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Interview of Joseph Polchinski by Dean Rickles on 2009 March 18, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/33729
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In this interview Joseph Polchinski discusses topics such as: family background and childhood; undergraduate studies at the California Institute of Technology (Caltech); Richard Feynman; Tom Tombrello; Gerard t'Hooft; Stanley Mandelstam; graduate work at the University of California, Berkeley; D-branes; Dan Friedan; Bob Kahn; string theory; quantum field theory; renormalization; quantum gravity; Stanford Linear Accelerator Center (SLAC); Lenny Susskind; supersymmetry; working at Harvard University; Sidney Coleman; Mark Wise; Bryce DeWitt; working at the University of Texas at Austin; Richard Matzner; Kavli Institute for Theoretical Physics; Michael B. Green; John H. Schwarz; Edward Witten; David Gross; Chris Hull; Paul Townsend; Juan Maldacena; Cumrun Vafa; National Academy of Sciences.
There is a fairly wide spectrum of topics to cover. We may go over into a second session; we will try to keep it to one session. Before we actually start, do you collect any of your old workbooks and notebooks?
Yes. I have all of my notebooks going back to graduate school.
That’s fantastic! You’ve got them all archived. That’s very nice to know. Do you know if there are archives set up at Kavli about the running of the place?
There are obviously the archives of the web talks. But if you mean a history of the institute, there’s never been a formal history. The people who founded the place, who first devised the place are still on the faculty at UCSB, although I guess they are all retired now. They should write down their stories. That’s a good idea, because it’s a fun story. But I don’t believe there’s anything written, no.
I may contact them. The usual way to start these oral histories is to start with some early biographical details about parents, sibling, early childhood, and these kinds of things.
I was born in New York in 1954. My father, also named Joe, at that time worked for Shenley. He worked in the business office of a large wine firm. And he actually moved around a bit. He was a troubleshooter, so we moved around the East Coast a bit. My mother, Joan, as was common then, was a housewife. She stopped working outside the home when they got married.
Was this in White Plains? I’ve got that you were born in White Plains.
Yes, I was born in White Plains. We lived in White Plains on and off until I was ten. My dad had stints in New Jersey and Baltimore as a troubleshooter. But we basically lived in and around White Plains for most of that time.
Did your parents have an academic background?
No, there’s not really any academic background in my family. My mom had a year or two of college. My dad, I think, had an undergraduate business degree. He was curious about science. In those days the route to a scientific career was not as well paved as it is today. He was curious. He told me that his lack of being able to speak German kept him from majoring in Chemistry. You know, that was one of those little things.
Were they born in America?
Yes, they were both born in America. My grandparents were born here. My great-grandparents were among the huge group of Irish and Polish and so on that immigrated to New York in the mid to late 1800s.
Were you the first in your family to go to a university and get a degree?
The first to get an academic degree. Again, my father had a business-type degree. I guess I would be the first to have a real academic focus.
It was a well-to-do-ish family?
Yes, definitely a well-to-do family.
I’m wondering where the physics came in. When did that start?
I had a natural curiosity for science when I was a kid. My biggest treat was these How and Why Wonder Books of Science. Those are what I most looked forward to as Christmas presents. There was one on atomic energy, one on dinosaurs, and one on rocks and minerals. I remember the one on atomic energy making a big impression on me. They showed a dime and said there is enough energy in this dime to power a city for a year. So it was a curiosity I had since I was very young.
What age are we talking about?
I think I was reading those books in second grade. I can remember because there was one Christmas in second grade when I misbehaved, and I didn’t get to read my How and Why books for a few weeks. [Chuckles] In school, even through high school, science is not usually that well taught. Even if it is well taught, you need a pretty exceptional teacher to reach someone who is going to become a professional scientist. Science in school never seemed as interesting. The school subject that stood out for me was math. Not surprising I’m a theorist, because I never had such an inclination to take radios apart. I just had a natural affinity for math all through school.
Not a tinkerer?
No. I wish I had been. I wish I had the ability to do things, but it just wasn’t my inclination.
Did you find the math teacher a bit more useful?
When I got to junior high school I had a math teacher who I think was fired because he didn’t fit the mold. He was great for me because he knew what I needed and just kept giving me more, so I went through four years in one. But he couldn’t keep order in his classroom, so he had trouble with the administration. That was a great time for me and I really took off. Isaac Asimov’s books were another great influence around that time. He would have these collections of essays that were his take on astronomy or evolution or a subject in math.
He was really good at popularizing it. It’s interesting, Hubble had a similar thing because of Jules Vern novels and he was obsessed — Jules Vern and exploration and these kinds of things.
You asked about siblings, and I should mention my sister three years younger than me who is a retired police officer and horse-breeder. She’s a very bright person, but not academic in her inclinations.
Just one sibling?
Just one sister.
Did she go to the university?
She went for one year and it wasn’t her interest. I should say, I was a tremendously shy kid. That was another thing. Probably one reason I developed in this direction is I was not oriented toward people that much, but really numbers.
Could this be from moving around? Obviously people who tend to move around a lot…
That’s a good question, because we actually moved to Arizona when I was ten. I hated moving because it was hard enough to make friends. Certainly moving exacerbated it. In my sister’s case, I think it was not a problem. She is a people person. She knows thousands of people and she just loves it. She’s just a different kind of person than me.
You mentioned Arizona. Tucson, Arizona. What was the name of your school before you moved to Arizona?
The main one was the Virginia Rhodes School. The one thing that sticks out about the school was that there was a new math that was supposed to be the response to Sputnik. I don’t know if it was really good for most people, but it was perfect for me. I loved it, but then we moved to Arizona, which did not have the new math.
You were noticed by the math teacher in the old school. Was this kind of information passed on to Canyon Del Oro?
Yes. The math teacher was actually at — Canyon Del Oro at that time went from seventh through high school, so that’s where the math teacher was. I took the school’s math for a couple of years. I took calculus. At that time, calculus was not taught commonly in high schools; at least it was not taught in that one, so I went to the local university, the University of Arizona, for calculus and took it as a night class. It was not so well taught. I actually spent a couple of years at high school having no math or science, just playing chess, which I regretted. Obviously it didn’t hurt much in the long run. The funny thing is I’m not that good at chess. Obviously I’m pretty good at it, but there were people who were just much better than me.
They must have started younger.
I don’t want to waste your time going off on too many tangents, so stop me if we are. I was one of the better high school students in the state, but there was a guy who was a level above me at my school, and he walked up to me in the lunchroom in 8th grade and challenged me to a game, and he beat me. Which, you know, I wasn’t used to being beaten, so it was very upsetting, so I said, “Okay, we are going to play again.” And he beat me again. Over the years I won a few games from him. But I ran into him a few years ago, and what amazed me is he still remembered the moves of the first game we played. Not every move, but he remembered the general strategic flow of the game.
You shouldn’t feel so bad about losing! [Laughter]
He just had a knack I don’t have.
How old were you at this stage?
This was 8th grade; I was about 13 or 14.
Did you see yourself in competition with him?
Oh yes, because I was one of the best players in the state, but he was the best.
It’s good to have competition.
It’s good to have competition, but it was frustrating because I worked hard at it and I couldn’t catch him.
Can you remember what you were reading at that stage?
I was reading a lot of science fiction. Not much else. I was pretty narrow in what I would find interesting. Maybe I read some history and biography, but pretty much science fiction.
Did that start tweaking your interest more toward physics away from math?
I remember I would ask questions about things like the speed of gravity. I was curious about these things, but I didn’t have a clear sense of what the different fields of science were.
You probably didn’t know they were physics because the physics is very different at school.
Yeah. I took physics. I remember being blown away by the idea of how understanding electromagnetism tells you what light is. I’m still blown away by that. There were things that impressed me, but getting to Caltech was just a complete…you know, you step on campus and you immediately find out what theoretical physics is, and it was obviously the thing that I was supposed to be doing. It was math and all the kind of questions I wanted to think about: gravity and energy.
Did you go to a college in between?
No, I went straight to Caltech from this high school. I first heard about Caltech from an article in Reader’s Digest about the pranks they play. The fact that they have a student to teacher ratio of around two to three, and everything I heard about them was great, so I applied there. I got in and I was so happy. It was a great direction for me. I immediately got there and found that theoretical physics was what I was meant to do.
So you enrolled into physics? [Yes] What math did you have under your belt by then?
The only formal mathematics I had before Caltech was calculus. At Caltech I actually took very little. I took Advanced Calculus and Complex Variables. I somehow got the impression that it wasn’t useful for physics. No one straightened me out. It’s such a beautiful subject. All the math I know now is the stuff you get from reading physics papers. You know, somebody explains group theory physicists, differential geometry physicists. But unfortunately, my formal mathematical background, I didn’t have the sense to develop when I was young and had the time.
So you didn’t do this at Caltech either? You didn’t do differential geometry or these things?
I took general relativity from Kip Thorn using his big black book, but it’s pretty much impossible to learn from that book. It’s all pictures — I couldn’t turn the boxes into equations.
A good person to be taking general relativity from.
I probably would have gotten more from it a few years later. Weinberg’s book was the right book for me. It’s lacking in geometric intuition, which so am I, but it has in some sense the physics. I guess I think more like Weinberg than Kip Thorne or…
Did you get a scholarship to Caltech?
I had some small scholarships from my father’s company. That was the main one. Tuition then was less than tuition is now, and my parents were well enough off to pay my tuition. I actually discovered years later that they had to borrow to pay it; they never told me that at the time.
At least you did them proud.
I felt really good while I was there. I really felt like I knew it was a lot of money, but I’m getting my money’s worth.
Was Feynman there at the time?
Feynman was there. I actually graded for him as a senior. He was good because he had this Physics X where he would take questions from freshman and sophomores once a week for an hour. I was too shy to really interact with him closely. I asked him once about renormalization, and he didn’t want to talk about it. I had to figure it out for myself, which was fine. I asked him if there was any physical meaning to renormalization and he said, “No.” That surprised me, because by that time Wilson had already given his perspective on it. But Feynman knew what renormalization was. He knew that it wasn’t just this magic subtraction of infinities, but that you were just parameterizing your ignorance. I don't know, maybe he was — he just said, “No.”
Did you do courses with Feynman?
Just the one I graded, which was a quantum mechanics course.
Was Gell-Mann there?
Gell-Mann was there. He had very little to do with undergraduates by design.
Who made the biggest impression teachers wise?
My fellow students made a big impression. I have to mention one. I think it was practically the day I walked on campus I met Bill Zajc, who went on to become the spokesman for the PHENIX (Pioneering High Energy Nuclear Interaction eXperiment) detector at RHIC (Relativistic Heavy Ion Collider) and who had read the Feynman “Lectures on Physics” in high school. He was the one, more than the professors, who helped mentor me as to what science was, what physics was.
Was he about the same age? Did you enter at the same time?
Same age. We got there at the same time.
Were you in Blacker House?
He and I were both at Blacker House. He married my sister, but that didn’t last very long. He actually, of all the individuals at Caltech, made a bigger impression than any of the faculty.
Why do you think that was?
I think it’s actually often true that your peers make a bit — The faculty are remote, whereas your peers are around you all the time. (I’m making a bigger impression more by reputation than by anything else.) I had some professors who were great in terms of human interaction. Tom Tombrello — I worked with him several summers and did research with him. The research was actually in nuclear physics.
RF- Losses in peak fields.
That one, right. It helped break me in, but it actually didn’t turn out to be too closely connected to what I did later. He was great for the human side of science. He was someone who cared about the students, and really supervised the undergraduates.
It’s very nice of him, and odd to write papers with undergraduate students, right?
And he did it year after year. He had a number of students when I was there, and over the years a number of students.
These are the students as well? I’ve got G. Fox and Rolfs?
No, Rolfs was a graduate student. Fox was actually a professor, Geoffrey Fox. Tom explained to me the problem and had me think about it a day. I had some thoughts. But Rolfs, the graduate student, did most of the work. He wrote the code, and I mainly read the code, understood it, and tweaked it a bit. This one was my first paper. Jancaitis was another undergraduate. Jancaitis and Tombrello had done — This is kind of a fun project where you try to understand that in a cavity with a helical coil the electric fields. There’s no analytic solution, so it was about finding the best approximation numerically when methods at the time were very crude.
Good number-crunching practice.
Yes. Tombrello and Jancaitis had done a simpler version, then they brought me on board, and I joined them to do a more complicated one where I was largely explaining their earlier methods.
This got quite heavily cited. Both of them are actually pretty heavily cited. That isn’t bad, is it?
I did not know that. That’s interesting.
Even recently it’s still getting cited.
Spires doesn’t catch those citations when they’re in other journals. Thanks. That’s fun! I’ve got to add that to my total, add it to my CV [laughs]. Those were fun experiences. Then there were a lot of problems that Tombrello had been working on that I failed to solve, but they were interesting too. Those are my only two undergraduate papers, I think. But again, the unsuccessful projects were as interesting as the successful ones.
Did you know at this stage that this kind of research wasn’t your cup of tea?
No I didn’t. It was interesting to me. We knew that Feynman was working on the deeper questions, “What is everything made of?” I didn’t think at the time that an undergraduate could — Undergraduates generally don’t get to work on that.
Unless you’re Schwinger.
Unless you’re Schwinger, right. Even Steven Wolfram, another undergraduate at Caltech. He was an undergraduate when I was a graduate student. That’s pretty exceptional. I didn’t think I’d be able to work on that as an undergraduate.
Did you know about string theory at this stage? Was Schwarz there?
Schwarz was there. Schwarz had this research position where he didn’t teach. I don’t think, as an undergraduate student, that I knew who he was. As a graduate student I went back to Caltech and met him and we talked a bit. He was always saying to people, “Look, we have this beautiful thing, and it needs to be thought about.” I looked at the papers, and the way it was presented then, the beauty wasn’t evident. It was pretty technical, so it was hard. I didn’t really take him up on it.
It’s interesting that you kept intersecting all this amazing string stuff that was going on. Then you went to Berkeley.
We had just two weeks ago a celebration here for Mandelstam’s 80th birthday, and it was interesting because I worked with him during the one brief period when he wasn’t working on strings. He had helped create them. And then when gauge theory came along, he really asked some very deep questions. In fact, the first paper he gave me to read is still the first paper you read if you are going to work on the Langland’s program because it explains this notion that there are dual groups. He had the idea that confinement was due to duality. So most of the problems he gave me to solve were too hard, and they are still the kinds of things that people here are —
Can you remember the paper?
It’s Goddard, Olive, Kuyts, in which they show that monopole charges are classified by a dual group. One of the simplest versions of duality is this bosonization, where the fact that you take a bosonic theory of strong coupling becomes a theromatic theory and vice versa in two-dimensions and Mandelstam had shown how in the bosonic theory you can construct the fermionic operators. This is a prototype for what we would still like to do in gauge theory. In gauge theory we believe that in electrical gauge theory, in terms of some set of electric variables can be re-written as a dual gauge theory of some set of magnetic variables. My whole thesis problem, which I didn’t make much progress on, was to try to construct the magnetic variables the same way that Mandelstam had constructed the dual-variables for theory. It’s still what people are trying to do with this meeting. You’re still trying to construct the T’Hooft operators. Kapustin made some progress on the T’Hooft operators just three years ago. About three years ago he finally solved part of my — I mean that’s how forward-looking Mandelstam’s work was. He was working on gauge theory, thinking very far ahead, and then he returned to string after I left.
Let’s go back to before you went to Berkeley. Did you choose to go to Berkeley or was an offer made?
I applied to the obvious places: Princeton, Harvard, Stanford, and Berkeley. I didn’t have the sense to realize that the real revolution that had happened was centered on the east coast at Harvard and Princeton. I didn’t have the sense that physics was so regional. The chair of Harvard was recruiting me, he tried to tell me, “No, no, there’s East Coast physics and there’s West Coast physics, and you want to be doing East Coast physics.” This turned me off, even though what he was saying turned out to be true.
Did West Coast physics mean Chew (Geoffrey)?
I think that’s what he meant by it. In my case I liked California. I had a Hertz Fellowship, which at the time was moderately more lucrative than other things, and at the time had a very restricted set of places you could use it. I probably would have gone to Berkeley anyway, but that made up my mind. I did find someone who was working on gauge theory and working on very interesting problems.
Did you know about Mandelstam beforehand?
No I did not. I didn’t have a good sense of who was doing what. There was no web then, so it was much harder to learn.
All the dual-resonance model stuff, dispersion relations, and so. You didn’t know about?
I didn’t know if I wanted to do relativity at that point or particle theory. Maybe even plasma physics I was curious about. They did that at Berkeley.
You started this in ’75?
I went to Berkeley in ’75. Right. Tombrello, my undergraduate mentor said, “Look, here are these papers by Hawking,” who was visiting. Actually some of his fundamental black hole papers were actually Caltech preprints because he was visiting there. I remember Tombrello handing me these preprints, saying, “Joe, you’ve got to think about this.” I did read them and was fascinated, but I didn’t really come back to them for another ten years.
Did Mandelstam choose your topic or did you?
He did. He had this clear direction, which was that he wanted to understand confinement by means of electromagnetic duality, which was pretty much right. And he wanted to make it precise so he gave his various students different pieces of this project. My piece was to try to understand the T’Hooft loops.
You had a scholarship for this?
I had the Hertz Fellowship. That paid for my entire time as a graduate student.
Did you not get an NSF scholarship as well for your PhD?
I did, but you couldn’t take both of those, so I had to decline the NSF. The Hertz Fellowship was more money.
The topic stayed the same throughout your whole PhD. It was chosen and…
Well, the topic was vague. There was not a sharp — Some advisors have a series of small problems and you do that and you meet with them. This, it was the same focus. There wasn’t a clear goal. I read T’Hooft’s papers. T’Hooft was working on a parallel track, and I learned a lot about solitons. Mandelstam is a fairly quiet fellow. He is very generous with his time, but he is not — He’s shy and he’s not going to push you. He provided gentle guidance rather than, “Do this. Do this. Do this.”
How do you think this relates to your later work? I tried to read your thesis recently, and it looks very similar to a lot of stuff that…
It’s more similar to what people doing in Langland’s and then my own later work. I went back to it. I’ve always felt that my dissertation didn’t really have any results in it in some sense, because the problems were rather big and ill formed. I went back and looked at it for this celebration we had, and I think it’s still true. Kapustin solved part of my thesis problem 20 years later. That’s how hard the problems were. I think Mandelstam is someone who very much followed his own direction. He avoided the mainstream. He wasn’t working on calculations, as many people were doing then. He had a sense that there were these important ideas. I think I picked up that style from him. I have tended to avoid fads. If too many people are working on something, I don’t work on it. That particular style worked out well for me, because the work with D-branes was very much of that type. I think I picked up the science itself from my advisor, which might have just been natural for me anyway. I think if I had been Howard Georgi’s student, I would be a very different physicist.
It’s interesting how many of Mandelstam’s concepts bear fruit, if you see what I mean — they suddenly spark a whole load of research. And you could say the same for D-branes and your renormalization too.
Renormalization was another outgrowth through graduate school, not Mandelstam. For some reason I didn’t ask him so much my questions about renormalization. When you read about renormalization in Bjorken and Drell, it’s this very complicated graphical argument. Wilson by that time had done his work and basically turned it into dimensional analysis. I actually learned this not from Mandelstam or from any faculty, but from Dan Friedan, who was a graduate student with me. We had a reading group, and Friedan was a year or two older than me, but also somehow had a better grasp of what was important. He passed on to me the Wilsonian lore. I’m not sure any of the faculty at Berkeley then were aware of how Wilson thought about renormalization, but Friedan had somehow learned it, and he taught it to me. We had a reading group. Just the idea. I mean these days you talked to physicists that something is dimension 4 operator, dimension 6 operator. That language was language that I learned not from any of the professors, but from Dan Friedan. It was a few years later when I was in Harvard as a post-doc auditing a quantum field theory lecture by John Preskill on renormalization, and he said this is the theory, but there is no good proof of it. When he said those words I said, “Oh, I know how to prove it because Wilson’s point-of-view is the basis for proof.” As soon as he said those words I knew that I could provide a new proof.
Let’s go back to Berkeley. There was a lot of string theory going on at that time. You mention in your thesis Bardacki.
Bardacki was another ex-string theorist.
So he left it behind?
He had. In fact, he and also Mahiko Suzuki. Mahiko taught a course on dual-resonance, which I took, but he began the course by saying, “This is the last time this will ever be taught at Berkeley.” People who had worked on string theory were naturally inclined to think about Wilson loops. Bardacki, like Mandelstam, was trying to find the dual transformation. It seemed like the right thing to be doing, even though we still don’t know how to do it.
Also you mentioned that Bob Kahn took over when Stanley Mandelstam left in your final year.
Mandelstam was on sabbatical and Kahn was there. Kahn scientifically had very different interests, although he was interested in quantum field theory. We talked about quantum field theory. The main thing I remember about him was he was very good with advice about post-docs. He was a more hands-on person than Stanley.
Just at the right time. Who else was at Berkeley at that time?
David Jackson, the author of the textbook. I remember two conversations with him. One is when he had us all over for a party, and he showed us, “This is the house that the textbook bought.”
He did that well.
I was at a party at Ramond’s house a few years later and he showed me the telescope that his textbook bought. And my book bought me a car, so it’s between the telescope and the house. Jackson had a big impression on me because at one point I was floundering and wasn’t sure what I wanted to do. He sat me down and said, “Joe, it’s not enough to be smart. You also have to work hard. You’ve got to focus more.” I remember that’s when I realized I really had to be more systematic and start to be more focused. I think that’s when I started saving my calculations, and not try to go in five directions at once but to really work on depth.
You got your first real publication out of this, “The Green’s Functions”?
[The Green’s Functions of Vortex Operators] was a reduced version of my thesis, though it didn’t actually appear — I think it was actually submitted around the time I left Berkeley. When I was applying for jobs I had no papers.
When you were accepted at various places you still had no papers?
I had no papers, which was pretty amazing.
You must have had some good letters.
I had some decent letters. I obviously was not the first person to get post-doctoral offers. In fact, I was probably sixth or — You’re very aware because the first people get offers then you wait, then the second people. I think I was sixth or seventh on the list that year. There were some good people. Mark Wise and John Preskill got the junior fellowships that year. Obviously they had both done tremendously impactful work as students, so it’s no surprise.
How did you get to know about all the new developments at this stage? We still don’t have pre-print archives or…
In those days, people would mail pre-prints out. In the LBL Library they would get a stack of maybe 20 pre-prints. Every day I would go to the LBL Library and look for that day’s pre-prints. That was a big part of…
Was it experiments and theory or…?
It was. It was both.
Did you do a lot of traveling?
Not as a student. As a post-doctoral student, yes, but not as a student. I traveled very little. I went to one conference at Caltech. That was when I had that conversation with Schwarz that I mentioned where he said, “This is something you should be thinking about.”
And you thought “no” at that stage?
I did look at the papers, but I wasn’t thinking about gravity. The real killer application for string theory at that time was gravity, and I was not thinking about gravity. Basically if you ask what I did as a graduate student, I spent my graduate years trying to understand quantum field theory. I don’t know much formal math, but I know a lot…quantum field theory is the center in my understanding of science. So I took quantum field theory from several different people and learned about all the different ways to think about renormalization. So I wasn’t ready to work on quantum gravity or string theory.
When did you get married? Did you meet your wife around this time?
My wife was a graduate student at Berkeley. She had had a boyfriend who went to Caltech, an ex-boyfriend. Was he even a boyfriend? I’m sorry. It was her boyfriend’s best friend who was at Caltech. We were all at Berkeley. He introduced us. We met on the volleyball court. We played doubles. She was on the other team. We both like sports. Our kids like sports, so that was a natural fit and it’s worked out well.
It’s an interesting way of meeting.
It’s funny because the volleyball court was in the basement of a church, so sometimes I tell people we met in church. The truth is that was just an accident. We met playing volleyball.
You have two sons, Steven and Daniel. Are they into physics and math?
They both were strong at math as high school students. Neither of them had a passion for physics. Steven, the older boy, likes numbers, but he likes units of dollars rather than Newtons, so he’s an economist. I’m someone who focuses on one thing at a time. So does my older boy. My wife is aware of everything. If I want to know where something is, I ask her. If she’s not around then I ask the younger boy, Daniel, because he’s the same way, he’s aware of everything. He’s taken an interest in biology. I think it’s largely an outgrowth of being a wrestler in high school. He became very scientific about nutrition. Wrestlers have to cut weight, so they are always starving themselves eating energy bars. But in his senior year, every single thing that went into his mouth went in for a reason. He would eat protein and carbohydrate at specific times for specific reasons. He became kind of a family legend. You couldn’t eat a candy bar near him because he would glare at you for putting it in your body.
Count the calories going in.
That has grown into an interest in biology.
Are they both at the University?
They are both at Berkeley. They share an apartment.
Did it alter the way you do physics or your productivity when you had your children? Did you do more work?
It’s funny. You might ask first about teaching, because I started teaching a few years before I had kids. The fact is that somehow all of these things, teaching and kids, people take them in stride. I’ve always been a hands-on dad, but the truth is moms put more of their energy in general. It’s true in our case. I think it was definitely Dorothy puts more of her energy and thought into it, but I put a lot of time into it. Somehow you manage. There are pictures of me with my son on my sitting on my lap and I’m doing calculation. You find ways. In experimental science and more hands-on fields, I think it’s harder, but you know, I can do work in the shower.
You can’t take your kids to an atom smasher and do an experiment.
Even today when you are really into a problem, you go to bed with a problem in your mind and you wake up in the morning and you’ve made progress. As you get older it’s harder to find the time to get that level of concentration. Just the past few weeks I’ve been fortunate that I’ve been able to do it, and it’s great when you get that.
Post-doctoral days. You’ve had two post-doctoral positions: one at SLAC [Stanford Linear Accelerator Collides] from ’80 –’82, and then Harvard from ’82–’84. Starting with SLAC, this was 1980. Why did you choose SLAC?
At that point, my wife had two more years with her Berkeley Ph.D., so I applied everywhere. In fact, I had an early offer from Caltech, and they were very generous in extending my deadline. I explained that I really wanted to stay close, and they were very generous in extending my deadline, and the offer from SLAC came through at the last possible minute.
Did you have to have a project in mind for these post-docs?
No, in those days you just…I mean even now…In this field you are very much a free agent in terms of whom you work with and what you work on. In my letter I think I said I wanted to work on perturbative and non-perturbative aspects of gauge theory — very general.
Is that what you did?
Yes. I got there and I actually spent the first two-thirds of the year continuing to write a proof of confinement, which was a mistake—you are not supposed to keep working on your Ph.D. But I was really captivated by this. Fortunately, eventually I started lurking — Lenny Susskind was a tremendous influence. He was someone who works by talking, so he was always surrounded by three or four of his post-docs, and he didn’t mind if another post-doc would lurk in the background and listen. He was learning supersymmetry then, and that was certainly a natural thing to learn, so I learned it, and eventually lurked my way into a project where they wanted — My first paper with him — He knew this is an important thing. It had to do with naturalness. There’s this whole question of whether the large ratio of the quantum scale, the natural Weak scale, has to be natural, whether there is some magic reason why the large quantum corrections of things would actually cancel. There had been a paper by Witten, which made it seem as though there could be natural magic cancellations between quantum effects that we see at the Weak scale and quantum effects up at the great unified scale. What we showed was that there was no magic, that the cancellation was separate at each scale. I was able to quickly develop the technical tools we needed to prove this, so I got to be a co-author on this paper. It was great. Then Lenny and I wrote several more papers together.
You wrote a paper with Susskind when you were still at SLAC?
Yes. First I wrote a six-author paper on this one result. Then he became interested in — There was the idea that supersymmetry might have broken at much higher energies than the Weak scale, and he and I worked on this together. It was a lot of fun. We went to his house, we worked at his kitchen table, and we wrote the paper together. I cannot work in such a hands-on way with my post-doc students. I have to go think. With him you would talk it out in real-time. It was a hugely fun experience. I learned a lot of physics. There’s a lot in that paper that people have re-discovered 20 years later, and I’ll occasionally be annoying someone by saying, “We knew that 20 years ago. Look at our paper.”
Do you have the title of that paper?
“The Breaking of Supersymmetry at Intermediate Energies”. It’s a good one. It’s gotten a few citations over the years. Things were much easier then. When you write the first paper on supersymmetry and intermediate energies, there’s just so much low-hanging fruit. Lenny was interested in general principles but not specific models, so we wrote this paper then we moved on. There’s a lot in that paper that could have been brought further.
I didn’t know you were working on supersymmetry at SLAC. Was Susskind — he wasn’t a post-doctoral student, he was a…
He was a faculty member there. At the time they had a much smaller group. SLAC had a big group. In the Stanford department he was the only one in his field, and he would come to SLAC two days a week. That’s how I got to hang out with him.
Who else was in the theory group?
The faculty were — Peskin had not arrived yet. Sid Drell ran it. He was more administrative, and a great human being. He really cared about the post-docs. Helen Quinn, who was more phenomenological. Fred Gilman, more phenomenological. Marvin Weinstein. Several of these people were working on a big lattice gauge theory project, which I talked to them a lot about. I was interested in lattice gauge theory, but I wasn’t interested in doing research on it.
Did this have any influence later, because the phenomenological, paying a bit more attention to data and such things?
Not really. Susskind was the huge influence.
I also noticed that you were in a race, the Slackers.
The Slackers. SLAC is great. They have a race around — the accelerator is, what, 3 kilometers? So my wife and I and a bunch of the post-docs trained and ran around it. We then did the Beta Breakers, the seven-mile race from the east side of San Francisco to the west side. That was fun. We then thought we could do a marathon, but we quickly realized it wasn’t our thing.
Do you know what number you came in during the SLAC race?
I do. It was 42nd. There was somebody who was slower. Jeff Bodwin apparently won.
Jeff Bodwin was another post-doctoral student. He could run five-minute miles forever.
An amazing time.
Yes. He was amazing. He’s still around. I ran into him a few years ago.
Jerry Ehlers came in 45th. Is that Jurgen Ehlers, or are these two different people?
I don’t know. Another person I know well is Steve Park, who might be on the list close to me. He ran it. He and I had different scientific interests at the time, and I guess we never have overlapped here scientifically. There’s this whole subject these days of calculating gauge theory amplitudes, of summing up 10,000 diagrams in one expression. This is the program that Burr and Casour — The fact that quantum field theory, that Feynman’s way of thinking that you just add up the diagrams, although it’s right, the sum is simpler than the individual parts. It’s been a huge enterprise to develop this technology. Steve Park, not at the time but a few years later, he did it first. He was the first person to say, “Hey, we can take 1,000 Feynman diagrams and write down the answer on one line.” He was not so much of scientific interest; his interests were different. Being at SLAC was great because obviously being at a laboratory like that; you were made aware of experiments. There was a summer school, and I remember sitting in the summer school and they were talking about top and bottom quarks, and thinking they are much heavier then the QCD scales, so they are similar to each other. Can we take advantage of this in some way? I didn’t have the sense to talk to someone who would know more about that at the time. But later on we’ll come back to this.
Let’s talk about Harvard. Was that a choice again, or were you made an offer?
It was a choice. I had another year at SLAC, but my wife was done with her PhD so we were looking for jobs together. They go straight into faculty jobs out of graduate school. They tend to go into visiting faculty positions. She got a visiting faculty offer at MIT. I knew that I wanted to see the East Coast and so I took a position at Harvard.
Your wife does Germanic studies?
Germanic studies. Her interests are more the linguistics and teaching of languages in general than German per se. She’s been in German departments because that was the first language she learned, although she’s actually trying to transition finally into an education department because it’s a more natural fit for her interests. But at that time she was looking at German departments.
Who was at Harvard then? Was Coleman?
Coleman was. Coleman was the obvious draw. Georgi. Weinberg had left. He was at Texas by then. And Glashow was there. But the ones who were most involved with post-docs were Georgi with the phenomenologists and Coleman with the theorists. What was amazing about Harvard was the tremendous of number of really outstanding young people.
As undergraduates you mean?
No, as post-docs. I could bore you with the list. There were probably 20 people who to this day are leaders in phenomenology, in formal theory, in lattice gauge theory. Paul Ginsparg, creator of the archive, was there. Lawrence Krauss, who was more famous for his Star Trek books than his science. You look a little bit like him by the way. When I first saw you I said, “That’s Lawrence.”
He came over to Sydney recently actually.
Lisa Randall was a graduate student then. Ann Nelson, David Kaplan, Alisha Manohar, Greg Moore. Wise and Preskill I mentioned were junior fellows, they were still there.
Was Smelding there at that point?
No, he had already left or not been there yet. Jacques Distler was there.
He’s pretty interesting. Did you interact as well?
Yes. There was a tendency to segregate a bit into a more formal and phenomenological. The faculty would have their families, and there was a formal family and a phenomenological family. Still it was an environment where you talked about everything. I wrote a paper with Steve Sharp about — Oh and with Mark Wise. Mark was a key figure in my education. This was the first place where I was around people who were interested in data. While I was at Harvard supersymmetry was discovered. I don’t know if you knew that. It was undiscovered about six months later. It was great, because when there was a rumor — there were more rumors then — you immediately learned about it, figured out what it might be, and wrote a paper. It was a much different style from Mandelstam’s style of thinking about impossible questions. It was great to be exposed to the style of really reacting quickly to new developments.
Which version of supersymmetry is it?
These were the mono-jets. Even now, the main signature of new physics is that something escapes from your detector, and so you see an unbalanced event. So mono-jets were one jet going in one way and nothing else, and these turned out to be not a very good signature because there are backgrounds in the standard model, but at the time the backgrounds were not understood, the way that you could get false signals was not understood. UA1 Carlo Rubbia announced discovery of super-symmetry. Also, a Cornell group at the time announced the discovery of some new resonance at HE called the zeta. It didn’t fit into the standard model, so we thought might be a Higgs. So again I wrote a paper with Steve Sharp, a very phenomenological paper on the way different bottom balance states with EK. It was a great experience to be exposed to other kinds of physics.
I did notice your publication rate and your collaboration rate suddenly goes through the roof at Harvard.
Harvard was where I finally learned how to do physics in how you identify your project and write a paper. I still didn’t quite have that from Lenny. Lenny was great because he asked important but doable questions, but I still didn’t understand. I have a lack of common sense. I didn’t understand how you pick out an interesting problem to work on, and finally at Harvard I learned that from the milieu of young people and a bit of Howard’s influence. Howard had this dictum of “No more than one half of an idea per paper.” Which was a good contract to what I had come in with. Mark Wise was good because he actually mentored me a bit.
But he was a post-doc.
He was a post-doc. But somehow knew I needed mentoring and took it on himself. Just a fantastic time of the year.
You did a paper with Zumino as well at Harvard.
No, that was actually at Aspen. Maybe it was even while driving from SLAC to Harvard; I visited the Aspen summer school for the first time. It was a nice experience, sitting around relaxed, talking.
Do you know what year that was?
’82. I met a lot of people. I think Larry Hall was on that paper, Mary Guillard, Graham Ross and Dogwa [?] (I forget his first name). In Aspen you just sit around and talk. We were all interested in supersymmetry and understanding its implications. I don’t remember that paper very well. It was one of these papers where you assemble your conversations. I don’t think it had a real good central point the way most of my other papers do.
More than half of an idea. At the Aspen meeting, was there any string theory starting again there, or was it still zero?
No, it was still zero. While I was at Harvard, Witten especially was taking an interest in higher dimension anomalies, and he wrote a paper with Alvarez-Gaume, who was another one of the young people at Harvard, on the idea of gravitational anomalies. That was when string theory came to more general awareness because there was this magic cancellation for 2D supergravity, which was the descendant of 2D string theory.
You did a paper with Alvarez and Wise in ’83. Did he tell you about the anomaly three?
No. Well, I don’t remember the precise order of things. I think Wise also took Alvarez-Gaume under his — I think Wise took it on himself to take these formalists and have them work on real physics. Mark is a great guy. He’s very low-key, and does great work while not having a big ego about anything. He got Alvarez-Gaume to work on some early supersymmetry model building, and he pulled me into the project to extend it to supergravity. Wise was clearly the leader of the project. He was the one who assembled the rest of us. Alvarez-Gaume actually I think worked on the anomalies after that. They didn’t really overlap.
It must have been quite soon after that, because that was 1984.
It was sequential: one project ended and the other began probably.
Was there any kind of excitement when that finally happened? Do you remember what happened?
It was higher dimensional. At that point, gauge theory as a unifying idea had been explored. Kalvza-Klein theory was around, but it was more exotic. It was not something that most people were thinking of. In my case, at this point I realized that renormalizability of gravity was important. It seemed to get worse in Kalvza-Klein’s theory. So I’m not sure. It made a localized impression. For most people it wasn’t clear that it was important.
That’s interesting, because the usual story you get is that the floodgates suddenly opened when this paper came out. It looks like from the publication citation it suddenly goes from 50 to 1,200 in a period of a year.
I think this is the point where the bifurcation between Princeton and Harvard happened. Because Ed [Witten] had gone to Princeton. [David] Gross was at Princeton. I think that the impact at Princeton was more — Harvard there was still this phenomenological…even among the more formal people there was still a bit of a phenomenological bent.
Glashow was there?
Glashow was there. He wasn’t so much of a hands-on leader the way Georgi was. Georgi took an interest in people.
So you weren’t immediately swept up with the string theory?
No. I was slow. I moved to Texas in ’84, which is a little bit out of the mainstream. This was the place where Dorothy and I had the best joint offer. She had a better offer at Illinois, and I had a better offer at Princeton; but this was the best joint offer, so we went to Texas.
It looked like a planned decision because it turned out to be — Was Duff there at that time?
Duff. No, he was not there yet. He was in College Station, a two or three hours drive away. It was not an easy drive to make.
You weren’t completely coming for Texas?
No. It was totally a two-body thing. When strings took off, I was not involved. A lot of young people were. Strominger and Horowitz were on the Calabi-Yau paper. Shenker and Friedan were in my generation. They were on the covariant anomaly paper. I’m sorry, no. They were working on anomalies, but they got scooped by Green and Schwarz. And then you had Harvey and Martin, other young people on the heterotic paper. I was a couple of years behind in learning string theory. It was partly not being on the East Coast and partly my own tendency to avoid fads. When I learned string theory, I started working on open strings partly because nobody else was. Everybody was working on the heterotic string.
The ones with gravity.
They had gravity, but open strings have closed strings too. They still have gravity, but they have the group SO(32) and not E(8)x E(8). E8, of course it was amazing, because breaks right down to SU(5) and to exactly the groups that you seem to need in nature. This is still a great puzzle because now we have more and more string vacuua, and it’s still the simplest ones that work the best. I partly worked on open strings just because nobody else was. This contrariness took a while, but it paid off.
Your renormalization group paper comes at the end of Harvard.
That was at Harvard.
That’s quite odd because you’d been doing super-symmetry, and then it’s quite a shift, it seems. Why did you start to work on it? Ah, the Preskill incident.
The Preskill incident. I really was unhappy about renormalization. The one question I ever asked Feynman at Caltech was, “What’s the physical meaning of renormalization?” I spent my entire graduate career reading the different proofs and not being happy that — And it was when Preskill said there was no satisfactory proof that — by that time, I had been using Wilson’s point of view in the first paper with Susskind on intermediate SUSY breaking, and it’s all about how you organize things in a Wilsonian way, and working with “Wise” There are just tricks that — Just the effective field theory way of thinking, I didn’t fully pick it up until Harvard because people just weren’t using it quite so consciously at Berkeley and even Stanford. But at Harvard you couldn’t help but pick it up. When Preskill made that statement, it just gelled that all the pieces were there; you just had to write them down. I went and did that for two weeks and wrote this paper. It would have been better if I had for three weeks written a clearer paper, but it had its impact.
It did very well. We’ve just done it for a reading group, by the way. Can you explain a bit what you did in that paper?
When you read most proofs of renormalization, they’re tremendously graphical. It’s all about one part of the reducible graphs, and you need these combinatomic theorems about graphs and sub-graphs. Yet then you hear a Wilsonian point of view. There are dimension four operators that scale in an energy independent way. Higher-dimensional operators to scale down; lower dimensional operators scale up. Renormalizability is just dimensional analysis. What I realized was that — First of all, you want to think the way Wilson does, differentially. You want to think about what happens when you go from this scale to this scale, and then there are no infinities. Infinities are things that accumulate when you scale from high energy down to low. Renormalizability is a statement that the low energy theory forgets about all of the details of the original theory except for a small number of parameters. So it’s just a statement that this flow has a small number of interesting directions, and all the rest of the directions scale away. The statement of renormalization becomes just this differential statement: How many positive and how many negative eigenvalues does the flow operator have? Since the flow is differential, it only has a narrow range of energies. Everything is finite. There’s no UV divergences, there are no IR divergences. And so you can say the eigenvalues at small coupling have to be very close to the values at zero coupling because there’s no place for any big effects to come from. And that’s the whole content. The whole content of renormalizability is that higher dimension operators scale away, and the dimensions of operators behave smoothly as the coupling goes to zero. The whole story of Weinberg’s theorem and nested sub-graphs and everything else is a consequence of those statements. And that’s how I structured the paper. The paper is structured somewhat pedagogically in that I first studied a simple system in with just two operators, one is renormalizable and one non-renormalizable, just to explain what the principle is, and how you turn the principle into a proof. The second half of the paper is the proof. It could have been made even more transparent, but I’m…yeah.
That’s good. Well it’s now called the Wilson-Polchinski method.
It’s funny, because I thought I was using Wilson’s equations. It’s often called the Wilson-Polchinski equation, which embarrasses me a little bit because my version of the equation is conceptually the Wilson equation. It’s a mild rewriting, which I guess is useful for many purposes. The real thing that I took pride in that I thought was new was that I had provided a proof. I was able to elevate dimensional analysis to a proof without any combinatorics. It’s funny, because nowadays the Wilsonian way of thinking is ubiquitous, but it wasn’t quite so ubiquitous then. I found out when I gave a seminar about this from Edward Resan [?] that Wilson had told people that his work was a proof for renormalization, and he was right. I was just writing down the steps to what he knew, but that hadn’t percolated into the community as a whole, that what he’d done was actually a proof of renormalization.
This often happens in science, though, where somebody makes it clearer and gets a bit of credit for it, which is fair enough. That paper did enormously well, actually; it’s a very highly cited paper.
Yes, it’s had good staying power.
Now we’re at Texas. Was DeWitt at Texas?
DeWitt was there. He and I wrote two papers together, which was fun, the sigma models.
Did that get you back into string theory?
Well I got to Texas just when string theory was just taking off, so we were all trying to teach ourselves string theory in our own way. I focused on some questions, which were not initially so fruitful. Weinberg, Fischler, Candelas, and I independently learned string theory in our own way, in a different way. Candelas had been on the Calabi-Yau paper, and continued over the following years to develop those geometric methods. He did these incredible things when I was there. He discovered mirror symmetry, he discovered the conifold transition. I kick myself that I didn’t appreciate when I was at Texas how good the work he was doing was.
So he was at Texas?
Yes. He was doing this beautiful stuff, but unfortunately he was using methods that were more mathematical and I didn’t know them. I would sit at his seminars and say, “This is beautiful. I should think about this more.” Fischler was collaborating with Banks, and Susskind learned it collaboratively, which was his style. For Weinberg everything follows from a small number of general principles; you know unitarity. And he was going to derive string theory from general principles and wrote a very — ran his papers, in which he really sort of lays down, “These are the principles. What can we derive from them?” Just the way he writes his field theory books — the way he writes all of his books. And I sort of focused on a couple of little paradoxes, which is what I’m prone to do, and try to resolve them. The first paper I wrote that had any impact was this paper on the path integral for the string on a totur. What got me into that was partly I think because Weinberg was insisting that you couldn’t normalize path integrals. But here was a path integral where the normalization had a physical meaning. It’s a small technical point, but it was the kind of thing that would get me going. It was useful because a lot of the thermal physics of strings, although I didn’t appreciate it as T-dualities in that paper, it took someone else to point it out and say, “Look…”
Where does T-duality come from? What date is that paper?
It’s funny, because I think it’s one of those things that people knew implicitly before they enunciated it. It was enunciated by people around the same time, by Sakai and by Senda. It’s one of those things that although it’s such an important thing, it trickles into the community instead of hitting like a thunderclap, the way you think it might have. Then there was a paper by Wilczek and Strominger and others who helped to explain the breadth of the idea. That certainly got a lot of attention.
Was DeWitt there? How long was he there in Texas?
He had been there when I got there and recently passed away. He was there the whole time I was there.
Was he a big figure there?
He was a big figure there. He was someone who, like Weinberg, had strong principles. He was fun to talk to because you wouldn’t always agree with his principles, but he was a good foil for argument.
Did you get any knowledge of the other approaches to quantum gravity from DeWitt? Did you know about…?
DeWitt had famous papers on two of approaches, the canonical and the path integral. I tried to read them, but they were much too long. I absorbed a little bit of them. I wrote a paper at Texas on canonical gravity with Fischler and Morgan. You had those wormhole pictures, so there was this question of what are the rules for wormholes? We were trying to back up to first principles, which was canonical gravity. We did some tunneling calculations. In fact, the entire wormhole story, although it was motivated by string theory, the main intellectual origin was Hawking’s path integral approach to gravity, and so you can count that as a different approach to gravity. It was all said in that. We were trying to connect it to even another approach, which is the canonical approach, because the path integral approach, it wasn’t clear what the rules were.
Do you have any idea what DeWitt thought of string theory? Because it’s not really written anywhere?
DeWitt had a specific technical objection, which was not well founded. He thought that when a string split that there should be an infinite pulse of energy from the splitting point.
Because if you do quantum field theory on a fixed space that bifurcates you do get such a pulse. It’s a boundary effect. When you are doing quantum gravity you are summing over all times that the split will occur and that has the effect of restoring energy conservation. It’s a small technical point that he was fixated on. I can’t blame him because I’ve also fixated on technical points that were not relevant and it slowed me down. I remember this was something we had a fair number of discussions on. So my work with DeWitt came later. He became interested in a question, which is the same question actually that Weinberg was interested in, in just a different way, just what you might call asymptotic safety. The question is does quantum gravity — just quantum gravity, path integral over metrics — does that make sense as a quantum theory? In Wilsonian language the question is does it have a UV fixed point? DeWitt’s way of addressing it was to simulate it on the computer. To simulate gravity on a computer is hard; there are a lot of degrees of freedom and a lot of symmetry. So he was simulating a simpler system, which he thought had the key features.
In mini- spaces?
No. It’s a non-compact sigma. In canonical gravity, the metric is a matrix but has two pluses and a minus, and so because of the minus the space of all metrics is non-compact. Gravity has more than that. It has general coordinate invariance and so on. But he thought the non-compactness was the key feature. So what he wanted to do was to simulate a different theory, basically a theory of a non-compact integral over matrices. Basically just metric tensors, but without coordinate invariance. It was non-renormalizable. It had the two things he thought were important: non-renormalizability and non-compactness. I got interested in that because — Although I had written a few papers on lattice gauge theory over the years, it does go back to Mandelstam that I’ve always been interested in the lattice approximation as a way to think about quantum field theory, and about the lattice also as a way you could also make strong coupling expansions. I realized in what DeWitt was doing, you could do part of the calculation analytically. So basically the path integral reduced in the end to two variables: an angle and a radius. You could do the radius analytically and just be left with an angle. It was really fun, because he was doing these complicated simulations, and you could reproduce part of his data just from a calculation on a piece of paper. It was useful to him because he and his students could do half as much work. They could do the radial part analytically and just simulate the angle. It never led I think to a conclusive answer to his original question, but it was a fun thing that pulled together a lot of different pieces of physics.
I’ve not seen these published. Did he publish anything from these?
We have two papers together.
The sigma model?
That was it. I don’t know if he went on and published anymore from there.
Were you teaching at Texas in your first year? [Yes] Then you get an Alfred Sloan Fellowship. Does that release you from teaching?
No. The Sloan Fellowship is nice, and it’s one of the real honors you get as a new faculty member, but the actual dollar amount is pretty small.
Yes. It doesn’t even really pay for a student. I think I used mine for a computer, for some books, and for some travel. It goes pretty fast. It’s more the honor than the money.
You’ve also done some work on cosmic strings at this stage. In ’88 you started looking at cosmic strings. How does that come in? When did you start?
Richard Matzner, who was in Bryce DeWitt’s relativity group, had become interested in numerical simulations of classical field theories, and he gave a colloquium on what happens when two cosmic strings collide. This was in the early days of cosmic string phenomenology when they were still possible candidates for being the seeds of galaxies, so it was important to know what happened when cosmic strings collide. What he found is that when they collide they always reconnect. No matter how fast you send them through each other, they don’t pass through; rather they exchange ends and reconnect. This is critical for the evolution of the strings networks. He gave a colloquium, and I was thinking, “We have strings too, and this is a question we could ask.” When they pass through each other, what is the probability that they reconnect?” It’s clear the fundamentals are going to be different, because when you turn off the coupling they always pass through each other, so the answer is different. But you should be able to calculate what’s the probability that they do reconnect. At the time it was just a fun thing. It was something I wanted to know the answer to.
Did cosmic strings and string theory start in completely different areas? [Yes] So it wasn’t the idea that we have these fundamental strings and they get stretched? [No]
When you have quantum field theories with spontaneous symmetry breaking, you can get defects. The most obvious defects are monopoles and strings. Monopoles are annoying because if you get them they have too much mass and they over-close the universe. This was a very nice piece of work that John Preskill did as a student. Cosmic strings are potentially more interesting because if you get them they don’t over-close the universe. It turns out, and Tom Kibble showed this, that they hang around. They don’t go away completely, they don’t dominate; they hang around with just enough density to be interesting. In particular, they have just enough density that their gravitational fields could be the seeds of galaxies. This was around the same time people were developing inflation as a theory of the origin of galaxies. Cosmic strings came along as an alternative theory, which at the time was equally consistent with the data. So there was a significant early effort on it, which was eventually killed by the data when the power spectrum of the microwave background was measured more precisely, you know, the acoustic peaks, it fits beautifully with inflation — it doesn’t fit the string predictions at all. That was an enterprise that I wasn’t involved in, but it came and went. Now Witten, shortly after ’84 asked the question — So these were strings, which again are magnetic flux tubes in a field theory. Witten asked the question, “Could fundamental strings also play the role of cosmic strings?” He pointed out three reasons why they couldn’t: the tension was too high, they are not stable, and they would never form.
They just wouldn’t get long enough.
Yeah, and if they ever did, they would produce inhomogeneities much too large. For me that was just a question of technology: can I duplicate Matzner’s, can I repeat Matzner’s exercise, but for fundamental strings? At the time it had no obvious consequences; it was just an exercise.
It’s kind of made a comeback now.
After branes came along, and after Arkani-Thamed, Dvali, and Dimopoulos and Randall and Sundrum pointed out they could lower the string scale, then suddenly Witten’s objections went away. Henry Tye’s students are the ones who pointed this out. As a consequence of one of our programs dealing with string cosmology, he became highly interested and worked on various aspects of the problem.
So where did we get up to? We got up to the end of Texas, I think.
You’ve left off some big parts of Texas. The parts I’m referring to are the parts that lead up to D-branes. The first piece was this paper was with Johan Kai [???]. So there’s the Green-Schwarz result that the anomalies in string theory cancel only in SO(32), and we wanted to understand in detail how that happened because we had already, which turned out in the end to be fallacious that we should be able to cancel them for any group. So he was one of my first two grad students, and I have to say that especially at Texas when I was learning string theory, a lot of my important work really benefited from having students. Some of the most important papers came about because I needed to keep my students doing something, had to think of interesting projects for them. This one kind of began that way, and it ended up being a very difficult paper. It’s like 70 pages.
What’s the title?
The title is “Consistency of Open Super-String Theories”. We in the end understood that the anomaly arose because a certain closed string field, a Ramond-Ramond field, so a field in a certain sector of the string, had an equation of motion that couldn’t be satisfied. Now in the modern language, we had discovered that Dirichlet nine-branes carry Ramond-Ramond charge, so this is an important fact in the modern language, and it is fundamental to every talk you hear. But in those days, the idea of branes was not around; and secondly, the importance of Ramond-Ramond charge was not around, so we had resolved why would couldn’t catch the anomaly, but anyway, it just sat there. The second predecessor work was this work with Dai and Leigh. The title is “New Connections Between String Theories”, early 1989. I wanted to call it “Fun With Duality”, but Rob Leigh was a serious guy and wouldn’t let me do that. So again, the T-duality was around, and by that time people were talking about it quite openly as evidence that strings had a minimum length size. But no one ever asked what happens if you apply it to — And the whole focus on heterotic string, everybody in the world was working on heterotic string because that was the one that seemed to be connected most closely to nature, and nobody ever asked what happens if you apply it to any of the other string theories: Type I, IIA, IIB. And it turned out these had interesting answers, because if you apply it to open string theories, then there’s the story that the T-duality involves the winding modes of the closed string, but the open string doesn’t have them, and in the end the only way you get a consistent picture is that the T-dual of the open string theory is a theory with a D-brane in it, and so in particular D9-branes are dual to D8-branes, D7-branes, D6-branes, and so on through a series of T-dualities.
So you essentially had D-branes back then.
We gave them the name back then. In that paper we named D-branes and also orientifolds, which is another word you hear a lot these days. No one had ever asked what’s the T dual of an unoriented theory, and again it’s non-trivial — instead of being a smooth space now has an object in it, but it’s sort of one of these orientifold planes.
Were other people not concerned about the profusion of these string theories, about these other string theories?
The focus on heterotic string was so great that the other string theories were just not — I mean people would occasionally have a look at them. But I think probably more than one person had the thought that they were all dual. In fact in this paper, we showed actually that the other three theories, IIA, IIB, and Type I, were all dual to each other. Actually the IIA, IIB duality had been discovered a few months earlier, we later found, by Dine, Huay [?], and Seiberg. So we actually condensed the space of five string theories down to two, and actually by similar T- dualities, the other two had been condensed. So instead of five theories, you already had two.
Did you have that in mind when you were doing this, the uniqueness?
No, it was a discovery, not an input. Our question was a thought experiment. We know if we put a closed string in a small box we get T-duality in a big box. What if we just repeat this to the other string theories? And it partly began as a way to keep my students occupied, but it became a really interesting question when the answer wasn’t obvious. So we didn’t pre-suspect, but we discovered that these theories were all dual to each other.
I’ve head you mention this strings in a box thing before, this thought experiment. How does it work in a bit more detail?
If you put a particle in a box, and again, the box is periodic, then in quantum mechanics you can only have integer numbers of wavelengths in the box, so as you make the box smaller, these wavelengths get very short, and therefore the energy gets very large. So if you ask what states remain at low energy when you make the box small, most of these states go away. The only states that are left are the states in which the wave function is just independent of the dimension. So you basically lose the box. You shrink the box to a point and it’s gone. For a closed string: now there’s the center of mass motion of the string. There’s a wave function for the center of mass of this string, which does the same things we just said. If there is any center of mass momentum, the energy gets very large, so we only have zero center of mass momentum. But a closed string can do something a particle can’t; it can wind, and it can wind many times before connecting back to itself. And as you make the box smaller, these states don’t have much energy because the string is not very long. What you find is that if you calculate the energies of the states in a very, very small box, the energy of the winding states in a very small box are exactly equal to the energy of the momentum states in a very large box. So this was the point of Sakai and Senda, and then again Wilczek, Strominger, and the others explained it wasn’t just the spectrum but the interaction as well. You cannot shrink the box. It’s interesting, because it’s a thought experiment where you have the mathematics — you can do all the calculations, but then at the end of the day you have to look at it and say, “Hey, the physics is this.” And the physics is that you try to make the box smaller, but past a certain point what happens is a new space time emerges and the box gets bigger. That’s T duality. But if you do this with open strings, they can’t wind, and so what happens is when the box gets big, you have both open and closed. The box gets big, there’s a D-brane in there. And if you have unoriented strings, strings that can wind but they don’t have a direction — this actually is the part that puzzled us for the longest time — then you get a box with a wall an O plane. So in some sense, these pictures were completely implicit in the original discussion. They were implicit in the technology of string theory, but no one had ever asked what is the actual physical picture that goes with the mathematics, and I have to say, there was parallel work. Mike Green, Peter Horava…
Was Townsend doing this stuff by then?
No. I’ll come back to what Townsend was doing in a second. But Horava and Green made parallel discoveries. Actually even as early as 1974, Warren Siegel was thinking about these D-branes, but in a way that didn’t catch on, partly because string theory was not popular then, but partly because he didn’t have enough of the picture. He’s probably kicking himself that he didn’t return to it a few years later. So many, many people had pieces of this picture. And we actually didn’t pursue it much because we also believed that the heterotic string was the theory of the world. This was just an exercise that we were doing. But we found these D-branes and these O planes. After this paper, I wanted to write a paper with the title “There Is Only One String Theory” because five theories had condensed into two, and it really seemed as though…
Yes, but I had no idea how to do it, and I was totally taken by surprise by Hull, Townsend, and Witten, that work. The Hull, Townsend, and Witten duality work is in some sense — You mentioned Montonen-Olive duality and having watched my Ph.D. advisor try to prove Montonen-Olive duality and in effect it failed, I was convinced it wasn’t true. So I wasn’t ready for a weak/strong duality. It took me by surprise. So anyway, this was in 1989. And I told a few people and I gave a few talks about it, but what I really thought the next thing to do with it was to extend it to the heterotic string, and that was the wrong question, and it took five, six years later for the right question to emerge.
What was the right question?
The right question was what are the objects that carry a Ramond-Ramond charge that are required by duality? This is the question which grew out of the work by Hull, Townsend, and Witman. That’s the end of Texas. We’ve jumped forward to 1995.
Before we shift properly, the super membrane paper as well, did that…?
That was another thing where it had a funny origin. It kind of began because at that time, BPS states, the idea of states that break half the super symmetry, they play a very big role in that, especially. The difference of that was not as universally appreciated as it is now. It was known by some people, but it was not as universally appreciated. But with my students, we realized that the fact that strings are BPS states is interesting and kind of strongly constrains their dynamics. Having done that for strings, we realized it should also work for higher dimensional objects, and we were actually not so much interested in the brane as something embedded in space time. We actually were interested in the possible realizations of supersymmetry in our universe. So we were actually initially thinking of the brane not as a brane but as our whole universe, and we realized that the same construction that had led to the Green-Schwarz superstring could also lead to higher dimensional objects. For us it was primarily an exercise. We didn’t have a specific application, and I was surprised when people started to work on this subject as a possible fundamental theory in its own right. That’s not something I expected because the Worldsheet theory of the membrane is non-renormalizable, unlike the Worldsheet theory of the string, so it didn’t seem like it had a right to succeed. The membranes aren’t fundamental the same way strings are, but they are still important. They provide a key piece of the duality. So it really started as just an exercise in realizations of supersymmetry; it wasn’t directed at anything of what it became.
It became what, 11-dimensional supergravity?
We did it for like two branes in six dimensions. Duff and people had the idea we should classify all the examples, and they found this nice classification in which the most symmetric example is 11 dimensions, and they also knew that there was also the maximum dimension for supergravity. So they felt correctly that they had something good. Also in those days, 11 dimensions just seemed like it was uninteresting — people didn’t appreciate that 11 dimensions was actually going to be important. String theory could only live in 10, and the 11 that an 11th dimension might open up in some limit was something you would talk about in the coffee room, but…
So people were talking about the possibility of there being this extra dimension that pops up M theoretic kinds of ideas?
The supergravity people clearly had something like this in mind. In fact this important paper (and I’m not getting the authors right, but Duff is one of them) shows that you can descend from the 11-dimensional supergravity membrane theory to 10-dimensional string theory. But the string theorists didn’t believe that this is where their theory came from. Now we know we were wrong. There’s this funny bifurcation where the whole supergravity community was doing great stuff, and the string theorists, those of us working on perturbative string theory didn’t recognize its importance. And one of the things that happened in ’95 is all of these two huge bodies of knowledge came together, and it was remarkable.
We’ll come to that in a moment. You mentioned Willie Fischler already, who obviously went on to write the matrix theory paper. How come you didn’t get drawn into this?
The matrix theory paper was late; it was ’96.
’96, so you’d already gone to Kavli by then. Were they already thinking along — no, nothing.
No. It’s interesting, because the supergravity people, Nicoli and others, had written down matrix theory then — this is again, ten years before the actual theory — but without understanding the physical setting for it. I mean they had here this nice theory, but they didn’t understand the way that theory fit into the larger picture of supergravity. The ’96 matrix theory paper explained what that theory was good for.
On the matrix model, rather than the matrix theory paper, the E-print archive starts off in ’91 purely to focus on this matrix model. Were you one of the few people who were involved in this, who were on the list?
I was on the email mailing list, yes. At the time, matrix models were one of the first glimmers we had of string theory beyond perturbation theory. In retrospect they are a pretty limited glimmer. Even now, they’re kind of a very special situation which doesn’t seem to teach too many broader lessons, but at the time they were all we had, so it was definitely something we had to think about. So I was down in Texas originally joining in calls, and then receiving Paul Ginsparg’s mailing lists.
Did it have an impact on the collaboration that was being done? Was it used quite a lot straight away, or did it take time?
I’m trying to remember. It’s not like I would immediately print out every paper. Some of the most interesting papers were stuffed and compressed — things were not as easy to use these days. So it’s not like today where the day’s listings are a part of your routine. Plus it was a small enough field that the papers were more sporadic. I don’t remember the exact sequence, but I guess it grew into a true archive even before the web. With the daily email listings and you could by email request a copy of papers. It was pretty usable, even in pre-web form. Ginsparg got it up and running fast.
Interesting how string theory keeps getting involved in these technological breakthroughs. The superstring book was the first book written in LaTech, and then we’ve got to the first usage of an archive in string theory. There’s a really nice quote from Mermin, which is the archive is string theory’s greatest contribution to physics. [Laughter] Which sounds derogatory initially, but if you think about it, it’s quite a huge contribution. So any other bits from Texas that I should cover?
I’ll mention one thing, and it’s probably not so important, but I told you that I had while at SLAC had been thinking about heavy quarks. In ’89, right at the same time I did the D-brane work, I thought about this a bit more and talked a bit to Mark Wise about it, and I probably would have worked with him on it except that’s exactly when I started my book. So it turned out to be really interesting, and Mark actually, it became a big subject, heavy quark symmetry. We all have things that we kick ourselves that we were around at the beginning for, or could have been and missed. This is one of my big kick-myself things. There’s also my book. I don’t even want to talk about that. That was…You know, I used to read biographies of Michelangelo, and he got into this thing where he agreed to build this tomb for Pope Julius that was just way too big and there was no way he could possibly do it, and I always thought when I read those biographies what a fool — how could you do this? And then I did the same thing when I undertook to write a string theory book, because that was a blight on my life for the next ten years.
It took ten years! So when did you start writing that?
I taught the class the school year ’87-’88, and I wrote my notes neatly and thought I’ll sit down in one year and type them up in one year. And I worked three months, research called me away; worked three months, research called me away. Every year I’d work on it basically in the summer. But it kept getting bigger because I wanted to do more things. Also, I kept rewriting it because I wasn’t happy with it, and I’m still not happy with it.
It’s a fast-moving field, and maybe you think that you really ought to include…
Well, string theory was pretty slow moving between ’88 and ’95. There were things happening, but their significance wasn’t appreciated. Actually it was not a good period to be writing.
So you were writing just as the Green-Witten-Schwarz book came out.
It appeared. It was a great book, but it really appeared at the beginning of the first superstring revolution, and there are a lot of things it didn’t capture. In retrospect, it was a fine book and I shouldn’t have wasted my time trying to — It’s a big subject; there’s room for many books.
Was the two-volume thing to match the Green-Witten-Schwarz book?
No, the original contract said 500 pages, and at some point it was just too big. I cannot believe I did that.
Well, you got your car!
I got my car. I wish I’d worked on heavy quark symmetry instead. I’ll tell you why I did it. I liked writing papers. I got feedback from people that they enjoyed reading my papers, that they were physical, they were logically put together. And I thought okay, I can write a book. Scaling up from a paper to a book — it’s huge! It takes a different — It just doesn’t scale so well. So I don’t think my book is as readable as my papers.
It must have helped you learning string theory.
Somewhat. I learned corners I wouldn’t have learned otherwise. The funny thing is, although I did the first D-brane work in ’89, I didn’t even have that in the first draft of my book because I didn’t appreciate the significance. It was a nice thing in the end because it’s something my book has that the Green-Witten-Schwarz book doesn’t have. The big regret is that when I started it, my intention was that it would be easy to get into, and it’s absolutely not. I know it’s not. I rewrote it six times and it’s still not easy to get into. I rewrote and reordered the first six chapters repeatedly, and could not — Barton Zwiebach wrote the book that I thought I was going to write, and it’s a great book, but you can’t do everything in one book.
Why do you think there were all these textbooks appearing on string theory but not any other approaches?
For many years there were not textbooks in string theory. What is very striking is the huge number of quantum field theory textbooks. On our own faculty, Tony Zee and Mark Sr. each have very distinctive books; David Gross has a book in his drawer. You can go around the country and half the people in the field have quantum field theory textbooks because it’s a subject that is big, and different people think different things are the most interesting. Sting theory, for the longest time there was Green, Schwarz, and Witten, and then ten years later there was my book, and only in the last couple of years have more books appeared. There is still a tremendous need, not so much for books on string theory, but for books on duality, on geometry as applied say to string theory. There are huge, huge subjects where you could write a great book, but there are very few people who could do it really well, and they are too smart to write a book anyway. So I think what we need are a lot more books that cover, not so much try to do what my book did, which is to cover the subject from the beginning, but when cover specific big aspects to the subject which are hard to find anywhere else.
Michael Dine’s book on supersymmetry I think is really nice.
Mike’s book is unique because for one thing, he uses two different dimensions for the metric of the two halves. But he tried to cover standard model, beyond the standard model, supersymmetry — he tries to cover everything at a certain level, so it does something that probably no other book does. And again, he has a perspective that combines formulas in the chronology that very people did. Yes, it’s very distinctive.
So Kavli in ’92. Was it UCSB who hired you?
At the time Kavli had not yet become a donor, so it was the ITP. I was Frank Wilczek’s replacement. The ITP had at the time four faculty members that the university assigned that basically paid the bulk of their salaries, and they were to facilitate the activities.
Did it used to be in the normal physics building?
No, it used to be actually in a building across campus. It’s always been separate from physics. It needed a lot of space for all the visitors — it’s always been a visitor facility, and physics wouldn’t have had room for it.
Were they employed by UCSB, ITP employees?
Yes. The four permanent members are also physics department faculty: we were tenured, we take on students, and we teach a little bit.
So that was in ’92. Why did you leave Texas?
Partly family reasons. My family and my wife’s family by now had all moved to the West Coast, and we always thought of California as home for that reason. So that was always in our mind. And of course, this is a unique place because of the tremendous number — Everybody comes through here. Our programs have — You see what it’s like. This week it’s exceptional because there are three talks because of the conference.
Was it like this when you arrived?
It’s actually doubled. David Gross has effectively doubled the size. We moved to a new building, and then we enlarged the building from money from Kavli. But he also is consciously increased the number of different activities, and also increased somewhat the size of each activity. It’s great. The demand, the number of good ideas for programs remains huge. Every year we have a challenge of how to schedule all of the interesting activities. Being here, though, during this program I’m running, we have 100 people passing through, maybe 120 passing through in five months, and if you try to interact in any fashion with that, that’s just a lot of good people. Most of those people will give talks. So it’s a lively place to be. There’s not another place like this in terms of being exposed to a lot of good people. So it was an easy decision to make.
Were you made an offer?
Yes, in ’92 I was made an offer.
So the ITP knew you were about to move or looking?
No, they approached me. I was not looking, and it was partly — Well, I was not looking to move in any hurry. Texas was great; my wife had a good job there. But they found that the German Department had a need for someone like Dorothy, which made it a two-body hire. We’ve tried to do a lot of hiring, and it’s always a challenge to find something that makes two people happy.
So was there much interaction between Kavli and the Physics Department?
Yes. During programs it’s even wilder, but even when there’s not a program of direct interest we have two or three joint seminars a week, on top of faculty meetings and anything special. I’ve collaborated with most of the people in Physics, and my students with their students and so on. It’s very close, yes.
So you still have to teach.
I teach, but only one quarter per year, which is good because there is no way I could teach this quarter with 100 people passing through.
Is that the same for all the permanent members of the ITP? [Yes] So what were you working on when you first…?
Actually my first year here, I was working on condensed matter physics. I had always been puzzled by the theory of superconductivity, why it worked so well, and I think it was my last year at Texas I was teaching quantum mechanics and decided I would think about this, because it was part of the course, and I realized that the Fermi liquid theory on which it was based was just an example of Wilson’s renormalization group. The condensed matter people understood this, but they didn’t use the same language. They used the language of quasi particles, which particle theorists don’t use; we say effective field theory. So when I learned that quasi particles were the same thing as effective field theory, I was very happy because suddenly I could understand all of this condensed matter physics. So I spent a year and did a lot of reading of papers and textbooks. I had this idea that I could solve (high TC superconductivity), because it’s a non-Fermi liquid theory, and I thought okay, I can bring in a new perspective to this subject and figure out what is it that high TC superconductivity can do, and I became one of the hundreds of people over the years that believed I could solve it and didn’t.
Did you have a student working on this as well, or was it just…? I know the paper is single author.
When I moved here I had three Texas students still finishing, so they all had some form of…I think actually Eric Smith, one of my students, took an interest in the subject and did some work related to mine on there on his own. But since I had just moved, they were senior and they kind of were all going their own way at that point.
Then there’s a nice set of Les Houches fluctuating geometry and statistical mechanics. Did that come about because of the Fermi liquid thing, or was it completely another topic?
They just had the idea for that school that they would invite some string theorists and this and that other people who were working on strings. What was the chronology; those lectures were in what year? [‘94] So maybe they were influenced…I don't know how they chose their speakers, so they might have been influenced by that.
Might have thought you could do a string theory perspective suitable for…
Yes. Ended up those lectures were part of my book. They weren’t exactly pulled out of my book, but rather morphed it, and then it morphed again back into one of the many revisions of the book. I worked on condensed matter physics for probably a full year, but eventually I realized there was a lot to learn, and although I had a different language, I didn’t really have that much of a perspective that was unique.
In the introduction to those lectures, it looks like you were put on your path to the non-perturbative aspect of string theory because of these lectures. You mentioned you reread Ken Wilson’s account of his early work.
Those lectures were part — The problem was always there. In the 1990 matrix models, even then Wilson was the motivation. And still is. I mean I still use that same title, “What Is String Theory?” for lectures, because we still haven’t answered Wilson’s — We’ve learned a lot, but these lectures…I’m giving next week in Trieste my latest take — not the whole thing, but particular aspects of what is string theory.
Since you’ve been at Kavli for quite a while now, have you noticed any major changes in structure or function, or what gets taught, what programs get…?
It’s changed hugely, and the great majority of the changes are due to David Gross. For the first 15 to 20 years, there was a new director every five years. That was the policy. Now David has been here going on 15, and he came in very consciously saying this place is good, but can be made better. It now serves at least twice as many scientists per year as it used to.
Was it originally multi-disciplinary?
It was always multi-disciplinary. It was fairly rigid in its structure in that it had four programs a year using one condensed matter, one astrophysics, one particle physics, and then a wild card.
Biophysics or some such?
We tried — although for many years it was hard to find good bio programs. This is something that has developed in the last ten years. But for many, many years, biophysics sounds good, but what would people actually do? There wasn’t much of a subject there. And now happily that’s changed. Sometimes it would be plasma physics, sometimes we’d have two condensed matter programs because it’s such a big field. And now we have more like 11 or 12 activities a year because they’re of all different lengths. The schedule is more flexible, with the enlarged building we can have three activities at once instead of two, and each of the three is larger than the two used to be. So it’s just a much, much higher level of activity. We now do, like this week, much more math/physics. We usually have a math/physics every year or two. Now we do have one or two programs in biophysics a year, and we added a permanent member in biophysics. We even do things like geology. We do more atomic physics. So it’s a great range of science, more people. David had also introduced programs to bring though grad students, and also to bring through physicists who are doing research at undergraduate institutions.
Do they become members of Kavli?
They come for two weeks a year for three years, so it’s a way that they can stay in touch with research.
Do you have Ph.D. students who are kind of affiliated with Kavli?
I have Ph.D. students, but they sit in the physics building for space reasons, and partly just because that’s where the other students are.
So it was the ITP when you arrived, and then became KITP?
David’s plans needed cash. They needed a building that was 50% larger, and he started a fundraising effort, and eventually was in contact with Kavli. Kavli was interested in becoming a major benefactor to science, and this was his first major donation, and now he has 15 institutes around the world.
Did he just let David Gross get on with it himself, or was he involved in any way?
No, he is fairly hands-off in terms of how the money is used. With that many institutes he’d have to be, right. He puts a lot of effort into where to put his money next, but not so much into — once he makes his choice, he’s not hands-on.
Whose idea was it to put the seminars on the web, the talks?
I think it’s fair to say it was David’s idea, but the fact that it worked so well, a lot of the credit goes to Doug Eardly who was a permanent member here originally. He’s one of these people that takes an interest in computers, and to make something like the web work you really need somebody who is hands-on and smart. He was in a point where his research had slowed. He was going to step down as a permanent member, so he’s now a physics faculty member, but he also makes the web work.
Do you think we’ve covered enough about D-brane?
You haven’t asked about the ’95…
Yes, the Dirichlet Branes and Ramond-Ramond changes report. I’m interested in why it was that paper that suddenly sparked all the explosion of interest.
I am too, yeah. String theory went through this tremendous wave of activity in the ’84 to ’87-’88 period. From ’88 to ’95, there was a perception that it had slowed down. Now in retrospect, huge amounts of stuff were done in those days: near symmetry, D-branes, Neveu-Schwarz branes, supergravity. Huge amounts of stuff being done, but nobody knew that it all fit together. And so at the ’95 string meeting, there was still the sense that the field was not moving forward. Then Witten gave this talk, I think it had 70 transparencies, in which he basically said the organizers had told him to try to think about big questions, and so he was going to solve all string theories. That’s paraphrased, but that’s how it struck me. He proceeded to present this idea of duality, which had been around, but he presented it in a way that was so compelling that there was no doubt. That is, duality at the time had seemed to be a very sporadic and random thing. He explained how every single string theory had a strongly coupled dual, and how you would figure out what it is. So suddenly it became a framework and not just some oddity.
Is this the higher symmetry and string theory; is that the paper it became?
The paper was written in April of ’95.
Maybe the talk was different than the paper.
The paper that appeared in the conference proceeding was actually something different. He’d already published the talk. I can’t remember the title, but it was similar to what you just said. [The title was: “String Theories in Various Dimensions”.]
So was he mentioning D-branes in his talk?
No. Well, yes, but nobody knew it, because he said that the web of dualities requires that there be non-preturbative effects, which instead of being like instantons, which are e-1/g2, they are the square root of that. And also that there should be particles whose masses instead of being e-1/g2 like a soliton, they’re 1/g. And I knew that D-branes had these properties because I had worked on them before. In fact I had written a paper just the year before about e-1/g effects from D-brane instantons. Mike Green, who had worked it on and off since ’89, also recognized that, and we looked at each other and said, “He must be talking about D-branes.” And this was in March. For some reason neither Mike nor I returned to it for like four or five months. One of the reasons in my case was that his talk was so rich in ideas that I made myself a list of homework problems just to understand all of the ideas. The bit about open strings where the D-branes would enter was like 12th or 13th on the list, so I didn’t get to it until August. So the distinctive property was that he needed particles that carried Ramond- Ramond charge — he needed branes that carried Ramond- Ramond charge. I was really obtuse, because I had this paper with Kai which showed that 9-branes carry Ramond- Ramond charge and the paper with Dai and Leigh which showed that 9-branes are T dual to all of the other branes. And so the results were in those two papers combined. You know, you get mental blocks when you think that something has — I can’t even now reproduce why I didn’t put this together. I somehow had the sense that the D-branes were stuck to the orientifold. I had some silly idea that makes no sense now. Then in August I started working through Ed’s dualities for open strings, and actually I thought I found a contradiction. I thought I found that one of them was impossible, so I emailed Ed and we worked on it together. That was actually the first time I ever collaborated with him. But in the course of that collaboration, and also working through my homework problems, it suddenly was obvious. It’s one of these like the renormalization where all of the pieces were there, and suddenly you know that they’re all there. So I emailed Ed and said, “Oh, by the way, these D-branes carry a Ramond-Ramond charge, and they have these other properties.” I thought it was neat. But I was not prepared for the response. He appreciated much more than I did how important this was.
Did he tell you to publish? Or were you already…?
I knew I was going to publish, but the way he reacted I immediately put aside all other things and spent two weeks…There was one thing I had to work out, which is there are some magnetically charged D-branes, some electrically charged D-branes. So Dirac showed that the product of the charges has to be quantized. And it’s a funny calculation to check that. So I had to do that calculation, and the first few times I got factors wrong, and in the end it came out to exactly the right number. That was the thing that made me so happy. So I put it out. You know, Ed is so quick. Like within two days he put out his paper that followed up on my paper. He probably could have put it out before—he was waiting for me. So in his paper he pointed out the non-Abelian structure, which I kind of was aware of but didn’t appreciate the importance of. His paper following my paper is what really attracted attention to the whole subject. I knew it was neat, but I had no idea how sweepingly important that it was going to be. It was important for black hole entropy; it led to gauge/gravity duality. It leads to all these connections to mathematics. You know, I don't know any formal mathematics, but mathematicians talk about D-branes all the time. It stuns me.
How quickly did these start? I know in black holes they were almost immediately used.
Within weeks of my paper, Vafa and Douglas and Sen had all pointed out important implications. I don't know of any episode like it in my experience where there had been such a change in a field. It’s weird, because although I felt like I pulled the cork out of the dam, I didn’t have any sense — it just blew me away. Why are they so important? Well, of course we suspected for a long time, and it was clear in my — I mean in my Les Houches lectures, I explained why string theory is not a theory of strings, and this was before any of this happened. It’s clear that whatever the fundamental formulation of string theory is, D-branes are closer to it than strings. If you sort ask today for what is our most complete formulation of string theory, either matrix theory, the Banks et al one, or AdS/CFT duality, in both of those it’s the degrees of freedom on branes that are the fundamental degrees of freedom. So it’s pretty remarkable that there’s all this stuff underlying string theory. D-branes have literally been the subject to close to 10,000 papers. You could be working on the subject and not know it! The fact that we could have worked on this subject for ten years and not suspected this is amazing. I was the one who was fortunate enough, partly because I was in Texas looking at oddball questions, like my advisor taught me. I was the one who sort of noticed these things.
And then the context became just right, and off it went. What’s the importance of these e-1/g? So the branes are giving a physical interpretation of these?
That’s not their only — The e-1/g effect, so in quantum field theory you weight tads by the action, and with the sort of natural way to scale things in quantum field theory, the action has a 1/g2 effect, and so non-perturbative effects are like that. I think it was Steve Shanker who first pointed out — I guess this was one of the enduring lessons from the 1990 matrix theories that the growth of perturbation theory in those models is faster than quantum field theory, and the fact that perturbation theory is diverging faster implies or suggests that there were larger non-perturbative effects. So in some sense, this is a clue that string theory is not a field theory of strings. It doesn’t tell you what it is, but it tells you that when you find these effects, then you’ve found something more fundamental than the strings. So that was the first clue that they were important. It’s not the only thing that’s interesting about them. Probably the most interesting thing about them is the Maldacena duality, the fact that the…Yeah.
So Witten just didn’t know what these things were.
It’s interesting, because Witten — and when I say Witten, Witten was the one who I heard it from and I think most of us heard it from. But he said in his talk, and it’s true, that a substantial fraction of what he said was already in Hull and Townsend’s paper. So Hull, Townsend, and Witten were aware that these states had to be there, and they conjectured that they were black holes, because in some sense you can always make a charged black hole; you just hide the charge in the singularity. And for a few years after D-branes came along, there was this tension between do we think of them as black holes or do we think of them as D-branes? And there was kind of an understanding that when the coupling was weak they looked like D-branes, and when the coupling was strong they looked like black holes. That was the essence of the Strominger-Vafa paper on entropy counting, the fact that you can take a black hole and you can reduce the strength of the coupling and it is no longer black; it becomes D-branes. Then Maldacena came along and made more precise. He said that if you look at the low energy physics, that there’s a single picture of the gauge theory that captures both limits.
How important was that entropy counting in terms of how people viewed string theory at that stage?
It’s interesting, because it was important for the following reasons. If you list the problems of quantum gravity, string theory was first of interest because it solved one of the problems: the problem of renormalizability. If you list the other problems, there’s black hole entropy, black hole information, cosmological constant, and then space time singularities, initial conditions, and so in some sense this is the first time that string theory had solved a gravitational problem beyond the first one that it hit. It had mostly been interesting for particle reasons—it provided a framework for grand unifying particle physics and gravity. But I think it’s fair to say this is the first time that — Actually, going back to mirror symmetry, there was very interesting work, but in gravity there’s a question that space time can bend, but can it break and its topology change? In the context of mirror symmetry, it was shown that in fact in string theory the topology of space time can change, that you can have a geometry, you can go through a phase where there is no geometry, and then you can get a different geometry. Now that was neat. I don’t think you can say that solved a problem that people were waiting to see solved, but it was a neat discovery. With the black hole entropy counting, it solved a problem that had been sitting there for 20 years, and so it represented a solution to a new problem of gravity. Now, I have to say that the information problem always bothered me a lot more than entropy problem. The information problem is a sharp paradox. Hawking points out via this thought experiment that some law of physics must break down. He says it’s quantum mechanics, and you can argue with that. But the fact that some law of physics must break down, nobody has ever gotten around that. That’s a much sharper…that’s much more annoying than just the statement that black holes look like they have an entropy, how do you count it? In fact, you could say that the fact that black holes look like they satisfy thermodynamic laws is a pretty neat thing, and there should be a stat mech. But the information paradox is much more of a sharp poke.
It’s a paradox.
Yes. So after entropy people kept looking at these things, thinking if we entertain entropy we can entertain information as well. So they started studying what happens if you throw stuff at a D-brane, and this led to Maldacena’s recognizing this duality between gauge fields and strings. This entropy counting is neat, but the gauge/gravity duality is amazing, because it really says that gravity and string theory are not anything new; they’ve always been present in the framework of quantum field theory or gauge theory, if we simply knew how to read the code, and Maldacena told us how to read the code. This has many implications. One is it does resolve the information problem at least implicitly, because it shows that you can formulate the quantum mechanics of the black hole in terms of the gauge theory which is purely quantum mechanical — it satisfies the ordinary laws of quantum mechanics. It shows that Hawking was wrong about the breakdown of the laws of quantum mechanics. What does break down in some sense is locality. The fundamental degrees of freedom in the gauge theory are not local in space time. So this is now the third of the big problems in quantum gravity, which I would say that string theory has resolved. I would also count the fourth, the cosmological constant, but this is much more controversial.
Because it requires a landscape? [Yes] So how does the black hole information problem get resolved? I know it is something involving the theory on the boundary.
There are two questions. The first question that we always ask ourselves at conferences is what happens to the information? It’s lost; it comes out, it remains in a remnant. Basically those are the three choices. In any AdS/CFT duality, a black hole is just the dual description of a gas of hot gluons, and this is very satisfying because the black hole thermodynamics is telling us that a black hole behaves like a thermodynamic object. In this duality, it is literally dually described as a gas of hot particles. And the Hawking evaporation is just ordinary evaporation in this case, and so the information just comes out with the evaporating gluons on it. Now, there is still a puzzle, which is the following. We now have this duality. We know then that there is one quantum theory which has two descriptions, one in terms of gauge fields and one in terms of gravity, and we do the calculations in terms of gauge fields, which we understand very well, and the information comes out. We do the calculations in terms of gravity, the way Hawking first did it 25 years ago, and we still get Hawking’s answer. We still haven’t found Hawking’s mistake. It wasn’t a trivial mistake; I mean it was a very — People still come back to this. I was working on it just last year. My feeling is that the most important thing about the information paradox is that it led us to discover AdS/CFT duality. A paradox is like a thought experiment and the lesson of the experiment was AdS/CFT duality. But I think there is still more to learn, if we could understand in some sense how in the gravitational variables do you do the calculations.
So you can solve it in the dual theory, but you don’t know how these are…
Right. You know, Susskind draws pictures of strings getting big as they fold through the horizon. There is some logic to that. I think that it’s possible to give a sharper answer to the question. I mean you can simply say that the gravitational variables are bad variables, like trying to use baryons as fundamental fields in QCD instead of quarks and gluons. But you should still be able to explain why they’re bad variables.
Because they’re still dual, right?
They’re still dual, right. So you should be able to see why that picture is breaking down. So I believe that there is a sharp question there. I spent some time last year and failed to do anything interesting, but I’d like to return to it.
We are about at the end; there are just a few more things I’d like to talk about. Prizes and recognition is one of them, and maybe the quantum gravity wars, strings and loops and these kinds of things. What first? Maybe the quantum gravity wars. You recently reviewed these books, Smolin and Voit, and it’s stirred up quite a bit of heated debate. Why do you think we’re getting into this kind of heated debate?
It’s interesting, because I used to compare physical humanities. In the humanities, there are these many schools of thought, and they argue with each other, and nobody can ever decide which is right. In science that tends not to happen because we have experiment as an arbiter. But it’s of course much harder. In quantum gravity, we have no positive experiments, and even in particle physics the rate of new information has slowed down. Actually, in spite of this, we’re mostly not like the humanities. In spite of the appearance of wars, there’s actually an enormous consensus in the people who have tried to work on different aspects of quantum gravity, that string theory is, one can’t say right, but the thing that one should be thinking about. When I say a consensus, you can look at all the mathematicians we have here, you can look at people coming from particle phenomenologists, heavy ion physics, inflationary cosmology, especially you have people like Linde, Guth, and Steinhardt, all the people who created inflationary cosmology, and they’ve all in the recent past worked with string theorists and are working ideas coming from string theory. You might have thought, a priori, that if somebody was going to tell you that you were not going to have much experimental information for many decades, that science would just come to a halt, and we’re fortunate enough that we seem to have put enough of the pieces together. I mean in the end, we think that there is some theory that unifies everything that we know. That’s the experience in physics, electricity and magnetism and light are all the same thing, and so on. If you have a theory that unifies that many things, it should be not so impossible to find, because there are many places to grab onto it. In fact string theory, I mean it was originally grabbed on to in a fairly funny way in terms of dual resonance models, but you could also grab onto it from AdS/CFT duality or from matrix — I mean there are actually other ways that you can think about discovering it. If you ask me what is the strongest sign that this is the right thing to have been thinking about, it’s AdS/CFT duality, and it’s the fact that it’s not a new idea at all; it’s really been part of quantum field theory all along, and we simply had to understand quantum field theory better. It fits with the general principal that nature is fairly economical; it tends to use a small number of ideas over and over again. I would think it would be a rather bizarre situation if there were one theory of quantum gravity that happens to be dual to gauge theory, and a completely different theory of quantum gravity that happens to be the one that describes the real world. I think it’s hard enough to imagine even one theory of quantum gravity existing, much less two.
Can you see no possibility of getting dualities in these other approaches?
Well, let me first undercut my entire argument by giving you an example of when I would have answered that question incorrectly. If you had asked me 15 years ago whether supergravity and membrane theory was going to be dual to what we were doing, I would have said no, they are working with some kind of mutilation of the real theory, when in fact they had just as good of a grip on it as we did, just on a different piece of it. So having said that, you can ask could this happen again, and there is no question that there are ideas that are missing and that they will very likely come from people who come from a completely different point of view. But you know, this has already happened. Inflationary cosmology and the internal inflation in the landscape weren’t thought of by string theorists; it was thought of by Andrei Linde and the cosmologists. It turns out that it’s realized in string theory, but they are the ones who provided the idea and the framework and the anthropic principal. The idea of large dimensions, again, is now central in string theory, but it was provided by the particle phenomenologists. So in fact the subject has definitely developed due to ideas that came from outside. I think your question was this specific group of outside people who go under the Rubic of Loop quantum gravity, there are many, many small ideas. It’s almost non-string gravity. There are many people with their own individual approaches, and is it possible that somewhere out there is an idea we need? Certainly. The main thread of Loop quantum gravity I have a specific objection to, which I voiced to them. Loop quantum gravity in its kind of central form is an outgrowth of canonical quantum gravity, and when you can canonically quantize, you give up Lorentz invariance. Now the question is, is this a bug or a feature? There’s a general thought that Loop quantum gravity really is not Lorentz invariance, that it predicts at some level violations of Lorentz invariance. But if you have a theory that predicts violations of Lorentz invariance, you have to have a pretty good argument that it correctly reproduces successive Lorentz invariance in all the vast number of places where it succeeds. This is not easy. In the standard model, we measure 20 couplings, and if you give up Lorentz invariance there are 20 more, but they are all zero because these have been measured. But in some sense, the big thing that Feynman and Schwinger contributed to quantum field theory is they explained how you could do calculations in a manifestly covariant way. If you backtrack before then, it was all canonical, you had the Hamiltonian, and covariant results appeared by magic when you add up many, many non-covariant things. So if you’re going to start with a non-covariant description of your theory, and then start modifying it, there’s a big burden of proof to show that you actually reproduce the successes of Lorentz invariance. And I don’t believe that quantum gravity does. So, if you think about it, the Weinberg-Salam theory was discovered almost purely on theoretical grounds. There was the Fermi theory of the weak interaction, and it was natural to suppose that it was a vector bozon that mediates it. But that it was specifically a vector boson of spontaneously broken symmetry, that it was of this specific type, is required because of renormalizability. The Fermi theory has infinities, and in the quantum field theory of infinities, it’s very hard to fix the infinities while retaining Lorentz invariance, and that’s because it’s easy to fix the infinities if you smear things out in space. But Lorentz invariance requires you smear things out in space and time as well, because it’s Lorentz invariance, and when you smear things out in time you lose causality or you lose unitarity. So it’s easy to make a finite theory of gravity or anything else. There are a billion of them, if you’re willing to give up Lorentz invariance.
Have they not attempted to patch these problems by causal…?
The problem is you can give up any one of finiteness, causality, unitarity, or Lorentz invariance and it’s not a problem. The problem is when you try to keep all of them. And there are many different things that go into the rubric of loop quantum gravity which are not the same thing. So there is interesting work by Ambjorn Loll on dynamical triangulations, there’s interesting work by another group that suggesting that gravity may have an ultraviolet fixed point.
Reuter, right. The other things I would look at more closely if I felt I was stuck, but… So just for me personally, what would make an idea coming from that field interesting, first of all, we have this AdS/CFT construction of quantum gravity, but also we have this holographic principle, and again, this is one of these things that didn’t come from—this is another one that came from outside string theory. All the other principles came, first of all, from Hawking’s black hole paradoxes, and more specifically from T’Hooft. And now it seems to be a central piece of the idea, that the fundamental degrees of freedom are not local. And we know what that means in a specific context, a space that has anti-de Sitter boundary conditions. We don’t know what it means for a space that has more general boundary conditions or a space that’s expanding or a space that has eternal inflation. So if I saw a paper appear that came from any community, it could be any community, which seemed like it addressed this question, it would be interesting. Many of the things that people are doing, though, are not holographic, they’re actually pretty conservative. They’re very much stuck with — You know, string theory very clearly has been discovered, not invented, so there is this structure there. For example, the D-branes are not something that I thought, “Hey, why don’t we add something to the theory.” It’s something I discovered in the thought experiment that I described. Once that’s pointed out, it’s clear from many points of view they have to be there. The holographic principle was again discovered via a thought experiment with black holes. And with many of the ideas that people explore, one gets the feeling that they’re more invented than discovered. There’s nothing wrong with that, but things that are invented are generally too conservative. You know, they’re generally — I’m stating my own prejudices here, but if you think about there are certain kinds of ideas, like open sets that mathematicians need to construct things in a precise way. Those are the language that people use. I don’t believe those are fundamental to the way nature works. So many of the ideas — A litmus test I have when I see an idea is in some sense, if it looks invented… (I’m not getting that across), but it will often have a feel like this is some superstructure that a human added to nature; it’s not part of nature.
Something that Dirac might agree to?
He had a lot of opinions, many which I agree with. This distinctly discovered invention is an interesting one. I don't know if he addressed it in particular, but it comes up a lot.
I don’t suppose that he thought that he invented his hole theory. I mean that sounds like a discovery.
A discovery, right. Anyway, I think it is an interesting question whether quantum gravity might have a UV fixed point. Even if it does, well, it’s actually puzzling. If quantum gravity has a UV fixed point, it means that quantum gravity is in some sense a normal field theory; it’s not holographic. So that means that it can’t explain black hole thermodynamics. I feel like we’ve moved beyond this question. Back when the goal was to understand the ultraviolet problem, the UV fixed point and string theory were both candidates for that problem. But now string theory has gone on to provide a holographic description of black holes. I don’t think a UV fixed point can do that.
So you think they’re dealing with problems that have been made redundant possibly by string theory?
I think it’s an important question to as whether quantum gravity has an ultraviolet fixed point, and if it does, it’s important to know. Being a skeptic, I suspect it doesn’t, but that’s just because I tend to be skeptical of anything new.
Would there be any plans at KITP for a joint program?
We had a joint three-week workshop on singularities, and Ashtekar and Bojowald and Thiemann were here. We have Gary Horowitz and Don Marolff in the department, and they are very valuable because they both are educated as general relativists, not as string theorists, although they’ve both become interested in string theory. For that reason they are more broad in their perspective. On the right subject, yes. You know, programs we have coming up are, for example, applications of AdS/CFT heavy duality to ion physics and condensed matter physics? That’s not something that they could contribute to. We had a joint program; I forget the exact focus of it, about 12 years ago. It will surely happen again. It’s one of the things that KITP tries to do repeatedly is to bring together different communities. Like in one case, cosmologists and quantum field theorists. In the program this summer, condensed matter physicists and string theorists. But part of what makes KITP work is they put a lot of effort into figuring out if it’s really useful. That is, if there’s common ground, and if there is some set of problems that are ripe for progress. The subject of quantum gravity has been around for a long time. The three weeks Singularity workshop and I worked up was good, because three weeks is just enough time to kind of find out what other people are doing. You don’t expect in three weeks to… But in any case, I’m sure it will happen again. I don't know when yet.
Final point on this string wars thing. The issue of background independence seems to just constantly arise. Why do you think it is so divisive? What do you think it has claimed to be so divisive?
So I’ve told you some of the reasons why I don’t find many of the ideas of loop quantum gravity appealing. If you ask someone from that community why they don’t find string theory appealing, I think the thing for which their intuitions is least appealing is the fact that certainly the way it’s presented in textbooks, you start from a fixed space time. In a sense, this is because in a classic way of describing string theory — So duality means you have a theory with many classical limits, and in the classic way of describing string theory, you’re expanding it around the limit in which the metric becomes classical. The background is fixed. For many years, that's all we had. Now, I personally like to emphasize the difference between language and physics, so even though the language is not background independent, the physics I think always has been. I think if you ask string theorists what’s most lacking in this, they would say it’s a non-perturbative formulation. Now in some sense, a non-perturbative formulation, you could say they were asking for the same thing in different words, although a non-perturbative formulation is more general, less specific. I have a specific problem with the notion of background independence, which is the following. You are defining the theory by what it doesn’t have: it doesn’t have a background, as though you could get to the theory from where you are by a process of subtraction, and I think that’s the wrong way to think. The theory is more than we know; it has pieces that we don’t know. And when you start thinking about finding the theory by this process of subtraction, it takes you back to this notion of invention. You look for book keeping devices which sort of look better. But we’re looking for the real physics, not the language, not the bookkeeping. So this is why I think that it’s not been a fruitful way to think, because it takes you in the wrong direction. Also, I think a lot of thought experiments. The black hole information problem is a thought explanation; the string in a small box is a thought explanation. When you do an explanation, you usually choose a background to do it in, and so it doesn’t bother me to —You know, if I want to understand what the smallest things are, I’m happy to choose a background, like the one in this room, and smash them together really hard.
For convenience. Now there’s another aspect to background dependence. AdS/CFT duality means that if we put string theory in the anti-de Sitter box, we know everything about it in the box. Now, a lot can happen in the box. You can have black holes form, you can have Planck-scale gravitons, you can have space time even tear little pieces inside. So except for the walls of the box, you have a background independent description of the interior. Now, that’s a lot but not everything because we don’t live in such a box; we live in an expanding universe, and so to understand our universe, you need more. In local quantum field theory, if I tell what the theory is in a box, it’s a pretty trivial thing to get rid of the box. I’ve told you the equation’s of motion and the boundary conditions. Because the theory holographic, changing the boundary conditions change the theory completely. And so the fact that we can’t leap from AdS space to eternal inflation, say, is a consequence of the holographic principle. It’s important — it’s something we need to do. But the thing to focus on I think is not the sort of negative statement that you shouldn’t have a background, but the positive statement that you’re looking for a holographic description of physics.
I think we’re just about to wrap up. Well, prizes. Recognition came fairly swiftly after the D-brane papers. Two recent ones were the Daniel Heinemann Theoretical Physics prize and the Dirac prize, both of which you shared with Maldacena.
Right. And the Dirac prize also with Vafa.
These have got a very strong string theory background, especially the Dirac prize.
You mean previous winners? The Dirac prize is neat because it’s not just high energy, through; it’s been given for turbulence and for sort of all those theoretical physics.
Did you know you were going to receive it?
I actually didn’t know about either one. Heinemann I was more aware that it existed because I think I was on the jury for a different APS prize when I won that, so I was very aware that these prizes existed. But the Dirac prize I was less aware of.
In 2005 you were elected to the National Academy of Sciences, which is fairly major. Do you do anything as part of that?
The National Academy has a big role in terms of advising the government. It does studies through the National Research Council that are extremely valuable, and they’re respected for being nonpartisan. I would love to contribute to that. We’re occasionally solicited to serve on these panels, and to be honest, I don’t feel like I have a lot of the kind of broad practical knowledge that most of those panels call for. As I get older, I wish I had devoted my life maybe to something more useful, but…
It’s hard to tell, it might be very useful. [Laughter]
So that’s an important role of the National Academy that I haven’t gotten to participate in. Besides that, the process of electing new members is Byzantine, and that’s its other main goal.
Electing new members — that’s Feynman’s complaint!
Yes, it’s Byzantine. These prizes cause me some embarrassment when I think about the people who are more deserving that haven’t won them.
Any final things that you wish I’d have asked you?
What are you working on now? What students do you have now to finish off with?
I’m actually very happy, because just last night, I think, with a post-doc, Pena Donase and two students, Itsa Hinesburg and James Sully, I’ve done something I’ve wanted to do for ten years, which is the following. In AdS/CFT, you have this ten dimensional theory holographically embedded in a four dimensional theory, so it means that you can have two particles on top of each other, or they think they’re on top of each other but they can’t really talk to each other because they’re secretly far apart. How this can happen we have some good pictures of, but it’s never been made precise how it can happen. If you list the ways in which AdS/CFT duality has been tested, it’s really a very beautiful set of — I mean there’s very precise supersymmetric ones, there is symmetry breaking, there’s even experimental tests, numerical tests, high resolution… There’s a long set of tests, but there’s no direct test of this funny property that the theory really is holographic in the very strong sense that things can be on top of each other without talking to each other. It’s kind of technical, but one of the striking implications of AdS/CFT duality is that when the theory gets — Suppose you got some system, and you turn on some coupling constant very strong. You might think that the energies of all the states get very big because you have these strong interactions. In a supersymmetric theory, there will be some states which don’t get the energies because of supersymmetry, but all the other ones do. There has been a long-standing kind of conjecture, people haven’t thought about it so sharply, but I think it’s kind of sitting in the air, that the holographic property can in fact be derived from this other, more natural property that you have this big gap in the energy spectrum. It’s not real obvious how those to things would be connected, but what happens is when you do sums over complete sets of states in the quantum theory, you have a much, much smaller set of states to sum over, and so the theory is very highly constrained. So I’ve had a long-standing conjecture that in some sense that the reason for the holography is…or that you can derive the holography from this property of the spectrum. And I think just last night we’ve proven it in a very special case. I’m very happy because it’s something I’ve wanted to do for ten years.
Was this a genuinely collaborative kind of…?
Oh yeah. Especially the post-doc Pena Donase had exactly the right set of tools to do this. If he weren’t here, we couldn’t have done that.
Why it’s good to collaborate.
Yes. Being at the KITP, you have a lot of opportunity. One of the problems is that you can’t collaborate with everybody, so you have to kind of control the flow. Some of my best collaborations have been people who really at the start had a very different set of intuitions, so it took some time and effort to even get communication going. But once it got going, then we really had complimentary parts of the problem.
Like Einstein and Grossman — Grossman had exactly the right set of mathematical tools to complete the job. Okay, I think we can end there.