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Interview of Scott Tremaine by David Zierler on February 1, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Scott Tremaine, emeritus professor at the Institute for Advanced Study in Princeton. Tremaine discusses his current affiliation with the University of Toronto, and he provides a historical overview of the boundaries between astronomy and astrophysics. He recounts his childhood in a town north of Toronto, and he explains his early interests in science. Tremaine describes his undergraduate experience at McMaster, the opportunities that led to his graduate admission to Princeton, and the exciting developments that compelled him to focus his thesis research on astrophysics. He describes his dissertation on the dynamics of galaxies done under the direction of Jerry Ostriker, who at the time was focused on the earliest research on dark matter. Tremaine discusses his postdoctoral term at Caltech where he worked with Jim Gunn and Peter Goldreich, and he explains his decision to take a second postdoctoral position at the Institute of Astronomy at Cambridge. He describes his appointment at the Institute for Advanced Study, his decision to join the faculty at MIT, and he explains his ongoing research collaboration with Goldreich on studying Saturn's rings. Tremaine describes the intellectual origins of his book, co-authored with James Binney, Galactic Dynamics, and he explains his decision to join the University of Toronto to become the director of CITA. He describes his interests in the origins of comets, his contributions to black hole research, and his appointment at the Institute for Advanced Study. Tremaine discusses his work on exoplanets, and at the end of the interview, he surveys the importance of increasing computational power over the course of his career, the exciting advances that have been made in understanding galaxy development, and why the "three-legged" stool upon which cosmology rests - namely, on inflation, dark matter, and dark energy, is problematic.
OK, this is David Zierler, Oral Historian for the American Institute of Physics. It is February 1st, 2021. I’m so happy to be here with Dr. Scott Tremaine. Scott, it’s great to see you. Thank you so much for joining me.
To start, would you please tell me your title and institutional affiliation?
I just retired last summer, so I’m an emeritus professor at the Institute for Advanced Study at Princeton, and we moved to Toronto where I have an appointment as a professor at the University of Toronto.
Did you have the professorship before leaving the Institute, or it was sequential?
No, no, this is the sort of professorship where they don’t pay you so—
—it’s not quite as good a professorship as I had before, but I’m happy to be here.
Did you accept the professorship before or after the pandemic hit?
We planned to move before the pandemic hit, but the pandemic made everything considerably more complicated. You know, moving internationally in the midst of a pandemic is not really to be recommended.
[laugh] What was the plan in terms of your responsibilities at Toronto?
Well, I have an appointment that gives me access to university resources such as the library, and allows me to supervise graduate students. It allows me to hire postdocs. I have a grant from the Canadian research funding agency (the Natural Sciences and Engineering Research Council), and so I can keep most of the best parts of being a faculty member.
Did you make a habit, Scott, of having graduate students during your tenure at the Institute?
I had several graduate students while I was at the Institute, but not as many as I had when I was at Princeton University. That’s partly because the Institute has a very strong program of postdoctoral fellows, and so I was spending more of my time on postdoc mentoring, and less on graduate student mentoring. It’s also partly because, with the best will in the world for everybody concerned, the graduate students see less of you if you’re not walking up and down the hall every day, and you’re not teaching them in their courses. And so, your opportunities to attract graduate students are reduced a little if you’re not actually in the same building.
In what ways, or not, have you remained connected with the Institute since you retired?
Since everything’s being done virtually at the moment, I split my time about equally between talks in Princeton and talks in Toronto. So I’m staying as connected as possible with both institutions. Nevertheless, I’m looking forward to a reopening because it’s been difficult to meet all the people I don’t know here.
Scott, on that point, I’d like to ask a very in-the-moment question with regard to research in the pandemic. To what extent have the past 10 or 11 months been productive in the sense that you might have more time to focus on things that you were never able to get to before, and to what extent is the opposite true where the value of in-person interaction, not just over Zoom, is so vital for the science and may have put things on hold that you were planning to accomplish?
I have certainly seen some advantages to the huge increase in virtual activities. One is that I can keep up with what’s going on at coffees and talks at two institutions just as easily as I could normally keep up with them at one. I’ve attended conferences that have been held virtually, and, in some ways, they’ve been more effective than conferences held in person. But I do miss the face-to-face interactions. It’s been harder to get to know people. It’s been harder to develop collaborations and to get new ideas. I think that’s been generally true of most of my colleagues, and I think the majority of them have also found that it’s been harder to concentrate. In some cases, that’s for obvious reasons, like issues with childcare or the lack of a good space in your home to do virtual work, but even with the best work environment I think most of us have been less productive.
In my case, I’ve certainly found it harder to concentrate on research. Fortunately, early on in the pandemic, I decided that it would be a good thing to use the opportunity to try to write a graduate textbook on dynamics of planetary systems, so I’ve been spending a significant fraction of my time on that. That’s easier to do because it requires a little less focus than doing research.
Well, Scott, before we take it back to the beginning, I want to get our terms straight because this is something that’s going to come up as we develop the narrative. The distinctions between the terms ‘astronomy’ and ‘astrophysics’ mean different things to different people. They mean different things at different institutions. They may even mean different things generationally. So, I’d like to ask you where are the boundaries between these disciplines as they relate to your career and your research interests?
I’ve tended to straddle the boundary. All of my undergraduate and graduate degrees are from physics departments rather than astronomy departments, and I’ve always tended to think of myself more as a physicist than an astronomer, although most physicists probably think of me as an astronomer. In the broadest sense, you can think of astronomy as being a descriptive science like botany, and astrophysics as being the application of the laws of physics that you understand from the laboratory to astronomical systems.
In the first half of the 20th century, that distinction was very much borne in mind. I was told that even up until the 1940s, the library at Princeton University had a section for astrophysics and a section for astronomy, and the books were in either one section or the other, and the two weren’t mixed. And that one of the first things Lyman Spitzer and Martin Schwarzschild did when they were hired was to deliberately mix up the two sections. I think one of the big stories in both disciplines in the 20th century was the development of astrophysics—the application of physics to astronomical systems—in a way that simply wasn’t there in earlier times.
My guess is that these two branches are going to continue. But one of the developments that we’ve seen in the 21st century is a re-emergence of astronomy as a growing discipline relative to astrophysics, simply because the technology has enabled such enormous quantities of data collection. A discovery made through data-mining large surveys really is classical astronomy rather than astrophysics. If you think of the two as being in competition, you can argue that the 20th century belonged to astrophysics, and maybe the 21st belongs to astronomy again.
[laugh] Very good. Well, Scott, let’s take it all the way back to the beginning. I’d like to first start with your parents. Tell me a little bit about them and where they’re from.
My father was Vincent Tremaine. He was born in 1915 and raised in Kenogami, a small town in Northern Quebec. His parents emigrated from England in the early part of the 20th century. He had to stop his education before he finished high school because of the Depression. During the Second World War, he volunteered to join the Royal Canadian Air Force, and was sent overseas to Britain during the war. When he returned, he decided to stay in the Air Force, and that’s where he made his career.
My mother’s family mostly came to Canada earlier, sometime in the middle of the 19th century. She spent most of her childhood either in western Ontario or in British Columbia. She was a schoolteacher, and again, when the war started, she volunteered to join the Air Force, and met my father there after the war. As was usual in those days, after they got married in 1947, she resigned from the Air Force and became a homemaker.
Where did your parents meet?
They met in Toronto through some set of duties associated with their work, but I don’t know the details.
And where did you grow up? Where did you spend your early years?
I spent most of the time north of Toronto in an area called Thornhill that used to be a village but now is part of the urban sprawl surrounding the city.
Did you go to public schools or private schools growing up?
—just the local public and high school.
Were you always interested in science from a young age?
I was always very interested in science, although I wasn’t particularly interested in astronomy. I thought it was a descriptive science and I didn’t find descriptive sciences like astronomy or botany appealing. It wasn’t until I took a course in astrophysics when I was an undergraduate that I began to realize that it was pretty interesting.
Was there anything specific in your childhood that may have sparked a curiosity in physics or the universe, like the Space Race or Sputnik or anything like that?
I was seven years old when Sputnik went up, and the Space Race, the astronaut program, and going to the moon were all extremely exciting for any kid who was interested in technology. My father’s job in the Air Force had to do with aviation medicine, and so it was something that I heard about from his work as well. But I wouldn’t say that any of those events specifically turned me on to astronomy. Broadly speaking, my astronomy colleagues are of two kinds: there’s the ones who were interested in astronomy since they were 6 years old, had a little telescope, looked at the stars whenever they could, and so forth; and then there’s the ones who had no interest in astronomy, went into physics, and eventually chose astrophysics as an interesting sub-discipline of astronomy. I was one of the second group.
Did you have a strong curriculum in math and science in high school?
Yes and no. Secondary and post-secondary education in Canada was and is much more homogeneous than it is in the US. Very few people went to private schools, and the curriculum and quality of teaching was pretty uniform, without much dependence on the local district you were in. So, I don’t think I had a particularly strong or particularly weak background in physics and mathematics.
In terms of your interest—in terms of perhaps your family’s financial capacity—what were your-- what was available to you when you were considering undergraduate programs?
Again, the Canadian system was and is different from the US system. All the universities are public. There are no private universities. The best of them are not as strong or as well-funded as the best US universities, and the worst are far better than the worst of the US universities. There was no issue of whether we could afford or whether I could get into a private university, and both I and most of my contemporaries just picked one of the local universities, and [I] went there. In retrospect, I should’ve been perhaps more ambitious and thought about attending a private institution in the US, but it just wasn’t on our radar screen for me or anyone from my background at the time.
No very good reasons. Again, in contrast to the current situation for young people in the US, where choosing your university is an extremely serious, almost full-time occupation—
I think the perception was that most of the universities were comparable in quality, and it didn’t matter so much where you went. McMaster had one of the two or three strongest undergraduate physics programs in the province, and they offered me more generous financial support than anyone else—although I think I could’ve covered my costs on my own with summer jobs.
So that’s to say, going in, the plan from day one was for you to pursue a degree in physics?
I was aware that there were a lot of interesting disciplines in university that I hadn’t been exposed to in high school, and so I was open to the idea of switching. But my default plan was to go into physics, and I never saw any reason to deviate from that.
Were there any professors as an undergraduate who were really formative in terms of your intellectual development?
As in most places, I had a number of extremely good professors, and some that maybe were a little less inspiring. I was fortunate because one of the faculty, Tom Timusk, initiated the first course in astrophysics for physics majors while I was there. I took the course, I liked it, and so it sensitized me to the idea that astrophysics was an interesting specialty within physics.
To what extent were you aware of the binary in physics between the world of theory and the world of experimentation as an undergraduate?
I was certainly aware of the distinction. At that time and in that location, the opportunities for doing research when you were an undergraduate were quite limited, however, and I figured that I would learn which was the better fit for me in graduate school rather than as an undergraduate.
Did you have exposure to either instrumentation or lab work that might’ve helped you refine your research interests?
We certainly had laboratory courses, and they were useful. But I never really had the same kick out of the lab courses as I did out of some of the more theoretical courses.
What kind of advice might you have gotten about the best graduate programs to apply to?
The level of expertise I had when I made that decision was far below the level of expertise of current undergraduate students, and so my decision-making process was pretty crude. I was sure that it was best for me to go outside Canada for grad school. I was also pretty sure that the system at the American graduate schools was a better match than the system at, say, British or European graduate schools. So, it was really a question of finding the right place in the US.
I did the same homework that anybody else would do. I talked to my professors. I looked at graduate school rankings from the National Research Council, and it became pretty obvious what the best half-dozen graduate schools were. I was uncertain what I wanted to do, so I wanted to be in a fairly broad program where you had the opportunity to spend a year or two before you made a decision about what specialty you wanted to go into. And so. I simply made a list of the half-dozen I thought would be the most interesting places, and applied to them.
So that’s to say Princeton was more as a matter of reputation than a desire to work with any particular professor?
Yes, very much so. I had no idea of any professor I wanted to work with or even what discipline I wanted to be in. And I’ve continued to feel uneasy with students who say that they know exactly what subject area they want to go into or even what professor they want to work for. They go to graduate school, they start working immediately with that professor, and they stay with them throughout their whole graduate career. That just seems to me generally—of course, there were exceptions—not to be the best way to learn how to do research.
Oddly enough, I think one reason I chose Princeton was because they really didn’t seem to be very interested in recruiting me. I was originally on their waitlist, and only got an offer late in the season, and apart from the letter of offer they made no effort to contact me. I guess I figured that they must be really good if they can afford to treat prospective students that way. I later learned that the students who had been admitted were largely treated the same way—basically, the department ignored you for two years, which you were supposed to spend preparing on your own for a pretty brutal set of general exams lasting a week.
Before we leave McMaster entirely, one question on the social side, being an undergraduate anywhere in the late 1960s and early 1970s has a degree of interesting capacity to it. I’m curious if McMaster saw much of the counterculture, the antiwar movement, and if you were at all political during your time as an undergraduate?
The huge difference between being an undergraduate in the ‘60s in the US and not in the US is that your country wasn’t at war and you weren’t seeing your contemporaries being drafted. Although many of my contemporaries had strong opinions about the war in Vietnam, you never really felt either that you had a dog in the fight or that any protests that you made were going to attract a lot of attention either in Washington or elsewhere in the country. So, the antiwar movement much more muted outside the US.
When you got to Princeton, I’m curious, this was a particularly exciting time in the world of particle physics, and so many particle physicists would soon go on into cosmology and astrophysics. Did you perceive that trend? Did you take part in those interests at all?
I was certainly aware of the importance of particle physics. I took courses in particle physics, and I considered doing a thesis in particle physics. But I eventually decided not to, for a combination of reasons. One is that experimental particle physics was entering a period that was maybe a little less exciting than it had been in the ‘50s and ‘60s. There was a feeling that the field had gotten quite large, and maybe it didn’t have as much promise as it had had 10 or 20 years ago.
The second reason is that at the same time astrophysics looked really exciting: this was within 10 years of the discovery of pulsars, the discovery of quasars, and the discovery of the cosmic microwave background. Astrophysics was a small field that looked like it was really blossoming with new discoveries. I also thought astrophysics was a better fit because it was smaller, because and less dependent on huge experimental facilities.
What was the process for you going about choosing and developing a relationship with a graduate advisor?
Once I decided what area I wanted to go into, I did what any reasonable graduate student would do: you make a list of the people working in that area who are at your university, you rank order them in terms of what sort of things they do, what their reputation is, you talk to the other students they’re supervising, and then you go around and talk to the supervisors and see if they’re taking on new students, and see if you have any chemistry with them.
And so before you decided on a graduate advisor, how specific were you in your research interest in terms of what you wanted your thesis to be?
Oh, not at all. I talked to Jim Peebles, but he was going away on sabbatical the next year, and [he] didn’t want to take on any new students. Then I eventually decided that Jerry Ostriker was the person I wanted to work with. I had a few conversations with him, and he agreed to take me on as a student.
The first two or three months after I started working with him were really quite remarkable. I would have an appointment once a week, with no idea what I wanted to do for a thesis, and he would just throw out ideas. He would say, “Well, you could do this,” and he would describe some idea, and why it was exciting, and give me half a dozen papers to read. Fifteen minutes later, he would say, “Or, you could do that,” and he would give me another topic with another half a dozen papers. It went on like this week after week. It was an extraordinary learning experience: I would go away and read the papers and try to come back and give him some comments, and he would just come up with some new ideas.
What was Jerry working on at the time you got to know him?
He was doing some of the most influential and important early work on dark matter. He was exploring the idea—really for the first time—that there were large amounts of dark matter in galaxies, maybe 10 or 100 times the mass in baryons, in stars. This was the most radical revision in our models of galaxies in several decades. He had in his head all of the fragmentary observations they had available at the time, and [he] was trying to figure out if these were consistent with or provided evidence for this new view of galaxies. And of course, it turned out that this new picture was basically correct; it’s revolutionized astronomy and, by now, has revolutionized physics.
Scott, as you tell it, in terms of the various topics he offered for your consideration to do thesis research on, it suggests that his style as a graduate mentor was such that he was OK if you ended up doing research that wasn’t entirely and directly related to what he was doing at the time.
That’s certainly true. I think that he had sufficient breadth and energy that he was happy to pursue several different things at a time. He had other students working on very different things from what I was working on. He was really able to keep quite a few different balls in the air at the same time. Even when he became Provost at Princeton much later, he always claimed that being Provost was just a hobby and continued to work on multiple research projects.
And what ultimately did you settle on for your thesis research?
The thesis was on problems related to dynamics of galaxies. Quite unusually for the time, Ostriker was happy for the thesis to be a collection of journal papers stapled together. And so, I wrote two papers on formation of galactic nuclei; one on evolution of the orbit of the Magellanic Clouds; one on stability of stellar disks. These were all subjects that he was interested in, but they weren’t directly connected and, again, it’s a sign of his really exceptional that he was able to supervise in many different areas at the same time.
To zoom out for a bit, just to get a sense of the broader advances in both theory and observation at that point, what were some of those observations that—and theoretical advances that made your thesis research possible, and where did you see the field—there was real fundamental work that needed to be done?
One of the features of theoretical research in galaxy dynamics is that one only needs Newton’s laws, and so there’s not really anything you do that couldn’t have been done a decade or even a century earlier. That has its advantages and disadvantages. I liked it because it always seemed to me more interesting to come up with a good idea that anyone else could’ve had 10 or 20 years earlier, rather than one that responds to a new observational result that came out yesterday. If you think of research as a game, it’s a little more of a challenge.
Despite this, my thesis research was driven by one new result. There were observations of the centers of a couple of nearby galaxies from a balloon-based telescope called Stratoscope, sent up by a group led by Martin Schwarzschild at Princeton. Stratoscope had some really wonderful observations but wasn’t terribly successful. It was too far ahead of its time, but its high-resolution images were the precursor to the images sent by the Hubble Space Telescope, and now, 50 years later, people are starting to look again at balloon-borne optical telescopes.
Scott, what were some of the central conclusions of your thesis, and how were they responsive to some of the broader questions in the field at that time?
Dense stellar systems or galactic nuclei are found at the centers of many galaxies, and the thesis proposed a mechanism for making them. The idea was that star clusters orbiting in the galaxy attempt to reach equipartition with the stars in the galaxy, and in so doing they lose orbital energy, spiral in, and gather at the center. That particular proposal is still one of the two or three competing explanations for galactic nuclei. Probably the correct answer is that the nuclei are formed by a combination of processes, and this is one of them.
Another conclusion was that the orbits of the Magellanic Clouds are decaying by the same process. In a certain sense this was one of the first hints that mergers among galaxies are common.
Scott, did you spend much time at the Institute as a graduate student? Were there sort of must-have lecture series to go to during your time then?
There were two levels of relationship between the University and the Institute. At the first level, both places had interesting seminars and all of the grad students went regularly to the Institute for these. At a higher level, the Institute was much more cautious about its relationship with the graduate students than it is now. For example, they had a weekly astrophysics lunch called Tuesday lunch following the colloquium, which the graduate students were not invited to (at a later stage in my career, I investigated this—John Bahcall told me that it was Lyman Spitzer’s policy, and Lyman told me it was John’s policy). As a result of these restrictions, although the Institute was a tremendously valuable resource you really did feel that the center of your intellectual life was located at the university.
I’ll test your memory, Scott. Who was on your thesis committee?
The person on my thesis committee who was most interesting and impressive was Freeman Dyson. As you know, Freeman is interested in everything. And although I never would have had the nerve to ask him, Jerry Ostriker said, “Well, part of your thesis is related to stuff that Freeman’s interested in. Freeman’s a really smart guy. Why don’t you ask him to be on the thesis defense?” I asked him with some trepidation, but Freeman is a wonderfully courteous guy, and he said, “Certainly.”
The only problem was that there was a particular result in my stability analysis that I was convinced was correct and had verified numerically, but that I’d never been able to prove analytically after working on it for some months. And when I went to the thesis defense, of course Freeman said, “Oh, by the way, I was able to prove this analytically.” Afterwards he gave me a stack of notes maybe a dozen pages long, showing what the analytic proof was. Fortunately, of course, he reassured me at an early stage that this was not going to affect his judgment of the thesis. That was the first but not the last time I learned how smart Freeman was.
[laugh] Scott, after you graduated, what opportunities were available to you for postdocs?
You’re testing my memory again. I applied to four or five places, and got offers from most but not all of them. I’d never lived on the West Coast, and I thought it would be nice to go there. At the time, Peter Goldreich, Kip Thorne, and Jim Gunn were all at Caltech. They were all extremely strong at astrophysics theory or general relativity or a combination of theory and observation. So, it wasn’t too hard to make a decision to go to Caltech.
Scott, once you got settled in in Pasadena, I’m curious, coming from Princeton, these are of course two institutions with very well-defined physics cultures or physics identities. What were some of the key differences as you noted when you got to Caltech from Princeton?
I think there was a certain amount of generally healthy but occasionally unhealthy rivalry between them. I arrived just as Ed Turner was completing his thesis at Caltech, and he was going to Princeton, so we had a lot of conversations about the differences in culture. I think the most fundamental difference was that Princeton had a rather weak program in observational astronomy, while Caltech had an exceptionally strong group of theorists, but a very small one relative either to its observational staff or to Princeton.
I think Princeton—more specifically Spitzer and Schwarzschild—made a decision in the 1940s and 1950s to concentrate on theory, figuring that it was a niche that most other places weren’t exploiting, and they could thereby build up a first-rate program without huge investments in hardware and facilities. Caltech, at the opposite extreme, has always invested very, very heavily in observational facilities. Each strategy had advantages and disadvantages. The disadvantage of the Caltech strategy is that their very good faculty spent a significant fraction of their time worrying about and trying to manage these facilities, which was a problem that really wasn’t present at Princeton.
There were other differences. Princeton was a smaller institution, so it was easier to get to know everybody. It was less siloed into different groups that weren’t talking so much with each other. But Caltech had a huge advantage, even for a theorist—if you had a question that had observational implications, you could always find somebody there who was expert in what the observations actually said, and they were usually quite willing to listen to tell you what they knew.
Scott, how well developed were your plans prior to getting to Caltech in terms of knowing what collaborations to join, what you wanted to work on next?
Once again, I didn’t have any very good plans. I was fortunate that there were things that looked interesting that were more or less directly in line with what I’d been doing in my thesis. But I was also very much aware that Caltech had an extraordinary number of good people working in areas that I wasn’t familiar with, and [who were] using techniques that I wasn’t familiar with.
My point of view [is] that when you’re a postdoc you have time for unrestricted research, and you probably don’t want to spend it continuing to do stuff related to your thesis that you know how to do anyway. And so, my only plan was that I would try to talk to lots of new people, and I would aim to spend maybe half my time working on things directly related to my thesis, and half my time doing something different.
What collaborations did you end up joining? Who were the key people you worked with at Caltech?
I wrote several papers with Jim Gunn but the closest collaboration I had was with Peter Goldreich.
Did you know Peter before you got to Caltech?
No, I’d never met any of these people before. I was a little nervous about talking to him the first time because he had something of a reputation of not tolerating fools gladly.
I talked to him a number of times, and eventually just said, “Look, I’d like to learn something new. Can you suggest a project on something that’s different from what I’ve been doing?” And he said, “Why don’t you think about Saturn’s rings because the Voyager spacecraft is going to go there in a few years, it’s going to bring back lots of good data, it’s an interesting physics problem, but if you do, I don’t guarantee you’re going to get anywhere. I’m happy to think about it with you, but I don’t know if the ideas I do have are going to work out.” And I thought it was a great subject, so I was delighted to start thinking about it.
What was Jim Gunn like as a person? What was it like to work with him?
Jim is full of energy, and extraordinarily broad, very quick, very sharp. If you went to a seminar on just about any subject, he was likely to ask the most interesting question. He was also pretty busy. He was really trying to juggle a lot of balls at the same time, both theoretically and observationally. The view I had, which I think was shared by most of the younger people at Caltech, was that if you had an idea or you had a question, Jim was the best person around to talk to about it, but there were two challenges.
First of all, you had to find him because he was often in his lab or his office or at the telescope, and [he] was pretty hard to locate. But if you could find him and get half an hour with him, it would be incredibly valuable. You would come out with lots of good ideas. The second problem was that he could give you all these ideas, but he might not do any work on it until you found him again because he was just trying to do too many things.
In one of my meetings with him I made a more-or-less random remark that phase-space density in a system governed by the collision-less Boltzmann equation could never increase. Jim immediately said: “You should apply that to massive neutrinos as a dark-matter candidate,” and then went on to other subjects. So, I thought about what he meant on and off for a couple of months and then wrote a paper on what is now called the “Tremaine-Gunn limit.”
Scott, given that you perceived that the opportunities for greater research, better opportunities in observation in the West Coast was available, to what extent did you take advantage of that?
Perhaps not as much as I should’ve. I talked to the observers as much as I could. I tried to learn about the observational techniques. I tried to find a project that would involve some observations that I could participate in, but I was never really successful. I did get to go to Palomar Observatory once, and I had a trip to National Radio Astronomy Observatory in West Virginia because of a project I was working on with Jim Gunn and Jill Knapp which eventually led to a couple of papers.
As your postdoc was coming to a close, and you were contemplating your next opportunity, were you thinking perhaps about returning to Canada? Were you thinking specifically that you wanted to make a life in the United States?
I wasn’t strongly motivated to return to Canada. I had had really had a good time both at Princeton and Caltech, and didn’t think that I could find an institution in Canada that would be comparable. I was interested at the time at getting a different perspective, and I managed to arrange a year’s position at Cambridge just to be somewhere else and see what another scientific culture was like.
This was a decision, a conscious decision, not to enter the job market, not to look for faculty positions. You wanted another postdoc experience?
The job market in physics of course has its ups and downs. It had had huge growth in the 1960s and was suffering a hangover from that in the 1970s. When I went to graduate school at Princeton in 1971, they had cut the size of the entering graduate class to about half of what it had been before, partly because they were reluctant to train a lot of graduate students for whom they thought there would not be jobs. In a related story, several years earlier Princeton had gone through an awkward situation where they had promised draft exemptions to graduate students in physics, and then changed their mind after the students had arrived. So, something like half of the first-year graduate class in physics disappeared into the draft, and many of them were returning the year I arrived, which also motivated them to cut the number of new admits.
Scott, I wonder, just as you saw an opportunity to do something different, to live on the West Coast, if going to England was particularly attractive to you?
I thought it would be good for me to get acquainted with the European culture in astronomy and astrophysics, and at the time it was pretty clear that Cambridge was the most interesting place to go in Europe, so it wasn’t a very difficult decision.
What is-- Where does the Institute of Astronomy fit in with Cambridge overall? Is it like a school or a division in a US college?
I have never understood the way—
—Cambridge University works. It’s an extraordinarily complicated system with colleges and institutes and schools and other things. I think of the Institute of Astronomy as being somewhat like a department, but very much focused on research and on graduate teaching. But I’m the wrong person to ask.
Did this give you a window into astronomy that you might not have had before, given that you were operating mostly in an astrophysics environment?
Not so much, because Cambridge had challenges in providing observational facilities. Obviously, in contrast to Southern California, you couldn’t do optical observing locally except for very specialized programs. Cambridge had a strong radio astronomy group, but they were located in the Cavendish Laboratory, which was across the road from the Institute of Astronomy. So although there were some very strong observers in Cambridge, and I learned a lot from them, Caltech was better as an observational center.
What was your most important work at Cambridge? What did you do for that year?
I would say I was less successful in developing new subject areas than I had been at Caltech. Part of the reason for that is that the work I’d been doing with Goldreich was really interesting, and I continued to spend a large fraction of my time working long-distance with him.
When it was time to return to the States, what opportunities were you considering?
At the time, John Bahcall was running a very high-profile program at the Institute. When I was leaving Caltech, he had tried to recruit me for a five-year position at the Institute, but I was committed to go to Cambridge. We agreed that he would defer the position by a year, and so I had that position waiting for me, which made for a simple transition back into the US, except for visa problems.
[laugh] And you were working with Peter throughout your time at Cambridge? You were corresponding long-distance?
Yes, that’s right. Mostly by letter but I even sent him a telex once or twice when I had a good result.
Were you working on Saturn’s rings during your time in Cambridge, or that was after?
The work I did with Peter was mostly related to Saturn’s rings, rings around other planets, and a spiral structure in disk galaxies—all of which used many of the same techniques. I continued to collaborate closely with Peter for a large fraction of my time over a period of six or seven years. John Bahcall, who was my mentor at the Institute at the time, was convinced that this was a bad idea strategically. He said, “You might have a hard time getting a faculty job because you’re not doing independent research. You know, everyone will think you’re just doing calculations for Goldreich.” I was somewhat irresponsible—my response was, “Yes, but this is so much fun, I’m not going to give it up,” and so I just kept working with Peter.
Was his characterization of the nature of your collaboration fair? Was that really all that you were doing for Goldreich?
I think I contributed some fraction of the good ideas, but probably not most of them. When I first started working with Peter, he would come up with ideas or insights that I didn’t have, and I would just say to myself, “Well, he’s more senior than me. He’s been thinking about this problem longer. That’s not surprising.” And then, you know, after we had been working on the same thing for two or three years, he was still coming up with insights that I hadn’t had, and I said, “Well, he’s still been thinking about these problems longer than me.” But after I’d been working with him for about five years, I began to realize I could work with him for the rest of my life, and he would still understand most things before I did. So, in that sense, working with him was a very good but humbling education. It’s always good to meet someone who’s definitely smarter than you early in your career.
Scott, another administrative question, the title “long-term member” that you held at the Institute, is that more like a glorified postdoc, or is it more like an assistant professor level?
Well, [laugh] it’s definitely not an assistant professor position in the sense that it’s not tenure track. The Institute doesn’t have a tenure track. It only hires professors with tenure. It’s probably accurate to call it a glorified postdoc, but John Bahcall was always very self-confident and very proud of the Institute and the people who worked there, and he said that a five-year position wasn’t equivalent to an assistant professor—it was equivalent to an associate professor! But of course, without a tenure track--
--it’s not an associate professor.
Now, I’ll ask the reverse question, from your graduate school days at Princeton, during your time at the Institute in the late ‘70s and early ‘80s, would you spend a lot of time in the department of physics at Princeton?
I tried to visit both the Department of Physics and the Department of Astrophysical Sciences regularly. What I always told prospective postdocs at the Institute was that the only barrier to working with people at the university was how many times a week you’re willing to get on a bicycle, and bicycle 15 minutes to get to the other place.
[laugh] Scott, within the five-year appointment, were you on the job market, and was there an understanding if a good opportunity came along that you should take it?
I probably paid less attention to a long-term career strategy than people do now. I was aware that I had to make the transition to a faculty position, and I worry a lot about a lot of things, but for some reason, I wasn’t too concerned about getting a faculty position. I just figured that something would come up—that I’d start looking, you know, two or three or four years into the position, and it would work out one way or the other.
So, was this to say-- Did you apply to an open position at MIT, or they recruited you?
I was recruited. Rightly or wrongly, I was nervous about MIT as a place to go, and I took some convincing.
Because of its reputation for fierce competition?
Not really, although I was aware that they didn’t tenure the majority of their junior faculty members.
And at MIT, correct me, the associate level at MIT is not a tenured level?
Yeah, that’s correct. MIT had a very strong astrophysics program, but it was centered on X-ray astronomy, and focused on large space missions, which wasn’t something that I was well-matched to. Moreover, MIT focused on faculty members who could bring in large amounts of money and overhead because they were associated with large projects. And for theorists in the US, then and now, the grants are small and very competitive, and you can’t get stable long-term support in the sense that a big mission can.
At the time, MIT had a rule that you couldn’t apply for a grant unless the application included a fraction of your academic year salary. And the National Science Foundation had a rule that they wouldn’t fund academic year salaries. To apply for NSF grants you had to write the proposal with a request for a third of your academic year salary, send it to the NSF, have the NSF reject it and send it back, get permission from MIT to cross out the third of your academic year salary, and send it in again. And this, to my mind, was just dumb.
Another concern I had about fitting in to MIT that graduate students at MIT tended to be admitted to work in a particular group. Crudely speaking, the MIT admissions process at the time was that all the folders containing all the graduate applications were put on a table. Faculty would walk in and look through the applications. They would find one that they liked, they’d write a grant number on it, walk outside, hand it to the secretary and say, “Here, admit him. This is the account you’ll pay him from.” I just didn’t think that was a model that I was either comfortable with or that would work well for me. And that’s probably why MIT was the only grad school I applied to that wasn’t willing to offer me financial support, which didn’t endear them to me either.
Scott, who were some of the key people in astrophysics at MIT at the time you joined the faculty?
Although I’m complaining about all these features of MIT, in the end I thought it was a wonderful place. The undergraduate students were great. I had a number of extremely good graduate students. It was really good for my physics education to teach courses, and I enjoyed meeting a lot of the other faculty members. I would have been happy to stay there indefinitely.
Scott, what were your research interests at this point? What were some things that you were working on by the time you arrived at MIT?
Well, I’m sorry to say that I was still working on Saturn’s rings with Goldreich, and still having a good time doing so.
This is to say that there was a lot to keep you busy with regard to Saturn’s rings?
Yeah, it was a wonderful intellectual experience. We got a lot of interesting results, and I really enjoyed working with Peter. I was also trying to develop research on comets. I had a very good student at the time, Julia Heisler Indik, and she did her thesis on comets, so we thought about that subject quite a lot.
I had another excellent student, Martin Weinberg, who was working on more technical problems with the response of galaxies to external forces, you know, how the full dynamical system of a galaxy behaved when it was kicked. I was trying to learn about planet formation as well, but at that stage—and perhaps even now—it was a pretty primitive subject, and I didn’t get very far. I also started thinking about long-term stability of the solar system.
This was probably perhaps the first time since being a TA as a graduate student that you were teaching undergraduate courses?
In fact, I wasn’t a TA as a graduate student either, so--
So, this is really your first time in front of students as a professor?
That’s right. I arrived at MIT, you know, they just said, “Well, you’re teaching a recitation section in this freshman physics course.” And said, “Well, OK, so what am I supposed to do? What training do I get?”
“What support do I get? How do I do this?” And they just said, “Oh, your first class is Monday at 10 a.m.”
[laugh] Good luck.
I’ll ask a question at this point, but it’s a general theme in terms of your approach to teaching, to what extent is teaching valuable for your own research, and to what extent is it a diversion from the things that you’re more interested in pursuing?
I regret that I haven’t done more teaching. I was Director of an Institute for 11 years and got released from most teaching, and was at the Institute for 13 years where teaching isn’t required. Having said that, of course, teaching is really time-consuming if you try to do a good job, and if you don’t have to teach for a term, it’s a lot more relaxed than if you do.
I also found that I liked teaching physics a lot more than teaching astronomy, partly because astronomy is more descriptive and constantly changing because the observations are changing every year. You have to spend a lot of time getting up to speed and keeping up to speed on the latest changes in the observations. Whereas—if you’re teaching statistical mechanics or you’re teaching electricity and magnetism—once you’ve grasped the concepts you just have to think about how best to present them. The course you’ll teach next year is not that different from the course that was being taught 50 years ago.
I think that the answer to your question depends on what you’re teaching. If you’re teaching a basic undergraduate course that you’ve taught a number of times before, it’s not going to help you with your research very much. If you’re teaching a special topics course in some subject area that you haven’t worked on, but you would like to, you’ll learn an enormous amount. And whether you have to teach in an informal lecture or presentation, or through working with a graduate student or postdoc, the preparation concentrates the mind wonderfully, and I can focus a lot better on reading a paper and getting the meat out of it if I have to present it in some forum.
Did you take on graduate students right at-- right away when you arrived at MIT?
Pretty much. At MIT the number of graduate students you could take was directly affected by how big a grant you could bring in. I supervised three PhD students, some undergraduate students, and I think one or two short projects for graduate students when I was there. So, I think I did a reasonable amount of student supervision, but obviously other people have done more.
Scott, when did you first meet James Binney, and what were the intellectual origins that would become Galactic Dynamics?
Oh, I forget when I first met Binney—although he was located at Oxford, he visited Princeton quite regularly, so I’m sure I ran into him at one of those two places. The intellectual origins of the book were a little complicated. Back in the late ‘60s, Dimitri Mihalas wrote a short textbook on galactic astronomy with Paul Routly. Then, about 10 years after that, Jerry Ostriker thought that there was a real need for an up-to-date graduate textbook on galactic dynamics. So, he approached Mihalas and suggested that they collaborate on writing a larger, more up-to-date version, and they agreed to do so. Some years down the road, I think they realized that they were too busy to get this done, and they needed a more junior co-author who might have more time, so they added Binney.
Some years after that, they recognized that they still didn’t have enough time, and it wasn’t getting done. So Ostriker recused himself, and Binney went looking for another author, and asked me. Then when I was trying to decide, I thought, “Well, this is going to be so different from Mihalas’s original textbook that I don’t want to put a lot of effort into something that’s just a modification of Mihalas’s book.” I said I’d be happy to write something new with Binney, but I didn’t want to collaborate on revising Mihalas’s book. So, all this shuffle ended up with Binney and me doing it.
Scott, unlike so many graduate textbooks, Galactic Dynamics is appreciated because, relatively speaking, it’s pretty accessible. Was one of your goals that this would be something that could be useful in high-level undergraduate courses, or even for the lay public that was really interested in these ideas?
Certainly, the book was never designed for the lay public. It was intended for graduate students and for some advanced undergraduates. And the feedback I’ve gotten is not completely in agreement with yours. I’ve had a lot of graduate students tell me it’s pretty rough going.
I think the reason it was successful is that, first of all, it had no real competitors in this niche. Secondly, the field expanded in interest quite a bit after we wrote it. And third, once we’d written it, it poisoned the ground around it so nobody wanted to write a competing book. We were surprised at the success—by now it’s sold four or five times the volume that we expected.
To what extent did the book really foster greater interest in these issues, and to what extent was your book sort of very timely because these were things that were becoming of more and more import?
I think we were lucky that the book was so timely. I would like to think that it led to some of the growth of interest in the general subject of galactic dynamics, and certainly good graduate textbooks are lacking in a lot of similar research fields. But I think that a better explanation is simply that the book resonated with the progress of the research field. Cosmology was one of the hottest fields in physics, and cosmology is intimately connected to the nature of dark matter and the large-scale properties of galaxies. We were simply fortunate that we were well-positioned to be a part of that subject.
Of course, the book is self-contained, but I wonder how self-consciously you wrote it in a sense that you were addressing gaps in the literature, gaps in the knowledge based on what was available before?
This is hardly news, but I think the number of good textbooks at this level is small. It’s been hard for some time and is getting harder to find good textbooks that can bring you up to speed in a new research field. There are a variety of reasons for this, which I’m sure you understand as well I do. But I think if you look at the suite of graduate courses in astrophysics—or in any other branch of physics—and ask how many of the lecturers in these courses think they’ve got a good textbook that covers most of what they want at a reasonable level and with reasonable clarity, it’s going to be no more than one in three.
Scott, what were the circumstances leading your decision to join the faculty at the University of Toronto?
Well, the story really emerges from Canadian politics. Canada had a very small community in theoretical astrophysics, and it suffered from a similar problem that the US theoretical astrophysics community suffers from, which was that it’s much easier to make a case for a large investment in a new observational facility than it is to make a case for a large investment in theory. I’ve been on several of the decadal surveys, and we’ve tried very hard to make this case. We’ve twisted and turned the logic in every way we could think of, and I think we’ve never succeeded in making that case.
Several of the theorists there had the idea of setting up a national theory Institute—arguing that this was a way to provide support and cohesion to the national theory community that they simply couldn’t get any other way. They were successful in getting the Institute funded, and then they went looking for a director. I was skeptical at first. I was perfectly happy at MIT. I felt that MIT had advantages and strengths that the University of Toronto didn’t have, and so I took some convincing.
But I had elderly parents in Toronto. The Institute looked likely to provide opportunities for me that I would never get at MIT, such as having a flexible budget that was far larger than anything I could hope to get from MIT or NSF or NASA. And so after a lot of agonizing, I finally decided to try it.
Scott, to go back to a previous question about your plans to return or not to Canada after you were done at Caltech, and you said there were really no institutions that you thought that were going to keep you as interested as what was available in the United States, I wonder the extent to which CITA’s stature, and the people who were working there, was really exciting for you, that there was really important and good stuff that was happening at that time?
I’d phrase it more in terms of opportunity rather than existing reputation. They went looking for a director very early in the formative stages of the Institute. So when I arrived in Toronto, they had three or four offices and a couple of newly arrived postdocs, and that was it. I thought there was a tremendous amount of potential, but they didn’t lure me with an established reputation.
What was your game plan in terms of just making a contribution to CITA in terms of being aspirational, in terms of where you thought it could be going?
My idea was that there were a lot of very good theoretical graduate students finishing their theses around the world, and a very small number of postdoctoral fellowships at good institutions. Moreover, most postdocs were funded by specific grants, and therefore the postdoc had to work on problems associated with those grants, which might or might not be interesting. So, the game plan was to invest heavily in postdocs, put the postdocs together, and let them cross-fertilize each other’s work rather than working under the direction of a senior faculty member. I thought that would be the fastest and most effective way to jump-start a new Institute, rather than trying to attract graduate students or faculty.
The postdoc who are perceived to be the strongest are going to get offers from a Harvard or a Berkeley or a Princeton, and it’s hard for a new Institute to compete for them. But with postdocs the error bar on their evaluation is pretty large, so you can often get people who turn out in the end to be as good as or better than the ones going to Harvard. Moreover, even if you make a mistake in hiring a postdoc, two or three years later they’ve moved on to something else, and so you can try again, which means you can afford to take risks. I just thought that this was the best niche in the ecosystem for a new institute.
I think this game plan worked well at the time because it was largely a buyer’s market for postdocs. Since then, a lot of universities have established “prize” postdoctoral fellowships—fellowships that allow you to work on any subject that you want independent of a particular supervisor. Thus, the competition for postdocs is much stiffer; it’s more of a seller’s market. In some sense CITA is a victim of its own success: its model has worked so well that other places with more resources have copied it.
What exactly was the nature of your appointment at Toronto? Was it one appointment with multiple responsibilities, or was it-- were you dual-hatted?
Well, Canadians like complicated politics. I was the director of the Institute, and I had cross-appointments in the departments of physics and astronomy that allowed me to supervise graduate students. In addition, there was an independent corporation to give the CITA an existence independent of the University of Toronto so that, in principle, the Institute could simply pack up and move to a different university if it wanted to. One of the standard Canadian tropes is that Toronto hogs everything, and doesn’t leave out anything for anybody else, and so they wanted the Institute to be able to move elsewhere if it wanted to. This was a complicated structure, but it was set up by several clever people, not me, and in the end it worked very well.
What were some of the challenges in terms of having all of these responsibilities including administrative responsibilities?
One challenge was that because of the peculiar Canadian political situation, the model would only work if you had buy-in from people in the theoretical astrophysics community working around the country. And persuading somebody in Vancouver that an Institute in Toronto is really benefiting them is not so easy. And so, the challenge was to develop programs and procedures that kept the majority of the national community on board. I think we were reasonably successful, but keeping the community onside requires constant effort.
How international were the pool of postdocs? Was it mostly a Canadian operation, or there were postdocs from all over?
Probably we got about a third of our postdocs from the US, a third from Canada, and a third from Europe and the rest of the world. I think Canadians were over-represented, not because we were deliberately trying to hire them, but rather because lots of people are reluctant to take a position outside their home country, often for very good reasons. We had any number of people who we made offers to who said, things like “Well, I’d love to come but my partner is a doctor, and the licensing requirements take so long that my partner wouldn’t be able to work while he was there,” and so forth.
Scott, was it during your time at the University of Toronto when you became interested in the origins of comets?
I started working on comets when I was at MIT, but I had two advantages at Toronto. First, I was able to get funding to support a postdoc working on that subject, which I’d never been able to do in the US. And second, one of the faculty at Toronto who was roughly my contemporary, Martin Duncan, was interested in the subject as well, and we were able to set up a very good collaboration. By now Martin has contributed far more to our understanding of comets than I have.
What were your interests in the origins of comets? What did this connect to in terms of the broader research that you had done up to that point?
I’m afraid nothing at all. I couldn’t articulate an overall research strategy for you. I think I worked on them just because the questions of where they came from and how they behaved looked quite interesting. They were questions that I had the tools to investigate. Comets were a little bit of a backwater, perhaps, and so it was easier to get started in the subject than it would’ve been for something more active.
To what extent was this research specific simply to understanding comets, and to what extent was it geared toward answering larger questions?
At the time, it was really just relevant for understanding comets. Fortunately, some of the puzzles that we were trying to solve turned out to be relevant to puzzles that came up later in the context of extrasolar planets, star clusters around supermassive black holes, and a variety of other subjects. So, I certainly didn’t have any grand plan, but if you’re lucky, the insights you get from studying one subject turn out to be relevant for others.
What were some of the principal conclusions you drew from this research on comets?
The comet population can be divided into two classes: the comets with orbital periods less than about 20 years, and the comets with longer orbital periods. The principal distinction between them is that the comets with shorter orbital periods seem to have low inclinations relative to the plane of the planets. They’re concentrated in the same plane as the solar system, whereas the comets with longer periods are roughly isotropic. And one of the things that we realized when we did N-body simulations, which was obvious in retrospect, was that you can’t take a spherical source, subject it to planetary perturbations, and turn it into something that’s flat. And that says that the short-period comets had to come from a flattened source, which was distinct from the source of the long-period comets. The only thing we could think of some sort of belt of comets in the outer solar system.
About five years later, that belt, now called the Kuiper Belt, was discovered by Dave Jewitt and Jane Luu at Hawaii. A sign of how obvious all of this was came when I talked about this work at Princeton’s Tuesday astrophysics lunch. I got through my first couple of sentences on the isotropic distribution of long-period comets and the flattened distribution of short-period comets, and then Jerry Ostriker—who had never worked on comets—interrupted me and said “you can’t make a flattened distribution out of a spherical one, so you need a separate source.” A humbling experience.
One of the nice things about studying comets is that you can construct a simple standard model, which just says that comets started off as small bodies in the plane of the planets, and then follow the orbits of all these small bodies for the age of the solar system under the gravitational influence of the Sun, the planets, passing stars, and the Galactic tidal field, and see what’s left and what distribution it has. That residual distribution actually matches remarkably well with the observed properties of the comet cloud. So, like the Standard Model in particle physics, it’s got a lot of flaws, it doesn’t predict everything, it’s still got some shortcomings, but it goes a long way towards explaining the observations with very few assumptions.
To return to an earlier question now that we’re 20 years out from your PhD dissertation research, what were some of the advances both in observation and theory that may have made this work possible at this time?
In the case of comets, I would say the observational advances have been a lot less dramatic than in many other disciplines. It’s still true that a large fraction of the comets are discovered by amateurs, just going out in their backyard and looking through telescopes. However, the development of large CCDs has enabled telescopes, both amateur and professional, to search much larger areas of sky with higher sensitivity, uniform backgrounds, and well-defined selection criteria.
Take, for example, the discovery of the Kuiper Belt. As soon as we made the prediction, we were aware that it would be a good thing to go and search for it. But it was only discovered some years later because Jewitt and Luu had access to a big new CCD that was able to cover a larger area per unit time than previous detectors. The next big advance is going to occur two or three years from now when the Rubin Observatory comes online because it will make a larger, deeper, and more uniform survey for comets and other moving bodies than has been possible so far, and I’m optimistically looking for a big jump in understanding at that point.
Who were some of your prominent graduate students during your time at Toronto?
I had fewer graduate students at Toronto than I did either before or after my time there, although that’s probably small number statistics. One of my students, Paul Wiegert, who did his thesis on comets and is now at Western University, was responsible for the discovery of the first asteroid co-orbiting with the Earth. He branched out into the study of other small bodies in the solar system, and has done very well with it.
What were your decisions in terms of moving over to the Institute for Advanced Research in cosmology and gravity?
Well, it wasn’t really a move. About the time I arrived at Toronto, a group of academics founded what they called the Canadian Institute for Advanced Research. This was an institute that was driven by the peculiar nature of Canadian politics. Their view was that rather than funding a bricks-and-mortar institute, they would set up a sort of virtual institute to support people distributed around the world working on a common problem. They would be brought together at regular intervals to collaborate. The two central aspects of this vision were that the Institute would have no physical presence, and, that the projects would last for only five years. They might get renewed for additional five-year periods, but there was no expectation of permanence.
That model has now been operating for several decades. I think it’s worked remarkably well, and I got involved with it simply because one of the programs they set up early on was a program in cosmology. Somewhat to their embarrassment, I think, given their expectation that programs would have a limited lifetime, this program has now lasted for over thirty years, successfully passing a pretty rigorous review every five years. Its focus of activity has shifted over the years somewhat away from cosmology and towards other aspects of astrophysics. At one point, I was asked if I could act as the director of the program, and I agreed to do so.
Were you thinking about black holes at all at this point?
I’d always been interested in what goes on at the centers of galaxies, but most of my thinking about black holes arose because of the Hubble Telescope. One of the original goals for the Hubble Telescope was to look for black holes at the centers of galaxies. With the higher spatial resolution of the Hubble Telescope you had the potential to see the changes in the distribution of stars in velocity and position that are expected close to the black hole. Since the Hubble Telescope observing time was allocated through competition, some group of people had to design the program, write the proposals, and analyze the observations, and we decided to do so.
In the mid-1980s a group of mostly observational astronomers—led at the time by Sandra Faber at UC Santa Cruz—decided that they should set up a collaboration to do this. Faber assembled an extremely good team and, fortunately, I got asked to join the team as one of the theorists. That collaboration was active for at least two decades, and I think did a significant part of the work of establishing the now-standard view that there’s a supermassive black hole at the centers of most galaxies.
Scott, so I understand the timing currently, you retained your directorship in Canada when you joined the faculty at Princeton. You sort of had both appointments at the same time, at least for a period of years.
I retained the directorship of the program at the Canadian Institute for Advanced Research, but at the end of my five-year term, I just said, “Look, I can’t really do a good job at this at the same time I’m department chair.” So, I stepped down from my administrative role in the Institute for Advanced Research, although I’m still involved in the program as an advisor.
Physically, were you shuttling back and forth, or were you located mostly in one area or the other?
Oh, I was mostly located in Princeton. The Canadian Institute for Advanced Research directorship involved travel half a dozen, maybe a dozen times a year, but not more than that. I was shuttling back and forth a lot, partly because of that job but also partly because my parents were in failing health at the time.
Given how familiar you are with the department of physics at Princeton, what was new or different about joining the department of astrophysical sciences?
The two departments have quite different traditions and are run quite differently. When I was a graduate student in the physics department, it very much operated on a kind of sink or swim principle. They were very casual about whether you took any courses or not. The courses didn’t have grades and had no particular connection to preparing for the general exams.
The astrophysical sciences department was much more hands-on, so they matched students up with supervisors for short research projects when the students arrived, and then they rotated them through a sequence of other research projects with different faculty for the first two years. They gave courses, which they took quite seriously, which they graded, and which were matched to the subjects you would want to learn for your general exam. They also had a pretty rigorous policy of getting people out quickly. The rule was, roughly speaking, that when you arrive, we’ll guarantee you funding for four years of your graduate study, and we guarantee that you won’t get any more funding if you don’t finish your thesis in four years. That was more than a year faster than the average US PhD in astrophysics.
And of the two, I very much preferred the model in astrophysical sciences to the model in physics. Its detractors sometimes called it the “milk and cookies” model of graduate school, but I thought it was really good. It wasn’t, of course, established by me. It dated back to when Lyman Spitzer was chair from the 1950s onwards.
Scott, what years were you most active in studying planetary migration?
I was never really very active in studying planetary migration. When Goldreich and I were working on planetary rings, one of the byproducts of the research was the recognition that satellites or moons always exchange angular momentum with nearby rings. In fact, one of the ways that you make narrow rings is by exchanging angular momentum with two nearby small satellites, and this process squeezes residual material in the disk into a narrow ring. We realized that this exchange process was quite generic, and so should also apply to a planet like Jupiter in a protoplanetary disk. The consequence of this angular momentum exchange was that Jupiter would spiral into the sun in an extremely short timescale, much less than the million-year lifetime of the protoplanetary disk.
At the time, we just said, “This is obviously a physical process that should happen. It seems a little strange because it predicts that we shouldn’t have a Jupiter, but here it is.” And we wrote a paragraph or two about it in one of our papers. If you then fast-forward 15 or 20 years, the observers began to discover giant planets (“hot Jupiters”) orbiting much closer to their host star than Jupiter, and they recognized that it would be hard to form such planets in situ. They were trying to interpret how they might’ve gotten there, and here was a ready-made process.
I’ve never been totally comfortable with using this process to explain the hot Jupiters. The process is so efficient that every Jupiter should spiral into its host star, and there shouldn’t be any planets anymore. That’s obviously not true, but no one understood then or I think really understands now why the migration happens sometimes but not always, or why and where it stops. It’s turned out to be quite difficult to understand completely and quite sensitive to small details. I’m sure that disk migration plays a big role in determining the current distribution and properties of planets, but I’m not sure how it does so. Thus, it was really just dumb luck that Goldreich and I got associated with this early on.
Scott, given the significance of your administrative work in Canada, when you accepted the position of chair in 1998, how well-defined were your goals in terms of the things you wanted to accomplish for setting the tone of where the department should be headed?
That administrative position was quite different from the one I took on at CITA. CITA was a newly formed institute, whereas Princeton’s department had been functioning for decades as one of the premier astrophysics departments anywhere. There was a lot more freedom at CITA to decide on the goals, and the direction it should go in. And in a certain sense, there was less pressure because you were starting from nothing, so you didn’t have the standard of past chairs to be compared with.
Princeton had many advantages including excellent faculty, a reputation that attracted very strong graduate students, a relatively large and flexible budget, and so forth. One problem I had when I arrived was what to tell someone who wanted to start a new initiative that I didn’t think was a good idea. At Toronto my standard response was “it’s a great idea, but unfortunately we have no money”, and I realized soon after coming to Princeton that I needed a different excuse!
One concern when I arrived at Princeton was the Sloan Digital Sky Survey, which at the time was undergoing serious growing pains. Although the scientific conception behind the survey was excellent, and in retrospect proved to be revolutionary, the project had grossly underestimated the budget and the time needed to make it happen. Nobody was quite sure when it would get on schedule or what the final budget would be. So, one issue was trying to manage that.
The second concern was that although Princeton had typically had very strong undergraduate and graduate students, it had a very small postdoctoral program at the time. And so, the other primary goal was to strengthen the department’s program of postdoctoral fellows.
What do you see as your key achievements, looking back, during these years?
Well, you mean my key good achievements or—
—or should I include bad achievements as well?
It’s an open-ended question.
I had two signature bad achievements. We had a faculty hire with a shortlist of two. I argued to hire candidate A, and although I’m proud of the decision to hire A, candidate B did get a Nobel Prize. My other signature bad achievement has to do with telescope building. Each of the past chairs built a telescope: Lyman Spitzer was instrumental in building the Hubble Telescope. His successor was Ostriker, who was instrumental in building the Sloan Digital Sky Survey. And his successor after was Draine, who was only there for two years but at least got a 12-inch telescope built on the roof of the building. And I didn’t get any telescopes built, so everything went steadily downhill over a succession of four chairs.
More seriously, we were very successful in building up the program of postdoctoral fellowship fellows. Princeton has gone from, I think, two to fifteen or so when I was chair and to something like fifty postdocs by now. And I think they improved the atmosphere and the level of activity in the department enormously, although perhaps by now the number is even too big.
As I have said, the Sloan Digital Sky Survey turned out to be extraordinarily successful. I’d say my only credit was to spend a lot of time in meetings in windowless rooms at O’Hare Airport, tamping down some of the controversies and continuing to support the project so that it didn’t crash and burn. The success is entirely due to the extremely talented people who built the survey, and the equally talented people managing the survey who continued to be committed to it, and [who] found ways to make it happen.
I’m also pleased that despite my administrative duties I supervised probably nine or ten graduate students in PhD theses. They were a spectacularly talented set of graduate students, and their theses have been very influential. We also made a number of faculty hires, and I think the quality of the faculty in the department now is as at least as high as it was when I got there. The department was in extremely good shape when I got there, and I’d like to think it was in at least as good shape when I left.
Was it during these years that you were focused most intensively on the Andromeda Galaxy?
The Andromeda Galaxy is a wonderful galaxy. It’s the nearest large galaxy to our own, and we have an unobstructed view of the whole galaxy which in many respects is much clearer than the view we have of our own Milky Way. Part of the work I did for my thesis was on the center of the Andromeda Galaxy. Once Hubble provided higher resolution images of the center, they found a surprising and curious structure: it looked like the nucleus of the galaxy was double, which is very puzzling and hard to explain because two separate nuclei in that location should merge in a time much less than the age of the galaxy. When I was on sabbatical in the mid-90s, I was able to focus on that puzzle, and spent several months thinking about it, and came up with a model in which what appears to be double is really a lopsided disk of stars that looks double when you view it with limited resolution.
What did you do next when you stepped down from chair? What did you want to do in terms of the research in 2006, 2007?
I had started doing administration relatively early on in my career. I moved to the directorship at CITA when I was 35. And although I was able to keep my research going pretty well at that time, and when I was department chair, I figured it was probably about time for me to step down and see if I could re-invigorate my research activities if I spent less time on administration.
David Spergel was in the department. He was a decade younger than me, I thought he would be a wonderful chair, and I wanted to make sure he got a chance to do it sooner rather than later. I was offered the to do administration at some higher level at the university, but I declined.
So this is the origins of your decision to move to the Institute? It would be the best place for you to accomplish these goals?
I was really curious to see what I could do if I had a larger fraction of time to do research.
Given that so much of your career was based on your work with collaborators, I’m curious who at the Institute at that time might’ve been particularly attractive for you to work with?
I was mostly looking forward to interacting with the strong and large group of postdoctoral fellows at the Institute. One consequence of the move was a switch from working mostly with grad students to working mostly with postdocs. The advantages of postdocs are that they’re already up to speed, they have a lot of experience doing research, they have tools at hand, etc. The disadvantage is that they don’t have to do what you tell them to do. With a graduate student, you would just say, “Well, go away, and calculate this,” with a postdoc, you have to persuade them to do it.
And it’s not always so easy.
Once you were able to shed these responsibilities and focus on the research, what was most compelling to you at that point, circa 2007, 2008? What did you want to work on then?
The study of exoplanets had really been booming for the last decade or so, and was about to take off because of the Kepler spacecraft, which was just starting to return results. So, although I wasn’t part of the Kepler group, I was really interested in the results that they were getting, and in trying to understand them.
In the end, I think Kepler was an extraordinarily valuable mission, but it didn’t yield the sort of data that was susceptible to the kind of clean predictions or physical puzzles that have shown up in other subjects like cosmology. Planet formation is a lot messier than the formation of structure in the universe.
I’m curious, during this time at all, if you were involved in advisory work with telescope projects?
I was involved in a lot of advisory work, but I’ve steered clear of large telescope projects in general, except for my role in the Sloan Digital Sky Survey. I’ve been on a lot of advisory panels like the decadal surveys that have prioritized telescope projects for the National Research Council, but not really on the telescope projects themselves.
[laugh] I’m curious what are some of the things you may have learned with your work as an advisor for NASA?
My only real roles have been to advise on things like fellowships or grant selection, or to prioritize for the decadal surveys. I’ve found that I’m comfortable offering advice on academic matters, such as how to manage an academic department. But I probably don’t have the patience for either the long lead times of NASA missions or for the kind of internal politics and bureaucracy that’s associated with them. I have a lot of respect for the people who do, but it’s never really been one of my strengths.
Now, on the decadal survey for the NRC, given that it was over 10 years ago at this point, and given how important these surveys are in terms of setting the tone, setting the agenda for the research, how well has that decadal survey aged? How well have the things that you and your colleagues were thinking about 10, 12 years ago, how well have they played out?
I think the answer is mixed. The first thing to recognize is that a big part of the mission of the decadal survey is to avoid recommending bad projects. There are plenty of opportunities to screw up in a decadal survey, and a badly managed decadal survey could recommend a project costing several billion dollars that was of really marginal scientific interest but consumed most of NASA’s resources for astrophysics for 10 or 15 years. So, the fact that none of the projects that were recommended have turned out to be disasters or turned out to be scientifically obsolete is a huge positive point in favor of the deliberations of the decade survey.
Obviously, the other recommendations have been moving disappointingly slowly. Part of the reason is that the survey was instructed to ignore the James Webb Space Telescope. It was to be taken off the table because the tradition is that you don’t comment on projects that have already started construction. Over the past decade, the cost overruns in JWST ate up a significant fraction of the NASA astrophysics budget, and made it very difficult to move forward on the other recommendations of the decadal survey. So, in that sense, it’s much more slowly than it should have, but that’s a consequence of the ground rules that were imposed on us rather than bad decisions by the survey.
Decadal surveys are asked to do a lot of things. They’re asked to prioritize a few large projects. They’re asked to prioritize dozens of medium-sized and small projects. They’re asked to comment on the health of the community and a variety of important societal issues within the astrophysics community. And I think they’ve been extremely effective at some of these tasks, but not all. They’ve been remarkably effective at prioritizing large projects, and thereby getting the community to back a common set of goals and projects, and avoiding the infighting that can cripple progress. I think they have not been anywhere near as successful in prioritizing and steering small projects, or in making recommendations that have had a significant positive impact on the community as a whole.
Scott, given that how you were so heavily involved in advisory work that had policy implications, during years that transitioned from the Obama to the Trump administration, from your vantage point, did you see shifts in support for basic science during the past decade?
I don’t think it’s an Obama administration versus a Trump administration. Obviously, the two administrations had very different attitudes towards science in general, and different degrees of respect for and attention to scientific advice. But I don’t think these have had a big effect on the actual experience of people in astrophysics trying to get research funding.
I think that the overall situation for science funding has been getting, I would say, slowly and steadily worse for quite some time. The issue is partly that, in many respects, budgets have not kept pace with inflation, partly that once you build a ground-based telescope, it continues to do useful science far longer than most other scientific instruments do. And that means that unless you make very tough decisions to turn telescopes off, your budget eventually gets over-stretched. I’ve sometimes joked that we’d be better off if all new telescopes had bombs with a twenty-year fuse buried in their foundations.
I think the other issue is that both NASA and the NSF are being asked to do much more. They’ve been asked to take on a lot of extra roles without a commensurate increase in budget. So the NSF, for instance, is asked to spend a lot of effort on improving the broader impacts of the research that they support. I think that’s a very worthwhile goal but, of course, to the extent that you do that, it’s that much less funding that’s available for supporting the actual research.
I’d also say that the bureaucracy has gotten steadily more demanding. When I was at Princeton, I came across an old file containing a proposal from the mid-60s to build a telescope on the Princeton campus. It was one double-sided page, double-spaced, including the budget, and it was for an amount that today would be equivalent to several million dollars. Nowadays, I think, first of all, the chances of writing a successful proposal like that are essentially nil. But if there was one, the proposal would be probably 100-200 pages long.
Scott, to bring your narrative up to the recent past, the last five years, what are some of the things you’ve been working on, and to what extent was your original plan to move to the Institute, to unburden yourself from other responsibilities, how well has that played out?
I’ve continued to be interested in comets, and in what you can learn from them about the outer parts of the solar system, including possible extra planets outside Neptune. I’ve been paying a lot of attention to the results from Kepler and other exoplanet surveys to try to understand what we can say about the overall distribution of extrasolar planets. I’ve become very interested in the structure of galactic nuclei, partly because of their role in determining the likely signals from low-frequency gravitational waves. I’ve been watching closely the results from the Gaia spacecraft, which have dramatically improved the precision of measurements of the positions and velocities of the stars in the Milky Way galaxy. I’ve been looking for simple, clean problems with interesting physics in them, but of course you can’t always find such problems. The main conclusion from Gaia, for example, is that the Galaxy’s pretty complicated, and not entirely in equilibrium. I’m not quite sure how to extract clean results from that complexity, and I’m not quite sure anybody else is either.
Well, Scott, now that we’ve worked our way pretty much up to the present, I’d like to ask for the last part of our discussion some broadly retrospective questions. The first is, and it’s something that we’ve touched on briefly before, the extent to which advances in computational power and in instrumentation have really been vital to the things that you’ve been able to do over the course of your career.
They’ve been absolutely vital. The most important change for observers has been the growth in size, and decrease in cost of CCDs, which has dramatically improved the efficiency of telescopes, the ability to do large surveys, the ability to look for transient events, and the ability to do accurate measurements.
For the theory community, I think the growth in computational speed, and the falling cost of memory, have been the primary drivers of progress in theoretical astrophysics. I think my generation of both observers and theorists has been extremely fortunate. You know, in an oversimplified crude sense we’ve been able to surf along doing exactly the same thing throughout our careers. It’s just that every five years, you take the same problem or object, and you observe it again with a much better instrument or run it again on a much better computer, and you get much better results.
Eventually, those trends are going to stop, and it’ll be very interesting to see to what extent the theory and observational communities are able to adapt to that change: will progress actually slow dramatically or will there be advances in software, different advances in instrumentation, etc. that will enable us to keep going? No exponential growth can continue forever.
The cost of state-of-the-art telescopes in many cases is now over a billion dollars. If you extrapolate just the energy costs of the largest supercomputers, they become prohibitive within a few years. We are facing limits, we’re facing them soon, and the community will have to readjust to work within those limits. I think those trends have affected me less, because I don’t do observations, and I haven’t done very much in the way of large-scale computation. I’ve tried to focus more on looking at simplified problems that provide some insight that hasn’t been available before. But I’ve certainly benefited indirectly from the new observational results.
Scott, what do you see as some of the major advances in galaxy formation, spanning from the beginning of your interest in this all the way up to the present day?
In a broad sense, the most important advance is that we’re now able to put galaxy formation in context. We have a well-defined model of cosmology involving cold dark matter. There are still some parts that we don’t understand, and it may turn out not to be the whole truth. But that model of cosmology provides a very good idea of the initial stages of linear and non-linear structural formation in the dark matter. The complication emerges once you form stars, and once you have supernovae, once you have shocks, once you have gas cooling, the problem becomes much more complicated. But we now at least have the framework in which to do those calculations. You’re not scratching your head, and saying, “Well, let’s imagine a uniform, homogeneous gas cloud, and ask what happens when it collapses.” We know with extraordinary accuracy what the initial conditions for the complex parts of galaxy formation are.
The other critical but more worrying advance in understanding is that we now know that there’s a very important interaction between supermassive black holes and galaxies. The simple way to phrase it is that the mass of the black hole in the centers of most galaxies is only a few tenths of a percent of the mass of the galaxy. But the energy released in forming the black hole is many thousands of times the binding energy of everything else in the galaxy. And that means that the fate of the gas that’s trying to form the galaxy depends critically on whether 1% or .1% or .01% of the energy released by the formation of the black hole is effectively coupled back into the gas surrounding it. This is typically called the feedback problem. The nature of galaxy formation is extraordinarily sensitive to the details of black hole feedback, and I think that’s a pretty worrying sign for hopes of quick progress on a complete theory of galaxy formation.
Scott, these are fields that have operated in the periphery of your career and, of course, in our conversation. I’m curious if you could answer broadly the extent to which advances in both cosmology and general relativity have been useful for your research interests, and inversely how your research interests have advanced these fields?
Advances in the theory of general relativity have had a pretty modest effect. That’s not to say that there hasn’t been tremendous progress in general relativity. There was Penrose’s work in the ‘60s that showed how robust black holes were. There was work by many people on numerical relativity, culminating in Pretorius’s calculation of the collision of two black holes. But as far as astrophysical implications are concerned, the theory that we need to understand black holes and most of cosmology is mostly fifty years old or more.
The advances in cosmology certainly have been relevant for galaxy formation, as I said. The good news is that the effect of dark matter on galaxy formation and cosmology is almost independent of its properties, so long as it’s collision-less, cold, and non-baryonic. And, of course, this is also the bad news, since it makes it very hard to constrain the nature of the dark matter without actually detecting its non-gravitational interactions.
Scott, it would be impossible to talk about all the awards and recognitions you’ve received over the course of your career, but I’m curious if any single one stands out that’s most personally meaningful to you.
Two have been particularly meaningful: one was a fellowship in the Royal Society of London. It meant a lot to me, first, because it was one of the first major recognitions I got, and second, as somebody who grew up in one of the former colonies, when you get a pat on the back from the people in Great Britain it always seems a little extra special.
And last year I got the Russell Lectureship from the American Astronomical Society, and I was very pleased that the citation said that part of the reason for it was mentorship of young people.
Scott, last question, looking forward, we’ve talked about this before in terms of your awareness of theoretical or observational limitations. What do you see in terms of those limitations, looking forward to the next decade or two decades, and what are you most curious about and optimistic about in terms of overcoming those limitations in our ongoing understanding of how the universe works?
What worries me the most is that we have this extremely successful model of cosmology. The trouble is that the three legs of the stool holding this up are inflation, dark matter, and dark energy, and we have absolutely no idea what any of the three are. And if the current theory continues to successfully predict everything, then it’s going to be extremely hard to get additional insight into the nature of those three legs of the stool. We might find at the end of the next decade or even at the end of this century that we have an extremely successful model of cosmology that’s based these three basic principles that we don’t understand at all. I think that’s the real long-term challenge and opportunity in cosmology.
How are we going to figure it out? I don’t know. It could be that we’ll get some clues from particle accelerators or laboratory experiments to look for dark matter. It could be that we’ll find anomalies that don’t fit the current cosmological model, like this possible anomaly in the Hubble Constant that people are putting a lot of work into these days. It could be that some brilliant young theorist will sit down with a pencil and a piece of paper and work out a beautiful theory that is so compelling that we have to believe it, and it explains all these things.
More generally, I think astronomy is facing a similar problem to what experimental particle physics has faced earlier on: the next step always requires a more expensive, more ambitious instrument with a longer and longer lead time. And eventually we won’t be able to afford the next step.
And for you, Scott, personally, what do you want to accomplish? What are the things that are most compelling to you personally where you feel like there’s a reasonable chance for ongoing fundamental success?
On the subject of exoplanets, I’ve always found that my colleagues can be split pretty cleanly into two kinds. There’s the people who study exoplanets because they want to discover the nearest habitable planet. They want to see oxygen in the atmosphere. They want to see a biosignature, and they want to think about how we could go there, and ideally, they’d like to find intelligent life.
The other group of colleagues wants to understand the diversity of planets. What do planets look like? What are the possible varieties of planets? How many are there? What sizes are there? Do they have atmospheres? How far away from their star are they? Are there interstellar planets? They want to be able to understand the richness of worlds that are out there.
I think the first group of colleagues may be disappointed with our progress over the next couple of decades, but I bet the second group will be really satisfied. And fortunately, I’d place myself in the second group.
Scott, it’s been great talking with you. I want to thank you so much for spending this time with me, and for sharing your perspective and insight over the course of your career. It’s really important historically and for so many reasons, so thank you so much. I really appreciate it.