Laurence Yaffe

Zierler:

This is David Zierler, oral historian for the American Institute of Physics. It is August 3rd, 2020. It's my great pleasure to be here with Professor Laurence G. Yaffe. Larry, thank you so much for joining me today.

Yaffe:

My pleasure.

Zierler:

All right. So, to start, would you tell me your title and institutional affiliation?

Yaffe:

I am professor and department chair in the Physics Department of the University of Washington.

Zierler:

And to make this sort of hot off the press, what is UW and the physics department doing for September with COVID?

Yaffe:

Most classes will be taught remotely. That's true campus wide. In physics, I think it's true for perhaps all but one exception.

Zierler:

What does that mean for lab work this upcoming semester?

Yaffe:

It means it's compromised. But, for introductory labs, we're looking at schemes where students get basically a box of parts of some sort that they- they're still trying to convey similar skills about problem solving and so on, but it is inevitably different than an in-person lab.

Zierler:

Right. Well, let's go back to happier times. Let's take it right back to the beginning. Larry, first, tell me a little bit about your parents and where they're from.

Yaffe:

Well, my mother grew up in Minnesota, mixed European background. My father spent his childhood in Ohio, sort of mixed Eastern European background. They both met in graduate school when they were both in Ames, Iowa. They both got PhDs.

Zierler:

That would be Iowa State? They were at Iowa State?

Yaffe:

Yes, and later spent a few years at the national lab in Oak Ridge, Tennessee.

Zierler:

Mm-hmm.

Yaffe:

But they looked around and decided to come to California in the early fifties, where my father got a job at the Stanford Research Institute, SRI, and my mother started teaching. It was then San Jose State College, now San Jose State University.

Zierler:

And what were their fields of study? What were their specialties?

Yaffe:

They were both chemists. They bought a house at the edge of what is Los Altos, right on the edge of what later became Silicon Valley in between Palo Alto and San Jose.

Zierler:

I hope they held onto the house for as long as possible.

Yaffe:

My mother is still in the house after all these years.

Zierler:

Oh, excellent.

Yaffe:

However, my father passed away very early, when I was about 4.

Zierler:

Oh.

Yaffe:

A very rare, unusual illness. So that inevitably had a big impact on my childhood. That was basically me, I have one older sister, little bit older, my sister and I and my mother, she was teaching, typical academic, not terribly well paid, but you have summers off. So we got by.

Zierler:

Do you have any memories of your father?

Yaffe:

Scattered images but nothing terribly substantial.

Zierler:

Mm-hmm. Did your mother ever remarry?

Yaffe:

She did eventually, but not until I was in college.

Zierler:

So obviously you didn't grow up with any father figure?

Yaffe:

Not in a sort of direct central sense, no.

Zierler:

Mm-hmm.

Yaffe:

But there was Boy Scouts, there was later rock climbing with the Sierra Club, there was a lot of travel that started even earlier, visiting Death Valley in the spring, trips to Yosemite, car camping, driving up to Alaska in 1964 just months after their giant earthquake, travels back east visiting Minnesota. So that was also a big part of my childhood from not too long after my father passed away until I went to college.

Zierler:

Now, obviously, chemistry is in the blood. Did you feel like you had innate abilities and you naturally inclined towards the sciences yourself, or did you get special encouragement to pursue that from your mom?

Yaffe:

I, from pretty early days, was clearly interested in science. I mean, it wasn't terribly specialized, but I went through the phase when I was interested in dinosaurs and I started learning about paleontology. There were later periods where I was reading just a variety of science, looking at Scientific American. But it wasn't just until I went to college, but actually even later in college, where I really ended up deciding what part of science I was going to go into.

Zierler:

Now, in high school were you a standout student in math and science?

Yaffe:

To some degree. I was a bored student. Yeah. I did very well in math and science classes.

Zierler:

The classes were not intellectually stimulating for you?

Yaffe:

It varied. There was a physics class I took where the teacher was basically content to let me and a couple friends hang out in the back of the class and just keep reading ahead. And as long as we were learning stuff, he was happy.

Zierler:

Yeah.

Yaffe:

That was good. Math was kind of interesting in that after the usual algebra, trigonometry, whatnot—, was just public school at the time, and the school district, they didn't offer calculus in every high school. They did in a high school that was not my regular high school that was a ten or fifteen-minute bicycle ride away. So, I and a couple other students, one of whom was Steve Jobs, by the way, we would ride over to Fremont High School for calculus. And then we'd ride back, and we'd be late at our next class, which, in my case, was a bit of Russian. And so, I'd show up every day fifteen minutes late for Russian, but the teacher was very accommodating.

Zierler:

Was this sanctioned between the two school districts that you were essentially crashing the calculus course?

Yaffe:

It was the same district so, yes, it was sanctioned. Two different schools, one district.

Zierler:

That's so odd that even within the same district that a math curriculum would be so varied.

Yaffe:

Well, I think it was just they didn't have enough students to offer calculus at every school. So they did make it available, but they had to bring students from more than one school together.

Zierler:

Now, when you were thinking about college, were you thinking specifically about chemistry programs?

Yaffe:

No, not at all.

Zierler:

What schools did you apply to?

Yaffe:

Oh, gosh. I went to Caltech, of course, so Caltech, I looked at Harvey Mudd, I looked at Pomona. I can't remember if I applied to some of the UC schools. I just honestly don't remember.

Zierler:

So clearly you wanted to stay somewhat close to home? No East Coast schools?

Yaffe:

For this round, I'm honestly not sure. But what sort of settled it was that, at the time, Caltech sent an alumnus to interview every prospective student.

Zierler:

Right.

Yaffe:

And somebody who, unfortunately, I don't remember their name, came to talk to me gave me a good impression, and I should emphasize this was at the beginning of my junior year in high school. I dropped out to go to college. And that was not something that a lot of colleges were comfortable with.

Zierler:

To be clear, you didn't graduate early, you just dropped out?

Yaffe:

That's correct.

Zierler:

(Laughter) Did you ever go back for your GED?

Yaffe:

Well, bit of a funny story here. Not in any real sense. And, after a while, I got some amusement out of telling people, hey, I'm a high school dropout. But, unbeknownst to me, at some point, my mother went to the principal of my high school, saying, "Look, you haven't said that he's graduated because he needed some other year of English or social studies or something like this." And then she said, "Well, look, he took a quarter of some sort of humanities class at Caltech. He took a quarter of some sort of social studies. Maybe you should regard that as equivalent and just give him a high school diploma so you can count him as a success instead of a failure." And somewhat to my later frustration, because I was not party to any of this, they did so.

Zierler:

Caltech was not bothered by the fact that they were accepting a high school dropout?

Yaffe:

They could not care less.

Zierler:

I've heard from many people who had that experience of meeting a faculty recruiter from Caltech, that the recruiter would convey just how fun science at Caltech would be and that really stood out. I wonder if you had that same experience?

Yaffe:

I certainly did. I mean, both from the recruiter and from the actual experience later.

Zierler:

Now, you said you didn't major right away in chemistry; you didn't declare that?

Yaffe:

Well, I didn't focus on chemistry because I was convinced I wanted to do chemistry. But, at the time at Caltech, if you were to be a physics major, I mean, everybody at Caltech had to take, at the time, two years of math, two years of physics, I think it was one year of chemistry. That was just a given for everyone. But, beyond that, to be a physics major, there was freshman physics labs where I saw the students around me spending three or four hours in a lab every week doing it, and then twelve or fifteen hours writing it up and doing error analysis. And there was classical mechanics and there was classical E&M. And it just seemed terribly painful before you got to the fun stuff, which was quantum mechanics, which was modern physics. In contrast, to major in chemistry you had to take what every student at Caltech did, and you had to take a one credit class in how to give a seminar, how to stand up and talk. And, beyond that, you had to plan a program that your advisor approved of. And I got along extremely well with my advisor who was a theoretical chemist by the name of Bill Goddard, or William A. Goddard, III. He was very, very helpful, great role model, and he understood that I was going to take a mix of stuff that seemed interesting. I did take an organic chemistry lab, but I took molecular quantum mechanics from Bill, and then I took regular physics quantum mechanics from Richard Feynman. I took general relativity from Kip Thorne. All this was fine with Bill. And after a couple of years, it was clear that I was focused more on learning physics than anything else, but it was the fun physics. It was general relativity and quantum mechanics and the beginning of field theory.

Zierler:

Was this as much the fact that you were exposed to people like Feynman as it was you discovering your own interest in physics that you might have realized no matter where you might have learned it from?

Yaffe:

I'm sure it's a mix. I was exposed to Feynman early on. At the time I was there, he was meeting with freshmen in something that wasn't a formal class, it was just Feynman walking in and talking about physics to a bunch of freshmen. And that was fun. I was certainly influenced by the other students around me who were a year or two earlier, more advanced, and just seeing what they were doing. Yeah, it was a whole mix.

Zierler:

Did you consider just switching altogether to do a bachelor's in physics?

Yaffe:

I never did. I would've had to take all those labs that I didn't want to have to spend time on.

Zierler:

So, I wonder, when thinking about pursuing a physics degree for graduate school in what ways not having a full undergraduate experience in physics might've been a challenge?

Yaffe:

Well, when it came to applying to grad school, I did apply to one chemistry program, so I must've had some degree of uncertainty, but then I applied to a number of other excellent physics departments. I'm pretty sure I took the chemistry GRE and not the physics GRE, and that didn't actually seem to bother anyone. And I got a letter from John Wheeler when he was at Princeton, but it wasn't the substance so much, it's just that he took the trouble to write a letter to this kid who is applying.

Zierler:

Did you have any idea what it meant to get a letter from John Wheeler at the time?

Yaffe:

Not at all. But I had spent a summer working in a company that did chemical and radiological analysis that was run by someone who had known Wheeler when they were students much earlier. So, we had a mutual contact that I think brought my name up. That was right before Wheeler actually moved from Princeton to Texas, so I never actually interacted with him. But his letter positively influenced me to think kindly of Princeton.

Zierler:

Where else did you apply? And did you think about staying at Caltech?

Yaffe:

No. It just seemed like it was good idea to go someplace else.

Zierler:

Mm-hmm.

Yaffe:

I know I applied at Harvard. I know I applied to UC Berkeley. I think there was one more place that I forget.

Zierler:

So, Larry, a lot of people, when they have the full undergraduate experience in physics, many, although not all, have a pretty well-defined idea of the kind of physics they want to pursue in graduate school. Minimally, if they think of themselves more as experimenters or as theorists, and I assume that you probably did not have any of those things wrapped up at that point?

Yaffe:

Well, only halfway. I was very clear at that point that I was more inclined to theory than experiment.

Zierler:

How so? How did you come to that conclusion?

Yaffe:

Well, partly going back to what I describe as my initial impression of my fellow students slaving away in physics labs and feeling like, do I really want to do this? It's not so different seeing students in physical chemistry labs.

Zierler:

Right.

Yaffe:

And early on, what I really enjoyed was learning about how quantum mechanics works in the context of molecules. My advisor was a theoretical chemist which, in chemistry, theory is a much smaller component of the overall field than it is in physics.

Zierler:

Sure.

Yaffe:

So, in a sense, that was fortuitous, but I really got an early look at what it was like to be doing theory work involving quantum mechanics, admittedly for chemists, but that's close. And then, even as an undergrad, clearly learning quantum field theory, learning about modern particle physics was an interest. I had a field theory course from Steven Frautschi. I had a course from Richard Feynman on elementary particles not too long after he was doing all this work on understanding the J/psi, and we did a lot of fun stuff with that, and the beginning of scaling. So all that influenced what was, to me, the interesting part of where physics was at the time.

Zierler:

And probably on some level, I mean, your tenure at Caltech, '72 to '76, you could make the case that that was some- if you had to pick four of the most exciting years in theoretical physics, you could make a strong case that those four years would be up there.

Yaffe:

They were certainly good years.

Zierler:

So you knew you wanted to pursue theory. When did you develop a relationship with David Gross; was that right away or that came in stages later on?

Yaffe:

That was pretty quick. At Princeton in graduate school, they had some exams you had to pass. That was a big hurdle for everyone. And, rightly or wrongly, one could say that I was a little cocky. I decided I wanted to get those exams out of the way immediately. Most people take them at the beginning of their second year, but you're allowed to try at the beginning of your first year. So, that's what I did. It was interesting in that, of course, one of the core portions was graduate level electromagnetism. I had not studied electromagnetism since I was a sophomore undergraduate out of Feynman's lectures on physics. So, I read or at least skimmed Jackson's E&M book for a week telling myself that there was no point spending time focusing on anything that I couldn't remember in short-term memory for one week. And it was good enough. So, I managed to pass my exams, and at that point I started walking around and just walking into various professors' offices saying, "Can you tell me what you're doing?" I remember doing that with Elliot Lieb, who was doing statistical mechanics, and I didn't know enough about it at the time to really appreciate some of the really deep things that Elliot was doing. But I also did that with, I think, David quite early on, and a few others. At this time, there was a lot of back and forth from Princeton University and the Institute for Advanced Study. So I'd join some other students and I'd go to hear seminars over at the institute and got to meet Roger Dashen.

Zierler:

And who was giving some of the most significant seminars at the institute at that point?

Yaffe:

Oh, gosh. Big mix. I mean, at the time, exaggerated but only a little bit, it's like everybody interesting in theoretical physics, in the particle theory side of it, would be coming through Princeton.

Zierler:

Yeah.

Yaffe:

At the time, Jean Zinn-Justin was on sabbatical, was spending, I think, a whole year there. He gave a special course about critical phenomena that I sat in. But then there were people like, oh, gosh, you name it, John Collins, Sidney Coleman, coming down from Harvard, some of the key Russians coming through at various points, Gribov and others, Faddeev.

Zierler:

And you soaked it all in?

Yaffe:

Yeah. And David was interesting. I mean, David was somebody who intimidated a lot of people, students, visitors, you name it. And somehow, I learned really, really early that, while David did not like to waste time, and one could say he does not suffer fools gladly, if one had a concise, succinct question, he was actually very helpful. So I can't really say exactly when this started, but over the next few years, I spent a tremendous amount of time actually in David's office just listening when any visitor was there talking to David, which was practically on a daily basis, hearing what they were talking about, occasionally injecting a question or two. And it was a wonderful experience for me. I thought David was a terrific advisor.

Zierler:

And what was he primarily working on during your time with him; was it confinement?

Yaffe:

Well, it was the beginning of really trying to understand what's called nonperturbative properties in QCD. So it was the beginning of what's referred to as instantons or topologically nontrivial excitations and their role in different theories. There was a hope that this was going to lead to really understanding confinement.

Zierler:

Mm-hmm.

Yaffe:

In QCD itself, that didn't ultimately play out, but it still taught us an awful lot, things about theta dependence that connects to what's known as the strong CP problem, things that ultimately did connect with confinement in supersymmetric theories that are much more tractable. But it was sort of the beginning of that whole era of really trying to understand phenomena that require something more than just calculating Feynman diagrams.

Zierler:

And in terms of developing your own dissertation, did David essentially hand you a problem to work on or he expected you to come up with an idea and pitch it to him?

Yaffe:

In a certain sense, I think it's fair to say that I preempted even having that discussion with him. I just started coming in with things.

Zierler:

Mm-hmm.

Yaffe:

And at some point, I started writing my first serious paper and then it went through him and he said, okay, it seems reasonable, improve this a little bit. Much of what I did as a graduate student was really triggered just by seminars. I heard a seminar by Gerhard Mack, who had written a paper—it was Mack and Petkova—exploring confinement in the context of an SU(2) gauge theory. And I thought it was pretty interesting and I just sat down and said, how might this generalize if you weren't doing SU(2) but you're doing SU(3) or SU(N); can I understand how it generalized nicely? And that turned into a nice paper. Later on, actually very late in my time as a graduate student, Malcolm Perry came by to give a seminar. This is when Malcolm was working with Stephen Hawking. He was talking about Euclidian black holes, gave a seminar about some of their recent unfinished work, and some of the things that were said just didn't seem right. This was related to the fact that Euclidian black holes are not actually minima of the action. They're not actually stable in that sense. They have certain directions of physical instability. And there was this heated discussion with Malcolm and David and me in David's office right afterward, basically arguing up at the blackboard. And this led to a later paper that the three of us cowrote. And those two things together were a big chunk of what I was thinking of at the time. There was also the beginning of my work on finite temperature. I think that was also just triggered by hearing a seminar and that's something that Rob Pisarski, who was another fellow student of mine working with David, he and I worked on together. And that was related to understanding how instantons behave when you turn up the temperature.

Zierler:

Who was on your committee?

Yaffe:

David, of course. I'm quite sure Roger Dashen was there. I remember that Barry Simon was the person from farther afield. There was an experimentalist that I think was Frank Calaprice. I'm probably missing somebody, but that's about it. I remember Barry Simon especially, because he asked me the last question at my final defense. I talked about a part of my own work, specifically this work that was connected to lattice gauge theories, that was an extension of what Mack and Petkova did. It's basically statistical mechanics. That's what lattice gauge theory is. So Barry, at the end, says, "Well, let's think about something simpler. There's this thing called the Ising Model. Do you know what the Ising Model is?" And I said, "Yes, I've heard of it." And it's the simplest model about how spins in some material can tend to line up. And, depending on the temperature, you have a real phase transition from a magnetized phase to a higher temperature unmagnetized phase. It's a classic model. But Barry wanted to pin me down and say, this is some effective description that says spins want to line up. Where does that come from? I mean, can you explain that? And I've been at the blackboard for over an hour and I'm feeling kind of tired. And I sort of stopped and was trying to say, well, where did that come from? What's the logic? It actually connects to stuff that I learned in my early chemistry days. And I said, "As I recall, it's nothing direct." And the whole committee just broke out laughing and said, "Good enough." Which is an in-joke because there are direct Coulomb interactions between electrons and there are so-called exchange interactions. And the fact that spins want to line up is all about the exchange interaction, it's not the direct Coulomb interaction. But, at that point, Barry was content to just say, "Good enough. We can quit."

Zierler:

(Laughter) Larry, to the extent that you were capable of grandiose thoughts at the time, what did you see as the contributions of this work to the broader field?

Yaffe:

Well, I think the only way I can answer that is similar to the way I'd answer regarding much of my later work. I see it as incremental progress learning what can you calculate. When you say you have a theory like quantum chromodynamics, you have a definition of a theory you can write down; that doesn't mean you have effective means of answering a vast number of questions that that theory ought to be able to answer but it is too hard to calculate. So, there is a boundary between things we know how to calculate and things we don't. And I tend to think of progress as just pushing back that boundary, looking for opportunities where the tools that you understand, if you put them together in a clever way, will allow you to understand and calculate some physical property that hasn't been done before.

Zierler:

Where, if at all, do you see any theoretical limitations to those boundaries?

Yaffe:

Oh, gosh. They are deep in everything we call theoretical physics. So if you jump to string theory, we do have compelling reason to believe that string theory is a consistent way of combining quantum mechanics and relativity and ultimately describes gravity or can describe gravity. But if you actually simply ask, do we have a really well-defined definition of string theory, the answer is no.

Zierler:

Mm-hmm.

Yaffe:

If you say, what can we understand about string theory after three-plus decades of work, it is tiny corners, tiny, tiny corners of the space of phenomena that string theory should be able to tell us about, but we don't know how to get the information out.

Zierler:

I've learned in talking to so many people that there's a binary and a range you can establish on string theory. On one extreme is people who have lost all patience altogether that string theory has anything relevant to say in the physical real world, and there are others who say, we need to be patient because fundamental and important work could well be on the horizon. Where do you see yourself on that range?

Yaffe:

Oh, definitely on the latter. So you undoubtedly talk to people who will tell you or talk about what have essentially been sociological wars in the theoretical physics community.

Zierler:

Absolutely. Sure.

Yaffe:

Do you believe in string theory or do you not? I think that's simply framing the question entirely wrong. It's not relevant, it's not a matter for belief. You struggle to understand what you can understand. There has been over-enthusiasm leading to overly optimistic projections of how things might develop, and extremely foolish talk like "theory of everything." It's just not helpful. I mean, it's undeniable that string theory has led to all kinds of deep connections with mathematics. It's undeniable that it has led to much deeper understanding of connections in quantum field theory. Dual descriptions of quantum field theory, what's referred to as holography, the fact that a theory that you write down that looks very similar to QCD and has nothing to do with gravity can have a description that involves gravity in higher dimensions. So there's no question that string theory has taught us all kinds of things. Saying does it describe the universe we live in, and can it predict numerical parameters of things we measure, well, not today. Who knows if that will ever be true, but that's not the same as saying string theory hasn't taught us a lot.

Zierler:

Is this a viewpoint that has been consistent since you've learned from David?

Yaffe:

It's evolved. And to my mind, the sociology of it has evolved even more than the actual physics, but key for the sociology in the nineties was the development of what's referred to as AdS/CFT duality or gauge/string duality or holography. This realization that the space of nongravitational quantum field theories is not separate, is not disjoint from the space of gravitational theories or from string theory. So it became, to my mind, a moot question saying, are you a string theorist or do you think string theory is garbage? As long as you're interested in quantum field theory, there are connections to string theory that are important.

Zierler:

Where are you on multiverses and people who say it can't be science if we're not operating in a system where things can't be proven or disproven?

Yaffe:

I like to harken all the way back to the early days of understanding how to deal with infinities in quantum electrodynamics and Feynman diagrams. And one of the earliest lessons in that generation's study was if, as a theorist, you're trying to calculate and answer a question which you could not even imagine measuring experimentally, then the question is foolish. And so that is pretty much my perspective on what I regard as theological arguments about multiverses. I mean, I'm willing to say we do not understand what really happens in the earliest moments of the Big Bang. We have this picture of inflation, but when you combine it with quantum mechanics and this notion that you can have fluctuations which develop differently, there are some deep questions there. I don't purport to understand them. And I don't think anybody is going to understand them in my lifetime.

Zierler:

Yeah. I was going to say, you're in good company, at least (laughter). Larry, to get back to the narrative, when you defended your dissertation, what kind of opportunities did you have? What were you considering at that point, and what kind of advice were you getting on where to go next?

Yaffe:

So, times were a little different. I'm sure I talked with David. I wanted to basically keep doing the sort of stuff I was doing. That meant I had to get a postdoc. I know I talked with David who basically said, well, if I wanted, I could go to the Institute for Advanced Study. But he suggested it would probably be interesting and helpful to change environments. So I don't remember exactly where I applied. Again, it was not like today where people apply to twenty, thirty, fifty different postdoctoral advertisements. I think I sent queries to three or four places. And I have to admit I was influenced by factors which didn't have a lot to do with science, but Caltech had these nice named, endowed postdoctoral fellowships which paid a little better than ordinary postdocs at the time. And I was also influenced by the fact that I grew up in California. I liked being able to visit mountains. I'd just spent four years in Princeton. Princeton's an interesting place. But every summer I viewed it as essentially uninhabitable, and my first priority was, where am I going to spend most of the summer? (laughter)

Zierler:

The humidity; you didn't bargain for the humidity?

Yaffe:

No, no. It's always been tough on me. So, I don't really remember what else I was considering at the time between the two that I mentioned, but it just seemed like, yeah, why not go back to Caltech where they offered me this nice named postdoctoral fellowship? I'd be sort of familiar with what I was getting into, but this would be stepping in, of course, Caltech physics. It'd be quite different. But I already knew Feynman was there. It was not the most active period for Feynman, but he was still a very, very interesting person to talk to. Also Murray Gell-Mann. It's not like I had any clear plan in front of me. It was just, okay, two years, let's see how it goes.

Zierler:

And did you go with the specific research agenda in terms of what you wanted to work on? Did you see unfinished business from graduate school to continue or was this an opportunity to take on new projects?

Yaffe:

I went there finishing up this paper with Malcolm Perry and David Gross I already mentioned. That didn't actually get finished until I was there. But there was- how shall I put this, there was interesting stuff in the air connected with what's called the large-N limit of QCD where you think about this not as a theory of what's referred to as three colors in an SU(3) gauge group, but you just let 3 be N and you imagine what happens when N becomes very large. And, in simpler theories, in much simpler theories, there was a whole list of examples where this could give you something that, again, you could calculate, you could solve it in the limit where this number of degrees of freedom goes to infinity. Slightly counter-intuitive. Normally, you think you add more degrees of freedom, you make a theory more and more complicated, right? In freshman physics we start talking about oscillators with one degree of freedom for a reason. But in this class of theories, it's a little counterintuitive. They get simpler as you actually increase the number of degrees of freedom. And there was clear work by Gerard 't Hooft showing at the level of doing Feynman diagrams in QCD that something simplifies as N becomes large, although you couldn't actually calculate things, but you could understand that there was a sense in which the theory became simpler. And somehow or other, due to my awareness of a bit of the work that Barry Simon had done in statistical mechanics, which is a very distinct community, he had written a paper that involved some of the same ideas in the context of spin models but introduced a certain mathematical framework. And so, quite early on in my postdoc years, I just got interested in saying, does that sort of general framework that worked in spin models, does it generalize to theories like QCD, to Yang-Mills theories? And that took a fair amount of work, but that became a big Reviews of Modern Physics paper that I wrote as a postdoc, and that influenced a lot of my work subsequently, providing a general way of understanding how this large-N limit is actually a type of classical limit. And if you look at it in the right sense- well, it is literally a classical limit where quantum mechanics and quantum fluctuations effectively disappear. And there was a new description which is classical dynamics.

Zierler:

What does the word "classical" mean in this context?

Yaffe:

In the same sense of Newtonian dynamics.

Zierler:

Mm-hmm.

Yaffe:

That you have a precise set of equations of motion and you just integrate them forward in time. And if you know the initial state of the system, that tells you the final state. More technically, there is a phase space and there is Hamiltonian dynamics on that phase space.

Zierler:

Larry, you're well positioned. I'm curious, both culturally and scientifically, how did you see Princeton physics versus Caltech physics? What were some of the big differences that may have jumped out at you or key similarities?

Yaffe:

That's an interesting question, which has tentacles that go off in many different directions.

Zierler:

But in many ways they both sort of set the tone for two very different ways of approaching physics.

Yaffe:

Yes.

Zierler:

You could even say to the extent that there's an East Coast physics and a West Coast physics, they're best represented by these two institutions.

Yaffe:

To some degree. So, as you surely know, Caltech, in a sense, had a heyday when both Gell-Mann and Feynman were in their prime, even though they're enormously different in what they did and how they did things. And they were such dominant personalities I think it was really tough on everyone else around them. I was there as a postdoc. They were in a rebuilding phase. They were not enormously strong. They still had these two hugely important, remarkable individuals, but there wasn't a sense that this was the most happening place in theory. So, from '76 to '80, when I was a grad student, in terms of QCD-related theory, it was Princeton, it was Harvard, it was East Coast stuff. So, at the time, Caltech was a place to sit and think hard.

Zierler:

Mm-hmm.

Yaffe:

I had lots of interesting discussions with Feynman. He would come in on the weekend to see who was working, and he'd walk into your office and say, "Tell me what you're doing." I had lots of fun discussions with David Politzer.

Zierler:

Uh-huh. That must've been fun to hear his perspective on asymptotic freedom being a Gross student.

Yaffe:

He is such a self-effacing individual. He didn't really spend time on that, but he was very interested in what I had learned at Princeton, learning about things that hadn't had a big presence in Caltech. There were other interesting people around. Steve Frautschi was still there. George Zweig was there except he then spent time at Los Alamos. This was after he got interested in hearing and biology, and I actually ended up renting his house for a while. And I should say that John Schwarz was there. He was there in a research position, but he was "Mr. String Theory."

Zierler:

Right. He is to this day.

Yaffe:

Absolutely. And he was bringing in postdocs in string theory who were doing tough stuff, well, supersymmetry especially. Jim Gates and Warren Siegel were there just down the hall doing these incredibly complex calculations on pads of paper that were 2 feet x 3 feet and they would fill every page with tiny little scribblings. So it was kind of a funny environment.

Zierler:

And where did you fit in with all of this? What did you add to the mix?

Yaffe:

I was sort of bringing in a bit of that East Coast quantum field theory perspective.

Zierler:

Mm-hmm.

Yaffe:

Talking with a number of people, David Politzer in particular. Stephen Wolfram was there. Stephen and I actually gave sort of a special topics class, which was kind of interesting, where I just ended up talking about critical phenomena, some of the stuff that I'd learned at Princeton from Zinn-Justin. But critical phenomena is all about conformal field theory, it's all about renormalization group flow. This has been sort of a big theme in theoretical physics for a long time.

Zierler:

When was it time to move on from Caltech? Did you have a set term or at some point you realized other opportunities awaited?

Yaffe:

I had a postdoc position for two years and I think I must've been more than a little naïve in this sense, but I didn't have any notion that one, like, should go out and query about opportunities or talk to people about that. I just had this notion, oh, you wait around and see what opportunities appear. So, in my second year, at some point, I don't honestly remember when, on the one hand I got a query whether I wanted to stay at Caltech, and I got a query whether I wanted to go back to Princeton, in both cases as assistant professors.

Zierler:

Wow. That's a real binary choice right there.

Yaffe:

It was a really-

Zierler:

And you know exactly- there's no question, you know exactly what the opportunities- what each are going to be like?

Yaffe:

I was familiar with both places. I was aware that assistant professors at Princeton or, for example, Harvard at the time were—

Zierler:

Tenure is not happening, let's just be blunt about it.

Yaffe:

Yeah. I mean, there's a possibility. There's a percentage. And, okay, it's tenure track in the sense that percentage is non-zero, but it's surely down in the single digits.

Zierler:

And how does that compare to Caltech?

Yaffe:

At the time, it was pretty obvious that they needed to rebuild. Nothing was said, but it was certainly conveyed that there was a good chance that I would be able to stay.

Zierler:

So, reading the tea leaves, you thought that your prospects for tenure were better at Caltech, and, yet you chose Princeton?

Yaffe:

Yes. For reasons that had very little to do with physics and a lot to do with Pasadena and smog and just feeling like you're immersed in this sea of humanity that stretches for more miles than you can count.

Zierler:

Right.

Yaffe:

And you either like it and you like that lifestyle or you don't.

Zierler:

Princeton's a much smaller place, too, if you're thinking about population centers.

Yaffe:

Right. But I honestly wasn't thinking about, am I going to stay in Princeton forever? The improbability of that was sort of a given. It was more like, can I imagine myself living in Pasadena forever? And Pasadena is a much nicer place now than it was back then.

Zierler:

Sure.

Yaffe:

I mean, it's had a lot of redevelopment and the air is better. But just to sort of cycle back, when I was in high school, many days after school I would jump on my bicycle and I'd just go bicycling up into the Santa Cruz mountains, ride up to Skyline boulevard on one road, ride along it, come down another. I had a bunch of friends. Steve Jobs was one of them. We'd do a lot of this bicycling. Then I go to college, my first year, I take my bicycle with me because, you know, this is what I do for fun. And practically the first afternoon after I've moved into my little dorm room at Caltech, I jump on my bicycle and I do what I always do, I look for a road that goes uphill and I bicycle up to Altadena. And I get to the end of the road and I'm coughing and I'm feeling terrible! And that's nearly the last I bicycled in the LA basin.

Zierler:

(Laughter) Suddenly, humidity in New Jersey is not looking so bad.

Yaffe:

Well, the whole notion was you leave, right? You go to Aspen. Aspen is a nice place to spend the summer.

Zierler:

Right.

Yaffe:

So I didn't know what my future held. All I knew was that I was just having trouble imaging myself at the time living in what I saw of Pasadena around me.

Zierler:

Larry, did you see any challenges to coming back to Princeton and being accepted as a member of the faculty and not as still a kid graduate student to some degree?

Yaffe:

It's not something I spent any time worrying about.

Zierler:

Mm-hmm.

Yaffe:

I'm sure it was there, but it was just a continuation. When I was at college, I was the youngest kid. When I was in graduate school, I was the youngest kid. I was concerned that there were people around me that I could talk to that were interesting, and that was certainly true. And there were other young faculty at Princeton.

Zierler:

Now, did you find yourself gravitating back to David's office or did you self-consciously try to avoid doing that?

Yaffe:

Well, clearly a good question. This was kind of another pivotal moment. I didn't because this was the period when David got deeply into working on string theory. And there was a lot of interest in it. David was working on it. Ed Witten was getting interested in it. It's when what's known as the heterotic version of string theory was first developed. And it didn't terribly much appeal to me. Not that it wasn't interesting, but it was back to my notion of what physically interesting questions am I going to be able to calculate? And that was so far off, there was already much understanding that what we knew about string theory, which was the analogue of how to do Feynman diagrams, was never going to be sufficient to identify what's the correct vacuum state; how does it explain why we don't see supersymmetry in the real world? How could it ever lead to something that looks like the standard model? There was a whole list of questions that it was just clear you were never going be able to understand without really understanding string theory dynamics at a much, much deeper level, that was analogous to learning about the difference between what you could calculate in perturbation theory and what was the nonperturbative dynamics associated with chiral symmetry breaking and confinement in QCD. Vadim Kaplunovsky was one of the postdocs at Princeton at the time, and he was somebody that I remember pushing this message very strongly. And I found it convincing, and therefore I just didn't see that questions that I regarded as the really interesting ones were going to be achievable. And so, I deliberately chose not to spend my time joining the group of people that focused on string theory for the next decade.

Zierler:

And when you delineate the interesting questions, what are some of the deeper questions beyond those questions that help you decide this is what's interesting, this is what's not interesting?

Yaffe:

At some level, I think there's always just an element of your gut reaction, but anytime you think about physics beyond the standard model, you've got this whole host of questions. Why are the quark and lepton masses what they are? Why are the number of quarks and leptons what they are? Why are the gauge interactions what we see? Then there are things like we understood that QCD has this parameter theta that is a measure of CP violation in strong interactions. It's unmeasurably different from zero, but why? There's no good answer for it. And that's before you get to the big cosmological questions, right? Why is the apparent cosmological constant so small? As soon as you're talking about a theory that's supposed to explain gravity, you should be thinking about those cosmological questions because that's part of the gravitational dynamics of what we see around us. But if there's no good prospects of answering those questions, there's this feeling of frustration.

Zierler:

Did you take on graduate students during your Princeton years?

Yaffe:

I did.

Zierler:

Who were some of the more successful graduate students you've had from that time?

Yaffe:

Well, it's only a few. And they all sort of went their own way. My first student, Frank Brown, chose to go to Wall Street in the period where Wall Street was hiring a bunch of theoretical physicists. A later student, Ulf Lindqwister, ended up going to JPL before he headed off doing quite different things. One of my students, Tom Dickens, went to work for oil companies who were very interested in how waves propagate through disordered media, and that has a lot of connections to the functional integrals that are used in quantum field theory. And one of my students, Bill Somsky, ended up going into computer software and is actually here at the University of Washington doing IT support.

Zierler:

Hmm.

Yaffe:

All of this was influenced by the fact that, at the time, I was really focused on trying to explore a scheme for really doing calculations in what I referred to as the large-N limit of QCD or Yang-Mills theory, that really involved attempting to develop controlled numerical approximations to a very difficult infinite dimensional minimization problem. So it was a lot of very unusual scientific programming that was essentially a mix of numerics and symbolic computation that was a lot tougher than I initially anticipated.

Zierler:

In what way?

Yaffe:

I told you about how, on the large-N limit, it's a classical limit and things reduce to a phase space. It's an infinite dimensional phase space, and that, in and of itself, isn't necessarily a big deal. There's a classic mathematical methods problem where you say, for example, what's the shape of a suspended chain hung between two endpoints? So, it's some curve, and how do you calculate the shape of that curve? Well, you can do it by simply minimizing the energy, but it's infinitely many degrees of freedom because it's the position of every little point on that chain. But I mention this just to say it's really easy to say, how can I approximate it by a finite number of degrees of freedom where I can actually do a finite calculation that I can teach a computer? So you do a crude approximation and you calculate it and you do a better approximation that requires basically dealing with bigger matrices, and you do a sequence and it converges really quickly. So, that's an example of where you have a problem that's a variational problem with infinitely many degrees of freedom, but it's amenable to straightforward numeric calculation because you can have a sequence of truncations that work well. So what I had understood was how to reduce solving large-N Yang-Mills theory to a minimization problem in a classical phase space, but it was infinitely many degrees of freedom, it's a complicated geometry. So, you had to think about, how can you truncate it or approximate it with a finite number of degrees of freedom, and that turns out to be really hard, harder than we were really prepared to deal with at the time.

Zierler:

Now, at what point are you starting to read the writing on the wall in terms of your tenure prospects at Princeton?

Yaffe:

I'm realizing that my assistant professorship at Princeton is almost surely a typical one.

Zierler:

How? How do you do this? Are you divining the stars? Are you talking to people? How does this play out?

Yaffe:

You're pretty much divining the stars. But it became especially clear when I learned that they were offering promotion and tenure to Jeff Harvey, who was an assistant professor just as I was. And if they were going to keep him, the writing was not subtle.

Zierler:

Mm-hmm.

Yaffe:

And at some point, Sam Treiman calls me into his office and just says, yeah, this is the way it is.

Zierler:

Do you have a plan B at that point, or you go on the market after the fact?

Yaffe:

I was still essentially unaware of any notion of a real job market at the time. It was a rumor mill. It was a grapevine. And if a place was going to offer you a job, they'd come and tell you. So, my plan B, quite honestly- let's turn it around, my plan A was to wait around for some opportunity to live in a place that I would be happy with within academia. My plan B was to leave academia and go get some software job, which was clearly achievable.

Zierler:

And not a great intellectual leap for you personally? You could've jumped right into that?

Yaffe:

In terms of finding such a job, yeah. Whether one I'd be happy with, I'm not so sure about.

Zierler:

Yeah.

Yaffe:

But I was—

Zierler:

And to be clear, you loved theoretical physics, you wanted to continue in a career in theoretical physics?

Yaffe:

I did. But, at the same time, pushing the boundaries of learning about what you are able to calculate also involves really understanding what you can do just in purely computational terms. And so, yeah, I taught myself to program, but, more than most people, I really thought about it. I had Donald Knuth's, Art of Computer Programming, the full series, up on my shelf. I was learning about things which are not normally a focus of computing in physics that was more computer science, how do you do lookup tables and different types of hashing or trees, and what they're good for. Because that was part of the work I was doing in this large-N project, where basically you have to deal with lots of bookkeeping, representing and manipulating loops on a spacetime lattice. Literally all possible closed paths, and how do you describe them, and how do you teach a computer to compare them, and how do you tell when two of them are related by reflections and so on? So it was really a mixed bag of computer science and programming skills on top of the pure theoretical physics.

Zierler:

So how did UW come about?

Yaffe:

There had been a tradition of occasional summer schools in theoretical physics in Seattle, one of which I had attended as a graduate student, but, at some point, the faculty here, and that includes some of the people you've talked to recently, Lowell Brown, David Boulware, Marshall Baker, they had tried to hire a group of young string theorists the year before I was coming up, I think, and they got turned down. So they were clearly trying to rebuild the group. They'd made one effort. They were trying to do something different. And this was the era where I think a lot of hiring amounted to people calling up somebody at Princeton or somebody they knew at Harvard and basically asking, who should we look at? All I know is I got invited to come out and give a talk. I came out and gave a talk.

Zierler:

On what? What'd you talk on?

Yaffe:

I think this was high temperature QCD, which was also one of my longstanding interests.

Zierler:

Mm-hmm.

Yaffe:

So how does QCD behave at high temperatures? How do instantons work at high temperatures? And I spent time talking to everyone. As I recall, nothing immediate was said at the time. I went back to Princeton. I just continued waiting, and I was getting nervous. This was spring of the year when I was- well, no. I guess I was told I could've had one terminal year. It was that usual arrangement. But it was the spring of my sixth year at Princeton, and, yeah, at some point I get a call and heard "we'd like to make you an offer."

Zierler:

Larry, was your sense—were there sort of larger than life people, like, was there a Feynman of UW that sort of was bigger than the department as an individual, founding fathers, people like that, or was it more like you described, people that came from other places where it didn't work out for them but these were still top-notch people?

Yaffe:

Oh, pretty much the latter.

Zierler:

Mm-hmm.

Yaffe:

In particular, I think at least three of my colleagues were all ex-Harvard.

Zierler:

Mm-hmm. And, on the other side, if you want to square the circle between avoiding smog and humidity, you could do a lot worse than Seattle.

Yaffe:

Yeah. And this was a big part of my thinking at the time. I had pretty clearly said, if the price of staying in academia is living in a place that I'm not real eager to live in, I would prefer to be in a place that I like living in, even if it means doing something different.

Zierler:

But even in terms of doing something different, it's an excellent program, and that must've been apparent to you at the time, as well.

Yaffe:

Yes, very much so. And a good part of that, well, I was familiar with some of the work of most everyone here, but, in particular, Lowell had written some beautiful papers that I was very familiar with.

Zierler:

And it must've dawned on you right away, I mean, Lowell, he's just as smart as they come anywhere.

Yaffe:

Mm-hmm. Yeah. It was clear that he was the sort of person that I enjoy talking to, he just thinks really clearly, thinks deeply, thinks broadly. Yeah. I certainly enjoyed that.

Zierler:

Larry, I wonder in what ways it must've been liberating to be at a place that didn't have the intellectual baggage of a place like a Caltech or a Stanford or a Princeton that ate up and chewed out its assistant professors

Yaffe:

It was different, but there were still- you've talked to Lowell Brown. I haven't talked a lot about David Boulware, who was another very smart, very strong personality. There was a level of games they played with each other. There was Jim Bardeen, who has a more famous brother.

Zierler:

Right.

Yaffe:

Jim is very self-effacing, very quiet, not always easy to talk to, but is also just scary smart. So it was an interesting environment. It was not one that chewed up assistant professors. It was like the senior people were all doing their own thing. They hired me at the same time they hired my colleague, Steve Sharpe.

Zierler:

Mm-hmm.

Yaffe:

They were certainly interested in keeping young faculty, not booting people out. And I came here with tenure, so-

Zierler:

What projects did you take on when you got to Seattle?

Yaffe:

It was a bit of a funny period because I was looking around thinking what's next, and I did not have really obvious things to jump on.

Zierler:

What loops were there to close from your Princeton research years?

Yaffe:

I was certainly still interested in high-temperature behavior broadly. That's been true forever. But the things, the equilibrium properties of QCD that had been the focus of early work had been sort of played out for the time being. One of the first things I got involved in here was the beginning of thinking about what's referred to as real-time dynamics. Conceptually, imagine you had a box of really high-temperature quark-gluon plasma, which is hot QCD. If you just let it sit there, you're asking questions about its equilibrium thermodynamics. But then you imagine shaking it, disturbing it, and say, how does it respond? That's a whole different set of questions which require fundamentally different calculational tools. So very early on when I came here, I got a good student who was the survivor of my first effort teaching quantum field theory, trying to convey as much as I could that I'd learned in Princeton (laughter), cram as much as I could into a ten-week period that was a beginning of a long process of learning about what's actually achievable. So, for those initial students, yes, there was a mismatch between my expectations and what I could really expect of them. But there was a Korean student, Sangyong Jeon, who took the class and then started talking to me and ended up being my first PhD student here in Seattle. And he did a really, really—well, let me back up a second. Some faculty can talk to a student and they can give them a little warm-up project, go do this calculation, it should take you a few weeks. And hopefully they come back, and the faculty member says, here's a more interesting project, it might take you a month. I have never been good at this at all. I'm downright terrible, because I tend to think about either questions where it's already been done and that's not a good project for a student, or interesting questions where it hasn't been done. So, I tend to talk to students saying, I'm kind of interested in this. I don't really know how it works out. Why don't you go think about it and see if you can figure something out? And the reality of that is, in many cases, I've asked students to go spend a month on something which turns out to take a year and a half or worse. So, Sangyong started the same way. I suggested he start thinking about these real-time response questions in a simpler theory that's not realistic, just a theory of spinless particles bouncing around, or what we refer to as a scalar field theory. And he managed to basically take that project and run with it and turn it not only into a thesis but the beginning of really learning much more deeply how what's called kinetic theory connects with quantum field theory. And that was the start of a lot of later work that eventually led to my collaboration with Peter Arnold and Guy Moore learning how to do analogous but much, much more complicated analysis in theories like QCD, but was part of understanding for the first time, how do you calculate something like viscosities of quark-gluon plasma? That had never been done correctly. And so, the beginning of what was really a decade and a half effort began with my first student here in Seattle.

Zierler:

Seems like a pretty productive way to start in terms of learning what works best going forward.

Yaffe:

When you get the right student, yeah.

Zierler:

Who have been some of your other successful graduate students over the years at UW?

Yaffe:

Well, let's see. Pavel Kovtun is now in Victoria. He was a fun student. Mithat Ünsal is a very interesting theorist now at North Carolina. He was not technically my student while he was here. He was technically David Kaplan's student, but he and I spent a lot of time together while Mithat was here, and I continued to collaborate with him on a number of nice projects afterwards, so he's at least partly my student. Paul Chesler is a really unique individual who is currently at the Black Hole Initiative at Harvard.

Zierler:

Mm-hmm.

Yaffe:

Paul was a really remarkable, headstrong, at times difficult, very self-directed but extremely productive student. That was much later, but that's when I started getting into this work that involves so-called holography or gravitational descriptions of theories which are similar but not exactly QCD. Holography provided completely different calculational tools to address somewhat similar questions about real-time response about what happens when you take one of these theories that's mostly too difficult to calculate and ask, if you really had that stuff, how does it respond? And in that context, that turned into learning how to do numerical general relativity in five dimensions with boundary conditions that don't have anything to do with studying colliding real world black holes, which had been the focus of the normal numerical relativity community but doing entirely different type of relativity that uses this connection between five-dimensional gravity and quantum field theory.

Zierler:

Who have been some of your most significant collaborators over the years at UW or beyond, overlapping with your years there?

Yaffe:

Well, early on, definitely, Peter Arnold, Guy Moore, and that's been ongoing. Mithat Ünsal I already mentioned. Recently, I've had ongoing collaborations with several people who began here as postdocs but are now in faculty positions: Aleksey Cherman at Minnesota and Srimoyee Sen at Iowa State. I've greatly enjoyed collaborating with my colleague Andreas Karch who, unfortunately, is about to leave the UW for UT Austin. I spent a chunk of time in Germany, as I'm sure you saw in my CV, thanks to the Alexander von Humboldt Foundation

Zierler:

Mm-hmm.

Yaffe:

And that's led to some really valuable collaboration with Andreas Schäfer and some of the other people that passed through Regensburg, including the lattice gauge theorist-Gergely Endr?di. So kind of a mix.

Zierler:

Overall, let's sort of take the view of quantum field theory from 35,000 feet. How has the field evolved over the course of your time researching in this area?

Yaffe:

That's a good question, but it—

Zierler:

I mean, the topline question is: What fundamental advances have been made in quantum field theory over the past thirty, forty years?

Yaffe:

Well, looking down, we now have reasonably decent understanding of how confinement works in QCD. Much, much broader understanding about how related theories work, an enormous explosion in supersymmetric gauge theories. The beginning of huge understanding about how different theories are related, if you look at phenomena on different energy scales, how one theory that you say you're interested in may have long-distance dynamics that is described by a completely different theory. And that completely different initial theories might end up having a common long-distance description that is also referred to as a type of duality. Huge advances in understanding what are referred to as conformal field theories, which is really, in a sense, that's an insider way of framing the question. A more general way of framing it is, in a sense, much more basic. When quantum field theory began with quantum electrodynamics, it was understood certainly back in the early fifties in the work of people like Dyson, even though we could do Feynman diagram calculations, there was compelling reason to believe that you could not actually send what's referred to as the ultraviolet cut-off to infinity, which amounts to saying you could not actually make sense of the arbitrarily short-distance behavior of this theory. This is directly connected to what you've—or will be talking about with Marshall Baker. So asking about what quantum field theories truly, genuinely, have sensible short-distance behavior is the same thing as asking about what quantum field theories truly exist as consistent combinations of relativity and quantum mechanics. And one of the things we've learned is that the answer to that question is interestingly sparse. And as you go up in dimension, it gets sparser and sparser. And so, this is directly connected with the explosion of work in what's known as phase transitions and critical phenomena, where huge progress has been made understanding the answer to this question in two dimensions. And by two dimensions, I mean one space and one time. Much more recently, there has been tremendous progress understanding the space of consistent quantum field theories, or, in other words, conformal field theories, in three dimensions. That's huge progress.

Zierler:

Mm-hmm.

Yaffe:

It's still not in four dimensions, which is the spacetime we appear to live in, but, again, that's progress. It's the sort of progress that's going to last. Foundational information about what is the space of consistent theories. And then, the whole connection with gravity, which ultimately is connections to string theory, which is the not settled fundamental understanding of what is the space of consistent string theory vacua, and how do you describe it? That is far from being settled, but to my mind that is still clearly fundamental progress that's going to have lasting value.

Zierler:

But not fundamental progress leading to what you previously called crazily the theory of everything?

Yaffe:

No, absolutely true.

Zierler:

So what's the roadblock there? Why does that not sort of lead—if we integrate gravity, why does that not lead to the so-called theory of everything?

Yaffe:

One can imagine universes described by physical laws, or more technically theories that we could write down, that are not the universe we live in, right? So, at a technical level, even though I'm trying to be sort of picturesque about it, the space of imaginable, consistent, potential theories that could have quantum mechanics and relativity is big. There had always been hopes that if we understood what happens when you put gravity together with quantum mechanics, it will seem like there's some unique answer which will suddenly be the right answer. And that led into some of this hype about theory of everything. I think it's fair to say that one of the things we've learned today is that was just hopelessly naïve.

Zierler:

Hopelessly naïve-

Yaffe:

These people are talking about-

Zierler:

I'm sorry. I just want to understand. Hopelessly naïve in the sense that this is beyond human capacity to put together, or hopelessly naïve that in the foreseeable future we just don't have the tools available to us to put it all together?

Yaffe:

Definitely the latter.

Zierler:

Mm-hmm.

Yaffe:

I make no prognostication about what might be understandable many generations hence 'cause I just don't think that's productive.

Zierler:

Yeah.

Yaffe:

If you run that game going backwards-

Zierler:

Right (laughter).

Yaffe:

-you'll see how ridiculous it is. So I think the only thing you can do is say, where are we today and what do we see on a legitimate horizon? And I think sometimes you just have to say, here's a list of interesting questions that would be fantastic, that are deeply puzzling, engaging, but we don't have adequate information or techniques. We don't have ideas that are going to let us solve them.

Zierler:

So, specifically with regard to tools, in what ways have the astounding advances in computational power been relevant for really fundamentally pushing the ball forward, and where are those limitations that you alluded to that, even with computers, they can only give us so much?

Yaffe:

Good question. The growth of computing power has certainly gone hand in hand with our ability to calculate and quantitatively understand some properties of strong interaction physics. This is the history of lattice gauge theory, which began with sort of qualitative explorations on extremely small lattices that were very crude representations of anything that resembles the real world but began to let us qualitatively explore, what does it mean to have a theory that confines, where quarks bind together into mesons or baryons? It's been three decades, but today there is a list of questions that's not infinitely long but at least it's interestingly big, where we have well-controlled answers, questions like what is the mass of these mesons or these baryons or- I'm getting some funny networking issues on one end or the other here.

Zierler:

You're clear.

Yaffe:

There's a list of observables, you can calculate certain matrix elements or, in other words, measure certain properties of mesons or baryons, now with quantitative control where you can honestly say that this is the right answer to within a percent. That's a huge development. But, at the same time, there's a huge list of questions for which those computational methods simply do not apply. And that's leading to a new wave of, in my view, wildly over-optimistic predictions of what quantum computers someday might enable us to do that we cannot calculate today on classical computers. But we're really not there yet. It's very easy to write down a quantum mechanical theory and ask questions about its real-time dynamics: if you start with this initial state, what state does it develop into? Where, if you talk to a mathematician, the question is perfectly well defined and yet you say, good, role up your sleeves and calculate the answer. And, once you have an interestingly large number of degrees of freedom, we simply lack any good calculational technique.

Zierler:

And when we start to talk about quantum computing, where does deep learning and artificial intelligence come in where the computers can start asking the questions themselves?

Yaffe:

In the sort of physics I do, not at all. It's very different in some other areas.

Zierler:

That's interesting. What's different here?

Yaffe:

Well, think about, as an example, collider physics.

Zierler:

Mm-hmm.

Yaffe:

Experimentally, you're throwing a couple of protons against each other so hard that thousands, tens of thousands of short-lived particles come out. And most of what comes out is boring physics, is physics that's maybe hard to calculate but it's correctly described by quantum chromodynamics. The reason they're running these big experiments at CERN is to say, well, occasionally maybe something comes out which is unexpected. So that's a pattern recognition problem, and there's many, many ways of trying to approach that, saying how do you separate unusual events from trillion times more likely, less interesting events? And that's something where there is potential for machine learning. People are working on it.

Zierler:

Another sort of very broad question about the impacts on theoretical work, where has the world of experimentation been specifically useful to the big questions you've been involved with over the years?

Yaffe:

Well, let me give you a slightly circuitous answer. For the field as a whole, that would be things like the observational discovery of what's called dark energy or the accelerated expansion of the universe. It would be the discovery of neutrino mixing and the measurement of mixing angles. It would be the discovery of dark matter and evolution of galaxies and clusters of galaxies, all of which is continuing to really push the search for new physics beyond the standard model. But much of what I myself have worked on, I've chosen not to spend a lot of time on this speculative, or what's referred to as the phenomenological side of particle theory, saying, you know, we might have new physics described by these new interactions, new particles, new fields. And, if we did, they might produce these signatures that might be observable in some class of experiments. That's a big part of elementary particle theory, but it's not the stuff that I talked about which is pushing the limits of what we can calculate.

Zierler:

Meaning that you haven't spent a lot of time on what could be known had the SSC been built, for example?

Yaffe:

Yeah. In other words, if it was built and it made great discoveries, or if there are great discoveries at LHC in the next handful of years, which really give us much clearer ideas about what's there beyond the standard model, that will start a whole new cycle of looking at possible theoretical descriptions and being able to test them, and that could involve new interesting dynamics, but we're not there. So on that side my attitude is to wait. So, the direct answer to your question for me is the whole development of relativistic heavy ion collisions, which was something that was of interest to the nuclear theory community long ago that I was not an early advocate of or found especially promising. There was hope that if you collided relativistic nuclei at sufficiently high energies, you would basically liberate quarks. You would undo quark confinement. You would produce a quark-gluon plasma. And that level of understanding goes back to the very, very early days of QCD. But there was a period where politics led people to pursue a pretty naïve notion that, oh, those liberated quarks and gluons could be treated as weakly interacting, so weakly that we can just do simple perturbative calculations and that's what we should be looking for. And those of us who had really been studying how the strength of strong interactions changes with energy scale and thinking about confinement, at least this was the attitude that I learned from David Gross. That just seemed ridiculous.

Zierler:

Right.

Yaffe:

It was never going to be nearly weakly interacting quarks and gluons at anything like accessible energy. So how were you ever going to describe it? It's just going to be a mess. It's going to be hopeless. And the big surprise, and in a sense, what has made the experimental program on heavy ion physics as successful as it's been, is a discovery that the reality was not that initial hope for nearly weakly interacting plasma; no, it was the exact opposite. It was so strongly interacting that it could be modeled as a nearly ideal fluid. And that's a different simplifying description that turns out to be stunningly successful, and then presented a theoretical puzzle; how could that be? So saying it's nearly a perfect fluid is saying that its viscosity is very, very small. But if you did simple estimates based on what we knew about perturbative interactions, you found it very, very hard to understand how the viscosity could be that small, how the stuff produced in these collisions could be so well modeled by very simple ideal hydrodynamics, almost from the earliest moment of the collision. So that's been something which connects very directly with trying to understand real-time dynamics in QCD and related theories. And that turns out to connect very, very directly to this whole story of AdS/CFT duality and gravitational descriptions, because those only work in the limit in which the interactions of the underlying field theory are extremely strong, exactly the opposite limit from where you can do Feynman diagram calculations.

Zierler:

That's interesting.

Yaffe:

So that's what motivated a lot of work on my own part and many other people applying these techniques of gauge/gravity duality or AdS/CFT to questions that are motivated by heavy ion physics. What's the viscosity? Or, ultimately, just, can you model the collision where you bang two Lorentz contracted nuclei together and just run it using this gravitational description and see what comes out? And that's what my ex-student, Paul Chesler, and I eventually managed to do.

Zierler:

Now, you've played a strong role in the ongoing conferences on strong and electroweak matter. What's the state of play in that field, and how has it evolved over the past twenty years?

Yaffe:

That conference series has essentially turned into a gathering of people who are interested in interesting, nonperturbative dynamics and quantum field theory. So it's really kind of evolved. The original title, Strong and Electroweak Matter, applied when one of the big questions was, can we understand baryogenesis, or the asymmetry between the number of baryons, that's protons and neutrons? In the world that we live in, we see protons and neutrons. We don't see antiprotons and antineutrons. But the fundamental laws are almost symmetric between matter and antimatter. Why is it that the universe we live in is matter? We still don't have a good answer. But there's a thread that goes way back that said, could the dynamics of what is essentially in the standard model, that is electroweak interactions plus strong interactions, combine in a very subtle way to bias the production of matter over antimatter in the evolution of the early universe? So this conference series, there was a time where that was a key question. It also brings together people more broadly interested in cosmological questions that involve interesting aspects of field theory, structure formation, magnetic fields in the early universe, people interested in heavy ion physics. Right now, I think we're again in kind of an in-between period. Heavy ion physics continues but the low-hanging fruit is what gets plucked first. It's not really clear what's next.

Zierler:

So, what is that low-hanging fruit now?

Yaffe:

Frankly, I don't know. There will be further studies of heavy ion plasmas but connecting that to fundamental theory is a challenge, 'cause the plasma, it's an exploding fireball. You don't have perfect control over it. It's really tough to make detailed connections between what's measured and what we're able to calculate. On the cosmological side, there's going to be new generations of measurements which may tell us more about how dark matter might interact with parts of the standard model. There's lots of people looking at that. But there's no obvious answers now. There's going to be new data coming in that involves gravitational wave observatories and we have already observed some collisions of neutron stars with neutron stars. And it is hoped that further study of neutron star collisions will allow us to understand properties of the very dense matter at the center of neutron stars, which will then connect to fundamental questions about how well can we calculate what happens in dense QCD. Literally, you take nuclear matter and you squeeze it, and is there a transition to when it's no longer best thought of as dense nuclei, but instead just as some dense soup of quarks? And what's the nature of the transition between them? So those are all things that are potentially on the horizon, but where is there going to be some breakthrough? I've got no crystal ball.

Zierler:

I wonder if that speaks more broadly to some sort of bigger existential questions that are being raised in theoretical physics now about, what else is there to do? If there really isn’t that—as you said, there aren't the obvious low-hanging fruits that might've been sort of ripe for the picking 45 years ago, for example.

Yaffe:

I mean, this is a question that every single practicing theorist thinks about constantly, because it's how you decide, what are you going to spend your time and effort on?

Zierler:

And I'm asking to historicize the issue, is this self-reflection of asking these questions, is this more intense now than it was at the beginning of your career?

Yaffe:

No.

Zierler:

It's not?

Yaffe:

No, I don't think so. There are periods from what I talked about in the early days of focusing nonperturbative properties of QCD, there was the development of lattice gauge theory, there was studying supersymmetric theories, there was the whole development of the phenomenological side. There was grand unified theories. There was the first wave of string theory, there was the second wave, so to every one of these, you can think of as some recognition that it's now possible to do something that you couldn't do before. People jump in. They work, they work, they work. The number of papers goes up, and then things get harder. And the easy calculations have been done, or the easy consequences have been explored. It doesn't mean that everybody stops working on it, but people then start saying, is there maybe something else I should think about as my next big project? And some people work on a few-year timescales, some of us have more like decade-long timescales because you get into it and there are things to do. It seems like there is a contribution you can make. Right now, I think what I alluded to in the progress of understanding three-dimensional conformal field theories, connections between quantum entanglement and actual dynamics of quantum field theories, connections between quantum entanglement and gravity, these are areas where I think really interesting progress is being made. And I think, calling them ripe is too strong, but at least not hopeless. Really interesting questions where there has been some real progress, and hopefully that'll continue.

Zierler:

So, perhaps a really concrete way to answer that question is, in your capacity as department chair and a mentor to graduate students, for people who are at the beginning stage of their career, what do you think the most promising areas are to work on these days?

Yaffe:

I think that's almost a dangerous question in the sense that, when you're talking to a particular student, the answer to that question really depends on the student.

Zierler:

Right, right.

Yaffe:

So, for students interested in what is referred to as the more formal side of theory or quantum field theory, even though I rather hate that label, "formal." I really think it's inappropriate.

Zierler:

Mm-hmm.

Yaffe:

But it's still sort of quick and dirty, right? The side which focused on questions that are motivated by fundamental puzzles not directly tied to recent or current experiment. I would mention the same things I just said, dualities, connections between entanglement and gravity, deeper understandings of conformal field theories, deeper understanding of connections in the space of quantum field theories about when they can be describing the same phenomena. But if I was talking to a student who was more interested in developing calculational skills, numerical skills, I might point them in a different direction that either involves current studies of lattice gauge theory, that's just essentially a tool for calculating hydronic properties, or some of the stuff on using numerical relativity to continue to explore nonperturbative dynamics. But I'd warn them that that's an area where it's gotten hard. The easy, interesting things have been done, so now it's hard.

Zierler:

Just to bring it, Larry, back full circle, I was drawn to the idea that when you were a graduate student, you didn't need David to present you with a problem. And it seemed like-now, maybe this was a particular talent you had or you understood intuitively what the most exciting things were to work on, but the sense is that either the field had fundamental work on offer or you were just really well positioned to identify the work itself. So, do you think that students operating today who might have that same level of, I don't know, precociousness, does the world offer them the same opportunities that it offered you?

Yaffe:

There is still no shortage of interesting questions. In some sense, I think things have gotten harder, in part just because the global community of theoretical physicists is bigger than it once was. One thing that certainly hasn't changed is that the environment one is in can be very important in terms of broadening a student's understanding, learning where are the interesting questions on which actual progress is being made? The challenge is never to say, where are the interesting questions? There's the interesting questions and there's the answerable questions.

Zierler:

Right.

Yaffe:

And you're always looking for the sliver of intersection between those two sets.

Zierler:

(Laughter) And, on top of that, you want to ensure that that sliver hasn't already been done, as you mentioned.

Yaffe:

Right. So certainly, during my time at Princeton, I benefitted tremendously from it just being a very vibrant period with a lot of interesting people around. Now, in the portion of my career I'm in now, this is a focus of being a department chair, doing what I can to make this department an even more vibrant place for students with a healthy balance of research activities going on where there's synergy between what different people are working on.

Zierler:

So, in terms of assessing those different kinds of questions, what are the areas that have that Socratic effect to them? In other words, we now know all of these things, but that has simply opened up a whole new category of things that we didn't used to know that we didn't know, and now we can work on them. And in what areas has so much fundamental work been done that any new work really yields diminishing returns?

Yaffe:

I view it almost solely as the former.

Zierler:

Uh-huh.

Yaffe:

Now, to some degree, that's because your goals can evolve. So, for example, people doing lattice gauge theory calculations of hadronic properties today, one could say the methods have been developed, we know how to set it up, it's just a question of how big the computer is and how long a calculation will run. And so the specific question about what's the value of this matrix element of this particular current in a proton, let us say, why is that something worth investing a lot of time and effort; is it in what you just referred to as the second category? But what evolves is then you say, well, but certain matrix elements are needed in order to search for physics beyond the standard model. So, suddenly your motivation for doing a calculation, even though it purely involves quantum chromodynamics, is grounded in connections to the search for physics that's outside of quantum chromodynamics. And that's true of a lot of issues that involve searches for new physics. Jumping to a different example, hydrodynamics is no longer routinely taught as part of physics. It's engineering, fluid dynamics, but there's still fundamentally interesting questions in turbulence, and there is a community of people in physics working on that, and they just think it's because they don't understand turbulence deeply enough. So, in some sense, what I'm trying to say is I don't know of any area where you can say it's done because you've answered everything.

Zierler:

Right.

Yaffe:

But portions of the community may say it's no longer of such central interest and let's instead focus our attention elsewhere.

Zierler:

So, some institutional history, certainly now as your capacity as department chair, and looking back to where the department of physics at University of Washington was when you started, does it still retain that character of a gathering place of assistant professors from the Princetons and the Harvards and the Stanfords?

Yaffe:

To a somewhat smaller degree, but there's still a substantial dominance of people from a relatively small number of schools being those who end up staying in academia in the field. I think it varies, and it would be easy to overstate that. My department, when I came here, it was especially strong in theoretical physics.

Zierler:

Mm-hmm.

Yaffe:

And there was a period where our strength in experimental physics really declined 'cause generations retired, and we really needed to rebuild in experimental physics. And that's something that I was pretty seriously involved in on lots of search committees starting a dozen or more years ago. And I learned that, in condensed matter experimental physics, there's probably an even greater dominance that the cutting-edge things that are driving what's perceived as the most interesting work is coming out of a relatively small number of places. And breaking into that group is tough.

Zierler:

What's the draw when you're looking to recruit top talent to come to the University of Washington, people that might be looking at a Stanford or a Princeton even knowing that they might not make tenure there? What's the draw, what's the recruitment to say, give us a look?

Yaffe:

It's pretty easy. So one point is what you just said. Now, of course, the people we're trying to recruit tend to be people who have a good degree of confidence in their own ability and contributions, because, if they didn't, they might not have accomplished as much as they already have. So most of the people that I have been involved in recruiting are not that focused on their odds of getting tenure wherever they end up. They're focused on what the intellectual environment will be where they are. Who else is going to be near them doing similar work, whether they will have a hand in building a place that's stronger than it is than when they arrive, and, to some degree, similar issues of environment that I alluded to in my own path. Seattle's a nice place to live. We plug that as strong as we can. And then there's the reality that Princeton and Harvard and Stanford simply can't hire every really good person. Right? So there are going to be really good people who are not able to get a job at those institutions.

Zierler:

Right. And University of Washington is probably uniquely situated right atop that next tier and you can use that absolutely to your advantage.

Yaffe:

Exactly.

Zierler:

And also, who's to say that Harvard or Stanford or Princeton are the ultimate arbiters of who should stay and who should not go?

Yaffe:

That's exactly right.

Zierler:

Who might be- I would only ask you to name names in a very positive way, but who might exemplify that, as somebody that really exploded at the University of Washington? That what their contributions were absolutely could've been recognized with a chaired position at a Harvard or a Stanford or a Princeton?

Yaffe:

Oh—

Zierler:

And if the answer is just lots, that answers the question.

Yaffe:

Well, it's certainly a number. And it really just depends on how far back I go. Some easy answers are on the experimental side.

Zierler:

Mm-hmm.

Yaffe:

Hans Dehmelt got a Nobel Prize, but more recently we brought a group of people here who were working on neutrino physics. And Hamish Robertson recently retired, National Academy member and so on. He could've easily gone to other places. He liked it here. On the theory side, David Thouless chose to spend time here. He was offered positions elsewhere. The department managed to retain him. But if I could, I'd just jump around and emphasize it's not necessarily the people who stay put. Sometimes there's critical influence that I certainly think was very important for somebody's career who passed through this department, even though they went on elsewhere.

Zierler:

Like a Bob Cahn, for example.

Yaffe:

Bob Cahn from an earlier time. But a story I like to relate is Dam Son. Many people know Dam Son today. He's worked in a variety of areas. He's very interesting and a very profound thinker, but he was an unknown grad student in Moscow when I went to a conference organized by Valery Rubakov and I happened to hear Dam give a talk. It was a good talk by a student, a really nice student talk. A little technical calculation, but just very clearly presented. And the next year my group here was doing a search for a postdoc so we made him an offer. He came here, spent a few years as postdoc, then moved on I think to MIT and then to Columbia and then he came back here as a faculty member in our Institute for Nuclear Theory. And who knows how things would've played out if things had gone differently. He's a smart guy and I think he would've done well on many different paths, but his coming to Washington certainly played a key role in how things developed for him. I was sorry we lost him to Chicago.

Zierler:

Larry, on the policy side, I'm curious how you found yourself in this position to be a program manager in the High-Energy Physics division at Energy? You're not a high-energy physics guy, so how did this come about?

Yaffe:

Well, from my- Since I first went to Princeton as an assistant professor, I have had research funding from the High Energy Physics Division of the Office of Science at DOE. They fund a very wide range of theoretical work covering physics related to particles and fields, and the fundamental nature of space-time and the universe. So, I have a long history of writing my part of a grant proposal, sending it in, occasionally having people from DOE's High Energy Physics office come visit, both early on when I was at Princeton and then later when I joined the group here. We've always been supported by DOE-HEP. So, I got to know, to some degree, not extremely well. but I got to know P.K. Williams, who, for many, many years oversaw all university funding in high-energy physics from DOE. And, of course, over time you get sent grant proposals to review and so the program managers at DOE get to know who they can send proposals to and get a review back which they will find useful. So P.K. Williams ran the university program for ages, and then, at some point, there was a reorganization and he retired, and they began doing things differently; they put all support of theoretical physics into a separate budget, covering theory groups at both national labs and universities. And a fellow, CN Leung, was overseeing the theory budget for a while, but he was at DOE temporarily, basically on leave from his position at University of Maryland, and he needed to return to his university position. At some point, I got a query from CN, would I be interested in coming to DOE to help manage the theoretical high-energy physics program for an indefinite period, and I said no. Then the query morphed, might I be interested in coming for a few years? And I said, I think that's too long. Basically, we got down to might I be willing to go there for the two years, but it turns out they can only actually pay a dislocation allowance for one year. So eventually I agreed to go for one year, because it sounded interesting. At the time there was about fifty million dollars a year in the theory budget, within the High-Energy Physics division of DOE. There was an opportunity to have a hand in how it's allocated, to speak up for work which I thought deserved funding. And I don't know if you know much how this works. The way funding works in DOE is rather different than at NSF.

Zierler:

Mm-hmm.

Yaffe:

At DOE, grant proposals get sent out for review. They bring in physicists to form in-person review panels to compare different proposals, but the judgements of those reviews and those panels are merely advice to the program manager; they're not marching orders. So, the decisions ultimately about, do I fund this grant, and how much funding do they get? Do they get funding for a postdoc? Do they get funding for a student? It is all in the hands of the person managing the theory budget. This opportunity from DOE happened to come at a time when I wasn't in the midst of a long-term research project. I wasn't really sure what my next thing was going to be, and as I was fond of telling people, managing a program budget at DOE was a way of having more friends than you've ever had (laughter). Because I could visit any place that had a high-energy theory group, and everyone is happy to see me and they're happy to spend time telling me what they're working on. That was the interesting aspect of it, to be able to go on site visits and just talk to people, talk to the students, what are they doing? And to do things like assemble the panel that assesses early career awards, which is really fun. I mean, a lot of review panels you're making tough decisions about who's not going to get funded because there just isn't enough money for all worthwhile proposals. Early career panels, in a sense, are fun because you're saying everyone who applied is good, now who's the best that you can give this very special award to? Assembling the review panel for that brought together some of the best people I know. That was great fun. Washington, D.C. is also culturally interesting. I'm not sure I'd want to live there forever for similar reasons which have to do with summers and humidity.

Zierler:

Sure. Even worse than Princeton.

Yaffe:

It really is. But my wife and I, we were happy to go there for a limited period. We rented a little apartment in Georgetown. We were able to walk down to the Potomac Boat Club and go rowing on the Potomac River practically every morning, even if that meant I had to commute by metro and bus out to Germantown, Maryland, where this part of DOE is. It was fun to spend a year in D.C. going to museums, bicycling around, and so on.

Zierler:

Now, were you a DOE employee? How did that work? Did the DOE employ you or were you on leave from UW, what was the arrangement?

Yaffe:

The arrangement is a special program. The buzzword is being there as an IPA. I can't even tell you what it stands for anymore. But the way it works is you are not technically a DOE employee. They are paying your regular institution, so your funding is going through your regular institution and you're essentially on leave or a partial leave. Your institution is expected to pay a fraction of your salary but the bulk of it is covered by DOE. And it's a limited term thing. They can only do it for a few years and then they have, in the past, often had people who come in on these positions who decided to stay at DOE and become federal employees.

Zierler:

Was it a good experience? Did you like it?

Yaffe:

I just highlighted all of the good parts, and it was mostly good. And it was interesting. It was interesting to see how hard the other people in the office work just to keep the system going. And a lot of people don't realize that. The downside is the bureaucracy which is pretty brutal.

Zierler:

Were you operating as a physicist in any regard?

Yaffe:

Doing my own research?

Zierler:

No. I mean, when you were at the DOE doing what they wanted you to be doing for them.

Yaffe:

I would say yes. I mean, that's even what I was called there. But that was operating as a physicist with enough understanding of what is being written in every single grant proposal coming in in order to have a good basis for saying, who should I send this out to for review? Or sitting in on review panels and asking appropriate questions to make sure that any concerns are properly raised and addressed. And ultimately, making a plan for how funding is going to be allocated. Technically, I made recommendations to the next levels up, but to be quite honest, they delegate authority down, and most of my boss's time and effort and his boss's time and effort was all directed upward basically to relations with the OMB and relations with Congress and what's going to influence the total funding coming into DOE in the next year. And then they really let the levels below them focus on, well, what are we going to do with this money? What's the best way of allocating it? What was challenging is that I was comanaging the theory budget with somebody that they brought in who started at the same time I did and who was a permanent DOE employee but is not a theorist. And over time the two of us found it more challenging to work together.

Zierler:

What are some big takeaways you learned from that experience?

Yaffe:

I knew it in advance, but I really saw clearly how tough decisions have to get made and it's not easy to make those decisions.

Zierler:

Mm-hmm.

Yaffe:

In particular, if the field is going to adequately support young people starting out, it's got to make sure that there are enough resources earmarked for that, and that can mean reducing resources earmarked for some very well-established people in the later parts of their career. It doesn't mean they're not good physicists, it doesn't mean they're not doing good stuff, but at some point you can't support somebody earning a very high salary who's not as productive or at least doesn't have as much potential as some young person starting out. During the time I was there, the High-Energy Physics Office chose to put a cap on summer salaries similar to what NIH has done for many, many years.

Zierler:

Mm-hmm.

Yaffe:

But it was a new thing, and it was not well received in some quarters.

Zierler:

Sure. Larry, I wonder how this experience navigating the bureaucracy and the budget and the personnel was sort of useful training for becoming a department chair?

Yaffe:

I mean, certainly on the budget side and dealing with spreadsheets and even, dare I say, dealing with Microsoft software, that was pretty direct.

Zierler:

And just to bring the narrative up to the present, what have you been focused on in recent years on the research side?

Yaffe:

Most recently, it's been, well, two things, really. One is this work on understanding properties of dense QCD, not hot but dense, and connections between what you could call quark matter and what you could call nuclear matter. There's been a longstanding question about could there be just smooth, continuous evolution between those two regimes or does there actually have to be some genuine thermodynamic phase transition separating them? So my young collaborators and I have just put out a nice paper basically putting our stake in the ground saying, here are the reasons there has to be a phase transition. It's a fairly subtle argument. It's different than standard lore about how to identify phase transitions.

Zierler:

Which is what? What is that standard lore?

Yaffe:

It goes back to Landau and what's referred to as spontaneous symmetry breaking. Are there observables that you can argue are strictly zero in one regime, strictly zero for some reason that involves some symmetry, and then are non-zero in some other regime? And then, basic understanding of complex analysis, functions of a complex variable will tell you it is impossible for any function to change from being strictly zero in some open interval to being non-zero in any continuous smooth or technically analytic fashion. And that means there's a phase transition. But these cases I was referring to with dense QCD, there's no simple symmetry that would play that role or, as the buzzwords go, there's no local order parameter that will immediately distinguish these two different regimes. But my colleagues and I argue that instead there's what is an unusual nonlocal order parameter that is probing the physics of superfluid vortices. Both nuclear matter and quark matter are superfluids, but the way superfluid vortices behave in the two phases is different in a discrete identifiable fashion.

Zierler:

Mm-hmm.

Yaffe:

So, that's one thing that I've been doing, spending quite a bit of time on in the last few years. Combined with that is some continuing work on the gravitational duality and numerical relativity side of things. It's an outgrowth of some of my collaborations with German colleagues, one of whom is a student I worked with in Regensburg who will coming here as a postdoc in a year, and he's interested in that sort of stuff and I'll be working with him.

Zierler:

Larry, for the last part of our talk, I want to ask one sort of broadly retrospective question, and then that will flow into my last question, which will be forward oriented. And so I want to ask, for somebody in the world of experimentation, you're easily anchored either to a laboratory or an accelerator or a telescope, and that itself sort of provides the narrative through-line that connects so much of a given person's career. But in the world of theory, what do you see as the major narrative through-line or the major research questions that, in one way or another, have informed and connected all of the many areas of theoretical research that you've been involved in over the decades? What do you see are those connecting points or narrative through-lines?

Yaffe:

In a slightly technical sense, the umbrella is just dynamics of quantum field theory, interesting dynamics of quantum field theory. A big part of that is grounded in, how will we understand QCD and strong interactions? But for me, a big part of it isn't so tied up in whether some question is actually going to be measurable in some accelerator experiment or what have you, it's just literally, is this an interesting question? What happens when you take a box of this sort of material and you shake it? Are there different—are there phase transitions which separate different types of matter? Why are they different? What about their properties make them different? And that connects with being at this juncture of quantum field theory and statistical mechanics and condensed matter theory and kinetic theory and hydrodynamics. What I find interesting are questions which blend all those portions of theoretical physics. And, in a sense, that's been a theme throughout my career.

Zierler:

How do you know, and looking forward, the future is always one of limited resources, either relating to time, interest, energy, budgets, in the world of theory when you're asking the interesting questions, what's most interesting, how do you know what project to take on next? What are the sort of feedback mechanisms, knowing that you only have so much time in a day or a year, how do you know what are the most productive areas to work on both for yourself and the people who rely on you now that you are a senior scholar in the field?

Yaffe:

I wouldn't say that one ever knows the answer to—

Zierler:

But you have a nose for these things, right? A successful career in the world of theoretical physics means by definition you can't know but you have to have a good sense of what's going to be productive.

Yaffe:

It's true, and some people really have a much more narrowly focused answer for long periods of time.

Zierler:

Which is why I'm asking you that question, because you don't have- your interests have been so varied, the challenge is, how do you know what to do next? And I'll ask that question specifically, not respectively but in the next X-number of years; how will you make those determinations on what to work on next?

Yaffe:

Well, in that more focused sense, the honest answer is I signed up to be department chair for better or worse. And I told everybody who tried to congratulate me that condolences were in order. And there is an awful lot of truth to that. It is an entirely different set of challenges, but they're interesting in their own right, dealing with the large group of people which constitutes a physics department. But in terms of research, the reality is that I'm going to have a lot less time to focus on research. And therefore my answer to your question is going to be grounded in things that I just generally find are interesting to me where there may be a potential for progress, but it's going to be highly influenced by the individual students or postdocs who walk in my door and start talking and I'll try to see what most interests them that, again, might be in this intersection between the interesting and the computable.

Zierler:

So, specifically, if I were to corner you and you weren't department chair and you didn't have that to say, my plate's full right now, what's most personally interesting and exciting to you that, in a world of unlimited resources, you would throw your all into it?

Yaffe:

I think these deep connections between quantum entanglement and the emergence of spacetime and gravity are wonderful questions. I would love to be able to contribute more to progress in that area. I don't know if time is really right for that. I would be happy to spend a lot more time than I've had available to really study in detail some of the interesting work that has been done and then do what I always do, which is just think about, can we push something? Can we do something that builds on what has already been done that pushes the curtain back a little bit? That's the way I tend to approach things more than asking is there some giant question that I think I'm going to be able to answer?

Zierler:

Mm-hmm.

Yaffe:

Because it's just not worth making those sort of bets.

Zierler:

Well, Larry, instead of offering condolences, I'd like to say that I hope your tenure as chair is as productive and short and possible so that you do have maximal opportunity to get back to those questions.

Yaffe:

Thank you. We'll see how it goes.

Zierler:

(Laughter) Larry, it's been great talking with you. I'm so glad we connected on this level. For the record, I just want to state that we first connected what seems like years ago at the beginning of this pandemic where I thought I would just be doing these calls for a couple of months, and I'm so glad not only did you connect me to some really incredible physicists, but, as a matter of institutional history, these interviews collectively are going to demonstrate that University of Washington Physics is really a powerhouse, not only in the country but in the world. And the people there, it's really remarkable what's been done at a university that might not be synonymous with elite physics in the way that a Princeton or an MIT has been. And I think that's just a remarkable part of the historical record and I'm so glad we've been able to work together to do our part to get that story out there, so I really do appreciate that, as well.

Yaffe:

Well, those are very kind words. I thank you. I really appreciate the whole series of interviews that you've been doing to try and preserve some of the history of interesting people and interesting developments in the field.

Zierler:

And on that note, I'll be excited to report back when I speak to Marshall Baker later this month.

Yaffe:

Excellent. Well, if you ever need more recommendations, there are more interesting characters.

Zierler:

Absolutely. Okay.