Notice: We are in the process of migrating Oral History Interview metadata to this new version of our website.
During this migration, the following fields associated with interviews may be incomplete: Institutions, Additional Persons, and Subjects. Our Browse Subjects feature is also affected by this migration.
Please contact [email protected] with any feedback.
Credit: Harvard University Department of Physics
This transcript may not be quoted, reproduced or redistributed in whole or in part by any means except with the written permission of the American Institute of Physics.
This transcript is based on a tape-recorded interview deposited at the Center for History of Physics of the American Institute of Physics. The AIP's interviews have generally been transcribed from tape, edited by the interviewer for clarity, and then further edited by the interviewee. If this interview is important to you, you should consult earlier versions of the transcript or listen to the original tape. For many interviews, the AIP retains substantial files with further information about the interviewee and the interview itself. Please contact us for information about accessing these materials.
Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event. Disclaimer: This transcript was scanned from a typescript, introducing occasional spelling errors. The original typescript is available.
In footnotes or endnotes please cite AIP interviews like this:
Interview of Andrew Strominger by David Zierler on September 16, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Andrew Strominger, Gwill E. York Professor of Physics at Harvard, with affiliations at the Black Hole Initiative and the Center for the Fundamental Laws of Nature at Harvard, is interviewed by David Zierler. He recounts his childhood in Saint Louis, Madison, and then Boston, as his father, the prominent biochemist Jack Strominger, moved academic positions. Strominger discusses his undergraduate education at Harvard, where he started at age fifteen, and he describes his experience living on a commune in New Hampshire and hitchhiking to classes at Harvard during the week. He describes what he thinks string theory is, and is not, capable of describing as a representation of physics reality and the significance of the Calabi-Yau paper. He explains why the fact that the universe exist must be proof that there is some theory that can allow for gravity to be incorporated in the Standard Model, and he addresses criticisms that string theory deals in realms that are not scientifically testable. Strominger describes his graduate research at MIT where he began his work on quantum gravity, which he continued as a postdoctoral researcher at the Institute for Advanced Study and then at UC Santa Barbara, where he spent twelve years on the faculty. He discusses his long-term collaboration with Stephen Hawking, and he muses on the likelihood, in his view, that there are other universes even if there is no scientific way to confirm or disprove their existence. At the end of the interview, Strominger reflects on the unique importance of the concept of belief in science.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is September 16th, 2020. I am so happy to be here with Professor Andrew Strominger. Andy, thank you so much for joining me today.
All right so to start, would you tell me please your title and institutional affiliations? And I put an 's' on the end because I know you have more than one.
I am a professor of physics at Harvard, Department of Physics, that's my main affiliation. There's also the Black Hole Initiative, which is an interdisciplinary group that I'm part of here. And there's also the Center for the Fundamental Laws of Nature, an informal group of high energy theorists. Yeah.
Now, you're the Gwill E. York Professor of Physics.
The Gwill E. York chair is my full title, the Gwill E. Professor of Physics, yes.
Now, do you have any connection to York? Do you have any idea where the connection to this chair comes from?
Well, I know her. She's an avid supporter of the sciences, a wonderful woman who's interested in black holes, astronomy, theoretical physics, and gave a chair to the university to support that.
And you're the inaugural holder of this chair? Are there others in the department?
I am the inaugural holder of this chair, yeah.
Okay, well Andy, let's take it back to the beginning. I'd like to hear a little bit about your parents first. Tell me about them and where they're from.
So my parents are very interesting. My dad was... I'm just going to close the door here so I don't bother my wife.
She knows all this stuff.
She knows all this, but I don't want to bother her, I forgot to close the door. So my dad was from a Jewish family in Flushing. And my mother was from a-- grew up on a farm in northern Minnesota, where her father had cleared the land and built the house. And so they both came from very non-academic backgrounds. My father's now a pretty well-known scientist.
NPR recently did an interview of the two of us. But he came from a family in which, as he likes to say, you know, his joke is when does a Jewish boy become a man? It's when he finishes medical school.
And he went to medical school and they were very upset when he decided, as he puts it, he didn't like sick people. (laughs) So he went into research and it turned out he had a real talent for it, and they thought he'd wasted his life until they saw his name on the front page of the New York Times, and then they thought, "Okay, maybe he's done something worthwhile." My parents were both disowned when they married. Because she was from a Catholic family and he is Jewish... So they were very rebellious, and she had a dream, as, you know, she came from a very poor family in northern Minnesota. None of her relatives had, nobody had ever gone to college, and she had a dream to become a doctor, which was a completely wild thing. And she pursued it and somehow met my dad, and they were both these people who had become interested in the life of the mind, even though they came from backgrounds that didn't encourage that. Well, they got married, then he was a postdoc in Cambridge, England --right there with Crick and Watson. They were all friends, right when I was born in Cambridge. My dad was working on cells then, and they were working on other things you know about. And then my dad moved around. He was in Saint Louis, and then Madison, and then moved to Harvard, whatever, what, 50 years ago I guess now, was it? Yeah, 50 years ago.
And where did you spend most of your childhood?
So they moved to Saint Louis before I turned one, from Cambridge. And then when I was seven they moved to Wisconsin, Madison. And then when I was, just when I was turning 13, we moved to Boston. To Lexington, yeah.
Andy, did you sort of feel like you grew up in a scientific family? Was science all around you? Was that a path that you saw yourself on from early on?
Well, yes and no. I had three brothers. And we all loved science, and on Saturdays we often used to go with dad to his lab. He was a pretty hard worker. But sometimes he would take us to the lab. And I remember two things that we liked to do. We liked to slide the dry ice around on the floor. I loved to play with the old lab equipment. The other thing we used to do, which is really shocking, is we used to love to play with mercury. You know, they didn't know (both laugh), but we had these bottles of mercury.
You know, nobody knew about mercury (laughs). Okay, so we played with mercury. I remember there being in Madison a huge room of used lab equipment, and lab equipment was much cooler in those days than now. You know, it had lots of gears.
You know, you could look at it and pick it up--
It's an analog environment.
Yeah, yeah, yeah. It was. And there were glassblowers. His glassblower made a Klein bottle for me, which I-- I guess it's in my office. We were all very interested in science. We used to talk about it. We had a map of the solar system, a big poster in our bedroom. In second grade was reading Mr. Tompkins in Wonderland and I think in fifth grade I went to the library and checked out Newton's Principia. Then, okay, then I hit adolescence and kind of went in a different direction ... I was interested in many things and wasn't really thinking of being a scientist until my last year in college, actually.
Did you go to public school or private school in Boston?
I finished high school when I was 15. In Madison, I went to public school. I went to private school for three years in Boston.
Why'd you graduate so early? Did you skip a lot of grades?
I skipped some grades. I was widely regarded as a troubled kid. I got in a lot of trouble. And you know, drugs and everything. It was the 60s and you know. But I did very well in my classes... and I sort of zoomed through all the coursework. I had read through all the Feynman lectures and I just said, "Okay, I'm done with high school." And I applied to Harvard and got in.
That's fairly precocious of a high school kid to have read through all the Feynman lectures.
I was precocious. Yeah.
So you knew you had a natural interest and aptitude for physics? You weren't thinking, you know, as a career, even as a line of study, as a high schooler, though?
No, it was just fun.
It was just fun. It was just really fun. It was also easy for me. But I also was doing many other things in high school. I spent more time drawing, I wrote poems. I was interested in politics. It was really the politics. And then I was-- I mean, there's a very long story there which probably we don't even have time for-- but you know I always did whatever. We're talking about when I guess I finished high school in 1970. So you know, my last year, there was a lot going on (Zierler: Oh yeah.) in the world at that time.
A lot going on and a lot going on in Boston specifically.
In Boston specifically, and I was thick in the middle of it.
Now, would you go out to campus protests, even as a 15-year-old? Would you get involved in all that stuff?
Oh yeah. In fact, my whole family went down to, including my parents, went down to march in Washington. But even in Wisconsin, I was hanging out with kids in the campus at 12 or 13 and going to various protests. My whole family went down to Washington for the march on Washington in, I'm not sure what year that would be, '68 or something?
There were many. I don't know which one you're referring to.
Yeah, yeah. There were many. Yeah. So...
So your parents were politically engaged as well.
You know, that's interesting. My parents, and my dad, they were and they weren't... They're not especially political.
But like at the dinner table, would the concept of voting for Nixon, would this be like a topic of horror at the dinner table, kind of thing?
Yes... but the thing I was going to say is, even people who weren't very political were going down to Washington.
I mean, everybody was going.
They did have some very political friends. I didn't have political disagreements with my parents. I had all kinds of other disagreements, which we'll... but not political ones.
But they were very interesting people, and they had a lot of wonderful people through the house and it was a very great way to grow up. Yeah.
Now, did you want to go to college young? Did you want to take some time off? What did you want to do as a 15-year-old?
Well, okay, so I felt I was done with high school. There was nothing more for me to learn. My high school had a policy that if you got high enough score, you didn't have to go to class. So, I had essentially been excused from all my classes for a year or two, but was doing very well and so I just felt like I was done with it. And so I applied to Harvard and got in, which was somewhat unusual. They didn't usually take people that young. But then after I was accepted, I went -I don't know how much of this you want to know- but--
This is what we're here for. I want to know.
Okay. So I went and lived on a commune in New Hampshire.
So I left home when I was 15 and went and lived on a commune in New Hampshire. By the time the fall came, I was completely uninterested in Harvard, and I basically lived on that commune for three years. The first year, freshman year I would commute down for the week. The commune was a very interesting place. It was, okay, it was probably the most educational experience of my life. It really shaped my life.
Andy, was the commune a means for getting away from the drugs and trouble of Boston, or getting into a new, different kind of drugs and trouble?
Okay. There were a lot of communes in those days.
Within a ten-mile radius, there were three other ones. So there was the, you know, Hidden Springs-- there was one like five miles away where everybody was still taking lots of drugs and looking up at the stars, the commune I went to was a, everybody had been through their drug thing, was over it, and we were going to change the world. We were going to live off the land, grow all our food. And share all our property. Okay? I know it sounds crazy now, but I believed it at the time.
What about your parents? Were they supportive of this? Did they think you were crazy?
Oh my God, they were horrified. But they'd been completely horrified with me since [age] ten, because I had been... I was the kid from hell.
What did you do with your Harvard acceptance? Were you able to defer it, or was there a risk there?
No, no. I went back for classes, I just was up there on the weekends and I would... those were the days when everybody hitchhiked. I would hitchhike or I would take the bus, or get a ride when we delivered vegetables. And so I was back and forth between the commune all year. I didn't really socialize with any Harvard students during that year. And then when the spring came and we planted the crops I just left, I didn't go back to Harvard the next year. I was up there for three planting seasons, during which the entire commune fell apart financially and morally. By the end of the thing it was a total disaster. So I was the kid, right? I was 15 when I came. Most of the commune leaders were like in their early 30s, and the place was, I mean, they were all having affairs with each other, nobody was really working in the fields. Our financial model was a disaster. So (both laugh) it was like, and everybody was mad at each other. People were even mad at me. I still can't figure out why. And it was a real learning experience. It was my first failure. Well, there was a huge sense of failure, right? Because everybody went... It was just at that moment, we're talking, when is this? This would have been, I went up in the spring of '71, and it was just as the hippie idealism had kind of faded and was trying to become more concrete, you know? And this was the way to do it. There was this vision everybody believed in, even though in retrospect it was like crazy naive. People...yeah. It's interesting. I mean, I love all the ideals of the 60s, but if you go back and you look at some things that we said and were doing...you know, it was pretty wacko, you know? And so anyway, so that was...
Did your own politics change over this period that you were with the commune?
Okay, one thread through my life is I'm not -and this has served me very well as a physicist; I'm not really a `believer’. I’m a seeker. You know? Even my best ideas in physics, you know, I'm not sure they're right until absolutely proven. I did a lot of string theory but I was never a `believer’ in string theory. I was never as much a believer at the commune as some of the other people, although in my whole life, that's probably the thing I believed in most. The commune. And it was--
Belief in what? The ideal of being able to live outside of a capitalist system?
The vision that the world would become a better place if everybody were closer to the land. That was the theory. That we'd lost our roots, and we were eating junk food all the time, and people needed to work and be close to nature and the land, and that would solve everything. That goes back to my family which is big on the outdoors, and my mother of course grew up on a farm. We did a lot of camping and so on. And so the whole outdoors and back-to-the-land aspect really resonated with me. At the time when the commune went bankrupt, whatever it was, three years later, at that time - again it's hard to reconstruct this - but at that time, there were all sorts of reports coming from China about the communes in China and all the amazing stuff they were doing. And of course, this is already well into the cultural revolution and now we know that all kinds of horrible things happened then. It’s unbelievable how little we knew then. You don't look old enough to know this, but at that time, everybody, all good liberals, thought Mao was great and China was doing something fantastic, you know? That was the accepted wisdom among the academic left. A little hard to believe now, but it was. And so I decided I wanted to go to China. I went back to Harvard and learned Mandarin. Then I went to China. I was very intrigued by communism, socialism, as a way for the world to become a better place.
What was your plan when you got to China? What were you going to do or be affiliated with or study with?
Well, at that time nobody went to China. And...
This would have been what year?
This was, well I was originally approved to go in '74, but then it was a year later by the time we went. What happened was, have you heard of Chou En Lai?
Yeah, I'm a historian, yeah. (laughs)
You're a historian, okay. They were opening up to the West, and Chou En Lai wanted to have a group of 20 Americans under 30 come and live on the communes and work in the factories. He was organizing it through the Hinton family. I got wind of this and I went down to their farm in Pennsylvania and begged and they took me along. And so I went to China. Apparently, I was the first American teenager to go to. Now, the Hintons are a very interesting family-- Bill Hinton was on the Long March with Mao. The whole group were communists, right? It was an incredibly interesting experience. I worked in a factory and a commune, then I was a newspaper reporter for a while for a communist newspaper. My Mandarin was completely fluent at that point. I got back and was thinking hard about becoming a reporter -New York Times had offered me a position because I had gotten access to things that no one else could. But then, communism, it has some appealing features to it, but should we have a communist revolution in America? Is that what I believed? I didn't know if I believed that. And I just didn't know what-- and of course, at that time the China star was rapidly fading, and communism was in trouble. Marx had a theory, there was some experiments, and they failed. That’s how I would describe it. There were already signs of that failure. And so I didn't know what to do, I didn't know how I could contribute to the political world. Then I came back to Harvard and I'm like, "What am I going to do?" I knew that I could be a good theoretical physicist. Even though I hadn't taken very many courses.
Andy, to get back to this idea that you've never been a believer, maybe at some point, intuitively, you recognized that in science, you could side-step all of that and just focus on the observations and the facts?
Well, but there are also believers in science.
You mean like a true believer in string theory, for example?
There are believers in string theory. There are a number of them.
There certainly are. There certainly are.
And I am unhappy about the fact. I don't think most people who work on string theory are `believers’ in that way—though there are a few. And I'm unhappy about the fact that those few have been taken to represent the field.
And by the way, there's a big difference between believing that string theory describes the real world and believing that studying string theory in one way or another is a reasonable and useful thing in trying to figure out what's next in physics.
I do believe that we're learning something.
I don't believe that we've - I don't think it's ruled out - but I don't believe that string theory is the final word on physical reality. But I think we've learned some stuff.
I like to use as the spectrum, you know, on the true believer side, you have a John Schwarz, and on the other side you have a Shelly Glashow, right? That's sort of the extremes in terms of belief. And so, another way of looking at it is simply a matter of patience, which is, how--
Although, I have to say, you used Shelly as an example, and Shelly said to me - I have a lot of respect for Shelley- and I think he has respect for my work, and he said to me, "It's clear you guys are on to something. It's just not clear what you're on to."
And that's the point, though. It's a matter of patience.
The patience is where you have a true believer who says, "Stay tuned. We're going to do something amazing."
And others who would say, "You know, it's been since 1970. What have you got? I'm done waiting." Right? So it's really a matter of, even if there's some fundamental truth behind it, some people have lost patience, and they said if there's something that's connected to the observable universe, we would have seen it now. So that's how I like to encapsulate this concept of belief. Maybe it's not that somebody like a Glashow is not a believer, it's just that he's no longer willing to wait around to see what it might produce.
Okay, I wouldn't really put it that way. Because I think that producing an experiment that proves that the world is made of strings is not the measure of the success or failure of string theory, in my opinion.
But isn't part of the problem that there's no experiment that could do that? I mean, isn't the criticism one of testability? In other words, you might have needed to wait 100 years for gravitational waves, but throughout that 100 years, it was a matter of when's the technology going to catch up so that we can actually see if we can detect them? What's the experiment you could use?
There's such a public misconception about this -- which has been fueled by the “believers.” Now, by the way, I'm digressing a little, but let me tell you a...
I'm going to come back to answering this question, but I want to digress, because I'm also not critical of believers, even if not one by temperament myself. We need every type – most especially including the believers – to make progress in science. So I'm going to tell you a story. You probably know I did this work on Calabi–Yau spaces, right?
That was the best thing - I think many would agree-that string theory has produced in trying to lay a path of how string theory might be an actual theory of nature. You know, it was very compelling because it was so easy and it was effortless. Like string theory dumped it in our laps. Now, about a year or two after that paper, I wrote another paper in which I say, and it's there in print 1986, super strings with torsion, there are a lot of solutions of string theory and we're not going to be able to make any experiment, this will be an obstacle to making any kind of experimental contact. And in my mind in 1986, 1987, I gave up on the idea that there would be in my lifetime an experiment which verified string theory. Now, you might wonder why I kept working on it. And we'll come back to that in a second.
But let me tell you my little story. In my whole career as a physicist, there was nothing as dramatic for me as what happened after Green-Schwarz anomaly cancellation and the Calabi–Yau paper. Before that, there were ten, 20 people in the world working on string theory. Two months after, there was 500 or 1000. I've never seen anything like that before or since. We could try to figure out exactly why that happened, but okay, anyway it did. And right after that, my coauthor Ed Witten and I were in Princeton, and he said to me, "What should we do next, Andy? How about we compute the ratio of the mass of the muon to the mass of the electron? That will impress people." And I'm like, "What?" I mean, Ed is a genius, you know?
And I'm like, "What? You think we can compute the ratio of... Okay." I still don’t know if he was serious or not when he said that. I should ask him if he remembers. Though not the most ardent of the string believers, he was certainly more of one than me. I didn't think it would work ... I think partly because of my experience on the commune, oddly enough. (both laugh) And with China, you know? The commune, we're going to save the world, you know. Communism, China, we're going to save the world. Calabi–Yau spaces, we're going to save-- Okay, not the world, we're-- you know, but maybe physics. So we started working on it and we wrote what is one of my best papers. It's a classic. It was one of the first papers in which the depth of the connection between algebraic geometry and topology and string theory was laid out. However, we did not compute the ratio of the muon mass to the electron mass! You would know about it if we'd done that, okay?
Now, (laughs) Freeman Dyson was at the Institute then, and Freeman and I used to have lunch together every Saturday, all right? I got annoyed with Freeman later with his climate change stuff, but okay he's a great man, and a very wise man. So I told him this story about my conversation with Ed. Freeman was a big critic of string theory. But Freeman always has something surprising to say. And I think when I told him this story, there was a little bit of my trying to impress the great man with "You see, I'm a little more level-headed than Ed is." You know? that I kept my wits about me when everybody else was lost in the frenzy. In fact, Freeman didn't applaud me for being so wise despite my youth. He said just the opposite. He said, "You know, that's just great." He said, "It takes people like Ed who are unreasonably believing and swept up in what they're doing. If you aren't like that, you won't be able to do great science."
It was a very interesting statement. And that's John Schwarz, right?
I mean, John, total believer, you know? And what he did, his perseverance --I'm a huge fan of John's. Huge fan of Ed's, all these people, they're all great scientists, and they all have different styles.
And so John wouldn't have been able to do what he did- of course you probably know the story-if he hadn’t been a believer. John's more of a believer than Ed and when Ed, who's 1000 times smarter than John, or me, or anybody else, wrote a paper in 1983 with Alvarez-Gaume showing that string theory could never give the type of parity violation that we see in the real world. I don't know if you know this.
He wrote a paper which seemed to kill string theory.
And using mathematics that nobody else knew then.
Now, how much was he thinking about QCD for this?
Not at all.
Not at all?
No. He wasn't mainly thinking about QCD. He was definitely thinking about string theory and supergravity. There are some very peculiar structures that occur in our world with the Lee Yang parity violation and so on. He proved that you could never get that kind of structure out of string theory. Of course, it's good that string theory can't give you whatever you want, you know, that it's limited. So he proved that, and John was like, "I don't believe this." Then Mike – also a believer - and John found a loophole. Ed’s proof was mathematically correct, but there was an assumption that was unwarranted. And the Calabi–Yau work drove a sledgehammer into their loophole and showed that not only could string theory give you the kind of parity violation that we observe in the world, but it naturally led to very similar structures. So you need the believers, you need the skeptics, you need everybody. You need everybody.
But Andy, let's get back to the testability thing.
Yeah, let's get back to that. Okay.
The experiment that would sort of be the LIGO for string theory.
Yeah. So, I don't think there is such an experiment we can do in the foreseeable future. And this isn't a new opinion, this has been my opinion for 30, 40 years. More people have come around to it in the last several decades. So we have some puzzles. Quantum mechanics and gravity don't fit together, black holes especially are problematic when you include quantum mechanics. There's Pauli's renormalizability. This is a central puzzle in modern physics. We have to solve it.
But does that pre-suppose the belief that gravity and quantum mechanics should fit together? Does that pre-suppose that there is some larger theory, grand unified theory, where these things should work with each other?
I would argue that our very existence is proof that there is some theory which consistently incorporates quantum mechanics and gravity. Because--
Our existence as sentient beings, or the fact that there's a universe?
The fact that there's a universe.
I am willing to assume that the universe is self-consistent. That's my starting point. I don't think you can get very far if you don't assume that. I'm willing to take the gamble. I'm willing to bet my life’s work on the assumption that the universe is self-consistent. If somebody wants to question that they're free to, but okay, I'm willing to take that bet. So therefore there must be some way that quantum mechanics and gravity fit together. Paradoxes like this have been the central driving force of every major development in physics. The incompatibility of Newtonian gravity -where if you wiggle the sun, the gravitational field is instantaneously transmitted across the universe - violates special relativity in which nothing can go faster than the speed of light. That was Einstein's main input into developing the general theory of relativity. There was no experimental motivation and no obvious experimental connection. Now, it's true in general relativity that weak evidence-there's questions about the data now-but weak evidence was given for general relativity by the Eddington expedition. Interestingly, that relied on the bizarre coincidence that the sun and the moon are almost the same size in the sky. If we didn't have a moon, the first real test would have been 1966, Shapiro time delay. It would have been nearly 50 years to confirmation of general relativity.
So it can happen. Of course it's better if you have experiment, but we want to understand this problem and we don't have an experiment now. I don't think everybody on the planet should be working on it, but it's a central contradiction in modern physics, and I felt it would be fruitful to work on it. Indeed, it has been fruitful.
And fruitful to work on, meaning that with string theory, you're a believer insofar as this potentially could square the circle.
The problem between quantum mechanics and general relativity, it's now more than half a century old. It was first noted by Pauli. It’s a very, very difficult problem. String theory is essentially the only viable on-paper resolution of it. There have been other proposals. Every last one of them has either been proven to be inconsistent, or more interestingly, several of them have been shown to be equivalent to string theory. So, if you're trying to solve a problem and there's one solution there, I'm not saying that that means that is the right solution, but I think it's pretty clear that we're going to learn something by studying that solution. So that's been my point of view. String theory might or might not be right, but we can learn something by studying it. Indeed, in my own line of work, there's this huge puzzle about black holes and quantum information puzzle. This is the puzzle that started with Stephen Hawking's work in the 70s. Nobody knew what to do about it. The field was stuck. Then in the mid-90s, Vafa and I figured out how to embed this problem in string theory where you can really understand everything explicitly. We solved a piece it for some examples in string theory. It was a problem that seemed not to have any solutions. We were backed into a corner. A black hole is a hole in space, where does the information go? The solution was incredibly clever. I don't mean we were clever, I mean string theory produced a clever structure that managed to be consistent from every angle. In one approximation it looks like information is destroyed, but then using another approximation scheme, you can see that the information was not destroyed. And so it gave us ideas about how information preservation might work in the real world. The goal in my view is to understand what's going on in the real world -to use those ideas from string theory as approaches for how the real world might escape the Hawking paradox and to try to resolve it without assuming string theory. That general approach has been very successful in a number of applications, not just for the black hole information paradox. There are major problems in mathematics that were solved this way. String theory gave suggestions for how you might count curves in various algebraic manifolds. This was a problem the mathematicians were working on and interested in, and ideas came out of string theory – particularly my friend Phil Candelas - which weren't fully mathematically rigorous.
But then in the end, all the structures that string theory predicted should be there were proven to be there just using the axiom of mathematics - without assuming the whole structure of string theory. Similarly in condensed matter physics, and in nuclear physics, there had been various insights into the behavior of superconductors, of strange metals and so on that have come by the strange relationships that these subjects have to string theory. So I've used string theory more as a tool, as a jumping-off point. Believing that it's a useful tool is different than believing it is the last word on the fundamental laws of nature. Also, we solved problems in quantum field theory, and [the] large N limit. This is a problem that ‘tHooft had posed in the early 70’s, what is the large N limit of Yang-Mills theory? This problem was solved by Juan Maldacena.
Andy, let me ask the question of testability going the other way, right? So you've stated that there might not be an experiment to prove string theory, but as you probably know, the breakthrough prize was just awarded to the experimental group at the University of Washington for proving that Fishbach's fifth force theory was not true, right? So is there an experiment that you can conceive of that could disprove string theory?
I guess I am not getting my point across.
You're saying that string theory is totally outside the world of experimentation.
Okay. There’s several different points here. There are mathematical truths about a wide variety of quantum systems that we've discovered from studying string theory. Not quantum systems in the real world, but models. Those are mathematical truths. You can prove them. Those are interesting in and of their own independently of whether, if we get a really powerful microscope, we will see little strings, we're about 20 orders of magnitude away from having the kind of precision that we would need to do an experiment that would disprove string theory for sure. Okay? I don't think that in my lifetime we will do it. Now, Bose-Einstein condensation, when Bose and Einstein discovered it, was similarly, about 20 orders of magnitude away from being measurable. As you know, it was measured, and the Nobel Prize was given for it. Bose and Einstein could not have conceived of the experimental apparatus that was used to measure it. It could happen that unforeseen things in 50 years or 100 years will make string theory measurable. But even if it were disproved as the fundamental description of nature, we've learned something from string theory about possible structures relating quantum mechanical systems that have been useful.
So yes, I don't think - not many string theorists will talk this way - but I don't think that we are in my lifetime -- and I'm planning to live a very long time -- going to get direct experimental evidence for string theory. I'm nevertheless interested in string theory because I think it is giving crucial hints for – what is one of my major goals in the rest of my life -- understanding black hole information. How it's stored in a black hole, how it goes in and out. Black holes are physical objects we can see. It’s not guaranteed that we can understand black holes without understanding physics at the Planck scale. But it seems like we should be able to, because they're big objects that we see and the paradox was derived using low-energy reasoning. So I'm putting my eggs in that basket. That we can understand black holes without knowing the laws of physics at the Planck scale.
Because of both theoretical and experimental advances?
I don't think they're going to be directly useful experimental advances for this problem.
We’ve only just seen a black hole directly for the first time.
At the Event Horizon telescope. To see the Hawking radiation coming off the black hole would be more than 40 orders of magnitude improvement. The event horizon telescope, I think it was three orders of magnitude improvement and it was a spectacular feat. So I don't think we're going to get the next 40 orders of magnitude soon.
And to extrapolate from Einstein to the present, it sounds like in your view, technological improvement does not necessarily move forward exponentially? In other words, to get to 40 orders of magnitude, it might take another century.
Let me give you another example, actually maybe a better one. Boltzmann. I love Boltzmann's work. So before Boltzmann there were whole books full of laws of physics. Every rate of change of temperature as a function of how much heat you apply, of every different material on the planet, was a different law of physics. They were the laws of thermodynamics. And he reduced all of them-- I mean, nobody has ever in the history of science had such a cleanup, you know? -- he reduced all of them to kind of nothing. They all followed from statistical reasoning. But, he had to make an assumption. He had to assume that all matter was made out of molecules. At the time that was an outrageous assumption. People didn't really believe in molecules at the time. Historians of science tell me, that people didn't really believe in molecules until Brownian motion was seen under the microscope. Boltzmann was already dead then. He went to his deathbed with few people accepting what he had done. So that's another example of how pure reasoning that can lead you to a deeper understanding. He wasn't helped by any experiment. Brownian motion would have helped him. But he had a very mathematically constrained problem. I'm guessing - but we won't know until the answer is there -- I'm guessing that we're in a similar situation. I could be wrong. We all have to gamble, you know? My gamble is to study black holes and string theory, understand how they work, and see if we can derive a similar structure in the real world without assuming anything detailed about what goes on at ultra-short distances that we can't measure. Which is just what Boltzmann did.
What is this connection between string theory and black holes? So many string theorists have moved into black hole research. Speak for yourself, but also for the field. How do you understand those connections? Those through-lines?
String theory is supposed to be a quantum theory of gravity. And black holes are the place where quantum mechanics and gravity are both very important. It’s a paradox we have to solve to see that the universe is consistent. I'm interested more generally in understanding the laws of physics at the most fundamental possible level, and have always felt that the black hole paradox is going to be our window into the next layer of understanding about the structure of the universe -- our equivalence principle. Indeed, though the paradox has not been solved, we have learned a lot along the pursuit. Up until the 90s, everything in string theory had almost nothing to do with black holes, it was all just perturbation theory. Weak fields. Gary Horowitz and I discovered the brane solutions of string theory while trying to understand black holes. All the developments around holography and the 80th [inaudible], we were driven to that, in the course of trying to understand how a black hole works in string theory.
This has happened over and over again in physics. When you have an actual physical question you have to solve, it forces you to think about things in new ways. I had another recent experience like this. I've been interacting a lot with the people who built the Event Horizon Telescope. When they saw their image, they were like, "Okay, what is this?" They were so busy carrying suitcases of data and atomic clocks around, performing magic feats of synchronizing telescopes around the world, they just really didn’t much time to think about the science of the black holes. They were asking, ‘What does this image mean? What can we learn from it? How should we arrange our telescopes next? Whatever.
So I started thinking about their questions. I don't know if you know but I wrote a paper with them recently which I'm very proud of. One of the amazing things about that experience was, it wasn't just me helping them with their general relativity homework. They glimpsed some very, very striking qualitative behavior of black holes. The whole photon ring story with the black hole mirrors. I had just really never thought about it carefully until they presented me with the question, "Tell us what we should see in our image and what are the interesting structures in our image? And how can we dig anything of interest out of this image with all the “messy astrophysics" - that was their word- the messy astrophysics of the junk swirling around of the back hole? How can we learn about general relativity? How do we get that out?" Now, it turns out that there's an answer to their question which is much more beautiful and intricate than I had initially hoped and has led to a new experimental proposal involving telescopes in space. You’ve got to have something to focus on. For me, already during my graduate career, my focus has been understanding how quantum mechanical black holes work. You can't just say, "I want to understand the world--" you need something a little more precise, but you also don't want to get too precise. Then it becomes uninteresting.
So I'm very, very happy about my choice. It's like the best decision of my career. Getting back to your earlier question, I would be very happy if we could do an experiment which would prove or disprove the string theory. But there’s this false idea out there which I have trouble shaking from people: That string theory is either right or wrong. I don't think it's either right or wrong. I think it can be partially right and partially wrong, and is almost certain to be neither completely right nor completely wrong.
Andy, not to say that this is an unscientific assertion, but would you accept that if this is the case, it puts string theory in a unique category in the realm of science?
No. No. Okay, so let me give you my example.
Yang and Mills invented Yang-Mills theory to explain the relationship between the proton and the neutron. That was wrong. But nobody would say Yang-Mills theory is “wrong”. The structure that relates the proton and the neutron is an echo of other structures, and Yang-Mills theory does correctly describe those other structures. So they weren't that far off the mark. Yang-Mills theory found its place, but their original proposal was neither completely right or completely wrong. So that's an example. And this is exactly what Glashow said to me: “It's clear you guys are onto something, it's just not clear what it is.”
And I very much agree with Glashow.
But Glashow took it so far as to say maybe we shouldn't even have string theorists on the faculty.
That was before they hired me.
He hired me, and I think he was impressed by what we'd done with black holes. That quote was before the work on black holes, [and] he might not have said it after that.
Because you see string theory solved problems that other people had posed. It solved a lot of problems in mathematics. Mathematicians love string theory. It didn’t solve, but it certainly totally redefined the field that Hawking started with his black hole information puzzle. Some condensed matter theorists like it. But I think it's extremely unlikely -and I say this as a co-discoverer of these things - that the world around us is a certain Calabi–Yau compactification of string theory with a gluino condensate. It would just be too weird if in 1985 we took a shot in the dark and hit the bullseye. That's like throwing a baseball off the Empire State Building and getting it into a beer mug on 34th st-- It just can't happen.
The other thing I want to say about string theory is that, while we still call it string theory, the name is an odd historical persistence. The theory we're now calling string theory, first of all it's not a theory of strings. The strings are emergent phenomena in some phases of the theory. So if you did some amazing experiment and found that we were in phase of string theory that didn't have strings, well that would still be a success of string theory. I guess. I don't know what you want to call it. There’s an inevitability of the structure that we've found. It's amazing that everything we've come up with in the last 50 years fits into one framework. So it would be very bizarre if one day when we finally understand what goes on at the Planck scale, that the real truth is an island that is unconnected with all the ideas of the last 50 years. It could happen. For the last 30 years, everything new that we've discovered, as long as we can relate it to the ideas in string theory, we call it string theory. So, if we continue to call everything that we discover string theory, it's virtually certain that-- (both laugh) It's certain that when we get to the answer, we'll call it string theory! So that's part of what I mean when I say string theory doesn't need to be right or wrong, it can be somewhere in between. It can be that some of the current ideas are important and others aren't. There could be some twists. I mean, a lot can happen experimentally in 50 years. And a lot can happen theoretically in 50 years.
I see now how well your formative idea of not really being a believer works so well for you in string theory.
Yeah, yeah, yeah. There are people out there who will advocate the point of view- I always disagree with them but they're advocating what to me seems hopelessly simpleminded - that the world we live in is some souped-up Calabi–Yau compactification of ten-dimensional string theory. And by the way, I'm very proud of that work. I don't mean to be denigrating my own work. It gave us ideas of how things might fit together. I don't think it was an unwanted detour. I think it was a passageway into other things rather than an end.
Andy, let's get back to the narrative. Back to Harvard as an undergraduate. The decision that what was right for you would be to pursue theoretical physics, right? We need to flesh that out a little bit, because it does not compute up to now in terms of the narrative. You're back from China, you have all kinds of ideas, and then what happens for you?
I'm back from China. I'm not really integrated into Harvard. My friends are all strange Chinese people or people from the commune or ex-hippies, or you know. And then I went and lived in the dorms. And made some really great, lifelong friends then. And I didn't know what I was most interested in-- I also went - we skipped over that - I went through a religious Buddhist meditating phase-- everybody had to do that in the 60s. (laughs) So I'd been searching around in all kinds of things. There was a lot of feeling of-- people were more socially minded then - wanting to change the society and try to make a more just world. But I didn't know what to do. I didn't have a clear way to contribute... And I loved physics, and I knew I could do it.
Who were some professors that you might have connected with that helped you along this path as an undergraduate?
Well, my advisor was Bob Pound. And he said to me, "You're not smart enough to be a theoretical physicist. Theoretical physicists don't waste two years living on communes and running around in China." (laughs)
But you were ahead of the curve. You were a 15-year-old. It's not like you were 25 as a sophomore.
Well, but if I had only been interested in physics, I could have finished Harvard at 18. I didn't do that. I have no regrets about that. Then I had some amazingly smart roommates who were in physics, Paul Ginsparg and Mark Robbins, who sadly just died. They had taken a lot more physics courses than me and gotten A+s and everything, Phi Beta Kappa and so on and so forth. But I knew I could do it, and I knew I would love it. What I did that year, my last year, was I worked really very hard, in an experimental group. I worked for Rubbia and Sessoms, who was a junior faculty member. I built a liquid argon calorimeter for Fermilab. It was a big success. I loved it, it was a lot of fun. Experimental physicists have a lot of fun.
Yeah. And between being told that you weren't smart enough for theory and realizing you were good in experimentation, it's curious that you did go back to theory.
Well, I worked in an experimental lab because there's no such thing as a theoretical lab.
I had the idea that, which is laudable, that to do physics, I should understand experiment too. And it's been good that I understand experiment. But I think that was just at a time when experimentalists began to sort of become victims of their own success. Everything in the standard model has been beautifully verified experimentally, and the next layer of things we've liked to understand have become exponentially harder to get at. That was probably just starting to be true when I was in college -- the W and the Z hadn't yet been discovered. There was a lot of interaction between theorists and experimentalists then, and there's less now-- Anyway, I wanted to learn about experiment and you could work in experimental lab as an undergraduate. There's not much a theorist can have an undergraduate doing. So I loved experiment, but there was never any question in my mind that I wanted to do theory.
Did you have a senior thesis?
I didn't. I published a couple papers on the liquid argon calorimeter. I don't think people wrote senior theses then.
I guess my question, though, is by the time at the end of your undergraduate experience, in terms of your identity as a physicist, had you at that point gravitated enough back toward theory that you knew that this is what you would pursue for graduate school?
I knew before I started working on the calorimeter experiment that I was going to go into theory.
And Andy, I have to ask, normally to go on to achieve what you did would require a certain amount of focus and putting away any ongoing desires you'd have to, you know, do things like go live on a commune or drop everything and go to China. Did you have any sort of moment where you said –
Wait, wait, wait. There was a moment when I came back from China. I'm like, okay, when I came back from China, I was already 20. And I'm like, okay, we all live under this fear-- I now realize it's a false fear - that the life of a theoretical physicist is over at 30, right? It's just a myth. But I was like, "Okay, if you're going to do theoretical physics, you'd better get going."
Right. That's my question. Bear down. Do you sort of self-consciously say--
"I'm going to do theory."
So I saw the working with an experimental group as part of doing theory.
Yeah. But I mean just in terms of your maturity, are you committed to sort of not taking flights of fancy anymore and going off and doing crazy things? Do you sort of at any point have that conversation with yourself, that now's the time to bear down and work exclusively in this realm?
Well, I think I did, because I had had such an interesting period from...
It's a pretty full life you lived up to age 20.
I had lived a very full life up to age 20, yeah. And I felt some sadness about what I was giving up. But you have to give things up in order to get other things, and I was really very, very focused. I really, really worked hard.
And in terms of your focus in applying to graduate school, did you think about professors specifically that you wanted to work with? How did you go about choosing the schools that you applied to?
Okay, so that is a very tricky business.
Particularly because at a place like Harvard, there's very much the mentality, "You're at Harvard. Why go anywhere else?"
No, not really, because in general, there's a feeling that you should go to a different place for graduate school. And--
That was the advice you were given?
That's generally the advice. I went around and got advice. But by the way I didn't really see myself as going to work with a professor. It was more I was going to do it myself. What happened was I got some advice that I should go to Berkeley. That was the standard advice, I think, at that time. Berkeley had some crazy, like eight Nobel laureates on the faculty, and that was the place you were supposed to go. You don't always get good advice about these things. And yes, there were eight Nobel laureates, but they were all semi- retired or whatever, and I got out there and there wasn't at that time - departments have ups and downs - there wasn't at that time any good candidate for a thesis advisor in theory. And –
However, why should that have stopped you so much if you went in not really wanting to work with anybody in a mentor capacity?
Well, you have to have somebody who officially supervises your thesis. But at that time in theory at Berkeley they were all kind of a little depressed. And somebody, I won't mention heir name, but they gave a talk to the incoming theory students, and he said, "I haven't taken on a student in 12 years. I wouldn't do it, anybody. Don't be a theoretical physicist unless you think you're Einstein." And this is just after my undergraduate advisor Pound telling me I wasn't smart enough to be a theoretical physicist. But I was pretty stubborn and determined, you know? Nothing was going to stop me. So then I was back to Boston visiting family and I went into MIT and I talked to-- Oh, actually, first I dropped in at Harvard and I talked to Shelley. And I said, "Is Harvard a good place to do a degree in theory?" And he said, "I don't think so." (laughs)
Do you think he was saying, "I don't think so" because it's no longer 1974 with Georgi and Weinberg and all of that?
Well, it wasn't that much after 1974. - we're talking now 1980 or '79 maybe. No, it was because at that time there was kind of a culture of the professors not paying any attentions to the graduate students. The graduate students had just written an open letter to the community telling them that Harvard wasn't a good place to be a graduate student. So that was a little strange. They didn't want to recruit anybody, and plus, you know, I wasn't like a theory star then. People thought I could be a good experimentalist. So then I went to MIT, somebody had told me that Roman Jackiw was a great thesis advisor. And he was.
Yeah, I've heard that many, many times.
Yeah. So he wasn't cuddly with his graduate students, you know?
He was really tough with them.
Maybe, Andy, he broke through with you because between being told you were not smart enough to do theory, you might have had a chip on your shoulder, and that's sort of what motivated you not to work with a particular graduate advisor. Maybe he broke through on that regard?
Well, so anyway, I walked into his office, and I wasn't even an MIT student. I walked into his office, and I said, "Are you interested in taking on students?" And he said, "Yeah. Sure. I'll give you a problem now." I mean, he hadn't looked at my CV, anything. He just like said, "Yeah, sure."
So you're in the office, he gives you a problem right on the spot.
Yeah. I told you about the colloquium or not?
Then I started working on quantum gravity. I wrote a couple papers as a student on my own. He told me not to work on quantum gravity saying I would never get a job. And then many years later, when he introduced me for an MIT colloquium, he said, "I told Andy not to work on quantum gravity when he was a student. Good thing he didn't listen to me." (both laugh) So yeah, he's a wonderful guy.
Yeah. And you learned how to be a mentee.
I learned how to be a mentee, but he was very... there's something very down to earth about the way he does physics. It was never vague, and he had just very good problems. I think he is really a great, great physicist. Really a master of Yang-Mills theory.
How closely was he involved in the development of your dissertation topic?
Well, so I'd say about half the stuff I did, I guess I wrote, how many papers did I write? I don't know. Half a dozen maybe? And maybe half of them were on quantum gravity. Which was the half I was more interested in, and those were all my own ideas. And then the other half were problems he gave me. And that was my official thesis. He didn't want to count the quantum gravity ones. So the thesis was the QCD stuff.
What did you look-- what were your principle conclusions in your dissertation?
There were two main results. One is I solved what were called the loop equations of QCD in two dimensions in the large N limit. I did a lot of stuff on large N, which of course came back much later. And then I also solved quantum gravity in the large N limit, which was something nobody had even tried to do before. In Yang-Mills theory N is the number of colors, and the field of Yang-Mills theory is an N by N color matrix. Also, in gravity, the field is an N by N matrix where N is the number of dimensions of space time. In Yang-Mills theory, the large N limit turned out to be to capture a lot of the, or even most of the, important physics. Turned out to be incredibly interesting. But in gravity, the large N limit, even though I solved it mathematically, it turned out not to really capture what you wanted to capture. So it turned out not to be.-- I mean, it was a good work for a graduate student-- but it turned out not to be especially important in future developments of the field.
What did you want to do after you've defended?
Then I was working on quantum gravity essentially full time.
And there was a lot of discussion then of quantum gravity with extra terms. Higher derivative quantum gravity. This was kind of related to my large N work. I spent two or three years working on that and it's probably one of the least memorable phases of my career. It's not the right way to think about-- the idea was sort of to understand gravity as a renormalizable field theory by changing the dynamics a little bit, and okay. You don't want a lecture on it now. But it was not the right direction.
And were you working mostly on your own? You were collaborating with people?
Yeah, so I went down to the Institute for Advanced Study, and--
This was as a postdoc, you were there?
Yeah, as a postdoc. And at that time, the Institute, I remember Steve Adler saying, "We like to have people working on different things. We'd like to have somebody working on quantum gravity.” It was sort of a side thing that people thought about every once in a while. At that time, of course, John Schwarz had already proposed string theory as a solution to the problem of quantum gravity in, I don't know, '72 or something.
And so now we're talking, when I went to the Institute, that was '82.
This is the beginning of the second revolution.
Well, nobody was working on it in '82.
Well right, but this is like the origins of it.
Yeah, yeah. I started thinking "Okay, I have to understand this" even though I was very skeptical of string theory. Not because of the problem of experiment. I was already acclimated at this point to the idea that quantum gravity was a problem where experiment was not going to--
Yeah. And also, I mean as early as 1974, QCD had essentially killed off at least the first iteration of string theory.
And this was obviously the world that you were immersed in up until that point.
Right. So I thought of string theory as contrived. And there was another thing at that time, which is the people who developed string theory, they weren't general relativists. They didn't understand general relativity that well. It was approached from the point of view of a particle physicist. At that point I had already learned a lot of, and loved, general relativity, and I wanted a more geometric approach to quantum gravity. Which string theory eventually became, but not for another decade. And it's an odd thing, that string theory in its early days was all populated by particle physicists who weren't interested in things like black holes. I wrote some papers on classical general relativity during that period. For quantum gravity, I was looking for something conceptually new, some kind of different concept, you know, like the equivalence principle. String theory wasn't like that at all, you know? It was back then just a bunch of formulas. Here's a bunch of formulas where we're going to take particles aand replace them with strings-- it just all seemed bizarre. So I thought, okay, but they claim to have solved this problem I'm trying to solve. I should understand what they're doing at least. So I started studying it and somewhere around that time, 1983, I read their papers. Then Mike Green was visiting the Institute and they put him in the desk next to me for a week.
A strategic move, perhaps.
No, it was an accident, I'm sure. They had this light cone gauge and just the formulas just went on and on and I just wanted the new equivalence principle or uncertainty principle-- What is the conceptual thing? And I remember saying to him, "But isn't it just really ugly? All this tower of fields and these light-- isn't it just really ugly?" And he said, "Oh no. It's beautiful." (laughs) And you see, the thing about those guys, and this happens often, I mean there is no question that once you understand it, the mathematical structure of string theory is insanely beautiful. Of course, I eventually came to agree with Mike.
That's what I--
And there's the John Schwarz thing where the theory is smarter than we are.
Yeah, I would agree with that. The theory is-- I mean, the theory led us by the nose, or led me by the nose, to computing the black hole entropy. I'm like, okay, because now we understand string theory well enough, I can do this computation. If it doesn't give the right answer, string theory is out. But it did. The way that it gave the right answer on the first pass-through seemed insanely complicated. Only later did we start to realize that there were simpler and simpler and simpler ways to do the calculation. It kind of led us by the nose to this answer, and yeah, it is smarter than we are... It's sort of like a game. All along the way, there have been many, many places where string theory could have died. The conifold singularity was one that I'd worked on earlier that seemed to be an inconsistency in string theory. How is it ever going to get out of this corner? Violations of cosmic censorship. And then it was just like, sit back and, follow the math and sit back and watch this structure that comes out. So that's been kind of a repeating strategy I've used over the years is to look for a place where string theory looks like it's going to fail, and then just really shake it down and every, every single time it wriggles out in a way that just never could have been guessed.
And it sounds like you weren't even rooting for that. You were rooting for the other way, and that's actually just how it played out?
Well, by that time I'd been long convinced it has some truth in it. I wasn't trying to kill it then. Anyway, after I'd first studied string theory, this was like 1983, it was a year later when I did the work on Calabi–Yau compactification. So I had a head start by almost everybody else by a couple of years. Of course, there were Mike and John and all those people, but there were very few people who had bothered to understand what they were saying. That served me well. Yeah.
These were very productive years for you at the institute?
Well, you know, yes and no. I was there for six or seven years. The first one or two years I was working on the higher derivative gravity which went nowhere. Then I did the string compactification, which was a great success. But then my last year or two at the institute, I worked on string field theory which also wasn't great. So, about half my years there were fantastic, and the other half were only OK.
How much were you publishing during your time at the Institute?
I've been pretty constant - like eight papers a year my whole career.
Yeah. And was the Institute particularly good just for being in that intellectual environment where you could just hang out with people and bounce ideas off of people? Was that important to you personally?
Well, the Institute was... I had a lot of collaborators from around the world during that period. And I traveled a lot. You know, most of the time-- well, it changed. My first half of my time in the institute, nobody was doing anything or was interested in the area I was working on. The second half, everybody was interested in it.
And is Ed Witten the result of that transformation, would you say?
Well, Ed was not at the Institute while I was there. He was at the university. When I was at the Institute, it was a very unusual period. Steve Adler and Roger Dashen and Freeman Dyson were the senior faculty members, but none of them were sort of very active in high energy theory at that time. And there was a kind of incredible group of what they called a long-term members who were very active. There was Nati Seiberg, Greg Moore, Steve Wolfram, Mike Dine and me. The kids were running the place. That was good. We had a lot of intellectual freedom. So that worked well. It's a little different at the Institute now, but it's still a fantastic place.
And at what point were you sort of on the job market? Were you just hanging out and a recruitment offer came your way? Or how did that play out for you?
Yeah, I was hanging out and a recruitment offer appeared. Already in graduate school, my advisor had taken a sabbatical at what was then the ITP in Santa Barbara, and I'd gone with him for a year. I'd fallen in love with Santa Barbara, and that was where I wanted to go.
Yeah, but at that point, Santa Barbara is rapidly on the rise. I mean the ITP is about to become the KITP and it's... Exciting things are happening at that point.
They were starting to happen, and soon after I went out there, I became a principle investigator at the ITP and was--
Now, was David Gross there already?
I recruited David and then moved to Harvard a year later- something he's never forgiven me for. I recruited David, I recruited Joe, I recruited Steve Giddings. So I'll definitely take some credit for having built up Santa Barbara in my area!
It was probably also nice to get away from the Northeast. Santa Barbara's a pretty good place to be.
Yeah. I loved it out there and I'm a big hiker. I did a lot of hiking and I had little kids then and it was a great place for little kids.
When were you married? We sort of skipped over that.
Yeah, so I got married in 1985.
And how long were you in Santa Barbara?
I was in Santa Barbara for, let's see... It depends exactly how you count it, but let's say 12 years.
Mmhmm. So you were a full professor by the time you left?
Oh yeah, yeah.
And then how did Harvard come together?
Well, they called and offered me a job. I think I felt... I wanted to... Santa Barbara is a little bit of a small world there. And it was fine for me. I liked to hike, I had my colleagues, but otherwise socially it was very strange. A lot of rich people there, a lot of... you know, it feels culturally isolated.
Yeah. This is a comment about Santa Barbara, not about the university or the ITP.
No, about Santa Barbara, yeah.
And I always loved Boston. I mean, I had a lot of other job offers too, and we thought about moving other places, but Boston's like a really nice place to live.
Sure. And it's almost like you were coming home.
Oh, it was very much like I was. Which cut both ways.
(laughs) Which cut both ways. I mean I loved my parents, but... Yeah, it cut both ways. Mostly plus, but there were also some minuses there...
And Andy, during your 12 years at Santa Barbara, what were your major research achievements and specifically, what might you have been doing that attracted Harvard to recruit you?
Well, I mean, I guess there are several different things. Already the work on Calabi–Yau compactification had made an impact. I followed that subject partway into its many mathematical implications, including a conjecture with Yau and Zaslow which is a subject in mathematics now. So I had done that - it wasn't just the discovery of the Calabi–Yau compactification, but developing the connection between string compactification and algebraic geometry and various developments around that, which I think have probably been more important in mathematics than in physics. I think the Calabi–Yau, … what we showed and remains true is that string theory could in principle describe the real world. That success remains...there was also non-perturbative string theory and black holes...but I don't really want to be quoted or say why it is that everybody was offering me jobs!
Andy, we talked about black holes thematically but not chronologically. When did you start really working on black hole entropy?
Well, I was thinking about it in graduate school.
Did you know Stephen Hawking? Did you ever meet him?
Yeah. We were good friends. I guess we met in 1982. I was collaborating with him. We wrote a paper together. He visited me a couple of times at Harvard and I visited him many times in Cambridge. The last four or five years of his life, we were meeting two or three times a year. We wrote a bunch of papers together. I wrote an obituary for him you can find.
And the basis of the collaboration, what did he bring to the table and what did you bring to the table?
Well, so Stephen sort of drew a line. I think he liked string theory. But he didn't work on it. And certainly he was persuaded by our computation of his entropy - that was what caused him to concede his bet about whether or not information was lost. And he took all of that very seriously. He liked (AdS-CFT 2:17:59). Okay, I don't know how much of it you want to hear. Anyway, I made some observations and had a program in which I was trying to abstract ideas from string theory and see if we could find similar structures in the real world. There was some work that had been done in the 60s by Bondi, Metzner, and Sachs which was-- Have you heard of that? You're nodding.
Okay. So the general relativity community never made sense of that work. They didn't have a coherent picture. About seven years ago, I showed that it was the same thing as Weinberg's soft graviton theorem. And this was entree into a whole world of new structures that govern general relativity. Bondi, Metzner, and Sachs were missing some pieces, and they didn't understand that there was an actual symmetry of the scattering problem. I put it all together, and it turned out that it has enormous implications for the problem of black hole information –
Yeah, can you talk about that? What are those implications?
Let me first answer your other question. So anyway, I'd been going on retreats with Stephen for the last 15 years of his life, he had a retreat every year, which the Texas billionaire George Mitchell organized at his ranch out in Texas. It was Stephen and his students and a few people he liked to talk to. I was one of those. At one of these retreats, I talked about my observations and explained that I thought it might be relevant to black hole information. Well, first of all I showed that there was an error in his famous 1974 paper, a false assumption. And nobody had ever done that before.
What was it? What was the false assumption?
You could describe the false assumption in different ways, but one way of stating it is that “the vacuum is unique in general relativity”. In fact, the vacuum is not unique in general relativity. There are infinitely many degenerate vacua. That's what the Bondi, Metzner, and Sachs and Weinberg were really saying, though they themselves didn't appreciate this point. And the fact that they didn't understand what they were saying led them to say other wrong things and so on and so forth. But I sorted it all out. I think it is now widely accepted that what I'm saying is a correct perspective-that the vacuum in general relativity is infinitely degenerate. Different people have different opinions about how important that is, but that fact itself is not, I don't think, in question.
In terms of what? What are the theoretical implications, you know, of judging how important that question is?
Well, you have to do something with the fact to show it is important. Let me answer your question in a bit. So the vacuum degeneracy is related to a symmetry and a conservation law. There are in fact infinitely many conservation laws in nature, and they constrain the process of black hole evaporation. So there is an error in Hawking's original argument, as well as an observation that the conservation laws must constrain the process of black hole information. We know for sure that they imply that the radiation carries more information than you would have otherwise thought. The question is whether this is a small irrelevant side effect or the whole important effect. I gave a talk on this at a retreat and Stephen was like, "This is it." You know, "It will solve the problem." We started working together with Malcolm Perry immediately. He announced to CNN six months later that we'd solved the information problem and that the details would be in a paper shortly. (laughs) Stephen's a believer. Okay? So he believed in this. Now, I'd started it and I didn't believe it at the same level that (laughs) that he did. So I had to walk things back a bit... Anyway, when was this? This was maybe four years ago. 2016, yeah, maybe four years ago. I'm very excited about these developments.
But it's a program. There's a lot to do. It’s a way of trying to understand black holes which is very much in the spirit of Boltzmann’s understanding of thermodynamics. I'm not assuming an underlying string theory, yet my long experience with string theory has given me some sense of where the gold lies. Science isn't a science. Science is an art. You have to guess what the right path to take is, and I've guessed wrong and I've guessed right in my career. I'm not 100%. I feel like I guessed right on this one. Certainly, I've found some truths about how to think about BMS and soft theorems. It’s a big and growing story. We've already had a lot of interesting things to say about infrared divergences. People already like it, but the really big prize would be if this can solve black hole information or another, not-unrelated problem: we have this holographic principle in anti-de Sitter space. Do you know what that is, or?
Yeah, okay. So that's great, but we don't live in an anti-de Sitter space. We'd like to understand if there could be anything like this holographic principle in the world we live in. And the first indications of the so-called anti-de Sitter/conformal field theory holography, actually were from a bottom-up calculation by Brown and Hanal in the late 80s. They didn't assume string theory or anything like that, yet they discovered that there was this symmetry group. Much less concrete than what was done later on by Maldacena. But also, fewer assumptions. So, there is such a thing as a bottom-up approach to holography. The price you pay for insisting on only discussing structures that describe the real world is inevitably going to be that you don't have the same kind of mathematical control.
So string theorists are sometimes a bit addicted to perfect mathematical control -- maybe not all of them. If you don't have perfect mathematical control, if you can't say anything rigorously, then why are you talking about it? Well, if you're going to talk about the real world, you're going to be on flimsier mathematical footing. Is there a sweet spot in there where we can still say interesting things about the real world? With less than complete mathematical control but still non-trivial and interesting? My hope is that this world of structures that we've only recently uncovered, that have their ancient origins in BMS symmetry and soft theorems together with the analogies of how anti-de Sitter space works can be used to understand features of the world that we live in.
Andy, I wonder if you can talk from a more general level in terms of, how does your entire research agenda, how is that geared towards improving the standard model? Getting beyond the standard model?
Well, the standard model scale and the Planck scale are very far apart - now of course there are possibilities that the Planck scale is much lower than we think. Okay, that would be incredibly interesting. There's no reason, though, to expect it to be so. I don't want to pin my hopes on that. It's a good thing to think about from time to time. But Occam's razor would suggest that the Planck scale is the Planck scale and far from the standard model scale – not just beyond.
Okay, now there's presumably lots of physics between where we've tested the standard model and the Planck scale. I don't think we're going to learn too much experimentally about this physics in the near future. Okay, sorry, I don't mean that. We're going to learn stuff experimentally. We might discover dark matter. We might learn more about neutrinos. We could find some beyond the standard model particles. That's all great science. It's not what I decided to work on. And I don't--
Just because you thought that other people were better suited to focus on those questions?
Yeah, it's not that I've never thought about them. I've taken stabs at it from time to time. I think it's super important. I don't know if I thought that other people were better suited, I just really love this giant problem of reconciling quantum mechanics and gravity and it's exciting to me that... so it's a trade-off, right? You have to guess. Surely the problem of how quantum mechanics and black holes and general relativity are reconciled is a bigger, deeper problem than what is the next layer of physics just beyond the standard model. On the other hand, the prospects of learning something very concrete about what the next layer of physics are just beyond the standard model are greater. So we have to choose. Well, we don't have to choose. Some people work on both things. I've from time to time tried to do something in beyond the standard model physics. I've never hit pay dirt in that direction. I don't spend a lot of time thinking about it. I've felt like I was on a roll on the thing I'd been doing. So I'm happy with my choices.
Andy, I want to ask, not specifically about your graduate students, but as you looked to the up-and-coming generation of physicists, in what ways do you see that the questions that they are focusing on, sort of in generational terms. In other words, the kinds of things that would occupy a career over their next 40 or 50 years. What are some of the major trends that you see in the field, and what do you think is most promising and what are you most personally excited about for the next generation of physicists?
Well, I think that there's a good chance that we'll come to a completely satisfactory solution of the black hole information problem. Actually, at this point, we don't even know what a completely satisfactory solution means.
Hopefully you'd know it when you saw it. But I don't know if you could define in advance what exactly the features would be. Of course, there are many claims out there that it's already been solved, but those claims generally aren't accepted by anybody other than the person who made them. It’s not that it's science by vote, but a compelling solution will convince the critics. I'm very excited that we're entering an era where all of a sudden, we can see black holes in the sky. It’s not that I think we're going to see the Hawking radiation. But already we have found that just engaging with the observers has made us think about black holes in ways we never did before. We’re asking different questions. And it's leading to the uncovering of different kinds of structures: black holes are full of all kinds of crazy, miraculous structures. Miracles that nobody understands. In retrospect, it's obvious that if you look for a spherically symmetrical solution, which Schwarzschild did, that you can find the solution in closed form. It took Schwarzschild a month or two once he set his mind to it. But the Kerr solution, the rotating one, which is the one that describes real black holes, that took 50 years to find. The solution can be written down on one piece of paper: it just involves some very complicated set of sines and cosines and you can write it all out. Of course, I think there was a reason you had to be able to put it on a computer and solve it, but there was no reason that the solution had to be expressible in terms of elementary functions as it is. My mathematician friend Yau once said to me that he thinks it's the most nontrivial, exact solution to a nonlinear differential equation that had ever been found at the time.
Yeah. It's a miracle. And we don't understand why that miracle should have existed. And there are more miracles like that. Black holes keep regurgitating more and more miracles every year. For 100 years they've been doing this. There’s a lot that we don't understand about them. They are so fun. And mysterious. And now we are entering the era of precision black hole observation. LIGO and the Event Horizon Telescope, they're just the beginning.
Yeah, that's right.
I think that is going to be a very fruitful interface. It's going to be inspirational to pure theorists, even if it doesn't directly measure the Hawking radiation. I think that's an exciting area. But you know there are thousands of theorists out there, and everybody has a different opinion. I'm not a sage that can see the future. We've never been short of interesting questions.
That's profoundly exciting to hear you say. It really is. Andy, I want to ask you, I want to switch gears a little bit. And I want to ask you, in light of going back to this idea that you're not really a believer and you had this incredible early part of your life. I'm curious what level of appetite you might have for metaphysical concepts. And if you think much about those kinds of things.
Well, I did as a teenager. I thought about that kind of stuff a lot, and I wanted to understand the meaning of, you know, as every teenager does, to understand the meaning of life and why we were here and all that kind of stuff. And then I had to face the hard, cold reality that I wouldn't be able to figure it out. Didn't think I could do that. That is what led me to where I am. I started from there. I asked myself, what are the deepest truths about the universe that I might hope to make even some incremental progress towards uncovering in my lifetime? That is, what is the deepest question that one could ask that I might reasonably hope to chip away at? Everybody has a different take on what "deep" means, but what it meant to me is, where that landed me, was quantum mechanics and general relativity. How do they fit together? Now that's a very far cry from what's the meaning of life. There are so many questions about the universe that we live in that we can't hope to answer, but it's really kind of amazing that we can answer some of them.
But in your answer, you do seem to be limiting yourself. And again, I'm not asking, not in your professional, scholarly capacity, but just in terms of as a fundamentally curious person. In asking the deepest questions that are available to answer or investigate, do you accept that there might be some metaphysical bases for how the universe works that might be beyond the world of investigation, that are not knowable?
Right. Yeah. I like to think about those things. I'd like to know their answers. But I also don't want to be on a fool's errand. There's things that are way more basic than what I'm thinking about. Like why is there a universe at all? There's lots of far more fundamental questions than the ones I hope to answer. But then there's some that are just a little bit more fundamental, like are there other universes? And okay, I think intelligent people can disagree with me (laughs) but my view is that that is also a fool's errand. That we're never going to know if there are other universes-- I don't see what the tools are for all this discussion about the multiverse. People love it, but I just don't think that that is a question that we can definitively answer. You know, I actually secretly believe there are other universes. Why shouldn't there be? Believe is too strong a word. I'm not a believer. But if somebody said, " I'm putting a gun to your head. I found a magic way to answer this question. What do you think? Are there other universes or not?" Yeah, I mean why not? Why shouldn't there be? Occam's razor seems to suggest the answer that question is yes. But to me, it's not a scientific question. I like to do something where there's an equation that nobody can disagree with, you know? (laughs) I've achieved some of that, and to me those are the most satisfying things. Nobody disagrees with my calculation, of the black hole entropy in string theory.
Now, they could disagree about whether it's relevant to the real world, but there it is. It's a fact in string theory. I don't think many people disagrees with my discovery about the real world that the gravitational vacuum is infinitely degenerate. They might disagree on whether it's an interesting observation, or how deep it is or how important it is or something like that. If I worked on just beyond the standard model of physics, I might have the satisfaction of, had I done a good job of it, making statements about experiments in the real world, and that would be a different kind of satisfaction. I would prefer that to nobody can prove it or disprove it statements about other universes and... The stuff about other universes is also different in my mind than string theory, because in string theory, if you had unlimited resources, and unlimited time, you could build an experiment that would see a string. Or not. But the extra universes, I don't even know if you have unlimited resources and unlimited time how you would settle it.
I feel we've understood as a community a lot about the space of possibilities for what might happen with quantum mechanics and black holes. It’s amazing how much we've understood without having fully solved the problem. The problem is much bigger and deeper than we understood. If you had asked, I like to say this, if you had asked one of the believers in string theory in 1985 what their wildest hopes were for the next five years in string theory, they might have said, "We're going to find a Calabi-Yau compactification that has three generations and we'll predict the particle content in the couplings and we'll measure it at the LHC, and we'll know that string theory is a theory of nature." In other words, string theory was being viewed as the ultimate step in the reductionist program of understanding the laws of physics. That dream I think has been shattered.
What has happened in my view is even more interesting than that. To just add to your list on the wall of the muon, quark and others some more particles and say, "Okay, there we are, its done" is not as exciting as what's actually happened: new insights into possible emergent spacetime, the holographic principle, non-perturbative effects and strings turning into black holes. It's a whole zoo of wonderful phenomena that we've uncovered which just keep on coming and coming. We related some of them and put them together but we haven't yet made sense of it all. We haven't connected it concretely to reality, but it's a very rich and exciting enterprise, which just has been a real thrill to be involved in. Always with the idea that somehow at the end of the day... we'll wrap it all up. Of course, it would be a giant disappointment if I could be projected a century into the future to some history of science class in which first they talk about phlogiston for three minutes, followed by a whole lecture on how thousands of people were diverted into string theory and holography for 50 years, and that it had nothing whatever to do with reality. That would be terrible.
Well, we have it all on tape, so we'll have to see. (laughs) Andy, back in this universe, not in the multiverse, I want to ask you for my last question, you know, all of the things that you could work on going into the future, right? You will by definition have to limit yourself, time, resources, bandwidth, attention, all of those things. What's left for you personally? What are the things that you find most compelling, not just in terms of looking at all of the exciting stuff that's going on in the field, but for you in terms of devoting your own time? What do you want to accomplish as a representation of the kind of physics that you think is both most promising and most exciting for the next x number of years?
I can only say on a five or ten-year timescale. I can't really think any further than that. We have a new toy. We have a new tool which has grown out of this discovery of the degenerate vacua, which is related to BMS and soft theorems and there's a lot of things to understand about that. I'd like to use that fully. It's a very much a bottom-up approach. I'd like to use that to show that this concept of holography in emergent space time is not limited to anti-de Sitter space, but also occurs in flat space. Really everything I'm doing now in my current program does not require any assumptions about microphysics, and so it's a very different game than what I was playing before.
What changed? Why not make assumptions at this point?
I am not making assumptions about untested laws of physics but I'm making guesses about which approaches are going to pan out. That's a very different thing, okay?
I'm making guesses about which things look promising. We have to do that, everybody has to do that. I feel the time is ripe for the bottom-up approach, both because we've learned so much about the world, the possible theoretical worlds, that incorporate both gravity and quantum mechanics and because we've discovered this new and very rich set of tools for analyzing any theory. We've discovered some new symmetries and conservation laws which we can use. They apply precisely to those theories that describe the real world. Namely any theory of flat space with general relativity and quantum mechanics. It just seems like the right thing to be doing now. So, I'd like to understand that. In the program I started with Stephen Hawking and Malcolm Perry, I'd like to understand how these tools can be applied to the problem of black hole information. These programs are related because they're using the same tools. We've learned new things, it's really surprising - like going in one end of the fun house and coming out the other. String theory is all about physics at very short distances. But we've actually learned things about physics at very long distances in the process of studying strings. There's something called the Memory Effect. Have you heard of the gravitational memory effect?
So there's something I call the infrared triangle. There are three corners each with three different phenomena, and distinct echoes of these triangles throughout all of physics.. I connected the BMS symmetry corner, which general relativists study, with the Weinberg soft theorem corner, which quantum field theorists like to use, and showed they were equivalent. Then there's a third corner called gravitational memory, which observers hope to measure that is also a manifestation the other two phenomena. Memory in its earliest form was discussed by Zeldovich and Polnarev in '74. So if you look at say the mirrors at LIGO. Let me just describe it in terms--
LIGO is hoping to measure this. A gravity wave comes by and the distance between the mirrors oscillates. After the gravity wave has passed, the mirror doesn't return to its initial position. There's an offset. This offset is called gravitational memory. Rai Weiss told me he thinks they're going to measure it in the next couple of years. It's likely that- there's some papers on it- that they'll measure it in the next ten years. Now, the reason for this memory was fully understood only recently. Basically, it is a measurement of the fact that the gravitational vacuum is infinitely degenerate. After the gravitational wave passes, the vacuum afterwards is different from the vacuum before. So all of these corners of the triangle are connected. Now we have a much better understanding of this, and we've used our better understanding to find new soft theorems and new memory effects, not only in gravity but in QCD and in QED. In gravity there's several different kinds of memory effects. Zeldovich and Polnarev didn't find all of them. We’ve also predicted what we call the spin memory effect. It's an order of magnitude harder to find than the leading memory effect but it's there. And there's a little bit of discussion at LIGO on how to measure the spin memory effect that we've predicted based on our understanding of these degenerate vacua.
So this would be experimental predictions from coming out of our new understanding of the structure of general. So that's another answer to your earlier question, how useful is it? Not only that, some of my students and I wrote a paper with Venugopalan at the Brookhaven nuclear theory group about how this memory effect shows up in the strong interactions in heavy ion collisions. We published a prediction they're going to see QCD color memory at this future electron-ion collider. So another thing I want to do is to find more observational consequences. When you understand something new, there should be new calculations you can do that you couldn't do before, and there should be new experimental predictions: and we do have some. I'd also like to make a new observational prediction in QED and have had conversations with various experimentalists about the possibilities.
Another thing I'm very interested is all this fantastic-- I mean, these astronomers are amazing, what they can do now and I just love all this data that's coming out ... You know, it's pretty weird. I've been thinking about black holes since the early 80s, and I didn't realize until I saw the picture, that I had a little gnawing uncertainty inside … in 1980, a lot of people were saying that black holes didn't really exist.
Sure. It's like the late 50s and early 60s with quarks. Same thing.
Yeah. I was on a TV panel with Steve Weinberg, who I worship (laughs), in which he said he doesn't think they'll ever see black holes-- this was recorded six months before LIGO. He didn't think they're that interesting. So I had this gnawing feeling. What if I spent the last 30, 40 years thinking about something that didn't really exist?
But Andy, you really needed a photo to give you that assurance?
No, I didn't need a photo, but when I saw the photo, I cried.
I felt so good, you know…I've been thinking about these things and thinking of them in the abstract, they're up there, whatever. And then all of a sudden, there's nothing like a picture, you know?
It made it real on a gut level.
It made it real on a gut level. And so I love it. I love talking to the EHT people. I love being involved in the discussions of what the next experimental step is, how they should extend the telescope. So I'll keep being involved in that. I've written a couple papers, three or four papers, and I'm keeping my eye on various developments. Maybe more than three or four. Most of the real work there is developing the hardware and there isn't that much that a theorist can do, unless you're numerical. They don't need me to do numerical simulations. But I am very happy to have found a couple of places where some analytic work was useful, and it even fed back and gave me some other ideas about my quantum theoretical pursuits... It's a long road we're going down, you know?
To understand this stuff. And there's no shortcut. We have to have 1000 people or however many people are in the field going down every alley, exploring every road. Nobody gave us a map, you know? So by the time we get from Boston to San Francisco, we will have covered every road in between. (laughs) Nobody gave us the shortcut.
Well, Andy, it's been a trip we've been on here. This has been an extraordinary amount of fun for me, and--
Oh, you're fun too, yeah.
Oh well I really appreciate that. You have so many interesting perspectives on so many things, and really at the end, you might, you know, from the beginning, just to sort of put a loop on all of this, you might not be a believer, as you say at the beginning, and yet, your reaction to seeing that photo of the black hole, there's something deeper in you that goes beyond the sort of mental pronouncement that you're not a believer, because in your own way, you very much are.
I'll take that. I'll take that. The word can mean different things.
Absolutely. Well, Andy, thank you so much for spending this time with me. This is, I know already, this is going to be an amazing addition to our collection, and I'm so happy that we connected.
Okay, that was a lot of fun.