Daniel Kleppner

INTERVIEWER: Professor Kleppner, thanks very much for taking the time to come talk to us today.

KLEPPNER: Happy to be here.

INTERVIEWER: Let's start at the beginning. Tell me a little bit about where you grew up, how you grew up, your early years?

KLEPPNER: Well, I grew up in the suburbs of New York City. I lived in a little town called New Rochelle. And I had a sort of a normal, but a very happy childhood. My family had no particular background in science or engineering or mathematics, although I had an older brother who liked mathematics. But when I was a kid, I liked to build things. And I built lots of things, electronic devices. I sailed. I learned how to do woodworking, which has been a hobby of mine through my life.

And then in high school, I had a wonderful physics teacher, Arthur D. Hussey, and he kind of reinforced my interest in physics. So by the time I graduated, I had a very well-defined interest in physics. And it's certainly very helpful if you know what you want to do at an early age.

In those years, physics was extremely fashionable. These were the post-war years. And physicists were considered heroes for having brought the war to a quick conclusion with the atom bomb. And more importantly, although it wasn't as well known, for the development of radar. So physicists were public figures then. And it was easy to be carried along in this spirit of enthusiasm and appreciation for science. So that sort of the background when I went to college, which was Williams College.

INTERVIEWER: So tell me a bit about your parents? What did they do? And presumably they were supportive of this interest that you had?

KLEPPNER: Well, they were always supportive of it. And they were proud of it, but they didn't know very much about it. My father came from a Viennese family which immigrated to the United States around 1906. So he grew up really pretty much in poverty. And he went into the advertising business and ran a little advertising agency, a Madison Avenue advertising agency, although all my recollections have little to do with Mad Men, which is very popular series on that right now.

But he was a very kind and generous father, not only to us, but he helped bring in many relatives from Germany during the '30s. And was someone who was very well-regarded. But his education had been really pretty well focused. He did not have broad intellectual interests, although he was certainly an intelligent person.

My mother grew up in New Jersey and went to Barnard College. She was a lovely woman and a delightful person in every sense. And she was widely read, had a wonderful sense of humor, and sort of made our home a happy place. So we grew up, we being my older brother, myself, and my younger sister, in a home which was really a very happy home.

INTERVIEWER: So you mentioned the way that physicists were treated as heroes or celebrities at that point, not too long after the close of World War II, which must have captured the imagination of a bright young high school student at the time. Do you remember what kinds of problems or puzzles or things like that really captured your attention as a teenager?

KLEPPNER: Well, as a teenager I just liked building things. And that turned out to be very useful when I went on to become an experimental physicist. We'd build mechanical things. I did not build a telescope. I did build several rowboats and I had a sailboat. So that was a very important part of it.

And I had a crystal radio. So I could listen to radio over earphones. And the thought that the signals were just coming out of the atmosphere, I remember thinking totally remarkable. And actually, I still do. In fact, the idea of the electromagnetic field, although it's very well understood in physics, always seems like a miracle to me.

I had a close friend. We liked to build devices. One device I built was a slot machine. It was a box. And there was a window on the front of it. It was filled with nickels. And there was a little cup underneath it. And there was a lever and a place where you could put the coin in on the top. And there was a sign which said, "Nickels only." And we left that in the classroom. And you put in the nickel and pull down the lever and it lights up and says, "Thank you." So that's the kind of prank, a very benign, sort of harmless, humorous prank. But that's something that we took great delight in making.

INTERVIEWER: So you mentioned that you attended Williams College. Tell me a little bit about your undergraduate experience and how your intellectual interests developed during that time?

KLEPPNER: Well, I had a wonderful undergraduate experience. Although for reasons I'll explain, I actually left college early. When I went to Williams, I liked physics, but I wasn't all that sure about my abilities in it. In high school I was a good student, but not a star student by any means. And so I was uncertain as to how I would do.

Well, it turned out during my first term at Williams, I did very well in physics, which pleased me and surprised me. So that was a very nice, positive experience at the beginning of college.

What was even more important for me at college is I came across a number of teachers were very supportive of me. I went to Williams with an idea in mind that I wanted to build a device, sort of a forerunner of cybernetics. I just wanted to show that you could make a machine which could do anything, a programmable machine. Now, this was before the days of digital electronics and such. So in that sense, it was sort of an advanced idea, but what I was making was sort of a mechanical kludge.

And the physics department there gave me a little laboratory space in which to do this. And one of the teachers, Howard Stabler, who was quite expert in the electronics, spent a lot of time teaching me about vacuum tube electronics, so I could use vacuum tubes rather than mechanical relays in the device.

And this was very important to me, just doing this, all on my own. I remember the pleasures of working there late at night, sometimes over vacation time. This joy of just making things work, all by oneself, was something the college gave to me. So I was very appreciative of that. And the other teachers were good also.

What wasn't so good about Williams for me was the social atmosphere. At the time, it was something of a playboy school. It had that reputation. And I think the reputation was justified. Life there was--- nearly everyone centered on fraternities. And the fraternities were very cohesive, social institutions. I felt that they were archaic and in a sense anti-intellectual. And I know that many of the faculty agreed with me.

So I went to speak to the president of Williams in my sophomore year because it seemed to me that they must recognize this problem and were planning to do something about it. Well, the president of Williams, James Phinney Baxter III, to his credit, immediately made an appointment with this freshman who just wanted to come in and talk with him. But it turned out that he thought that these were the soul of Williams College. So there was a real misalignment of interest between this freshman or sophomore and the college administration.

So I thought I ought to leave, on principle. And actually my professors that I talked with sort of understood this and agreed with it. And we figured the best way to leave would be to graduate early. So I accelerated and graduated in three years, rather than four years. And then I got a fellowship to Cambridge University. So I went to Cambridge University for a couple of years to make up for my slightly truncated experience at Williams College. So that worked out very well.

I should mention also at Williams, an interest which I did not have before going to Williams, which really flowered there, was an interest in English literature and poetry. I had not really been exposed to that before. And that was very exciting for me. So I not only finished the physics major, I think I may have technically completed an English major also.

So looking back, I was really very busy in those years. And looking back, everything is in a rosy glow. Of course at the time, one is always suffering from anxieties and worries. But the nostalgia is really a very-- well, nostalgia is dangerous. But it's a very nice practice to indulge in. So I'm very nostalgic about my undergraduate experience at Williams College.

INTERVIEWER: So you mentioned that you went to Williams. I guess, first of all uncertain about exactly how you would do in physics. And secondly, with just a very strong desire to build things. But you came out of your undergraduate experience at both Williams and Cambridge more focused on physics as a profession. Was there kind of an aha moment along the way where you made that sort of determination?

KLEPPNER: Not really. Somehow, even from college, or from high school, I sort of felt that I was destined to spend a life in physics. And I never gave it much more thought about that. I was not terribly reflective. There are lots of other things one can do. And to some extent, it's too bad to narrow your focus too early, not to think about other directions which I might have gone. So you lose something for these early loves. But you gain a lot too, namely you have a direction and you know what you want to do. And if you're lucky, you're able to do it.

INTERVIEWER: So tell me a bit about then going to pursue your graduate studies?

KLEPPNER: Well, before I left Williams, I'd applied to Harvard University and was accepted there. I applied to Harvard because one of my principal advisers at Williams, a physicist named David Park, who we still see, he's quite elderly now, but I talked about where to go for graduate school. And his advice was very simple. Go to Harvard. So I applied to Harvard and I was accepted.

And then I got the Fulbright Scholarship. And they agreed to defer for two years. So when I went to England, I had a place waiting for me back in the United States. So I came back to Harvard.

But this is all terribly naive. There are other good schools too. I mean his advice was good advice, but it was not uniquely good advice. And to tell you how naive I was, when I was at Cambridge University, which has a tutorial system, my tutor in the second year was a young physicist named Kenneth Smith. And he told me about the research that he was doing with a recently developed technique called atomic beam molecular, or atomic beam resonance.

And in describing this work, he told me about how you can use this to measure properties of nuclei. And he showed me a little book, which I read, called Nuclear Moments, by a chap called Norman Ramsey. And he also told me about the possibility of making an atomic clock and that you might be able to make a clock which was accurate enough to measure the effect of gravity on time, which sort of excited me. I didn't do anything about it. I didn't rush out to say, that's what I'm going to do. But I remember that idea being fixed in my mind then.

And when I went to Harvard University and was walking down the hall, I saw the name on the door, Norman F. Ramsey. And I thought that's amazing, for a couple of reasons. I didn't realize he was at Harvard. I hadn't even bothered to look at the faculty.

Of course in those days, you didn't Google the faculty. You have to write some letters to find out who was doing what. But I was immensely pleased to see a name that I recognized. And somewhat surprised that a book had a real author to it. I'd never met an author before.

So I came with a predisposition for Norman Ramsey. And fortunately, I came along at a time when he had an opening in his research group. And what's more important, he'd just had a very good idea. So that's sort of how my career at Harvard got launched.

INTERVIEWER: So let's continue that story. Tell me a bit about the work that you did together and what resulted?

KLEPPNER: Well, Ramsey's work is in the subject called molecular beam magnetic resonance, although his interests turned out to be much broader than that. And one of the applications for the technique that he developed, for which he was awarded the Nobel Prize, the separated oscillatory field method, was that it enabled the development of atomic clocks.

Now, the clocks it enabled, actually he did not work on those himself there. In fact, the first practical atomic clocks were made here at MIT by Jerrold Zacharias. But he was very interested in clocks and precision measurements.

And he had another idea for improving the precision of atomic clocks. It ultimately became the hydrogen maser. And the idea was a rather radical idea. And at first-hand, it sounded nutty. And he agreed it said nutty. But you put in some numbers and it looked plausible.

The art of making an atomic clock is that you would like to observe a natural frequency in an atom and to govern the rate of an oscillator to equal that natural frequency. So the oscillator becomes the ticking device for your clock. Now, the precision with which you can do that depends how long you can look at the atom. The longer you look, the more accurate it can be.

If you're trying to compare two wrist watches and you only have one minute to do it, you can't compare them to much accuracy. If you can compare them for a year, you obviously get much higher accuracy. So you want to look at the atoms for a long time.

Well, in those days, these atoms were all on the fly. They're moving at thermal speeds. And so they go by pretty quickly. You try to make the apparatus as long as possible, but there are practical limitations on the length. Well, Ramsey had the idea you might be able to keep the atoms around a long time just by having them rattle against the walls of a bulb or a vessel.

At first glance that sounds kind of nutty, that here's this atom, you're trying to look at its frequency with very high precision. You have to treat them very delicately. And you're going to let them hit walls and rattle around. It's like trying to watch alarm clocks by batting them back and forth. But it turns out that for the hydrogen atom, this might work. And this was the idea that we pursued.

Now, we couldn't pursue it right away for technical reasons. You needed to have a pretty good idea that it would work well to do the experiment. And the reason is, in a maser if it doesn't work well, you don't see any signal at all. There's no way to kind of start from zero and work up. So we did a preliminary experiment, for which I did my PhD, where we tried the idea not on hydrogen, but using cesium atoms.

And I built an atomic beam apparatus. And we put a little storage region in the center of it and let the atoms rattle around. And we could see that they would rattle for a few times. And on the basis of that, we could predict that the hydrogen maser would work. And at that point, we went ahead and made it. So my PhD work was actually for this preliminary experiment on cesium atoms in a box. And then as soon as I got my PhD, I became a postdoc, and we built the hydrogen maser.

INTERVIEWER: So you stayed at Harvard beyond your PhD?

KLEPPNER: Yeah. I said I stayed on. I was a postdoc for a couple of years. And then I became an assistant professor at Harvard, so I started teaching also. But during this period, I was dominantly working on the hydrogen maser.

INTERVIEWER: So I wonder if you have any thoughts about, and this is maybe something we'll return to later, but about collaboration in science? I mean it sounds as if you benefited from some really terrific mentoring--

KLEPPNER: Yes.

INTERVIEWER: --at that point in your career?

KLEPPNER: Yes, very much so. I mean starting in high school--- Mr. Hussey was great. He gave me permission to work in the labs there, sort of out-of-hours. One of the out-of-hours time was when the whole school was having a pep rally, and I wasn't that interested in cheering football. So I stayed up and worked in the lab. And the high school principal, he noticed that I was in there and called me in and gave me a dressing down for lack of school spirit. But Mr. Hussey was not at all disturbed.

So this was wonderful mentoring. I mean he gave me the signal that you should follow what you think is important. And also I respected him highly because of his intellectual abilities. He was a victim of the Depression. He never went to graduate school. But he knew in the '30s, that quantum mechanics had been developed. And he didn't really understand it. He knew he didn't understand it, but he knew about it. And he told me about that. So that sort of put a bee in my bonnet on that.

Also in those days, calculus was never taught in high school. And I was interested in learning it and he taught me some. So he devoted time to me. So he was a wonderful mentor. And then as I've indicated, at Williams College, they were very supportive in encouraging what I wanted to do and basically making me feel happy. So that was good.

And it was sheer good luck that I bounced in on Norman Ramsey when I did because he was a great mentor in every respect. He was a role model.

He died very recently. And I've just been going back, reading his oral history transcripts. And from what all I know about him, he was a truly exceptional person. He was one of the towering figures in 20th century physics. So I was very fortunate to work with him. And we were close friends for life. So that's just a wonderful mentoring relationship.

INTERVIEWER: Just to pursue that for a minute, maybe it's hard to answer this in a constrained format, but what made him such a great mentor and a great scientist?

KLEPPNER: Well, first of all, I can't say what made him a great scientist. He was a great scientist. But he had wonderful personal characteristics. He was very generous. He was very modest. And he had this marvelous personality. He was an extroverted person. He was energetic. He made an impression on everyone who met him. So he had these unusual characteristics.

And he was very broad. At the same time he was running his molecular beams group at Harvard, which was a very active research group, he was involved in providing scientific leadership to the particle physics community. He personally was really essential in the creation of Fermi Lab.

And I didn't realize it at the time, it's only reading back now, that I saw all that he did. But he brought together warring scientific communities. He was the one person that they could turn to. They had confidence in him because of his absolute honesty, his absolute integrity, his fairness. And he had this very unflappable disposition. He was always good- natured. I never heard him speak a word of anger in life. So as a personal role model, as well as a professional role model, he was exceptional.

INTERVIEWER: So you stayed on at Harvard for several years after your PhD. You worked on the hydrogen maser. Tell me a bit else about your last years at Harvard and then coming to MIT, how that came about?

KLEPPNER: Well, first of all--- it came about--- Harvard didn't give me tenure. And I've since learned that there was a big battle in the department over that. But in fact, it would have been unwise for me to stay at Harvard I think. Because no matter how generous and good-spirited Norman was, he cast a long shadow. And it was good for me to be just the right distance. I see a lot of him. We continued friendship. But I was in a different institution.

I started some other research work while I was at Harvard. But I was still continuing with hydrogen maser experiments. And then I became seriously involved with teaching at Harvard too, which was very important to me. Well, it still is, although I'm not teaching in the classroom anymore.

So when I came to MIT, I had a pallet of experiments that I wanted to pursue and some ideas about teaching that I wanted to pursue. And the transition was very simple. I just crossed town. In fact, we had bought a little home in Belmont that we still live in. So we didn't even need to relocate our home.

INTERVIEWER: So you mentioned that you had a pallet of experiments and also some ideas about teaching that you wanted to pursue. Let's take each of those in turn. I'm very interested in hearing what you came to MIT and what you, at that point in 1966 you said, what you were hoping to accomplish?

KLEPPNER: Well, there was one experiment, a basic physics experiment. You can use the maser as a clock. But hydrogen is an interesting atom and there are a lot of other things that you can measure with hydrogen. And so we started an experiment to measure the size of what's called the magnetic moment of the proton.

This is important because in nuclear magnetic resonance, its protons you're looking at. And for many experiments, nuclear magnetic resonance was actually used to calibrate the magnetic fields. And one wanted to know the value very accurately. And we could do that with a hydrogen maser.

So I built a new type of maser at MIT. It worked in a strong magnetic field, and set out on a series of sort of basic measurements on hydrogen with a maser. And then I'd had other ideas too about a totally different area, sort of atomic scattering experiments. The techniques for atomic scattering were advancing rather rapidly then. And they were about to advance even much more rapidly. So with a student who started working with me at Harvard, David Pritchard, he came with me to MIT and we worked together on the scattering experiments. So those were the two areas that I started out working on at MIT.

INTERVIEWER: And you mentioned also teaching being very important to you and having some sort of things you wanted to pursue in that area. Tell me about that.

KLEPPNER: Well, I had been giving one of the freshman courses at Harvard. And at this point, I'd gotten some rather what I thought, in fact I still think, are clear ideas on how you go about teaching freshman physics. This is an ancient subject. Everyone has their own ideas on doing it.

But what I did when I got here, was I started a new course. And I started it with a colleague here, assistant professor, Bob Kolenkow. And this course, it was called 8.01S. It's now called 8.012. And it's still given. And it was designed for a rather special audience, namely very bright, ambitious MIT freshman.

So it wasn't taken by the whole class. But it was taken by 150 or so of the students. And it was a course which tried to teach the very basic ideas, but at a much deeper level than they're normally given. And we worked very hard on that course and developed it over a number of years. And I think it was developed successfully, because the course is still being given very much in the form that we gave it. And the teachers who teach it, enjoy giving it.

And from our first lecture notes, we developed a textbook, which was published in 1972, Kleppner and Kolenkow, which is still in print and still being used. And at the moment, we decided it's time for a second addition because the first edition is roughly 40 years old. But we're very proud of the book. When I visit people, I often see it on the shelf. That makes me feel good.

So teaching can be very rewarding. And that was one way it was rewarding. However, the book is a little bit old fashioned. We decided we shouldn't talk about slide rules anymore.

INTERVIEWER: So when you came to MIT in the mid-1960s, what was MIT like at that point? And what was the physics department like?

KLEPPNER: Well, first of all, the physics department--- it's a large department. One reason it's large is it encompasses astrophysics and also condensed matter physics. In many schools, astrophysics and astronomy are a separate department and condensed matter physics and materials science are totally in a separate department.

So this department was much broader than in other schools. But that makes it large. So there's always a compromise. When things are large, you lose the sense of cohesiveness that you can with a smaller department.

So it naturally breaks up into different areas. And so I was in sort of the atomic physics area of it. But I must say then that was slightly isolated. It's only in retrospect that I realized that. At the time, I was too busy doing things to appreciate the significance of that.

The department later reorganized, at least we're grouped together in divisions. And the atomic physics area, particularly in the past few decades, has grown. So it's an active community of its own.

And the department itself, I think, in spite of its size, has always been a very cohesive department. We meet together once a week for lunch, which everyone enjoys. And I think it's a pretty essential thing to do with the department of our size.

But my experience here has always been a happy one in the department. For example, when someone is proposed for tenure, I have never seen the proposal contradicted sort of purely on the grounds of intellectual partisanship. If a person is really good, you can tell that no matter what the field is.

There are lots of ways of judging candidates. And MIT's way is a very thorough way. And if the candidate is judged well, I've always seen everyone sort of rally behind him.

That's unusual. In many departments, you have factions and they battle each other. So my experience in the physics department has always been a happy experience here, and particularly in recent years, because my own area has grown so well and is such an exciting area. And now it plays a much more visible role in the department than it did when I first came here.

INTERVIEWER: So who were some of the notable colleagues or collaborators, mentors from that time at MIT when you first arrived?

KLEPPNER: Well, one of the things that I inherited or sort of took over when I came here, was a graduate class which you've been taught by George Benedict, who was working on well, on the optical and other ways to study the properties of matter. And he had a course, which was a very interesting course for me. And I adopted some of the material from that course when I started my own atomic physics course here.

The provost when I came was Charlie Townes. Now, the laser had just really been invented. In those days, it was still very young and developments were going rapidly. I must say I think I slightly missed the boat in not latching onto lasers sooner, because it rapidly became apparent that they were sort of powerful tools for doing all sorts of atomic experiments. But I do remember going to a number of his group meetings and listening to about the work going on there.

A young physicist, Michael Feld, was in that group, who later became a member of the department. And sadly died, I guess about a little bit more than a year ago. So he's someone else that I interacted with.

INTERVIEWER: So let's also talk a bit about how your research interests and the problems that you worked on evolved once you had arrived at MIT and some of the next challenges that you faced?

KLEPPNER: Well, the next big area that I went into was the subject of Rydberg atoms. A Rydberg atom, they're just like hydrogen atoms. In fact, they can be hydrogen. According to the Bohr theory, the Bohr model, which is no longer regarded as a rigorous way to look at atoms, but is very suggestive, you think of the proton, it's a planetary model, and the electron goes around like that. And it can go around in a bigger orbit. It can jump from one orbit to another and emits some radiation. And you can account for the spectrum that way.

Well, you can label these orbits, the first one, the second one, the third one, the fourth one. In the early 1960s, radio astronomers found radiation which came from hydrogen atoms going from say the 100th to the 99th orbit, a totally different world. It turns out these are enormously large in size, which is why you don't see them in the laboratory. They fall apart very easily. And the radiation that they emit is not in the optical regime, but it's in the microwave regime.

And they were discovered. And that was while I was still at Harvard. And I remember Ed Purcell sitting down with some of us, some afternoon, just thinking about the properties of these atoms, which are really, everything is different just because of this huge geometry. And thinking at the time, wouldn't it be nice, wouldn't it be fun to study those. But there was no way to make them in the laboratory at that time.

Well, I mentioned that the laser had been developed. And in the early '70s, I realized that you could use these lasers to create the atoms in the laboratory. And we had a weekly group meeting. And we started talking about those.

And then it was about I think maybe 1973, one of the postdocs here, Rick Freeman, went off to an APS meeting and came back and said, well, some other people are thinking about these too. We ought to get to work quick. And so we got to work quick.

And we started out to try to make these atoms in the laboratory. And it turns out they were quite easy to make. And what's more important, they were very easy to do to detect. The problems with atoms is that individual atoms are very difficult to detect. And the Rydberg atoms, well, they give out radiation, but not very much. That's a hard way to detect them.

However, the way you can detect them is that these atoms are so large, they're so weakly bound, that you apply a little electric field, you can pull them apart. And you have charged particles, which are very easy to detect. So we started to work on that. And the experiment started working. I think it was in maybe January '75, that we got the first results.

Now, I'd planned to take a sabbatical leave in Oxford in that spring. And in fact, we got a house there and my family went there. But these experiments had just started working. And I felt I just couldn't walk out at that particular moment. So my wife, who is very supportive and understanding, said, come along when you can. So I stayed behind. And we worked on these atoms. That was a very intensive period.

In fact, since we rented out our house here, I actually set up a home in a lab office and lived there for about six weeks. And I certainly don't recommend this to anyone. MIT is a lovely place to do research, but it's a rather grim home. But we lived there and got the experiments going. And then I went to England and joined my family.

INTERVIEWER: So do you have any, this might be a good time to ask--- I mean, starting with this sort of very early interest you had in building things, creating devices, and hearing a story like that, that kind of sort of commitment to seeing something through, what kind of advice would you give to experimental physicists or people who may be aspiring experimental physicists, more importantly, in terms of success?

KLEPPNER: Well, do things. If something interests you, follow through.

Now, I can't tell people what to do now because all the technology that I learned is now totally obsolete. I mean I learned machine shop practice. Well, machines are automated these days. I learned how to do vacuum tube electronics and vacuum tubes are museum artifacts right now. And I'm in awe of experiments, the techniques which are available now.

But graduate students, well, they're a little bit like young children. Young children just acquire language quite naturally and learn at a furious rate. Well, the graduate students I see these days seem to pick up all these new technologies. Optical technologies, electronics technologies, data processing, all those seem to be sort of second nature to them in a very short time. So I can't tell them how they should go about doing it.

But I think just the idea of pursuing your interests and trying out new things. I mean that's sort of a general principle.

One thing I find regretful is that there's so much schlock around, science kits where everything is done for you. I don't think children can learn that way. When I was young, we had chemistry sets and you could make things smell and burn. And of course, we live in such a litigious society right now, that nothing possibly dangerous can be done. So we've lost all that.

On the other hand, kids can get access to NASA data, astrophysical data online, and just do things in very different ways from before. But I think it's just, how can you teach a person to be energetic and enthusiastic? You can just try to support it when you see it.

But what they should do, I can't tell right now. I mean it could be that some young person can come up with some very important way to try to get a better hook into the problem of climate change, for example. Even though there's a huge community working on that, there's always room for new ideas and good ideas. So my advice is, well, all I can offer is platitudes rather that useful concrete suggestions.

INTERVIEWER: So I think you mentioned it was about 1975 that you succeeded in creating the Rydberg atoms in the lab. Was it '75 or so?

KLEPPNER: I'll have to check the date. It may have been '76 when that first paper was, I think.

INTERVIEWER: Okay. So all right, what did you tackle next? What drew your interest next?

KLEPPNER: Well, it turns out the Rydberg atoms are a world of their own. And they led into many new directions. The first was just trying to study some of the basic atomic properties of these atoms. And then I got particularly interested in the structure of these atoms when you put them in electric and magnetic fields. It's quite interesting. If you put the atom in an electric field, it can come apart. But before it comes apart, it shows interesting structure that you can understand.

And you put them in a magnetic field and you're in a new world. The effect of a magnetic field on the spectrum of the atom is called the Zeeman effect, which was discovered around 1897 or 1898. And turned out to be very important as one of the first pieces of evidence that there were charges inside of atoms. So the Zeeman effect was historically an important spectroscopic tool.

But when you put these Rydberg atoms in a magnetic field, the Zeeman effect is just totally blown to bits. You're really in a new world. And we were trying to understand that world and trying to really understand the spectrum. It's such a simple system, an electron, and the proton, and the magnetic field. But it turned out that the theoretical analysis was very challenging.

And we did have some theoretical collaborators in Paris, who were very helpful in this. And others got interested in it. And then it emerged that these atoms made contact with a subject which had really only recently emerged, which is the problem of chaos. Of course, MIT has very strong connections with that. It was Ed Lorentz over here, who really put chaos on the table of important physical problems.

Well, it turns out that these atomic systems, if you look at the classical motion, the motion can become chaotic. And the question is, what is the quantum structure of the atoms? And so the connections between quantum mechanics and chaos were very provocative and elusive. And we started working on that.

We learned a lot from another group in Germany, Karl Welge, who was working along the same lines. And so we became very involved with that for a number of years. So the next big item on the agenda there was quantum mechanics and chaos.

But I also resurrected another old idea, which you could do with the Rydberg atoms. And this is a subject which has become to be known as cavity quantum electrodynamics. And the thought is this, atoms can exist in various energy states. If you have an atom in one of the high energy states, it will inevitably, all on its own, radiate and go down to the lowest possible state. That process is called spontaneous emission.

And spontaneous emission is a very basic process in physics. It's a very important process. For instance, if you're looking at noise in quantum mechanical devices, the ultimate source of noise is spontaneous emission.

I pointed out, actually even before coming to MIT, that in principle you could stop spontaneous emission. If you put an atom into, I'll just call it a tuned cavity, you could prevent the atom from radiating. And the reason for doing that, again was this reason of measuring. You'd like to keep the atom around as long as possible to make measurements on it. And in the optical regime, your measurement time is always limited by the spontaneous radiation rate.

So I proposed that if you put the atom into a cavity which was too small, a little cavity, it's a chamber which is say, all conducting. So radiation can't get in or can't get out. And if the cavity is smaller than the radiation wavelength, there's no way the atom can radiate. That was the idea.

But there was nothing you could do about it for years and years. But then when we had Rydberg atoms, we could start doing something about it.

And we did an experiment which actually demonstrated that. One of my graduate students, Randy Hulet, demonstrated that in our lab. It was called inhibited spontaneous emission. And in a sense, this was a grandfather experiment, because this whole subject of quantum systems interacting with, resonated with classical systems, is now called cavity quantum electrodynamics, and is of very active interest now.

So we did the original experiment on that. And then we went on to do other experiments on it. Other groups became very interested in it. And so that subject has flourished.

INTERVIEWER: To return just briefly to the first thing you mentioned, chaos theory and the relationship with sort of quantum theory, again, I don't know if there's any way of helping me to understand it in the time we have, but why is that so important, chaos theory? I mean what we do what you mean when you say that and why is that important?

KLEPPNER: Now, that's a very good question because the subject of quantum chaos has been controversial. One of the real leaders in that, Michael Berry, a marvelous British mathematical physicist, had tried to introduce the term quantum chaology, because in fact there is no room for chaos in quantum mechanics.

The way I like to pose the problem is this, every one, every physicist believes in quantum mechanics. In a sense, quantum mechanics underlies classical mechanics. I mean classical mechanics is the mechanics of Newton and large bodies.

Now, it's well-known that quantum effects are only apparent when you're looking at very small bodies. Nonetheless, the belief is that the quantum laws are fundamentally correct. And that in principle, one ought to be able to describe classical phenomena using quantum ideas.

That's a very important principle. When Bohr first proposed his model for the hydrogen atom, he used that principle. He calls it the correspondence principle. If you go to very high quantum numbers, systems just start looking classical. Like these Rydberg atoms, when they radiate, they radiate, you can calculate all that just classically and get pretty good answers.

So this is the question, can you really describe classical motion quantum mechanically? Well, chaos puts that to the test. Because chaos is such a complicated phenomenon and can you really describe chaos quantum mechanically? So that's the essence of the challenge there.

Well, there are the number of answers to it. The basic answer is that in chaos, the smallest change to a system, say to the initial conditions of a system, can produce large effects later on. That's the so-called butterfly effect, that a small perturbation can lead to huge differences.

Well, quantum mechanics has a small size limit to things. Below a certain size, it doesn't make any sense to say, I'm going to change the initial conditions, because you can't establish them in the first place. So in that sense, quantum mechanics does not permit chaotic behavior in the classical sense. Nonetheless, there are many features of the quantum motion which reflect sort of chaotic behavior, the way functions can start looking in certain characteristic ways.

So I think the subject has been quite fruitful, but there's no simple answer to the question. And in fact, in chaos essentially all systems are different.

But the principles of quantum mechanics, they can be applied. Maybe the best way to put it is, you can apply them to understand chaos, but they're wildly inappropriate. It's not the language. It's not only the language of choice, you'd be a fool to try to describe the classical chaotic motion using the language of quantum mechanics. It's the wrong language.

INTERVIEWER: Let's turn now to Bose-Einstein condensation. I guess I'm just going to start with the most fundamental question, what is it? And how was it that your research interests turned in that direction?

KLEPPNER: Well, it's a good question, because that was the question I asked Tom Greytak. There was an article published in Physical Review Letters in April of 1976, by Lou Nosanow and Bill Stwalley. And it pointed out the possible Bose-Einstein condensation of spin-polarized hydrogen.

Well, I read this article. And what it pointed out was that if you have hydrogen atoms which are spin polarized, which means the electrons in the two atoms point in the same direction, these can bounce off each other--- these atoms. But they will not form molecules. And so they form an atomic gas. And that atomic gas has unusual properties, namely you could cool it to absolute zero temperature and it would still stay a gas. It would never condense.

Now, the reason that was interesting is if you can cool atoms, if you can get them cold enough at high enough density, they undergo what's called this phase transition, quantum mechanical phase transition--- the Bose-Einstein condensation. And what it means is, if you have a gas of all these atoms moving around like individual particles and all of a sudden they change their phase-- well, I shouldn't say all of a sudden-- as you cool them down, they start dropping into the lowest energy state in the system.

They stop behaving like individual particles. It becomes like one giant quantum system. It's one quantum system which has many particles in it. The easiest way to describe it is they just come to rest. Now, they can't be actually at rest because of the uncertainty principle. But they go into the lowest state which is available. And this article pointed out that you might be able to see this. Now, this had never been observed in a gas.

And I read the article and I thought what they were requesting was kind of preposterous. It meant hydrogen at much lower temperatures and at higher densities than had ever been seen before. So I thought it was a very unrealistic proposal. And kind of annoyed at my friend Bill Stwalley for making such an outrageous suggestion, at least without saying well, this is outrageous suggestion. But they didn't discuss that at all. They just showed the science.

Well, I had a conversation with Tom Greytak. Tom's background was in low temperature physics. It was in liquid helium, studies of liquid helium. And he asked about that article, whether it might be possible to do that? And I said no, that was totally absurd. And he said well, that's unfortunate because Bose-Einstein condensation is really quite interesting. And I asked him what is it, just the question you asked. And he explained it to me.

And so we had a series of conversations, in which they would start out by pointing out why you couldn't do this and then figure out a way why you could do it. I realized that some of the techniques which we used in making the hydrogen maser, namely storing atoms in a vessel, might be applied to make spin-polarized hydrogen, to get to Bose-Einstein condensation.

The art of storing hydrogen atoms in a vessel is to prevent it from sticking to the walls. And at room temperature, there are certain materials that hydrogen doesn't stick to-- Teflon, for example. Well, in making the hydrogen maser, we had often thought the ideal thing on the wall would be a film of liquid helium, because that's very non-interactive.