“Don’t be afraid if what you’re thinking does not look popular,” said Dr. John Hopfield ’54 H’92 in a conference call with The Phoenix. On Oct. 8, Hopfield received the 2024 Nobel Prize in Physics with “Godfather of AI” Geoffrey Hinton for their work in developing fundamental building blocks of machine learning. Hopfield’s work included developing Hopfield networks, or recurrent neural networks, which are modeled after the structure of the brain.
Hopfield’s academic career and life experience have been marked by his pursuit of answers to questions and solutions to puzzles, by his breadth of work and depth of focus. Hopfield reflected on his view of the odds of winning a Nobel Prize, as well as the experience of the past three weeks after receiving the honor. “I’m having great difficulty getting my arms around becoming the Nobel Prize winner for physics. It just seems so unlikely.” Hopfield recalled, upon opening his email inbox, that he had received hundreds of congratulatory messages. “I had never seen that many emails before in my life … And it was clear that this unlikely event had happened … And life has not been the same since. It’s been wonderful.”
Hopfield’s upbringing stimulated his physics-focused mind. Both of his parents were trained in advanced physics, and encouraged his pursuit of answers to theoretical questions. “I was in a position to get the answers to the questions a child naturally asks, the answers to which were actually part of the fields of physics. So I grew up thinking that everybody understood how the world worked in physics terms. They turned out not to, and there was a career.”
Hopfield’s years at Swarthmore were similarly influential for his life’s work and in fueling his curiosity. When asked of key people who influenced his academic career, Hopfield spoke of William “Bill” Elmore, who taught in the Swarthmore physics department from 1938 to 1974. “At the time, he was the head of a four-person physics department. In high school, I had taken the usual chemistry and physics courses … So when I put down my probable major on my application for Swarthmore, I put chemistry or physics. I went in to see my scheduled advisor, Bill Elmore, who was chair of physics, and … he got out his pen, drew a black line through chemistry, and said, ‘I don’t think we need to consider chemistry.’ He was so right, but that was a decision point. And the other decision point, again it was with him as an advisor, was, he saw where my interest in physics was.”
Elmore helped Hopfield identify his area of focus in physics, and nurtured this interest during those formative years at Swarthmore. Hopfield wanted to do something practical relating to physics, and Elmore helped him figure out in which direction to go. “I didn’t want physics as empty intellectual instruction. I wanted to do some things which were actually useful … I had always been interested in how things worked. So utility was part of my game.”
Not only did Hopfield’s undergraduate experience help him identify his interest in physics with tangible applications, but also, it allowed Hopfield to realize that he did not want those applications to be in nuclear technology development.
“I had found out from my undergraduate work that I really enjoyed working on the physics of the world around us,” he explained. Swarthmore’s liberal arts foundation affected Hopfield’s perception of physics. Yet, he shared that this influence was “more around the periphery,” surrounding his broader perception of the practical effects of studying physics. “For example, in the early ’50s, nuclear weapons were a real issue — in a way which they still are today, of course — but the times have changed greatly. It’s no longer that people don’t understand the physics of how they work. It’s much more of a political issue … which tended to keep me away from the physics of the small, which was going to take me towards nuclear weaponry, and in some sense, as part of it. I didn’t really want that as part of what I did. And so there was the real connection between the liberal arts and physics, over possible applications of what you might do in research ten years in.”
During Hopfield’s time at Swarthmore, he was greatly impacted by a Quaker iteration of academic inquiry: collection. “When I was there, collection was a once a week affair. Thursday, I think it was, classes at 10 o’clock didn’t exist and everybody went to compulsory collection, like compulsory chapel. What happened at collection was, after a minute of Quaker silence, generally a very distinguished speaker from the liberal world somewhere trying to address Swarthmore students.” Hopfield recalled that among such speakers were Patrick Murphy Malin, former director of the American Civil Liberties Union, and philosopher Bertrand Russel. These guests covered a breadth outside of the scope of Hopfield’s classes. “We heard all kinds of one-hour lectures, very broad things, which just were not part of the curriculum of anything in particular. Some people hated it. I loved it. I never took any of the allowed absences. I never cut collection. I always went and found it just engrossing. It was a big part of my education, of my liberal education.”
Collection allowed Hopfield an engaged space for intellectual discussion and inquiry outside of his immediate physics focus. To this end, interdisciplinarity has played a great role in Hopfield’s career. Crucial to the success of his wide-ranging work, Hopfield explained, has been an appropriately broad foundation. New fields, like artificial intelligence, continue to emerge and rapidly evolve. Hopfield said of machine learning, “It’s a field which didn’t exist twenty years ago. Twenty years ago, it was a subpart of computer science, and it’s going to be a subpart of something for a long time, but it’s not a good educational base. It’s not broad enough, in my view, to take seriously as, say, an undergraduate major, [that] you really need to know more broadly, [like] computer science or neuroscience, which are subjects to major in and base your education on. I think you always have to find something with that, with a historical existence, an existence which is likely to be ephemeral as fields mix and intermix and rename.”
Hopfield underscored the significance of a strong foundation, and how it permits wide-ranging academic exploration and intellectual freedom. “Breadth of base is something which you need if you’re going to try to make interesting changes between things which don’t have any apparent connection on the surface. I grew up in the physics tradition. I could still teach [first-year] physics pretty well. I never could be an appropriate candidate to teach [first-year] biology. You need to know the historic lore of a subject to be a good early-on college teacher of a subject. And you will always, to some extent, remain with the label of your undergraduate major, and just be sure to find a fairly broad label. That’s my bias.”
Hopfield explained that his general base allowed him to transition between disciplines and approach questions pragmatically. Elucidating his preference for a strong educational base, and advising students to follow suit, he said, “If you want to approach a problem which nobody knows how to find the answer to, [physics, mathematics, and molecular biology] are likely to provide more of a basis for starting at this, more of a basis looking for other systems which have some analogs to the behavior you’re trying to explain, and to the issue and its target.”
Hopfield received his Ph.D from Cornell University in 1958, where his doctoral adviser was Albert Overhauser. “He was actually a devout Catholic in a physics department, which is something we did not often find, and that put him slightly isolated … And he just very much marched to his own drummer.” Hopfield recounted the story of Overhauser’s most famous paper, about lithium metal and spin resonance. While all of the math was done correctly, the results weren’t intuitive. People did not believe his work was correct. “He was almost shunned for this. He got the paper published. Nobody believed it was true. And then the University of Illinois did the experiments. Overhauser was right, no question about it. And the same people who repeatedly said Overhauser must have made a mistake now said, ‘He’s right, but it’s trivial.’ Overhauser’s paper went from being wrong to being right but trivial in the minds of these few. Eventually it became an accepted part of physics, but it was so unexpected.” Overhauser went on to receive multiple honors for his work on the Overhauser effect. His advising style with Hopfield was centered around posing questions and offering puzzles to solve.
“[Overhauser] was a marvelous thesis professor for me, because he didn’t tell me what to do, he didn’t. He gave me a couple things that were puzzles which he didn’t know how to solve. I took over those puzzles and solved [them]. And that was my thesis. But he never told me what to do. He came in and listened. But I said, I’ve tried this, I’ve tried that. This doesn’t work, and so on and so on. He was a great listener … I’ve begun to treat my best students with the same passion he had, encouraging thinking for yourself about a system.”
Hopfield also cites the late astrophysics scholar Philip Morrison as an influence. “Philip Morrison was at the Los Alamos laboratory during the war, and he was a great liberal physicist, and saw physics very broadly, saw politics very broadly. He was one of the people who got in trouble with the witch hunts for communists in the State Department and so on, witch hunts having to do with Communists, the Communist-sponsored infiltrations into government, into Los Alamos, and so on. And Morrison ultimately, I think, lost out to the Cornell president, and Cornell would not defend his political views and so he left and went off to MIT [the Massachusetts Institute of Technology].” Hopfield spoke passionately of Morrion’s time at Cornell, during which Morrison always had his eyes on the horizon, looking towards “exciting new things like molecular biology.” Hopfield considered his understanding of Morrison’s unique position in the department. “He was the only person in the physics department who really had that kind of guts. And he kept it up for years. He wrote a column in Scientific American while he was a professor at MIT, once again, just because it was the type of thing that interested him. And he was a good example of: you don’t have to be narrowed by your field.”
Hopfield reflected on how his time at Princeton University as a professor of physics affected his relationship to intellectual expression. He went to Princeton in 1964 after working as a researcher at Bell Laboratories and an assistant professor at Berkeley, selected for his expertise in condensed matter physics. “That’s what I did for a while, until I found problems, I was running out of problems to work on, and I wasn’t replacing them with new problems I was finding, and that finally found me looking toward biological questions. That’s not really the purpose for which Princeton had hired me. They had a biology department. They had hired me to be in physics, not [to] be wandering. So at that point, I did wander …where I was between chemistry and biology, and free to do different things, free intellectually, I was no longer in a niche which Princeton put me in and I had agreed to.”
Looking back on his academic career and on his journey from insatiably curious child to Nobel laureate, Hopfield offers a framework to understand the world, aligned with his work in physics. Hopfield has braved the unknown through his life’s work of posing questions, pursuing puzzles, and embodying the courage to defend one’s convictions. He teaches Swarthmore students and the world alike to embrace the beauty of the interdisciplinary and to dive fully into the questions that may arise.
“As I’ve gotten older, I’ve gotten more secure in my own judgments and I can get away with knowing less because I make the right choices … wandering from discipline to discipline. But if you want to do very interdisciplinary things like Overhauser, you have to get used to the fact that people will say you’re wrong, and you have to get used to the fact that you either defend yourself under those circumstances, or if you can’t, you probably are wrong and you must accept it. It’s a tougher position to be in. I wander.”
Carefully written, like Hemmingway. I hope the Swarthmore I enjoyed years ago still vigorously lives.
Concerning the writing of non-fiction, Ernest Hemingway remarked,
“If a writer of prose knows enough about what he is writing about he may omit things that he knows and the reader, if the writer is writing truly enough, will have a feeling of those things as strongly as though the writer had stated them.” (E. Hemingway, 1932)
The PNAS length limitation forced me to be highly selective in what was said—and what was omitted. Had Hemingway been a physicist, he would have recognized the style. In hindsight, the omission of the almost obvious probably increased the impact of the paper. The unstated became an invitation for others to add to the subject, and thus encouraged a community of contributors to work on such network models. Successful science is always a community enterprise. You managed very well. Thank you. John