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Credit: JHU Department of Physics
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Interview of Marc Kamionkowski by David Zierler on August 26, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Interview with Marc Kamionkowski, William R. Kenan, Jr. Professor of Physics and Astronomy at Johns Hopkins University. He discusses his family heritage of Ashkenazi Jews who left Eastern Europe for Argentina, and his father’s medical research which took the family to Cleveland. Kamionkowski recounts his childhood in Shaker Heights, and he describes his undergraduate work at Washington University, where he switched from pre-med to physics to work with Marty Israel and Joe Klarmann. Despite his lack of preparation, Kamionkowski explains his admission to the University of Chicago, and he describes “the bug” that made him focus on physics and drive to succeed in quantum mechanics and understand quantum field theory. He discusses his thesis research under the direction of Michael Turner on energetic neutrinos from WIMP annihilation in the sun. Kamionkowski discusses his post-doctoral research at the Institute for Advanced Study where he was in Frank Wilczek’s particle theory group. He describes his first faculty appointment at Columbia and how experimental advances had opened up opportunities in cosmology. He explains his decision to move to Caltech because of its strength in theoretical astrophysics and where he became director of the Moore Center. Kamionkowski discusses his subsequent move to Johns Hopkins, and he surveys his recent projects on the Hubble Tension and early dark energy. At the end of the interview, Kamionkowski explains why he has always valued research that bridges the divide between theory and experimentation and why he expects this will continue to inform his broad research agenda.
Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It’s August 26th, 2020. I’m so happy to be here with Professor Marc Kamionkowski. Marc, thank you so much for joining me today.
Pleasure to be here.
Okay. Alright. So, to start. Would you tell me, please, your title and institutional affiliation?
I am the William R. Kenan, Jr. Professor of Physics and Astronomy at Johns Hopkins University.
And when were you named to the Kenan chair?
I think it was 2016.
And do you have any connection to Kenan? Do you have any idea why this chair is named in his honor?
He was some businessman. Early 20th century from North Carolina. He endowed chairs all over the place. And this is just some endowed chair that opened up at Johns Hopkins. Steve Chu is the William R. Kenan, Jr. Professor at Stanford University. There is—was—a biophysicist at Syracuse University who moved to Santa Barbara. Cristina Marchetti was the William R. Kenan, Jr. Professor at Syracuse University. And there’s also one at Bates College a very, very good numerical relativist; Thomas Baumgarte, at Bowdoin College as the William R. Kenan, Jr. Professor there.
Interesting. So, all of these Kenan chairs are in the physics world, broadly conceived?
I don’t think so. Those are just the physicists I know.
Oh, I see.
It’s not earmarked for physics. The foundation just endowed professorships at universities across the country. Probably sometime in the mid-20th century. It’s like the buildings named after Olin you find at universities all over.
Olin. O-l-i-n. Like every university has an Olin something or other.
Sure. Sure. Well, Marc, let’s take it 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 from Argentina. They were children of Ashkenazi Jews who came from Eastern Europe to Argentina. I don’t really know when exactly, but probably the ’20s or ’30s; before World War II.
Did they leave specifically because of anti-Semitism?
[Laughs] Yeah. I think so, although I’m not sure. Things weren’t so great for Eastern European Jews back then, but I don’t know if they left for economic opportunity or specifically because of anti-Semitism. I heard stories that my grandfather on my mother’s side was smuggling things across some border in Romania when he was 7 years old. My grandfather on my father’s side is from some small town from the east of Poland. My grandmother on my mother’s side—her story is kind of interesting. She lived somewhere near the Russian/Polish border, and when she was 15, she was sent to some other village to go to school. There was a war and the borders changed and so she never saw her family again after that. Interestingly enough, she kept in touch with a brother of hers. She had an older brother who, when World War II came, like many other Jews, fled east, and he wound up settling in Tashkent and had a family there.
My grandmother then moved to Argentina by herself, when she was still a teenager. And then many years after the State of Israel was created, she moved to Israel. Probably early ’60s, I’m guessing. Late ’50s, early ’60s. She kept in touch with this brother of hers in Tashkent, from sometime in the late ’20s, until the fall of the Soviet Union. And then she, when the Soviet Union fell, he and his family emigrated to Israel and she was reunited with him when they were something like 80 years old. So, they spent the last few years together.
Your parents met in Argentina?
Yeah. My father was from Buenos Aires. My mother was from Tucumán, which is a small city in the middle of Argentina. And she went away for college, which was very unusual.
Was there a Jewish community where you mom grew up?
There was a big family in Tucumán. She had a lot of relatives, a lot of friends, in Tucumán. My impression is that most of these friends were Jewish but not all. All the family were Jewish.
So, she grew up in Tucumán. Her father did assorted things in business; I don’t really know exactly. My father was born and raised in Buenos Aires. His father made furniture for a living. I think my mother grew up in pretty good circumstances for the time. My father’s, I think, was much more working class.
And where did they meet? They met in Buenos Aires?
Yeah, they met in Buenos Aires. My mother went away to Buenos Aires for college, which was unusual. People didn’t do that too often. But she lived in a convent because that’s where you sent [Laughs] a college-age girl to live while at college away from home. I think my parents met at a Zionist youth organization or something like that. My father was a doctor. He was in medical school at the time. And then after my parents got married, they moved to Cleveland, Ohio, in 1959.
Huh? No. The Mayo Clinic’s in Minnesota.
Oh, right. I meant the Cleveland Clinic.
Cleveland Clinic. No, he went to do a residency in gastroenterology at Mt. Sinai Hospital.
Were they intending to go back to Argentina?
So, at about that time, most of my mother’s family was moving to Israel. I don’t know the exact timing, but about the time that my parents moved to Cleveland, or shortly thereafter, my uncle moved to Israel. And, then, my grandmother and grandfather on my mother’s side moved to Israel. The entire family from Tucumán, I think, moved to Israel in the late ’50s, early ’60s. A lot of my father’s relatives moved to Israel as well, but his immediate family stayed there. I don’t know what the plan was. I don’t know if there was a plan. And I don’t really have a clear idea of why, exactly, they came to Cleveland. But I think my father was looking for opportunity. I think he was an ambitious guy, and he was looking for opportunities. They might have also wanted to get away from their family? I don’t know. [Laughs]
[Laughs] Did they stay in Cleveland?
So, they stayed in Cleveland. I have an older sister who was born in 1963, and I was born in 1965. And shortly after I was born, I don’t know exactly how much longer after I was born, the family moved to Israel. I think the plan was that they would stay there. They joined the rest of the family in Israel. My father was working at some hospital in Rehovot in Israel which is where the Weizmann Institute was. He had no affiliation with the Weizmann Institute, but that’s where he got a job.
Was your family religious at all?
I was raised conservative. My family was conservative. We went to conservative synagogue. I had far more Hebrew school than is good for anybody.
My sister went to Hebrew school.
And how long was the family in Israel?
The family in Israel is not religious.
No, but how long was your family, when your family went, how long were they in Israel?
It was about two years. We were still there for the Six-Day War, and I don’t know exactly why, but about that time my parents decided to move back to Cleveland. I’ve asked, and I’ve gotten answers, but I’m still not entirely sure.
Well, the Six-Day War was a pretty scary time to be in Israel, of course.
It might’ve been partly that. It might’ve been part wanting to be independent from the family. And, I think it might also have been that my father had better professional opportunities.
Sure. So, you don’t, obviously, you don’t remember any of Israel during that time.
No, when I first started speaking it was Hebrew. And the story I heard is that when my family moved back to Cleveland, they sent me to pre-school and I didn’t speak English. And back then they told my parents that you have to speak English at home so Marc can learn how to speak English with everyone else. Now we don’t do that, right? We tell immigrant parents, speak your language at home so the kid becomes bilingual. So, although when I first started speaking it was in Hebrew, I don’t really remember anything.
That’s interesting. Your parents would’ve spoken to each other in Spanish, I assume.
So, did you pick up Spanish as well?
No. My parents spoke to each other in Spanish. But they never spoke to my brother, sister, and me in Spanish. And I wasn’t curious enough about what they were saying to each other to bother to learn. In retrospect, it’s kind of a shame. I can, sort of, when Argentinians speak, understand. And I think if I were to go to Argentina, I’d probably pick it up. But, no, I never really learned to speak it. It wasn’t something you did back then.
Right. And where did you grow up mostly?
I grew up in Cleveland. I mean, all my childhood memories are from Cleveland.
Through high school you were in Cleveland?
Shaker Heights. When we first moved back to Cleveland, we lived in a suburb called Warrensville Heights. But that was becoming black. And, I don’t know exactly, but that was about the time of white flight from Warrensville Heights. But then we moved to Shaker Heights, which is actually an interesting place. I never really fully appreciated this until I went to college. And, then, when I was grown up, I was watching TV one day, probably 20 years ago, and I saw a documentary about it. Shaker Heights was interesting because it was about the time of white flight, but it was half black, half white. And my schools were integrated. My elementary school was not integrated for kindergarten, first, second, third grade. You know, I remember there were maybe one or two African American kids in my class. But starting in fourth grade, the Shaker Heights school system set up a voluntary bussing system. And, so, starting in fourth grade, I had many more African American kids in my class. And one of my best friends at the time was African American. And I didn’t fully appreciate how unusual this was until much later. And then my junior high school was integrated. And my high school was integrated.
At what point did you start to get interested in science? Was it even before your, sort of, formal education in middle school and high school?
I always liked math and science at school. I wasn’t doing anything special with it. I think when I was in junior high or high school, the plan was that I would become a doctor. Because I did, like math and science, and I was smart, and my dad was a doctor. And so many of my friends’ parents were doctors. It just seemed like that’s what you do when you like math and science. I didn’t really have friends whose parents were mathematicians or scientists or theoretical physicists. I did really like physics when I took it senior year in high school. I had a very, very good teacher. I didn’t fully appreciate how good he was until about seven years ago, when I was inducted into the Shaker Heights High School Alumni Hall of Fame. When I got this award, I went down the hall to Adam Riess who’s got a Nobel Prize and told him about it. He said, “Congratulations,” and I said, “Fewer people have been honored with election to the Shaker Heights High School Hall of Fame than have been recognized with the Nobel Prize.”
[Laughs] There you go. Marc, were you thinking about physics specifically when you were applying to college?
I went to college to be a pre-med.
Where did you apply?
Where did you apply to school?
I applied to Yale, Brown, Rochester, Washington University, and one other place. Somewhere else; I don’t even remember now. I didn’t get into Yale or Brown. I got into Rochester. I got into Washington University. And there’s one other place I’m pretty sure I applied. I can’t remember. So, I decided to go to Washington University.
At what point did you switch over into physics?
I really liked my physics class in high school. The high school did not offer a formal AP class, but several of us wanted to take the AP exam. So, our physics teacher let the five or six of us who were interested in taking the AP exam go in a separate room the second semester and just read and work through Halliday Resnick to study for the AP exam, so that’s what we did. We worked through Halliday Resnick, more or less on our own, second semester senior year. I took the AP exam and did well. When I got to Washington University, I placed out of freshman physics, and I decided to take sophomore physics just to have something enjoyable in addition to the pre-med classes.
The teacher I had freshman year for quantum physics was a theoretical physicist from Greece who had little concept of what undergraduate education in the United States is supposed to be like. [Laughs] His name was Nicholas Papanicolaou, and he was a chain smoker, which you would still see sometimes back then—[Laughs]. The first day he comes into the class, and he looks really grumpy, and he erases the chalkboard. And he starts writing the Schrödinger's Equation. And I’m copying it into my notebook. And I had no idea what the hell it meant. And then, as an afterthought—and this is my first 15 minutes in class in college—he writes, oh, in this class we’re going to use atomic physics units. So, h-bar equals c equals one. And I had no idea—[Laughs]—how in the hell you could set the speed of light to be equal to one. But I thought it was the coolest thing in the world. Anyway, I really liked the class. He was very entertaining. I mean, he was a nut, but he was very entertaining. The class was completely incomprehensible, but reading through the book and doing the problems I started to get it.
And I was taking the pre-med classes: biology, chemistry, and it was no fun. The sophomore-level physics class was primarily kids who wanted to be physics majors. And, maybe, a few engineers of various types. But it was a small class. It was more intimate; the people who were there were interested in the subject. And when you took a freshman chemistry or biology class, you’ve got huge lecture halls, hundreds of kids, and only very few of them interested in the subject. Most of the people I was surrounded with in those pre-med classes didn’t want to be there, but they needed to do it to get to medical school. It wasn’t enjoyable. And, I think, also I have a natural affinity for physics. I’m not very good at memorizing tons of things. One of the things about physics is you deal with simple systems. People always think that physics is something for very smart people; it’s very complicated. But, in fact, in physics, you always deal with simple systems. And, I think, the way my mind works, I think it’s well-matched to that kind of thinking. You know, biology—
Would you say specifically theoretical physics? Were you, as an undergraduate— was it theory that you knew you were particularly good at?
Nah. When I was a freshman, I didn’t even know you could make a living as a physicist.
[Laughs] I mean, it was something to major in. But I didn’t know you could be a physicist for a living. I didn’t know there was theory and experiment. I was an extremely naïve physics major when I started. So, I think, some combination of the chance that I wound up in a smaller sophomore-level class in freshman year. Some combination of that versus the freshman level classes I was taking: biology and chemistry. And just a natural affinity for the subject. After that I decided to take more physics sophomore year with an intent to major. And I kept with the pre-med program until the middle of sophomore year. I took the first semester of organic chemistry which was just—[Laughs]—horrible. The book is so thick and there’s just so much information. Isomers and monomers and all this stuff. And the middle of my sophomore year I decided I wasn’t going be, I couldn’t be, a pre-med.
And you probably knew at that point that physicist was an actual career to pursue.
I don’t know. I think I figured that out senior year. Certainly, by senior year, I figured it out. I wasn’t really too career oriented at the time. I wasn’t thinking too much about the future, you know.
At one point did you determine that you wanted…
I was having a lot of fun in college. [Laughs]
At what point did you determine that you wanted to pursue physics for a graduate degree?
So, what happened is that after my freshman year, between my freshman and sophomore year, I went home to Cleveland. I thought I was going to be a doctor. Research experience was useful for the med-school application. And, so, my dad found some gastroenterologist at University Hospital who was willing to take an undergraduate research assistant. I worked in that lab [because] I knew how to do linear regression. I could program this little calculator to fit lines to data, and so I did that for them. They thought I was a genius because of that. The lab was a lot of fun, but I didn’t really get interested in the subject. The people I worked with were really nice, pretty laid back. I didn’t really have much understanding or appreciation for what they were trying to figure out. And then, the second summer, between sophomore and junior year, it was sort of similar. I actually got a job at the beginning at an auto supplies warehouse, because I wanted to make money. [Laughs] I mean, this research job was okay, but I wasn’t getting paid a lot. And, somehow, there was some auto supply warehouse where I applied for a summer job and I got a job there. It was 7:00 o’clock in the morning to 3:00 in the afternoon, and it was paying well. But I have to say, it was just such an eye-opener. I did that for about a month or two, and I was working with people who did that for a living. You clocked in at 7:00 o’clock in the morning and stayed until 3:00 o’clock, and it was tedious.
And, you know, after I did this for a month or two…
You said, “There’s gotta be a different way.”
Yeah. And, so, I went back and spent a second summer at the gastroenterology lab. But then junior year, I didn’t want to go home. I wanted to spend the summer in St. Louis because I had all these friends who were gonna spend the summer in St. Louis. I went and talked to my undergraduate physics advisor and asked him if there was some way I could get some research job in the physics department. He said, “Yeah” and then found something for me to do. I wound up working in the experimental cosmic ray group there. There were two professors I worked with: Marty Israel and Joe Klarmann. Joe was my undergraduate advisor, and although Joe was older, Marty was the guy who was running the show. And there was also a research scientist named Bob Binns. I shared an office with a post-graduate student who was doing his PhD thesis on the project.
There was also, in the office next to me, the guy who did the computer, the IT for the group. He was actually a very smart guy who was much more; they should’ve given him a PhD. [Laughs] They gave me a problem to work on, and you know, it was kinda interesting. It was like a homework problem, but much bigger. Instead of, like, being this much it was like this. And there were always little chunks and pieces I could fill. I didn’t really fully appreciate the science at the time, but I did manage to get a good appreciation for the analysis that we were trying to do. This was a heavy nuclei experiment. They were measuring the abundances of heavy isotopes in cosmic rays, with intermediate atomic number. And I worked with them, and I have to say, the people I worked with were great. They had a group meeting where the collaborators from the University of Minnesota and Caltech came. And one of those guys was Ed Stone who later became a colleague of mine at Caltech. He was a very big man of science. He was the PI for the Voyager missions. I didn’t understand this at the time. But they came for the collaboration meeting and I got to sit in. And, then, they went over to Marty Israel’s house for dinner one night, and it was summer. And we were sitting on the porch. You know, barbecuing and grilling. And these guys were talkin’ about all the conferences they were going to. Oh, I’m going to Adelaide in Australia. Oh, I’m going to Bonn in Germany. I’m going to Paris. Marty Israel had a nice house. And it just occurred to me that this might not be a bad way to make a living.
Better than clockin’ in at 7:00 in the morning.
Yeah. You know, they were interesting people. They were clearly enjoying their work. They got to go all over the place. I didn’t really decide to go to graduate school at that time. But, anyway, my senior year started, and sometime towards the beginning of senior year, I ran into Joe Klarmann, my undergraduate advisor, and he said, “So, are you ready for the GRE this weekend?” And I had no idea what he was talking about. He said, “The GRE. It’s a test if you want to go to graduate school in physics.” I don’t remember the exact conversation, but I had not registered for the GRE. He convinced me that I should take it, just in case I decided to go to graduate school. And, so, I called up the GRE office and all the testing spots in St. Louis were booked. But they had an opening for testing at the University of Missouri, Columbia, which is about two hours west of St. Louis. And it turns out I had a really good friend who [Laughs] had also decided at the last minute to take the GRE to do anthropology in grad school. So, he and I drove there Friday evening; the test was Saturday morning. I had a 1980 Rabbit diesel. We drove there, and we didn’t make any hotel reservations. We thought we’ll just stop when we get close and just go to some random hotel. So, we drive and we start to get close, so we get off the highway to find a hotel. There’s some Best Western right off the exit, and we go in there and ask for a room and they say, “We’re booked.” And, then we’re like, “You’re booked? Why are you booked?” It turns out that that was the homecoming weekend at University of Missouri, Columbia. We actually had to drive back toward St. Louis stopping several times before we managed to find a hotel that had a room, and then we had to wake up extra early ’cause then we had to drive back that extra hour.
So, I went and I took the test. I didn’t do that well. [Laughs] I hadn’t studied at all, and didn’t have any appreciation for what the test was about. As I appreciated afterwards, not the kinds of things they teach you in classes. I don’t really remember when exactly I decided to apply to graduate school. But I did apply to graduate schools. I don’t remember everywhere I applied. I applied to Caltech, MIT, University of Maryland, University of Chicago, maybe a few other places. I didn’t get into MIT. I didn’t get into Caltech. I think I might have applied to Harvard. I didn’t get into Harvard. I got into the University of Maryland. And I got into the University of Chicago. The reason I got into the University of Chicago is that, at the time, they had graduate students involved in reading the applications and one of the graduate students who was on the committee was John Grunsfeld, who later became an astronaut and was a senior NASA administrator. John Grunsfeld was a graduate student in the cosmic ray group at the University of Chicago, so he knew the cosmic ray people at Washington University who had written my letters. I think I had a reasonably strong transcript, but it was by no means perfect. I was not very driven in college. When a class was interesting and I liked it, I did really well in it. But when the professor and/or the class was boring, I had no tolerance or patience for it. And, so, my transcript was probably not quite as good as it should have been. There were some physics grades that might’ve been a red flag for some admissions committees. That, combined with a not-so-spectacular physics GRE. I think that what happened is that John read the letters from Washington University, knew the people there, and was willing to take their word for it.
So, I got into the University of Chicago. I was trying to decide between Chicago and Maryland. And I had a girlfriend at the time who was going to go to graduate school at VCU, Virginia Commonwealth University. And, so, I was thinkin’ about Maryland pretty seriously, but I had a discussion, I remember, with one of my professors at Washington University. I asked him about the University of Maryland, because he had gone there for a PhD. He was going on and on about all the different things about Maryland that were so great. And then he said, “Where else do you have? Where else are you considering.” And I said, “The University of Chicago.” He said, “If you have a chance to go to the University of Chicago, go to the University of Chicago.”
And I remember my mother also telling me, “Do what’s best for you.” And, you know, you’ll figure it out with your girlfriend. So, I went to Chicago.
Did you have any idea, Marc, you know, even before you got there who you might want to work with or what kind of physics you would want to specialize in?
I thought I was going to work with the cosmic ray group because I worked with the cosmic ray group at Washington University. That seemed fun, so, I thought I’d work with the cosmic ray group. And in fact, when I arrived in Chicago, my first year there, most students were being supported by teaching fellowships. But I was one of the few first-year graduate students who was being supported by a research fellowship because that cosmic ray group had money to support students. And there weren’t a whole lotta first-year students who go to graduate school and say, “I want to do experimental cosmic ray work.”
So, they supported me. I worked with the experimental cosmic ray group there my first year. That was Peter Meyer, Dietrich Müller, and Simon Swordy.
Was this a separate project from what was happening in St. Louis?
Yeah. It was a similar type of analysis. Similar type of science. But it was a separate project. I don’t really remember it too much because I think I was so focused on my classes. If you remember in one of the earliest flights of the space shuttle, there was a big egg-shaped thing in the back. That was the project I worked on in graduate school. That was the University of Chicago—I forget what it was called. Ultra-heavy nuclei. Or, heavy nuclei experiment. They were measuring the abundances of nuclei, of heavier nuclei. I think ultra-heavy nuclei in cosmic rays. So, it was a very similar type of analysis. Very similar type of work, but there was, at the time, no collaboration between the Washington or Chicago groups, and it was a different project.
How long did you stay with that group?
Just my first year. That was fun. When I was a first-year graduate student, that’s when I got the bug.
The theory bug.
Well, the physics bug. And that’s when I figured out—that’s when I learned that, you know, there’s a difference between theoretical physics and experimental physics. And if you’re a theoretical physicist you wouldn’t have to [Laughs] worry about what the, you know, the transition radiation detector material was made out of. [Laughs]
What happened is that I really got the bug the first year in graduate school. You do nothing but physics. You take physics classes. You do physics research, and all your friends are physics students. And Chicago is very exciting. The professors were world-class scientists and, you know, they exuded confidence and excitement and enthusiasm for what they were doing.
Was there a particular professor or course where you could identify having caught the bug?
Just in general?
Yeah. My first-year classes [Laughs] for the most part, sucked. I mean, the University of Chicago pedagogy. [Laughs] It’s not high priority but I did very well. Most of the other students, I thought, had much better preparation than I did. Most of them had gone to college knowing that they wanted to be a physicist and taking things really seriously. And knew more about research and what was going on in the world of physics.
There were probably a lot of A-type personalities surrounding you at Chicago.
Yeah. And, so, when I first got there, I felt like I was behind because there were all these things that would show up in classes, and most of the other kids had seen something like or knew something about or knew why you should pay attention. Most of them had a good sense about what they wanted to work on. And the really smart ones knew they were going to be theoretical physicists, and they were going to work with the string theorists or the particle theorists. And I [Laughs] was kinda clueless. The students, sort of grouped amongst themselves to do homework problems, and the guy I was doing homework problems at first was also not too clear why he was there (he left after finishing a Master’s.) We were going through the problem sets and he’d be stuck, and I’d say, oh, yeah, this is how you do it. I remember, he said, “You’re pretty smart. You should be with those other guys.” [Laughs] After a little bit of time, I got the sense I could do this as well as anybody else. Even though these kids went to Caltech or Harvard for undergrad had placed out of quantum mechanics or field theory or whatever and knew so much more. By the middle—end of my first year, I got the impression that I could actually do pretty well with this. And I also decided that I wanted to do theoretical physics, because I just loved quantum mechanics.
I just loved doing problems in quantum mechanics. Working through the books. Figuring things out. Understanding things. Statistical mechanics. Towards the end of my first year, I remember I went into Dietrich Müller’s office. He was my research advisor. I told him what I was thinking about trying to find a theorist on the faculty to work with over the summer. And he looked at me and he goes, “I always had you pegged for a theorist.” [Laughs]
I started talking to various theorists about whether I could work with them over the summer. I remember I went into one particle theorist’s office and I told him I wanted to work him. He told me about all the things that he was working on. And then he said, “Have you had quantum field theory yet?” Nope. He said, “Do you know how to calculate Feynman diagram?” Nope. He said, “Well, there’s not much you can do if you can’t, don’t know how to do Feynman diagrams. So, why don’t you take quantum field theory, and then come back next year.” I went into somebody else’s office. He told me about all the great things he was doin’. All the exciting things. He goes, “Have you had quantum field theory?” I said, “No.” He said, “Well, can’t really get you started in particle theory until you’ve had quantum field theory.” And then I heard about somebody else—I failed a few times. Then somebody told me, well, why don’t you go talk to Bob Sachs. He’ll often be more receptive. Bob Sachs at the time was a retired nuclear theorist. And he had an office in the experimental particle physics building, because he wanted to be with the experimentalists.
He was this cantankerous, but still sweet, old guy. I went in his office, and he told me about all the exciting things he was working on. Gave me a copy of his new book on CP and time reversal violations. Then at the end he said, “Have you had quantum field theory?” And I knew that if I said no [Laughs]—
—you know, what the answer would be. So, I lied, and I said, “Yes. Of course.” [Laughs] Then he said, “Do you know how to calculate Feynman diagrams?” I’m like, yeah. In that case, “I guess you can work with me.” I then went straight to the bookstore and got a copy of a quantum field theory book by Mandl and Shaw that was recommended to me. It’s a nice book because it gets straight to Feynman diagrams without skipping any steps. But it does it in the quickest possible way.
So, as an undergraduate, Marc, Feynman diagrams was not even on your radar. You had no idea, right?
My girlfriend was an art student. And she had read about astrophysics in her spare time. I think that’s the first time I ever heard about the Hubble Constant. I had no idea. My second semester, senior year, I did take a general relativity class that Cliff Will taught. And there was some cosmology in there. But I was not paying a whole lot of attention second semester senior year.
But you got this book, and you figured you could do it on your own.
Yeah. I’m pretty good about that. At that time, I was very good about getting through books. Reading through. Doing all the problems. Making sure I understand. So, I went through it. I learned quantum field theory. I would say that my gut understanding of quantum field theory still comes largely from that. He gave me a project to work on. It was pretty simple and straightforward. I wrote a short, single-author paper by myself. It got completely ignored for about 10 years. And then there was a brief flurry of interest in that paper, for a short period of time. But I worked with him over that summer, and I got increasingly confident. I don’t remember the details. I can’t remember if it was towards the end of that summer, or the beginning of second year of grad school. I was looking for a genuine thesis advisor because Bob Sachs said he shouldn’t be my advisor because he was retired. I remember going to talk to some of the particle theorists but somehow didn’t get taken on, although I don’t really remember the details. I did, though, find out about the cosmology group at Chicago. At the time, there was a brand-new cosmology group, which was one of the first early universe cosmology groups. It was a collaboration with Fermilab, and the faculty were David Schramm, Mike Turner, and Rocky Kolb, and there were research associates at Fermilab; Andy Albrecht and Josh Frieman had just arrived there.
You know, by this point, I was talking with lots of different graduate students and getting a much better sense about what was going on in the department. Although I was fairly naïve at the beginning of first-year graduate school, by the beginning of the second year, I had a pretty good sense about things. And it was an exciting—that was clearly one of the exciting groups. I’d go hear research talks, and one of the things that I liked about that kind of astrophysics and cosmology is that you got to use all kinds of different things in physics without having to specialize. They used gravity and general relativity. There was statistical mechanics that would show up in the things they were doing. There was particle physics that showed up in what they were doing. There were interesting phenomenological things, but it wasn’t too detailed at the time yet. So, it seemed like a fun thing to do because you could use different areas of physics but didn’t have to specialize in one. At the time, it seemed that if you were a particle theorist, you just did quantum field theory and calculate cross sections. And, at the time, that if you were condensed matter physicist you had a very specific thing that you focused on. But I liked all the different aspects of physics. I liked my quantum mechanics classes. I liked thermodynamics. I liked general relativity. I liked particle theory. And, you know, one of the things that was interesting about this group was that they seemed to use ingredients from all these different areas of physics. And the other thing, it was clearly a very exciting group.
What were some of the big questions that the group was asking at this point, given that it had this multi-disciplinary approach?
The big questions that they were interested in were dark matter and inflation. This was late ’80s, so inflation was still in its first decade. David Schramm, Rocky Kolb, and Mike Turner, between the three of them, were thinking about inflation, they were thinking about dark matter. We heard a lot about big bang nucleosynthesis. That group did a lot with that. And the other thing that they were spending a lot of time thinking about was baryogenesis. But there were also graduate students working on phase transitions in the early universe. Topological defects. There were all kinds of different things that sounded really cool floating around. I went to talk with Mike Turner, and he told me couldn’t really take any students now. At least, he couldn’t commit to taking any students now. But here’s a calculation that I’m thinking about. Maybe if you wanna take a look. At that point, I was savvy enough to realize, you know, that if I wanted to work with him, I had to show him that I was going to be good.
By that time, I’d learned how to calculate Feynman diagrams, but most of the students who went to work with him had not yet acquired a whole lot of facility with particle physics and quantum field theory. One of the calculations he was interested in doing actually required that you understand quantum field theory and how to calculate cross sections. And, so, he said, so you know how to calculate cross sections? Here’s something I’m interested to do. And he gave me some papers by Olive and Srednicki on supersymmetric dark matter that were limited to some regions to the parameter space, and he thought it might be interesting to explore the full parameter space. In retrospect, an obvious kind of thing you would give to a beginning graduate student, the kind of project that you don’t care whether it gets done really quickly, but if it gets done, great. You can write a paper.
By that time, I was savvy enough to realize that if I wanted to work with this group, if I wanted to do theoretical physics, I had to show them I was going to be good, and so I did nothing but work on that calculation and got results relatively quickly. Also, I was collaborating on this project with a post-doc named Kim Griest who was also interested in this problem. I was talking with Kim about the details, and I learned a lot from Kim. I did a lot of reading. And I did a shitload of calculating [Laughs]. And I calculated a lot of really long cross sections. At about the same time, I don’t actually remember the order, but about the same time while I was talking with Kim, who was teaching me all these different things. He had heard something interesting; it had been noticed that cross sections for dark matter annihilation seemed to go down with energy, and somebody had some idea that this might be related to unitarity. As I told you, I was really into quantum mechanics and I understood how you could get this result. I understood how you could show why it is that these cross sections are decreasing with energy based on an argument involving unitarity. So, Kim and I worked that out and wrote a short paper that we got published in PRL. I think that might have been my first published paper in cosmology. And at the same time, I was doing all these cross sections, and I think Mike Turner realized that, you know…
You could do it.
—yeah. So, I did that short paper on unitarity bounds on dark-matter masses.
What was the central conclusion of that paper?
We were able to show that unitarity bounds the magnitude of cross sections. And the bound decreases with the mass. And the cosmological abundance of that type of dark matter particle goes up as the cross section goes down. At the time, there was an upper limit on the mass density of the universe, which is about factor 10 weaker than it is now. But from cosmological observations we knew that omega h-squared had to be less than one, and we were able to use that to show that the dark matter particle mass couldn’t be any greater than some number, which at the time was 300 TeV. People still look at that paper. After that I became part of the group. I stayed in graduate school for four years. I wrote three or four more papers with Mike Turner on various subjects. All of them involving particle dark matter.
Was Mike your advisor?
What was your dissertation on? Was it on, basically, what you were doing in that group? Or was it a distinct project?
My PhD thesis was on energetic neutrinos from WIMP annihilation in the sun. At the time, it had been realized that if the dark matter consists of weakly interacting massive particles they could collect in the sun. And inside the sun they could annihilate to a whole bunch of things and produce energetic neutrinos that you could then detect in experiments like Kamiokande or, at the time, Irvine Michigan Brookhaven, IMB, a neutrino detector in a salt mine under Lake Erie.
And what was the state of neutrino research at this point? Had it been discovered that they had mass?
Not yet. That happened around the early 2000s. This was about, a bit more than a decade before we knew that neutrinos mix. So, actually a lot of the calculations in that paper had to be re-done later on. But, anyway, people had pointed this out, and for my PhD thesis, I sort of did a very detailed and comprehensive analysis of the whole thing. I wrote a long single-author article, paper, on energetic neutrinos from WIMP annihilation in the sun and the earth. That got published in Physical Review D. The University of Chicago was willing to accept for the PhD thesis, a single-author paper published in a refereed journal. So, I actually didn’t write a PhD thesis. That paper was my PhD thesis.
Did you have to even go through an oral defense?
Yeah. There was an oral defense. I can’t remember who was on my committee. I think Nambu was on my committee. Mike Turner was on my committee. Maybe David Schramm or Rocky Kolb? I don’t even remember. All I remember about my thesis defense is that Mike Turner’s wife was about to give birth to their first child that day.
He was a little distracted?
[Laughs] Yeah. So, okay. Good. Sure. Any more questions? Okay. Good. Congratulations. See ’ya later. [Laughs]
And with Nambu, were you thinking about—was string theory part of your world at all?
The really smart kids all did string theory. I wasn’t one of the really smart kids. I didn’t do it. It was two things. I don’t have much affinity for mathematical formalism. I don’t have patience for it. I don’t know that I’m particularly good at that type of abstraction. So, I didn’t become a string theorist. But there was a very active string theory group at the time. Steve Schenker, Dan Friedan, Hirosi Ooguri, and Emil Martinec. There were four faculty members when I was there. There was a group of three which was Steve Schenker, Dan Friedan, and Emil Martinec. It was a fairly new group. They were very young. They were doing very exciting stuff. They were doing work that was getting a huge amount of attention. Mike Douglas was a post-doc in the group. He was doing the work that made him famous. And Hirosi Ooguri was an assistant professor there for one year. Actually, since I had aspirations to be a particle theorist when I was working with Bob Sachs, I had an office with the particle theory group and I was actually sharing an office with a string theorist for about a year, half a year, something like that. So, I knew the string theorists, and I’d hear about what they were doing, but I never got into it.
How did the opportunity at the Institute come up for you?
[Laughs] I was applying for post-doc positions, and I wish I had saved this cassette tape. Maybe I still have it? I was applying for post-doc positions, and I had no idea how it would go. I had no idea what my prospects to get a good postdoctoral position were—I was pretty confident I would get a post-doc somewhere because that group—Rocky, David, and Mike Turner—had produced a number of students who had been quite successful with post-doc positions. Larry Widrow was a couple years before me. He got a post-doc position at Harvard. Ted Russell got a post-doc position at Livermore. Josh Frieman had gotten a post-doc position at Stanford, I think. So, I was confident that I’d get a good post-doc position. But I had no idea, whether I’d get a good one or where exactly. I did get one offer early on from Steve Weinberg at the University of Texas, Austin. I was out late one night, and I came home, and we had one of those cassette recorder answering machines. There was a message from Steve Weinberg on the cassette tape. And I thought, “I’ll never throw this cassette away.” [Laughs]
And then I got offered a job at the Institute for Advanced Study. That was pretty interesting because that was the premier place. Ed Witten was there, and Ed was at the peak of his influence and reputation. And Frank Wilczek was there and he was at the peak of his influence and reputation. And, you know, although Ed Witten was focusing entirely on string theory, just a few years before he had written four or five papers that were landmarks in cosmology. And you know, Frank Wilczek had broad interests, but just in the year or two before I applied, he had written four or five papers that were landmarks in cosmology. And it just had the aura, you know, of the Institute for Advanced Study.
I remember when I got offered that position I was floating on air because I felt so smart.
Was there someone in particular who drove that offer for you to get, for you to come there?
Is there someone in particul—I don’t know. What do you mean?
I mean, it’s not like the whole Institute says, “Come, we’ll accept you.”
It was Frank Wilczek. It was Wilczek.
It was Wilczek. Okay. Right.
I was feeling so cocky because that was the first time anyone from that group had been offered a job at the Institute for Advanced Study. And all the string theorists, you know, they were, like, [Laughs] Ed Witten was God, you know. And I was being inducted into heaven, or something like that. I was walking around Chicago feeling so proud of myself. I’m so smart. I’m so smart. But then I also had an offer from University of California, Berkeley. Actually, I didn’t have an offer. I got an SSC fellowship to use at Berkeley, and I was trying to decide whether to take the SSC fellowship to Berkeley or take the post-doc at the Institute for Advanced Study. Berkeley seemed really cool also. They had the Center for Particle Astrophysics. Joe Silk was there. Marc Davis. Bernard Sadoulet. They were doing dark matter experiments. So, I wasn’t sure where to go. I decided to visit the Institute for Advanced Study before accepting. I remember I got there on some random day of the week, and it was lunch time. And I went to Michelle Sage’s office because she was the administrator. “I’m Marc Kamionkowski.” She says, “Oh. You got here just in time for lunch. I’ll take you to the cafeteria. I’ll point you to the cafeteria. You can go down there and find Ed and Frank. They’ll all be having lunch there.” And I remember, I went to the cafeteria and while I was walking down the stairs to get to the cafeteria at the Institute for Advanced Study, I saw Ed Witten walking up the stairs. And he seems to be thinking. And I knew it was Ed Witten because I’d seen him give a talk. And, I thought, “I should stay out of the way because he’s clearly thinking about something important. If I get in his way, I’ll set physics back God knows how many years.” [Laughs]
So, I try to stay out of his way going down the right side. And he’s coming up. And then he looks at me, and he goes, “Are you Marc Kamionkowski?” I said, “Yeah.” He goes, “Oh. We’ve been expecting you. Why don’t you come join us?” [Laughs]
He then takes me through the lunch line. You know, helps me find food. Buys lunch for me. Takes me back to the table. And I find myself sittin’ there at a table with Frank Wilczek and Ed Witten and Murph Goldberger, who was the director at the time.
And they’re just shootin’ the shit. [Laughs] And I’m sitting there talking with them. And they’re talking, you know, about whatever it is that, you know, people at the Institute for Advanced Study talk about. Then afterwards, when I’m done, Frank says “Why don’t you come back to my office, and we’ll sit down, and we’ll chat.” And, so, I’m sitting there in an office with Frank Wilczek and Ed Witten. I was a very good graduate student. But there’s a limit. [Laughs]
They started making small talk with me and asking me about cosmology, and asking me what the other people at Chicago are working on. And I tell them about what my office mate was working on, some extended inflation model. Like a good graduate student. I knew what I did very well. I could answer any questions about that.
But I didn’t really have any sense about what the other people were working on. And they’re asking me these questions, and I’m answering. And then Ed said, “Well, if that’s the case, then wouldn’t you expect that…?” And I’m, like, [Laughs] completely stumped. And after a few minutes there, Ed and Frank looked at each other, and, I swear to God, they gave each other the is-it-me-or-is-this-guy-an-idiot look. [Laughs]
When I went to Princeton, I felt like the smartest person in the world. And when I came back, I just felt like really, really small.
I was just completely reduced. I felt like such an idiot. And I was sure that when I got back to Chicago there would be another message on that cassette tape saying, “Um…”
“We take it back.”
Yeah. Yeah. [Laughs] “Sorry, Marc, but, uh, we must’ve made a mistake.” [Laughs] But, they didn’t do that. I was trying to decide between Princeton and Berkeley, and David Schramm was fairly unambiguous. He said, “If you go to the Institute for a post-doc, you can go to Berkeley afterwards. But it’s not so clear if you go to Berkeley that you can go to Princeton afterwards.”
Maybe it’s sorta similar to the advice about choosing between Maryland and Chicago?
Yeah. Mike Turner was kinda cagey. He was like, “Well, at this place you do this and you can do this and you can do this. This place you can do this, you can do this, you can do this.” And in retrospect, I would’ve done fine at either place. Both of them were great opportunities. But David Schramm was unambiguous. So, I went to the Institute.
And, so, what’s the day to day like at the Institute? Is it mostly hanging out in each other’s office and just shooting ideas back and forth? Is it all conferences and papers and presentations? What is—what does the day look like there?
I don’t know how it is now. It’s probably not too dissimilar. I was a post-doc, so I didn’t have anything specific to do. I didn’t have any specific projects.
Yeah. So, what do you do? How do you plug into projects? Are you required to come up with stuff on your own?
Yeah. What happens is, you go there, and they give you an office. There are seminars a few times a week that you go to. Frank Wilczek had a group meeting at his home one evening per week. And then there was tea time and we all went to lunch together. So, nobody ever came into my office and said, “You do this.” But I had to find something to do on my own. Either you go to a professor and say, “Do you have any ideas for me?” or you think of something on your own. I would say the first year was very demoralizing and difficult because, at the time, I was nominally in Frank Wilczek’s group. I was in the particle theory building. My office mate was another post-doc with Frank Wilczek. The other people in the building were string theory post-docs. Other particle theory post-docs.
And, at the time, Frank had a group of students and post-docs who were very focused on a specific type of theory, and he had one or two other post-docs who were doing condensed matter theory. I was the only post-doc in that group, or in that building doing cosmology, and it seemed that I should be doing what they do if I wanted to get anywhere. I felt like an outsider. And I spent a lot of time trying to read what they were working on and learn about what they were doing, and playing catch-up at the same time. I continued to finish off projects from graduate school, and I did have ideas for more work on supersymmetric dark matter. Also, during that first year, I don’t remember precisely when, but probably closer to the end of the year, I started spending more time going to astrophysics talks. I started talking with John Bahcall and with David Spergel, in particular. David Spergel was an assistant professor and he had tons of ideas.
What was Spergel working on at that point?
He was spending a lot of time working on topological defects and the cosmic microwave background. I found some project to work on with him towards the middle or end of the year. I feel like the first semester, I feel like I accomplished nothing. But then towards the end of that first year I got three projects done that turned out to be great. And maybe even a fourth. And then by the, you know, …
What were those projects?
One of them was a paper I wrote with David Spergel which was my first paper on the cosmic microwave background. I would characterize it as an exercise for myself. But in the course of going through that calculation, and reproducing what everybody else did, by the time I was done, I knew about as much as anybody else. And the paper I wrote after that turned out to be very important and got a lot of attention. There was another paper, two papers, I wrote with John March-Russell who was a particle theorist in Frank Wilczek’s orbit. He was officially at Princeton University. I don’t remember exactly how it came about, but we realized that quantum gravity effects would spoil the Peccei-Quinn solution to the strong CP problem. It also had implications for topological defect models of large-scale structure formation. John was a real shark at the time. He knew how to write quick-and-dirty papers. And so, he and I wrote two short papers, both of which got a lot of attention. And, I think it’s about that time, towards the end of my first year, that I also wrote a paper with Katie Freese. I don’t remember how that got started. Maybe at Aspen or something like that? Something completely different. Something involving first-order phase transitions.
The first semester was really, really demoralizing. It was very, very difficult because I felt that I was in over my head. Everybody else seemed to be working together and doing this particular kind of science and I didn’t seem to be able to latch onto that. But then, towards the end of my first year, I found other things to do, and I realized that I was getting things done. The other thing that I did was to go talk with John Bahcall. At the time there was a particle theory building and an astrophysics building, and the particle theory building was Frank Wilczek and Ed Witten. And John Bahcall was in the astrophysics building. John had a very strong group of astrophysics post-docs, but I was not one of his post-docs. But I sort of had interests in astrophysics, so I went to talk with him and ask him if he had any ideas about things to work on. I think the other post-docs in astrophysics felt a lot of pressure because they thought that John was watching over them, making sure that they don’t waste their post-doc slot at the Institute for Advanced Study and, actually do something that’s gonna make them all famous. But I was not one of his post-docs, so I thought that if I went in there and I did something useful for him or interesting to him, that that would be a plus. But if I didn’t feel like I was gonna piss him off if nothing came of it; I wouldn’t be wasting one of his post-doc slots. I wound up working with him on something completely different, several, projects involving solar neutrinos.
So, by the time I had finished my post-doc there, I had written a bunch more papers on particle dark matter; the kind that I had written before with a bunch of different collaborators than I had in grad school though. I had written this paper on first-order phase transitions in the early universe. I had written these two papers with John March-Russell on quantum gravity effects and global symmetries. I had written papers on the cosmic microwave background with David Spergel. And I had written several papers on solar neutrinos with John Bahcall. Again, when you’re a post-doc you never know how well you’re doing. I had no idea what my prospects were for a good faculty position. But I knew I was doing well—I was starting to feel confident.
Would you be going into, you know, Frank’s and Ed’s office and getting the sense that they’re not concerned that you’re an idiot at this point? I mean, at a minimum, are you getting that kind of feedback?
At some point, but I don’t remember when. I felt at first that I was just wasting a spot, because at Frank’s group meetings, it seemed like everybody knew what they were talking about except for me. [Laughs]
Maybe everyone else felt that also?
And I never actually worked with Frank Wilczek. I was nominally his post-doc for three years, but I never actually wrote a paper with him. I don’t remember when, but at some point, somebody told me, maybe it was John Bahcall, that Frank thought highly of me. At some point, I realized that I wasn’t a disappointment. I don’t know whether he thought I was a genius or something, but I didn’t feel nervous going to ask him for a letter of recommendation.
And how does the Institute work? I mean, is the post-doc sort of open-ended? Or, you have a couple of years, and then it’s, you know, you get a letter saying it’s time to start thinking about moving on?
At the time, they had three-year positions and they had five-year positions which were a bit higher paid and more prestigious. So, the ordinary geniuses would get three-year positions, and the real super geniuses would get five-year positions. I got a three-year position there, but in the third year, when I started applying for other postdoc positions and some faculty positions, they promoted me to a five-year position. I then only used one semester of that because I got offered a faculty position at Columbia University.
Were you actively on the market at that point? Or, you were recruited?
I was applying for jobs. But in some sense, I might’ve been recruited. I didn’t apply for that job at Columbia University because the advertisement was for a particle theorist and the job description went on and on about QCD: perturbative QCD, lattice QCD, relativistic heavy-ion collisions. And it just didn’t seem like they were looking for somebody like me. But then at some point I got a phone call from Mike Turner, and he wanted to know why I didn’t apply for that job at Columbia. I said, “Well, they’re looking for a bona-fide particle theorist. They’re not looking for someone like me.” And he said that, I think, Erick Weinberg had called him up and asked why I hadn’t applied. That they’d be interested in considering me. So, I applied, and I got on the short list. I went there and I gave a talk, and I remember I was in Erick Weinberg’s office and he said, “How come you didn’t apply?” I said, “I thought you were looking for a particle theorist.” At my job talk there I talked about supersymmetric dark matter. He said, “You’re the first candidate that we’ve had that’s actually talked about a particle.” [Laughs]
He said, “Everybody else was just thinking about strings at the time.” So, they offered me that job. I don’t think I had any other job offers at the time. I had the option to stay at the Institute for two more years. I did get a phone call from the Technion in Israel. They didn’t offer me a job but they said they wanted to offer me a job. They asked me whether I was serious about it. They didn’t want to waste their time if I wasn’t serious. And I thought about it for a few days and decided I didn’t want to move to Israel. I don’t think I had any other offers at the time though. So, I went to Columbia University, as an assistant professor after one more semester at the Institute.
It was clear that you should take the Columbia position?
It wasn’t quite so obvious. Columbia’s tenure record at the time was really, really crappy. I remember Erick Weinberg telling me, “You should not take this job expecting to get tenure.” “If you take this job you should look at this like a place you’re going to live and work for five to eight years and then look for a job elsewhere.” Because nobody was getting tenure at Columbia at the time. At the time Columbia was hiring assistant professors to essentially be glorified post-docs. And that was understood. And I had conversations with some people who told me I shouldn’t do that. Because I’m gonna go there and I’m not gonna get tenure, and, you know, at that point I’m gonna be a mid-career scientist, and, you know, it doesn’t look good when you get turned down for tenure. And, you know, when you apply for tenure jobs you can be competing against tenured people all over the place. So, some people actually warned me not to go.
So, Columbia, like a Harvard and a Stanford, it had a reputation of not promoting from within?
At the time, when I went to Columbia, they had maybe about, I think, 12 assistant professors. Every single one of them, except for me, had been either a post-doc or a graduate student there. I was the only, out of the 12, who just came from the outside. And all of the rest of them were clearly affiliated with a senior faculty member. You know, there was a senior faculty member who they were working with. They were, you know, in fact, assistant professors.
The tenure rate was one out of five. Nobody went there with the expectation that they would get tenure. I really wanted to live in New York City. It seemed like a really exciting thing to do. And, I thought, you know, if I’m so worthless that after five or eight years nobody wants to touch me with a 10-foot pole, maybe I should find something else to do. So, I went there knowing it was a risk, but I was very interested in moving to New York City.
Did you take on graduate students right away when you got to Columbia?
Yeah. I had three graduate students, I think, my first year. Yeah, my first year. There was one student who I recruited before I even got there. He was a Stanford undergrad and he was admitted to Columbia but he wanted to do cosmology. I remember talking with him. He wanted to make sure that I would actually be there and be taking students. So, that was Ari Buchalter. Catherine Cress was officially a student with David Helfand in astronomy, but I worked with her, as well. And then, the third student I took, I think from the beginning, was Xuelei Chen who had just arrived from China. So, I had three graduate students. I don’t remember if it was my very first year or the second year, but all three of them got to PhD before I left. And then there was a fourth student, Alex Refregier, who had also been working with David Helfand and a few other people. I wasn’t his advisor, but I did a research project—advised him on a research project in weak lensing of galaxies, which is what he wound up doing for a living.
I’m curious, Marc, given that there was some degree of confusion about how you identified yourself and what Columbia was offering in terms of posting the position. Did that affect the kind of physics that you wanted to pursue in that faculty role?
No, but I’d say I felt that I was under a huge amount of pressure when I was at Columbia.
Now that I’m tenured I understand how these decisions are made, but at the time, I didn’t really know. The expectation was that I wouldn’t get tenure, and there were no senior faculty doing what I did. So, I wasn’t sure that any of them would appreciate what I did.
Or understand it.
Or understand it. I wasn’t sure how they would evaluate it. I didn’t know how they would identify letter writers. One way to say it is that I had never met anybody from Columbia at a meeting before I went there, and I would probably have never met any of them, you know, at a professional meeting had I gone somewhere else for a junior faculty position.
And would you say that for a department that had the stature of Columbia that this was a real blind spot on their part? That they didn’t have senior people working in this field in the way that a Caltech or a Stanford definitely would have?
No, actually there were very, very few top departments hiring people like me when I went to Columbia.
When I went to Columbia, cosmology was sort of a fringe thing. And I think that Erick Weinberg and Jacob Shaham and other people realized that there might be some interesting things going on, and so it might be nice to have one person doing it. But they weren’t going to build a group there.
So, it’s actually the opposite. That sort of Columbia, in your hire, was sort of ahead of the curve on these things?
I think so, but then things changed when I was an assistant professor. I was hired at Columbia in 1994, but over the next five years everybody started hiring cosmologists.
Well, first there was COBE. And then WMAP, or what became the WMAP, was selected by NASA. And I think that people realized that there were just lots of very, very strong post-docs choosing to work in this field. And I think that people realized that something was going to come out of this. We saw similar things with gravitational astrophysics, although that still took a while. It didn’t build up quite as much as it should have until after LIGO.
So, in 1998, when you’re named associate, forgive me, I don’t know how this works at Columbia. Is this like a Harvard where that’s a glorified assistant or is that a tenured promotion?
That was untenured.
That’s an untenured promotion.
That was their way of letting me know that they liked me. That I was doing a good job, maybe because I was very nervous about my tenure prospects. As I said, I didn’t know who in the department would be interested in my work. They had very strong groups in QCD. A very strong group in formal field theory. They had a very strong group in nuclear theory. It wasn’t clear who my advocate would be. The other thing is that when you’re an assistant professor you hear all kinds of things about tenure decisions but don’t really know what to believe. It has to be a unanimous and enthusiastic vote of the faculty. Even though the faculty are unanimous and enthusiastic they send it to the Dean. Then the Dean sends off for 20 letters from people across the country, you know. It seemed like I would have to be known to every astrophysicist and particle theorist in the entire world. Seemed that everybody in the department would have to like me. And then you’d hear that at the end of the day it’s T.D. Lee who decides, and I had very little interaction with T.D. Lee when I was there. In retrospect, though, he gave me a chance. He was the head of that group that I joined.
So, I had no idea what my prospects for tenure were. Then in 1998 I got the Warner Prize, and I realized, “Well, maybe I might actually get tenure.” And, I don’t remember the exact history, but about that time, I was invited to be considered at Caltech. Typically, at Columbia, at the time, you’d spend five years as an assistant professor, then you’d get promoted to associate without tenure, and then after three more years you’d get considered for tenure. So, it was an eight-year tenure track. And I was promoted a year early to associate professor, I think, as an indication that they were serious about tenuring me. I went to interview at Caltech, I don’t remember when exactly, but it was 1998. And then it took a long time before I heard back from Caltech. It was spring 1999 when I was offered a job at Caltech, but I think Columbia had already gotten the wheels for the tenure review rolling.
And Caltech’s offer was with tenure?
Was that an easy decision for you?
Not as easy as you’d think, because I really loved New York City. I couldn’t imagine living anywhere else. And Los Angeles, seemed to be—[Laughs]—I had a very New Yorker view of Los Angeles, let’s say.
Sure. And it’s not even like, you know…it’s Pasadena Los Angeles. [Laughs]
Yeah. The idea of living in Pasadena when you’re living in New York City, it’s just completely ridiculous. So, it wasn’t a foregone conclusion, but the offer from Caltech was just so strong. It was just so exciting. There were so many people there that had interests that I think overlapped, to some extent, with mine.
Like who, for example? Who would you have been excited to work with who was already at Caltech?
I was joining a theoretical astrophysics group that was: Kip Thorne, Roger Blandford, Peter Goldreich, and Sterl Phinney.
Yeah. [Laughs] Right.
There was nobody like that in theoretical astrophysics at Columbia.
Were you involved in LIGO at all?
No. My office was right around the hall from the LIGO director’s office, and there were lots of LIGO people in the building. So, I always heard about it, but I had no connection with it.
Did you bring graduate students with you?
No. As it turned out, all the students that I had were graduating that year. I think it was that year. I can’t remember if I graduated all four students that year or whether I graduated one the year before. I don’t remember if Catherine Cress was 1998 or 1999. So, I didn’t take any graduate students with me. But Ari Buchalter, who had been my graduate student at Columbia, had already been offered and accepted a post-doc position at Caltech. A Prize fellowship at Caltech. So, when I went to Caltech he was there as a post-doc. Then during his first year there he got interested in hedge funds and making money. [Laughs] So, he left science after two years at Caltech. And now he makes a ton of money, but he remains interested in science and now offers a prize for research in dark energy.
How did your directorship of the Moore Center come about?
The Moore Foundation, and separately, Gordon Moore, gave Caltech $600 million about that time. And $300 million was for student support or infrastructure or endowment. But I think the other $300 million was to support research. And, so, essentially, the Moore Foundation gave the Caltech provost a huge pot of money to use for research initiatives around Caltech. Our division chair at the time, Tom Tombrello, was very proactive about trying to get as much of that for our division as he could. And, I think it was his idea that we should try to get some of that for a cosmology center. He recruited me to write up the proposal and be the director. And as far as proposal writing goes it’s pretty easy [Laughs] because we weren’t competing with universities across the country. It was more or less a description of the center rather than a proposal. And, so, that’s what happened. It was just, essentially, money that the Moore Foundation gave to Caltech and they distributed a huge amount of money to a bunch of different faculty members.
And what were some of the projects that you wanted to work on in this position?
The commitment was half-a-million dollars for 10 years, and, the faculty numbers in theoretical cosmology at Caltech were relatively small. So, we used that, essentially, to hire two more faculty members doing similar things. Research faculty. One of them was Andrew Benson who is more on the astrophysics side of cosmology. And the other one was Sean Carroll. And the timing was just absolutely perfect. That Chicago decided to not tenure him; I think made a big mistake there. They decided to not tenure him just at the time that this position was opening up. And, it also allowed us to support two post-docs. So, we got two research faculty, support for two post-docs at any given time, and there was also a lot of money for visitors. So, we had all kinds of people come through Caltech for sabbatical stays supported with that money. So, it was a good thing, but it’s also a two-edged sword.
In what way?
When you’re a scientist, every dollar that you get in terms of grants takes some amount of your time. If you’re an experimentalist it might be necessary in order to do your research. But if you’re a theorist, it’s not so clear. You can work by yourself. You can work effectively with a handful of students and a post-doc or two. But once you start getting more money you start supervising more people. Many of these people work effectively on their own, but even so, every dollar that you get requires some amount of your time for administration.
You’re moving away from the science?
Yeah. At some point, more of your time is spent being an executive scientist rather than actually doing theoretical physics. More of my time, was spent, you know, working out logistics for peoples’ visits. You know, getting CVs, writing proposals, worrying about office space….
Was this part of the equation for you to want to go to Hopkins? Where the idea was that you would just be on the faculty; you’d be a civilian, so to speak.
That was a complicated decision with lots of different factors, personal and professional. You know, to a first approximation it was a two-body problem. But it wasn’t that we had to go there. My wife, Rebecca, had an offer at USC also. We could’ve both stayed in California. So, we had an opportunity…
Is she in physics also?
She’s in chemical/biomolecular engineering.
So, she got through the organic chemistry book that you didn’t get through? [Laughs]
No, actually not. [Laughs] Her PhD is in computational and neural sciences. I don’t know. Maybe she did? No, no. I don’t think she did. She was a math major in college.
But in the end, for you, joining the faculty was not having these additional responsibilities was an important factor?
It came into play.
Did they reach out to you or were you looking? I mean, you could’ve just sort of stepped down from the directorship and just been on the faculty at Caltech, too, I imagine?
Yeah. I didn’t move to Johns Hopkins just because of that. I didn’t move to Johns Hopkins just because of the two-body problem. I didn’t move to Johns Hopkins for any specific reason. It was some combination of factors….
So, Marc, just to bring the narrative up to the present. What’s some of the research that you’ve been working on, you know, since you’ve been at Hopkins?
A lot of stuff actually. I mean, one of the things that surprised me is that I think in terms of my research productivity, I’ve been doing just as well here, if not better than I did at Caltech. Probably a narrower scope, but I think the work I’ve been doing is probably more interesting than what I did during my 12 years at Caltech. I’ve managed to attract really good students and post-docs here. We’ve done work on the Hubble Tension and early dark energy that’s gotten a lot of attention. We spent a few years exploring the possibility that black holes may make up the dark matter in the universe. That work got a lot of attention. During the first few years here, there were several ideas for large-scale structure and mathematical formalisms for large-scale structure that my student post-doc and I worked on. We didn’t actually get a whole lotta mileage out of it for a few years, but over the past few years I’ve written lots of papers based on the tools we developed. I wrote a paper on interstellar dust that is far more astrophysical than anything else I’ve ever done. So, those kinds of things.
Well, Marc, I think for my last question I want to ask you, you know, looking toward the future, you’re involved in so many big question marks like dark matter, for example. What are the things in the future that you’re, sort of, most excited about? Where you see greatest opportunities for advancing even fundamental understanding that has been, sort of, out of reach for the entirety of your career?
Say that again.
[Laughs] The kinds of things in the future, you know. We’re still struggling to understand dark matter, right? So, I’m curious, what are the things that you’re most excited about working on that you think are most compelling in terms of, you know, really moving the ball forward on discovery?
I don’t know. I always get asked these kinds of questions, and I always tell people I really don’t know. Which I think they’re surprised to hear.
I mean, the theme of our talk has been, there hasn’t been any grand plan in your research agenda over the course of your career. So, maybe, it’s more about, like, you know, using your powers of extrapolation thinking about how you’ve come to look at any given project? How you became involved in that. And think about what might be the next projects to work on that continue to advance discovery.
I think one of the things that’s been relatively constant in my work is some connection between theory and experiment. I don’t really write theory papers that are read only by other theorists. I do like to try to maintain a connection between theory and experiment or observation. I like to try to learn new things. I’m always trying to find exciting new areas of science that I haven’t worked on before to see if I can figure out something interesting to say. For example, I tried my hand at ultra-high-energy cosmic rays a few years ago. I’ve been writing papers on pulsar timing arrays over the past few years, which I’ve never worked on before. And then a few years ago, I wrote some papers on interstellar dust, which I’d never really worked on before. So, I think I’m gonna continue to look for new areas that seem like they’re interesting, where I can learn something new.
But beyond that there are two other constants. I’ve always worked on dark matter, on and off in various different ways. So, I’m sure I’m going to continue to think about dark matter and how we might actually try to figure out what it is. And the other thing that I’ve been workin’ on fairly steadily for the past 20-something years is the cosmic microwave background. And clearly there’s going to be a lot to do with the cosmic microwave background and large-scale structure. The Vera Rubin Observatory is going to start in a couple years. There’s going be CMB-S4 and/or other experiments like that coming along within the next decade. And there’s just tons of things that you can do with these measurements. The people involved in the projects are outstanding, and the data are just going to be incredibly precise. It’s going to be an incredible wealth of data. For a couple years I was not thinking much about this subject because it seemed like there wasn’t much more that except for more of the same. But just the past year or so, I found some real interesting papers about new things that you can do, and my post-docs, students, and I have come up with several more ideas for interesting, new types of analyses that you can do with these data.
There’s also early dark energy and the Hubble Tension. I don’t know how much I’m going to work on that. I wrote a paper a few years ago that got a lot of attention, but the obvious things to follow-up require analysis tools that I don’t have a good grasp of. I’ve had students and post-docs who are great at these kinds of things and if others come along maybe I can work on these things with them? But I don’t know. I don’t ever really know. There seems to be two types of people in science. There are some people who knew they were gonna be scientists all their life, and they are driven by some big question; they go to bed every night thinking about that big question. But for me, it hasn’t really been that way. I didn’t know I wanted to be a scientist all the time. A lot of the twists and turns in my career came about by accident. I don’t think that I’m motivated by the answer to some specific big question. I try to find an interesting problem, within the quest to address the big questions, that I can solve or contribute to or figure out, hopefully in collaboration with a student or a post-doc. Yes, I’d like to know what the dark energy is, but you know, if I just sit here staring at the walls, it’s not going to help me figure it out. But maybe I can find some way to contribute to the endeavor. For example, some people are trying to use lensing time delays to address the Hubble tension, and a student and I figured out some interesting little calculation that we could do to say something about that. So, I think that it’s more that I’m always trying to find interesting little problems which may help move things along.
Marc, given how you emphasized how you straddle the world of theory and experimentation, how much are technological advances part of the equation for you, in determining what those, the most compelling smaller questions or problems are, out there, not only to work on but that are feasible in terms of better technology that is going to give us better opportunity to understand this stuff better?
I don’t think it’s much in the way of detail, but I think one thing that I’m aware of when I do my work is that things that are seemingly unimaginable now are going to be done within my lifetime. I’m eating right and I’m exercising, so [Laughs] I hope that will be true. But I’ve been around long enough to see that it’s possible. When I was writing these papers on the cosmic microwave background in the mid-’90s they seemed like fun theory problems, and you know, here are these cool equations. And we found this interesting thing you could say if you could someday measure this. And maybe someday someone will try, but it just seemed so futuristic. I thought that maybe it might get done, but if so, it’s going to take a really long time. But many of the things that I thought were ultra-futuristic have already come to pass. And some of them came to pass much more quickly than I ever imagined. Some of them turned out to be much better than I had ever imagined.
What’s a good example of something that came to pass much more quickly than you would’ve thought?
WMAP and measurement of small-scale CMB fluctuations. I started working on this towards the end of COBE, and COBE had taken more than 20 years from conception to completion. This was mid-’90s and my collaborators and I, and other people, are writing papers about what you might someday do if you had another satellite mission up there. As it turns out, many of the things that we thought would take 20 years to do were done by 2001, first By BOOMERanG, and then by 2003 WMAP, and then even Planck, in 2013, which was still less than 20 years after I wrote those papers. It was far, far more than we ever imagined you could do. So, that actually happened. And the other thing is there were some fortunate accidents. The baryon density turned out to be larger than we thought back in the mid-’90s, and, so, the amplitude of the small-scale fluctuations was bigger than we thought. So, the signal-to-noise of the experiments turned out to be higher, in part because the noise was a little better than anticipated, but more so because the signal turned out to be much bigger than we had anticipated based on what we thought in the mid-‘90s what the universe looks like.
So, you never know. That’s the idea.
Yep. I have no shame in writing papers about what someday might be measured, even if it seems completely inconceivable that you could do it now.
Well, Marc, on that note, I want to thank you for spending the time with me today. It’s been a lot of fun talking with you, and this is going to be a great addition to our collection. So, thank you very much.