Notice: We are in the process of migrating Oral History Interview metadata to this new version of our website.
During this migration, the following fields associated with interviews may be incomplete: Institutions, Additional Persons, and Subjects. Our Browse Subjects feature is also affected by this migration.
Please contact [email protected] with any feedback.
Credit: Dept. of Physics, UIUC
This transcript may not be quoted, reproduced or redistributed in whole or in part by any means except with the written permission of the American Institute of Physics.
This transcript is based on a tape-recorded interview deposited at the Center for History of Physics of the American Institute of Physics. The AIP's interviews have generally been transcribed from tape, edited by the interviewer for clarity, and then further edited by the interviewee. If this interview is important to you, you should consult earlier versions of the transcript or listen to the original tape. For many interviews, the AIP retains substantial files with further information about the interviewee and the interview itself. Please contact us for information about accessing these materials.
Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event. Disclaimer: This transcript was scanned from a typescript, introducing occasional spelling errors. The original typescript is available.
In footnotes or endnotes please cite AIP interviews like this:
Interview of Philip W. Phillips by David Zierler on April 8, 2021,
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 Philip Phillips, Professor of Physics at the University of Illinois Urbana-Champaign. Phillips recounts his early childhood in Tobago and the circumstances of his family’s move to Washington State. He conveys his bemusement at having no degree in physics, as his graduate work at the University of Washington was in chemistry, where he completed a PhD on fluorescence lifetimes in single molecules under the direction of Ernest Davidson, and where David Boulware provided the intellectual entrée to physics. Phillips explains the opportunities that allowed him to pursue postdoctoral work at Berkeley and learning RG from Orlando Alvarez. He describes his first faculty position in the chemistry department at MIT, some of the research challenges given that his primary interests were in physics, and his feeling that MIT was at the time not a very inclusive atmosphere. Phillips discusses his work on the random dimer model and the happenstance opportunity that led to his faculty appointment at Illinois. He explains getting involved with the National Society of Black Physicists and his efforts to make the department more diverse. Phillips describes the research that was recognized by the Edward Bouchet award and why Tony Leggett is among the few physicists who truly understands Mottness. He discusses advances in strongly coupled electron systems and he explains why he dislikes the term condensed matter and prefers solid-state. Phillips reflects on STEM’s response to the racial strife over the past year, and he discusses his current interests in pseudogaps. At the end of the interview, Phillips conveys his dream to solve the Hubbard model and to make advances in high-Tc research.
OK. This is David Zierler, oral historian for the American Institute of Physics. It is April 8th, 2021. I’m delighted to be here with Professor Philip W. Phillips. Philip, it’s great to see you. Thank you for joining me.
You’re welcome. Thank you for having me.
To start would you please tell me your title and institutional affiliation?
I’m professor of physics at the University of Illinois Urbana-Champaign.
Philip, a question we’re all dealing with right now. For your science how has the past year plus in the pandemic been going for you? What have been some of the challenges and perhaps some of the unforeseen benefits of the very new style of work we’ve all been experiencing?
Well, I really miss being able to walk into my students’ offices. And my postdocs. I work very closely with them. I benefit a lot from having them hear my wacky ideas or helping me get unstuck on an equation. And I’ve been coming to my office recently and I now resort to working things out by writing them on the board. [laugh]
Rather than just walking into my students’ offices. So, yeah. That’s definitely a minus. And so we have meetings online. A plus is I’ve probably been more productive. It’s a funny thing really. I mean when the pandemic hit, we had just gotten a major result published in Nature Physics. The numerous implications of this paper now occupy my time as we push forward now with reduced interruptions. But I really do benefit from people’s input. Now all that takes place over Zoom but sometimes the quality of Zoom questions surprises you. I never thought people concentrated that much in this brave new format. So, I guess that would’ve happened anyway. But you’re more focused because you’re not travelling. So, you have more time to do your work.
Philip, let’s take it back to the beginning. Let’s take it back to Tobago. Tell me about your family and where they come from.
I was born in Scarborough, Tobago, the capital. Fairly uneventful. I’m the third of five children. And my father was a teacher. And my mother became a teacher in the high school eventually, but she didn’t start out like that. She was taking care of all of the five kids. It was a fairly academically oriented family. You know, my father was a history teacher and then became a principal of the school. So in the West Indies, things are very education oriented because there are no luxuries as in the states—nothing to fall back on. [laugh] OK.
[laugh] Philip, when did you get interested in science? Was it early on?
Not really. I was in high school. The subjects I was good at were math and English. I did not do well in physics. It’s in college that I really caught on to physics.
What opportunities did you have? What kind of places did you apply to for college?
I didn’t apply to any other place except the one my father was teaching at. Because it was essentially free.
What college was that?
Walla Walla College.
Did you go in intending to pursue science or you were open to any number of studies?
I started out an English major. And I wanted to be a poet.
And then I realized well, you have to have real talent for that. And I’m not sure I could survive as a poet. [laugh] The stuff I was writing took too much out of me. It was very confessional. So, then I decided that’s not the sort of thing that’s a long-term career. And I started shopping around.
Did you take a class in physics specifically that changed your mind?
I started with a biology class, modestly interesting but kept taking math classes, differential equations, probability and statistics, complex analysis. Then I took chemistry and the fact that there was structure behind the periodic table quite intrigued me. That really cemented my interest in science. Then physics came along and I had taken so much math that it seemed quite easy, unlike my high school experience.
Did you ever pursue experimentation? Or once you knew it was physics it was always going to be theory?
No. I got an NSF summer fellowship at Washington State University doing experimental work. I didn’t particularly like it. I found a professor down the hall who gave me quantum mechanics problems to solve and I really liked that. So it soon became clear where my natural leanings were. Ultimately I knew this before hand.
Philip, at what point did you realize that you wanted to go to graduate school?
Senior year was when I made the decision. All my friends were going to med school. I was the only one heading to grad school. That felt weird.
Were there any opportunities to stay at home or you knew you needed to go to the states for this?
This was all in the States. We moved here when I was 10.
So, we were already in the states.
Where did you grow up in the states?
Boston. Well, actually it was Brookline. OK. But you know, the Boston area. We spent a couple of years there and then when I was an adolescent we moved to Walla Walla, Washington. And that’s where my father got a job teaching at a college there. And that’s where I spent my teenage years and college.
Growing up did you feel more West Indian or more American, would you say?
Oh. Definitely West Indian. [laugh]
I mean it really is a function of your parents. And West Indian parents have a way of maintaining the West Indian… you eat different food. I mean, it’s… you speak differently. So, they did not want us becoming Americans. They didn’t think Americans had values. Or they had values, but they were just so different from the ones that we had. With us, it was education, education, education. And did I mention it’s education? So, right.
What graduate programs were you considering? Physics and chemistry?
Well, it was just chemistry. I settled on majoring in chemistry and math and took a lot of physics courses but was senior lab short of a physics major. It’s hard to fit all that in in 4 years.
This is amazing.
Yeah. I have no degree in physics.
[laugh] I mean senior lab is pretty extensive. And I couldn’t fit it in.
Who were some of the professors at the University of Washington that you were close with?
My advisor was Ernest R. Davidson, the Davidson known in computational physics for the Davidson method of diagonalizing large matrices. And I guess from him I learned how to get things done quickly. He was an incredibly efficient man. And he set a high standard. He was not a particularly nice man. So, he wasn’t someone you would feel warm and fuzzy with. But we knew what the standard was. And you know, we all knew we could never become Ernie Davidson. He was on a different sort of level. He was really an applied mathematician. His imprint on his field was quite large. Aside from diagonalizing large matrices, any number of methods had his name on them. None of us who were graduate students at the time could see a way of equaling Ernie.
Alvin Kwiram I was also close to. He was a family friend. But probably my biggest inspiration came from a physics professor, David Boulware. I took math methods, classical mechanics, and quantum field theory from him. His lectures were things of beauty. I mean, he used to walk into a class without any notes. It was just absolutely inspiring. And he would give perfect lectures. I now lecture without notes largely because of David Boulware. He was a student of Julian Schwinger’s. Julian Schwinger was as you know a legendary lecturer. So, we learned things in math methods that only Julian Schwinger knew. And you know, his students were his disciples, Gordan Baym for example was a student of Julian Schwinger’s. It was inspiring to know that we were being taught inner Schwinger. One day in lecture I asked if what we were doing was equivalent to Feynman path integrals. He said yes but don’t tell anyone. The Schwinger-Feynman competition was clearly passed down to his students.
Philip, where is condensed matter in all of this? That’s what I’m waiting to hear.
Well, I’ll tell you. Its quite a roundabout story. My exposure to physics was through David Boulware. He is a high-energy theorist, really a relativist. He exposed me to the ideas of ’t Hooft and big ideas on unification. Around that time, Howard Georgi came up with his SU(5) paper. I can still quote the beginning of this paper. I actually met Howard when he came to U of Washington to give a talk. Let me just put a bracket around that. But I was doing rather mundane stuff, quantum chemistry. Not really big question stuff. I became competent at it. I viewed the Ph.D. as a means of getting research experience. Figuring out how to get things done. But I didn’t view it as the thing that would give you the training to solve the problems that would engage my mind for the rest of my life. So, I took a very utilitarian approach to the Ph.D.
And your thesis reflected that?
Yeah. It was on fluorescence lifetimes in single molecules. There is nothing condensed matter about that at all.
Who was on your committee?
David Boulware. Ernie Davidson. And I think George Halsey. And I think there was Alvin Kwiram as well. But Boulware was my huge mentor in physics, I mean that was a really inspiring thing, you know?
Did you hang out in the physics department at all? I mean UW physics is a pretty amazing place. Did you experience any of that?
Sure. David Thouless was hired when I was there. The KTN invariants came out when I was a graduate student. So it was clear something was going on. I took a class from John Rehr, a condensed matter person. It was clear that physics not chemistry was where it was at. So I wanted to finish my Ph.D. as quickly as possible because I didn’t feel as though my research at the time would really engage me for the rest of my life. So I finished in 3 years. I had basically solved all the problems my advisor gave me and then some so it was time to leave. I recall him saying in my third year: “Philip, I think you need to just end it here. You know? There’s nothing I can give you right now.” So this was after 2.5 years.
Then I started to think about what’s next. I told him “You know I really want to do condensed matter physics” [laugh] He responded, “Philip, that’s just like you to go and choose something like that in which you have no real background. But you’re gonna screw up your life if you do that. You should really stick with quantum chemistry.” But I had no interest in it. So I started looking at various fellowships for postdoctoral work. I needed a fellowship that would give me some sort of independence. So, bless his heart, he nominated me for the Junior Fellowship at Harvard and also the Miller Fellowship at Berkeley. And that’s how Howard comes up again. Because he was a senior fellow at Harvard, he was one of the people who interviewed me and he remembered me from earlier at UW. So it was clear to me that I would not continue with single-molecule quantum mechanics. I got my PhD in 1982. Those were incredibly heady days in condensed matter physics. You had to not have had a pulse to see that something was exciting there.
Quantum Hall and Laughlin’s gauge argument, quasicrystals and a few years later high-Tc. I mean you had to have been braindead not to have realized that the real intellectual depth of physics was condensed matter. I mean high energy had some great ideas in unification but string theory really hadn’t started and experimental confirmation or non-confirmation was quite afar. So, condensed matter was where it was at. I didn’t get the Harvard fellowship but I did get the Berkeley one. I wrote a proposal that was inspired by some of the things I learned from John Rehr about phase transitions. And you know Ken Wilson had just done RG. It was an amazing time.
Philip, what specifically about RG? What was so incredible about that to you?
[pause] The fact that you could understand the behavior of a system on different energy scales, which means you understood basically the long distance and the short distance physics from a single equation by looking at how the interactions were changing as a function of energy. It told you that systems were behaving universally. So, the fact that from a complicated system it came down to just a few differential equations was to my mind an astounding thing and that that carried with it the short and long distance physics and in quantum chemistry you were just doing short distance stuff. You never asked a question about what happens at long distances.
I guess I always wanted to work on the important ideas. It was clear to me that quantum chemistry couldn’t even tell me why water expanded when it froze. What was really bothering me is that I could not tell any story in quantum chemistry that really mattered. So, condensed matter appealed to me because it was about real stuff. It was about big scientific questions. It wasn’t clear quantum Hall was about a big scientific question. But ultimately you know, it’s a universal phenomenon. So, and no one understood it. So, it was about something really deep. So, I went to Berkeley and I started working on many-body physics. And that’s when I started gaining the background to become a condensed matter physicist. So, it was a painful process because I didn’t know a whole lot. I worked non-stop.
Did you take classes or were you mostly an autodidact as a postdoc?
What was interesting [laugh] is I sat in on one class and half the Berkeley physics faculty were in the class. [laugh]
[laugh] Who was teaching that class?
Orlando Alvarez. Orlando Alvarez had just come to Berkeley from Cornell. He was a postdoc of Ken Wilson’s. And so, he was teaching an RG course. And everyone was in the course. Mandelstam. You know, all of the high powered Berkeley faculty were in the class. All of the graduate students who wanted to do condensed matter. Orlando Alvarez had been taught by the oracle, Steven Weinberg. So, when you have Weinberg and Wilson in your background people pay attention—and while I was at Berkeley, Wilson won the Nobel Prize. So imagine such a class with the Mandelstams of Berkeley there and Alvarez as the teacher.
Did you interact much with Marvin Cohen during your Berkeley years?
Not at all. I didn’t know who he was. I knew Orlando Alvarez. And so, I talked to him a lot. He’s an extremely accessible guy. And you know, although I didn’t have the perfect background, I read a lot. And I could derive equations. One thing I learned how to do because I was a math major is, you know, I trusted my math ability. And that’s what allowed me to move into a new field without having the physics background. I think I always relied on that. You know, I didn’t know the physics but I knew I could learn the math. And to a large extent, if you can do that you can become a theoretical physicist.
So, what was interesting is that at the time when I was at Berkeley, chemistry departments took note of the fact that there’s a lot going on in condensed matter. And they started hiring people who were doing postdocs in physics groups. Tony Haymet, who worked at Harvard with David Nelson, got hired at Berkeley. Jim Skinner who was writing papers on Anderson localization was hired at Columbia. So, it seemed as if I hadn’t shot myself in the foot by going to Berkeley and expanding my horizons and having more of the physics focus. Even though my advisor thought, “Phillips, you’ve definitely toasted yourself here because you should keep going with the hot hand.” But to me it wasn’t…it was not the hot hand.
But it seems like—just to fast forward a moment here—you could apply to chemistry department faculty positions as an undercover condensed matter physicist.
Yeah, yeah, yeah, yeah.
Were you not competitive at that point for physics faculty positions?
No. No. I mean physics at that time was…remember there were very few jobs in both fields. And it was the mid-80s. You know I was applying for jobs in ’83 and high-Tc wasn’t discovered yet. There was quantum Hall but not many were doing this. So, the big influx of ideas into condensed matter didn’t happen until high-Tc. And then everyone gloomed on; the field kind of blew up then.
Did you interact at all with the Stanford group? Were you aware of what like Mac Beasley and Ted Geballe were doing at Stanford?
Remember, I had limited knowledge of physics types. I didn’t even know Marvin Cohen. I knew some of the Berkeley people. I went to the colloquia and of course I was there for Bob Laughlin’s Quantum Hall colloquium. By far the best talk I have ever heard. I mean I still remember the colloquium, you know? And it was clear he understood it. No one else understood it. And he would win the Nobel Prize for it.
What did he understand at that point that no one else did?
He explained the integer quantization with his gauge argument and he also explained the 1/3-effect or as it is known now the fractional quantum Hall effect. He had just written down the wave function for it. He gave the colloquium a few months after writing down the wave function for the fractional Hall effect that described what is now known as the Laughlin liquid. So rarely do you go to a colloquium about a result that is just fresh off the press.
How long were you at Berkeley?
And what kind of jobs were you applying to when you were on the market?
So, I applied to, I think I applied to four jobs. There were four jobs in theory and chemistry. There was a job at Florida, MIT, Yale, and Dartmouth. And I got interviewed at Florida, MIT, and Dartmouth. I did not get an interview at Yale. I got offered a job at Florida. I interviewed at MIT. And I didn’t hear from them immediately, but then they said, “It’s coming down to you and this other person.” And by the time I interviewed at Dartmouth, MIT decided they would offer me the job. So, right.
From Brookline earlier in your childhood did you feel like you were coming home at all to Cambridge?
Absolutely! I mean, Boston, when you move to another country, another state, and the first place you move to is pretty, pretty formative. So, it did feel as though I was going back home. Could you hold on a minute? I want to get some water.
Philip, did you make clear to the department, the chemistry department, that you thought of yourself as a physicist at this point and you intended to pursue a physics agenda?
Um, no. I didn’t understand that enough about myself at the time. I didn’t really understand what had happened to me at Berkeley. Partly because there were several of us. I mean, look at Ted Kirkpatrick. Do you know him?
He’s at Maryland. You know, they were working on similar things. I didn’t feel as though you had to declare it. You know? He was working on the quantum Lorentz gas. I was working on the quantum Lorentz gas. I mean that was just one of the problems. It was clear I was interested in quantum condensed matter. Some of those problems were of interest to chemists. But I began to figure out that perhaps going down this path would be problematic. I remember telling one of my colleagues, Jim Kinsey, when I was at MIT when he asked me what is the most important problem. I replied “The metal insulator transition.” Saying that to a chemist that’s problematic [laugh]. I remember that particularly. And he said, “Oh. OK.”
But you know, I looked at the people who were hired in similar years. You know, Dev Thurumalai at Maryland, Jim Skinner at Columbia. They were all doing things which weren’t far away from what I was doing. In fact, they were doing pretty much the same thing. Lots of people were working on Anderson localization at the time. The scaling argument had just come about. People understood now that there couldn’t be a metal in one dimension but conducting polymers were around. So, to my mind, that suggested that there’s an obvious inconsistency. Interestingly, conducting polymers were being made by chemists. I remember Pimentel from my Berkeley days. Chemists were working on them. So, it didn’t seem my interests were that far out.
What about in the physics department at MIT?
I talked to Patrick Lee a lot. In fact, he was sort of a semi-mentor to me. But I have to say, I wasn’t very politically aware. You know, I came from Tobago. I didn’t come from Trinidad. The big difference between them is Tobago is this small, idyllic island. Trinidad is an industrial place. You know? A million people. I think at the time 20,000 people lived in Tobago. [laugh] So, it’s a much smaller place that’s not as worldly. And definitely I inherited a sense of separation from the world I would say. My parents certainly didn’t do anything to defuse that. What I did notice is that those hired when I was at MIT started to tailor their work to topics more in line with traditional chemistry. They started doing big computer simulations. Because of my background in quantum chemistry, that sort of thing did not appeal to me. So I just kept going where I thought the fundamental problems were because that’s what motivated me.
Did you take on graduate students who reflected your interests?
That was one of the things that was a concern. I worked with I think five graduate students at MIT. Only one was in chemistry. They were all from the physics department.
This can’t be very good for your tenure prospects.
It wasn’t. It wasn’t. I didn’t think the chemistry graduate students were very competent. It’s interesting to run into some of those students now. I gave a colloquium at Toronto and ran into a student I taught at MIT, Jamie Schafer. He commented, ``You know, Philip, we remember the lectures you gave us in quantum mechanics. They were on tight binding models, localization and Hubbard models.’’ He kept the notes because that was his first introduction to all of this. I guess I didn’t realize how much I had crossed over. But to me what I was doing didn’t seem like much of a stretch. After all, given my research interests, that stuff was just the bread and butter. He said, ``It was obvious to us that you were not a chemist.’’ So, I guess I was sort of the last to know that, at least to understand it.
Another aspect of being an outsider, of course. Where was MIT at that point generally on diversity? Did it feel to be an inclusive place to you or no?
No. [laugh] Certainly not. I was an outsider in all sorts of ways. I was raised in a cloistered West Indian family. I attended UW where I thought my advisor’s view of science did not appeal to me. I managed to get a job at MIT and then I started working on things that were not central to the department. On top of that, I was one of a few Black faculty in any of the sciences. Certainly none were tenured. So I began to see that there were potential problems up ahead. One of my colleagues at MIT was the late Irwin Oppenheim. I don’t know if you know him.
There’s an APS prize named after him. He was a very important person in my life. He would take me aside and tell me various things. And he’d say, “Philip, race is never neutral. It either helps you or it hurts you.” So, it certainly never helped me at MIT.
Philip, tell me about your visit to Oxford. How did that come together?
Ah! That was another interesting thing. One of the people who was very influential in my early life was, I mean early scientific life was Peter Wolynes. Peter Wolynes was the first scientist I met outside my research group. I went to a theoretical chemistry conference in--I think it was Boulder. He attended my poster and we have been friends ever since. He had a postdoc named David Logan. He got a job at Oxford. He was working on localization as I was. He was the UK counterpart to me, a chemist doing physics. So, there were other people who were doing the sort of things I was doing. So, it didn’t seem so inconsistent. And he invited me to come to Oxford for a semester. Ultimately, it came about because of Peter Wolynes.
Sort of a general question to bound your MIT years. Are you operating primarily in a theoretical world or are there advances in condensed matter experimentation that are really relevant and that are pushing theory forward?
Absolutely. I profoundly believe that physics is an experimental subject.
OK? And we have to pay attention to that. So, I’ve always motivated my work from experiment. In fact, I always viewed my focus on experiments as one of my angles. That’s not the motivation for lots of theoreticians. Granted I do a lot of math but the experiments are the motivation.
Yeah. That’s right.
OK? So, you know, if you view yourself as an outsider you always have to have an angle. And my angle has always been well, I’m the guy who pays attention to experiments. I am quick to jump on new things. A few examples are the Kravchenko problem, the key problem in the ’90s and the one that led to my focus on strongly correlated electron physics, iron-based superconductors, and twisted bilayer graphene. If there’s something new that is paradigm changing, I’m there. That’s what motives me with strange metals. Such problems require different thinking. Looking for things that are outside the box is what appeals to me, in no small part most likely because that’s where I am.
Another way of measuring this is you know, for your scholarly output during your MIT years, where are you publishing? Are you publishing in Phys Rev Letters or are you publishing in chemistry journals?
[laugh] What do you think? [laugh] It was Phys Rev Letters.
Phys. Rev. Lett or Science.
Did any senior people in the chemistry department say you’ve gotta get out of here because you’re not a chemist?
Well, when I came up for tenure, that was certainly an issue. I began to look at what happened to my other colleagues who had a similar sort of physics background who were hired to do physics but as they moved through the ranks, they all switched to doing more traditional chemistry things. I never switched. Ultimately that was a mistake if I wanted to remain in chemistry. You know, you look at the Thirumalais and the Skinners of the world. They all started doing things that were much more in line with experimental chemistry background. I never did that.
Now you’re promoted to associate professor, but of course at MIT associate is not tenured.
But is it a sign that you’re moving in the right direction? I mean, are you getting competing narratives here?
OK. Now, very interesting question. I see you’ve done your homework. [laugh] OK. At the time, I had done the exceptions to localization. They seemed to understand that I had done something unique. And this was the basis for my promotion to associate. When my case was presented at science counsel at MIT that’s when the problems started. There people began to wonder, is he really fitting into the bigger picture and I don’t think the person who was arguing the case particularly understood anything about theory. And so, it was clear I’d done something. But they wanted it to be more chemistry oriented.
So, after that I was told by the chairman, “You need to do something more chemistry oriented.” After that I decided, oh, I’m going to apply this to conducting polymers. And then that’s when I came up with the second Phys Rev Letters, applying the random-dimer model and then we had this big Science paper. This new mechanism for conducting polymers. And then everyone said, “Oh. Well, he’s done it now.” [pause] And it was clear I’d done something. OK? But I think my letters were from physicists. I had published in physics journals. There were letters from people in physics from MIT who were helping out.
Was Bob Birgeneau one of them?
Bob Birgeneau was the dean. He really tried to help me.
I thought he might have.
He is a stand-up guy.
Yes, he is.
I was shocked this case ended up on the negative side. In the lead-in to the tenure decision, I had to give the colloquium in physics. The topic was of course the random dimer model and applications to conducting polymers. I remember a question from Ed[mund] Bertschinger, a relativist. The question was quite sharp concerning the generality of the model but I was ready for it and he visibly and audibly agreed with the answer. That was huge as it sort of took the negative energy out of the room. After all, it isn’t the case that too many chemists give the Physics department colloquium. I think I was the first. Patrick Lee introduced me. So the talk went well and Birgeneau in particular sought me out after the talk and said how much he enjoyed it. But you know, it is complicated. Because you know, to get tenure you really have to get into the main stream of a department. And I was not really playing that game. I was just doing what I found was intellectually interesting.
Why not a simple lateral move to physics at MIT? Was that discussed?
Apparently, MIT had a rule that if you were not tenured in one department you couldn’t move to another.
You mean if you were denied tenure or if you were not yet tenured?
If you were denied tenure in one department you couldn’t move to another department.
Alright. So, Philip, clearly at this point you’re not a grand strategist with career moves, but why did this not occur to you before you were denied tenure?
Not even for career moves. Just for intellectual happiness.
You know, my neighbor said something to me recently which sort of surprised me. She said ``You just want to do something important—the career is secondary.’’ I guess that was always my strategy. So, that was always my motivation. I have no political aspirations. I never want to be chair of my department. I just want to be always in the trenches moving things forward. That’s what motivates me. But you’re right. Why couldn’t I have seen this as a survival issue? Well, and that’s part of the reason I went and applied the idea to conducting polymers. And you know, it wasn’t like I hid the papers. They were in Phys Rev Letters and this big seven-page article in Science about the whole thing. So, I thought I’d done what I needed to do. And you know, things went down badly. But Patrick Lee thought it was a wrong decision. Bob Birgeneau thought it was a wrong decision. Irwin Oppenheim did. Very big people thought it was a bad decision. But they couldn’t change anything.
Was there a negative byline in the works? Like, First Black Professor Denied Tenure…was that part of the discussion also?
I have no idea. the thing I took away from Irwin Oppenheim’s comment on race is if you are different, things are harder. On top of that if your work is even more different, then it’s curtains.
And certainly, I saw it happening. I mean a Black guy comes along and doesn’t do traditional chemistry, essentially thumbing his nose at well established problems, how are people going to sign on to what he is doing? [laugh]
You know, there are always things I’m doing that are not making it easy for people. Was I aware of this? Not until the end.
To stay in the happy place on the academic side, not the political side. By the time your tenure, or your non-tenure I should say, at MIT was coming to a close, just in terms of your body of scholarship, what were your key accomplishments up to this point?
The random dimer model. It is the general exception to Anderson’s localization theorem. When I got to MIT in 1984, the scaling theory of localization was on everyone’s mind since it was published in 1979. It says that in any dimension less than or equal to two, disorder kills metallic states. I came up with the exception. It went from being obviously wrong to trivially right overnight because it’s a simple thing. Should I tell you what the result is?
Ok so here’s the model. The simplest model that gives you Anderson localization is the random binary alloy. You have two different site energies and then you put them down randomly on a line. All states are exponentially localized. No transport. But when I started learning about Anderson localization, I had worked on classical diffusion problems. And I knew that you could have diffusion in a random system classically. So, I always had in the back of my head why did quantum mechanics kill the diffusion pole? It didn’t seem like quantum mechanics itself should just be able to do that. So, once again faced with a prominent result, I thought there must be a problem. Yes an exception to the gang-of-four scaling argument. [laugh] And I took that on as a research program. You know, most people would not view that as a winning research program trying to come up with the exceptions to localization. But I did because that’s what I do.
And so, here’s the exception. Don’t put them down one at a time; put them down two at a time. So you have a random system made out of dimers. Two site energies neighboring one another is now the building block for the disorder. So the only difference is that you put the random site energies down two at a time not one at a time. Well, everyone will tell you well, it’s still a random system. Why should that be any different? And then I would explain to them that a-ha! Two sites can have a resonant energy and if a wave goes without scattering it doesn’t matter how many of these you have. It will be unaffected through the whole system. And that’s the random-dimer model. So anytime you have a plane of symmetry in your defects, there will always be a resonant unscattered state. That’s how you get around localization. And then what we did is we showed that the defects that you have in polymers are always of the random-dimer type. And then we showed that this resonant energy always occurs at the Fermi energy in systems like polyaniline and others. That’s why we were asked to write the paper for Science.
So, that was my big result that I had come up with. No one had ever come up with that before and I applied it to conducting polymers. So, that was what I came up for tenure on and people knew it was right. And I remember when I explained this result to Phil Anderson. So, I was going around giving talks. Lots of talks in physics departments.
And I remember I walked into Phil Anderson’s office and said, “I’m working on exceptions to localization.” And he goes, “Really?” He said, “There are cases in which localization doesn’t work.” And I go, “Really? So, what are these, Phil?” And he didn’t come up with any. I just derived the result on his board. The math is simple: elementary quantum mechanics. And so, it was clear that it wasn’t a really shocking result. What was clear is that it was missed and it stemmed from a simple organizing principle. It created this field of correlated disorder. So, that was my result.
To go back to the question about experimentation and theory to invert it, in what ways were these advances useful for experimentalists in condensed matter?
Ah because then people started looking for random dimer models conditions in nature. If you can make a disordered system that transmits waves at one particular energy you have a filter. So, people started experimentally designing these things. That was one of the applications. Fibonacci disorder we showed is an example of this. We applied for a patent. [laugh]
That was my next question. Obviously, for you it’s very much a basic science framework, but I was curious if there are some applications that come as a result of this.
Yeah, yeah, yeah. The only patent I have I got when I was at MIT.
And what has it been applied to?
Narrow pass-band filters.
Which are what? What do they do? What are they good for?
Yeah. You know, it’s funny. Everyone thinks that when you get a patent you get lots of money. No. Only people developing it. And people can take the idea and run with it. So, I never followed it. Once again, I don’t have any political aspirations, so I never followed up on it. OK? Probably my mistake, but you know, I’m not in this for the money.
What was your game plan after you were denied tenure? Were you very proactive on the market? Did you get recruited elsewhere?
I was very depressed, actually. I was very pissed off. Because I mean I come up with the exceptions to localization. You know, Phil Anderson won a Nobel Prize for that. [laugh] I don’t have a job, so it was like…so, I had a friend in the math department, Gian-Carlo Rota. And he was a sort of a confidante and I told him I was thinking of not going to this conference that I got invited to in Sweden. And he said, “You have to go to the conference and you have to give your talk.” And I don’t really feel like it, you know? I feel like, when you don’t make tenure at MIT you feel really shitty, you know? It’s like the whole world knows you didn’t get tenure. And I took it very personally. It’s hard to figure out how to take it any other way. You were denied tenure, you know? And you were at a place like that. I was very young. You get there because you were driven and so your whole intellectual world is collapsing.
Philip, were you not able to depersonalize the glaring issue of you being a physicist in a chemistry department working on physics problems, publishing in physics journals?
No. It didn’t help because I didn’t have a job, you know? And so, your whole life is upended. I wasn’t able to do that. You know, there were a couple of chemistry departments that were interested. Michigan State. But I didn’t have a real plan. My plan was just to hide. So, then I went to this conference in Sweden and that’s what changed my life. Because I gave a talk and at that point I just decided to tell people you know, if they know of any jobs I’ll be completely game. [laugh] And one of the people who this was told to was the chairman of the physics department here.
Ah. Who was that at the time?
And in fact, he came to my talk.
Did you know Campbell before?
I had no idea who he was. I’d only met Tony Leggett and Mike Stone. And Tony Leggett heard my talk on the random-dimer model in Trieste. So, he knew all about the result. And Mike Stone I had met in an airport. So, David Campbell asked me to sit with him at his table during dinner and I don’t think anything of this. And then at the end of that he said, “Well, I’ll go back to Illinois and see what the chemists say. And maybe there’ll be a possibility in physics.” And I knew what the chemists would say. I mean, you know, Illinois chemistry views itself as MIT chemistry. So, it was a nonstarter. So I never thought of this conversation. I literally never took the conversation seriously.
Did you permit yourself even a moment of giddy anticipation that Illinois and condensed matter, maybe it’s the best place in the whole world to be?
I’m being very honest here. That thought never crossed my mind.
You’re serious when you say you’re not political. You are devoted.
It never crossed my mind because how is a guy supposed to go from chemistry to physics. For one thing, do you know of anyone who’s ever made that move?
[laugh] No. I mean, this is a unique story.
No, no one. No one ever makes that move from a chemistry department to a physics department. Why should I assume that I could do that? I need some more water.
This time I’ll get a cup.
So, when did this offhand comment that you thought nothing of become something real? How did that play out?
I think several months later. Three months later I listened to the messages on my answering machine at MIT and he said, “This is David Campbell at University of Illinois. Don’t accept any jobs ‘til you hear from physics.” And he goes, “That’s right. Physics at Illinois. We’d like you to come out and give a colloquium and a condensed matter talk.”
I like how he emphasized physics to make sure you heard it correctly. You didn’t have to confuse yourself.
That I heard it correctly. Yeah. Until then literally, I didn’t even think about it.
It was the best place I could’ve ended up. I was so done with chemistry. They were involved in the recruitment process, but—and the question always came up. I mean, you did this great work. Why didn’t you get tenure? So, it didn’t make sense to a lot of people.
Well, Philip, what were the stakes at this point for your career? Was it sort of like you’re either going to pursue a top tier opportunity or you’re gonna bail on academia altogether? Or would you have gotten you know, a teaching job at a state school, a community college? I mean what were your options at this point?
Well, I mean, my options were U of Pittsburgh in chemistry. Continue going down the line of chemistry. I didn’t apply for anything else. I mean, I technically didn’t apply. I was asked to apply at various places. So, I hadn’t really considered anything except academia. There’s something I am sort of hiding here. I never really liked MIT. I wanted to get tenure at MIT in chemistry and move to the chemistry department at Berkeley because I always thought that that was a lot more open place to the kinds of things I was trying to do. And if you look at people like Brigitta Whaley, her work is all over the map and she is in chemistry. So, I didn’t think that I was so different would stand out so much at Berkeley. But that didn’t work out that way. So, I was just looking for something reasonable to come along. But ending up at a top place in chemistry was just not an option.
What was your job talk at Illinois? What did you present on?
The random-dimer—I gave the colloquium on the random-dimer model and applications to conducting polymers. That was my big result and a lot of them hadn’t heard the result.
Who was in the crowd that sticks out in your memory?
Tony Leggett. He asked a very important question.
What did he ask you? Do you remember?
Well, it was…yes, I do. [laugh] He goes, “Isn’t the equation you’re using a classical equation?” It was relating the conduction to the diffusion coefficient. But you’re dealing with a quantum mechanical problem. And I go, “You are correct, sir.” And then I said, “But given that I’m trying to explain diffusion it seems like a reasonable result. It’s a reasonable thing. Yes.” [laugh] I did say, “You are correct, sir.” [laugh]
Not that it really matters. Tony Leggett is Tony Leggett. But was he a Nobel Prize winner at this point?
No. He was not.
He was not.
I certainly knew of the eminence of Tony Leggett.
And I thought after this question I was hosed.
So, I remember I came up with a better answer and called him up and told him because I thought that that would be relevant.
When you got to Illinois, just a general feeling, did you get the sense that it was a nice place? People were nice? It was a supportive place?
Yeah. Yeah. I mean that was, it was such a breath of fresh air from MIT. I mean, MIT’s an incredibly competitive, cold place. And this is just the opposite.
What about on the inclusivity side?
That’s something I started.
Were there any other faculty members of color at that point?
No. There were no Black graduate students.
So, that’s something I started addressing when I came here. I would go to the NSBP meeting. I would start recruiting.
Were you active with NSBP before Illinois?
No. Remember I was a chemist so…
Right. But you’re a physicist undercover. Why not include NSBP in your research agenda?
Because I’m you know, I don’t have any political aspirations. I never claimed more than I was able to claim so…
…how can I claim I’m a physicist when I’m in a chemistry department. That would only be correct if was in a physics department.
What was the offer? How quickly did it come after the colloquium?
So, I gave the colloquium on Thursday and then the next day I gave the condensed matter seminar. And then they asked me for people who might be able to write letters. And then I don’t remember the exact sequence, but I think by December or so I knew I was going to Illinois. And I think the colloquium was in October or November. So, they moved pretty quickly.
Did you go there initially on the basis that if they were to offer you something it would be tenured or would they fast track a tenure clock?
I don’t think I was savvy enough to even ask that question. I was just so over the moon that a place like Illinois was interested in me. So, they knew I had a tenured offer from Pittsburgh. They knew the salary. And that was part of the negotiation. So, I think they knew they had to make a tenured offer.
And they did.
And they did. And they did.
What was your immediate emotion upon receiving this news? Did you feel like it was a lifesaving event?
Oh, absolutely. Absolutely. It was like I had been saved from oblivion. I think if I’d gone to Pittsburgh I think it would’ve been another similar sort of thing to MIT where things that engaged me would not be engaging to chemists. And I was interested in Luttinger liquids at the time. I was interested in applying Bethe ansatz to spin problems. I mean I couldn’t do that in a chemistry department so a huge weight had been lifted. I could finally take a deep breath.
Did you indicate from the beginning that you wanted to include diversity things as sort of part of your overall portfolio? Or did you sort of do that quietly on your own and allow it to build its own momentum?
That’s what I did.
Quietly. I did not know that at the time. There’s a correction. There was one Black graduate student when I came here. Rodney Green. And he was a friend of a postdoc that I hired. And from Rodney I understood how alone he felt. So, it’s at that point that I began to do something.
Philip, was there anything going on at Fermilab that might have been relevant for you? Was that something that you could’ve fed off of?
No, no. I mean, I was not really doing any high energy stuff at the time. At that time condensed matter and high energy were fairly separate.
Yeah, right. That’s right.
Now when you start to make these diversity moves obviously you need allies. You need people, fellow faculty members, administrators. Once this thing had a momentum to it who could you turn to say, “I need help with this”?
The director of graduate studies was always supportive. And these were different people since I’ve been here. And yeah. They trusted me. And if I said somebody is good and they would admit the person—and the people I had suggested all came along and did very, very well. So, they had good reason to believe me. So, that’s how it really started. Lance Cooper was one of the DGSs. You know, the director of graduate studies is always someone who’s fairly savvy interpersonally. And so, I never had a problem. Yeah. I never had a problem. There could’ve been huge resistance, but there wasn’t.
Tell me about the research that led to the Edward Bouchet Award.
Well, included in that is the early work I did here at Illinois along with the random dimer model. The first project I worked on at Illinois was the size dependence of the Kondo effect. Those were experiments in which people noticed that the Kondo effect depends on the size of the system and the disorder. And that’s an unexpected effect and we showed how that can happen.
What were the theories up to that point that suggested it was unexpected?
The exact solution and the Wilsonian RG argument. [laugh] The Wilsonian RG argument. And so, it was an exception to that. I’m always interested in the exceptions, you know? Because that’s when you learn something. So, that’s a common theme. I wonder why it is that that’s what I do. I’m always you know, looking for things that don’t quite mesh with everyone’s understanding.
I wonder if the Bouchet Award helped in sort of turning you into an insider or at least that you began to think of yourself as one.
Well, what made me think more in that way is that I was finally in the department in which my main intellectual effort was mainstream. And that was never the case at MIT chemistry.
How did you feed off of that? I mean, on a day-to-day. Just in terms of your interactions, the way you went about your research. What was possible for you besides the obvious swimming upstream that you were doing constantly at MIT? What made that different at Illinois?
The graduate students wanted to work with me.
And I was able to direct them. Before I would have to sort of like do lots of song and dance to get people interested in my work. At Illinois, I was an obvious choice for students to work with. And then the problems I chose were main stream. I was working on this Kondo not yet main stream. After I gave a seminar at Stanford, one of the graduate students asked, “Why aren’t you working on high-Tc?” And I go, “Well, I’ve nothing to say about high-Tc.” And then he kept pressing me. And then he said the bottom line was you’re smart enough to work on high-Tc, why aren’t you working on high-Tc? And that’s something I took very seriously and I ended up working on high-Tc. But it was sort of indirectly because the next problem I worked on was the Kravchenko problem. So, this was an experimental problem discovered by Kravchenko and Myriam Sarachik whom I’m sure you know.
This was the problem of an anomalous metallic state in a 2D electron gas. Once again this was an exception to the localization theorem. No metallic state should be possible. But there it was experimentally. The resistivity went down as you cooled the sample down rather than increasing. And so this didn’t fit once again the standard localization theory of Anderson. As this was an experimental problem, I had an angle as to how the system might be circumventing localization. So being in a physics department led me at an increased rate to pursue new experiments that were part of the mainstream of the field. I felt as though I had a license to go and do that which I didn’t have at MIT. So, operationally I was the guy who worked on new experiments in the field. And that’s part of the role I play at Illinois.
When did you start working with Robert Leigh?
Ah. OK. So, that was like 2005 I think. In the aftermath of the Kravchenko problem, I started thinking about strong interactions and superconductivity and from there a linear extrapolation to high-Tc. My angle was discerning pairing from strong repulsive interactions. So then I started looking at the Hubbard model. There were lots of graduate students who kept coming around at that time. In actuality, I wanted to avoid high-Tc because the word on the street was that David Pines had solved the problem so there was nothing to do. What I realized was that the Mott problem was wide open.
So, I started working on the Hubbard model. that’s how I developed Mottness. And I realized that the key thing about Mottness is that you can’t separate the energy scales. As a result Mottness presents an obstruction to carrying out the Wilsonian program. A single equation allows you to relate high energy to low energy. The Mott problem wasn’t like that. The thing about Mottness is that when you remove an electron near the chemical potential, you affect all energy scales. It’s not like a Fermi liquid in which things just change things right around the chemical potential. All energy scales are affected.
So, I kept telling this to Rob Leigh, a string theorist. When I moved to Illinois his office was next to mine. And he’s a shy guy so I tried to draw him out to find out what was on his mind. I always liked him; I knew how to bring him out of his shell. And then one time I went and talked to him and told him about this problem with integrating out the high energy scale that seems like there are other degrees of freedom living at low energy. And I kept telling him about this. And one day he showed me something he’d sketched on a piece of paper. And I go, “Rob, I think that’s really good. Let me see if I can whip it into shape.” And that’s what I did. And the first paper we wrote together was how to integrate out the high energy scale in the Hubbard model. It led to new degrees of freedom at low energy, revolutionary at the time. I mean, to my mind it’s not revolutionary because the system is strongly coupled. [laugh]
So, what was actually revolutionary and what was not?
OK. It was revolutionary. So let me tell you exactly what the result is. If Wilson’s idea is correct, you integrate out the high energy stuff, it doesn’t affect the number of low energy degrees of freedom you have. Because in all of those problems although high energy stuff exists, it really does not mix with the low-energy stuff. There is a 1967 paper by Harris and Lange in which they showed that the number of low-energy degrees of freedom in the Hubbard model depends on the hybridization among the sites. Hence, the number of low-energy degrees of freedom are dynamically generated. A.B. Harris is the Harris of the Harris criterion. This is the only paper he wrote on the Hubbard model and there is a reason for that. If the number of low-energy degrees of freedom is dynamically generated, then the Wilsonian program is not well defined. All of this arises because the kinetic and potential terms do not commute. The number of electrons the lattice holds does not depend on the hybridization. As a result, there are degrees of freedom at low energy that are not enumerated by counting electrons. Harris and Lange must have realized that and that’s why they abandoned the problem.
The most important thing I learned from Rob is that in a strongly interacting system the propagating degrees of freedom don’t necessarily have a particle interpretation. Which means they don’t have a local interpretation. This means that if you were to now remove them from the system, you might not just be removing a clump of energy of some well-defined place. It might be affecting the spectrum over all energy scales. And to my mind, that is the real definition of a strongly correlated system.
So what Harris and Lange really showed is that there is UV-IR mixing in the Hubbard model. That’s what the disconnect between the electron count and the number of low-energy degrees of freedom is telling you. Well I think people knew this, but I was the first person who really understood what it meant or so it seems as it is a key feature of Mottness.
So, yeah, realizing that, that was a major by-product of working with Rob. Since he is a string theorist, I kept asking him do I have to pay attention to all the activity in AdS/CFT? I asked this because I started seeing papers by Sachdev and Hartnoll and all these incredibly smart people on gauge/gravity duality. He initially said no but one day he said, “You need to go and learn this.” So, which meant here I am in 2007 learning about relativity and string theory. Herein likes my preparation from the classes with David Boulware. All those Schwinger tricks came in handy and on some level I found high-energy fit my more natural inclination. Eventually when I had learned enough, we started writing papers on gauge/gravity duality and condensed matter physics. This was a paradigm-changing collaboration.
Now you’re really a physicist.
Yeah. So, I mean and of course I could never have done that in a chemistry department. So, I had to leave chemistry to become the scientist that I could become. There’s no way I could have stayed a chemist.
That’s a great quote. I love it.
Yeah. I could never have stayed in a chemistry department.
How much did you interact with Tony substantively?
Oh! Tony is one of the few people who really understood Mottness.
Tony’s big idea in high-Tc is that the mid-infrared is doing everything. Low Tc materials lack a mid-infrared band. In the Mott insulating state, the mid-infrared is absent. Only upon doping does it appear, which means it arises from spectral weight transfer from the upper to the lower Hubbard bands. Since the central idea of Mottness is the connection between the high and low energy sectors, Tony was quite receptive to my ideas. He has served as a quite useful sounding board. Tony is no knee-jerk physicist.
That’s for sure.
He thinks very deeply. He understands that strongly correlated systems can function under their own rules and we shouldn’t impose Fermi liquid theory. So, you know, look. He came up with this harmonic oscillator model for decoherence. So he understands things on a deep level. I was very intimidated by him when I first got here and then I realized you know, he’s stuck on some things that we’re all stuck on. So you see experiments have a way of leveling the playing field.
Philip, sort of a broad question during these years. What were some of the advances in strongly coupled electron systems at this point?
Well, I would say they were mainly experimental. T-linear resistivity. The fact that superconductivity was next to a Mott insulator. I thought that people understood the Mott problem when I came here. Then I realized they didn’t really understand very much about the Mott problem. At that realization, I pursued the Mott problem tirelessly. We were one of the first people to do a two site self-consistent cluster calculation on the Hubbard model. A few years later DMFT, DCA —all of that took over and I turned to AdS/CFT as a way of approaching the UV-IR mixing problem. All of these numerical techniques were a big advance and were not available when we started. I never intended to be a numericist so I don’t have any hard feelings about the success of these techniques. The fact that the real computational boys came in and took that whole problem over, that’s fine. My game was to understand what the physics was and then try to come up with models that get at the core.
In ’97-’98 Juan Maldacena was making big splashes of course. How was what he was doing relevant to your research?
At the time, not.
It was not. OK. How did you make this connection? Or did he make the connection?
No. It was in 2007 or 2008 when people like Sachdev started working on gauge/gravity duality. That’s when I started making the connection. So, look. The key thing about Mottness is the lack of separation of the energy scales. In gauge/gravity duality, the quantum field theory lives at the boundary. The bulk contains all the energy scales in the whole system. So, I thought well that’s clearly the way to go then. That clearly has all the ingredients to sort of give you something like Mottness. You see it was a quite evolutionary process. It is sort of fitting as I was originally drawn to high-energy physics but I was stuck as a chemist and here I’m able to, you know, complete the whole circle.
Philip, who was the second Black physicist to join the department? And were you involved in the recruitment and hire of that?
Absolutely. Nadya Mason.
Oh. That was Nadya.
Yeah. I told the department, “You have to hire her.” And then Myron Salamon said, “We should make an offer to her before she leaves today.” And I don’t think that was done, but shortly after. I was, yeah. I always made sure that I looked out for her, and I knew of her work because one of the things I’d worked on extensively while I was here was the insulator superconductor transition.
I’m curious about some nomenclature. The textbook Advanced Solid State Physics.
Solid state is self-referentially a throwback. It’s a bit of an anachronistic term, circa 2012. Soft matter physics is fully developed. Everybody says “condensed matter.” What were you thinking with the title Solid State?
I hate the name condensed matter physics.
Yeah. Because I feel as though if you say you are a particle physicist, people know what you do. Ditto for when you tell someone you’re a string theorist, they know what you do. You tell someone you’re a biophysicist, they know what you do. You tell someone you do condensed matter, they have no idea what that means.
So, I’ve been campaigning to change the name of condensed matter physics to something else, for example quantum matter, because that’s much more descriptive of what we do. At the time I wrote the book, I didn’t think of quantum matter. I simply went with what I thought was more descriptive.
Does the maturity of soft matter physics make it easier to call it solid state physics? In other words, there was no soft matter physics, right? It’s a new field.
Condensed matter is an attempt to capture everything. But in so doing, it conveys not very much. So, I’m about names that really say what it is that you’re doing. Like Mottness. You know? That tells you it’s about what was in Mott’s head. You still have to delineate what was in his head which I can do, but names should really convey something descriptive that is useful and I feel as though this is not what is going on with condensed matter. The title was an attempt to be descriptive. Well, and there isn’t anything in the book that isn’t about solids.
What kind of support are you getting for this nomenclature campaign to throw back to solid state, not condensed matter?
As with all my ideas, there is huge pushback.
It’s going nowhere.
When you jumped into high-Tc with both feet what were some of the real theoretical walls that you and everyone else was hitting up against at that point?
How do you compute? How do you compute? And that’s…yeah.
And obviously you mean classically. We’re not yet, we’re not talking about quantum computation with high-Tc.
Right. Right, right. I actually meant by pen and paper, you know?
The problem is how do you compute in the strongly coupled regime. When I came to Illinois, as I have said, the word on the street was that David Pines solved the problem. But I soon realized that it was all phenomenology. What surprised me as I dug deeper was how shaky the footing was for most of the ideas I had assumed, looking at the problem as an outsider, were well established. With high-Tc, the strength of your personality really determined how well your ideas sold.
Hence, with high-Tc, it really was not the marketplace of ideas that determined much. Take my guru Patrick Lee. When I was at MIT, his work certainly seemed convincing. Going to Illinois and talking to a wider group of people, I began to see the problems close up. All was not as it seemed. So to my mind, the hunt for computational accuracy was a must. Is there an exactly solvable Mott model?
And where are dirty bosons in all this?
OK. It’s in the context of Nadya’s problem.
Bosons should either superconduct or insulate. Early on (1989 or so) people such as Alan Goldman showed that at low temperatures, destroying superconductivity in 2D led to a metallic rather than an insulating phase. I worked on explaining this result for 5 years. We came up with a model involving glassy disorder. We wrote a big review in Science on it entitled, “The Elusive Bose Metal.” So my take on the whole problem is that disorder is relevant, hence dirty bosons.
What have been some recent advances in the fractional quantum Hall effect? Particularly with regard to topological insulators?
The closest I have come to the FQHE recently is twisted bilayer graphene. I would say I have never really worked on the fractional quantum Hall effect. I did one paper on it. It was in Phys Rev Letters a long time ago. It was on high filling factors. Not low ones. So the FQHE is not something I have focused on.
Is AdS/CFT a gift that keeps on giving? I mean there was this burst of excitement, but does it keep giving good things to you for your research agenda.
For me yes. Initially, there was this burst of energy and I thought that oh my God, someone is going to solve the Hubbard model using AdS/CFT. If that happens, then I’m gonna be out of business.
So I said that person has to be me. I worked on that for a few years and soon realized that lattices were near-to-impossible using AdS. I thought the Matthew Fishers of the world would jump in on this problem. But only two of us did, the other being of course Subir Sachdev. In Europe, we are joined by my good friend Jan Zaanen. The easy problems in the probe limit all got solved, Fermion correlators in a curved background. Back reaction has proven to be a real problem. But I continue to mine this field because all my deep ideas have come from my exposure to it. The whole idea of anomalous dimensions for conserved currents came from my exposure to AdS/CFT. I think if we solve high-Tc, we are going to have to give up on particles and locality. Perhaps AdS/CFT is a way forward here.
Philip, a snapshot of just where the field is now. So, to go back to the way you were contextualizing the research scene at Berkeley, right? This is like, it’s like particle physics in the early 1970s, the things that you were talking about. Everything was foundational. Massive discoveries every day, right?
Where is the field now? In the way that you contrasted particle physics in the early 80s, right? Where is your solid state, all the things that you’re involved with now? Is your sense that the graduate students today are operating in an equally fertile research environment that you found yourself in all the way back at Berkeley?
I would say no. The field that most graduate students are going into right now is one involving no interactions. It’s topology. The whole field started with a quite interesting question: how do you get edge states without breaking time reversal symmetry? That’s the question Charlie Kane asked. Then he came up with topological insulation. This is turning out to be a fundamental feature overlooked in the development of band structure. This field has a definite end, I would think, as it is a non-interacting problem. By contrast, the field of strong correlations seems to have no end in sight because of the computational intractability of this problem. Few students are being trained in this field. As a result, the basic problems are still there: what happens when you dope a Mott insulator? How does superconductivity obtain? So the field is wide open. Ditto for grand unification. After Georgi’s paper on SU(5) in the ’70s, the field is still wide open. Look at the anouncement yesterday from Fermilab that the g-2 experiment deviates from the Standard Model by 4 standard deviations. Our now on strongly correlated physics is on models that could potentially re-invigorate the field.
With all this new stuff who’s the graduate student that’s ideally suited to work with you?
It sort of depends on what part of the group they join. Our group is evenly divided between string theory and condensed matter and some do both. So ideally, all incoming students know general relativity. We have one combined group meeting so everyone is exposed to everything. In fact tomorrow I’m giving the group meeting on Bethe Ansatz. [laugh] As you know, this is mathematically technical. But I’m trying to solve a new type of Kondo problem in a host of interacting conducting electrons that I think is exactly solvable. We’ll see.
A very in the moment question just because it’s exciting across the board. This muon wobble that everybody is all excited about at Fermilab.
What for you might be exciting there?
The fact that the Standard Model doesn’t get it all right. So there is something to learn. At present, we have no idea what the impact will be on other areas of physics. The reason I was drawn to high-energy physics is that it took the long view. That creativity was rewarded in and of itself. Look no further than Georgi’s SU(5) paper, a truly great paper but we know now it is not the final story. Howard is always interesting.
[laugh] That’s how Howard became Howard, being provocative, always asking the right questions and coming up with daring ideas that have huge experimental implications. The proton is unstable. But people went and found just the opposite. So I feel as though high energy physics rewards creativity more than condensed matter does. It’s partly because our field does not take the long view. Simple tabletop experiments can falsify claims made quite readily.
Between your requirement that graduate students have to have a background in general relativity and your general adventurousness with your research agenda, what might you or solid state physicists generally who work the way you do, what might they contribute to the really big question of figuring out how to unify general relativity and quantum mechanics? Because the people who work on that full-time, they’re not getting anywhere with this. So, maybe there’s like an all hands on deck kind of approach we need to take here.
I’m writing a paper right now. The first line of the paper, you won’t believe what the first line in the paper is. [laugh] Let me get the paper up. Overleaf. There you go. It’s in Overleaf. Strange metal. Here’s the first line of the paper: The simple thought experiment of two electrons impinging on a black hole reveals the essential tension between gravity and quantum mechanics. [laugh]
There you go. [laugh]
So yes. As a result of the work I’m doing, quantum gravity is always in the mix. That’s pretty much why I’m still working on AdS because it offers a window into this problem. An overarching theme of my work is the role non-locality plays in introducing state-dependence of the boundary quantum field theory and the role of such state dependence in constructing the interior of black holes. I applied to the physics division of NSF to support this work. So the foundational questions in quantum gravity are at the core of what I am doing.
That’s exciting to hear.
Yeah. So, and I’m fully aware that my foray into this—motivated by condensed matter—is allowing me to work on just a pure high energy problem. But I don’t view it as a pure high energy problem really because I think it affects everything.
So, as you say and I’m sure you’ll continue to insist, you remain apolitical in these academic issues. I’ll say it for you if you won’t say it yourself. You’re eminent at this point in your career. You’re well respected across the board. Do you have the platform to push against departmentalism? In other words, the thing that almost sunk your entire career from MIT was being trapped not intellectually, but trapped departmentally, right?
Where you bring value to your current research it seems it comes from your math background, right?
Are you looking politically, maybe even against your own best interests that if what you’re after are just the best answers to how nature works, boxing people up into academic departments might not be the best way to go about that?
Yes. Quinn said it beautifully. Divisions at a university aren’t those of the universe.
[laugh] That’s great!
You know who told me that? Howard Georgi.
There you go. Perfect.
Now Howard and I go way back because he interviewed me in my failed attempt to become a Junior fellow at Harvard. Regarding departmentalism, I think Illinois has been very forward leading in this way. We hired Shinsei Ryu as a condensed matter person who has serious credentials in high energy. In fact, he was stolen from us to Chicago and then Princeton hired him as a high energy person. We knew him as a condensed matter person because we take a view that if it’s interesting it is interesting. Illinois is unique in this regard. Regarding being boxed in, I mean there are political things I have gotten involved in and you know some of them. For example, getting the APS to boycott cities that are engaging in Neanderthalish policing practices is a great step and a true sign of leadership by the APS.
Let’s talk about last year.
Physics, STEM in general, had a moment of reckoning. But it’s a moment of reckoning that scientists of color certainly didn’t need themselves.
How is what you have been pushing at Illinois ahead of the curve, ahead of other departments?
Yeah. Without someone like me, it is difficult to recruit students. But there are places that do recruit at NSBP meetings. So other departments are doing what is necessary. But numbers matter and Nadya and I both being here helps. I have been asked to talk to other departments about recruitment and there is really no big secret here. Students are drawn to where diversity is already present. Getting it started just takes a few students and that any institution can do by recruiting in the right places.
Philip, a question specific to you, of course. There’s a duality. Going back to what I asked you. You know, you moved to this country as a 10 year-old. Do you identify more with West Indian culture or American culture? You didn’t bat an eyelash when you, very clearly, said it’s West Indian culture. But I wonder how you navigate the fact that in the middle of Illinois you present presumably as an African-American man.
Right. See. That’s the thing you can’t lose sight of. It doesn’t matter.
Is that an opportunity for you? Does that provide a layer of complexity? How do you navigate these things in terms of if the prize for you is demonstrating that diversity is good for science, that a plurality of ideas, a plurality of experiences is actually good science? So, how do you operationalize the fact that you have an internal identity that might not match externally how you might be perceived?
OK. That is the problem. That is the thing that I don’t think people really appreciate. That with me, it’s a different layer. But I’ll tell you something. You get stopped by the police, that internal dialogue goes out the window.
You’re just Black. So, you can’t be fooled by that stuff your parents told you that you are different. But I still have a contorted internal dialogue of where I really belong. In America, I appear Black. Only in Trinidad am I truly West Indian. In the States, I get stopped more than most people. Now, I know I tend to speed, but you know, I get stopped for things that I never even knew were laws. Did you know it’s a law that you’re supposed to move over to the left-hand lane on a freeway if a policeman is in the breakdown lane?
I did not know that one.
I didn’t know that was a law!
But you got stopped for that and maybe I never did.
I had to go to court and a $340 fine. And who’s in court? Everyone looks like me.
OK? I ask my colleagues about this law and they haven’t a clue. So, look. The States is a very sobering place. And it’s comforting to have dual identities, but there’s one that’s forced upon you more than any other identity. That is, how you appear to others and the social baggage that entails outside the West Indies. So, you know, when you raise your kids you have to make them aware of this reality. That they can’t hide behind something else. I guess that’s the thing that I was not made aware of. My parents sort of thought we could just be West Indians and not worry about what’s going on. But you can’t. You live in the States. That’s the reality.
How will you ensure or at least be part of the conversation to ensure that #ShutDownSTEM was not just a day on the calendar? And that it’s you know, life back to “normal” again? I remember very clearly on that day thinking to myself you know, this is central to everybody, but it’s also 24 hours.
Yes. That’s right. So, that’s why I wanted to do something that would not go away after one day. There was huge pushback to my proposal. I’m surprised that the APS actually adopted it. So, that is something that came out of that whole movement. And it’s living on. I try to do things that matter. And it’s amazing that the APS did it! Jim Gates was quite instrumental here. You know, this idea’s been floated around to other organizations and they’re not biting yet.
Yeah. I was just gonna ask you about Jim Gates. So, obviously a point to emphasize here is that there’s diversity in diversity.
And there’s a generational component to this as well. So, somebody like Jim Gates and Ron Mickens, right? They have a very different perspective than the Particles for Justice crowd who are postdocs and assistant professors. You’re kind of right in the middle. You’re sort of a bridge generation there.
Yes. Yes. That’s right.
So, in that case, in that way you’re sort of well-positioned at least generationally to appreciate both sides. How do you perceive these distinctions and how do you put that diversity to productive use so that everybody benefits?
OK. The thing that we have to realize is that everyone’s right here. That’s the hard thing. Ron Mickens I think had it worse than all of us. He came up when it was very difficult. I think having the ability to stand in other people’s shoes and see it from their perspective is what this generation has to be able to do. Certainly the things we’re talking about now might not have even been possible in previous generations [laugh] OK? ShutDownSTEM! No, cause it’s not gonna happen. So, but the awareness that something has to be done is the thing that has always been the motivation. So, I think every generation makes it possible for the next to be more direct. I think that’s the bottom line.
And direct even to border on confrontation.
Yeah. Absolutely. Cause it is confrontational.
Its what Ghandi said, “Confrontation cannot be avoided in changing someone’s mind.”
Philip, let’s end our talk going back to a happy place. Let’s go back to the science.
Some broadly retrospective questions. What have been Eureka moments in your career? A moment where like you didn’t understand something—not necessarily a moment where you moved the field forward, although I’ll want to ask you about that as well. But for yourself, in your own understanding. What stands out in your memory as moments when you were studying a problem and something just clicked? And it really moved things forward for you?
There are two, actually three. The random-dimer model [laugh], zeros, and the most recent paper we wrote in which we calculated exactly the superconducting instability in a doped Mott insulator. The latter was definitely a Eureka moment. Mottness has been on my mind for a long time and the realization that you could solve for a superconducting instability exactly in any model for a doped Mott insulator surprised me. Zeros. I remember that one specifically because I was driving down a freeway talking to my postdoc and we were talking about zeros. Something he said made me finally get the significance and then I saw how they were at the heart of all properties of doped Mott insulators. It was at that point that Mottness became a quantitative thing.
OK. To expand, Green functions are one over something. And when that thing is equal to zero, the Green function goes off to infinity. That defines a particle. Now what happens if the Green function is actually equal to zero? What does that tell you? Because Green functions are a causal thing, you can calculate the real part directly from the imaginary part. So, the thing I realized is that the real part vanishing can only happen if the stuff at high energies or above the chemical potential is cancelling the stuff below the chemical potential. Which means zeros are about exactly the thing I’d been trying to understand for a long time. The high and low energy physics all mixed together. All of this comes down to whether or not something is zero.
That was a very “a-ha!” moment. As a result, I’ve written several papers on zeros and the community understands things about zeros now. Yeah. The most recent paper we wrote, no one had ever calculated exactly the superconducting instability in anything other than a Fermi liquid. And we did it in an exactly solvable model for a Mott insulator. I mean I should’ve thought about this a long time ago and I didn’t. And this model was written down in 1992, so it wasn’t like the model wasn’t there. Something else, this model has zeros. It has all the things you need to define Mottness. So, it’s sort of a historical accident that people ignored this model. So, I’d say those are the three things that stand out because they were sudden.
To an outside observer, you’re all over the place. I mean what are you not involved in?
I know. It looks like I’m all over the place.
So, what’s the through line? What connects everything from Berkeley to now?
I’m interested in things that break paradigms. That’s the through line. Things that break paradigms tell you there is something new to learn. And if there’s something new to learn I feel as though I have a chance.
Because time is such a precious resource. Because there’s only so much that you can spend on any given one project. What have you learned about when you’re after that question? What are the feedback mechanisms that you rely on where you say keep going? Or where you say I’ve hit a wall, I’m going on to something next?
Excellent question. OK. [sigh] Generally, I give something up if I don’t have an angle, I am outmanned (outgunned), or if I’ve solved it and then there’s nothing else for me to do. So if I haven’t solved it, that’s the key motivator for staying with it. I feel as though you have to play the hand you are dealt. So quitting would not be a good thing to do. I never like to leave something unfinished. That’s sort of a cardinal rule with me. I start something, I finish it.
In all of your collaborations, do you play the same role no matter who you’re working with? Or is it very dependent on who you’re working with?
Typically, I am the guy who drives it along.
This goes back to your graduate advisor.
Yeah. Yeah, yeah, yeah. And it’s always because I have some math insight into something. And what am I doing tomorrow? I’m giving the group meeting on the Bethe ansatz in the Hubbard model. Very technically demanding thing and you know, I’m doing it because I think we have a chance to solve an interacting host with a Kondo impurity. That’s never been done. If it’s possible I’m going to be the first one to do it. I don’t know if we’ll succeed, but you know, it’s worth trying.
You say that physics is fundamentally an experimental discipline and your research is always alive to what’s going on in experiment. So, right now what are the things that are most compelling to you that may drive the theory forward?
OK. In high-Tc one of the big questions is what’s the origin of the pseudogap? Where does it terminate? How is that related to the strange metal? There’s an experiment out of Stanford that tells us now that the termination of the pseudogap might not be what people had thought it was. It doesn’t seem to be a continuous transition; it seems to be a discontinuous transition. No theory of the pseudogap can give you a first order transition. First order discontinuous transition. So, those experiments coming out of Stanford I think are very profound. The question comes to mind: Are all binding-unbinding transitions necessarily second order? What happens if asymptotic freedom is absent? Could that be a way of circumventing the second-order nature of the transition? Hence, the experiments might be telling us about the absence of asymptotic freedom in this problem. Asymptotic freedom we know won a Nobel Prize.
In the cuprates the short-distance physics is not free. This implies an absence of asymptotic freedom. So perhaps the experiments are fundamental. So I sense a big problem around the horizon and it is the experiments that are leading us forward.
Dale Van Harlingen tells me that there’s a quantum revolution happening at Illinois. Are you part of that revolution and where is it headed?
Yes to the first, no to the latter. The department looks quite different than it did 27 years ago when I was hired. Building a quantum computer is now a real physics problem.
I hope he is right. This is quite forward leaning thinking and Illinois is at the center of it.
What unique insight might you have to existential questions such as what is quantum computing even good for?
Oh, I think that’s an easy question. If you could really get a quantum computer, many algorithms now become simple. I mean, you could really get at problems that are now limited by the sign problem. Simulating the Hubbard model with Quantum Monte Carlo can then be pushed realistically to incredibly low temperatures. Yeah, if someone gave me a quantum computer I’d like to make sure that they didn’t give it to anyone else because then… [laugh]
[laugh] So, you have pretty well-defined ideas of what you’d do if you got your hands on one?
Oh, absolutely. I would try to solve the Hubbard model. But everyone would do that. I mean, that’s a no brainer, you know?
Why is it so clear that the Hubbard model is unsolvable through classical computation?
The sign problem is insurmountable at present. This limits the temperature and size of system.
So, Philip, last question. Looking to the future, right? All of these things, let’s just extrapolate. Where is your entire body of research headed? What are the things that you’re willing to let go of? What are the things that you’re right in the middle of? And what are the things that you haven’t even started but you want to?
OK. Things I’m willing to let go of: twisted bilayer graphene. I think I’m being out-personed. [laugh] You know, we had one of the first papers on that. I don’t like the way the field is going. People just keep doing Hartree-Fock, weak-coupling stuff, when the problem really is strongly coupled. That’s just what they can do. So this is a problem I will have to let go.
In terms of new directions, I always judge my success by if in a year or so I’m doing something I couldn’t have seen myself doing before. So, as long as I keep doing new things I feel as though I’m fine. The big thing that I think we’re going to be up against is quantum gravity. And I’d like to have something to say about that. So my research seems to be converging from many different angles to this problem. But you couldn’t just have worked on quantum gravity or you would be. But what has made me have staying power is that I’ve always drawn on different angles. That is, my insights aren’t just informed by being a card carrying whatever. I’ve really managed to combine things. And that sort of parallel processing is something that I think more people need to be able to do. So, that’s one reason I’m raising this crop of graduate students to be parallel processors not necessarily crowd followers.
In your expectation they should have their Misner, Thorne, and Wheeler down by the time they come to you?
Yeah. Absolutely. They have to have certain basic things down. I mean I can teach them these things, but it would certainly help.
The last part of the question is what is the stuff that you’re not even yet involved in, but you want to be?
Well, a direct approach on quantum gravity I would say is the thing I’m not yet into but would certainly like to solve. Also, there are certain experiments I would like to see happen in condensed matter. We have this idea that what’s really going on in the strange metal is a new electromagnetism. The Aharonov-Bohm effect should be fundamentally different in such systems if we are correct. Bringing such experiments to fruition would be a huge accomplishment, one that I hope to pull off. After all, solving the strange metal is the big kahuna.
Where will that lead if you get there?
Solution to high-Tc. High-Tc comes out of the strange metal.
The coefficient of T-linear resistivity tracks the superfluid density. As a result, explaining high Tc cannot be done without explaining as well the strange metal.
OK. Well, good luck.
Thank you. Thank you.
Philip, this has been a tremendous amount of fun for me. I’m so glad we were able to do this. So, thank you so much.
I’m glad we were able to do this. This was very, very good. You are an excellent interlocutor.
Oh, why thank you very much. [laugh]