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Credit: Daniel Addison
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Interview of John F. Hawley by David Zierler on July 5, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with John Hawley, John D. Hamilton Professor of Astronomy, and Senior Associate Dean for Academic Affairs in the College of Arts and Sciences at the University of Virginia. Hawley discusses his responsibilities as Associate Dean and he conveys his ongoing interest in black hole observational work and in the future findings of the James Webb Telescope. He reflects on his career’s overlap with the rise of computational astrophysics and he explains why he is agnostic on the hypothetical value of quantum computing to the field. He recounts his childhood in Maryland, then Kansas, and then northern California, in support of his father’s work as a minister, and he describes his undergraduate education at Haverford where he developed his interest in astronomy. Hawley explains his decision to work with Larry Smarr as his advisor at the University of Illinois, and he describes the origins of the Supercomputing Center. He describes the opportunities that led to him to Caltech to work with Roger Blandford, who was working on jets and active galaxies, and where he pursued synergies between analytic and computational analyses of black hole research. Hawley emphasizes the proximity to NRAO that influenced his decision to accept an offer from UVA, and he discusses his foundational collaboration with Steven Balbus on accretion disks. He explains his motivation to write the textbook Foundations of Modern Cosmology, what it was like to win the Shaw Prize, and how his administrative responsibilities gradually and mostly overtook his research agenda. At the end of the interview, Hawley reflects on the complementary nature of his technical collaboration with Balbus, and why he thinks terms of numerical and analytical approaches as separate endeavors.
This is David Zierler, oral historian for the American Institute of Physics. It is July 5th, 2021. I am delighted to be here with Professor John F. Hawley. John, it’s great to be with you. Thank you so much for joining me today.
Great to be here.
John, to start, please tell me your title and institutional affiliation.
I'm at the University of Virginia. I'm the John D. Hamilton Professor of Astronomy, and Senior Associate Dean for Academic Affairs in the College of Arts and Sciences.
John, on the administrative side, what are you doing in your capacities as Associate Dean?
From 2012 to 2020, I was the Associate Dean for the Sciences, and then I moved to the Associate Dean for Academic Affairs; basically, the portfolio covers the three divisions of the College, which are Arts and Humanities, Social Sciences, and Sciences. I'm one of the most experienced associate deans in the College right now. I assist the divisional associate deans, and I'm sort of the expert in policy and procedure. Right now, I'm working on improving college policy and procedure as it relates to equity and inclusion, amongst other things going on.
How is University of Virginia dealing with the pandemic? Is the plan to go back full in person, in the fall, or is it adopting more of a remote hybrid policy?
We're planning to go back pretty much to full in-person classes. We do have a vaccination-required rule, so the students are required to be vaccinated. There’s a similar rule for faculty and staff. The administration is optimistic that they will be able to run something that looks more like a normal semester in the fall. I hope that that’s true.
Now with your administrative duties, are you able to maintain an active research agenda in astrophysics?
It has tapered off. Since becoming the Academic Affairs Dean, I haven't been doing too much in astrophysics, I must admit.
How long do you plan to stay in this role, and do you have a plan to return more full-time to your academic specialty?
I would say right now probably I'm looking towards retirement, probably at the end of another session as the academic dean. It’s getting towards that time.
[laughs] To the extent that you're able to keep up with the literature, what you are interested in right now, and what are you following more broadly in the world of astrophysics?
I’d say right now I'm following the observations of black holes, mainly. Of course, the gravity waves that they're observing with LIGO and Virgo are [laughs] pretty amazing. When I was at Caltech, which was 1984 to 1987, LIGO was a dream [laughs] and they just were creating offices for LIGO and had submitted the big proposal to the NSF at that time. So, I was thinking at the time, and postdocs and graduate students were thinking, “Boy, this is a long shot.” [laughs] That is, actually getting this project to go, and to actually detect anything. I think there were some cynical remarks about maybe if there were a couple of neutron stars that smashed together out around the orbit at Pluto, they might see it. So, it’s sort of amazing to me, with those observations now, what they're able to see. And of course, the Event Horizon Telescope is actually beginning to image the near-hole region; that also is quite amazing, and something that certainly we didn't anticipate years ago.
That’s right. John, a nomenclature question as it relates to your area of specialty—where do you see the boundaries between astrophysics, astronomy, and cosmology? And have those boundaries shifted at all, over the course in your career?
I would say if they have shifted, they’ve become more unified. When I started years ago, cosmology was sort of its own thing. And before I started, cosmology was even more of its own thing because it seemed a little speculative, and observations to back it up were pretty primitive. And similarly, with black holes, when I got started, it was still in the time when Kip Thorne and Steven Hawking had their famous wager going as to whether or not black holes actually existed. And x-ray astronomy was young, in its infancy as it were. When I was a graduate student, the VLA had come online, and they had just begun to take snapshots, images of active galaxies, and you could see that there were radio-emitting jets in those galaxies, but you didn't know what was going on. And so things were a little compartmentalized.
And I think nowadays, we have enough data and observations that cosmology is now unified with the rest of astronomy. Black hole studies are just another aspect of stellar history, and they're also tied to cosmology, because black holes seem to form so early in the universe. And general relativity has gone from an esoteric mathematical exercise to observations. And so, it really seems to me that the artificial boundaries, as it were, between the subdisciplines, have really broken down, and people see everything now in a more holistic way than they might have 30 or 40 years ago.
John, a related question that’s equally broad—obviously all advances in the field require a productive interplay between theory and observation and experiment. Today, where do you see the leader in the field generally? Is theory leading observation, or is observation leading theory?
I would say right now observation is probably leading theory, which is a good thing, actually. [laughs] Again, back to cosmology, and again taking the long view, 40 or 50 years ago, what were we talking about in cosmology? Well, we knew the cosmic background radiation existed. We knew that there were external galaxies. We knew some of them were active. And nowadays, we've got data from the COBE, WMAP and the Planck experiment. We've got the exquisite observations from radio telescopes, new radio telescopes that are more powerful. We're just beginning to see what, for example, the MeerKAT observatory can do. We have the hope of the James Webb Space Telescope coming up. I think at the moment, the richness and the capabilities of what the observations are able to show us are probably now getting out ahead of theory.
Certainly, for the gravity wave observations—theory is trying to keep up. But I would say the observations—to take an example from cosmology—the Hubble question, the difference between the Hubble constant as determined, say, from the CMB observations and from the more traditional galactic distance observations, is challenging theorists. People are talking about using gravity wave observations to obtain a more accurate value for the Hubble measurement. And I would say right now theorists probably are a little bit—not quite at sea, but a little bit lost—I haven't seen any good theoretical reasons for why the Hubble constant should be measured at two different values, when it should be all converging to one value. I think the theorists have a little work cut out for them at this point.
John, looking to the future, what are you most optimistic about, regarding some of the ongoing mysteries in astrophysics, that might be resolved in a timeframe when you'll still be active and able to process what has been understood?
Gosh. I guess I'm very hopeful that the James Webb Telescope could give us images and data about the first galaxies as they are forming. There’s still a question of what goes on during the earliest moments of galaxy formation and how that might relate to black hole formation. I remain personally blown away by the gravity wave observations, and there’s no telling what they might see, as they continue to improve LIGO and other gravity wave observatories. Current thinking is focused on the so-called multi-messenger idea, where the different measurements are all being brought together—radio, optical, and now gravity waves. So, I would like to see multi-wavelength, multi observations of something like highly detailed black hole neutron star collisions, or neutron-neutron collisions. Some of these events are ones that we dreamed of over the years, but always wondered what they would look like. I guess from my perspective, those are the things I’d really like to see.
John, an administrative question as it pertains to UVA—if I understand correctly, the Department of Astronomy is a standalone department, not included within physics. What are some of the advantages and disadvantages to that breakup?
Well, I think historically it’s an advantage to an astronomy department to be separate, because then they can govern their own fate, as it were, and set their research priorities. I suppose in a Physics/Astronomy department there could be a tendency for astronomy to get subsumed in the larger world of physics. Historically, UVA’s Physics Department has been very active in particle physics, for example. They're associated with the Jefferson Lab. They've got people doing experiments at Oak Ridge, at Fermilab. They're also big in condensed matter, AMO physics and some quantum physics. But they have now, in the last few years, hired people into general relativity, experimental general relativity, as it were.
I think if an astronomy department can manage it, I think it’s to their advantage to be in control of their own fate. But it’s nice to have a Physics Department next door, so that the two sides can continue to be stimulated by each other, and to work on things like general relativity, when you have people in, say, both camps. I think it has worked out well for astronomy at UVA, particularly since Charlottesville is the headquarters for NRAO, with which the Astronomy Department has been associated and whose faculty have collaborated with the scientists at NRAO. And in recent years, the Department has associated and collaborated with the Department of Chemistry on astrochemistry, which also relates back to NRAO and the sort of science that ALMA is doing. The Department having its independence has been good, but it also enables them to determine where they would like to have interdisciplinary, interdepartmental efforts. I think it has served them well over the years.
A broad question as it relates to technology—given your area of expertise in computational astrophysics, in what ways has computational power grown over the decades that allows the field to do things now that weren’t even imaginable 30 years ago?
Well, if you look at my career, which pretty much spans it, the first computer I did anything on in college was a PDP-8, which was a lot of fun. You flipped a bunch of switches to boot it, and you had a teletype to communicate with it. We ran little programs in BASIC to solve the Schrödinger equation in a non-analytic potential [laughs]. And it would sit there and compute, compute, compute, and then print out your numbers for you. Today, you could make an app to do that [laughs] if there was a market for a one-dimensional Schrödinger equation app, I suppose. Anyway, from there I went to punch cards and to the centralized, so-called big computers that universities had in those days. Then I cut my teeth on a Cray-1, which had a million words of memory, which was astounding.
It was capable of 100 megaflops, which was also massively astounding. Today, of course, you go to the biggest centers in the world, and they're talking petaflops, even exaflops, and the amount of memory—people say, “What? This node only has 120 gigabytes of memory? What kind of low-quality stuff are you trying to pawn off on me?”
So today the problem isn’t getting the computational power, so much as getting the software and figuring out what problem to run that makes sense. Computing has come an enormous distance over the course of my career, at least.
Is there a generational component to it as well, in terms of contemporary research? In other words, are graduate students and postdocs really ahead of the curve, even from professors, in terms of what they're capable of doing?
[laughs] To some extent, that’s probably always true, because the postdocs and graduate students may be a little bit more daring, as it were. I think that today, students are used to computers. Nothing fancy, but I don’t know that they necessarily have a leg up, except that, without naming any names, a lot of older faculty have difficulty dealing with the relative complexity of the computing today. They still tend to think in terms of software that was maybe state of the art 15, 20 years ago. Whereas the young people probably are a little more willing to learn. So, yeah, the young people probably have a certain advantage. I've been involved, also, with computer administration at UVA, and one of our problems that we have right now is getting full professors to understand the capabilities of the systems we have. They're not probably going to run anything on the computers themselves, but they don’t even know what the computers can do at this point.
Looking to the future with regard to computers, are you optimistic that quantum computing, if it ever comes to pass, will be valuable for astrophysics?
Well, I don’t want to say no, because [laughs] we don’t know what quantum computing will look like. You could ask, “Well, will GPU computing be valuable to astrophysics?” And, “Maybe.” There are some people who have put a good deal of effort into adapting astrophysics codes to run on GPUs, but it hasn’t been a game changer. Will quantum computing be a game changer? I guess I don’t see it, but maybe? Maybe I just lack the vision to see what quantum computing could do.
Again, I think right now the issue is more one of modeling and simulation software, and basically knowing what you can do. Even if you had a really great code, what exactly is the problem you're going to set up, and what are you going to learn from it? Because you can’t just type, “Model galaxy,” press return, and be done. You have to set the problem up in some way that makes sense, and then you have to run the problem, and then you have to analyze the problem in a way that leads to insight. So even if you had the most powerful computer in the world, [laughs] knowing what you want to model and then getting insight out of it are the things that still rely on you, not the computer.
Maybe another way to get at the question is, are there obvious gaps in classical computing that quantum computing may resolve?
Again, I guess I don’t see it, necessarily. I think there could be something in signal processing or data reduction that might be more interesting. But just in terms of simulation and modeling, unless there’s some sort of interesting breakthrough in how we solve PDEs that goes beyond the sort of techniques we have today, I see that as being more the limit than what the computer might be capable of. But that could just be because I'm not thinking expansively enough about it.
John, let’s take it all the way back to the beginning. Let’s start first with your parents. Tell me a little bit about them and where they're from.
My parents are Bernard Hawley and Jeanne Hawley. My father was a Presbyterian minister for his career. My mother got a degree in music, and she basically took care of us, but she did teach piano and direct the choir in the church. Their roots come from the Midwest, although the Hawley family goes back into New England, of course, where all the Hawleys are, and upstate New York was sort of the immediate generation before my paternal grandparents. My grandparents on my mother’s side were in the Arkansas, Kansas, Missouri area. In fact, my grandmother’s family side, if I remember correctly, was on the Confederate side, and my maternal grandfather’s family side was on the Union side, which was reflected in the fact that my maternal grandfather always voted Republican, and my maternal grandmother always voted Democratic, for the whole of their lives, which was up to about age 90. I would say that both sides of my family were fairly rooted in an unexceptional sort of normal ordinary existence, I guess.
Now, did you move around a lot, in support of your father moving from pulpit to pulpit, or did he spend most of his career with one congregation?
He spent most of it in one congregation. I was born in Maryland, but at age six we moved to Salina, Kansas, where my father became the head pastor of the First Presbyterian Church of Salina. And he stayed there basically until the mid-1980s, where he took a job with a Presbyterian church in San Marino, California; that was 1987. And then he took a church in Palm Desert, California, before retiring, sometime in the early 1990s. I don’t remember the exact date. So basically, all of my “wonder years,” as it were, from age six until I left for college, I was in one house. We didn't move around and I had a pretty stable, secure existence.
Would you say your father’s theology was more on the evangelical or fundamentalist side, or more on the liberal side?
He was very liberal. [laughs] I've never heard the complete story, but somehow he got into some battle—I guess it was either when I was very young, or before I was born—with I guess one of the big evangelical preachers—I don’t know who—about some point I think involving racism or civil justice. I never really heard the whole story. But he was [laughs] advised—he got advice from a lawyer friend that—“Don’t bother with this fight, because they're not interested in learning from you, and you're not going to win.” He was quite liberal, I would say. Ran a modernist church, active in basically trying to promote, to the extent he could, social justice.
Is that to say that, growing up, there was no apparent conflict between religion and science in your household?
No, I would say not. No. He was very interested in science. And he always supported our education—my parents gave us kids lots of books to read. My mom took us to the library all the time so we could check out books to read. They encouraged us in science. From the very beginning, they encouraged us to learn everything we could. My siblings are Steve, Diane and Jim, and we all were interested in reading and learning. They have all followed intellectual pursuits. More specifically, my older brother was into astronomy before me. In fact, when we lived in Maryland, I have a memory of his receiving a telescope, I think at Christmas time. He went on to become an astronomer, and I don’t know if you knew that he was actually a space shuttle astronaut. Steve Hawley.
So he had an interest in astronomy from very early on, which obviously my parents encouraged. He built telescopes when he was young. He did lots of observations. He went to the local university and talked to astronomers there. Obviously, that was somewhat influential on me. [laughs]
What’s the age difference between you and your brother?
Eight years. Yeah, so he’s the admired older brother, so of course I'm going to be very interested in understanding more about the things he was interested in.
Would you say that in high school, your high school offered a strong curriculum in math and science?
Honestly, not particularly. It was okay. I did get calculus, but it wasn’t AP calculus or anything like that. We didn't have AP courses. Physics was so-so. The only interesting thing there was my class was the last class in that high school to learn the slide rule.
Because when I was taking physics, the physics teacher, who was also the wrestling coach, got himself an HP calculator, and at that point, he lost interest in the slide rule.
So we spent a bunch of weeks learning the slide rule, which I don’t [laughs]—I certainly don’t resent. I mean, the slide rule is kind of cool, actually. But it sort of marked the transition, in some sense, between one generation and another.
John, where did your brother go for undergraduate, and did that affect your decision at all?
He went to the University of Kansas. His decision actually did affect my decision in a certain way, which was that as the eldest son, my father was keenly interested in getting him the best possible education. And so we took a college tour when he was in high school. I was just tagging along, obviously; it was a family vacation. But we visited high-quality, small liberal arts colleges in the Midwest, like Carleton and Beloit. I can’t remember them all now. And my father was very much hoping that Steve would pick one of those fine, small, liberal arts colleges to go to. And he decided to go to the University of Kansas. And my father I guess realized that, “Well, I need to accept that my children can make their own decisions. It’s not up to me to tell them what to do.”
But, that influenced me, because I was impressed by my father’s passion for small, liberal arts colleges. And even though I wasn’t really fully aware of the struggle, as it were, between the different points of view, it did impress on me the desirability, perhaps, of going to such a college, which is what I ended up doing. In fact, I was the only one of the four kids who ended up going to a small, liberal arts college. Everyone else went to the University of Kansas.
Did you appreciate at the time how strong Haverford was in the sciences?
Yeah. It was a factor that I knew they had an astronomy department, which was unusual for a small liberal arts college. And I was impressed by how Haverford was discussed in a book we had, which I believe was called The Insider’s Guide to U.S. Colleges and Universities, I think, where they described Haverford as their model for the ideal small liberal arts college. I found that attractive—I looked at that course catalog and everything, before deciding to go there, and saw they had a lot of great science courses. But at the end of the day, the decision for me, at least, was straightforward.
And was it astronomy from the beginning? Was that what you knew you wanted to pursue?
Astronomy and physics, yeah. I did major in both. But I did want to take astronomy—I particularly was interested in taking cosmology. Because at that point, cosmology just totally fascinated me, having read some essays about it, and there were Scientific American articles about cosmology that I had read. And at the time, of course, the main question of whether the universe was open, closed, flat, was wide open. There were even some lingering shreds of the steady state theory floating around. It was all very unknown. But like I say, I just found it totally amazing and fascinating. And the prospect of being able to take a course in cosmology and astrophysics just basically was a great attractant for me. And indeed, the course I took at Haverford was quite good. It was taught by Bruce Partridge, who did his thesis work at Princeton on the cosmic background radiation. He was a renowned educator at Haverford, and also involved with the very early days of studying the cosmic background radiation; he certainly knew cosmology.
Given that Haverford places such an emphasis on teaching, who were some of the mentors that really exerted a formative intellectual influence on you, as an undergraduate?
Well, I did mention Bruce Partridge. In a way, he was so revered on campus that he was a bit intimidating. But the other astronomer at the time was Keith Despain, and he was very influential for me. In fact, he had done computational work with Willy Fowler at Caltech on nuclear reactions in stars. And so that was sort of my introduction. And he taught the numerical methods course, and the stellar structure course. He introduced me to computational astrophysics through stellar modeling. My senior thesis project was with him to compute a nuclear reaction network in a shell-burning region of a red giant star, to see what sort of isotopes it would produce and in what ratios. He got me started with a passion for computational astrophysics. There were some others, as well, but I would think he was probably the most influential.
This sounds like you were exposed to some pretty advanced stuff as an undergraduate.
Yeah. [laughs] In a sense, that was kind of the advantage of being an astronomy major. There were four of us—four astronomy majors—and two professors. So you could get a lot of [laughs] attention, if you wanted it. And of course, Haverford did try to involve students as much as possible, in various research projects. I had some friends who were physics majors that did some things with faculty—a physicist named Jerry Gollub was working on turbulence studies, and some other ones worked on something involving condensed matter. At Haverford, the scientists were research active, but they didn't have graduate students, so undergraduates were their assistants in the lab, as it were. There were a lot of research opportunities for students who were interested in them.
Did you have any summer internships that gave you a broader perspective as an undergraduate?
No, I didn't. [laughs] My brother, Steve, had gone to NRAO for a couple of years, and did some other things. I applied to all that stuff. I applied to Arecibo and NRAO, maybe some others, and I didn't get in to any of them. [laughs] I don’t know why! But that was the way it went. So I spent my summers back home, and I had a job at a medical lab. So [laughs] I logged samples—when they came in from the hospitals, logged them into the big ledgers that we had, and then helped with entering patient information into the computer system that did the blood chemistry exams. I was sort of involved in science, at that point, but it was a bit more on the clerical side, surrounded by test tubes and other things.
John, what kind of advice did you get about graduate programs to apply to, or even specific professors to work with?
I don’t remember anything specific. Certainly, I would have discussed it with both the professors at Haverford. I got some advice from my brother on what to do, and what to look for. And I sent in my applications to a large number of graduate schools. And I guess in the winter of 1980 was when I did the grand tour of visiting a whole bunch of graduate programs. I think at this point I lose track of things—I don’t remember the details of a lot of that.
I do remember visiting the University of Texas, because Craig Wheeler was my host, and he picked me up and took me to the hotel, where the visiting graduate student applicants were staying. And for some reason, I had been assigned a suite. I think maybe it was the last room they had or something. And Craig Wheeler, he said, “Well, hope this is big enough for you!” or something like that, which at the time amused me greatly. I certainly got to know Craig Wheeler better later in my career. After that visit, I went directly from Texas into the teeth of some sort of snow/rain thing at the University of Illinois. If there was going to be a reason to go someplace based on weather, I certainly wouldn't have ended up at Illinois.
But Larry Smarr was the graduate student recruiter there that year, and he put a lot of effort into it. He showed me some work he had been doing, early work on simulating black holes, and black hole accretion, with a computer. And I thought, “Oh wow, I want to do this.” So that was the end of that, for me.
Was Larry deep into supercomputing at this point, or that’s earlier?
He was getting there. This was before the Supercomputing Center. But his thesis had been to calculate the gravitational waves produced by two black holes colliding head-on, and he had used a computer at Lawrence Livermore Lab, working with Jim Wilson, who was a researcher there. Larry basically pioneered numerical relativity—that experience convinced him that computational techniques were the way to make progress. By the time I got to Illinois, he was dedicated to computational astrophysics and already thinking about how it could be possible for U.S. universities to get more computational power than they had at the time. I’ve got a lot of stories about this, if you want to hear them.
Well, like I said, Larry had done his work at Livermore, and he was aware, at the time, that basically the only way an American researcher could get access to the top-of-the-line computers was at the weapons labs. He had a Q clearance, and a number of other prominent researchers at the time also had Q clearances and would go to the labs in the summer to work on their research. And of course, he was working with Jim Wilson. Jim Wilson is somebody who history probably should remember better than it does. I don’t know the degree actually to which they do remember, but he was a researcher, like I said, at Livermore, and in his spare time, he did astrophysics, and was really a pioneer in computational astrophysics. He was interested in supernovae, and he did have an impact there; he was kind of the first person to realize that neutrino flux from the core could reenergize the shock wave coming through the supernova.
But he and Larry got into computing gravity waves from black holes in the mid-1970s. Of course, people do that all the time now, but that was the beginning, and all these decades between then and now—to get to where we are now—represented huge amounts of effort by lots and lots of people, in developing and refining the techniques for numerical relativity. But Larry was right there at the beginning. And he had done computer animations of the gravitational waves that were produced. This was one of the first films of a computer simulation, certainly of black hole gravitational waves, and this was very pioneering.
But Larry was very appreciative of the fact that the labs had these state-of-the-art computers, including the new Cray-1, and the university had some old CDC computer of some sort that was just barely above the punch card era. He began to collaborate with Karl-Heinz Winkler at the Max Planck Institute in Garching bei München. And in 1981 he spent the summer there. And what Max Planck had was a Cray-1. Not one of the very earliest ones, but certainly a very early one. And if you’d like to see that Cray-1, it’s now preserved at the Deutsches Museum in Munich. But they were working on simulating astrophysical jets along with Mike Norman, who obviously went on to have his own stellar career in astronomy. They were using the Cray-1 and producing simulations at a sort of refinement and resolution that nobody in astrophysics had ever seen before.
And so, I visited Max Planck in 1982 for a NATO conference and then again to do research in the summer of 1983, and then in the spring of 1984. That’s where I did my simulations for my thesis. But the fact was that Larry wanted to get a Cray-1 at Illinois—and then it became a Cray X-MP. But that’s where he started really getting involved in the need for supercomputing access for American researchers. And in 1983 he submitted the unsolicited proposal to the NSF for a Supercomputer Center at Illinois. It became known as the Black Proposal, because it was in a binder that had a black cover. But it was an unsolicited proposal, and the NSF said, “Great idea, but let’s do this right.” And so that was the beginning of the NSF’s supercomputing program, and Illinois was one of the recipients. Larry launched NCSA, the National Center for Supercomputing Applications, in 1985. As a footnote, user IDs on the system began at 10000, and I think I was UID 10001.
Now, was this development relevant for this thesis research, or this was sort of a separate track?
It was separate, but in some ways, I provided the narrative that Larry used. His elevator line, as it were, was “An American researcher has to go to Germany to use an American-made supercomputer.” And I was the researcher that went to Germany [laughs] to use the American supercomputer. I supplied him with lots of nice images of black holes with matter around them, and I made movies of matter falling into black holes, to demonstrate the power of computing that could be done on something as advanced as the Cray. So in that sense, my research added to the narrative—he was off to the races [laughs], just on the basis of what you could do with a Cray compared to anything else at the time. It was so remarkable and could easily convince one that these new supercomputers will really make a complete, tremendous difference in what we're able to do in terms of simulation and modeling.
What were some of the broader research questions in the field at that point, and how did you see your thesis research being responsive to them?
Well, if we just confine our attention to black holes at the moment, sort of the big question began where—sort of following on to Larry’s research—was what sort of gravitational radiation is produced by black holes interacting with each other? What’s the amplitude? What do the wave forms look like? And can we compute them? As I'm sure you're aware, in general relativity the equations themselves are hideously complicated if you start actually trying to solve them. They're very simple to write down in their basic tensor form, but if you actually try to solve something, you quickly run into problems. There’s a famous story of Einstein not imagining that anybody was ever going to get an analytic solution to them. And of course, Schwarzschild solved one sort of before the words left Einstein’s mouth. Gravity waves were a big area of research. The big question was, what are the sort of numerical techniques we can bring to bear on the problem to manage the complexity of basically solving for the gravitational space-time, when you also have to solve for the coordinate system—you have to start with a mesh, but the mesh has to be part of the solution in GR.
For black hole astrophysics, the questions were basically, what is the structure of the accretion disks around black holes? A lot of the fundamental contours of the problem were laid out by Shakura and Sunyaev in their seminal 1973 paper. The structure of a thin disk is determined by internal stress of unknown nature, but which could be parameterized as a function of the pressure. And from that follows the accretion rate and what sort of radiation was being produced; these were the big theory questions. At the same time, x-ray observations had revealed black hole candidate systems and their diverse properties. And more to the point, the VLA was producing high-resolution maps of radio jets emerging from the cores of active galaxies. And so something very energetic happens in the cores of active galaxies. What is it? How do they produce jets? How do they produce the radiation we see? Those were all the big questions.
My little niche on that followed from Larry’s fundamental belief that the answers to these questions could be obtained through numerical simulation and modeling. If you could calculate the behavior of matter orbiting around a black hole, then that would be the route to understanding how black hole phenomena seemed to account for what was observed in quasars, active galaxies, and how they produced jets. Much of this stuff was unknown. At the time, lots of ideas flying around, lots of notions, people putting forward ideas, and it was difficult to know for sure what was going on. It was a very exciting time.
Besides Larry, who else was on your thesis committee?
There was Bob Wilkinson from Atmospheric Science. Because [laughs] people over in Atmospheric Science were the other people at Illinois doing simulations. Bob was actually simulating tornadoes and severe weather. He was sort of a fellow traveler with Larry in terms of believing in the power of computational simulation and the need for greater capacity to improve resolution. He brought some additional computational modeling expertise to my thesis committee. If I remember correctly, Jim Truran and maybe Ron Webbink were the other members of the committee. But it’s kind of lost in the dimness of time. [laughs]
What opportunities were available to you for postdocs?
My recollection here was there were quite a few. But I wanted to go to Caltech. [laughs] And that was because of Roger Blandford, whom I'm sure you've heard of.
Because I had met him at some conferences, and he was of course very interested in active galaxies and jets. He had some interesting ideas. And I think in the same vein, while he didn't do numerical simulations, he saw the promise of them. I saw that as being in a place where there was a lot of astrophysical theory going on, about the radio observations, and also the optical observers there were doing a lot of stuff with observations of active galaxies. Of course, they were also experts on space observations as well. It just seemed to me to be sort of like ground zero of black hole research. So whatever other opportunities I had as a postdoc, I [laughs] was primarily interested in Caltech.
What was Roger working on specifically at the time you joined him?
I would say jets, if I remember properly. Very interested in jets. And active galaxies. He wrote a Rev. Mod. Phys. article with Martin Rees and Mitch Begelman on the Theory of Extragalactic Radio Sources, that I read many times. And jets were quite the mystery at the time. They're still not completely solved today, I would say. But I think Roger was certainly an early proponent that magnetic fields had to be important. This was a time when most people—well, at the risk of generalizing—most people were reluctant to bring magnetic fields in. I remember somebody saying something like, “The strength of the magnetic field is proportional to our ignorance.”
Sort of like you bring in the magnetic fields when you don’t know what else to do, but we should be able to solve this without the magnetic fields, first. And I think Roger realized—I'm sure he wasn’t the only one—that magnetic fields were probably essential to lots of phenomena. And I think he was fairly convinced early on that magnetic fields were primarily needed for jet acceleration and collimation in active galaxies. It wasn’t when I was at Caltech, but shortly after I had gone to Virginia and I saw Roger at a meeting, and I was telling him about my efforts to improve my general relativistic accretion code, and he said to me, “Forget general relativity. Focus on magnetic fields. You can use Newtonian gravity. It’ll be good enough to get a handle on what’s going on. The magnetic field’s what’s really important.” And obviously since I remember a remark he made to me in 1989—I remember that to this day—it obviously had an influence on me. And indeed, I did start focusing on non-relativistic MHD at that point.
John, did you see this research during your postdoc as more of a continuation from your thesis research, or more an area of new opportunity?
It was sort of a continuation, in that I continued simulating stuff related to black holes. But things got a little less focused on the black hole per se, and a little more focused on the physics of things that might be going on in an accretion situation. I think I mentioned earlier that one of the problems with—the problem with accretion disks at that time was that nobody knew what the internal stress was, or the viscosity of it. Many ideas had been put forward with very little success. People became comfortable with simply parameterizing it and moving on.
But when I got to Caltech, one of the things that was sort of hot, as it were, was a global instability that had been identified through a linear analysis by John Papaloizou and Jim Pringle. It was called the Papaloizou-Pringle instability. And it was all a global analytic study of an accretion torus, which is basically sort of a big inner tube-shaped structure around a black hole. They had done the analysis, but nobody knew exactly what the instability did, or whether it was important or not. But anything that was unstable in an accretion disk was considered important to investigate. At Caltech, Peter Goldreich, Jeremy Goodman and Ramesh Narayan were studying the instability in a particular configuration, where they made a very slender torus, like a little narrow inner tube. And in that simplification, you could reduce the problem to a vertically integrated 2D system. And they did a stability analysis on that system and were able to basically understand better the nature of the global unstable mode.
At the same time, I took that configuration and ran a computer simulation on it. The computer I was using at the time was a Cray XMP in Culver City, owned by an early digital effects company called Digital Productions. The NSF had bought time from them as a stopgap while the centers were getting going. Anyway, the first time I simulated a ring, a narrow ring that was orbiting around a Newtonian mass, and I got the growing instability. Peter, Jeremy and Ramesh said, “Oh, wow, that’s great. And it’s got the right growth rate and everything.” And then I just let it run, and the ring broke up into four separate blobs. And so I brought them an image of that, and said, “Look, it breaks up into these blobs.” And Jeremy Goodman said, “Oh, I think I can model that.” And they went away, and like a half hour later, they came back and said, “We have an analytic model for these blobs.”
And so that was sort of an example of the synergy that was possible between analytic analyses and computational analyses, that they could work together to bring insight, and the analytic models helped you understand what was actually going on. As a footnote to this, I think at the end of the day, we and other people did come to understand the nature of the Papaloizou-Pringle instability. And at the time, we concluded pretty much that it wasn’t going to be important for accretion disks. But the fact that in the narrow ring the instability causes a breakup into these blobs has remained of interest, even up to more recent times, as a possible mechanism in a protoplanetary system for forming the initial things that might go into making planets, although that’s a bit speculative.
What were some of the most important telescopes that were central for this research, both land- and space-based?
Well, there are always important telescopes that are pathfinders to things. Penzias and Wilson’s microwave horn was one of the most important telescopes in history. You could say the same thing about Galileo’s telescope. In both of those cases, they opened up a whole new realm, but they weren’t able to get much in the way of details. So with all due respect to the predecessors of each of these telescopes, I would say the VLA was probably one of the most important, because it gave you the first high-resolution radio observations. I mean, really detailed radio observations of jets and active galaxies. It was such a breakthrough in resolution that it really changed people’s thinking. Similarly, of course the Hubble Space Telescope. I usually like to remind everyone that Steve was the person who launched it in 1991, just in case anybody has forgotten. And this is just a joke—you may recall that when the Space Telescope was launched it had a bad mirror [laughs]. The images were blurred, because of an error by the people grinding the mirror. I always said, “Well, it must be your fault, Steve, because you were the last guy to handle it.” He thought that was really funny. Anyway. So obviously the Hubble Space Telescope.
For black hole observations, it was able to resolve in more detail, down into the core of active galaxies, demonstrating to people once and for all, these things really are on black hole scales, and quasars were at cosmological distances requiring huge luminosities from a compact source. They're not star clusters or anything like that, in the cores of active galaxies. The early x-ray satellites, probably XTE and Einstein, certainly Uhuru—were all important in my area. I don’t know which one I would want to single out. Just call it the X-ray astronomy in space. So let’s see, I got radio, UV, and optical with the space telescope. And X-ray with the X-ray satellites. Those certainly at the time were the big ones, I would say.
So it really sounds like you were relying on all kinds of telescopes for this work.
Yeah. Again, the thing about the black hole systems—well, stellar-sized black hole systems—they are luminous in UV and X-ray. And the earliest X-ray satellites detected things like Cygnus X-1, and similar. X-ray satellites were kind of absolutely necessary to study the stellar mass black holes. But in the cores of the active galaxies, you're producing UV, infrared, and radio, and the jets were all mostly radio. They had some optical jets, but mostly radio. So you really did need the full spectrum to understand active galaxies and high-resolution radio to get the structures of jets.
Tell me about the Bantrell Prize that supported your postdoc at Caltech. How did that come together for you?
[laughs] I think at Caltech almost everything is named for somebody. Almost every professorship is endowed. [laughs] I'm probably exaggerating a bit, but they have a long history of philanthropic contributions, as well they should. They’re in an admirable place in terms of being one of the top research institutions in the country. So as far as I know Bantrell was just somebody who endowed a fund to support postdoctoral research in astrophysics. Sort of the nice thing about it, for me at least, was in addition to providing me with a position, was that it came with a little research fund for the postdoc to use at their own discretion. So that was very valuable in allowing me to attend conferences and programs during that time. There was a lot of really interesting stuff going on at these conferences. In addition to just being very nice for me, it also allowed me to learn a lot, by basically going to these various programs and such, while I was a postdoc. So that was a nice thing to have.
Now, by the time 1987 came around, were you looking for a second postdoc, or were you ready to look for faculty positions?
I applied mostly to faculty positions at that time. I do remember asking Peter Goldreich, “Do you think I can get a faculty position?” And he said, “Well, I think at least you're at a position where you're the sort of person that might get one.” Or something like that.
That is, he wasn’t making any guarantees, but he wasn’t ruling it out, either. So I applied to a bunch of places. And spent the winter of 1986-1987 doing interviews at a bunch of places. And I got two offers. One was from Virginia, obviously. The other one was from UC Irvine. And I chose Virginia at the end because they had NRAO. Actually, Charlottesville, in some sense, had more astronomers than you would normally expect in a town of that size, just by the virtue of having NRAO there. So it was more than just the Astronomy Department at UVA, but also the fact that NRAO was there, and of course radio astronomy is very important for black hole research.
When did you first interact with Steven Balbus?
After I got to Virginia. Steve was hired about the same time I was. And for whatever reason, we kind of hit it off. And [laughs] in some ways we were different, because I was computational, and he was analytic, both of us perhaps to a fault with our own sub-area. But like I said, my experience at Caltech had shown the power of combining analysis with simulation. You calculate something, and then you simulate what could be calculated analytically to make sure you're on the right track, and then you go beyond that, into simulation, and you take it back to the analysis. Now you've seen what the outcome is. Can you understand that? Can you make sense of that from the analytic point of view? And so Steve and I complemented each other pretty well.
As I remember, the first thing we were working on was cooling flows, which were the idea, at the time, that certain galaxies seemingly had enough gas at their center such that it should cool rapidly. And the question was, since the cooling time was much shorter than these other times, it implied that there should be this big accretion rate of material into the core of the galaxy. If true, what could that mean? What was the structure of the cooling flow like? What energy sources could there be to keep it warm enough to not collapse? If it collapsed, did it form stars? So there were a lot of unknowns.
Steve had an interesting idea about gravity waves—not GR waves, but just waves created by gravity in a fluid—by interacting galaxies orbiting the giant cooling flow galaxy, and whether those waves could be a source of energy to keep the core warm enough to keep it from collapsing. We did some simulations of perturbed gas centers in galaxy models and he did some analyses. And he did an analysis of some gravity wave properties and thermal instabilities in cooling gas, that would be appropriate for cooling flows.
And [laughs] here’s where the so-called interesting part of the story comes in with it—somebody wrote a paper subsequent to his paper on this thermal instability of cooling flows, pointing out that if you add in magnetic fields, the stability relation changed completely. And Steve got all mad, and “Oh, I've got to figure out what’s going on here.” And he realized, yeah, this other author was correct, and it was because the magnetic fields can do things that pure hydro couldn't. And [laughs] you see where this is leading, of course—the lesson from this story, then, was that basically a magnetized fluid is not a hydrodynamic fluid with a little magnetic field stuck into it that doesn't do anything. A magnetized fluid can be a completely different type of fluid with novel behaviors, even if the field is weak. And obviously that insight served us well, going forward.
When did you know that this research would be so useful more broadly for accretion disks?
[laughs] That was actually a specific moment in history. [laughs] The story goes like this: Steve and I were interested studying some properties of accretion disks, and there had been a recent paper talking about gravitational waves in accretion disks. And of course, we had been studying gravity waves and cooling flows, and Steve said, “Well, accretion disks are going to be magnetized.” And based on the experience we had, we knew these gravity waves in disks are going to be affected by the magnetic field. Steve set out to redo the analysis including the magnetic field, because he didn’t believe that these waves are going to behave the way that they would in the purely hydrodynamic model. At the same time, I had been working a number of years on developing an MHD simulation code, more motivated by the fact, if you will, that it was almost certainly the case that black hole jets were magnetic in nature. So you're going to fundamentally need magnetic fields to model jets.
Anyway, Steve went off and did his analysis and then he came back and said, “Something weird happened here. There seems to be a simple instability that emerges when you add a magnetic field.” That was the moment—I said, “You're telling me that the magnetic field on an accretion disk is unstable.” And then I said, “That is extremely important.” For me, that was the moment, right? I said, “Aha!” And I went back to my office, took the simple configuration of a magnetic field and put it in an idealized rotating flow. Since I had been working on these narrow disks for the Papaloizou-Pringle Instability and I had been working on magnetic fields in general I had the model in hand. I just put the narrow disk configuration into the MHD code, and put some magnetic fields in.
At that time, I was using the NSF Supercomputing Center at Pittsburgh. I shipped it off to Pittsburgh to run a short job on the Cray there. I got back the results, plotted them on my screen, and the magnetic field had evolved—it had been straight vertical before, and it had developed little kinks in it. I got a printout of the little picture of the magnetic field with kinks in it, took it down the hall to Steve, and said, “Here. This is what happens after you evolve it a little bit.” And then Steve got excited. He kept that picture on his wall, I think for many years. And so, if you will, the rest was history. It was off to the races, at that point. I did better simulations, and we got sort of a nice view of the basic modes. And from that, we were able to understand at a more basic physics level what exactly was going on with the instability, which we eventually referred to as the magneto-rotational instability (MRI). Again, this interplay between analytic and computational modeling allowed us to figure out really what was going on at a pretty rapid pace.
Now, was this essentially a two-man endeavor, or was there a broader collaboration even beyond UVA for this discovery?
This was pretty much the two of us. Of course, I had been working with Jim Stone on developing the MHD code. So there was that. And sort of a funny note—Jim had also been looking at vertical fields embedded in an accretion disk. A number of people had done this. Uchida and Shibata in Japan, years ago, had taken a—this is actually interesting—they had taken a disk and threaded it with vertical magnetic fields and evolved it. But in order to see evolution, they just cut the angular momentum in the disk below the Keplerian value so that it collapsed. Because they were trying to make jets. And they had one remark in one paper where they said, “This is okay, because if we don’t do this, the disk collapses anyway.” But apparently they never thought to ask themselves, “Why does the disk collapse under those circumstances?”
Similarly, Jim stuck a vertical magnetic field into a disk around the same time Steve and I were doing our work, sort of a prelude to simulating jets and winds. Jim noticed that kinks were developing in the vertical field. And quite properly, he went back and put a lot of effort into making sure that there wasn’t any numerical instability in his code. I had the advantage, as it were, by coming at the problem from the other direction. Steve had the analytic instability analysis first, and the simulations came second. In Jim’s case, the simulation came first, and he needed to verify that the numerical technique actually worked properly before he could ask himself why this was happening. But really, at the end of the day, it was just that—me doing computations, Steve doing the analytic theory, and then combining them. Those were our first two papers on the subject.
Given the significance of these findings, John, what conversations did you have with Steve about appropriate journals to publish in, or conferences to share these findings with?
I think we discussed whether it should go like to ApJ Letters, or maybe Phys Rev Letters or something, but my recollection is we didn't want to be confined by the page limits there. We wanted to do a pretty thorough job on it before we put it out there. We did send pre-prints to people. But [laughs] our belief, at the time, which turned out not to be right, actually, was that as soon as this hit the street, we’d immediately be swamped with competition, for the next realization or the next manifestation or further analysis. And surprisingly, that didn't really happen. What we got initially was pushback—“This can’t be right.”
What was the source of that pushback? What was the concern?
I think the idea was, how could something so simple have been overlooked for so many years? [laughs]
Which—that’s a good question. I have some ideas about that, but—Ethan Vishniac tells a story on himself, that he got our preprint, and he said, “This can’t possibly be right. I'll show them.” And he sat down and did the analysis himself, and then said, “Oh. Yeah, it is right.” So that was good. Other people just assumed that we had blown it somehow. There was a paper that came out not too long after our paper where somebody had added a toroidal field and redone the analysis, and said, “Ah! It’s not a simple instability. It’s an overstability.” And that could be a problem, because when it becomes an overstability, the eigenvalue goes from being real to complex, which means that it’s no longer at just a growing mode; it’s a growing and traveling mode, so it becomes a global problem. Except that the complex component was tiny. So yeah, it changed it, you can argue, from a purely mathematical view, yes.
But before any wave would travel any distance, there were several e-folding times of the linear instability, so it didn't really matter, if you thought about it from a physical point of view, not a mathematical point of view. The other pushback we got was “how do we know disks have magnetic fields?”—the way I liked to put it was, everybody assumed that disks had magnetic fields in them, until we found a use for the magnetic field. And then they began to question whether or not there would be magnetic fields. And would they be the right strength? Or maybe they'd be too strong, or maybe there wouldn't be enough ionization. So, it was sort of interesting, that like I said, what I thought would happen—people saying, “Oh my god, this is it; I gotta exploit this right now, because it opens up a whole new area of research,” didn’t happen.
On that point, more broadly, what new questions were able to be pursued as a result of these findings?
Well, in principle, we could basically say what the origin of the internal stress in the accretion disk was. The next thing to do was to demonstrate that the instability really led to turbulence in the disk, and that the turbulence produced a stress that was on order of what was needed, based on observations of accretion disks, and that the whole thing was self-sustaining. And we did that. Basically, we had to go to three-dimensional simulations, which were barely possible with the computers at that time, but we did it in a particular idealized configuration, which simplified the problem, at the cost of some things we didn't consider as important at that time. Eventually, we got to being able to do full 3-D simulations of a disk orbiting around a black hole. We were able to demonstrate, in the next couple of years, that the instability did lead to turbulence, and it was self-sustaining. And we did a linear analysis with a toroidal field in place, and were able to characterize the nature of the instability when the magnetic field was more general. After that, it sort of became issues about the circumstances under different kinds of configurations, and what would happen when you made things more realistic—our first models didn't have vertical gravity, so we added vertical stratification.
And like I said, went eventually to full 3-D models of one type or another. The first paper with a full 3D global disk model was in 2001 with Julian Krolik; this was the beginning of a long and fruitful collaboration. And then people were interested—by this point, things were beginning to take off. There was some work done by Steve and others on the instability in the presence of non-ideal MHD. You know, with resistivity and viscosity. The more general analysis showed the effects of resistivity or viscosity on the instability. That was interesting and it turned out the ratio of the resistivity and the viscosity was important for the behavior of the instability. More complexities were uncovered over time. It became a much richer area of research, and it did begin to—[laughs] after a time, it did begin to be studied more broadly.
Tell me about winning the Helen Warner Prize from the AAS.
Oh, yeah! What year was that?
I believe it was 1993.
The Warner Prize is the AAS prize for a young astronomer, an astronomer under age 35. Which was me, at the time, but not Steve, he being older than me. But my colleagues at Virginia put me up for that award. Of course, I didn't have the foggiest idea they had done so. And Ellen Zweibel was the president of the AAS at the time, and she called me up shortly before the AAS meeting in January of that year, to tell me that I’d won it, and invite me to [laughs] come to the meeting, I guess. So that was a very pleasant surprise.
Now, was the recognition similar for later on what you would be recognized with the Shaw Prize, or was this different?
Yeah, it basically was for the MRI. Of course, the difference was there that the—Warner Prize, MRI was new at that point. And the citation included also some reference to the work I’d done just in pioneering computational astrophysics. Whereas the Shaw Prize was really awarded after enough time had gone by that the position of the MRI research in the pantheon of accretion disk studies was now fully appreciated.
When did you start to really pay attention to what LIGO was doing?
Well, I followed it. Like I said, it was born while I was at Caltech.
Did you interact with Kip Thorne or Barry Barish at all on that?
Yeah, Kip gave the postdocs and graduate students some seminars on the project. All the postdocs and graduate students would sit around in the interaction room and discuss it. And [laughs] they moved a bunch of postdocs and grad students out of their office space in order to build the LIGO project office. The graduate students were upset about that. [laughs] We weren’t directly involved, but we were observing the beginning of this thing. And of course, everybody had gone down to the basement to see the LIGO prototype experiment, which was being carried out by Ron Drever. Anyway, it was like—you just got to Caltech, right?
You haven't been in Robinson probably, yet?
I have not.
Yeah, there’s a bunch of levels in Robinson, including something called the sub-basement, which I don’t recommend going into.
The basement level was kind of awful, but at least the ceiling was higher than six feet. Anyway, Ron Drever’s experiment was down there. It was two several-meter pipes, vacuum pipes, with laser beams going down to mirrors, all set up on an optical bench.
What I remember was the leveling devices which held the tubes in place, he had used toy rubber cars as the shock absorber. So you had this vise, holding the tubes at the right level, and it was cushioned by these toy cars. And I was just sort of impressed at the high-tech [laughs]—quote “high tech” unquote—nature of that aspect of the experiment. Anyway, we weren’t directly involved in any way with the beginnings of LIGO, but we were there, seeing it happen. It was sort of amazing that the NSF was willing to invest in this. I actually knew Rich Issacson at the NSF. He was the program officer for gravity. Because he knew Larry Smarr. So I had actually met Rich, and he was a real nice guy and very forward-thinking. He was very supportive of my research. Anyway, he was the NSF guy that did the lion’s share of the work to get LIGO off the ground. And then LIGO went into the phase where they were constructing things, and then it began operating, but they were not expecting to see anything in LIGO phase one. I’d see these talks where they’d show the sensitivity curves, and the expected sources, and the prediction “With LIGO 2, we'll be able to get down to this sensitivity.”
So there were a lot of years where it was sort of still promise, in the future. After LIGO 2 came on-line it wasn’t at the top of my mind by any means. But then the rumors started circulating that the LIGO people were about to have a big announcement, and it was going to be the first detection. And it was like, “Wow, really? They actually did it? That’s amazing! Let’s see it, that’s amazing!” And then they showed it, and it was like, “Yeah! I can’t believe it! This looks so amazing!” When it was announced, I was teaching cosmology to undergraduate students. It happened to come at the right point in the class where we were talking about black holes, that it was like, “This is a historic day!” Tried to convey to them the excitement of this. I don’t [laughs] know how successful I was at that.
[laughs] John, on the educational side, did you take graduate students right away when you got to UVA?
Pretty soon after, yeah. Had a first graduate student—Eric Lufkin was his name. He worked on the cooling flow problem, and the gravity waves in the cores of galaxies problem. He actually went [laughs] off to do work for J.P. Morgan, I think, in New York. [laughs] They always like to hire lots of people who know math and know computation and know how to do things. So that’s where he ended up. I didn't have a lot of graduate students in my time—you had to be a student who was just passionate for computations. And when you admit about five graduate students a year, maybe one is going to be that way, at most. The others are going to be interested in radio astronomy, or optical astronomy, or other theory. Virginia does attract a lot of students in radio astronomy for obvious reasons. I didn't have a lot of students, but the ones I had were very good.
And on the undergraduate side, what have been some of your most enjoyable courses to teach?
Definitely the cosmology course. I never really did cosmology research myself, but fairly early on in my time, the chair of the department asked me to take on this cosmology course that had been taught by somebody else for many years. So I did, and I kind of redid it. And in doing so, I developed like 100 pages of text, which I had printed up and made available for the students. Because at the time, there really wasn’t a good undergraduate-level book on cosmology. There were graduate level books like Weinberg, but they weren’t suitable for this course, because this course was taken also by a lot of non-science majors. How you teach non-science-majors cosmology was the challenge, and to give them the real stuff, not “just-so stories.” And that’s what eventually led me to write a textbook, Foundations of Modern Cosmology, which I wrote with my wife Katherine (whose thesis project was on cosmology)—we did the first and second editions of that.
And you saw this book as filling a gap for non-science majors?
Yeah. It was intended to be largely non-mathematical, but still to do the concepts. Not dumb it down, but really explain the concepts. Do general relativity, and explain what the equations meant, but you don’t have to solve them. Discuss expanding space. And then after the COBE and WMAP data, then you actually had something to work with. [laughs] Then you could focus on sort of the unified model that we have today for cosmology. But it was really aimed at interested students who were not necessarily science majors. And so that was kind of a niche—there had been some books about cosmological ideas, like The First Three Minutes by Weinberg, a very good book for the first three minutes—
But that’s kind of where it ends! And Hawking’s book was popular but useless. And so there really wasn’t much—there were discussions of cosmology in the basic intro texts, the Astro 101, 102 texts. That was always a chapter, and at least at UVA, that chapter often would get left out, because you didn't have enough time to get to it. So it wasn’t common at the time for there to be cosmology courses for undergraduates, nationwide, as far as I could tell. But UVA had kind of pioneered a bachelor of arts program in astronomy for people who weren’t going to go to graduate school. The argument, as it was explained to me when they told me about it, was that people major in all sorts of things in college, and they don’t go to graduate school in it. So why not major in astronomy, if you like it? But you don’t need to have all the physics and math, because you're not going to graduate school in astronomy. But you're having a good time while you're an undergraduate. It’s a perfectly good major. “Yeah, that makes sense.” We did have a lot of people like that, who just took it because they were interested. Which is great. So that’s the aspect I enjoyed, was the students who were there not because they were relentlessly pursuing an astrophysics degree or anything, but because they just were interested in it, and thought this stuff was cool.
John, when you stepped into the role of Department Chair, what were some of the most important issues facing you at that point?
Well, a few years before I stepped into that role, the Astronomy Department had received a major gift from a grateful alum. And that major gift allowed the Department to buy into the Large Binocular Telescope and also to participate in the University of Arizona’s telescope system, as well as join the Astrophysical Research Corporation and the Apache Point Observatory. Which was significant because UVA had never had anything bigger than a one-meter telescope at their command.
But when I got in there, we were looking at the end of that gift period, and what was being supported by essentially one-time or few-time money now had to be supported going forward by some other mechanism. I learned a lot about Excel spreadsheets at this point, projecting out financial models involving fairly large sums of money going forward, where we didn't have any guarantee of income. It ended up in the creation of an endowment, plus getting an agreement with the College for additional support, so that we were able to come into stable financial shape after a few years. But [laughs] that kind of problem is quite different from [laughs] what you're trained to do. That’s not astrophysics; that’s accounting. That was the big problem I had to deal with.
Was the department in growth mode at that point? Were you able to bring on new assistant professors?
We did. Any department that has a major philanthropist who’s interested in it—I came to understand this—will attract the attention of the Dean’s office. You will be showered with more goodies, as it were. We were in a growth mode. I created an external review committee to come in and advise us. The number of our graduate students had gone up, and I remember that in particular, because the external committee said—I was bragging about how many more graduate students we had, and they said, “Well, how many graduate students are you aiming for?” Well, I never asked myself that question. I was just trying to increase the numbers. It’s a very good question, in other words. And so, we did have a period of growth and some change. And we certainly stabilized a foundation for the Department going forward. I served in that role for six years. Two terms.
Did you like the administrative portions of the job, and was that useful in your decision-making on becoming an associate dean?
I mean, yeah. I would say that I liked the aspects of trying to think strategically about the department, and to try to steer the ship. I didn't have a lot of personal ambition as to exactly how the Department should evolve. I was more interested in trying to elicit a vision from the faculty, and to help them as a team to realize that vision. I certainly didn’t have any plans to go into the next level up, which was associate dean. But the Dean at the time basically invited me to a basketball game in the president’s box. I said, “Oh, that’s nice.” That shows you how naïve I was.
Why does she invite you to the president’s box at a basketball game? Oh, just to reward you for a job well done probably?
No. She said, “Well, you may be wondering why I asked you here today. The reason is I want you to be the next Associate Dean for the Sciences.” And I said, “Oh. Why?” And then I said, “Uh, who else are you considering?” And she said, “No one. You're it.” And so I guess I—I accepted my fate.
More broadly, what did you learn about some of the bigger challenges at UVA that you would be expected to work on?
The big challenges in the sciences obviously—I think this is probably true generally—is that you're in a constant state of renewal, because you have people that run the gamut in age and in career stage. You're constantly needing to think about the future, what sort of new faculty should you like to recruit, what are your strengths in your departments, what directions do you want to go in. The way a field is changing is important—what are the trends in biology? Should we be adjusting our thinking in that way? But at the end of the day, I would say based on my experience, it really comes down to hiring really top-notch junior faculty, and equipping them as best you can for success. And then supporting them as they build their careers. And if you have done that right, then after a number of years, you're going to have this really strong group of faculty who are doing cutting-edge research and getting grants. You don’t always succeed with your hires, and sometimes you hire people and then they leave for other places.
But I would say on the whole, we've done very well. We certainly have some outstanding faculty that we hired during the time I was Associate Dean for the Sciences, whose accomplishments speak for themselves, basically. I feel that that’s like the most important legacy of my time in that role, the hires that I was able to facilitate. The Associate Dean doesn't write the job ads. He doesn't sit on the search committees. He doesn't review the applicants, though he interviews them when they come to campus. But he is the person that tries to make the Department’s goals come true. So that was sort of my role.
It’s not a science question at all, but I'm curious about your own personal experiences during the white supremacist rally in August 2017, and from your vantage point as associate dean, how UVA dealt with this.
[laughs] Well, it was a Saturday. There had been rumors that there might be some protests or something. But I certainly didn't have a sense, by any means, what was actually going to happen, nor did anybody really anticipate the sort of UVA-centric attack by the—the mob. So my memory is, it was a nice day, and I was sitting on the deck, having wine or something, in the late afternoon, and I get a text from my sister-in-law, Eileen. She says, “I hope you're all right.” I'm like, “Why wouldn't I be all right?” “You weren’t downtown or anything, were you?” Developments happened locally very rapidly, and I hadn’t really been paying attention. And then I would say nobody really anticipated what happened that evening with the white nationalists descending on UVA, and terrorizing the people who were there. Our Dean was great.
Basically, he knew that this was a shocking and painful thing for everybody in the community, and particularly students and faculty of color. We had some prompt messaging, from the Dean’s Office acknowledging that pain, which was good. Shortly after that, we had a chairs and directors meeting, and it focused on the events and how we would respond to that as a school and faculty. And it was also an opportunity for the chairs and directors to express their concerns, and to consider the broader question—I mean, basically, such an occurrence shakes us out of complacency, opens up a number of broader issues for discussion related to the legacy that UVA has, which is not completely unambiguous, and the state of current affairs with the faculty, and particularly our support for faculty of color. Diversity, Equity in a broad sense, and what we were going to do about that. And today, we are still trying to address these issues through a number of initiatives.
You mentioned, at the beginning of our talk, that you're involved in diversity and inclusivity efforts. Was this a part of your agenda before the rally, or was the rally really a beginning for this work?
Well, certainly for me, it was probably a beginning. We had done a few things like institute something called Directors for Diversity and Inclusion, which was envisioned to be a rotating position in departments with faculty members taking on being trained and then working to help with those issues, in the departments. It was thought that this would be kind of a grassroots way to do it. But I would say after the events, we got a lot more serious. And when I got into the academic affairs position, one of the aspects of that was a lot of stuff changed about that time, in the Dean’s office, due to some personnel changes and such. But what became clear was we needed to better understand and document our procedures and processes, and we needed to examine our procedures and processes to see if they were fundamentally equitable.
And what I learned in this was that it’s sort of like a thread—you see there’s a thread loose, and you start pulling on it, and you find it’s attached to everything else. [laughs] So if you say, “Well”—for example, “we have a long history of trying to run diverse and fair hiring for faculty.” But it doesn't end there, by any means, even though there’s a lot of focus on that. You have to look at everything, because it all fits together. It’s not just a matter of hiring a diverse faculty; it’s a matter of supporting them when you get them here. It’s a matter of supporting all your faculty. You don’t hire people into tenure-track jobs with the expectation that you're going to boot them out after six years. You want them to succeed, and so do you have the things in place to make sure they succeed? What assumptions are implicit in the programs you have? Do our evaluation processes recognize and support diversity? And then you start asking, “Well, what about our postdocs? What are we doing for our postdocs to make sure they succeed?” Are we supporting our graduate students properly? Do they have stipends that are adequate for their support without relying on personal or family resources that not everyone may have? Are we recruiting a diverse body of graduate students? Are our degree programs meeting their needs? How can they be improved? Like I said, once you start tugging one place, because it’s all connected, it becomes sort of an all-consuming job. I'm sure you can see how that would be true.
That’s right. To go back a few years, what was it like when you got news that you won the Shaw Prize?
[laughs] Oh, yeah, that’s a story I like to tell. [laughs] That was Memorial Day weekend in May, 2013. Shaw Prize was nowhere on my mind. I knew about the prize, but it’s not something I ever thought about, over the years. I certainly never imagined myself in that position. Anyway, I guess it was the morning after Memorial Day, Tuesday morning, I got up, checked my email, and there was some email there—well, there was an email from Steve Balbus saying, “See you in Hong Kong” or something like that. And I thought, “What the hell does that mean?” [laughs] Because again, I didn't know anything that was going on. And then there was another email from some colleagues and they said, “Oh, congratulations! A well-deserved honor.” I thought, “What are they talking about?” Because it turned out that the Shaw Prize people had been unable to get in contact with me. It had been announced and everything, but I didn't know it yet. I'm looking at these things, and beginning to piece things together from the messages I got from other people—and so I said to my wife Kathy, I said, “I think I won a prize.” And she said, “Nah, must be some mistake.”
She doesn't necessarily like it when I tell that story, but there’s somebody else who’s named John Hawley, who is actually in sports medicine, and apparently fairly prominent in sports medicine. And so she just assumed that there was a mix up—and we've been mixed up before, and I had—somebody had written to me once asking me about something related to sports medicine, and I thought, “Where did that come from?”
So of course that was a possibility—because if I had won a prize, wouldn't the people in charge of it have told me? And I went to their website, and there it is. There’s my name. Not the sports medicine guy!
Yeah, it turned out somehow that Shaw Prize notification email ended up in the bit bucket, so I never, ever got it. So that’s why it sort of came as a funny surprise to me. Instead of being told, it just emerged from the email.
Coming 20 years after the Warner Prize, I wonder if this recognition caused you to reflect on some of the changes in MRI research over the years.
Yeah, it did. Well, yeah, it took a lot longer than I thought it would at the time to really establish the properties of the MRI and its role in accretion disk physics. There’s a whole intermediate area of research, which came after the MRI, which was something I'm kind of proud of, in terms of research, but it wasn’t going to get recognized the way the MRI did. This was the fact that in accretion disk theory, for many years the prevailing feeling was that even though we didn't know how the disk would become turbulent, a hydrodynamic shearing disk with very high Reynolds number would nevertheless be turbulent. This is the so-called non-linear instability. Keplerian flow is hydrodynamically linearly stable, so it had to be a non-linear instability. And that notion was so prevalent.
To tell another story, shortly after, maybe a year or two after the MRI first was published, a prominent astrophysicist was visiting to give a colloquium here, and we went out to dinner. Steve and I were there. And he said, “Yeah, it was really nice work you guys did, but”—turning to me he said, “You know, if you would do your simulation without a magnetic field, I think you'd find that it would be turbulent anyway.” And I thought to myself, “I've done that, I think, and it’s not turbulent. But okay.” And I went back to the office, and I said, “Okay, I'll set up a simulation, high resolution, without magnetic fields, Keplerian shear, see what happens.” I ran it, and of course nothing happened. It was completely stable. But this notion was so prevalent that we tried to understand where the notion had come from and why people believed it. We traced it back to a remark by Zeldovich in a conference paper where he simply asserted that shear plus high Reynolds number would be turbulent. And then we studied it, wrote what I consider to be a really nice paper, where we took a Keplerian shear flow, local approximation, and adjusted the angular momentum gradient with a parameter.
You can do simulations where you do it at one value, and then you could tweak it. And so as long as the angular momentum is increasing outwards, you're stable, and if it’s increasing inwards, you're unstable. That’s the well-known Rayleigh criterion. And we gradually turned the knob back towards sort of a zero angular momentum gradient, which basically is equivalent to a simple shear flow. And lo and behold, that’s where this non-linear instability develops. On the other side of it, it’s completely unstable, by a linear criterion. What we uncovered was that as you went from the stable regime, to where it’s linearly unstable, you pass through a zero point with no angular momentum gradient, then into the unstable regime, which everybody knew was unstable. When you passed through this zero-point, the linear terms cancel, and so the only thing left are the non-linear terms. And because there’s no restoring force anymore, the fluid can become unstable through a non-linear means. So we thought that paper cleared it up once and for all! That paper was published in 1996 with Jim Stone. And people were still talking about non-linear hydrodynamic instabilities ten years later. I said, “I thought we put a nail in the coffin of this thing.” Steve said, “Oh, there’s so many nails in that coffin, if you try to drive any more in, you'll just be driving nails into nails.”
I don’t recall exactly when he said that. As I said, I really liked that work, because it was a really well-defined problem. We developed a simulation technique to explore it in detail, and we tied it back to analytic results that we could reproduce through analytic equations, basically everything about it. But still, people didn't believe, and they just said, “Well, your simulations, they're not at high enough resolution.” Okay. How much resolution do you want, right?
And how come I can get the non-linear instability in a simple shear flow with an eight-by-eight grid? I tried to show it the other way, right? Where I know it was unstable; how low a resolution can I go and still see it? Well, you can’t go to one zone, because that’s not enough, but certainly like eight zones was enough. So how much more do you want? It kind of came down to sort of a philosophical question about whether you could believe simulations or not, I guess. Anyway, it was kind of a sidebar on the whole thing, but I think what it led to over time was a very gradual loss of interest in the idea of non-linear hydrodynamic instabilities in accretion disks, but it never went away completely. Nobody ever said, “By god, you're right. This whole idea was stupid.” Young people today probably don’t think that. They probably say, “Yeah, it’s the MRI. Whatever.” So that’s fine, I guess.
Just to bring the conversation right up to the present, what do you want to do in your remaining tenure as associate dean? What are the issues that are most important to you?
Right now, I would say trying to improve the College in a broad sense, improve the way that we do things. Make things more equitable. Promote equity amongst the faculty and staff. Make sure our processes are fair. Make sure that we have processes that make it possible for everybody to succeed to the best of their ability. I’d like to see improvements in the graduate program. That’s not solely my domain of responsibility by any means, but I think that would be great, if we could do that. And I want to basically be in a position, when our current Dean steps down, that we in the Dean’s office will be able to hand off the College, as it were, to the next administration, with it much better and stronger than it has ever been in its history. That doesn't sound too grandiose.
It’s good! For the last part of our talk, I’d like to ask one broadly retrospective question about your career, and then we'll end looking to the future. I’d like to go back to your emphasis on the complementary nature of your technical expertise with Steve Balbus. I wonder if you could reflect a bit more broadly on what was so important about that complementary areas of expertise that you both brought to the table, and what lessons we might derive more generally about how these collaborations can contribute to future success in astronomy?
Yeah. Well, if I could think about this, not necessarily having thought deeply about it, it starts with the notion of a numerical approach and an analytic approach as two separate things—people historically view those as two completely separate things. And as numerics became sort of more prominent, in some sense they were deprecated. Like, “Oh, you had to use a computer to do that. Computer simulations have errors. You don’t know anything about those. Who knows what’s going on?” To give you an example, very early when I was a grad student, I went to a conference with Larry Smarr on general relativity, and somebody presented a talk where they had found some analytic solutions to a set of equations for gravity waves. And after the talk was over, Larry raised his hand and said, “You know, [laughs] in my year such-and-such paper, I actually solved these equations and published solutions to them.” And the speaker looked blank and then he said, “Well, but you did it numerically.” And Larry said, “They were ODEs. You could solve them to arbitrary accuracy.” Long pause, and the guy said, “You did them numerically.”
And as a naïve graduate student, I said to Larry, “What the hell is he talking about? I don’t understand that.” Larry said, “Where the hell does he think the value of sine comes from? You take sine of an angle of 39—sine 39 degrees, what is that equal to? You get it numerically. It’s an ODE.” So anyway, that impressed me obviously, since I remember that very clearly, however many years ago that was.
Anyway, so my point is that there exists this perception of a fundamental divide between a numerical approach and an analytic approach. And that people do simulations, they set up a big problem, they run it, and does it look like whatever you're trying to simulate, or not? But here, we can do this analytic approach, and you get an exact solution to this highly simplified problem. That’s done, then. And then of course the trouble with an analytic solution is you can solve something analytically that doesn't have the first thing to do with anything in nature. Or it can start OK, but you can push some analytic technique well beyond the point of where it makes any sense to do it. You see that a lot with stability analyses. People think, “Oh, let’s go to a higher order.” But if nothing showed up at lower order, it doesn't really help to go to a higher order. But you can spend your time as you like. Similarly, when you run a simulation and you show it, does it look like an accretion disk? Yes? Oh, that’s nice. Here’s a shock wave; isn’t that nice. But if you connect the two you may have something. The first thing you do with your simulation is to run it in the regime where the analytic solutions are valid, and you reproduce those, and adjust what you're doing to take it just a little bit beyond.
And can you understand that? Can you understand what your analytic theory means anyway? Do you learn something? I always used to argue that if you do a simulation of a problem, that’s a little like looking in the back of the book, in the physics text, for the answers, where it says, “Chapter three, problem six, the answer is 42.” Okay, well, that’s great to know, but if I write 42 down on my homework page, I'm not going to get credit for it, because I have to actually go through the math to show how you get that. It’s like the simulation can give you a hint as to what the system is going to do, and then you can go back and say, “Well, can I understand this better from an analytic point of view?” so that in principle, the two things can work complementarily.
The other thing with a numerical simulation is that you don’t have to simulate the whole problem. You don’t have to set up the whole global 3-D accretion disk. You don’t have to include all the terms. You can set up an experiment that just focuses on some specific aspect. That’s accepted if you're doing some sort of an analytic problem, you generally have to reduce—simplify it down to a set of equations, and maybe approximations—it’s only 1D, it’s only 2D, axisymmetric, something like that. You have to get to a place where you can actually do an analysis. And then the secret is to not take it too far. Similarly, you can do the same thing with a simulation. You reduce it to a simpler problem. You work in 1D or 2D. You keep in mind the limitations of your approximations when you get the results. But you can tweak the knobs. Okay, adjust this parameter. I'm sure nature doesn't do it that way, but we might learn something by adjusting a parameter. It’s an experiment.
And so numerical simulation should be thought more of as like a numerical experiment, than a model of a star or a disk. In my Papaloizou-Pringle research at Caltech, the first thing that I did was try to reproduce the analytic eigenmodes in the analysis of the system, and I couldn't do it. They wouldn't hold together. And I reported that back. My colleagues went back and looked at their analysis, and they found an error. The eigenfunction was wrong. They calculated modes wrong, because they had a minus sign here, instead of a plus sign. They said, “Wow, that’s nice. Your simulation helped us to find that error.” Then I was able to reproduce their results, exactly! So it impressed those guys, that a simulation can actually help you with your analysis. They're synergistic. I don’t know the degree to which that view is practiced today or not. There still is an awful lot of analyses that sort of go beyond the rationality horizon, and then at the other end simulations that just take a code and run it—we added every term we can think of! Our approximations here are well beyond the limit of where they're useful, but we're going to go ahead and run this simulation anyway, and we're going to tell you it looks like an accretion disk, or something.
For my last question, I’d like to recall back to the very early part of our discussion, where you noted that you see a convergence or unification in the fields of astronomy and astrophysics and cosmology. I wonder where you see more broadly the advances that you contributed in MRI to that overall trend in unification.
Well, when you get right down to it, the MRI is not restricted to accretion disks by any means. It’s just an instability that arises in a magnetized rotating fluid. It actually had been—a form of it had been found before by Velikhov in rotating Couette flows with magnetic fields, which was something related to tokamaks. But it was kind of an obscure paper, and it was never really followed up on. The instability probably can be seen in tokamaks if you're in the right regime. It has been proposed that it may have an important role to play in supernova explosions, because you have a collapsing, magnetized rotating fluid. And this is a way that in a dynamical time scale you have an exchange of angular momentum and energy. Which I'm sure you're aware has been an issue—supernova explosions are funny [laughs]. You wouldn't think that the physics involved in such a major thing would sort of hang on the hair’s edge between blowing up and not blowing up. You’d think if nature does it, they blow up real big, and it should just pop right out of our equations, but that’s not the way it worked out. So the MRI may have a role to play there.
Like I said, since it’s just a fundamental instability, there’s no telling where it might crop out. A lot of interesting stuff has been going on in areas like protostellar contexts, where you have non-ideal fluids. I'm aware of some work where possibly the MRI may have a role to play in planet formation. So, as we begin to understand more systems, it may have a role to play—because it is just a very basic piece of physics. It’s hard to say. But it would be sort of nice, I guess [laughs] from my perspective. The more useful things you can do with it, the better. By the way, we have had some people say, “You were involved with that medical thing?”
“The other MRI.”
Yeah, I only wish I was involved with the medical one.
That’s it! Well, John, it has been a great pleasure spending this time with you. I'm so happy we were able to connect, and I really appreciate you spending this time with me.
Okay, great. I've enjoyed it.