An Interview with Prof. William G. Hoover

The Scholars’ Avenue recently had the opportunity to meet Prof. William G. Hoover, an eminent computational physicist from USA, and his wife, Prof. Carol Hoover. They were on campus for teaching a course on ‘Nonequilibrium statistical mechanics and molecular dynamics’ offered in GIAN during 5-16 December 2016. Dr. Prasad Matukumilli, a student of Prof. Baidurya Bhattacharya took the interview.

Here are some highlights:

TSA: Can you briefly tell us about your family and background?

WH: Well, my father was an economist. He taught at Harvard, the University of Michigan, Carnegie Mellon, and Pitt. His main interest was classical music, outside of his academic work. He built about 30 string instruments, ranging from small viola da gamba to a large Tromba marina, which looks the Eiffel tower in general shape. It just has one or two strings, and it’s about seven feet high. My mother was a social worker. She was particularly interested in unwed mothers and worked for several years in a place that took care of them. She did not go to Harvard, but she went to Radcliffe, which is now combined with Harvard. She and my father graduated at the same time, in 1928. They met in a rather curious way. My father was walking through Cambridge, Massachusetts and he heard some beautiful violin music. Curious to know where it came from, he walked up to the house and knocked on the door and met my mother, So, one thing led to the other and here I am.

TSA: Could you describe your education and your student life in general?

WH: Well, I went to high school in Washington D.C. I had three years at Sidwell Friends, where Barack Obama’s children have been in school. Then I moved to the public school, Woodrow Wilson where I had a very good Chemistry teacher. It was there that I decided to pursue a career in science. I really never looked back from that point. I went to Oberlin College, which I would not at all recommend anymore because it has become distressingly liberal. My thoughts are quite opposite of those of that college. I did graduate work at University of Michigan, Ann Arbour, in the fields of Chemical Physics, sort of in between Chemistry and Physics. So I was moving to Physics at that time. I had a post-doctoral year with one of John Kirkwood’s students at Duke University, Jacques Poirier. And I read an article in the Scientific American magazine, by Berni Alder and Tom Wainwright on molecular dynamics. A 1959 article called molecular motions left a strong impression on me. I really wanted to generate pictures like that myself. So, after that postdoctoral year, I went to Livermore, California. I also interviewed at Los Alamos, but the salary at Livermore was twenty percent greater than the Los Alamos one. I thought they both were very good places, so why not follow the money? Another fringe benefit of Livermore was the opportunity to teach at the Larry Clowton University of California. One of my students was Carol (his wife), so I am really happy that I made the choice to go to Livermore instead of Los Alamos. And somehow or other, that has brought me here. Just by the good fortune of the internet, I got acquainted with Puneet and Prof. Bhattacharya through their work published online.

TSA: So, when did you know that you wanted to be a physicist in particular?

WH: Probably about 1957. I was around 23 years old.

TSA: Who influenced you the most, when you were young?

WH: Probably my father when I was very young. He taught me Calculus at the dinner table which was very valuable to me and we always had some good books in the house, like Mathematics for the Millions and One two three … Infinity by Gamow. So, the background at my home was very academic, that probably helped me to go in the direction of research.

TSA: Moving on to your work, you have developed the Nose-Hoover thermostat for simulating molecular systems at a constant temperature. Can you briefly explain why it is so important?

WH: Well, I think natural processes occur isothermally and isobarically. There’s mechanical equilibrium when the pressure is everywhere about the same and thermal equilibrium when the temperature is everywhere the same. So if you like to look at what actually happens to complicated molecules like proteins, it’s very useful to do the simulation of those things that correctly incorporate the fluctuations due to the material being in a thermal bath. So I think the thermostat, that at first something looked like interesting mathematics, turned out to be very useful indeed and several people have got Nobel prizes for their work in biomedical problems, which has used that type of thermostat in their research work. So it’s really very difficult to predict and project the consequences of fundamental research but in this case, it seemed to be something useful, which made me very happy.

TSA: Can you please describe three of your major contributions to statistical mechanics or theoretical physics?

WH: Only three? (laughs) Okay, I suppose. I like very much the reformulation of Mayer’s virial series, because Francis Ree and I were able to introduce a new kind of diagram that was a little more specialised than Mayer’s that made the work of computing the virial series a lot less arduous; made it possible to go from a five-term series to a twelve-term series. And so that was certainly something I was very pleased of. I guess another thing was that I noticed, looking at the virial series and the corresponding expansion for the pair distribution function that if you look at the function with the Boltzmann factor missing, you can calculate exactly chemical potential- the work that it takes to insert a new particle into a fluid or even a solid- directly from the extrapolated radial distribution function. That’s called the Wilhelm method, but actually, we did that a year before Wilhelm.

TSA: More recently, your active research area has been the smooth particle applied mechanics. Can you please tell us about that?

WH: It’s a way of doing continuum mechanics with particles and it’s a lot simpler because the equations are ordinary differential equations.You don’t have to calculate spatial gradience to solve them. I saw a fellow, in the theoretical division of the Livermore laboratory where I worked, who knew about smooth particle mechanics. I asked him to teach a couple of my graduate classes at the Department of Applied Science. He gave two lectures on smooth particle mechanics and I liked them very much. So I spent about ten years working at smooth particle problems and discovering that the interpolation method, namely using a weight function to calculate many-body averages of field quantities-was useful for molecular dynamics and for Monte Carlo simulations as well. So there’s a fringe benefit of the cross-fertilization between the continuum mechanics and molecular dynamics made possible by the smooth particle weighting functions.

TSA: What are your views on India and the Indian culture?

WH: Well, Indian culture appears to be, you know, tens of centuries, thousands of years, like China. So I’d say that’s a tremendous amount of historical background and intricacy, exemplified maybe, by these elephants and the proud fellow in front. It’s very great to have a tradition like that, one that goes back for a long time. United States is just totally different. A little more than a couple of hundred years ago, it was inhabited by Indians, you know, different type of Indians. So, we don’t really have a history like that. I can simply admire it. It’s a little bit like reading the Bible, staring at the Pyramids or looking at the Great Wall of China. What I like most about India, I think, is the people. They are so friendly, and nice and welcoming. They have such good food and such good science. I won’t say anything about Ramanujan, but we are so happy to be here. My father visited in 1950 for several months, so I am glad to see what brought him here. I am very delighted to be here.

TSA: A paradox of our times is that, as society tends to depend on scientific and technological advances as never before, the interest in basic sciences is diminishing. Do you find it alarming for the rapid advancement of science? What do you say to a young undergraduate to make him take his career in science?

WH: Well, you showed me yesterday at the museum a really nice quote of Nehru, I would like to get the exact wording and use it in future when I think back on this visit because it described the value of science to the world, “ The fact that we can’t really expect to survive as a human race unless we keep pursuing scientific answers to our questions.” I think for a young person, the main idea is to follow what you find to be interesting. You know, if you are curious about something, or it doesn’t make sense to you or you get excited about it, I would simply follow that. I have been very fortunate myself for probably 50 years or so to follow the research direction that seemed most interesting to me. I very much enjoy the academic career where you don’t really have a person directing you what you must do. You choose your own way, so, if you can manage that I think it’s probably the best way to live.

TSA: How do you see the progress of science in India or Asia comparison to other parts of the world? Do you see any uniqueness about Indian science?

WH: I don’t really know that much about Indian science. I guess I may not run into many very successful Indian scientists without coming to India because there are many in the United States. I think I met some at the University of Michigan in graduate school and some in Duke University during my postdoc. Yogendra Gupta is probably the one I knew the best, who was department head of Physics at Washington State University, looking at shock waves and running a shock dynamics laboratory. So I think Indian science is very good.
The GIAN program bringing in people to foster collaborations with other countries is a very positive step.
For me personally, travelling to places like France and Germany and talking to the scientists there really stimulated my imagination and allowed me to be able to do things that would’ve been impossible otherwise. So, this particular program that facilitates the movement of people back and forth is quite valuable. You know, knowing first Puneet and then Prof. Bhattacharya and then the rest of you has been a very pleasant experience for me and very stimulating.

TSA: Any comments on administration in scientific institutions in general, on the importance of bright people to take up important chairs a single person can make a significant change?

WH: As long as they’re willing to do it, I think it’s a sacrifice. It’s quite amazing actually. Not everyone is willing to make that kind of sacrifice. Some people stay in academia, they don’t aspire to be the department chairman or Dean or Chancellor or President. People like Willard Gibbs, who had a huge influence on science, never took up an administration position. They’re happy doing their research. So you can serve humanity, either way, doing your research or climbing the ranks and directing the funds to others doing research. Dimitri Kusnezov was an eminent researcher at Yale for about 15 years but went on to take up a position in the Government as an administrator because he felt that by distributing research grants, he’d be more effective. It was a very selfless act. Some do that, some do not.

TSA: There are many instances in history where couples have done great science like Pierre Curie and Marie Curie (Nobel 1903), May-Britt and Edvard Moser (Nobel 2014), Gene and Mildred Dresselhaus etc. Any special advice to couples working in science?

WH: Anyway, it’s almost too late by the time you are a couple because usually people meet and there is something in the hormones, racing around their body that causes you to fall in love with somebody else. They might be a biologist or they might be a sociologist or they might be a photo technician or something; you really can’t predict. If you are lucky enough to meet somebody in your classes and fall in love with them and marry them, and they are interested in the same thing as you are, then it’s a really spectacular nice thing. It’s extremely rare, I think. Joseph Mayer and his wife Maria Goeppert Mayer are really an excellent example of that. There were a couple of organic chemists at Harvard also that published several books together, in that same situation. So it’s a nice thing but unless you work very hard at it, it’s very difficult to accomplish. Anyway, I wish good luck for all those alliances. I see there are not a lot of women in the room, there are some.

CH: Do you want me to comment? Well, first I’ll like to comment that it was 15 years after the class that we met and decided to get married. It’s a joy to be able to share your research experience with your partner. So, I’ll recommend it to those who fall in love with someone of the same field. It is also useful because you can understand each other’s work, where in other marriages they may be quite different in their fields. So, I think I’m lucky.

WH: Yeah, me too.
[Laughter]

TSA: There is this saying by Oren Harari “The electric light did not come from the continuous improvement of candles.” In that context, what are your suggestions to young researchers in attempting problems from a uniquely different perspective?

WH: You are saying that people get tired of what they have been doing and would like to do something, a little different. Peter Debye, who was invited to a dinner with my thesis advisor and me at the University of Michigan during my graduate years gave me advice on that question which I think was very good. He said, simply go to the library (You can now go to the internet, you don’t have to go to the library) and start to read and when you find something that doesn’t make any sense to you and seems like it may be wrong, that’s a good thing to study and pursue and try to understand. I found that to be very good advice. I found a couple of things at school that way. In graduate school, where people made mistakes on pretty tricky problems; by thinking about those problems some more, I got interested in various theories and as a result, I was able to make some contributions there. But, that wouldn’t have happened, had I not gone to the library and started doing some reading.

TSA: In the campus, almost half of the students are Ph.D. research scholars. Any specific suggestions to them when they’re just entering into some scientific career on how to formulate a research problem and how to pursue it at the early stage of their career?

WH: Yeah, I guess the idea is to try to do what Thoreau suggested, I think his most famous quotation is just a repetition of the word simplify, he said- “simplify, simplify, simplify.” So if there’s a problem you think is interesting, it seems to be useful to create a model that is as simple as possible, to try to understand it from a fundamental standpoint. It seems to me that if you are unable to do that, then probably it’s not the right problem for you; you should find something else to do. The idea is to cut away some clutter and try to develop some new insights into the problem, and then give those to other people. I’ve heard that sentiment expressed by a lot of scientists, and I think it’s pretty good advice.

TSA: Dr. Carol, would you like to add some advice to people, how they might promote their Ph.D. research?

CH: Love what you’re doing; that’s number one. Number two, he said to simplify. Will is well known for working on very small problems, like two and three particles, and gaining a step forward in understanding and his colleagues often kid him about it – “oh he’s gonna use another small system.” But I think there is a lot of merit in it. But I think also that if you’re excited about the problem, then you’re gonna wanna do it, you’re gonna want to spend the time.

TSA: We’re coming to the last few questions. As Einstein said, “God doesn’t play dice with the universe.” Are you a critic of quantum mechanics in some way, because your research completely involves classical approaches. Do you like to treat problems of nature from the quantum mechanics method as well?

WH: Yeah, I don’t really know if there is a quantum mechanics perspective. Because it’s rather complicated you know, it’s not like, springs and dashpots and collisions or that sort of thing. It’s a complicated calculus of probabilities and big matrices. It seems to me that if you’re interested in spectroscopy or the structure of the nucleus, problems like that, then you’re forced to use quantum mechanics. But most of the world, to us, because we happened to be composed of ten to the twenty four-twenty-five atoms, is on a totally different scale. We see things around us that obey classical mechanics to a very high degree of approximation. So for me, the quantum mechanics isn’t really that interesting. I went through courses in the two slit experiment, all of the various hydrogenic orbitals around the nuclei, black body radiation, all of that stuff. For me, it’s just not particularly interesting because it doesn’t have much to do with what I see around me in my day to day life. So, I don’t mind quantum mechanics at all, it’s absolutely essential to electronics and so on. Einstein did some classical stuff and some quantum stuff, so I certainly admire him; I am not even sure whether there is anybody rolling the dice or not. That’s another difficult question.

TSA: In 1985 Physical Review A paper, you published the modified set of the equations of motion that can generate canonical probability densities in the phase-space. How would be the journey after such a triumph? I always wonder how do accomplished people in science feel after they solving big problems. What are the reflections on how they feel at the other side of the mountain?

WH: Oh I see, ya. Well, that’s interesting.When I first read Nosé’s paper in 1984, I really couldn’t understand them; the idea of time scaling was unfamiliar to me, I hadn’t done much of Lagrangian mechanics at all. So I thought very hard and I wrote down a list of 12 question, and I took those 12 questions to Paris for I knew Nosé would be at a meeting. And I met him by accident 3 or 4 days before the meeting started; so we made an arrangement, to sit on a bench in front of the Notre dame cathedral and talk about his mechanics and the questions that I had. We spent several hours doing that. That was exciting for me so I went to Lausanne where Phillip had a laboratory. He connected me with a fellow that knew how to make computer graphics and we started pulling out trajectories. And seeing that the trajectories were not really ergodic they didn’t go everywhere. They seem to make toroid instead, that was really exciting for me. Then off to Vienna, meeting with Harald Posch and France Wesley and discovering the fractal nature of the distributions, that was quite exciting too. So in a way it was kind of an emotional rush all the way along. There wasn’t really anything about getting to the other side of the mountain at all, it was just the excitement of going up the mountain. And luckily enough Nosé’ invited us to come and study with him in Yokahama. So we were able to do that and learn a little bit about the Japanese culture and make some very good friends there. It all seems to be a natural progression, I don’t think it’s an up and down, it’s just an exploration of the infinite universe around us and, opening up new possibilities. It’s exciting as I think you know.

TSA: The last question. Even after retirement, you work so hard every day and are still publishing and still motivating young scientists with a lot of energy. What is the philosophy behind that?

WH: It’s just very selfish you know. It’s an exciting and pleasant way to spend time, particularly if you can do it with someone you love. Why would I want to do anything else? I can’t think of anything that would give me more pleasure. If I could I’d probably do that.

TSA: Thank you, Professor Hoover.

WH: Oh, thank you so much; Those were quite moving questions.

Prof. William G. Hoover and Prof. Carol Hoover

Prof. William G. Hoover and Prof. Carol Hoover

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