Joseph Silk

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ORAL HISTORIES
Joseph Silk

Courtesy: Joseph Silk

Interviewed by
David Zierler
Interview date
Location
Video conference
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Interview of Joseph Silk by David Zierler on March 31, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/47089

For multiple citations, "AIP" is the preferred abbreviation for the location.

In this interview, David Zierler, Oral Historian for AIP, interviews Joseph Silk, Homewood Research Professor of Physics at Johns Hopkins, Researcher Emeritus at the Institute of Astrophysics in Paris, and Senior Fellow at the Beecroft Institute for Cosmology and Astro-Particle Physics. Silk recounts his childhood in London as the child of working-class parents, and he describes his early interests in math and his acceptance to Cambridge. He discusses the influence of the fluid dynamicist George Batchelor and the gravitational theorist Denis Sciama, and his decision to pursue graduate work at Manchester before enrolling at Harvard for his PhD research under the direction of David Layzer. Silk describes the revolutionary discovery of the cosmic microwave background and some of the observational advances that were driving the young field of cosmology and galaxy formation. He discusses his postdoctoral appointment with Fred Hoyle back at Cambridge and his next research position working with Lyman Spitzer at Princeton, and with Jerry Ostriker on black holes and pulsars. Silk describes the circumstances leading to his first faculty appointment at Berkeley and the excitement surrounding the high red shift universe, the birth of X-ray astronomy, and he describes Berkeley Laboratory’s gradual emphasis on astrophysics over his 30-year career at UC Berkeley. He discusses his long-term research endeavor to verify the prediction of the Big Bang theory and the incredible results of the COBE project. Silk describes his budding interests in particle astrophysics, which he considers a discipline distinct from astronomy, cosmology and astrophysics, and which grew from cosmic inflation. He describes the import and future prospects of supersymmetry, how his namesake contribution “Silk damping” came about, and he conveys his excitement about moon-based telescopes. Silk draws a distinction between understanding the very beginning of the universe (t = 0) and the tiniest fraction of time after that (t = epsilon) and why an understanding quantum gravity will be necessary to make advances in this field. He discusses the current controversy around the Hubble constant, he describes his decision to transfer from Berkeley to Oxford and how this led to his current slate of affiliations, including his appointment at Johns Hopkins. At the end of the interview, Silk discusses his current interests in the moon telescope project and what the legal ramifications of a permanent moon presence might look like and why, in his popular talks, he finds it important to project a sense of awe about the universe.

Transcript

Zierler:

Okay, this is David Zierler, Oral Historian for the American Institute of Physics. It is March 31, 2021. I'm delighted to be here with Professor Joseph Silk. Joe, it's great to see you. Thank you for joining me.

Silk:

Hello, there.

Zierler:

To start, would you please tell me your current titles and institutional affiliations? And you'll notice that I pluralize everything because I know you have more than one.

Silk:

Yes, it is slightly complicated. So, I am Homewood Research Professor of Physics at Johns Hopkins University. So that's my major position. But it's only part-time, currently just two months per year. I'm also Researcher Emeritus at the Institute of Astrophysics in Paris. That’s where I am most of the time. And finally, I'm a Senior Fellow in Oxford at the Beecroft Institute for Cosmology and Astro-Particle Physics. So, I basically try to spend my time in three locations. The problem now, of course, is that with virtual meetings, I have probably three times more seminars than I used to have. Because in principle, I can virtually be at three places, where before, it was one at a time. So, I'm busy trying to cope.

Zierler:

What exactly is the arrangement between your work in France and your main affiliation at Hopkins? Why not be emeritus at Hopkins and do what you're doing since you're in France sort of full-time?

Silk:

Well, I have an official position, a part-time position at Hopkins. And everywhere else, I'm just emeritus. So that's basically why I arrange to keep my connection with Hopkins and incidentally with the U.S.

Zierler:

Do you currently have graduate students?

Silk:

No. I work with graduate students. I don't actually have any at the moment. My last student finished last year. I expect a new student to begin later this year. I will be a co-supervisor.

Zierler:

Well, you already alluded to it, but a question we're all dealing with right now, how has your science been affected, for better or worse, as a result of the pandemic and remote work? In other words, in what ways has the past year given you more time to spend on some problems that you might not have been able to deal with, and in what ways has the inability to see your colleagues and collaborators in person hampered the work that you might have gotten done over the past year?

Silk:

I think the pandemic has had a negative effect, actually. Because a lot of my interactions are informal. That means at coffees or seminars. And it's just not quite the same when you give or attend seminar now. If you're speaking, you don't see the audience. And when you ask questions, there's no room for any audience participation. It's very much restricted. People, in fact, don't speak very much, I've noticed, in Zoom seminars, apart from the speaker. It's just different. And I find it's a negative effect because there's just less stimulation going around, exchanges of ideas, basically. I think we're missing a lot of that now. And it's even worse at the big meetings, which we still have, but are done on Zoom. The discussions, I think, are very much restricted from what they would otherwise be. So, I'm hopeful things will change before too long. But not this year, probably.

Zierler:

A very broad question that I think will come back as we develop our discussion, and that is, I like asking about terminology because it means different things to different people, even in different countries and in different institutions. And for you and your research agenda, it would be very interesting to hear your take on where you see the boundaries between astronomy, astrophysics, and cosmology. And have these boundaries been more or less fixed over the course of your career?

Silk:

Well, first of all, I would even add a fourth topic, which is particle astrophysics, which I very much have gotten into over the years. And the boundaries are very soft between those areas. Cosmology, certainly, is a blend of all of three, astronomy, astrophysics, and astro particle physics. The other themes are slightly more distinct. But everything, really, is now being driven by the observations. And so, we mostly deal with big projects in astronomy to try to tackle the data, which is incredibly difficult to do now. That because when you look at the universe for evidence of dark matter or whatever, it's increasingly difficult to find anything at all that you can really chew on for a while. So, I would say the connection is that its highly data driven. And the data is sparse. That's my experience.

Zierler:

And that's to say, of course, in the history of physics, there's always the interplay for when theory is driving experimentation and for where experimentation drives theory. And so, your sense, now, is that the observations are really out in front of the theory at this point.

Silk:

Well, listen, if I was a string theorist, I might even not agree with that because I think for someone like me, who's inspired, basically, by the data, there's some puzzle, some paradox out there that strikes your imagination, and you want to explain it. And then, you look in these other related fields. Something that you see in astronomy would have implications, maybe, in the particle world as well. And you try to draw the line between the two. So, that's very much the way I tend to work, actually.

Zierler:

Well, let's take it all the way back to the beginning. Let's start, first, with your parents. Tell me a little bit about them and where they're from.

Silk:

So, I grew up in London. And it was not at all an academic family. My parents were very much working class. And my father was a tailor, actually. My mother, just a house maker. And we lived in a suburb of London. I did well at school, and when I got to my secondary school, high school, I suppose, in American parlance, I was the first pupil ever to win a place in Cambridge. My school had been around for thirty-odd years but had never had such high aspirations for its students. I just had a wonderful mathematics teacher, actually, who really inspired me. I think that was the reason I got enthused and did well in my examinations. But even before that, I would say, the English education system was pretty good in those days. So, coming from a background with absolutely no academic heritage whatsoever, I was determined to break through and do something a bit different. And it was mostly by accident that I just passed my examinations and eventually got into Cambridge.

Zierler:

What were your parents' and grandparents' experiences during the war?

Silk:

So, my parents were second-generation immigrants, I guess you could say. My grandparents fled from, basically, Russian pogroms. And during the war, my father worked on essential war duties. We were evacuated from Central London where we lived, to a nearby town. And it was just as well because just before we evacuated, I'm told, that bombs were falling around us. And when the air raids sounded, my parents had to rush to the nearest tube station and go underground. I was their first child. That lasted until around 1943 or so, I think, when they moved out of London for the rest of the war. And I remember when we came back to London, the bomb shelters in the back garden, that sort of thing. The holes in the streets. It was all quite a different place. Well, I didn't know it before the war, of course. But it was just very different than what it is now. The scars of war were everywhere those days.

Zierler:

Do you know if Silk is an anglicized name?

Silk:

Yes, it is. My father anglicized the name, I think, just before I was born. And so, the original name is, basically, Polish-Russian in origin. Polish-Russian Jewish.

Zierler:

Was your family Jewishly connected at all when you were growing up? Were you members of a synagogue? Were you bar mitzvahed? Things like that?

Silk:

Yes, that's right. I was. They were not very orthodox in their religion. But I think the most that they would do would basically go along for the high holidays once or twice a year to the synagogue. What was fun was that we did, every year, have a Passover service. That was very traditional, very nice. With the special meal and all. And so, that was maybe the main family religious social event that my parents liked to pursue.

Zierler:

Beyond mathematics, was there anything else as a boy that got you interested in science? Like, for example, the launch of Sputnik, the early space race, or even just gazing up at the stars?

Silk:

Actually, living in London, you don't really see the stars. So, I don't have any memories whatsoever in my youth of looking at a starry sky. That's not quite true because I was in the Boy Scouts for quite a while. And we would go on these wonderful camping trips, and then we would look at the stars. That would be quite amazing, sleeping under the stars. So yes, I did get very enthused by that. This was when I was twelve to fifteen. But that never led to any serious intent when I went to university. I was studying mathematics with absolutely no idea of what I would apply it to. But it was just mathematics.

Zierler:

And of course, in the British system, you declare the major right away. There's a commitment from the beginning what your major course of study is going to be. And for you, it was mathematics.

Silk:

Yes, I had this wonderful teacher in high school who really inspired me in mathematics. And so, when it came to presenting my case at university, I chose mathematics because I did well at it. He strongly recommended it. And it was interesting. It was problem-solving. They emphasized that in the examinations. And that's a very practical approach, actually, to mathematics.

Zierler:

If Cambridge was available to you, was Oxford as well? Did you consider attending Oxford?

Silk:

Well, the choice wasn't really made by me. I guess I just followed my teacher's advice. So, Cambridge was the option at that time, and this is probably to some extent still true, its more dominant in physical science than Oxford. Oxford is very famous as being the center of philosophy and many arts subjects. That, of course, has changed a lot since. But Cambridge had this amazing tradition of physics and mathematics from giants like Isaac Newton, for example. So that was, I think, why my teachers at school pushed me to apply to Cambridge. I probably did apply to Oxford. I'm pretty sure I did. But Cambridge responded first. It was quite interesting the way that went because normally, the English system is that from age eleven to fifteen, that's the first phase of your high school education, you take a set of examinations at age fifteen in probably eight or ten subjects, actually. And it's only after that that you specialize. And normally, the way it works in what we call the sixth form is that you take two years to pass your advanced examinations, and then if you want to go to a university, especially an elite university, you typically spend a third year preparing for a special scholarship exam to get in. And that's how Cambridge and Oxford operate. They have special scholarships to admit students. And I was lucky in the following sense, that at that time, Cambridge was going through somewhat of a radicalization. It hadn't quite got to the point of admitting women. That came decades later. But it was widening its catchment area from predominantly public-school boys, that is private school education, to the public sector, grammar schools. And so, I was recruited by one of the very forward-looking colleges, one of the first, in fact, to enlarge its catchment, one of the first to ever admit a colored person, for example. This was Clare College. And so, I took their examination a year early. I was encouraged to do that. I didn't pass it, but they were sufficiently impressed that they offered me a place immediately and said, "Well, you don't need to spend a third year preparing for the scholarship examination. We'll take you right now." And that was good. I didn't get a special scholarship to go there, which probably was good for me, too, because it meant more or less, I didn't stand out as much as I otherwise might've done, which might've been a bit too much, actually, in those days.

Zierler:

Coming in with mathematics, when and how did physics enter the picture for you?

Silk:

Much later, actually. Well, first of all, in mathematics, you have to choose between a pure option and applied option. I chose the applied mathematics at university. And so, that automatically involved applications to physics. I was taking courses in fluid dynamics and quantum theory, as well as topology and complex algebra, that sort of mixture of things. And so, this was actually a very good preparation. In fact, the department that ran the mathematics courses I was taking is the department of applied mathematics and theoretical physics. They combined both in their department. So, the teachers of the courses came from physics as much as mathematics areas. And so, I was lucky. I had some wonderful courses at Cambridge. And it's only a three-year course, actually, to finish there. And that happened. I was not the best of students because it was my first period away from home, so I suspect the social challenges distracted me more than they otherwise might have done. And I didn't end up getting a terribly good degree, actually.

Zierler:

Who were some of the professors at Cambridge that you became close with or exerted a significant development on your intellectual maturity?

Silk:

Probably the primary person was George Batchelor, who was world-renowned for his work in fluid dynamics. So that, of course, had a strong impact on me later. I think much of my early research was based on stuff I learned in his course. John Polkinghorne was another person I remember well. Taught quantum mechanics. Another was Denis Sciama, whose specialty was gravitational theory. In retrospect, he had an immense influence on my later choice of a research direction. So those were, perhaps, three of the highlights from my Cambridge days.

Zierler:

As you were considering graduate school, how well-defined was your identity as a budding physicist in terms of experiment versus theory, in terms of the kind of physics that was most compelling to you?

Silk:

Completely undefined. So, the way it works is that one spends year one at Cambridge taking part one of what is called the mathematical tripos. They called it tripos because long ago, the students used to sit on a three-legged stool when they were questioned by the examiners. And so, you passed part one, which includes pure and applied mathematics, a mixture of things, and you specialize. I specialized in applied mathematics in part two, which takes two more years. And then, midway through your third year, you have to decide what you want to do afterwards and whether you want to continue. And so, I first considered going to work in London. And I remember considering a couple of options. One was in management consulting. Another one was with an actuarial company. And actually, I took short internships in my Easter vacation in each of those areas. I would say both were a disaster. I just didn't get enthused about what I was asked to do, what I learned, or even by the other people or students I was with. For example, the actuarial work was just totally, totally boring. Tables of life expectancy, etc., applying those. The management consultant work was intriguing. They put a group of us up, a big company, in a hotel in Northern England. And we had events with the management over the weekend. And the one interesting thing they did was, they gave us an unlimited liquor bill. We could charge all we wanted, on an unlimited tab. I suppose this was designed to test us, to see if we actually survived the weekend. And I have to admit, I was probably among the dominant set of people who did not do very well in that weekend, for whatever reason. Anyway, time went by, and this was Easter vacation. And then, I took my final exams, and I applied to do a PhD. And entrance to a PhD depends very much on how well you do in the exams. And so, my first choice was to stay at Cambridge, and I ended up getting a second-class exam. So normally, it's first class, second class, third class. Second class is very broad, and you could get into the PhD program if it's a good second-class degree. And it's all totally unofficial. Your supervisor knows how well you did. And so, my supervisor was an astronomer, actually, my mathematics advisor. Donald Lynden Bell later became quite famous in the field. He advised me that, "Well, you're going to have a hard time staying on, getting into the PhD program here at Cambridge. I suggest that you go elsewhere for a year and take the equivalent of a master's degree and see how you do." And that's what I did. I went to Manchester; I was accepted there to spend a year doing a diploma course in astrophysics. I actually went to see the head of physics there when I interviewed for that position. And at that point, I had no idea what I wanted to do. And it was he who pushed me gently in the direction of astrophysics, astronomy. And so, that, I did for a year. And I did well at Manchester, and I never looked back.

Zierler:

And this is very characteristic of a Cambridge undergraduate. The going assumption was, you should remain at Cambridge for graduate work.

Silk:

It was considered the elite place to be. That's right. Oxford was your second choice. That was down the road, and you were considered much of a traitor if you went there, from a Cambridge perspective. And there were the other universities, and in those days, they had a very derogatory name because they were newer. At Oxford and Cambridge, they were called red brick universities. And they were, basically, from the nineteenth century. But now, of course, they're excellent universities. Then, they were, too, as I found when I went to Manchester, for example.

Zierler:

So then, when did the United States open up for you? When did you even think that Harvard would be a possibility?

Silk:

So, at Manchester, I was in my diploma course. I was sitting in the same room as all the PhD students and talking a lot to them and the professors about what to do next. So, the option came up of exploring my options in Manchester. It was a very special place, and they had a tradition in having pioneered the beginnings of radio astronomy in Jodrell Bank, nearby. And they're very strong in things like the interstellar medium, the physics of our Milky Way galaxy, that sort of thing. And another curious thing was, the head professor there was an expert on the moon. That was his big thing. There was nothing quite like cosmology, as I recall. But I began reading around and found an intriguing set of articles by a professor at Harvard, who was a cosmologist, and I just found that somehow, when he talked about the universe, and how he worked on the formation of structure in the universe and the Big Bang, I was fascinated by this. And so, I thought, "Well, that's somewhere I'd like to go for a PhD." But it wasn't so easy from England. You had to apply and get accepted. And so, my big breakthrough was that I had a research project that year working with a radio astronomer in Manchester, Roger Jennison, very famous in the field of radio astronomy for discoveries he made about the principles of interferometry, which is how radio telescopes of a very large size resolve images. But he decided to do some fundamental physics, that was his dream. And he employed me to do a research project in his lab, where he was building a machine to test the spin of the electron, basically. It was a rather bizarre analogue project. And I helped him with the theory behind this project. He was a great hands-on person, so I wrote short notes for him explaining what I thought was the theory of what he was trying to do with his experiment. It was a giant centrifuge type machine that was designed to model the structure of the atom. He was apparently impressed by what I did, and so I applied for a fellowship to go to Harvard. That was at the time of the beginning of the European Space Agency. It had another name in those days, the European Space Research Organization. But they had a new fellowship opportunity to send potential graduate students to the U.S. with the aim of eventually bringing them back to Europe to be space scientists, basically. And so, I applied for this scheme, and I got a fellowship to go to Harvard.

Zierler:

And this was 1964, '65?

Silk:

This was 1964.

Zierler:

And you heard the term cosmology? You remember that specific term when you were intrigued by what was going on at Harvard?

Silk:

That's right, that's right. The person I engaged to work for to do my PhD with at Harvard was David Layzer, who died just a year or so ago, sadly. But he was a very eminent cosmologist in his own way. Very individual, very fascinated by the large-scale structure of the universe. He was interesting in several ways. He was a polymath, someone who worked with flair on several different things. He'd originally been working on multi-level atom spectroscopy in the sun, for example, theoretical calculations of that. But he also got fascinated at some point in the structure of the universe and fascinated by the Big Bang. And he was working on this in the early sixties when there was an amazing controversy raging between Fred Hoyle and other people on the steady state universe and the pursuers of the Big Bang theory. And Layzer was convinced by concepts of evolution and structure formation that emerge naturally in the Big Bang. He pursued that. And he had this intriguing idea, which was one of the papers that impressed me, that the universe must have begun from some incredibly dense and cold phase. The theory postulated that it was initially like a gigantic, highly dense crystal. And as it expanded, it will inevitably fissure and crack. So, he said, "Aha, these cracks are how structure forms." And he was following in the footsteps of a famous Russian cosmologist, Yakov Zeldovich, who had similar ideas. This was in the early 1960s. And Layzer went on with these ideas to see how structure would turn into galaxies, eventually. He pursued the ideas of the flow of energy in the universe. This is what fascinated me. Entropy inevitably was increasing. This means disorder and chaos is increasing. Yet somehow complex structures formed, like galaxies and stars. And so, he wrote very readable papers on energy development in the universe. He’s famous for something called the Cosmic Virial Theorem, having invented that. That's the balance of energy and gravity, basically, in the expanding system. So, I'd read these papers, and I said, "I want to work with this guy." He accepted me to be his student. And we figured out that my thesis topic would be the formation of the galaxies, how galaxies were made. And I was supposed to put in some details to flesh out his theory, beginning with cracks in the cold universe. And so, one of my first problems was this, that just after I'd arrived at Harvard, there'd been a major discovery in cosmology. It happened in the preceding year, really. This was of the cosmic microwave background. And so, this was the fossil radiation of the Big Bang discovered by Penzias and Wilson, not that far away on the East Coast in New Jersey and further developed by the group at Princeton nearby. And it led to the concept of a hot Big Bang, a Big Bang full of radiation. And they'd discovered the fossil glow from the Big Bang, the so-called three degrees Kelvin cosmic microwave background. So, if this were true, it, of course, meant that Layzer's ideas about a cracking cold universe didn't work at all. Once the universe was hot. We had to redevelop the theory. Matter had experienced this incredible heat. It would've been very early on when it had cooled down and become the cold microwave background that we observe today. So, that was the problem I set myself. And the amazing thing about David was that he was totally open, so he said, "Fine." But he personally had this rivalry with the Princeton side of the Big Bang debate. He refused to believe in the hot Big Bang. He never really accepted that at all. Many decades, actually several decades, probably, passed before he abandoned the cold beginning. But he wanted to encourage me to do what I wanted to do. Meanwhile, he, of course, was trying very hard to explain the microwave background by filling the universe up with dust, having lots of stars radiate and having the cold dust absorb and reradiate the fossil glow, it's a model that never really worked, but he pursued it for decades. Meanwhile, I was busy using the hot universe to make galaxies. That was my main preoccupation for those three student years.

Zierler:

All of this is happening, of course, in the days before inflation. What did inflation add to all of these discussions after the 1960s?

Silk:

Well, let's see. Probably, we want to just go back a moment to the interaction of the formation of structure with this background radiation, the fossil radiation. That had an incredible physical effect on structure in the universe. So that was the stuff I worked on. Inflation was a much later concept. We suddenly fast-forward to 1981, roughly a decade later. And that opened up a new concept in understanding other aspects of the universe, such as why it is as large as it is and the properties of the universe. There was a major breakthrough. Maybe its biggest idea was the seeds of the structures, the galaxies originated as tiny fluctuations that were at the quantum level. And only when inflation occurred were the fluctuations stretched out to the large scales that eventually could make galaxies and clusters of galaxies. So that was the big imprint of inflation. But from my point of view, it didn't matter at all because I considered a later phase of the universe, long after inflation was presumed to have occurred, when all one had was ripples in the density and radiation, and then the gradual effects of gravity took over, which concentrated matter. Eventually, along came the epoch of galaxy formation. So, inflation was just a later attempt to understand better where everything came from. Actually, it’s an incomplete attempt. We're still trying desperately to make sense of how inflation began. But it certainly had a major impact in cosmology.

Zierler:

To fast-forward to current times, where observation is really driving the field, where was observation during your graduate school days? What was happening in the world of observation that may have been relevant to these discussions?

Silk:

What was intriguing at that time was that there were already hints about dark matter. For example, there were vigorous discussions about the nature of what held galaxies together and drew galaxies towards each other, that sort of thing. So, this was something that was just beginning to become a subject of discussion among astronomers. But probably the most important thing was that telescopes then were trying to determine the expansion rate of the universe. That was one of the major goals. And it was hugely uncertain, actually. In fact, it was almost as if there were two schools of thought. There was one that campaigned for a very low value of Hubble's constant, which means a large universe. Another that campaigned for a high value, a very small universe. And the data was bimodal, according to the leaders of these two efforts. And many of my early years were spent hearing their arguments about who was right, who was wrong. Only much later did the compromise vision arrive that the truth was exactly between the two. So that was one big observational debate that affected me. The other one, of course, was to do with forming galaxies. So, in the universe, as you look for a way to look back in time, you should see traces of forming galaxies as you look further back with the bigger telescopes. Now, in those days, we were just beginning to crack the redshift barrier. One of my colleagues decades later at Berkeley was the pioneer of using radio galaxies, identifying them and finding objects very far away that were identified with the more youthful versions of the Milky Way. So, that was something that hadn't really begun when I was a graduate student. But we were limited to very nearby galaxies with our bigger telescopes then. We didn't really see any evidence for evolution. This was to come a bit later. So, that was one major experimental change. And the final one was the subject that was a central part of my doctoral thesis, which was the microwave background. One of the key parts, I should say. Because one of the aspects of forming galaxies was that the fluctuations, which must've been there to form the galaxies, so that gravity could then take over, first seed the clouds that form the galaxies. There were initial fluctuations present in the very early universe. And these had to leave their imprints in the radiation, even though they were incredibly weak. We couldn't see them today because they had turned into galaxies. When you look at the fossil microwave radiation, there are predicted to be these variations from place to place in the sky. That was what I developed in my doctoral thesis. It was only experimentally much, much later that measurements became feasible. I think my work initiated the first discussions about what experiments we should be doing. I could tell you one story about that. But afterwards, it took years for progress to happen.

Zierler:

As a graduate student, was your sense that cosmology as its own field was sufficiently mature for you to pursue this as an academic career? In other words, were there professor positions that were advertising specifically as cosmology at that point?

Silk:

I don't think people took cosmology as a field that seriously at the time. But certainly, extragalactic astronomy and astrophysics were very much important fields. Cosmology, in those days, I would say was a subset of extragalactic astronomy. There was considered to be little hard-core physics in early universe cosmology, and that was only to change much later, when, largely thanks to theories like inflation driven by particle physics, new ideas entered the fray. Of course, nuclear physics even then had a key role. To do this justice, many years before I was a student, there was a huge input of nuclear physics into cosmology. This is the work of George Gamow and his students and collaborators. So, that had a large impact, and this probably, in my early days, was the major reason cosmology was taken fairly seriously, because it was a means of understanding where the helium in the universe came from. And that was a very important part of cosmology. But all the evidence for this was to do with nearby stars, nebulae, and so forth. So, the distant universe only came into the game much, much later.

Zierler:

What about particle physics? Of course, this was a time of tremendous advances in theoretical particle physics. To what extent were these advances relevant for the kinds of questions you were asking as a graduate student?

Silk:

As a graduate student, not at all. I don't think I interacted at all in those days with the people on the opposite side of the Harvard building complex where physics was centered. I only met the particle physicists in later years. At that point, I think, I had little idea of what was going on. My physics colleagues there were very much into collider physics, the implications for the structure of the proton, and so forth. There was a beginning phase of my interest. This happened when I did sit in, I remember, at one of my most impressive courses. I attended, in those days, when I was a graduate student, a lecture series by Julian Schwinger. So that did sort of blow my mind. More for his lecturing style than anything else, how he was able to go to the blackboard with no notes and just derive the integration of electrodynamics into quantum electrodynamics by covering the blackboard with incredibly dense writing. Erasing in one hand, writing in the other. Just incredible. And I was at his lecture, actually, the morning the Nobel Prize was announced. And we gave him a standing ovation. He nodded to us, and immediately resumed his lecture. So, I did interact, but I didn't integrate any of this into my research at that point. But it was very good to have this as a background much later.

Zierler:

Were there some fellow graduate students in your cohort that you've remained close to over the years?

Silk:

So, I do see a number, occasionally. Ben Zuckerman, for example, was one of my contemporaries, a radio astronomer. An officemate was Jay Pasachoff, who's an eminent solar astronomer. So those are two that I've kept some contacts with. Not very many more, actually. Oh, I should add the Cesarskys, Diego and Catherine. They were also my officemates. I've kept up with Catherine probably most closely of all, actually, over the years.

Zierler:

Who was on your thesis committee?

Silk:

My thesis advisor David Layzer, of course. James Wright, who was a relativistic astrophysicist in those days at Harvard. He worried about testing general relativity. I recall one faculty member from those days, who was not on my thesis committee, but in those days, you had to give a public thesis defense. His name was Henry Mitlers. When the floor was opened for discussion, anyone could ask a question on any topic. That was the system. I remember, he asked me, "How does the sun shine?" Which I have to admit was a challenge. There are subtleties in that question.

Zierler:

When you wrapped up, 1968, this is a very interesting time to be in Cambridge, Massachusetts. What were your impressions of all of the tumult that was happening on campus with regard to the Vietnam War, Civil Rights, Women's Liberation?

Silk:

I escaped from the U.S. as all of that was beginning, actually. I left in early June, before even summer break, and I got married that year and was distracted. And so, I ended up traveling for a couple of months, and then was in England for a year. Only when I returned to the U.S. one year after that to go to Princeton did I run full blast into the Vietnam protests.

Zierler:

What were you doing in Britain for that in between year?

Silk:

I was a postdoc with Fred Hoyle at the Institute of Theoretical Astronomy, as it was called in those days, in Cambridge.

Zierler:

What was Hoyle working on at that point?

Silk:

So, he was very much still working on the steady state theory, I think. Even though this was a time when it was seriously challenged by radio astronomy, by people finding evidence for many more radio source counts in the universe than expected in the steady state universe, depending on how far away are the sources. He was a fascinating person, though. I remember he gave a course on nuclear astrophysics when I was there, which I attended. The way he gave a course was totally unique. He invited one of the guests who was spending a sabbatical that year with him, Ed Salpeter from Cornell. They would sit on a sofa in the coffee room and have a conversation. They would do this for two or three hours every week in term. This was the course. I must say that we young post-docs gathered around, breathlessly listening to every word they said. So, it was an inspiring course, actually.

Zierler:

Was your intention always to come back to the States? Or you thought, perhaps, you might pursue faculty opportunities in the UK?

Silk:

I tried, at first, to get a faculty job in the UK. I did not succeed, actually. And what drew me to Harvard had been David Layzer. What drew me to Cambridge was the chance of working with Fred Hoyle. And what drew me, then, to Princeton was the chance of working with Lyman Spitzer, who invited me to come as a postdoc. Basically, in those days, getting a postdoc was so different from today, you were almost invited to come. So, I remember that one day in my final year at Harvard, there was an incident. David, my advisor, called down to me from his office on Observatory Hill. He shouted to me, "Hey, Joe, I've got Geoffrey Burbidge on the line." Burbidge was a close friend of Fred Hoyle. "Would you like to go to work with Hoyle for a year in Cambridge?" Just like that. I hadn't applied. And it was somewhat similar in later going to Princeton. I must've written letters vaguely expressing my intent. But then, Lyman calls me up one day, says, "Would you like to come for a year?" So, I went to Princeton.

Zierler:

Why specifically Lyman? What were you interested in with regard to what he was doing?

Silk:

So, I'd worked a lot on the interstellar medium when I was in Cambridge. That was one of my interests along with cosmology. In fact, I'd not worked on that subject before. I was intending to do something new. And the beauty of Cambridge was that you were put in an office with your contemporaries. Hoyle had many, many associates, postdocs, and students. So, you didn't see that much of Fred, actually. But you were always a student with three or four contemporaries my own age. And down the hall, there were people like Martin Rees and Stephen Hawking. All at the same level as me, basically. First-year post-docs, everybody. And we all talked together a lot. And so, one area that fascinated me was the interstellar medium, what the ionization sources would be. Again, this was driven by observations. That was a time when there was a new satellite experiment that was illuminating our knowledge about ultraviolet aspects, seeing ions in the interstellar gas that we hadn't seen before. What caused these? That was the question. And so, I worked on the theory of that with one of the other post-docs for a while. And Spitzer was fascinated by similar aspects. In fact, the Copernicus satellite was basically launched by his colleagues at Princeton, and he, himself, was a major player. It was the first ultraviolet space telescope. All of this eventually led to the Hubble Space Telescope, of course. So, it was the right place to be, given those interests.

Zierler:

And this was a second postdoc, essentially?

Silk:

That's correct, it was just a short post-doc.

Zierler:

And how long was this for?

Silk:

In those days, it’s interesting, postdocs were typically one year long. I guess there was less federal money around, perhaps, but also far fewer people looking for jobs. And so, it was one year, and during that year, I applied for jobs and had various offers to consider. I ended up going to California to Berkeley.

Zierler:

What was it like to work with Lyman?

Silk:

He was fascinating. He could think on his feet. He would just speculate about different things. I guess he had so much encyclopedic knowledge behind it all that he was able to just reason in front of you, drawing on his experience, and say, "Why not consider this aspect? Maybe such a star is going to collide with another star or whatever. And these are the implications." Things that you ordinarily wouldn't think of. It would just be a step too far, a speculation too far. But hearing him speculate from this great comfort zone seemed to make all the difference. It encouraged a younger person to give it serious attention, he'd actually want you to fill in the details. That was really his approach. We never actually worked together, but I was just inspired by his style, actually. I did take a course he gave. Again, it was a wonderful course on the interstellar medium. Because he was writing the key textbook in this field at that time. And his textbook came out of notes that he made as a result of his course. And the way he made those notes was fascinating. He would basically assign some text, and he wouldn't actually lecture on the text, but he would ask students to talk about the chapters successively. Have a discussion, that was his lecture. And from that, he would, then, improve his notes, and figure out all the flaws, and so forth, and refine the notes. And eventually, a textbook emerged from this. And again, it was just a great experience to see how you're not presented with a solid black and white case for something, but you can see the story emerging. I think that was a wonderful part of his teaching philosophy, actually. And previously, that was, to some extent, what I'd learned earlier in my Harvard days. But that was amazing to see this style in practice at the research level in Princeton.

Zierler:

Who else did you interact with at a senior level at Princeton during the year? I'm thinking of people like Bob Dicke or Jim Peebles.

Silk:

Dicke was in physics, I was in astronomy, astrophysical sciences. So, for historical reasons that I never fully understood, there was a slight rivalry between the two. And in retrospect, it would've been far more productive to have had more interactions. But I did visit them. I remember talking to Dicke and Peebles, and their associated students at the same time. It included people who are now famous in the microwave background community. I gave a seminar in their gravity research group and told them about my ideas about the fluctuations of the microwave background. And in those days, we were so far away from measuring fluctuations. The microwave background was a temperature of three degrees Kelvin in equivalent microwaves. I'd predicted milli-Kelvin fluctuations. We know now that it's ten times even smaller than that, as mapped by recent satellites. And we were so far away from measuring those fluctuations. This was what I was talking to them about. It was intriguing because in those days they were sowing the seeds of future experiments to look for just these things. Having missed out on discovery itself, of course, they were determined to get their hands on the fluctuations. David Wilkinson was one of the young people there at the time. I remember meeting him.

Zierler:

Did you cross paths with Jerry Ostriker? Or he hadn't arrived at that point?

Silk:

Oh, very much so. In fact, he was maybe my closest collaborator in my days. He was a young faculty member at the time that I was at Princeton. And so, we just worked together, incessantly, actually, that year on things like black holes and pulsars. Very new discoveries, or future discoveries, I should say. Neutron stars, it was, in those days, and similar things, which had just been speculated about. And even dark matter, all these many issues were just beginning to be hinted at in those days, really. And so, we worked together a lot on new ideas about where to go in astronomy.

Zierler:

And to come back to this interplay between theory and experimentation, what theoretical advances may have been relevant at this juncture in your career?

Silk:

I would say that one of the breakthroughs was certainly the expansion of the extragalactic horizon. This is around the time of the discovery of the quasars. There were these point-like radio sources at that time, which maybe a year or two before had finally been identified in optical counterparts. These were found to have emission lines and a very high red shift, it was finally inferred. And these objects were so luminous that before long, it was realized they just had to be supermassive black holes. And so, that was something that I was in at the birth of, as it were. And also, around that time, pulsars were being discovered. The first pulsar was found by Jocelyn Bell Burnell. And again, within years, it was clear that this couldn't be something as mundane as a spinning white dwarf star. It most certainly had to be a neutron star. And that, then, led to many, many interesting questions about things like, "Shouldn't there be a pulsar in the middle of the Crab Nebula?" That sort of thing. And excitingly, it could be the source of particles for all the mysterious emission we saw in the Crab. And ideas about, "In the future, we might want to look for X-rays." X-ray astronomy was a beginning field in these days. So it was that I was at the birth of quasar astronomy, X-ray astronomy, pulsars. All these things were beginning in those days. It was a tremendously vigorous field. And today, there's gravitational wave astronomy. We have a whole new dimension. The excitement we have there, then, it was in something else. But it was a similarly exciting time to be living in.

Zierler:

And to go back to that similar question about job opportunities, was there anything in the UK that was compelling to you? Or at this point, you know you wanted to stay in the States?

Silk:

I married an American girl, and she was a graduate student in botany. She spent a year with me in England, was not accepted as a graduate student, was a research assistant for a year, and was not terribly happy. So, I would say that personal reasons probably forced me to focus on the States more than scientific reasons. I definitely did apply for lecturing jobs in the UK. But it's possible I didn't do this wholeheartedly enough. I'm not sure. I didn't get offered one. Had I gotten offered one, I might well have considered staying. I didn't take it seriously enough for that. But in the end, it was the U.S. that beckoned. And the faculty job offers came in very soon after I returned to Princeton.

Zierler:

Where did you consider?

Silk:

Well, it was a choice between my alma mater, Harvard, and Berkeley, basically. So that was my choice.

Zierler:

I'm sure you were aware of the culture at Harvard of not promoting junior faculty.

Silk:

Yes. I don't think at that age, at that time, I ever thought about it. I don't think I asked that question.

Zierler:

Why did Berkeley win out then?

Silk:

I think it was that there was a very, very persuasive chairman at Berkeley. A guy called George Field, who you've probably interviewed. His interests covered much of the same things I'd worked on and was someone whose work I'd admired and wanted to work with. I would say that was probably the decisive factor. And he promised me a number of things that maybe Harvard didn't really explore in enough detail with me. So, I felt that there wasn't really that much of a choice in the end but to go west.

Zierler:

Beyond Field, what else was happening at Berkeley that was exciting to you?

Silk:

So, the other exciting thing at Berkeley was the high red shift universe, which was just being explored. So, in those days, it was a major triumph to get to red shift one or beyond. And that's where Hyron Spinrad was taking us. He was an optical astronomer who would get identifications of radio sources in the sky and look at the galaxies nearby, the possible candidates, and painstakingly get their red shifts with the biggest telescopes he had access to. And he was the one who made a breakthrough in the red shift barriers beyond one that first really showed there were some amazing things going on far away in the universe at early times. And so, that was a major effort. And the other big thing at Berkeley in those days was the birth of X-ray astronomy. And so, that was pioneered. And ultraviolet astronomy. That was being pioneered by a group at the Space Sciences Lab led by another colleague, Stuart Bowyer. So, this was fairly low energy X-ray astronomy. And that was really very important X-ray astronomy. It was complementing the work being done at the same time on harder X-rays by people like Riccardo Giacconi and so forth on the East Coast. There was this East-West Coast rivalry, and Berkeley was focusing more on soft X-rays, which are a great way of understanding our galaxy, basically, and all the hot gas around it. That's what they were focusing on. It was, again, a great strength of Berkeley in those days.

Zierler:

Obviously, nowadays, Lawrence Berkeley Lab has become a significant player in astrophysics. Was there anything happening at the lab that was relevant for you when you arrived?

Silk:

Not at first. It took years, but I was at Berkeley for thirty years. So, after fifteen years or so, halfway through, then the lab started playing a very major role in activities that I was involved in, peripherally at first, at Berkeley. This was, for example, the lab was very intrigued by high energy astronomy. The first high energy cosmic ray experiments on balloons were pioneered in part of the lab, some of them. There was a big interest in gamma rays, neutrinos, all these sorts of things as possible targets for futuristic experiments. But above all, maybe, dark matter. That turned out to be one of the most interesting things that people at the lab were experimentally searching for. By building detectors, I would say. That was their big strength, detector building. The cosmic microwave background was my other great interest. The lab was heavily involved in building detectors that were used by a group led by Paul Richards at Berkeley in physics. And intended to measure, with a greater sensitivity, the fluctuations of the microwave background. This research was to lead to some of the first detections of the fluctuations, which were the seeds of galaxy formation that I worked on a long time before. So, there's a whole story there, of course.

Zierler:

As you say, you missed the sixties at Harvard and then experienced it full force at Princeton. I wonder, by the time you got to Berkeley in 1970, what was going on at Princeton seemed staid to you.

Silk:

It was curious. In a sense, when I arrived in Berkeley, there was still the whiff of tear gas in Sproul Plaza. It was the heady days of Reagan as governor. But one adapted quickly. And Reagan went on soon to other things. So, Berkeley was actually a very open environment. It was very nice. Culturally, it was very, very different from anything I'd grown up with in the UK. And actually, quite different from Princeton. The major thing I noticed at first was that the faculty all wore jackets and ties on the East Coast and in the UK, but Berkeley was a much more casual atmosphere.

Zierler:

Was your sense contra Harvard that junior faculty at Berkeley were really supported with the intention of promoting them?

Silk:

Well, tenure track is an interesting issue we could debate for a while. So, the philosophy is different. Harvard has its own particular way of doing things. I would say they do support junior faculty at Harvard, it's just that they have to compete against an international pool if they want to get tenure. That can be very, very difficult. So, at the University of California, one doesn't have quite this level of competition. You're really competing against yourself, but this also means you have to have very high standards, of course. And so, they judge you on your level more than your relative competence. Well, that's part of it, of course. But it's more on your own level. And I find that, being judged on your own merit is something that, I think, is a fair way to go about this. I would say that. I never really had much doubt when I was junior faculty that there'd be any problems like that. I think it's the usual story. You publish papers, you do research, you give conferences, etc. If things seemed to go well, then at Berkeley, it was just well-known that most that could happen was that junior faculty might get delayed. I think in my thirty years there, I remember just one case of someone not getting tenure. And the person who didn't get tenure there is now very famous elsewhere.

Zierler:

And really your primary affiliation was in astronomy? Or was it physics?

Silk:

It was astronomy, but I was also in physics after a while.

Zierler:

So, you had a dual appointment at some point.

Silk:

That's correct. In other words, it was one hundred percent FTE in astronomy, zero percent in physics. That meant I had occasion to teach in physics. That was the major responsibility. And then, of course, take on research students in physics.

Zierler:

And so, most of your teaching responsibilities, your graduate students, largely, it was in the astronomy program.

Silk:

Well, the teaching was largely in astronomy. My graduate students, in retrospect, were mostly in physics. And the reason was simple. Physics was a top-five department of the U.S. in those days. Still is, I think. Just about, anyway. And had excellent students, amazingly good students. Astronomy students were good, but somehow, they were not as selected as competitively in astronomy as the best physics students were. So, I was, in fact, sought out by physics students to a greater extent than astronomy students. And that's possibly a reflection of the work I did, of course, which was astrophysics, not astronomy. And there was very little theoretical astrophysics being done in the physics department. So, I think something along those lines was sort of a natural combination.

Zierler:

You mentioned the rivalry at Princeton between the two departments. Was there anything similar to that at Berkeley?

Silk:

There was, but much less so. And in fact, as my time at Berkeley went on, we tried to integrate the departments by having people with zero FTE from each other's department. As long as their work was relevant to ongoing research. So, I think that was a much more amicable atmosphere at Berkeley than I've seen at other places.

Zierler:

What were some of the major research projects you became involved with during the early years at Berkeley?

Silk:

So, one of my early projects was working on how the properties of galaxies developed and evolved. And so, one curious part of the story is that a galaxy begins as a big cloud of gas, which then collapses under gravity and breaks up into stars, etc. But if this were all there was to it, then you have to ask certain questions. One is, we know that galaxies are not arbitrarily massive, not arbitrarily small. There's a range. In fact, if you take the galaxies, and you plot their numbers as a function of the mass of their stars, you find there's a typical value for a galaxy. There aren't vast numbers of big ones, there aren't vast numbers of small ones. There are lesser numbers. And there's the average galaxy. It's well-defined as a median value. And this average number, this median mass of a galaxy or luminosity of a galaxy is roughly that of the Milky Way. Actually, slightly bigger than the Milky Way. And the question is, why? What is the physics that gives rise to this? So, one of my early projects was to try to understand this. It turns out to be a tug of war in a gas cloud between gravity, which wants it to collapse and fragment into stars in the galaxy, and the rate at which it can lose energy. Because if it can't lose energy, then it can't collapse. So, it loses energy by cooling. Cooling of atoms rubbing against each other and exciting energy levels and so forth. Energy leaves as the electrons jump down to lower energy levels. So, you have this battle between the two. And so, it turned out, I showed that there's a natural limiting place where this battle is won by the antigravity forces, actually, that gravity doesn't work anymore. And that's roughly this magic mass, the typical mass of a typical galaxy. So, that was one of my early results. And that's still, I think, generally accepted as the fundamental understanding of why galaxies have the masses they are. It's all a question of the ability of the initial cloud in which they form to get rid of its energy. For example, in a cluster of galaxies, you find it's full of hot gas. That hot gas does not collapse to make a galaxy. It stays hot because it's just too massive a configuration to be able to cool effectively. A galaxy is just right. It's the least damage. That was one project. Another one from those early days was understanding the nature of the fluctuations in the microwave background radiation. Basically, when you look at the sky with enough sensitivity, you see tiny variations. It's not completely smooth. This is when you subtract off all the noise from our galaxy in the foreground, from the dust or whatever’s around us, even from other galaxies. You see these tiny ups and downs in temperature by milli-Kelvins, basically. Compared to the three degrees Kelvin of the background radiation. There are hot spots and cold spots. And these represent the initial density fluctuations from which the galaxies themselves condensed a long time ago. One of my early projects was to try to predict how large these fluctuations should be because this was critical for the astronomers. They were building experiments, incredibly sensitive ones, to look for these things. They had to know what the target was. And at first, they had no idea. And so, I was able to calculate for them, along with a graduate student at the time, what the strength of these fluctuations should be so they could better design their experiments. And it wasn't just that we calculated the strength of the fluctuations. That was maybe the main thing. But I also figured out the shape of the fluctuations in the sky. That is, again, if you looked in the wrong place, as it were, in terms of too low an angular resolution of the telescope, you wouldn't see anything at all. There was a natural squiggly spot to look for in terms of the size of these fluctuations. There was one result, and then maybe the equally important result was how strong they should be. So, those were results that emerged from our research, and from other people's research that basically went along with our work. I had post-doctoral fellows working with me at the time. We had a group of researchers and other people around the world doing a number of different things at more or less the same time. And we ended up with a prediction that eventually led to the first dedicated balloon experiments. I mentioned one of them that was at Berkeley. There was another one at Princeton. And then, finally, along came the satellite experiments that mapped out the microwave background sky and finally measured these exquisite fluctuations in detail.

Zierler:

What were the key questions behind the balloon experiments? What specifically were you after?

Silk:

So, the balloon experiments were simply designed to look for the fluctuations that had to be in the universe because of structure–the galaxies had formed. We could not be in a homogeneous universe. It had to be a rippled universe. Because the density fluctuations are like ripples, long ago–this was less than a million years after the Big Bang when the matter and the radiation were closely connected. This was where you have the radiation scattering off the electrons, etc. And eventually, things cooled down. And the radiation, then, basically, flowed to us. We see it as the microwaves in the sky. But it should've carried the imprint of the density fluctuations just because in some places, there was a little more matter that scattered slightly more. In others, a bit less. And so, the idea was to measure some imprint that proved to us that galaxies had been made as the Big Bang tells us they should've been made. We need to verify this prediction of the Big Bang theory. That was the goal behind everything. And it was the formation of structures in the universe, notably, the clusters of galaxies that leads to the inevitable presence of temperature fluctuations. They have to be there. And until we measured them, we just couldn't be one hundred percent sure that the Big Bang was the right theory. And this was even after we had inflation and so forth. And it was the COBE satellite in 1990 that made the ultimate breakthrough, actually, that measured the first traces of fluctuations. And its successor satellites WMAP and Planck that filled in the details.

Zierler:

When did you become first aware of what John Mather was doing?

Silk:

I was first aware of what John had done in detail only at the press conference I attended in 1990, when he released the amazing results on the blackbody spectrum of the cosmic microwave background. That was an incredible surprise because only years before, his former PhD supervisor, Paul Richards, and another graduate student had flown an experiment on a balloon that claimed to measure huge deviations from a blackbody. It was an amazing thing to have, which would've meant trouble for the Big Bang, as it were. But it was really wonderful to have the Big Bang black body prediction confirmed by experiment. That was the first I knew of this major result. Of course, over the years before, I knew he was working on the COBE experiment with the spectrometer. The PI of another COBE experiment, George Smoot, actually, was a longtime colleague at Berkeley at LBL, so I talked to him a lot. I knew about the experiment they were doing. But the results were closely guarded until they were released officially.

Zierler:

And what was so compelling about those results insofar as the questions you were asking up until that point?

Silk:

So, I mentioned the spectrum, which was entirely compelling. And then, as far as the fluctuations go, the major result was that the universe was simply not smooth as seen in the sky, the universe back a few hundred-thousand years after the Big Bang, which is where you're seeing it with these microwave photons. That's when they last scanned for matter. The universe was then not homogeneous. There were ripples in the universe, and these temperature fluctuations implied there were underlying density fluctuations, and that implied tiny excesses of gravity here and there in the early universe and deficiencies of gravity in other places. And those, as time went on, accumulated matter and eventually collapsed into giant structures, the galaxies and clusters that we see now. So, this was a subject I'd been working on for decades before, and it was good to see that experimental results were taking shape. Of course, COBE had an incredibly blurred view of the universe. It only had the ability to measure at an angular resolution of seven degrees on the sky. Detecting the smallest fluctuations was to take another decade before the details were mapped out.

Zierler:

Who were some of your most significant graduate students during your time at Berkeley?

Silk:

I have had a lot of graduate students. I had more than thirty. So, let me mention a couple of them. Wayne Hu, who's now at the University of Chicago, and a member of the National Academy, worked with me on the spectrum of the microwave background. We predicted there had to be some tiny deviations from black body. Which, by the way, have not been discovered yet. It's a prediction for the future. And then, he also worked with me on refining the theory of the fluctuations and making more precise predictions that eventually led to the refined experiments we've had in recent years. Another student was Max Tegmark, who's now faculty at MIT. Max is well-known for many things, but with me, he worked on what sources affected the temperature of the universe and the ionization state of the universe. We worked on the idea that stars, a long time ago, might've exploded into supernovae and ionized a very, very early universe, releasing plasma, and what traces these might leave on the universe we see today. Another source of fluctuations that would distract our cosmological search, perhaps, but something that we want to trace out, too. So, that was one project we did. He's since gone on to work on the dark ages of astronomy, twenty-one cm measures of gas at very high redshift, and on machine learning.

Zierler:

To go back to these boundary issues that we began with at the beginning of our conversation, cosmology, astrophysics, and as you say, particle astrophysics, when did particle astrophysics sort of enter the picture in its own distinct realm?

Silk:

So, this was in the early 1980s. It was a combination of, I would say, two different things. One was inflation, which we've mentioned. Inflation was, basically, theoretical particle physicists coming up with a new type of early scalar field that could do dramatic things to the very early universe for a tiny fraction of a second, and, basically, make it enormously bigger, and explain one of the biggest problems that had plagued astronomers like me, at the time, and others, that was the initial conditions of the universe. How on earth could the universe have gotten to be so big? And how on earth could it have these fluctuations, which were galaxies today, but must've been born on scales that were so large, that light wouldn't have had time to cross from one end of the fluctuation to the other? The universe was so tiny early on, and the distance light traveled since the beginning was tiny. So, how did these fluctuations come about? Inflation solved that problem. So that was one immense insight that particle physics was going to make an impact. But there was another side to the story, which was probably even more important from the particle physics point of view, and that today, has led to modeling of dark matter. So, the idea, also, was that dark matter was a major problem. We had no idea what it was. Plausibly, it was something highly weakly interacting, and logically, it might be a new particle that we hadn't discovered yet. There was a theory that was popular at that time, which involved unifying the fundamental forces of nature. The theory is called supersymmetry. But one of the byproducts of that theory is that as the universe cooled down, this is very early on, things will be left behind. We have relics left behind. These were the ideal candidates, the stable ones, at least, for dark matter. They even have a fancy name, they are called neutralinos. And there was a wonderful prediction attached to this in the sense that, as the universe cooled down, one could figure out the properties of these particles. Roughly speaking, if they interacted too strongly with themselves, there will be too few left over. If they interacted too weakly, there would be too many because they destroy each other when they interact. And their self-annihilations produce possible signals, wherever they occur today. Therefore, they had to be just right, like Goldilocks. And this meant you could figure out what their cross section was, their interaction rate was with ordinary matter. And still is today. And this meant you could make predictions about what you might expect to see. If you look around our galaxy, you look where there's dark matter at the center of the galaxy, huge amounts of dark matter. We're pretty sure of that, although it's slightly debated. But we see that the stars are moving faster, etc. There is dark matter in the center. Therefore, why not look there and look for tiny flashes, from these conglomerations of particles that interact with each other? Dark matter may not be completely dark. The particles annihilate with each other. It’s only a weak effect but there are so many left-over dark particles. So, the effect adds up. The result is tiny flashes of energy that might be detectable high energy particles like neutrinos, or gamma rays, or X-rays. So, one of my most important papers was when I predicted that these neutralinos should have a signature. One should look for glows in gamma rays, flashes of high energy neutrinos, and even antiparticles because when they annihilate, the dark matter particles produce equal numbers of particles and antiparticles. Positrons have high energy, this should come from their annihilations, maybe. It happened at that time, and it’s happening now. Wherever the dark matter is. Dark matter is really, very sparse, but there’s lots of it. Highly ambiguous signals have been claimed, but experiments were to get much better. And now, one of the main goals of astro-particle physics is to build experiments that look for dark matter signatures, such as high energy neutrinos. Another idea that was also developed at the same time was that the dark particles, which passed through the earth, were weakly interacting. They would pass through detectors, and if you put a detector in a sufficiently shielded place, you might see occasional interactions with ordinary detector material, just scintillations of light or whatever. Building experiments deep underground, in quiet, noise-free environments to look for dark matter has become a major industry, both in astrophysics and particle physics. And this began in the 1980s but was maybe the major explosion in experimental cosmology [and] astrophysics that has come from that time. It's a huge field now.

Zierler:

While we're in the chronological neighborhood of the early to mid-1980s, to what extent were you following advances in string theory and the so-called superstring revolution in 1984?

Silk:

At that point, it was in its own universe, I would say. There was no common language, even. I think it was only much, much later that string theory got to be able to go beyond making predictions, which have had little success, but to have applications in condensed matter physics, for example, where the models of string theory had their own sorts of analogues and proved to provide a very sound theory inspired by fundamental particle physics. And it was only at that time that the string theorists began looking for observational verifications of string theory, which they still haven't found, by the way. But at that point, when they began talking to observers or thinking of observations that might be relics of string theory, such as cosmic strings, for example, one of the many relics, the much wider community became involved. I worked on cosmic strings in the 1980s. It was a prediction of string theory, that when the higher dimensionality of superstrings went away, and we ended up with a normal universe. Tiny loops of cosmic string were left behind, trapped from this higher dimensional stuff that existed only at very high energy, and those could have observational implications. One was generation of a background of stochastic gravitational waves. And this was one of the interfaces of superstrings and the astrophysical universe. And there are many others now that people are desperately trying to pursue, but so far with little success.

Zierler:

And later in the 1980s, you mentioned supersymmetry. I'm curious if you were following developments with larger projects, such as the SSC, which would operate at high enough energies where potentially, supersymmetry could be something that was studied.

Silk:

We haven't found supersymmetry yet with the LHC, and we've looked for energies up to fourteen TeV, teva-electron volts, so far. So, we set very strong limits on supersymmetry, this mythical running together of fundamental interactions, which should happen, should converge, and should involve new particles, perhaps. This does not occur at the energies we have access to now. So, there's a huge push, now, to say, "Well, maybe it's happening at a higher energy that we haven't accessed yet. Let's build a collider of one hundred TeV.” Much larger than the LHC. There, supersymmetry may be hiding, waiting to be discovered. There may be mysterious particles, which would have no counterparts in known physics, detectable because when they're produced in collisions, there are jets, one-sided jets, evidence of nothing coming from one side or the other that would tell you that you've got something very new, a neutralino, maybe. Anyway, this is very much a hot topic, now, to build such a machine. Probably will happen in a few decades, I would guess. And so, that, to me, is the future of supersymmetry. Some of my colleagues say, "Well, this is already known to be unlikely because if we haven't seen it where we expected to see it, which is roughly in the Higgs mass regime or whatever because that's the boundary of current standard model of particle physics, then if you want to push it to a higher energy, you have to tune your parameters more and more finely to make it work. And this seems unlikely." But my response is, "We have no idea what fine tuning means. It could be anywhere. We have to look. The only place it could be hiding is at a higher energy. Let's look there." In a nutshell, I think we have to look for supersymmetry. It's such a beautiful, compelling theory. Maybe it's out there somewhere.

Zierler:

With respect to your contributions with the cosmic microwave background, when did you first get the sense that there would be something named in your honor? Namely, Silk damping.

Silk:

I never really got the idea, I suppose. I'm not even quite sure I fully understand why this is, actually, because it seems to me there were others that really made the detailed predictions of Silk damping. And so, I could imagine calling it, I don't know, dissipation damping or something. But I feel honored.

Zierler:

So, let's unpack that a bit. It was certainly named in your honor. So, let's sort of unpack that a little bit. What were your contributions that you think made it so significant that you had this named for you?

Silk:

Basically, what I worked on was the friction between radiation and ionized matter. Radiation and electrons, basically. So, photons scatter off electrons. And in particular, if I have a cloud of plasma, of ionized gas, as I do in the early universe, by a cloud, I just mean some slight over-density above its surroundings because that's effectively a cloud for our present discussion, it wants to contract under gravity. And it contains radiation as well because it's random patch of the universe. Early on, the radiation was compressed, too. It didn't have time to go anywhere. But the radiation wants to expand and move away, wants to smooth out. It exerts a strong driving force. Early on, this cloud was so much bigger than the light crossing time of the universe, but it didn't have time to react to itself. But once the universe is large and old enough, 100,000 years after the Big Bang, if you like, then the radiation can try to expand. It now has time. As it expands, there's a friction between the radiation and the electrons. The radiation is scattering, it’s pushing the electrons. And so, the electrons, therefore, must be losing some energy. It's like heat, if you like. And this gives you a tiny excess addition to the radiation. It's not black body because the universe is no longer dense enough to make black body radiation because it's not a perfect furnace anymore, so it gives you a slight distortion of photons from the perfect black body radiation. So that's the idea. And then, if these fluctuations are small enough in size, then they will have smoothed out completely and carry the electrons with them. And the protons follow closely behind. And only if the fluctuations are large enough could they last around to the epoch when the last scatterings occur. We hope to see them in the microwave background. So that, basically, sets a minimum size where you can expect to see fluctuations in the background radiation when they last scattered. And so, that critical size is what's come to be called the Silk mass. And in fact, when you look at the distribution of fluctuations, that we now measure beautifully with the Planck satellite, and balloon experiments, and ground-based experiments, you find that the fluctuations in the sky, they peak at a degree, and they gradually get smaller and smaller as you go to smaller angular scales. Then, when you get to a couple of minutes of arc, they basically peter out completely. And this is because of this damping effect, basically, this friction. They just haven't survived. They've left a little bit of heat behind that we haven't found yet. That'll be the next magic trick in this game, the major challenge to find that heat, to look for the deviation of the black body of the relic radiation. And we’ll measure the tiny scales, the fluctuations much smaller than those of the galaxies. The building blocks of galaxies. I hope we find that someday. But that's got to be decades away. It's a horrendously difficult experiment to do. That'll be the ultimate test. But so far, what we have seen are the damping of these fluctuations on galaxy cluster scales, and that relates to this early calculation I did. It's an aspect of it.

Zierler:

When you say decades away, what are some of the key observational, technological, or even theoretical limits that make this so remote right now?

Silk:

Well, here's what we have to do. We have to do so much better than COBE. John Mather's FIRAS experiment on COBE, his spectrometer, set limits on the deviation. To something like a part in 10,000 of a black body. It was an incredibly precise experiment. He had what is called a Michelson interferometer pointing at the sky. You compare the sky in one aperture with a perfect black body on board, calibrate a black body, and are able to look for the difference. He was able to set limits like that. So, to measure these predicted fluctuations, we have to do 10,000 times better than FIRAS. We're looking for parts in 100,000,000. And that's what's really, really difficult. It's true COBE was a small telescope. So, people are now developing plans to build bigger telescopes with much more sophisticated interferometers to basically compare the radiation with a calibrated black body, the background radiation, and look for spectral deviations in the cosmic microwave black body. But it's not going to be easy. The current state of the art is that people have proposed something to go on a NASA spacecraft call for explorer experiments. This was first proposed several years ago and led by Al Kogut and others, and they were turned down. And they proposed it a second time, they were turned down a second time. Basically, they were only able to get three orders of magnitude improvement on John Mather and COBE’s FIRAS, but no more. And that was one of the problems. They couldn't quite do well enough. So, we need to go beyond an explorer class mission, that's a fairly smallish telescope on a satellite. We need to go certainly bigger. And that would involve a major experiment. And those experiments are very, very expensive, apart from being difficult to do. And a spectrometer to measure this deviation of the black body spectrum would be in competition with, let's say, a giant X-ray telescope, for example. Or a giant set of gravitational wave telescopes equally. And science these other telescopes can do, I wouldn't say it's so much deeper, but it's so much broader. A gravity wave experiment, for example, could not just test Einstein's gravity, but it can also do things like measure black holes that have formed from stars, black holes that have formed from supermassive objects, and do many things. Whereas the black body spectrum deviation experiment is going to measure something incredibly fundamental, but it's one thing. So, when you compare these experiments, if you're sitting up there dividing funding, it's hard to sell one of these CMB spectral experiments. So, I think there is a solution, actually, which is going to come not quickly, but in the long term, and that is not to imagine a free-flying space experiment which will be out-competed by these other projects, but to put an experiment on the moon. And there, I think you have a real chance of doing something. Why? Because we're going to the moon. It provides a space-like environment and a large platform. That seems totally clear. We've done amazing things in terms of construction and many, many other achievements in space. But what we should also be doing, and one of the goals I've been trying to persuade people to adopt in recent years, is to plan to put telescopes on the moon. So, this is where our microwave background spectral telescope might be. It would be a module that wouldn't need to even pivot around the sky. You could just put it into a crater. And the moon, as it rotates, would scan the sky for you. And it would be an interferometer, with a small telescope that had multiple detectors, but would be a large enough telescope to give you the improvements that you certainly need. It would compare an accurate black body with the emission from the sky, correlate the two together, and try to get a differential signal. You would just put the proposed telescope into a dark crater. Why a dark crater? Well, many craters near the south pole, especially, which is one of the main goals of future moon missions, happen to have high rims, they're in shadow all of the time because the sun is always very low on the horizon, and many of them are so cold, as low as thirty Kelvin with ice in the centers. They're ideal places to build a far infrared telescope of the sort we need to look at the microwave background. It would be a terahertz telescope, basically. And what is more, the rims of these craters, some of them are so high that they're always illuminated by the sun. You have a perpetual source of solar power. They're incredible places to do astronomy. Probably to do other stuff, too. But I think astronomers have to grab one or two of these now when they negotiate what we'll be doing in ten- or twenty-years’ time when astronauts go back to the moon in number. And I think that sort of scale and place is the way you could mount the right experiment to measure these spectral distortions. And I'll tell you, there's one other amazing thing you could do. If you could really get this factor of 10,000 advance on COBE, which I think you could do this way, then why not go another factor of a few? And if you could do that as well, then you could do what to my mind is maybe the most amazing thing of all, which is to measure the atomic physics of the recombination of the universe. To think back to when the universe was expanding hundreds of thousands of years ago, eventually, it was destined to gradually cool down and become atoms. Well, as it recombined from plasma to atoms, it emitted hydrogen and helium recombination lines. The same lines are bread and butter to astronomy everywhere in the nearby universe. But with this experiment, we could see these lines near the beginning of time. We could see high orders of these lines, from the first hundred thousand years. We could measure the abundance of helium. The redshift is so large we’d be looking at far infrared frequencies. We could see Lyman-alpha, Paschen-alpha, and Brackett-alpha, and all the various orders. An amazing way of doing the physics of the very early universe in addition to measuring the predicted deviation from a black body. So, I think it's an irresistible challenge, but it's for the future. That's the problem.

Zierler:

Perhaps it's not a very important distinction, but I wonder if you would consider a telescope in a crater on the south pole of the moon to be a land-based telescope or a space-based telescope.

Silk:

Well, the moon has a space-like environment. There is no atmosphere. That is the trick. But there are also other problems, like dust. So, you'd have to think of clever ways to cope with that, too. So, there are issues local to the moon that would worry you somewhat.

Zierler:

Best case scenario, if this project comes to fruition, what would it tell us about the universe that we don't currently know?

Silk:

Well, in principle, it could help us confirm key aspects of cosmology. The thing that excites me the most is one step further away. Inflation is this incredibly successful theory, but it's highly phenomenological. We do not understand those initial conditions that led to inflation. It does make fundamental predictions that we're striving hard to test. Unfortunately, these predictions, I would say, are generally not really robust, for the most part. And this, for example, might be the polarization of the microwave background. We don't precisely know how large that should be. It’s a wonderful signal of the ending of early inflation and gives you gravity wave signals. That's one of the predictions. But in many models, it’s way too small to be detectable. There is one signal that excites me the most, which is a robust prediction of all inflation models. And this is the fact that inflation does imprint on the fluctuations a tiny twist or a non-Gaussianity that's primordial. It's hard to define the polarization. It at least has a certain gravity wave type shearing mode that you could see. The non-Gaussanity is something that’s much harder to detect, but it's something that really, really is predicted. And to do it, you have to go to low frequency radio astronomy. Because, basically, it's such a weak signal, if I count all of the pixels in the microwave sky, I just have a few million of them, and there's simply not enough information there to get to the signal I want to, to test inflation. I have to have many, many more pixels to be down to this totally low, non-Gaussian, guaranteed signal from the beginning. There's only one way to get those pixels. You target the gas clouds from which galaxies assembled long ago. You go to the dark ages. How do you do that? You go to the far side of the moon, where you don't have the earth interfering for you. You look at a red shift of fifty, that's a wavelength of ten meters for twenty-one-centimeter radiation, highly red shifted. And that's where you can see the building blocks of the galaxies. The gas clouds before there were stars to mess up the universe. And so, because every galaxy's got roughly a million building blocks, then suddenly, from millions of pixels, whatever the numbers of spots on the sky, or for that matter, the millions or maybe even hundreds of millions of galaxies among future surveys with space telescopes, I could suddenly get millions or even trillions of bits of information from the dark ages. And that, to me, is the ultimate goal of cosmology. First, the dark ages is our new frontier. We have to go there, anyway. We will go there. We have to search at really low frequencies, that means some thirty megahertz. The earth’s ionosphere is a serious impediment. The moon is by far the best place from which to look. The most radio-quiet place in the entire solar system is on the far side of the moon. And there is no ionosphere. Once you go to the moon you can build low frequency radio telescopes. It’s really just a collection of dipole antennae, deployed as an interferometer. The technology is well known, we are planning it for the Square Kilometer Array. Then why not go to the far side of the moon for this ultimate test of inflation? So that would be my vision for the future. Again, we're talking ten, twenty, even thirty years, but that's fine. That's nothing, really, compared to time scales for other projects.

Zierler:

And when you talk about the conditions leading to inflation, to what extent does that really mean understanding t = 0?

Silk:

It means understanding, I would say, t = epsilon, where for the moment, epsilon is something that's really small but not zero, and limited by our ignorance of quantum gravity and so forth. We have to get there, and then we can figure out what, if anything, came before. But we're not there yet.

Zierler:

And in terms of getting there with quantum gravity, does this mean integrating all of the forces?

Silk:

It means, basically, I would say, having a new theory. You might be looking at emergent phenomena that we haven't even begun to realize yet. I think, basically, it's a blank slate at this point. We desperately need to unify gravity with our fundamental forces. And so far, I think people who try to get to the beginning, basically so far are putting in classical physics as an intermediate step to try to hide whatever might've happened before. But I think we have to mount this obstacle somehow, whether it's a new type of cosmology or whatever, I have no idea. There are odd ideas floating around like loop quantum cosmology. But I think it's open, from my point of view. We just don't have a theory. So, I prefer not to speculate about the universe before time epsilon.

Zierler:

And then, in the late 1990s, dark energy enters the picture. What is the impact insofar as your research is concerned at this point?

Silk:

Well, I suppose the most interesting impact is that it's a possible beacon of something weird going on in the universe. I personally don't believe dark energy per se is such a major issue. It's otherwise known as the cosmological constant, which Einstein brought in, and it's a number. It’s an incredibly small number compared to anything that we might have expected. Where it comes from, why it's there–I'm happy to accept that it's another number. And for me, it's just part of cosmology. I'm not driven to understand the origin of this number because there are other numbers that I don't understand either, I don't think anybody knows the origin of the hierarchy of particle masses, such as leptons or hadrons. Or the fundamental constants, such as the fine structure constant. Or the remarkable weakness of gravity. So, it's one number among others. The challenge is for some future generation of theorists, perhaps. We need a theory. But there is a related issue, though, which is that we have this controversy in our field now about the Hubble constant. There are different indicators depending on whether you look at the far universe, or the nearby universe, and you get different values. There is a conflict here. And varying dark energy is one of the culprits that's being invoked to try to reduce this tension between the experimental groups. And so, it could be that the tension is a pointer to something fundamentally wrong with our current understanding of cosmology. Or it could just be, which is, again my point of view, that there are simply observational issues that the two rival teams, debating whether it's local variable stars on the one hand or the cosmic microwave background on the other hand as a preferred calibrator, have yet to fully come to grips with. So, we'll see.

Zierler:

Back on planet earth, what were the circumstances that compelled you to leave Berkeley and take the position at Oxford?

Silk:

I would say I'd been at Berkeley for a long time, and I think I was looking for a new challenge, basically. And then, there were personal issues that motivated my return to Europe as well. So, a combination of them.

Zierler:

Tell me about the Savilian Chair. Obviously, it goes back a long time in history.

Silk:

Well, yes. So, Christopher Wren was one of my predecessors there, among others. And perhaps Edwin Hubble was one of the most illustrious astronomy predecessors. He had his little observatory in Oxford, basically a hole in the roof, still preserved, actually, as an attic museum embedded now in student housing, where he supposedly noted the arrival time of the comet named after him. Oxford was an interesting place to be because when I went there, it had vivid memories of a glorious past of astronomy, centuries before. But its recent history was definitely less illustrious than it could've been. It was very much trying hard to catch up with Cambridge as a significant place in the UK to do cosmology, but its cosmology history was very much off of the main street. And I went there to try to get momentum going again. That's what I did there for a decade or so.

Zierler:

Did you bring graduate students with you? Was this an entirely new start for you?

Silk:

I brought post-doctoral fellows. And graduate students were hired.

Zierler:

And were you generally working on the same research projects, or did the move give you an opportunity to take on new projects?

Silk:

It was very much the same thing as before. That is, cosmic microwave background radiation and dark matter, the two major themes of much of my work. Along with galaxy formation. My major challenge was in hiring new faculty and opening up new research directions.

Zierler:

Culturally, did it feel like you were coming back home?

Silk:

A little bit, yes, it did. Oxford has its distinct cultural life. Much is centered in the colleges. I was based in New College, in fact one of the oldest of the colleges. This turned out to be a rewarding experience, socially as well as culturally.

Zierler:

Over thirty years, you were somewhat Americanized, I would think.

Silk:

Well, that's true, but also, I was commuting to Paris for several years, actually, after I got back to England. So, this helped make the cultural differences somewhat diffuse, as it were.

Zierler:

The commute to Paris was because you were already getting involved with the Institute?

Silk:

I had a visiting appointment at the IAP in Paris over many years. And at the same time, I got more involved with the institute, of course, to ease my trajectory to Paris. This took on a new dimension when I obtained a large European grant that enabled me to set up a research group in Paris when I retired from Oxford.

Zierler:

When did you become involved with Hopkins?

Silk:

That was at the end of my stay in Oxford, when I decided that I wanted to reestablish some roots, to some extent, in the U.S. So, the timing was good for that.

Zierler:

And what was happening at Hopkins that was attractive at that point?

Silk:

It was an interesting group that combined both astronomy, astrophysics, particle physics applied to cosmology, and at the same time, there was a close connection with the space telescope institute. The institute there was expanding, and it was closely interfaced with Hopkins. In fact, I was, for a number of years there, in both, actually. The department of astronomy and physics and the Space Telescope Institute.

Zierler:

And who were some of the key people that you worked with at Hopkins?

Silk:

Probably the key person who I worked with most there was Colin Norman, who, long before, had been my postdoc at Berkeley. And also, with Rosemary Wyse, another professor there. She'd been my post-doc, too, previously.

Zierler:

And what was useful about the proximity to Space Telescope?

Silk:

The fact that we were exposed all the time to the latest discoveries in the distant universe. Indeed, in the nearby universe, too, because exoplanets were being discovered as well. But largely for me, it was galaxies very far away, the notion of using galaxy clusters as telescopes in the sky, as lenses to magnify these galaxies, that sort of thing. All this was just exploding then at the Space Telescope Institute.

Zierler:

What are some of the contributions more broadly of Space Telescope? What are they providing that something like NASA itself would not?

Silk:

Obviously, it is funded by NASA, so it's part of NASA. And what it does, basically, is data reduction. The incoming data from remote targets is reduced and analyzed from Space Telescope and will be from the future James Webb telescope. It's a resource for that. And it employs scientists both to do the mundane stuff, the data reduction, but also to model and interpret the data, and thereby involves many interactions with universities all over the country, and the world. Many people come through to look at the data and give seminars. And it's just a wonderful focus, a center for astrophysics on the East Coast.

Zierler:

To what extent were you following advances with the accelerating universe during these years?

Silk:

Pretty closely. One of my colleagues there, Adam Riess, was a central player in that game, of course, so I would often talk to him.

Zierler:

And was there anything particularly relevant for this discovery for your work?

Silk:

The discovery of dark energy as a cosmological constant, or the discovery of the tension in the Hubble constant measurements? Because there were two discoveries.

Zierler:

Either one.

Silk:

Actually, dark energy was discovered long before I was there. We'd already encountered dark energy a decade earlier. The Nobel Prize came after I was there. So, I think it was an accepted fact, pretty much. Ten years ago, there was no tension in the field. We were zeroing in on the best value of the cosmological constant. It was, I would say, just one of the new regimes of cosmology, which would have implications for the distant future of the universe, etc. But today, of course, it's more detailed. The universe is accelerating, but not enormously accelerating right now. So, when we look far away, we try to measure the cosmological constant because it has a huge impact on what we see and on what we will see.

Zierler:

To get back to the question of the divide in the departments with physics and astronomy, how did that play out during your continuing time at Hopkins?

Silk:

Well, at Hopkins, I was in the astrophysics section of the department, but it was a stimulating mix. It was a joint department, physics and astronomy. So, we'd tend to have astronomers on one floor, physicists on another, but a lot of interaction between them, common seminars, coffees, whatever. So, it was a different feeling from my Berkeley days, but in some ways, not too dissimilar from Oxford, where at Oxford astrophysics, it's part of the physics department. So again, it was embedded in physics. That also made for good interactions. I think that's a very healthy pattern for astronomy to have.

Zierler:

And when did you start to become involved with your current affiliation in Paris?

Silk:

That was gradual over the years. I've visited for many years. I'm often spending summers there. And it was when I retired from Oxford. I decided to make Paris my main base. And this was greatly facilitated when I obtained a large grant from the ERC.

Zierler:

And this environment is primarily French or English language?

Silk:

It's actually mixed. I would say socially, it's French language. Scientifically, it's English language.

Zierler:

So, if you're giving lectures, you're delivering them in English.

Silk:

Yes, that's right. And scientific discussions at seminars are in English. But our coffee hours are mixed. But sometimes in French. And I give popular talks in French.

Zierler:

And in terms of your expectations, obviously before the pandemic, would you mostly spend the academic year at Hopkins and then the summers in Paris? Or you would mix it up more?

Silk:

No, my appointment at Hopkins is part-time. So, I would just spend a couple of months a year there. Often up to three months.

Zierler:

Just to bring the narrative up to the present, pandemic notwithstanding, what are some of the things that you've been working on in recent years?

Silk:

So, I have two major interests currently. One is propaganda for telescopes on the moon. So, I've been very much involved in organizing workshops and so forth on projects to go to the moon. This is a good time to design telescopes for the moon. Because we're discussing lots of infrastructure for the moon, commercial activities on the moon. We're going to eventually do things like mining and build hotels on the moon, apart from sending astronauts and tourists there in the near future. But I'd like to see us plan to construct serious telescopes on the moon. And so, I've been involved in thinking about radio telescopes, and infrared telescopes, and optical telescopes. All of these can do incredible things on the moon that we could never do on the earth or most likely even not do from space. And so, that's one of my major lobbies now, publicizing the different projects for the moon. And the other one is something more astrophysical. It's trying to understand the connection between supermassive black holes, quasars, and star formation activity. There seems to be a connection, but I think the actual details are a little bit fuzzy. And I have various ideas on how we should be pursuing this.

Zierler:

It seems that the project of building infrastructure would have both legal and technological challenges. So, let's start, first, with the technology. To what extent would these endeavors require partnership with commercial spaceflight agencies such as Space X or Blue Origin?

Silk:

Well, commercial partners are certainly being commissioned by the space agencies, NASA especially, to do various activities on the moon, launches to the moon, delivery of payloads on the moon. So, this is important, but I think the overall decisions come centrally from our administration. And they have to decide on the optimal planning. What are we going to do on the moon? How do we balance commercial activity with scientific activity? It's not obvious at all. Because, for example, I mentioned lunar crater sites. They might be a priority for commercial activities. Some of them are full of ice. Essential for rocket fuel. So, you might want to decide that you want to do some ice mining in the dark craters. And this is because ice is going to be invaluable as a resource on the moon for rocket fuel, liquid hydrogen, liquid oxygen. There'd be competition. So, somehow, we have to develop the right framework, the right persuasion, that we reserve certain craters for ice extraction. Perhaps for habitations. Dark polar craters are also ideal sites for telescopes. We see these problems of competition for resources beginning already. Even now, there are so many micro satellites in Earth's orbit, astronomy is beginning to be impacted and will suffer more in the years to come. So, we want to avoid these problems well before we run into them on the moon.

Zierler:

Presumably, there's a big difference between moon missions that are temporary versus those that are more permanent. And so, if the United States or any singular national space agency wanted to set up a permanent mission on the moon, even a scientific mission on the moon, what are some of the international legal ramifications for such existential questions as, "Who gets to own or control this particular crater?" These are obviously discussions that need to happen. To what extent are you involved in this, even if on an advisory level?

Silk:

This is something that comes up repeatedly. There is an outer space treaty. A little bit along the lines of the Antarctic Treaty. And then, this was signed many years ago, in 1967. I think 110 nations have now signed up to it. And this does do minimal things. It limits military activity on the moon, for example, and it says that no nation can own any bit of the moon. But there are many things it doesn't cover, such as competition, who gets mineral rights on the moon, what you do with rival claims, what if there were crimes committed on the moon? How are the rules enforced? But we have time. Unfortunately, international treaties are now not necessarily at the forefront of our thinking and planning. We have other concerns. But I would say that by the time we build bases on the moon, which will be in twenty years, probably, we hopefully will have developed a much stronger legal framework that will commit us to collaboration, to controlling pollution, to avoiding environmental destruction, and maybe even limiting not just physical pollution but radio pollution, optical pollution, that sort of thing, so astronomers can be happy in different sites. There are issues like these that I hope will be adopted. And they're being thought about, but I would say still not actively being treated at the highest level so far.

Zierler:

Do you envision terraforming or some version of that to be viable for the moon? Or will this be entirely a built and sealed environment?

Silk:

In principle, there are areas you could terraform. You certainly have the oxygen supply to do this. Even in the regolith. Maybe the best place to consider terraforming would be a giant lava tube, a huge cave, where one could even imagine putting many buildings. This has certainly been discussed. Lava tubes might be a possible site to develop. This would be an interesting but challenging arrangement. The beauty of the lava tubes is they have high protection against meteor impacts because they have very dense natural lava roofing.

Zierler:

Is radiation exposure a concern?

Silk:

It is if you wander about on the lunar surface for too long. And I think that we'd have a system of alarms, probably. Because the beauty of radiation is that its worst impact is from giant solar flares, and they take ten or fifteen hours to get from the sun to the moon. So, there'd be time to have a warning system if something dangerous is coming your way, as it were, to go to shelter. When you design buildings on the moon, one would certainly need to have roofing that would be meters thick of aluminum or something like that to shield against the minimal effects of radiation and cosmic rays, energetic particles, the sort of thing that comes from the sun.

Zierler:

Just to clarify, the attractiveness of the moon is simply because of its proximity, the logistical ease of getting there? Or is it possible that what you hope to accomplish would be better done on one of Jupiter's moons, but it's simply too far away?

Silk:

The moon is just logistically so much simpler. It's our nearest neighbor, and it's got a space-like environment. It's also the focus of an immense drive for tourism and commercial activities. No one is going to send tourists to Callisto or Europa, in the next century anyway. But the moon is actively being discussed. So, it's just the place to exploit. The only competition would be free flyers in space, and I think that if you really want to go for mega projects, mega telescopes, which can do incredible things, then the only realistic place you might consider putting them is on the moon. That's the future, I think. Eventually.

Zierler:

Now that we've worked up to the present, for the last part of our talk, I'd like to ask a few broadly retrospective questions about your work and career, and then we'll end looking to the future. So, the first thing that I'd like to ask you about is, between your public lectures and some of your books and writing, you definitely gear some of your scientific explanation to a broader audience that is interested in these fundamental questions. What are some of the most important or compelling things that you want to convey about the universe to people who are not your technical peers?

Silk:

I would say just a sense of awe, basically. And a sense of beauty. And there's so much out there that we do not understand. So, our ignorance is a source of mystery, too. But all of these things, we have access to one way or another. And so, all of these mysteries are something that we can't quite literally put our hands on, but we can access with telescopes. So, I think it's somewhere you could relax, you could think about the universe, and it just gets you to another space where there are no limits. It's a new experience.

Zierler:

Our conversation has been delightfully devoid of overly technological questions. And so, I'm curious, to what extent has the fantastic rise in computational power over the course of your career been relevant for your research?

Silk:

It's made a huge difference. Let me give you one example. I would say that when I began, in astronomy, we knew about the halos of galaxies. We were pretty sure in the early days they were dark and vast, and that one surrounded the Milky Way. But we had no idea that the halos were structured. Thanks to computer simulations, even the early attempts showed that if you fill the halo with what we call cold dark matter, dark matter in which intrinsically the particles have no velocities, which is the natural form dark matter should be in when it condenses out of the early universe because it cools down so effectively as the universe expands, then this matter's unstable. It fragments and makes lots and lots of clumps. So, the revelation from the simulations was that dark halos are full of clumps, full of substructures. Teeming with them. Thousands of them, many thousands. And we saw this in the simulations. And I don't think any theorists had predicted this.

Zierler:

Another thing to consider, over the course of your career, when we look at cosmology as being relatively young at the beginning for you, would you say that now, cosmology is a fully mature field?

Silk:

I hesitate to say that it's mature because we're still seeking the beginning. So, we can speculate a lot about the end, we have different views about that, where we're going. But the beginning, I worry about. There's a question mark there right at the beginning about the moment when the quantum physics and the gravitational physics have to match together somehow. So, I think until we address that, it's not fully mature. On the other hand, it's reached an area of so much precision, we're discussing percent precision when we ask what is any quantity in cosmology, that the measured precision level is a measure of maturity to some extent. So, it's had a huge change in my time in cosmology.

Zierler:

This is a tall order, of course, getting right to the very beginning. This is to assume, as a hard-nosed scientist, that you think that these things are knowable.

Silk:

I see no reason why we shouldn't have the ultimate theory of quantum gravity. And where that will lead us, I do not know. That will certainly tell us about the origin of time, no doubt. Time might be an emergent phenomenon, for example. Some people speculate that already, but without any real solid proof. But I think it's a well-defined aspect of fundamental physics, and we’re just not there yet.

Zierler:

On the question of mysteries and what's knowable, on the other side of that, surveying your long career, what are the areas or aspects of your research where you feel at the beginning of your work in the 1960s, things were really poorly understood but now are truly well understood?

Silk:

Let me give you an example. I think we've got an amazing way in understanding how galaxies are made. I began my career by trying to form galaxies. And my formation model would be a cartoon, basically, on a piece of paper. Plus a few simple equations. Nowadays, with powerful computers, we can make incredible simulations, and we can produce galaxies that you can't distinguish from the real thing. I can show you the Sombrero Galaxy or the Milky Way Galaxy, the image on a computer compared to the real thing, and maybe the expert can figure out there's a difference. But really, it's just not obvious at all at first glance. So that's incredible advancement. What worries me about this is that in doing these simulations, we're making certain assumptions. We have to somehow span the range from planets, to stars, to galaxies. That's a huge dynamical range. Astronomical units to megaparsecs, basically. We can't do that on the computer. Not yet anyway. We have to make assumptions. Now, on the large scales, we're guarded by observations of problems. But on the small scales, we have a real issue because we don't actually have a theory of star formation, believe it or not. Even though we observe so much of it locally. We have no fundamental theory of this. And so, we have to not only take empirical ideas but mold them into our simulation somehow. So, at the end of the day, the worry with the computer simulation picture of a galaxy is garbage in, garbage out. I don't think we're very far away from that, unfortunately. So, I just find these wonderful models are great for making catalogues that guide observers into astronomy. That’s an incredible change since the early days. But I don't think they're quite telling us yet what fundamentally is going on. How will we go all the way from forming planets to galaxies? I think there are gaps, some missing issues there that, one day, no doubt, we'll overcome as computers get ever more powerful. To me, that's the great challenge at the moment.

Zierler:

Perhaps it's purely a speculative question, which I know you'd want to avoid to some degree, and perhaps it's more of belief than deduction, but if you had to choose one or the other based one everything that you've learned, do you think we're in a multiverse or not?

Silk:

I think the arguments for a multiverse are very slippery. In fact, they've been called the slippery slope for good reason. Because you just extrapolate from the universe you see to the universe just beyond the horizon that you can't see, and so on, and so on, eventually, you end up in the universe you can never ever see. So, I just find that philosophy to be metaphysics, if you like. I just don't think it's anything that we can actually test by using counterfactual arguments guided by anthropic principles that are favored by some of my colleagues. I just don't think that's really part of physics. We are missing something. So yes, if I had to vote, I would vote for the universe we observe, not the ones we don’t. There's just a lot of stuff that we haven't seen yet, but that's fine. I'm not going to change the laws of physics to satisfy someone else's fancy.

Zierler:

For my last question, looking forward, as you've conveyed, there's so much work to do. There's so much that we don't currently understand that could be understood. It's a big field out there. Time is a finite resource. For you personally, what are you most optimistic about being a part of as you continue on this journey of fundamental understanding of the universe?

Silk:

I'm just happy with mysteries slowly being solved, new discoveries of objects that we had not even dreamed, that might've existed a few years ago before eventually being discovered. So much richness is out there in the realm of the telescopes. And we haven't even exhausted the types of telescopes we can build, the parts of the spectrum we can look at, types of particles we can imagine out there. So, to me, it's exploring the mysteries of the universe that I think is the driving phenomenon.

Zierler:

Well, I want to thank you for spending this time with me. It's been great fun hearing your explanations about the universe and all of your perspectives over the course of your career. I'm so happy we were able to do this. Thank you so much.

Silk:

Thanks, David.