Q&A with Professor Emeritus James Peebles GS ’62, 2019 Nobel Laureate in Physics

James Peebles GS ’62 was awarded the Nobel Prize in Physics on Oct. 8 “for theoretical discoveries in physical cosmology.” Peebles’s innovative and original work on cosmology has fundamentally changed how people understand the history of the universe. In addition to being a leader in his field, Peebles is the Albert Einstein Professor of Science, Emeritus.

His profound contributions to his field have previously been recognized with prestigious honors, including the Crafoord Prize in 2005 and the Shaw Prize in 2004, among many other accolades. His new book, “Cosmology’s Century, An Inside History of Our Modern Understanding of the Universe,” will be released next year.

Peebles sat down with The Daily Princetonian to discuss his career, teaching philosophy, and the next great mystery he is excited to investigate.

The Daily Princetonian: First of all, congratulations on your well-deserved Nobel Prize in Physics. Now that you have had some time to process the events of last week, how are you feeling? What is the significance of this award to you and your field of study?

Professor James Peebles: I am still a little numb. You know, I’ve been retired since the year 2000. I’ve kept working but at an ever-slower rate — more relaxed. I’ve enjoyed a quiet life until that phone call a week ago, last Tuesday. The time since then has been wonderful, but just crazy. So, the first feeling is, well, as I say, slight disorganization. I’m, of course, deeply proud.

I have been working in this field, cosmology, since 1964 — 55 years. I’ve seen it grow from a very small field. It was a real natural science then. There were observations, there were theories, there was work to try to bring the two together, but at the level of activity it was miniscule. It has grown, at first very slowly since the mid-1960s, grown with increasing speed over the last 20 years, and now has reached the status of a well established natural science. You pause to consider that that’s a remarkable extension of well established physical theory, from the smallest scales probed by CER-N [European Organization for Nuclear Research], to the immense scales of our observable universe, quite a triumph. So I’ve been working on this field for all that time. I’ve made many contributions through the years.

I hate self-promotion, and it’s quite uncharacteristic of me to say this, but I think the Nobel Committee made a good choice.

DP: Going back to the beginning of your career, was there a moment when you knew you wanted to become a physicist? What attracted you to the field, and what has kept your focus specifically on cosmology over the years?

JP: I learned that there is the profession of a physicist, and that you could be paid to do it, at the University of Manitoba as an undergraduate. I owe a lot to that place. The faculty in the department of physics there — my fellow students — taught me what physics is really about. They showed me a lot about how to do it — that’s the faculty and also the students. We learn a lot from our fellow students. My fellow students introduced me to my wife; they saw us marry and shipped off to Princeton.

My most important teacher in the faculty, Kenneth Standing, in my junior year, started talking about how I would go to Princeton to do graduate work. It was never a feeling of suggestion about it, but rather simply a statement. I had no reason to resist, so off I went to Princeton, knowing that I wanted to do physics.

When I got here, I soon ran across Bob Dickie’s research group and I soon after that saw the kind of physics I wanted to do is what he had started doing. It just fit my style, my instincts, and it was wonderful. I started working in cosmology a few years after I joined the group; that was at the suggestion of Bob Dickie. He gathered three of us into the attic of the old physics building, now the student center [Frist Campus Center]. He brought together David Wilkinson, Peter Roll, and me — three young members of his gravity research group. He explained that it had occurred to him that maybe the universe started hot and dense and expanded. His reasons for thinking that are kind of confused, but the guy had spectacular instinct.

He said to Wilkinson and Roll, “Why don’t you build a radiometer to look for this radiation, if it’s there?” And, he said to me, “Why don’t you look into the theoretical implications?” That set my career; I’ve been working on it ever since. It set David Wilkinson’s career. We, alas, lost him to cancer far too soon. He was on the faculty here, too. That was his career. Peter Roll escaped — he went into education and [put] computers into classrooms. But, nonetheless, you pause to consider how important a simple remark can be; [it] can change the world — at least a little fraction of it.

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DP: You have previously noted that you have a “preference for under-appreciated issues” in terms of selecting your work with cosmology. What did you mean by this, and what is it about these kinds of issues that draws you to them?

JP: Certainly when I started working in cosmology it was an under-appreciated field. A lot was going on in physics then in other branches — particle physics, condensed matter physics, even biophysics. Cosmology was, well, I should say the word, dismissed as “empty” or, close to it, “speculative.” “Why don’t you do something more meaningful?” I’d never had that said to me, but I had the distinct feeling that, well, better I should look into cosmology than them. It’s just so speculative.

I was very fortunate that, in fact, the field was under-appreciated, so I, as a junior person, could first get to know the handful of people doing meaningful research in this field — try that now — and I could embark on projects without much thought of competition. I guess that helped me develop the habit of working in fields in which there was not much competition, but it also goes along with my instinctive feeling that I would rather not join the crowd. If you join the crowd you can do very meaningful research, but it will be part of a group. That’s great; it’s very important that people do this.

But you tend to be stuck along a given direction. Often that direction was chosen because it was obviously the good one, and it then often works out that, yes, it was good. But you do, in operating that way, miss the chance that you have taken the wrong direction. Maybe not totally wrong, but maybe there are things to be thought about that haven’t yet been much discussed. And that has been the direction of my thinking since the early ’80s. As the theory I was introducing became popular, I became uneasy because it didn’t seem to me all that obvious that my ideas were right — they were just thrown out there.

Cosmology is now big science, and that’s great. Very meaningful research is being done by lots of people, most half my age — great. I’m not about to get into that. I couldn’t add much. For one thing, I don’t like computers … I am happy as a heckler these days. Let’s start in the early ’80s, and my habit in that direction has continued ever since. I have nothing to prove, and I have [a] deep fascination with the world around us, with cosmology, so good, it all fits. I have a happy life.

DP: Can you elaborate on those feelings of uneasiness you mentioned you had when you first started working on cosmology? As a leader and pioneer of your field, have you had to deal with any challenges in paving the way for others?

JP: The uneasiness is simple to explain. The theory of the evolving universe was then very little supported by hard evidence. It’s to my taste to look at the interface between theory and practice. That’s what made me uneasy. The challenge — there was not a challenge, really. I don’t remember ever being criticized for working in cosmology. I did notice undertones of wonder why I’m going in that direction, but no one ever said to me — including ... the senior faculty in this department — maybe you should do something more useful. No one ever said that, isn’t that remarkable?

So no, I did not find challenges at all. I felt uneasy only because of my instinctive desire to see that interaction of theory and observation, which is so magical, to think that you can make up ideas and then design experiments and see that the idea agrees with the measurement. And of course you know that practice is at the heart of physics and gave us these devices which are such a magnificent demonstration of how scientists and engineers can make nature operate according to its bidding, push around those little electrons, shape them in these little fluid molecules — it’s magnificent. That’s science. I’ve had the wonderful opportunity to do a little of that.

DP: As for your work that was awarded the Nobel Prize, how would you explain to someone unfamiliar with the field of cosmology why this topic matters?

JP: Princeton University’s mission has two parts: the communication of information and the accumulation of information. The first is easy to justify. We teach you, you go out and do good things. That’s good, that’s firm, explicit, and obviously useful. The second can be questioned. Why are you spending time and resources on curiosity-driven research? There is the very obvious answer. Some of that curiosity-driven research is going to pay off big.

Consider the Nobel Prize for lithium-ion batteries. You’ve seen the students roll around campus on those little scooters powered by a small battery underneath, it’s incredible. I’m not saying it’s a good thing always, and I think those scooters are — well, I think they are lovely, but I do fear that someone is going to run into something someday.

The other argument for curiosity-driven research is that we are satisfying the hunger to know where we are. I feel that that same hunger is satisfied by literature and poetry. Am I wrong that we all want to know what is our place in the world, in society, in connection with our friends in neighbors? And we discover that in part through beautiful poetry, literature, and, I insist, curiosity-driven research. We find that we are not alone in this solar system; there are others.

You pause to consider that the other part of the physics prize goes to the discovery of planets around other stars. Those planets are so far away that it’s a good bet that we, the human race, will never go visit them. It’s a good bet that fascinating things are happening on those planets and we’ll never know about it. Does that please you? It does me. We are never going to see what the universe actually does as it continues expanding. Never, I guarantee it. Does that interest you or distress you?

A member of our faculty, now alas passed, Rubby Sherr, used to on occasion talk to me about his research in nuclear physics and he would say, “isn’t that neat?,” and that phrase is just right to me. The evidence that the universe is evolving is at the same time profound, and isn’t that neat that we know such things even though that knowledge is never going to pay off in any material way, like putting food on the table?

DP: In addition to contributing to scientists’ understanding of the history of the universe, you have always kept teaching as part of your career. What is it about teaching that grips you?

JP: I think this is simply a result of my love of physics. Since I was an undergraduate, physics is, for me, neat, and I like to tell students about it. I think my enthusiasm is reasonably contagious. I certainly have profited from a technique that I didn’t consciously develop, but just somehow naturally evolved. I am in the habit of making wisecracks during lectures, and it has a miraculous effect: the students giggle, and they wake up!

I also, well, I guess I feel a little responsibility, I want to be prepared when I go into a classroom, so I do take care to remind myself what I think. Also, I hate the prospect of being asked a question that I don’t know how to answer, so I try to avoid that. The product seemed to work. And of course there is reinforcement there, isn’t it? My style grew in response to the students’ reactions to it, that’s part of the way we evolve, of course.

DP: Are there any wisecracks that went over especially well in lecture?

JP: Well, I do remember one that students always loved. I was teaching, on occasion, hour-and-a-half long classes. That’s a long time, and my habit used to be to break halfway through. And because — you’ve noticed I’m kind of skinny — I tend to get hungry so I go down to get an ice cream bar … This was particularly when I was teaching quantum mechanics, and you’ve got to pay attention there, so I would say, “Now I’m going to do something really difficult, I’m going to continue to lecture while eating this ice cream cone.” They always laughed.

DP: You have built a career and reputation based on generosity and a willingness to share and collaborate with others. What advice would you give to students as they navigate college and begin to take steps into the professional world?

JP: I can only make the most general of comments. Each of us finds our way, and each of us being a different, truly unique individual we will have a unique road to travel. So travel it well, with care and an open mind. Another comment I always make at this point is don’t judge your career by the count of prizes and awards received. Because, although I think this Nobel Prize is wonderful, awards and prizes are capricious. It’s wonderful to receive them, it’s not an insult not to receive them, it’s not a demerit in any way.

DP: What do you mean when you say to travel with an open mind?

JP: It’s easy to get locked into a line of thinking. It’s important to do that when you are doing research, you’ve got to focus. But a tight focus means you have blinders on, and so it certainly helps and is important, at least it has been important for me, to stand back and on occasion ask yourself, “Why am I doing this, why don’t I look around?” Most of the time, in my experience, you will receive the answer “Because there’s nothing much else to do in this line of research, nothing that looks promising.” But on occasion, you might find yourself answering yourself: “Well, there is something to do, so why not try it?”

DP: Your career has spanned several decades and your work has directly led to some of the most exciting developments in our knowledge about how the universe works. After such an impressive career, what is next for you? Are you working on anything now that particularly excites you?

JP: I have just completed a book on how we got to where we are in cosmology in the past century. It’s a scholarly book; it’ll have equations. It’s now at Princeton University Press, I get the copy-edited proofs tomorrow. That book is, at the same time, an attempt at a history — I’m not a historian — but my attempt in that direction, and my personal journal, my autobiography. They are mixed up, so I switch often from the impersonal historian to the personal … It should be published in the spring … This is not a book that is going to make any money for anyone, but it will please my colleagues. And it is aimed for students who are interested in physics, and I hope that will work well. After that, well, I look forward to getting back to a peaceful life. I intend to keep doing research always these days and for sometime, on the fringes, where I feel most comfortable.

DP: Is there anything specific you are hoping to explore?

JP: Galaxies are, to me, enigmatic. They have properties that are surprising within our current thinking about how galaxies form. We have a theory based on the theory of the expanding universe. That theory can be used to set up conditions to study how galaxies form. These studies are able to model objects that look a lot like galaxies, it’s impressive. They’re certainly on the right track, more or less.

But there are properties of the simulations that are just quite out of whack with the observations, as I see them, of real galaxies. That may be, simply, a result of the complexity of how galaxies form, or just possibly, it’s a result of the fact that the standard theory of the expanding universe is incomplete. We have that hypothetical dark matter. In the standard theory that dark matter is as simple as it could possibly be, an ideal gas, collision-less. It’s natural to try that first because it’s simple. So far it has worked quite well.

But you always are looking for discrepancies that might be pointing to the need for a still better theory or example of the dark sector. You’ve maybe run across the so-called Hubble constant tension. It is a discrepancy between some sets of measurements and others, the discrepancy arises through the application of the standard theory. Maybe that’s telling us that the standard theory is not quite right.

I’m sure it’s not quite right, it’s got to be more interesting than it is now, it’s got to be more complete. Maybe there too there is a hint to a deeper theory. That’s fascinating, but to me the properties of galaxies are so much more rich that they must have a lot to teach us about the way they formed. And that may have some hints to how we can find a better theory. So, on my desk is a debris, undisturbed since Tuesday before last, of plowing around through the literature for properties of galaxies that seem to me to be fascinating.

I will enjoy plowing through the literature, and the little “a-ha” moments where it’s not quite “gotcha,” but it is, well, you know you didn’t really build a good theory for this property of galaxies. I’ve been doing that for some time and I enjoy it. I started doing this in the early ’80s when popular estimates of the mass density of the universe seemed to me to be wrong. We needn’t get into the details, but I can just say I enjoyed going to conferences and lecturing people on why they had the mass density too high. Turns out I was right. But at the time, it was fun to go to conferences and make unpopular arguments that I felt could be defended — you don’t make an unpopular argument that’s silly. It annoyed some of my younger colleagues. I remember one saying to me, “You’re just doing that to annoy us.”

And I have to admit, I did have a little pleasure in annoying them. It’s the same game now with the properties of galaxies. I don’t go to nearly as many conferences, but when I do I point to these curiosities. Well now, as then, there was a tendency for them to say, “Well, there goes Peebles again,“ and then forget about it. That made my comments always seem kind of fresh, because they had forgotten that I was complaining about this, which is fine, that’s the way it should be. But when I made the remark much earlier about not wearing blinders all the time I had in mind just this sort of activity. Question authority.