'm not going to name names, but there are some of the people who I think are clearly the people with the most creativity and the most intellectual armature in my field who've been effectively inactive because they've defined too high standards of depth and importance to what they want to work on. I was very influenced by Bob Schrieffer. Obviously, Bob Schrieffer did work, he happened to do epochal important work as a graduate student. I'm sure this was very hard on him psychologically. I'm sure that there was a lot of things that he had to come to grips with. By the time I knew him, he had a strategy that I've tried to emulate, which is that, of course he wanted to be working on things that were ultimately going to be important, but he focused on very specific, rather small problems that seemed like they might have something interesting in them. But he- in identifying what he was going to work on, obviously, thinking about where it fit in things, played some role. But once he had identified something that he was going to work on, he worked on it, he worked on it explicitly, he worked on it not thinking about it in the broader context until he'd understood the small problem. And then he would systematically back off and say, having understood this small problem, what does this teach me about a somewhat bigger problem, and what does that teach me about a somewhat bigger problem? He was always thinking about explicit things. He was always trying to solve some problem, not trying to solve the most important problem in the universe. And that seemed to be a strategy that kept him productive, focused, and, in his case, certainly proved to be very effective.
I like to keep my eye on the bigger problems. I do much of my physics in conversation. I talk with people about what I'm doing and what they're doing. In conversation, I try to always be thinking about big problems and where things fit. But, in terms of what I do, I like to be focused on, I really want to solve this. I want to understand how this thing works. So, I've avoided your question, but this time there was some thought behind it.
What you're saying is, you keep your research agenda pretty tightly bounded, problem by problem?
To take on to that, particle physicists—
I wouldn't say bounded, I would say focused.
I hope focused. I'm prepared to let it move in directions that I didn't originally envisage. In fact, that's ideal if, as I study one thing, I suddenly realize that, oh, you know, that's really interesting in a totally different context. I've understood this thing. I was thinking about this class of problems. But this is relevant to something else. So bounded is the wrong word. I want it to be unbounded but narrowly focused.
But focused, right, right. Are there structural constraints in condensed-matter physics? So, in other words, a particle physicist can say, well, the SSC never got built and so there's some questions that, absent some major international advance, kind of leave us stuck in certain ways. Are there those kinds of structural constraints in condensed matter that you see, or is it just totally different?
To zeroth order, the answer is - it's totally different. What characterizes condensed-matter physics is an extreme breadth of interesting phenomena to understand and extreme breadth of different conceptual problems to grapple with. So, there's not the sense of a few key problems that everybody is going to want to think about, whereas in high-energy physics there really is a very narrow set of problems that you're focused on. And so, if something blocks progress on that one problem, it's very hard to recover from that. That being said, there are clearly some structural problems and, in particular, structural problems for condensed-matter physics in the United States having to do with material science. We've had a long structural problem with not being leaders in either discovery of new materials or the refinement of interesting materials. And it's a funding issue, but it's probably much more deeply a sociological reason, that the people who do material science for physics don't really fit well in general in either physics departments, because how is that physics to try to get a better crystal of something? Or in material science departments where, who cares whether this is a material that physicists are interested in?
There are, of course, very clear counterexamples to this, and I'm proud to say that the Stanford Physics and Applied Physics Departments are one of the places in the United States where materials really are developed. But, structurally, the best materials in both senses are disproportionately coming from China and Japan and Germany and even Canada. And this is something that there have been many National Academy reports written on, but that is a structural problem because advances really do require good materials.
Right, right. Well, Steve, as I knew it would be, this has been an absolute delight speaking with you. I really want to thank you for spending this time with me, and we'll be so excited to include this in our collection, so I really appreciate it. Particularly because of how busy you are, I'm really honored that you spent all this time with me, so thank you so much.
All right. You're welcome.