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Interview of Mark Trodden by David Zierler on September 8, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Mark Trodden, Fay R. and Eugene L. Langberg Professor of Physics, and Co-Director of the Center for Particle Cosmology at the University of Pennsylvania. Trodden describes the overlap between astronomy, astrophysics, and cosmology, and he recounts his working-class upbringing in England. He discusses his undergraduate education at Cambridge, where he focused on mathematics, and he explains his decision to switch to physics for graduate school at Brown, where he worked under the direction of Robert Brandenberger. Trodden describes the impact of the COBE program during this time, and he discusses his work on the microphysics of cosmic strings and topological defects and their effect on baryon asymmetry. He explains his decision to return to Cambridge for his postdoctoral research with Anne Davis and his subsequent postdoctoral appointment at MIT to work with Alan Guth. Trodden discusses his next postdoctoral position at Case Western, which he describes as a tremendously productive period, and he discusses the opportunities that led to his first faculty position at Syracuse. He notes the excellent graduate students he worked with at Syracuse, and he explains what is known and not known with regard to the discovery of the accelerating universe. Trodden describes why the theory of cosmic inflation remains outside the bounds of experimental verification, and he explains the decisions that led to his decision to join the faculty at Penn and his subsequent appointment as chair of the department. He discusses the work that Penn Physics, and STEM in general, needs to do to make diversity and inclusivity more of a top-line agenda, and he describes much of the exciting work his current and former graduate students are involved in. At the end of the interview, Trodden looks to the future and offers ideas on how physicists may ultimately come to understand dark energy and dark matter.
OK, this is David Zierler, Oral Historian for the American Institute of Physics. It is September 8th, 2020. I am so happy to be here with Professor Mark Trodden. Mark, thank you so much for joining me today.
It’s a pleasure, thanks for having me, David.
OK. So to start, would you tell me please your title and your institutional affiliation?
I’m the Fay R. and Eugene L. Langberg Professor of Physics. I’m a professor physics and astronomy and I co-direct the Center for Particle Cosmology at the University of Pennsylvania.
Now Fay and Eugene Langberg, do you have any personal connection to them or do they have a particular connection to the department of physics?
I’m woefully ignorant of that I should say. This is an endowed chair that has been in the department for some time. There were holders of it before me and there will be holders of it after me. So, I think it’s not like one of the more recent chairs where you have direct contact with people.
And when did you become co-director for the Center for Particle Cosmology?
The center was established as part of me coming to Penn in 2009. So I came to Penn in January of 2009 and, technically, the center began then, although that’s also when most of the work to make it into a real entity began.
I see. So your onboarding, that was sort of baked into the process.
And what were some of the original goals in creating this center?
Well, I think it’s been clear for some time, certainly pre-dating the formation of the center, that the sort of traditionally distinct areas of cosmology and gravity—where one usually thinks of gravity as involving large, heavy objects and in the context of cosmology, you know, planets, stars, galaxies, etcetera—and the physics of the very tiny, quantum mechanics, the kinds of things you look at in particle accelerators. Well, I think for a long time, we thought of those as very separate disciplines, but it became clear I think over the last 50 years or so that most of the large questions in physics involve some combination of the two of them.
And certainly, the big questions in cosmology—what happens at the beginning of the universe, why is there more matter than anti-matter, what is the nature of dark matter, questions like that seem to inextricably involve the connections between quantum mechanics and physics of the tiny and gravity and cosmology. And so, the sort of purpose of the center was explicitly to bring together people who work in both these areas at the interface, like myself, and give them a space and opportunity to interact and sort of try to probe questions that lie at that interface.
Now as you mentioned, of course, a major theme in modern physics is that there has been this merging interest of particle physics and astrophysics and cosmology, but the term particle cosmology is rather unique. Did you come up with that name or does that recognize a particular school of thought or intellectual heritage?
I would not want to claim that I invented that phrase. I think people have used variations on that phrase—and very likely that exact phrase—in many places. Before, you will see places referring to particle astrophysics, particle cosmology. Sometimes you’ll see them referred together in the same phrase, particle physics and cosmology. So yeah. I think it’s a very good descriptor of what we do, but I wouldn’t want to claim any origin for that story.
And just in terms of refining and defining our terms, there’s been—over the years—shifting understandings in schools of thought about astronomy and cosmology and astrophysics. And if you put particle in front of all of those, I wonder if you can tease out where you see some of the differences in those fields of discipline?
I think that’s one of those questions you would get a different answer from ten different people you asked it to.
Which is exactly why I ask it.
So, I guess the first thing I should say is that despite being a professor of physics and astronomy and directing a center for particle cosmology, I am pretty far away from being an astronomer, or what I think of as an astronomer. The truth is, I think, that astronomy, astrophysics, and cosmology all bleed into one another in various ways, and there’s no real hard line between these disciplines. When I think of astronomers, I really do think if people who predominantly use telescopes and that doesn’t—of course, as you well understand—no longer means looking into an eyepiece somewhere.
But people who really are looking at the skies and I would say most of the field—certainly not all of it—is involved in the properties of astrophysical bodies. Stars, planets, things like that. As you move through astrophysics, which you might think of being concerned with how those bodies behave and interact, through to cosmology which I think is more consumed with the very largest scales in the universe, obviously people’s focus changes. But part of what’s happened in the last 20, 30 years is that all these tools are being applied in very different ways. Things you normally think of as cosmological observations are being used to probe the behavior of even things as small as planets. And observations that observe planets in the solar system are being used to perform precision tests of things like gravity, which are relevant to cosmology. So, I think of these things as being intertwined and the distinction more one of taste rather than actual taxonomy.
Now, in terms of putting those ideas actually into an institutional format, what are some of the things that happen at the Center for Particle Cosmology that are unique? That might not happen at like, say, a KITP, for example?
Well, somewhere like KITP is very different, of course. The KITP is designed to predominantly be a place that brings external people there to work and to come together to address specific large questions, and I love the KITP. I just actually stepped down from the advisory board three days ago.
It’s a wonderful place and it’s great to have been involved with them. Our center, I think-- Unique is a very strong thing to say, but I think the role of places like the Center for Particle Cosmology-- I think first of all we are focused on these questions that lie at the interface. So, we have many people in the center who have part of their research that sits in a way that you might think of very firmly in particle physics or very firmly in cosmology. And it might be quite hard to think of it in particle cosmology terms, but everyone in the center has part of what they do that sits towards the interface. And the goal I think is to put them together in a way that they can feed off each other come up with new ideas and test them and try to sort of bootstrap that process.
So, I think it’s quite hard to speak in the abstract about things like this, but from my own perspective, when I came to Penn, some of the things that I was most excited about, in principle, are things that actually came to happen. So, I have colleagues who are very close to me—like my colleague Justin Khoury where we have many wonderful collaborations together—but we have also had collaborations where we’ve had ideas and drawn in people much closer to the data, like my co-director Bhuvnesh Jain, and having these people down the corridor next to large, collaborative spaces where we’re meeting our postdocs and students so there’s no physical or intellectual barriers to having those conversations. That’s really been the goal of the center.
And just administratively and physically, is the center located within the department, both in terms of its funding and its location? Or is it really its own entity in both regards?
So, there’s slightly different answers to each of those parts of the question. Physically, it’s easy. We are within the Department of Physics and Astronomy and we sit in renovated space on the fourth floor of the David Rittenhouse Laboratory. The funding is-- So Penn has been very generous to provide funding for the center, seed funding and ongoing funding for the center over the time that I’ve been there and it’s my hope that they’ll continue to do that, but what that enables us to do primarily is-- Well, two things.
One, it enables us to hire people, postdocs. We’re predominantly theory and people are the most important part of what we do. So we can hire postdocs that are not tied to specific projects or specific grants, but the flipside of that is it enables us to leverage grants that we bring in from the Department of Energy, from the National Science Foundation, from NASA and other places, Simon’s Foundation, to sort of beef up the kind of projects we do with that funding and also to forge new directions with our sort of seed funding from the center. So that’s where that funding comes from. It’s separate from the department as a whole.
And then administratively, I would say Bhuvnesh and I co-direct the center, we take advice from various center members as we make decisions about what to do, but we don’t exist as an entity that is entirely separate from the department. All our members are department members and, in some sense, we’re contained within the rules that run the department.
So last question—I’ll make it a concrete one—on the center, and obviously there are many good examples I’m sure, but I wonder if there’s one that jumps out in your mind of a postdoc who did a dissertation where both they were excited and you were excited in the sense that the questions that you’re interested in, absolutely we would love for you to be here and we know you want to be here. What would be an example of a dissertation or a research agenda that would be really perfectly located at the center?
Well, I think we actually have lots of examples of those. So historically—I should say over the last decade—we have been unbelievably lucky to attract really the very best people to the center and many, many postdocs have come, go through our center, and are now populating faculty positions in the US and abroad. And all of those people have worked somewhere on the spectrum that I talked about. So, there are many success stories, but to give you a couple of very concrete examples, in our last—I won’t mention names, maybe—but in our last round of postdoc hiring just a year and a half ago, we brought in two new postdocs. I think it’s fair to say that both of them had spent a little bit more time working closer to what you might think of as empirical cosmology, sort of data-driven cosmology let’s say, but maybe less so working on the underlying models that can explain the data.
And right now, I’m involved in three completely separate collaborations with those people that involve both my theoretical colleagues and my more observational colleagues to try to come up with new models, figure out how to rigorously test them against data, and at least one of these postdocs is also involved in some of the survey experiments that we buy into in the department and is feeding that information into the work they do with the survey. So, in some sense, that’s a real sort of head to tail involvement in the process of particle cosmology. And I would like to think that it’s the history of what we’ve done in the center that drew these people to Penn and the sort of excitement of what’s going on that enables them to be so successful.
Well Mark, let’s take it all the way back to the beginning now. Tell me a little bit—first of all—about your parents and where they are from.
So, I come from the United Kingdom. I was born in a town called Wigan in greater Manchester, although my brother would kill me if I didn’t also stress that it’s historically Lancashire and right between Manchester—
And is this a classical Manchester accent that I’m hearing now?
No. I moved around a lot. Also, by the way, my town is between Manchester and Liverpool and the accent is different entirely from both Manchester and Liverpool.
So, this is a hodge-podge.
Very much so, yes. So, I grew up there. My dad is a factory worker. My dad is retired now, but he was a factory worker. And my mother didn’t work when we were very small, and later on worked as a barmaid in a pub, which she had done beforehand. That’s where she met my dad. And then worked at a market stall in my hometown.
Would you say that you grew up in a working-class kind of environment?
Yeah. We grew up on a council estate in the United Kingdom, which I think translates as kind of a project in the US. Yeah, very much a working-class environment.
And I know the system is very different there, but to the extent that the binary is relevant, did you go to a public school or a private school growing up?
So certainly, growing up, I went to a state school, let’s say, to avoid the problem with public and private. And then I was at a local what’s called a comprehensive school until I did what were called O-levels at that time, although they were phased out not long after I took them. And then when I was 16, I had the opportunity to take an entrance scholarship examination for a private school. And I took that, and I scored very well, so I spent 2 years doing my A-levels, which is the two years you spend immediately prior to going to university in the UK, at a public school, which was very unusual for everyone I grew up with. That was not a standard path at all.
When did you start to realize that you were pretty good at math and science?
I think I was always good at them at school. I think I was pretty good at most academic things at school, including sciences. I certainly didn’t know very early on that I wanted to do, say, math and science versus the languages or anything like that, but by the time I had finished my O levels—when you do your A-levels in the United Kingdom, you have to specialize. There’s far fewer of them you take than in the sort of general education you get prior to that, and by the time I took my O-levels, I had decided I wanted to focus on the sciences and mathematics, and when I did my A-levels, I took two different mathematics A-levels—physics, and chemistry.
You must have done pretty well to go to Cambridge.
Yeah, I did well. I got all As and yeah, Cambridge was very competitive to get into and I always, quite frankly, wanted to go to Cambridge. So, I won’t deny that I was delighted to get in.
Now, were you thinking about physics and mathematics from the beginning in terms of what you wanted to focus on?
You mean at Cambridge?
Well, so in the UK, the system is very different. In the UK, you choose what you want to study ahead of time. So, you choose what I think in the United States we would call a major ahead of time. So, you apply to a particular university or a set of universities to study a very particular course. So, I applied to Cambridge to study mathematics and that’s what I went to do. And so for three years, all I did was pure mathematics and applied mathematics. And the applied mathematics had aspects of it that were essentially theoretical physics, but that’s all I did for three years.
Now to the extent that you had a grand plan—and you may not well have at all—were you thinking that this background in applied mathematics would serve you well later on to pursue a degree in physics? Or were you open-ended in your goals at that point?
No, I think I went to university thinking that I might want to be a pure mathematician. I loved maths more than I liked the other things, to be honest. So, I did not expect to end up as a theoretical physicist.
Why did you make the switch?
I don’t have some sort of big realization story about that. It was really a gradual thing. You go to university—at least I went to university—the first year, you take all the classes they tell you to take. Half of them are pure, half of them are applied. I really enjoyed them all, but I frankly enjoyed the applied ones a little more, and I think I was better at them, actually. And then as you progress through the next two years, there’s a little bit of choice enters in the second year. There’s some core courses and some choices you can take and your third year, you really just take whatever you want, and by the third year, I already knew that I wanted to focus almost entirely on the applied classes.
And the applied classes ranged from things that really you would think of as applied mathematics, like mathematical methods, all the way through to thinks that you really would think of as theoretical physics. General relativity, quantum mechanics, you know, three semesters of quantum mechanics, general relativity, things like that. And then at that point—Sorry, I think I obviously clearly had decided that I wanted to try to get a PhD in theoretical physics.
And I wonder if looking back, in what ways did having that mathematical background serve you well and then once you got into a full-time physics program? To what extent did you need to play catch-up among your cohort that might have been studying physics for those years that you were not?
Yeah. I think that’s a good question. I think looking back, I think it stood me in terrific stead. The work I do now has a lot of mathematics in it, and I am so glad to have all that mathematical background. To be frank, it’s still something I love about the subject. So when I went to graduate school, I was ahead of most people mathematically by quite some way, but it’s also true that there was a lot of physics-- Well, there were two things really. There was a lot of physics that I didn’t know, even though I had taken a lot of theoretical physics, there was a lot of what I now think of as physics intuition that I didn’t have. And part of that is just having taken fewer physics courses and part of it is not having been anywhere near a lab for my entire time in undergraduate.
And then the other thing is there’s a lot of, sort of—what to say—things that physicists just memorize. It’s not a central part of the field to just memorize things, but there’s things you memorize by accident. I just hadn’t had that kind of exposure. So, actually, when I took the GREs—'cause I was going to go to graduate school in the United States—I didn’t do very well at all in the subject GRE and part of that was I didn’t know some of the formulae, I just hadn’t focused on a lot of these physics basic things. So “held me back” is maybe strong, but even today I think there are small things that I feel like people still have embedded in their minds from their undergraduate physics education that I have to remind myself of going along.
I wonder if collaborators might come to you specifically because you do have this unique background in mathematics. In other words, that might be one of the things in collaborations that you bring to the table.
I would say it is one of the things I bring to the table in collaborations. There are many mathematically-minded physicists out there and there are many people who are accomplished theoretical physicists these days who did not have a background in mathematics, but yet whose mathematical expertise and training is extremely advanced nowadays. So, I wouldn't in any way have used the word unique about what my own background in this, and if you were to look at the UK and look at people who are theoretical physicists in faculty positions, I think you’d find a high number of those people who went through a mathematics background like I did. So, it’s not very unusual, but in my own career certainly I think that is a strength that I bring to my own collaborations. Yes.
I’ve come across this term Cambridge man and I wonder if you thought, initially, you might have been on that track and if you wanted to sort of not only switch fields, but also leave the country to avoid becoming a Cambridge man.
I didn’t have such a grand idea of my future, I would say. I mean, I had decided I want—So, I did my undergraduate degree at Cambridge and I stayed and did a master’s essentially. Something called part three mathematics, which was terrific. And then I stayed around for a year and supervised undergraduates, and I did research with someone at Cambridge and I stayed around for that year. You know, I had a girlfriend, I wanted to stay around, and I just did that. And then I knew I wanted to apply to PhD programs, but I didn’t know if I wanted to stay in the UK. And only over that year that I decided to apply to the US programs, and it wasn’t some grand notion like not wanting to stay in Cambridge. I’m very glad I did it. I think I’ve had a lot of luck in my life and one piece of luck was that things turned out that way for me—so I have no regrets—but it wasn’t that conscious need to leave.
Now, mathematics, of course, has its own spectrum from the abstract to the applied as well. I wonder what your thought process was in terms of theory as you transitioned to physics?
I guess I’m not entirely sure what you mean. I already knew—So, I had spent a year doing research at Cambridge and I had been working in this particle cosmology area, and in fact in--
You mean as part of your master’s, you were doing the research.
So I did my master’s, then I did a year of a research--
Oh, I see.
--at Cambridge. So when I did my master’s, towards the end of my master’s there was an option to write an essay. They called it an essay. It was kind of a small research project that counted towards your final grade, and I chose to do that. And there were many suggested topics, but the one I chose was supervised by a Cambridge faculty member, Anne Christine Davis, who was then a lecturer and recently retired, and in the interim became the first ever female professor of mathematics in Cambridge history. And Anne guided me in that research project, and I did very well in that research project and in the summer when I was thinking of staying for a year to do research if I could.
She had written on my final project that there was going to be a meeting on the topic of what I’d been working on, and I was welcome to come. And I went and she was very welcoming and so I asked her if I could work with her in that year, and she said, “Yes.” And that year I worked with her, and it was a tremendous opportunity. I am enormously grateful—to this day—for her giving me that opportunity, which she had no obligation to do. She had many of her own students and lots of things to do. And I worked with her, and we got involved in a research project that also involved a professor from Brown University, Robert Brandenberger. And towards the end of that year when I decided to apply to graduate schools, I had written a paper with them, and I applied. So by the time I went, I went to Brown and I worked with Robert.
So that was your entrée to Brown. It was that specific connection.
Yeah. That’s right. And by that time, I fully knew that I wanted to work in particle cosmology. I had already—in some sense—dipped my feet in the water of that area, but the transition from being a mathematics undergraduate to doing that—you know, I really loved general relativity, I still do to this day, and I really loved quantum theory. And I was loathe to give either of those up and there are really only a couple of directions you can go if you don’t want to give them up. One is to work in say quantum gravity or something like string theory. I was interested in that, but ultimately didn’t go that direction. Or to work in something like the early universe where you’re kind of forced to use them both, and that’s what I did.
Just to step back a little bit, in terms of where particle cosmology was in the early 1990s, what were some of the big questions in the field at that point and where did you see yourself situating your own research to address those larger questions?
Well, it’s funny, I took a cosmology class in my master’s and in that class-- So, that was towards the middle of 1991 and the final page of my notes from that entire class I remember very distinctly. We had just taken the level of density difference in the universe—galaxies versus where there’s no galaxies, if you like—and figured out if they grew from gravitational attraction, how big would the fluctuations in mass from one place to another have to be at early times in the universe and what effect would that have on the microwave background radiation? And literally the final few lines of those notes says the temperature of the microwave background should fluctuate from place to place at a level of one part in 10 to the five or six, and if we don’t see those fluctuations in the coming years, it will be a massive overturning of everything we know about cosmology. And within a year, the COBE Satellite measured those fluctuations explicitly.
And were you paying close attention to COBE? Was this really interesting and relevant to you?
Yeah, absolutely. That was the year I was doing research and it was fascinating. It was amazing. But I wasn’t working on that, I should say. But that was the time I think where people really started to focus on the notion that these things like the microwave background, which had been measured for 20 years or so, but only measured sort of gross level, we thought we might really be able to pick up features that would tease out interesting facts about fundamental physics. And so I was focused on that question. And at that point in your career, you’re not thinking about the grand scheme of things. You’re trying to work on the project that you’re working on and at the time, I was working on an idea called topological defects and these are a kind of object that can be produced by particle physics as the universe cools. And in a sense if these things exist in the universe, then they’re a sort of leftover fossil of the early universe. They’re a leftover fossil of the highest energies we could ever have access to.
So, I was interested in whether you could use things like the microwave background or other measurements as a tool to observe and gain insight into high energy physics through those at the time. And I think although I no longer really work on that exact area, for the most part, but that general philosophy is something I think that’s just stayed with me throughout my career.
How did you go about developing your dissertation topic?
So, there are many different styles that advisors have. I mean, I know people who go to graduate school—and certainly this was the picture I had in the UK—you meet your advisor on the first day and they say, “Here’s an interesting question that I think you should work on.” And then you work on it for three years and you produce a thesis. That was not my experience at all in graduate school, and it’s not the experience I give my own graduate students nowadays. So, I had written a paper already with my advisor by the time I got to graduate school, and he had a whole bunch of ideas that I could work on and I thought about them a little bit and I started to read about some of them and some of them I wasn’t interested in and some I worked on, but he had other students and other postdocs and we started little projects on our own, so I wrote papers in a range of areas.
And then along the way, he had been working, again, with this person—Anne Davis at Cambridge—on the notion of the matter-antimatter asymmetry of the universe, and I had been thinking about these cosmic strings and I came up with an idea of putting together two different aspects of this work, and see if you can have a completely new way of constructing the matter-antimatter asymmetry of the universe. And we worked together on that. And that-- My thesis has a very broad title that I actually can’t remember right now, but my thesis contains a lot about the microphysics of cosmic strings and topological defects and their effect on the baryon asymmetry of the universe and it also contains a little bit about nonsingular two-dimensional gravity as well. So, it’s a little bit of a hodge-podge. And a lot of my students end up with theses that have a similar flavor to them, I think.
Who was on your committee?
I don’t remember. [laugh] I mean, obviously my advisor Robert Brandenberger. I would have thought that—I should look this up, it must exist somewhere—but there was a relatively small, terrific theory group at Brown when I was there and I know who was there, I have it in my head, and I can imagine who I asked. So, I’m almost certain that Gerry Guralnik, who’s one of the people who’s famous for inventing and understanding the Higgs mechanism, was on it. And I would almost certainly imagine that Antal Jevicki was on it. I don’t remember who else. It’s sort of embarrassing, actually.
Well clearly, that means it must not have been too painful of an experience, which you definitely would have remembered.
That’s true. I mean I think for the most part, defenses, you’re terrified to do them, but if your advisor knows what they’re doing, you should be going forward to your defense with--
What was Robert’s style? Was he a pretty hands-on mentor?
He’s a wonderful advisor. He really led by example. So, he worked a ridiculous amount, so he was in his office all the time working. Early, late. He was always available when I wanted him. So, I never had trouble talking to my advisor, which is something you’ll hear people complain about occasionally. “I can’t find my advisor.” I never had that problem. I think I was a very motivated graduate student, so I didn’t need him to be pushing me, and he didn’t. I also think he would have been loath to push very hard on students. I think he strongly wanted you to find your own way. So, he was gentle and incredibly supportive and mostly led by example. I have zero complaints about it. He was an excellent advisor.
Now I’m curious, when you left Cambridge to go to Brown—and again, staying on the theme of not particularly having a grand plan—were you open-minded about where you wanted to sort of make a career? Did you think you’d be going back to the UK or were you specifically intent on making a life for yourself in the states?
So, I really had no plan. And also, at that point in your career and your life, you don’t necessarily know all the options or understand the forces that will come to bear on you and how a career is developed. I was around for three years and midway through my third year, I applied for postdoctoral positions, and I applied in the UK, I applied in the US, and I applied to a variety of other places like CERN. I think I applied to some in Germany, a couple of other places. A lot of places, I mean, probably 40 places or so.
After I sent out those applications, I met the person who is now my wife and who was also a PhD student in history at the time at Brown. And then as the process went on, I got an offer of a PPARC Fellowship in the UK, which is a fellowship that I had agreed that I would take to Cambridge to work with Anne Davis. So I was going to go back to Cambridge and I—in fact—accepted that position. And then Anne was on sabbatical at CERN and I was spending two weeks at CERN visiting her and I got an email from Alan Guth at MIT saying, “We have this postdoc position. We made a short list. It’s several people, you’re one of them. Would you be interested in coming to interview?”
And at the time, I knew that once you’d accepted a job, you should never say no. You should never not go. But Anne told me, “Look, you have personal reasons for wanting to stay in the US maybe and this is a fellowship so it’s not such a big deal, there’ll be someone else on that fellowship list. It will be great to have you at Cambridge but it’s okay if you want to go interview.” Which is a wonderful thing to do for someone.
I spoke at Anne’s retirement workshop last year and it was one of the very first things I mentioned about her is having been kind to me in this way. Anyway, I went to interview and I got offered the job at MIT, and I think MIT’s a wonderful place and I was very happy in the US at that point and I had just met the person who was going to become my wife. And so, I stayed and I went to MIT. So, it was much more haphazard than having some plan for where I wanted to be.
How well in terms of what was going on at MIT, how well did that fit into the questions that you were asking and what you wanted to work on next?
Yeah. That’s a complicated question. MIT is and was a wonderful place.
It’s huge, also. You’re entering a much larger environment.
That’s right. So it’s immense. So I was A, delighted to go to MIT because it’s MIT. It’s great. I was delighted to go to MIT because I could actually live with my fiancé in Providence and commute to Boston to do that, which was terrific. But I was also extremely excited to go to MIT because Alan Guth was there, the person who had originally emailed me. And Alan was one of the people who co-invented one of the dominant ideas of particle cosmology ever, the idea of cosmic inflation, and was kind of a hero. So, this was an amazing opportunity to go work with him. So in that sense, it fit extremely well. There was a lot of activity there, Lisa Randall was there as an untenured assistant professor, who was also terrific, and several other people.
In practice-- I have no regrets, I had a great time at MIT and these people were all wonderful and I see them today and think very highly of them—but in practice I didn’t actually work with any of these people to the extent that we wrote a paper together. I tried to work with Alan, but he was very busy and he was working on his book at the time. I tried to work with Lisa, but things didn’t quite gel. I tried very hard over a long time to work with Eddie Farhi and various other people and that produced some projects, but none that Eddie was actually on. And so, I primarily ended up working with other postdocs and students and that turned out fine, but it was-- I didn’t end up engaging to the extent that I wished I could have with what was going on at MIT at the time in cosmology.
Now in terms of, again, this grand plan or lack thereof, was this an opportunity for you to refine and improve on your dissertation research or were you looking to sort of expand your research space and take on new projects?
Well, I think it was a two-year postdoc, so for a two-year postdoc, you would be a little crazy I think to throw away your thesis topic because it provides you with something you can keep writing papers on. So, I did continue that.
So, in that vein, Mark, what were some of the—I don’t know what the term is—unanswered questions or unexplored areas? Obviously, no dissertation can do it all, right? So, I wonder what—in your graduate era research—was ripe for ongoing inquiry?
Well, there were ways to better understand the microphysics of what was going on in this model we had proposed, so we did some of that when I was a postdoc with my previous collaborators and another person, and then also there were ideas associated with what’s called superconducting cosmic strings and the way in which they might provide unique observable signatures in the universe. And so those directions I took and I worked with, but what I was about to say in answer to your last question was that that aside, this is one of the seemingly rare cases where I did have a plan, which is that I really did want to move into new directions, and I hoped that by working with Alan and Lisa and other people, I would move towards inflation and supersymmetry and things like that and start to expand what I was working on. I got out of grad school extremely quickly, which is great. I was able to move on in my career and I clearly did quite well quite quickly.
The downside of that is I probably could have been a little more intellectually mature by the time I got to my postdoc to enable me to take better advantage of those opportunities. And so even though I wanted to move into those areas, at the end of the day, I only did to some degree. So, I wrote more papers on the topics of my thesis and I wrote papers that were nothing to do with the topic of my thesis, but that had some intellectual connection to the topic of my thesis, if you know what I mean. And that was fine, as I said. And I moved on from MIT after two years, which is what would have been the plan.
What did you want to do after that? What was feasible after a two-year postdoc at MIT? Assistant tenure track jobs or were you looking specifically for another postdoc?
Well, I think if you’re either lucky or have achieved something truly remarkable or both, you might have had a shot at a faculty job at that point, but it would be very rare.
And what was the market like at that point?
It was bad. I mean, everyone will always tell you—unless that person is 75 years old right now, anyone you speak to will tell you that the market was terrible when they applied. It was terrible when I applied.
Yes. The binary, of course, is not good or bad. It’s how bad was it?
Yes. I actually thought it was really quite bad, but also, I think it was a time when people were somewhat interested in hiring string theorists for stringy jobs. And this thing that I told you about COBE coming along with the microwave background results in ‘92, so now we’re three years later, it’s ‘95 or so and really mining the microwave background had become a huge deal. And so, around the time that I was applying both for postdocs and faculty jobs, there were a lot of people getting jobs who were experts on mining the microwave background, doing things with that data; people who were interested in pure string theory, and there were fewer jobs, I felt, for people of the interface between those two areas. And I think that remained true through to beyond me getting a faculty job in 2000.
So, I didn’t really think I had a shot at getting a faculty job after that first postdoc and I applied for second postdocs, and I was lucky enough to get one at Case Western Reserve University in Cleveland, which had a very strong particle astrophysics group.
Now, just because we’re talking about the 1990s and string theory and the job market, I wonder if you were on the sidelines, or you were sort of in the middle of that intramural debate that was going on there where you had people like Shelly Glashow and his campaign to keep faculties string-free and things like that. What was your perspective on all those things?
I mean, I was on the sidelines, for the record, since I wasn’t working directly in that area. I mean, it was interesting. It’s a debate that we’re 25 years further on now and it’s a debate that still goes on. People have very strong opinions about string theory. I never really have shared these strong feelings. I think string theory’s fascinating, it’s done amazing things. It has a real shot of being the right answer. We absolutely don’t know that it’s the right answer, and so the answer to me seems do more work, and people are doing more work and string theorists are doing wonderful things and people who think string theory’s not the right answer are doing interesting things and challenging it and I think that’s the sign of a healthy field. So I sort of stood by and watched this storm in a teacup develop about string theory, but it has continued to be, essentially, that level of concern, I think.
The operative word there—as I’ve heard so many people share their feelings, strong held feelings about string theory—is really one of patience, right? That as you say, there are people who advocate, “There’s some really promising stuff here, more work needs to be done,” and other people who say, “How long as the field been around? What has it actually demonstrated? How is it actually applicable to physical reality? I’m sort of done waiting.” And for you, it sounds like you’re still patient, you still want to see interesting work being done because it certainly can lead to some pretty fundamental things.
Yeah, but I also take a somewhat organic view of the field and I think people have voted with their feet. When things cease to be interesting, people jump ship. And so, people keep working on it because it’s interesting. That’s not true of everyone, some people get locked into an area they’re working in and they can’t do something else, but you see a lot of people interested in string theory because it has remained interesting. It’s true that progress can be slow and it’s true that there are peaks and valleys in progress and it’s true that it may not be the right answer and it could be wrong. But I think if you look over the time that—so there were fantastic new things learned about string theory before I ever was in the field. That fueled a lot of new research in string theory.
There was something of a valley I think of a lack of results for a long time and then in the late 90s—partly due to the work of people like Joe Polchinski and partly due to the work of people like Juan Maldacena—a resurgence happened. The AdS/CFT correspondence was discovered. The D-branes were discovered and then that was a whole flurry of work in the field, including understanding things like black hole entropy and microstates that any theory of quantum gravity is going to need to explain and string theory may or may not be the right theory of quantum gravity, but it passed that test.
And nowadays I think we again saw a lull, and now we’re seeing some interesting new directions that have come out of that field like emergent space time and things like that. So, I just don’t tend to get het up about these things. I think you have to hire a diverse set of people. You have to allow people to work in the areas they think are most interesting and you just have to hope that that will take care of things. I don’t know another way to do it.
Yeah. A lot of people who are very much in defense of string theory—or are card-carrying members, as it were—will say a lot of the criticisms are sort of rooted in not understanding some of the mathematical sensibilities that underlie string theory. I’m curious if—with your very strong background—you feel comfortable understanding the work on its own terms?
Well, I’m not going to claim to understand the nitty gritty of all the mathematics that underlies a lot of the work in string theory, so I guess I won’t lay claim to some special point of view on that basis. Undoubtedly, in any field—and I don’t think string theory is special in that sense—there are people who are critics of that field who are critics partly because they don’t really understand that field. And that can be said of cosmology, it can be said of anything, really. I think there are plenty of critics of string theory who I think know enough about string theory that their problem is not that they don’t understand things, but they have a particular point of view, and I don’t necessarily agree with that point of view, but I don’t think you can just sweep them away by saying, “Oh, they don’t understand.”
Now to get to Case Western, was this the first time that you were teaching or were you teaching at all during graduate school?
I mean, I TA-ed during graduate school, but I would be a liar if I didn’t just say that everyone did everything they could not to teach as much as possible, but funding was somewhat scarce, so I did some teaching and I enjoyed it fine and I think I did an OK job of it in grad school, but it was all fairly low level things, TA—grading mostly, things like that. So, I can’t really claim to have done serious teaching there. Before that, the year that I was at Cambridge, when I did research, I funded myself by conducting supervisions for undergraduates. That was really more serious teaching than any of the things I did in graduate school.
But then when I went to Case Western, I had a three-year postdoc at Case Western and at the end-- The first two years, I did no teaching at all. And then the second year, I was asked if I would be interested in taking up a research assistant-- Sorry, that’s not true. A visiting assistant professor position, which would have me teaching a class and which would give me a little extra money and would also put some real teaching on my curriculum vitae for faculty jobs. And I wasn’t naive enough to think that a serious faculty job would care overly about me having had classroom experience, but my research was going well and I was writing a lot of papers. I had taught in the summer once, and I figured it wouldn’t be a huge amount of extra work and it would provide me with just a little more direct flexibility. And so, I did that, and it also extended my position to four years, which at the end of the day, I didn’t take because I left. But it had that advantage of that job security also.
But this was not a tenure-line job.
Absolutely not. And it was not intended to be and there was no promise of it becoming one.
And in terms of your research, what were you working on at this point?
So, I was working on a lot of different things. So, I was at Case Western for three years and in that time, I wrote—if you include conference proceedings—something like 22 papers. And I put that down to some small part myself becoming more academically mature and ready to instigate projects on my own, but in large part to an extremely nurturing environment there. You said that MIT was big—and it’s not a criticism, it’s just true—and so there are many postdocs and you’re not necessarily going to stand out just by virtue of being there. At Case Western, postdocs were like gold dust, and they were delighted to have me and it was clear that they really wanted to work with me.
And so I was hired by three people, essentially, in the theory group. Tanmay Vachaspati, Glenn Starkman, and Lawrence Krauss. And I ended up working with all three of them. And certainly, Tanmay and Glenn in particular were around every single day and we would have coffee twice a day and we would spitball ideas. And it was just a constant opportunity to have new ideas and to flesh them out. Also, I worked on some of my old ideas. I went to a conference the first month I was at Case Western and Sean Carroll who’s someone I’d been-- We had overlapped for a year as postdocs at MIT—was there and he and I came up with an idea at the conference and wrote it into a paper in three weeks when I came back.
I worked on extra dimensions with Glenn Starkman. I worked on the origins of inflation with Tanmay Vachaspati, I worked with Anne Davis—my former collaborator—on supersymmetric cosmic strings. I worked on a very broad area of things, and I loved it, I have to say. It was a terrifically rich intellectual time.
You must have been pulling many long days to be involved in all of these projects.
You know, I worked very hard, but I’ve always, I think, been pretty disciplined also. I mean I like having a life outside of physics and I continued to do that there, but yeah, I worked very hard. But I’ve never been the person who gets into the department at noon, works for half an hour, checks his email and has lunch. I mean, I always wake up early, I start work early, but I allow myself a decent chunk of the evening to live a life also.
Now because there was this four-year package opportunity for you, were you actively on the market at this point? Did Syracuse sort of recruit you out of the blue? Or you were actively applying for positions?
No, they certainly didn’t recruit me. I was actively applying for positions and the job market was tough and it was very hard to get jobs and I was absolutely uncertain that I would ever get a job, that’s for sure. So, I applied for jobs after my first year at Case Western and got no interviews at all, and I applied after my second year at Case Western and I got two interviews—one at Dartmouth and one at Syracuse, and Syracuse ended up offering me the job.
Now, in terms of the market and trends in the field because openings are always a very good indicator of the direction that physics and faculty want to take things, right? I’m curious what were the jobs that were—and they were few and far between, right, so it’s a limited dataset—but I’m curious what were the jobs that you thought you would be competitive for? What kinds of niches would you be filling in these faculty offerings?
So, occasionally people put out ads that were very specific. So, we’re looking for someone in string theory or we’re looking for someone to work with cosmological datasets. It was very clear to me there’s no way I would get those jobs. Most of the jobs I applied for were jobs either put up for someone working in particle physics or someone working in-- or a theoretical cosmologist. And I felt like, at some level, I fit both those descriptions, but I was also acutely aware of the fact that—depending on whose eye was on it—I very well might not at all fit those descriptions, depending on what you conceived of as being a particle physicist or a theoretical cosmologist.
So my hope-- I would apply to all those jobs and the way in which I wrote my applications was, you know, I tried very hard to lay out my chops if you like as an honest card-carrying particle theorist and as someone who knows about cosmology, but I also tried to point out that the questions that are coming out of data, like the microwave background and, by the time I was applying for jobs, the accelerating universe, are things that would only find answers in fundamental physics. And the people who answer those questions are going to need to be people who have great facility with cosmology and to really understand the particle theory. So, you need people like me. That was the story of myself that I told committees that were looking for faculty jobs.
And my hope was that the right eye would find that. I mean, so I was hoping to be hired either by a department that really wanted someone like that or a department that didn’t know they wanted someone like that until they saw my glowing application, which probably didn’t happen. Or to be hired by a department that really had a very broad conception of itself and was looking for a really good person and thought that I might be a good person and thought, “Well, he’s working on something interesting as well.” And that was the plan.
What were your impressions of Syracuse when you got there?
Well, I interviewed there in the winter. [Laughter]
Although it’s not like you were coming from Caltech. You knew the northeast, although Syracuse is its own animal with the lake effect snow.
I live in Philadelphia now, which is in the northeast essentially. Philadelphia gets 18 inches of snow a year. The first winter I spent in Syracuse, they came close to an eighth of an inch from the record where they got 191 inches of snow. So, it’s very different. Look, it was cold and snowy, but-- So my wife at that point had decided—she had just finished her PhD, didn’t want to go into academia, and wanted to be a journalist at the time. And we knew that we wanted to go to-- We were hoping that I would get a job in a place where it was big enough that it would have a serious newspaper, and Syracuse did.
So quite honestly, I didn’t spend an enormous amount of time worrying about what Syracuse was like. I wanted it to have a good university and a good department, and it did. And I wanted it to be a big enough place that we would be happy together there, and the rest of the things you might worry about about a place, I think, honestly at that point-- Most academics-- I did not want to go to a terrible department, and I did not want to go to a place where I could not imagine living. With that aside, I think you definitely were just hoping someone would hire you at that point, and so I didn’t give that much thought to it, to be honest.
And in terms of again, this idea of filling a niche in the department, what role did you take on relative to what other faculty members at Syracuse were doing that were sort of at the periphery of what you were doing?
Yeah. So, I mean, I think the people who were primarily responsible for hiring me at Syracuse was, amazingly, someone from the gravity group, Don Marolf, who is not there anymore, and someone from the particles group, Mark Bowick, who’s not there anymore. And I had overlap with what both those people did, but I don’t think they hired me with the notion that I really need someone to work with a specific person. I think they thought it was a good area of research. They were aware that the department, I think, was quite siloed into a gravity group and a particle group and that the future probably didn’t look like that for funding and intellectual engagement in fundamental physics, and I think they were looking for someone who could bridge those areas and it was a department where they were not getting hire after hire after hire, so I think they wanted to make the most of this hire in the sense of covering a somewhat broad intellectual ground.
And so, I’d never had sat down and had lengthy discussions with them, but I strongly felt when I interviewed with Syracuse that I had good connections to the intellectual content of what was going on there. And I also felt a very good personal fit with the people there and certainly when I came to Syracuse, it felt that way.
And this was a tenure line job, finally.
Yes. That’s right.
What kind of classes did you teach at Syracuse, and did you take on graduate students there as well?
Yeah. So, I was put into the second semester astronomy class, very large class. 4 to 500 people and it was team taught. So, I taught it with someone else. And it was a huge amount of work. So the first year, I had a semester off because I had just arrived. Second semester I taught it with Mark Bowick, this class. And we taught it together. So, he taught one section, I taught the other section, 200 and some people each. It fulfilled the science requirement, so not everybody had a burning desire to become an astronomer, let’s say. And it had athletes in there and it was challenging. But I had taught large classes before and I kind of enjoyed it, but it was a lot of work.
And then the second and third years I was there-- And I think the fourth year, although I’m trying to remember now—yeah, that’s what happened. The second and third years, I taught that class entirely myself, both sections, and then got a semester off teaching in reward for double teaching. And the fourth year I was there, I again team-taught a semester of astronomy. And then after that, I taught a small-- My former colleague, Don Marolf, had created a small general relatively class for undergraduates, which is a very challenging thing to do. Not upper-level undergraduates, sort of mid-level undergraduates. So, you can’t teach any of the mathematics that underlies general relativity. It’s all concepts and things like that. I took that class and I really loved teaching that to very smart undergrads. And I taught mathematical methods to graduate students.
Syracuse for undergraduates must have had a pretty good range. In other words, the top students at Syracuse probably would have been competitive anywhere, but on the other end, it was probably more of a regional school. So, there must have been some challenges in terms of teaching a fairly wide range of students regarding their interests and aptitude.
Yeah. I think you described it essentially the way I would. The most talented end was remarkable, but there was a long tail—both in natural ability and interest—and you had to deal with that. I was lucky to have had some teaching experience. The things I did at Case Western stood me in good stead, but also, I had good mentors. I team taught with Mark Bowick, who was very helpful and helped me do that and I think I’m a decent teacher and I’m enthusiastic and energetic and I was willing to engage with the students, and I think it went fine. It wasn’t without challenges, but I think being willing to just work with the students while maintaining standards goes a long way.
Mark, I wonder if now, that you’re sort of properly situated within a faculty, and you’re teaching and you have graduate students, in what ways or not did this affect your own research agenda? The kinds of projects that you wanted to take on, your ability to be involved in long range collaborations. I’m curious what impact—if at all—this might have had on those things.
So yeah. I’m happy to tell you-- If you don’t mind, let me just answer the last thing you asked me which was about my grad students that I took on at Syracuse because I would feel terrible if I didn’t mention those.
So, I was very lucky. I took grad students from the beginning at Syracuse, and I worked with my own graduate students and I worked with graduate students of other people. And I just was incredibly lucky. I had some terrific graduate students, and in particular, I had two Italian students early on—Antonio De Felice and Alessandra Silvestri—both of whom-- Antonio’s a professor of the Yukawa Institute in Japan in Kyoto and Alessandra is a tenured professor at the University of Leiden in the Netherlands. And they’re fantastic people. And I was very lucky to be able to work with them really early in my career and so that was a very important part of being at Syracuse. You know, in terms of how graduate students drive and affect your research agenda, it’s an interesting question. I think it’s very different for different people. For me, I think in the early days I mostly drove the research agenda of my graduate students. And that’s not a comment about them, it just is the way those collaborations happened to go. As I got--
Meaning that you would have a particular problem that you’re interested in, and you saw that for whatever reason, this would be something good for a graduate student to work on? That kind of relationship?
Yeah. That’s right. Not to say they didn’t work on other side projects, but I drove that more than anything else, I would say. But it’s varied over the years, and I think nowadays I’m really happy when it works that I start driving the initial ideas and then a couple of years in, they really start driving the ideas. And I’ve had some really outstanding graduate students in recent years who have fallen into that category. My student Garrett Goon is now a postdoc at Carnegie Mellon and my just graduated student, Mariana Carrillo Gonzalez, really drove me into areas that I was not intending to work in.
So, they’d go away to a conference or a workshop or a summer school and come back and say, “I heard about this thing, I’m kind of interested, would you be interested in talking about it?” And then we find some area where my expertise can say something about that area, and they can learn more about it and all of a sudden they’re driving the ideas and they’re driving the projects and that’s an exhilarating thing to happen and you feel very good when they can take that sort of skill forward with them to their postdoc or whatever they do next. So yeah, definitely grad students—when it works well, which I’ve been very lucky in that regard—[are] not only are useful to your work, but help drive you in directions you wouldn’t go.
Now, you were at Syracuse for long enough where I assume you think of it in terms of segments, right? So, if you can talk a little bit about how your research might have changed over the time you were there, in terms of your collaborations, in terms of the new questions you might have been interested in. Also, all of the exciting developments in the world of experimentation that you were following. I wonder if you could sort of reflect on that mini narrative of your career.
Well I think when I first got to Syracuse, two very big things that had happened in the last sort of three, four years before that were in the particle physics side. People had discovered and started to work extensively on this idea of large extra dimensions—a notion that there could be dimensions other than the three spatial and one time that we know of, which you don’t have to wrap up very small to hide their experimental effects from us here, and yet which can have profound effects on certain scales in the universe and in sort of understanding the symmetries that guide our four dimensional world. And people discovered that they could be much bigger than you thought in a variety of ways.
Some of these ideas came directly out of string theory and were sort of co-opted more broadly and others were separate. And then in cosmology, it was only a few years before I came to Syracuse—maybe two to three years—that the accelerating universe had been discovered. And so, I had begun working on both those things at the very end of my postdoc at Case Western, and I spent certainly my pretenure years working on a variety-- I’m always working on many different things. It seems unavoidable, but those two things were things that I think were the biggest intellectual drivers for me in that period. And then post-tenure, sort of 2004 onwards, I think I worked less on the extra dimensions. I felt there was less new intellectual depth to be found there, but I still worked a lot—and to some degree I still do today—on the sort of theoretical ideas that underlie the accelerating universe.
So, we were fortunate enough to have a very clever idea that got a lot of traction, and then that fed into other ideas and other collaborations. And so, I worked with my postdocs, I worked with my students there at Syracuse. I worked with some faculty collaborators at Syracuse, but I also mentioned I had written a paper with Sean Carroll back in 1997. We had written a couple of small papers since then, but after I came to Syracuse, we wrote a string of papers in this area that sort of took a number of years to write, and I think all of which were part of one thread to do with the accelerating universe. And that was probably the thing that followed through to about the point where I left Syracuse.
I’m just curious as a fun little side note, I wonder if in your early collaborations with Sean Carroll if you saw that this was a person who would become one of those sort of physics public intellectuals on PBS hour-long specials and things like that, if he was already sort of headed in that direction.
Yeah, I think Sean was always that person. In fact, we wrote a popular blog together for many years with some other people where he was certainly one of the primary drivers of everything that went on there.
Mark, I want to ask a sort of general question that you can fast forward and go back even within the narrative of your time at Syracuse, and that is sort of you said that you’re disciplined in terms of having a life out of physics and you keep coming back to, how many projects are you sort of juggling at the same time, right? Ten, twelve, twenty, whatever it is. So, my question there is not so much how you fit the hours into the day, but it’s more of a work style in the sense that on any given day, are you putting in a little bit of time on each of those projects, or is your work style such that you tend to block off chunks of time that you exclusively devote to any one or two projects?
Yeah. It’s complicated. I say I’m working on a lot of things, I mean, I think the most number of things I’ve ever been working on—there’s a corner of my board at work, back in the days when I could go to work—and I would keep a list of things that were even from very early stages of discussion through to almost a paper. And there’s never more than ten things on that board, and of those ten, the things that I would really count as things that are things I’m actively involved with are never more than, say, six.
This is still a lot, just for the record. It’s a lot.
It’s a lot, but not everyone is like that or comfortable with it. Sometimes it means the projects take longer, right? But the truth is I have a couple of grad students and I’m working with a bunch of postdocs and a bunch of other collaborators and sometimes they’re working together on a project, but sometimes we’re working on different projects together. So sometimes what’ll happen is you will say, “OK, on this project, this calculation needs to be done. You should do this.” And then I’ll check it later or vice versa. And when it comes to writing, we hand the paper back and forth. And so usually, at any point in a project, if it’s early, then we’re having Zoom calls where we try hash through the intellectual questions and work on things or we’re at a big blackboard—if we’re at the department—trying to work through things. And those early stages, when it’s exciting, you’re working several times a week you meet just for a couple of hours and just pile ahead with the project. But after that it’s more, “OK, the next thing to do is this. Why don’t you do that, and I’ll take a look.” And at the time I’m working on another project and vice versa.
And so, it’s a bit of both. Sometimes there are intense periods, sometimes it’s back and forth, and sometimes there are projects where I cannot do all the work. I mean, some of the interdisciplinary projects where for example there’s some clever, fundamental physics idea where we’re writing a new model and trying to make sure it’s mathematically sensible, but then we’re also trying to compare let’s say with some simulation or by running it against the data, whether it fits the data the right way. That part of the project I won’t pretend to be an expert on and I’m not going to write Monte Carlos to check things against data.
And so there are parts of projects, large collaborative projects, where you don’t do everything. You have to stand behind the results of everything, you have to understand them, but you don’t do every single thing yourself. And so my projects all lie in somewhere, some part of this space that I’ve described and every day, I certainly don’t work on every project. Every day I try to work on one or two at most.
Mark, given how important collaborating is to you—and some people, their style is that they enjoy primarily working on their own, and it seems like for you you definitely go in the opposite direction—what is sort of the through line on all of your collaborations in terms of the kinds of things that you are consistently contributing among your peers on any given project? Is there a commonality?
You mean do I contribute the same amount to every project or—
No, not in percentages. I’m talking about intellectually or your field, the kinds of—what you consistently bring to the project even though the projects themselves might be incredibly diverse in terms of the questions that are asking, the people that you’re working with, those kinds of things. I wonder if there are any narrative through lines that jump out at you in that regard.
Well, I think the two things that I think I do better than other things that I do—I’m not going to say better than other people—is I’m creative in terms of coming up with models and ideas, and I bring to the table the sort of skills and also real desire to try to bring particle physics rigor to the kinds of models we have in cosmology. So when I’m interested in a cosmological question like the accelerating universe for example, it is very easy to write down many models—and I use the word models very loosely here—that might explain that. It’s much harder to write down a model where you understand whether it’s mathematically self-consistent, whether it makes sense as a quantum theory, whether it’s a theory that the predictions you are extracting from lie within the validity of it treated as an effective field theory, things like that.
So, I think what I bring is the particle physics rigor and some creativity in creating the models. I think there are other things I’m decent at, but those are the things I feel like I find myself doing again and again on the projects that I do.
Mark, at this point I want to ask a more focused question since the accelerating universe is clearly of longstanding interest for you, and that is, you know, one of the harms of when research is recognized at the level of a Nobel Prize is that there’s sort of a public perception that a big discovery was made, you can wrap it up and call it a day because we’ve figured it all out, when in fact the reality is such that that’s just sort of the tip of the iceberg. That this has confirmed the beginning of something and now, you know, even more involved research can begin. So, I wonder if you can address that broadly in terms of where the research has been post-Nobel on the accelerating universe.
Yeah. So, you know, the Nobel Prize was given for the observations that established that the universe is accelerating, which was an extremely well-deserved Nobel Prize and really one of the most important things to happen in cosmology, I think, in the last hundred years. Having said that, the question that it throws up, why is the universe accelerating, remains a huge question and there’s two parts to that—there are many parts to it—two big broad questions here. One is what is the range of possibilities in our physical models that could give rise to that? What’s it telling us about fundamental physics?
The other side is how could you possibly hope to distinguish all those possibilities, one from another, and learn more about the universe in the process? So an observation question. And I would say there has been terrific progress on the observational side—which is not what I do, of course. And so, I can say a little bit about that. We’re very lucky these days to have many, many missions, cosmological surveys, they’re using many different techniques—weak lensing, the baryon acoustic oscillations, looking at supernovae, the microwave background, polarization of the microwave background, etcetera, etcetera—currently going on and planned in the future. And the new observations of gravitational waves that all of which, at some level, are going to tell us new things about fundamental physics. And all of which, at some level, have the opportunity to shed light on this question of what is the complicated fundamental physics that might be driving acceleration?
So post Nobel, which is-- We’re 23 years from the discovery of the accelerating universe and nine years after the Nobel, I think, for the accelerating universe. I think we’re in amazing shape. We have incredible observational program and people are still having amazing ideas about how to test these things.
Theoretically, it’s been a much tougher road and I think that’s partly because it’s a very complex question. There’s one thing we know that is very clean, in a sense, that could cause the universe to accelerate and that’s a cosmological constant. The cosmological constant is one of the things we understand the least in physics. It’s a single number that we put into our theories and we understand it the least, and the reason we understand it the least is that it is an object in our theories that is highly sensitive to quantum mechanics and the value that it would need to have to explain the acceleration we see is extremely different to the one which you would expect it to have in any sensible quantum mechanical theory.
And so even on its own, that is a problem that was known to physicists way before we knew the universe was accelerating and remains today, and it’s a fundamental question that we would love to solve. But now that we know that the universe is accelerating, I think it is important to try to understand the space of theories that could cause that, and I think there has been an immense amount of work on this in the last two decades. A lot of it is not very focused, but a lot of it is, and that focused work—I think—has taught us that general relativity is a very robust theory of gravity, that attempts to change it can run into a myriad of problems—theoretical problems, never mind observational problems—and that is driving us down various intellectual paths, which are becoming narrower. Narrower because we understand more about the theories and narrower because observations keep pressuring our theories.
So that is a very healthy situation to be in for a field where there are directions that you can still go in, but they’re getting pressure from theorists and pressure from experimentalists. And so I remain excited about the possibility that the accelerating universe is going to tell us something very fundamental and interesting about fundamental physics, but I also think—like any healthy field—it’s possible that that killer observation is going to come along or that killer theoretical development that will say, “Now we do understand this,” and it’s either this thing here that’s not very interesting, or it’s just not very interesting at all. But that’s what it should be like to work on something that’s cutting edge.
Mark, to go back to your initial interest in working with Alan Guth, I wonder if you could talk a little bit about sort of where inflation ends and expansion or acceleration begins, right? And there, the question—if I could refine it a little bit—is with the accelerating universe, how much are we talking about the early universe, sort of T equals 0 to 400 thousand years after or whatever that number is and when are we talking about the contemporary universe, 13.7 billion years later as it continues to accelerate? If you could sort of help suss out those distinctions.
So in this conversation that we’ve had so far, any time I have referred to the accelerating universe, I’m talking about today. So, the late universe we know is accelerating. We also know that the early universe—I don’t mean very early, I mean hundreds of thousands to billions of years after the big bang—was not accelerating. And then it began accelerating. So, it’s this late-time acceleration I’m talking about. The very early universe, the extremely early universe, there is an idea that is the dominant idea of the early universe, which is called inflation, during which the universe would again have accelerated.
So when I talk about that, I’m going to call it inflation. So, when I’ve talked about the accelerating universe, I mean today, essentially. You know, what do we know about the universe? The things we know about the universe that are incontrovertible are that a minute or so after the big bang, the universe was expanding—not accelerating. It was expanding and slowing down and it did so in such a way that it gave rise to a very specific distribution of the lightest elements in the universe. That’s, in essence, the first direct evidence of cosmology that we know of.
Then the universe continued on its path, and 100,000 years or so-- 300,000 years or so after the big bang, the microwave background ceased to really interact with the rest of the matter in the universe and streamed throughout the universe. And that is also a very—relatively early, but yet—quite late in a sense piece of data about the universe. That happened at a time when the universe was still slowing down. And then we know at late times the universe began accelerating again, what I’ve called the accelerating universe. So that’s the broad path. Again, the earliest thing in that entire story I just told you was about a minute after the big bang, so-called big bang nucleosynthesis.
That whole story—which I think, by the way, is utterly remarkable—that we have a very consistent story of cosmic evolution from about a minute to 14 billion years is ridiculous, and yet true, right? True and was developed in a shorter time than from when my grandfather was born. Nevertheless, there are huge questions that it throws out that require attention, questions like, why did matter clump in the way it did? What laid down the initial distribution of inhomogeneities? Why is the universe flat spatially? Why is it homogenous on scales that never should have been in causal contact if you just naively extrapolate this expansion backwards?
And all those questions—and many other ones—are what pushed people to come up with this notion of cosmic inflation. Cosmic inflation is a period when the universe expanded extremely rapidly, accelerated, and did so in the blink of an eye. I mean we’re not talking about years or minutes here, we’re talking about Planck times. Multiples of the Planck time. So really a very short time. And then at the end of that, the acceleration ceases, the universe has been accelerating and it stops accelerating and it keeps expanding but slowing down, and that’s what you would think of as a big bang.
So, the story people would give you is there’s a hypothetical notion called inflation that happened at early times that gives rise to the expanding universe that’ll later on cool down and give rise to the production of the elements etcetera, etcetera, and it’s these latter parts that we’ve been able to observe.
Now because, sort of sequentially in terms of discovery, inflation precedes acceleration—has our knowledge of the accelerating universe gone back and refined and improved our understanding of the inflating universe?
So, we should probably be precise about what we mean by discovered. So the universe was discovered to be accelerating today by observations. Inflation was a theoretical idea that emerged. It’s a great idea. If you were to ask most cosmologists, most cosmologists would tell you, “It’s our best idea and likely to be true,” whatever likely means in an answer to a question like that. But it has not been discovered in the sense that we know it to be a fact about what happened to the universe. So, they’re very different things in that sense. So, what I would say is that where there has been interplay, I would say, is that in our struggle to come up with theoretical ideas for late-time cosmic acceleration, we have been able to use many of the ideas that people came up with to try to explain inflation as a way to construct models for today’s cosmic acceleration.
We’ve also been able to take some of the very creative ideas we’ve had about late-time acceleration and use them to come up with new ideas about what could be driving inflation, but in terms of direct interplay between the two, I would say there’s less, partly because, again, we don’t know that inflation was correct. It is very powerful. If you wanted to be a booster for inflation, you would say the theory of inflation made predictions that turned out to be true, and I think that’s correct. The notion of inflation predicts that the spectrum of fluctuations from the microwave background should be approximately scale-free with a slight tilt and that’s exactly what you see in the data.
If you wanted to be a cynic, you would say, “Well, that’s very close to it being very, very symmetric at early times and there could be many reasons for that.” There are many ways to look at this, nevertheless I think it’s our most promising idea about early times, and it could be that there’s other things that are responsible for the reason the early universe behaved the way it did, and inflation is an idea that you get by taking a weakly coupled quantum field coupling it weakly to general relativity. One reason it’s very powerful is that we understand that theory very well. It’s all in a regime where we know how to calculate things.
It could be that there are things that go on in the early universe—in general relativity, or in quantum gravity—that generally we don’t know how to calculate today, that in fact are a much more plausible explanation for the earliest times in the universe, we just don’t know how to figure them out. So, inflation’s great and I’ve worked on it many times, but it sort of has a different standing than late time cosmic acceleration.
That’s a very important point. I’m glad that you made those distinctions. Can you imagine an observational or a technological breakthrough that would allow for the confirmation of an inflating universe to be confirmed in the way that the accelerating universe was confirmed? In other words, the shorthand is with the Nobel Prize, they wait until these things are borne out, which is why the inflating universe has not gotten a Nobel Prize yet—can you imagine an experimental or an observational breakthrough where this would go from our very good, best guess to sort of this is really how it happened?
So, it is tricky, I should say. I think there are people who are very diehard believers that inflation cannot be the right answer, and I think if you would ask this question to some of them, they would say no. That there is no observation that can tell you inflation is the right answer because the inflation idea can give you many other answers, why should I trust this one? So that’s a long conversation. Personally, I would say—I mentioned that when the microwave background was discovered and then became a tool, one thing that was discovered was that the spectrum of temperature fluctuations in the microwave background is approximately flat with a slight tilt that inflation predicts. I mentioned that.
So, another thing that the simplest inflationary models predict—but not all—and that’s an important thing to point out, not all. That’s the get-out clause for what I’m about to say. The simplest inflation models predict a similar effect for gravitational waves produced by inflation. So there’s a very particular type of gravitational wave, the so-called B-modes, that it’s hard to produce in other ways in the universe and could be a clean signature of coming from inflation. And I think if you could measure those and measure their spectrum, in the simplest models, there is a tight connection between the spectrum for gravitational waves and the temperature. And so if you measured that—which by the way is a very tall order, I’m not suggesting that this is going to happen tomorrow—you could do that and you found that that so-called consistency condition was satisfied, I think it would become a very hard thing for people to say that inflation was not the right answer. But some people still would.
Mark, let’s get back to planet Earth and back to the narrative trajectory of your career. I’m curious, how did the opportunity at Penn come together for you? You are promoted on a very impressive time scale at Syracuse. Were you looking at some point to leave? Were you recruited at Penn? How did that play out?
I wasn’t looking to leave. I’ve had some opportunities over the years and haven’t taken them.
Also, I want to ask on a personal level, was your wife able to pursue a career in journalism in Syracuse?
So, my wife did pursue a very successful career in journalism in Syracuse, but then a couple of years into that, she decided she wanted to be a firefighter and threw herself whole-heartedly into that, went through the academy, became a firefighter, got promoted to a lieutenant in record time, became the first ever woman fire captain in the department and is now the first ever woman district chief in the department in Syracuse. So—
Very cool. I was not expecting you to say that.
When I tell people these stories, no one ever asks me at a party about what I do after we get to that stage. They just care about that.
They’re like, “Let’s bring her in on the call.”
So, she has her own terrifically successful career in Syracuse, and so of course that is something that factors into these thoughts. As any academic will tell you, if there are two academics in a relationship—that two body problem as they like to call it, can often be insurmountable and we were very lucky to have jobs that we both loved in the same place. And so, I wasn’t looking to leave. One of my now colleagues of Penn had mentioned to me a few times that Penn was looking around for someone in this area and would I be interested in moving? And I had said I wasn’t a couple of times.
And then I was invited to give a talk there, and when I was getting ready to go down for the talk, they contacted me and said, “Oh, we set up a meeting with the chair for you, ‘cause we’re still thinking that maybe you might be interested in moving.” And so, at that point, I still wasn’t interested in moving, but my wife and I were on a drive back from her family in Indiana and we got talking about it and she was the person who said, “You know, this could really work. We could have a place in Philadelphia and keep our place in Syracuse and my schedule is so many days on and so many days off and yours is flexible and it could be really nice.” And so, we thought about it and I decided to at least explore it a little further.
And so I did, and the more I talked to Penn, the more I became excited about the possibility of moving. And then there was a period where I wasn’t sure it would work. They had conducted a search as well as a sort of recruiting and I didn’t know if they-- They had also found someone in their search they liked, and I didn’t know if they would get permission to hire me or to hire that person or to hire both of us and we didn’t really know. And I also started to have second thoughts at that point, a little bit, but ultimately it all kind of worked and they made me a terrific offer and they also hired this other—then younger, now senior—person in the field. And then we jumped and did it.
And I’m a theorist so it was all very well to theorize about how it would work for my wife and me, but we didn’t really know how it would be in practice, but in practice, it worked out just fine.
Now to come back to this initial conversation we had about the Center [of Particle Cosmology], were you thinking—before—to the extent that you were thinking about establishing sort of a long-term career at Syracuse—were you thinking at all about developing some kind of version for what would eventually become the center at Penn? Was that sort of on your radar? Or was that suggested to you as part of a package to attract you to Penn?
Well, I had been thinking about whether it would be possible to have some kind of center at Syracuse for some time. It would have meant starting from a different starting point. Syracuse has a very strong department, but relatively few people in the areas that would be relevant to a center like this, whereas Penn already had a lot of people.
And funding, I’m sure Penn-- There are deeper pockets at Penn than there would be at Syracuse.
There are more resources at Penn, and that certainly makes more possibilities, that’s right. Although Syracuse was very supportive of me and it’s not clear that they would not have been prepared to do something. As part of my discussions with Penn, I put forward this center, that if you wanted to get into this area, you would need to do this, and it would be important. And there was broad support from some of the people who were most interested in getting me to come to Penn. And there was a chair of the department at the time, Tom Lubensky, who is a very cautious person but also really was very good at recognizing opportunities and fighting for them for the department. And Tom helped me push this through and it became part of the package that I negotiated to go to Penn.
Now—as you well know because you so generously connected me to so many of the professors at Penn who have a much longer institutional history of the department than you do—I now, myself, have a very well developed understanding of the ebbs and flows of Penn throughout the decades, right? So I’m curious from your perspective, when you got there, where was Penn in that broader narrative of the areas that it was strong in, the areas where it certainly needed improvement? What were your impressions as you got settled in?
Well, I’m not going to talk about improvement, but I think Penn-- It’s a great department, right? Even now, I’ve been chair of the department for six years now and every year I get to look at what people are doing as part of having to review how the department’s doing that year and it’s just amazing, the department. I mean, we have world-class theoretical particle physics group, we have an unbelievably strong astrophysics group, which has just got stronger over the years. Focused on cosmology, but not only in cosmology. It’s a department which certainly has one of the finest soft matter groups in the country or the world, and hard condensed matter—probably the largest development in all of hard condensed matter, quantum condensed matter in the last 15, 20 years happened and was instigated at Penn by my colleagues Charlie Kane and Gene Mele, the discovery of topological insulators.
And then in particle physics experiment, we have one of the largest university groups working on the Atlas experiment at CERN. So, I felt that I was coming into an amazing department and I hoped that I could bring something that they-- Not so much that I would improve things, but I would fit into a hole that they had, namely this connection between their very strong particle physics and their very strong cosmology. And as I said, they hired me at the same time they hired an assistant professor, Justin Khoury, who is now a full professor, and that was incredibly—I had always been a fan of his, and so I really loved the idea of having a very close colleague right there at the same time, but it was also a big part of being able to have this movement to bring those two areas closer together. So, I don’t know if I want to categorize it as improvement, but we filled the hole and developed a program I think that had been in the background maybe more at Penn. And parallel to that, I think the individual particle physics and cosmology efforts and condensed matter efforts have just been increasing in quality for the decade that I’ve been at Penn. And it’s a terrific department.
Yeah. There’s no doubt. I mean, you summarized it beautifully. It really is remarkable, all of the concurrent, exciting things that are happening in the department right now, it’s a remarkable transformation and again, to go back to this institutional history that goes back decades, these are exciting developments and it’s just wonderful to see how all of these things have played out. Did you take graduate students with you? Did you have any who were sort of midstream from Syracuse?
I did. I had a couple and there were many models for how to address that. And I gave the students a lot of flexibility in that, but the way they chose to do it was to remain Syracuse students, but to move with me to Penn. So, they worked at Penn while getting their degree from Syracuse. And so, I graduated two students that way. And at the same time, I was taking on new students at Penn. Yeah.
Same kind of question as when you got to Syracuse, what opportunities—besides developing the center—what opportunities did you see in terms of pursuing new collaborations, new areas of research when you got settled in the department?
Oh, I mean there were too many. There are still opportunities to collaborate in my department they I’ve yet to take advantage of 12 years in. I mean, I’ve said several times, I have broad interests. So, I was very anxious to get involved in collaborations with my colleague Bhuvnesh Jain, who is a theorist, but who works close to the data. And we’ve done that successfully several times while I’ve been there, particularly testing some of these sorts of complicated models of cosmic acceleration, for example. This new faculty member who was hired along with me, Justin Khoury, we really do work in very similar areas and he’s just terrific and we’ve written quite a few papers together, including one this year and that’s just been a constant joy to have someone who was easy to work with and who was so smart alongside you.
But I hope to work with Vijay Balasubramanian, who works on gravitational aspects of string theory, and I think we still will. And we just hired a new person, three years ago, Jonathan Heckman. I’m still trying to develop projects with him, and I could name three or four other names of people who I have yet to work with, but there’s just so much going on, you have to get around to it and it takes time.
Now on the spectrum of people agreeing to become chair, on the one end, there’s a real sense of, “Oh, OK, I guess I’ve gotten out of it long enough, it’s really my time to do this.” And then the other side is, “There’s a real opportunity that I have here to reinvigorate and set the agenda and prioritize the things that I think are important. So, I’m curious when you were approached to become chair where you sort of saw yourself on that spectrum in terms of service and also opportunity.
Yeah. So, I should say we have a pretty baroque system for choosing our chair, which I absolutely will not bore you with, but I mean, I was not looking to be chair. That’s maybe the most important thing to say. And I had only been at Penn for five years at that point. I came as a full professor, so it’s not that I was a newbie in some sense to that idea, but it still-- I’d only been in the institution five years. So, I was actually more than a little surprised to be one of the people who ended up in the running to do this and was eventually asked to do it. I wasn’t looking to do it. I didn’t really want to do it.
On the other hand, I’ve always taken the point of view that someone good has to be chair. The chair does have to represent the department, you have to fight for the department, you have to understand and believe it and have skin in the game when it comes to the things that the department needs from the administration, and I don’t think you can have someone in that job who is doing it because they’re not doing research or doing it because they’re not doing something else. I think you have to be involved. And I know that I have some of the skills that it takes to be a chair. I’m not suggesting I’m the best chair, but I think I have some of the skills.
So, I was taking the point of view that you should not say that your time is more important than other peoples’ time. So I was not comfortable just saying, “No, I won’t do it,” when I was asked and trying to pass the buck. What I did do is say-- I didn’t say I would do it, I said I would talk to the dean. And I talked to the dean, and I made a case for the things that I thought would help me to be successful and allow me—at a relatively young age—not to screw up my research career while being chair. And I think—roughly speaking—I’ve managed that. So I haven’t been really teaching as chair and I’ve had some research support, which has helped me to deal with the hugely increased workload of being chair.
I did hope that I could-- I did not want to be chair, but I thought if I was going to be chair, I would try to do the best I could for the department and you’d have to ask others how successful that has been, but I think I have led the department through some big things, like an external review, and a lot of hiring, and of course COVID-19. And I’m hopeful that I’ll be able to look back at the end of it when I’m not chair and think that the department did OK while I was chair.
Mark, I want to ask you specifically, we are right now—in the physics community and STEM in general—it’s a moment of reckoning in terms of increasing diversity and inclusivity in the field, right? And so I wonder to what extent does 2014 feel like a very long time ago, in terms of putting these things atop the agenda as chair, in terms of setting the tone and in what ways do you feel like these were things that the department really needed to address and this was an opportunity—long before #ShutdownSTEM, long before Particles for Justice and things like that—where there was opportunity to address these issues even from the beginning?
Yeah. It’s a hard question, actually.
Because Penn has certainly made strides in terms of-- There’s obviously always more work to do, but Penn definitely has made strides over the years to become a more diverse and inclusive faculty.
That’s for sure, but it’s an extreme challenge in physics, I think. And it really is kind of a long game, I think, in physics. And so, I think you’re right, Penn had made strides. Penn certainly cares about this a lot and that’s part of the reason it’s a complicated question, I think even in 2014, this was very much being pushed to the top of the agenda. I think any department chair who became chair in 2014 would tell you this was a big item on things that the administration, for example, were pushing. The department takes it very seriously, I think. We’ve had a number of initiatives while I’ve been chair to try to increase diversity and to also at least be mindful of and work on the environment and tone and atmosphere of the department if you like, the culture of the department.
I’d like to say that I think, on the whole, we’re a welcoming department that doesn’t suffer from severe issues in this way. We suffer from the lack of diversity that all of STEM essentially suffers from, but particularly physics. And we’re doing our best to battle it, I would say. It’s a hard thing to answer, I think. We do take it very seriously. We do take everything else we do very seriously, of course. And so, this is one thing that factors into the many different forces that are at work to shape a department and to shape a university, but certainly it’s one and it’s important and as you say, it’s become very prominent, certainly in the last year.
Right. So, to historicize that issue where it’s clear that strides have been made and my question about to what extent 2014 seems like a long time ago, does the urgency of what’s been happening in the past few months—can that be?—of course it is a long game, but the long game is played out day-to-day, right? So, what opportunities do you see in terms of actualizing the urgency of the moment sort of going forward?
Well, I mean, I think the opportunities are somewhat limited just by the nature of how universities operate, and how the field operates, and how the field works on a pipeline of people coming through. I mean, we are taking this opportunity to foreground efforts that we’ve been working on for a number of years, like engaging with bridge programs and things like that to try to heavily diversify our graduate classes with the hope of using that as a way to increase diversity at the postdoctoral and faculty levels. So that’s something we’re taking advantage of and literally I’m working on this afternoon when we get off this call.
So, I think that’s an opportunity. When it comes to faculty hiring, I think it’s a difficult road. Diversity has many axes, and they’re not all equal. By that I don’t mean of equal importance, I mean they’re not all equally easy to address. We have made great strides in increasing the number of women in our department over the last decade or two, let’s say, but the representation of African American or Black faculty is zero. There was one person in our department, that person left to a terrific job elsewhere. But addressing that problem is extremely difficult, but it is something that we’re working on. So, to address particular opportunities at each level for each axis of diversity is a hard thing to do, but certainly in terms of our graduate program, it’s something we are working on a number of different initiatives.
And that of course is where the long game comes into play where these will be fruits that will be borne out over the next generation.
That’s right. And I also think that Penn—I’m very supportive of the particular issues of access and equality that have been highlighted through 2019, 2020. There are, of course, many other ones and Penn is also heavily involved in trying to make a difference there. I’m a first-generation college student in my family, I look around me at our faculty, and there are many people who look like me to look at, but are not like me. Most people at our faculty are not first-generation college students. And Penn is—just to give an example of another axis in which there’s been work on—is working very hard to try to increase the representation of first-generation college students, both as undergraduates and in the pipeline.
And I think that will have some effects on other aspects of diversity that we’re trying to address as well, but it’s just another example of one of the things where we’re trying to grab this moment and do something with it.
And Mark, just to bring the narrative up to the present, given all of your service and institutional responsibilities, not so much how have you made the time, but what’s the research that you’ve been involved with over the past six years, and given all of the other constraints on your time, I wonder if that has sort of helped to refine not what you’re really interested in, but what you find to be sort of most worthy of the diminishing amount of time that you have to simply just continue to do the science?
Well, I should say I don’t actually think I’ve changed the way-- Let me back up. What I usually tell people when they ask me how much work it is to be chair is I don’t teach, I do far less university service than I used to beyond being a chair, and with those two things in place working about 15 percent harder than I ever worked in my life, I keep my research constant.
OK. That’s great to hear.
So I told the Dean in the conversation we had when I became chair that above all else, if this job affected my research, I quit. Period. And so far it hasn’t, I would say. And so in that sense, I’m not suffering from a lack of time to work on my research, although being chair during the pandemic has been a completely different level of challenge, but that’s only one six month period of the six years. What is challenging when you become chair—or take any administrative position, I think—if you’re going to keep doing research, which I certainly am, is that the nature of the time you have available changes.
So when you’re teaching and doing service, you can block off that time. “Here’s where I prep for my class, here’s where I teach my class, here’s where I have office hours, here’s where I prep for this meeting, here’s where I go to this meeting.” Chair, it is just constant drip, drip, drip, drip, drip, drip. So, I could just spend my entire day dealing with things that come in that are streaming up my inbox behind your face here presently talking to you.
So, the two things that I had to learn were not so much finding time for research. Like I say, research is the most important part of what I do and I’m not going to give it up to be chair, it’s just not reasonable. But the hard thing is organizing the rest of my time so that I don’t screw that up. And so one thing I had to learn is; I am a very organized person, which is for most things in my life, has stood me in wonderful stead. I get things done on time, I organize them, I deal with them quickly when they have to be dealt with. I’m very proud of that about myself.
I had to learn to be sloppier. I had to learn practiced procrastination because on a time scale of a day or so, about 25 percent of the things just go away. Things that appear big problems eventually go away if you wait a day. And so, I try to do that and I try not to deal with things instantaneously and I just try to preserve some time when I’m going to do science. But you also have to get pretty good at jamming science into the gaps between things and everyone has to, so some degree of being chair, that was the biggest challenge.
Well Mark, at this point at the end of our discussion, I want to ask you a few broadly retrospective questions about your career and then a sort of forward-facing question to look to the future. So the first is given the diversity of your research agenda over the years—all of the topics you’ve worked on, all of the people you’ve collaborated with—what stands out, if anything, in terms of the most fundamental contributions you’ve been a part of? And what are those areas where—even if you might not have had time to work on it recently—there are things that you always want to make time for to continue working on because you think that those contributions are on the horizon?
Well, things that you’ve done and trying to point out really is in other people’s minds, I think, how important the things you’ve done are, so I’m a little hesitant to do that, but certainly the thing that I worked on that got the most external recognition is the work that I did with Sean Carroll and Michael Turner, and Vikram Duvvuri on the accelerating universe, where we tried to come up with an idea. I think the model as we articulated in our paper itself doesn’t work, but the general idea became a very important idea that you could modify gravity in a particular way to explain the accelerating universe and became what’s known as the f(R) models. That was important. I think it was important in that people started working on it when we wrote the paper 17 years ago and are still working on it today. And so, I’m proud of that and it had a big effect.
I’ve also worked on papers that took many years to get recognition, and which I’m really proud of. Tanmay Vachaspati and I wrote a paper on the origins of inflation that took years and years and years and years to get any recognition, but I always loved that paper. And so, yeah. I think the thing that I mentioned initially is probably the thing that I became best known for. I mentioned when you asked me about my PhD what I had worked on, and I had said that I got drawn into—back even when I wrote an essay in my master’s—doing these things called topological defects. And those are something that—they were very fashionable, if you want to use that word—for a long time because people thought they could be used for creating structure in the universe. And now that model does not seem to work very well. I never worked on them for that at all, but that’s one reason they were popular.
But another reason they were interesting is that the early universe was a very energetic place and a lot of the goal of what I’m interested in is trying to use observations about today’s universe to tell us something about those early, highly energetic times in a way that may not be accessible from say particle colliders or things like that. And it’s quite a hard thing to do of course because the universe used to be energetic, and is not today. So, it’s historical in that sense. So these objects—these topological defects—they’re still very possible that they could have been produced in any number of particle physics transitions during the universe and we would not yet have discovered them, and if they do exist, they’ll be going around the universe having tiny effects, but they’ll be carrying with them—in some sense—a piece of the early universe locked inside them. They’re an object that—for mathematical reasons—when the universe cools and becomes its low energy state, a small part of them remains in the high energy state. So the cores of cosmic strings, the centers of magnetic monopoles, things like that.
And I have to say, I’ve always had a soft spot for that idea. I’ve always—at some point—you know, one of those things on my board trickling off the end is usually some idea that involves these objects and sometimes it makes it into a paper, sometimes it doesn’t. And I don’t know if I’ll ever write another paper on them again, but that is the kind of thing that I’ve always found fascinating, that this very concrete example involving really only the kind of physics that you can see in a laboratory. It’s the same physics that leads to flux lines in superconductors and defects in liquid crystals, could happen in the universe and could bring a piece of the early universe to today’s universe. I’ve always found that fascinating. So that’s something that I think is always going to be interesting to me, but cosmology’s moving quickly and so is particle physics. And I think I’m more forward-looking than backward-looking, in that sense.
Very broad question regarding the interplay of theory and experimentation, to what extent have advances in technology, computational power, the refinement of instrumentation-- What are some of the areas of those advancements that have been particularly relevant and useful for your own research?
Well, I’m sure you understand that I’m very far away from those advances myself. So, anything I say will be really quite gross, but certainly modern observations of the microwave background—modern survey telescopes would be impossible without technological advances to do with CCDs, the kinds of things that come into-- think of them as a very, very large version of the camera in your iPhone. Those things have come an enormous way, and really, without them, you just would not be able to have survey cosmology at all. So, I’m very aware of those advances, but I have nothing to do with them.
Computationally, it’s amazing. The simulations-- Two things have happened which have been crucial; more computational power, which is what is really enabling us to do large simulations of the universe these days in a way that was really unthinkable when I got my PhD, and then parallel to that has been advances in algorithms. So if you look at one of the most impressive things that’s happened in the last decade, which is the discovery of gravitational waves by the LIGO collaboration, which now will be turned into gravitational wave astronomy and will become one of these tools that are in our toolbox for cosmology—really knowing that what you’re observing with LIGO is say binary coalescences of neutron stars and black holes or pairs of black holes absolutely required a massive increase in computational power to do the simulations.
And brilliant breakthroughs in how you do numerical relativity, how you come up with the right algorithms pushed forward by people like Franz Pretorius at Princeton and others. Those two things together at the right time were the things that allowed us to understand what LIGO was seeing. So those all have massive impacts on my research because the outcome of those experiments that they make possible the interpretation of then feeds into the kinds of models that I’m interested in constructing and then, of course, feeds back into more tests that you can do with those models. So they’re crucial, but I just stand in awe of the people who do that kind of work. It’s brilliant, technical, challenging work that is part of science and part of the science I’m interested in, and yet I know nothing about it to a first approximation.
And I want return very briefly-- We talked a little bit about your graduate students at Syracuse and some who came over to Penn, the graduate students that you’ve had at Penn, I wonder if you can sort of talk collectively about the kinds of things that they’re involved in and how that might represent where the questions that you’ve been involved with over the years, where they’re headed to the extent that this is really very representative of that next generation of physicists coming online?
Yeah. So, they’re doing a broad range of things as well, but to give just a couple of examples, you know, there are people I’ve worked with—So, let me give a couple of concrete examples rather than trying to speak as a body. So, my student Garret Goon, who graduated several years ago was a postdoc at Cambridge and is now at Carnegie Mellon, we’re still working together, but he’s-- The areas we’re working together in are areas where you’re trying to understand how you could look for new particles in the spectrum of the microwave background by using sort of fascinating new mathematical techniques and using symmetries of the early universe. And we work on that together, I work on other things, but Garret is much more expert on this now and he’s really running with that and he’s running with it with other collaborators, with other people, and he’s also running with it in our collaborations. So in that sense, he’s continuing working on things that he began working on when we worked together.
My newly graduated student, Mariana Carrillo Gonzalez, who’s going to Imperial College as a postdoc, she wrote most of her thesis on what’s called the double copy, which is a connection between gravitational theories and particle theories that we worked on together extensively because she brought those ideas to me. So, I will continue to work in those areas, and I’ve become—you know obviously it’s connected to what I do—and I’ve become more expert in those areas, but I expect her to just plow along in that area and make contributions in a way that I will not on my own in that area. So, I think that’s a fascinating area. So that’s in an area that is a much broader area known as amplitudes, which is a realization that’s come through a lot of hard work that some of the things that are intrinsic to the underlying structure of general relativity—let’s say of gravity—can be captured by things that are entirely intrinsic to gauge field theories. Theories of like the strong interactions.
Those are very different theories and the way in which they map into one another is very nontrivial and not something one can talk about over coffee, and yet it turns out that you can sometimes use calculations in one of these areas to cast insight on the other one and it’s being used, in fact, to try to understand some of the backgrounds for experiments like LIGO. So, Mariana’s fascinated by that. She will be driving that field forward, also hopefully still working with me on various things, but driving her field forward on her own and I think that’s one area that there’s a lot of room for growth in the field. And I’ll be keeping my hand in there as well for that reason.
Well Mark, for my last question as I said, let’s look to the future a little bit. By definition, the research that you’re involved with lends itself to massive long-term open-ended questions, right? So, over the course of the remainder of your career, what are the things that are most exciting to you in the sense that there are things that—maybe not tomorrow, maybe not next year—but that are within grasp of achieving that level of theoretical understanding of issues in astrophysics and cosmology that are not yet there yet? And I ask that particularly in the context of you know six years is a long time to be chair. Maybe you’ll be chair for another six years, but at some point—
I will be chair for two more.
You can take that to the bank.
Well, given that you’ve already brilliantly been able to carve out in a way that many other people have not, you’ve been able to carve out your dedicated time to research, it’s exciting to think about maybe that 15 percent, you’ll just be able to put that right back into the research and devote yourself even more than you have in the past six years to these questions. So, personally and as a representative of the field at large for all the things that you won’t be able to work on just ‘cause there’s that much out there, looking to the future—both in the short-term and the long-term—what are those things that excite you most in terms of relevance for real discovery and moving the field forward in fundamental ways?
Yeah. I should say the 15 percent—I’ve been hearing great things about this thing called sleep and I plan on putting that 15 percent into that, but I think it’s a great time to be a physicist. There’s a lot going on. So, look, one very concrete thing that I think-- Everything is a long shot, as you say. This is a hard area, and there are a lot of open questions and some of them are very long-term. It is not unreasonable to think that in the remainder of my career, we would discover definitively what dark matter is. Okay? It is also not unreasonable that we will not. But I think we have ideas, and we have ways of looking for those ideas.
Now, are you self-consciously leaving dark energy off the plate?
I will come to that. I am most certainly mentioning dark matter before I mention dark energy because, to my eye, it is more likely that that is a problem we would solve in the remainder of my career. Dark energy, I think, is much harder. We’ve made 20 years of observations. Nothing has pointed to something beyond a cosmological constant yet with any level that you can rely on, but we do have terrific new experiments coming along, and I think it is likely that before the end of my career, either we will discover what dark energy is, or we will discover that it is not a cosmological constant and we have no idea what it is.
Or that we will decide definitively that we are out of good ideas and it’s probably a cosmological constant. Probably there, I mean, I really don’t like it when people use the word ‘probably’ in this sense. By ‘probably’ I mean exactly the thing I said earlier about string theory and other things. People voting with their feet, right? People will stop working on this problem when there are no more good ideas and no more progress to be made experimentally, and that could come in the remainder of my career. I don’t know. But I think there’s a better chance that we find out what is dark matter than we find out what is dark energy in that period.
If we could just stay on that topic for just a second, one of the amazing things with dark matter is that there’s so many different kinds of physicists who are after the same problem, right? And so that begs the question, you know, from your fields—because you represent many of them—what—to the extent that dark matter is either going to be something that’s understood from many fronts converging or it’s going to be one particular field that has a breakthrough moment in understanding—what might your field or fields, as it were, represent in terms of getting to that understanding of what dark matter is?
So, this is a topic that I’m not working on every day, I should say. But I think most likely when we understand what dark matter is, the evidence will trickle in, right? We will start to get-- And I think everyone would agree that we’d be lucky to get one good, definitive measurement, OK? But if you really wanted an absolute undeniable smoking gun of what dark matter is, then you would hope for—in some sense—the holy grail, which would be you observe dark matter annihilating through telescopes, you see a signature in that dark matter annihilation, you find a new particle in a collider that has the same mass that’s required to do that, and you find nuclear recoils of something like that in terrestrial dark matter detectors that match the broad mass and interaction strength that you measure in these other experiments. And at that stage, I think you would either identify it in a model that we had or all anyone would do would be write down models at that moment.
Do I think that level of coverage is going to happen? No, it’s unlikely. But could part of that happen? Yeah. So, I think one of those things is what you would hope for. And there have been a number of times in the last decade or so when we thought we might have a hint of that, and it hasn’t turned out to be right. These are likelihoods that I don’t know how to evaluate, but I guess what I’m trying to give you is a relative likelihood while saying nothing about the actual likelihood.
But I also think on the theoretical side-- Sorry, let me stick with cosmology for the moment. Let me say I did say that there was this—you asked me what could prove inflation. I do have a lot of hope for measurements of polarization of the microwave background radiation, and it was sort of unthinkable when I got out of grad school that you would measure polarization of the microwave background radiation, and now it’s not unthinkable. I think people think it is unlikely that we will measure, say, the slope of the spectrum of the polarization, but who knows? My experimental colleagues are brilliant, and I am not the person to say what they will or won’t be able to do.
So I would love to see careful, precise measurements of the polarization of the microwave background radiation, and my hope would be that it could be before the end of my career, maybe, that we could either say, “Look, it’s probably single field inflation that caused the universe to be the way it is.” Or maybe something even more fantastic. The spectrum looks nothing like we thought, and it points towards a completely different understanding of the early universe. Either of those things would just be amazing.
Theoretically, I think I would love to see a real understanding of the cosmological constant problem. So I feel that if we understand that, we will likely understand the theoretical origin of cosmic acceleration as well and that could be because it is a cosmological constant in how we understood it, or it could be because the mechanism that makes a cosmological constant so small also provides the acceleration. I would love to see that. I’ve seen progress—tentative progress among those directions—in ways I never thought I would in the last decade. Will it happen before the end of my career? I don’t know. It’s a very large problem.
Well, as you said, Mark, it’s a very exciting time to be a physicist and for everyone’s sake, I want wish you a lot of luck at being at the center of all of these discoveries.
It’s been so fun talking to you today. I really appreciate our time together. So, thank you so much.
Thanks for having me. I’m very grateful to be included in this and I wish you luck with the rest of your interviews.