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Credit: Research Laboratory of Electronics, Massachusetts Institute of Technology
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Interview of David Pritchard by David Zierler on April 20, 2020,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/XXXX
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In this interview, David Zierler, Oral Historian for AIP, interviews David Pritchard, Cecil and Ida Green Professor of Physics at MIT. He recounts his childhood in New York and discusses his early interests in science and the influence of his father, who was an electrical engineer. Pritchard explains his decision to attend Caltech as an undergraduate, and he conveys the reverence that he and fellow undergraduates felt for Feynman. He describes his natural gravitation toward experimentation and discusses his formative summer job at the Cambridge Electron Accelerator. Pritchard discusses his graduate work at Harvard and he relates some of the stylistic distinctions of Schwinger and Feynman. He describes his research in the lab of Dan Kleppner, who was working on the hydrogen maser, and he discusses his dissertation project on laser atomic scattering. Pritchard explains the events leading to his tenure at MIT, and he describes the negative impact of the Vietnam War on federal funding for basic science. He explains the origins of his interests in aliens and alien abductions, and he discusses how he brought an academic physics perspective to this field. Pritchard discusses some of the metaphysical or spiritual implications of his work on alien research, and he shares his ideas on the concept that knowledge is a social construct. At the end of the interview, Pritchard explains some of the through lines that connect all of his theoretical and experimental work, and he considers the impact of technological improvements over the decades on his research.
This is David Zierler, oral historian for the American Institute of Physics. It is April 20th, 2020. It is my great pleasure to be here with Professor Dave Pritchard. Professor Pritchard, thank you so much for being with me virtually today.
It’s hopefully going to be a pleasure.
[laugh] OK, can you tell us your title and your institutional affiliation, please?
Yeah. I'm the Cecil and Ida Green Professor of Physics at the Massachusetts Institute of Technology, which of course is usually MIT. And I think everybody knows that it’s in the other Cambridge.
The other Cambridge, right. Let’s start right at the beginning. Tell us about your birthplace and early childhood.
Well, I was born in Doctors Hospital in New York, and it was a little trip for me to drive by in a taxi once and see—it’s pretty beaten up [laugh]—and see it. And then my parents lived in New York and then in Brooklyn, and I don’t remember any of that. My first several memories are from when my Dad was working on servomechanisms on the calutrons in the uranium separation plant at Oak Ridge.
Aha! He was a physicist, your father?
He was an electrical engineer. But he was an expert in servo systems, electronic servo systems. You want to run as much current as you can through the calutron, which is basically a mass spectrometer that you run in the plasma regime to get as much throughput as possible, and that gets unstable at high throughput. And so you have to have active feedback to keep the U-235 ions going into the collector for the U-235.
Now, were your parents native New Yorkers?
No. They were New Englanders. My mom was a child of Vermont, and my dad was really—although he was born in Somerville and raised in Medford MA - his family was from upper New York state next to VT. They were actually Welshmen brought over to do slate mining in the mid-19th century.
And how did they get to New York City, your parents?
My dad was the first person in his family to go to college. He went to MIT, and in fact got an advanced degree, got a PHD—it was ScD—in engineering. So he was an electrical engineer, and then he went to work in various companies in New York City or on the western end of Long Island.
Now growing up, how exposed were you to your father’s work? Did he keep those worlds separate, or were you learning from him at an early age?
I think I learned science from him at an early age at the dinner table. And I taught myself: I would have some idea—well, I always wanted to do experiments. Maybe I should mention my first experiment, which is one of my first three memories, which are not ordered in time. But it was on Sunday afternoon in Oak Ridge, and my parents had some dinner guests. And I got bored, so I asked to be excused. I was a very polite child. And I was granted this privilege, and I went out on the porch—we had a long porch. And I was riding my bike up and down it, and I decided, “Well, this is pretty boring. Maybe I'll see if I can ride it down the steps.”
[laugh] How old were you?
Three and a half.
And so I would go up with my tricycle to the edge and work out the plan, which was that I was going to reach for the railing on the right, and steer with my left hand, and just ride down the steps—we only had five steps, maybe six.
You had it all worked out.
Obviously it took a lot of nerve, so I distinctly remember riding the long length of the porch down, riding fast—“Oh, I'm going to do this”—turning around, coming back, and going real slow—“Gee, I'm going to go up to the edge”—and I'm only two inches, one inch away, and then I'll ride back where I can get my courage up. And finally, I tried it. I distinctly remember the bike turning to the left. Because the bicycle is not stable if you tilt it more than the angle of the offset of the central steering column. And so the wheel turned to the left, and the rail was receding to the right, and I remember steering and reaching and having the rest of the handlebar about by my stomach as I was going headfirst over the tricycle. And of course I wound up in the hospital, maybe getting a couple stitches, and I had a big, black front tooth when I was a kid as a result of that.
And I take it this was long before helmets came into existence. [laugh]
Oh, yeah. Yeah. So maybe I got some sense knocked into me, but not very much. Because the spirit of trying to do something new and planning it out and carrying it out—I think that’s a pretty basic instinct of mine.
Now your primary school and high school, you went to public school or private school?
I went to public school all the way until I went to Caltech.
And in high school, did you demonstrate any strong aptitude in math and science?
Oh, yeah. I was in the top 10% of the class academically, but I was really good in science and math. The other kids knew it. In fact, I won—the two things that I remember—first of all, was going to State Math Day and winning the South Jersey region, and being honorable mention in the whole state. That was pretty impressive because the kids in North Jersey had really good high schools and they had special preparation sessions, and they would drill at all these old exams. And the questions were tricky, but there’s only a limited number of questions with known tricks. So that was one thing. And the other thing was the senior year, I decided to take a second year of French rather than the third year of algebra. And so I was coming out of my French class, and I would walk by the kids who were coming out of the Algebra III class, because the classrooms were close, and we were going in opposite directions for some reason. And I remember the time that there was a special test—it was the Bausch & Lomb math test or something. And in spite of the fact that I hadn’t taken Algebra III, I was the top student. And so like the whole class was coming by and kind of nodding—“Gee, you beat us out, even though we gave you the handicap that we were taking Algebra III – good going.” So I’ll always remember that because that was how I learned I’d won the medal.
Were there any teachers in high school that were really inspirational to you, or you really connected with in math or science?
I had a good relationship with a lot of my teachers. I wasn’t necessarily—well, usually in science, I was the teacher’s pet. One of the experiences that I remember very distinctly was having done the geometry homework and just being totally bored when the teacher was going over the geometry homework in class. I was so obviously bored that it was annoying the teacher, and she called me on it a couple times. And finally in exasperation, she said, “Well, if you can’t pay attention to the class, would you like to teach the class?”
And I said, “No, but I will.” Well, of course, this made all the kids very interested. And the most interesting experience, which I regret as a teacher now, was that there was a problem where you had to find the area of a cone. So I wrote down two pi r h, because I'm thinking of it as a triangle whose base is the circumference times h, the altitude to the top. And the class gasped, because here the best student had made a mistake. Then, because the surface is a triangle, I divided it by two. If you had just remembered it, you would have written pi r h so you never would have written the two. But I didn't remember it; I derived it every time on the fly. And that would have been a tremendous teaching lesson, but it just passed by without my emphasizing the valuable lesson for the class.
I wonder if that moment stayed with you as you developed an interest in physics education later in your career.
Well, I wouldn't say it was inspirational, but I now look back at it as something to remind myself, “Pay attention to what’s going on in the class. You're not giving a lecture.” I mean, you might be giving a lecture, but you should be interacting.
I'm curious—why Caltech? Were you applying to the best science schools in the country and Caltech was just the best option for you, or did you always want to go to Caltech?
I applied to Harvard, MIT, and Caltech, and one backup place. Of course, I had 800 SAT scores—well, at least maybe in math, and science, and I had fabulous recommendations, and I had won the state math thing, so I got into all of those places. It really was between MIT and Caltech. And I decided to go to Caltech first for its sense of adventure, and secondly because my dad had gone to MIT and I didn't want to do the same thing.
So why was Harvard out of the running immediately?
I didn't really think of it as being that great a place to study physics or to study engineering—actually, I was more interested in engineering at that time. Because I had done a lot of ham radio and building—I made models, sailboats mostly, made my own arrows, did chemistry experiments, did electronics, tried to make a radio in a cigarette case that you could smuggle into school. And I was a ham radio operator. And I knew that transistors were going to revolutionize electronics. So that was what I went to Caltech intending to do.
And their engineering program was stronger than MIT’s, was your feeling?
No, no. But both of them were stronger than Harvard. But Harvard is still struggling with exactly how they're going to do engineering. They don’t have electrical engineering and mechanical engineering. They have like bioengineering and nanoengineering. So we'll see how that works out.
But then, I found that my first electrical engineering course was a big turnoff. Things like Thevenin and Norton equivalents, and Y-delta transformations—I mean, that’s just obvious! I found that I was really more interested in why a resistor should have the same value—same ratio of voltage to current over a wide range of current. What’s really going on inside the resistor such that that’s the result? And of course that’s physics. So I kept tantalizing myself with the idea that, “Well, I'll go and I'll do solid state physics, but then I can switch over and do electronic devices afterwards.” But when I got to Harvard, it was obvious that the atomic physics group of Norman Ramsey and Dan Kleppner was much better—had higher quality people in it than any of the condensed matter guys. And their condensed matter guys were like working on liquid helium, which didn't seem that relevant for electronics. Little did I anticipate Josephson junction quantum computers, although that route is not proving out, either.
So let’s go back to Caltech. So when did you declare the major in physics at Caltech? Sophomore year?
Probably in sophomore year, I declared electrical engineering, and then I had to change it to physics.
Looking back, what were your impressions of the physics department at Caltech? Who were some of the major professors who you had a relationship with? What were the fields in physics that were really emphasized at that time?
Feynman was there, and so was Gell-Mann. And so was Pauling.
It was a pretty high-powered place.
Did you recognize that at the time? When you interacted with Feynman, did you have an appreciation of who he was?
Oh, yeah. We all worshipped him. Because first of all, he was a big extrovert, and he was very concerned with the PR aspect of being a scientist. And yeah, we used to go—I remember very distinctly a couple of times —this was the two years before he started the Feynman lectures. In fact, he started teaching to freshman when I was a junior. And the freshman course conflicted with the junior course I had to take so I never attended the Feynman Lectures. But before that, when I was a freshman and sophomore, he had a thing called Physics X, and you could ask him anything. Like, “What makes the stars shine?” OK. Or how does general relativity account for which way a gyroscope will point? So I would go to that thing, and then he would give this argument that was just very, very clear, and I would understand it just great. I’d go back to lunch, because Physics X was 11:00—I’d go back to lunch at noon, and I’d be talking to one of my colleagues, and I’d say, “Oh, Feynman has this great argument for how resistors work, and here’s how it goes. It’s just three lines. The first line. And then the second line. Hmm. It’s not at all obvious that follows from the first line. What did he say? Geez.” [laugh] You know? And he had this way of looking at things with just the right perspective, and if you got off the track, there was just a whole forest of unknowns or alternative hypotheses that looked equally attractive branching off the road, and you had to be careful not to get side-tracked.
In terms of the kinds of physics that were emphasized at Caltech on a hierarchy, where were the experimentalists relative to the theorists among the faculty?
Well, obviously if you have Feynman and Gell-Mann, they're the top dogs, right? And Feynman, by far, because he’s always “out”—you know, some afternoons he’s out in his shirtsleeves arguing with a bunch of graduate students about something. And the experimentalists, they seemed—people like Leighton—and my mentor, a young professor named Ward Whaling—they seemed like very solid people to me—very sensible, solid people—but the only Nobel prize winning experimentalist was Carl Anderson, and his main reputation was for being able to sleep with his eyes open.
If you went to his office hour, you'd swear he was asleep but his eyes were open. And he lectured Advanced Mechanics, and his authority was, “Well, I'm using Jon Mathews’ notes, so I know it must be right.” But I don’t think he understood what he was lecturing on. So that’s hardly an inspiration for a young physicist.
And was your sense among your fellow students in physics—were most of them planning on going into the academy, or were most of them planning on going into industry?
Well, most were planning on going to graduate school, which is usually the route to the academy, but it also can lead to industrial laboratories—especially in those days, a wonderful place to go was Bell Labs. I really think that going to Bell Labs after you finished your PhD was the wisest move that a young physicist could make. But in any case—grad students were selecting subfields. High-energy physics and even nuclear physics were considered great, and atomic physics was like, “It’s been dead for 15 years, hasn’t it?” [laugh] And so when I went to graduate school, I already had a preference for small stuff. But the summer before graduate school, I worked in the Cambridge Electron Accelerator, which was right across the street from the dorm that I lived in, very close to the physics department at Harvard. And it was an interesting experience, and I got some good recommendations out of it, but I was really turned off. The first turn-off was that you didn't, as a graduate student, get to do the experiment. You had only one part. It might be the Cherenkov Counter and the electronics for that. And it was the postdoc and the faculty members that really were in charge. Secondly, you couldn't understand the experiment in any real detail. Now, that was because we were doing a QED experiment. We were probing whether the electron-electron interaction went like ~1/r2 at small distances. Which was really equivalent to looking at wide angle pair production at high momentum transfer. It’s the same thing. And at the department there, there was Julian Schwinger, one of the triumvirate who won a Nobel Prize for QED. But none of the guys in the group talked to Julian. We had to wait until Sidney Drell came by.
Because you couldn't talk to Schwinger? It was impossible to talk to him?
That's right. I mean, he would talk to you. He wasn’t unfriendly. It’s just that he would say, “Well, the way you do the hydrogen atom is with a Green’s function transformation on a harmonic oscillator.”
And I’d react, “Hmm, is that really right? I don’t know. Geez.” After half an hour later, yeah, I guess the math works out, but it doesn't give me any insight at all.
I want to ask you—we moved a little too quick to Harvard. Did you come out of Caltech with a fully formed identity of the kind of physicist that you wanted to be? In other words, when you started at Harvard, did you go there knowing what you wanted to pursue, or were you more of an open book, and you were willing to pursue whatever seemed right to you at the time, when you got to Cambridge?
No, I think I was pretty specialized—I knew I was going to be an experimentalist, and I wanted to be a small-scale one. I had my heart set on that. I mean, I had worked on an atomic clock over the summer at RCA, and—aside from nearly electrocuting myself, it was a very rewarding experience. But I did work at the electron accelerator to see what I was missing, in a sense. And what I found I was missing was being able to understand and do the theory for what you were doing, being able to call the shots, and having a fairly maneuverable experiment —“Well, I think this other avenue is pretty interesting. Let’s go down that instead.” And in high-energy physics, that’s just not the way it was. And it was also obvious to me that the accelerators were going to get larger and fewer—at that time, there were cyclotrons at every campus, and the Cambridge electron accelerator was one of the first multi-campus installations. And then of course Brookhaven and Fermi Lab and all those things were coming. And it was obvious that you were going to have to share credit—I remember arguing, “You're going to have up to 100 authors on a paper. And who are you to be one of them?” And still I underestimated by a factor of—I don’t know what the record is now, but it’s probably at least 30 times 100.
Did you enroll at Harvard right after graduation or did you take some time off?
And why Harvard? What was your decision-making there?
Well, Harvard had some pretty good experimentalists at that time. It had Purcell, Ramsey, then Bloembergen, Pound and Bainbridge. So it was well stocked.—I considered Princeton, too, and I considered MIT, but I think it was also a question of social prestige and the fact that my sister was going to Radcliffe at the time.
Oh! That’s nice.
I had a close female friend at Caltech - Andrea Hasler, a fine arts major at Scripps - and on the afternoon I departed, I told her how my sister was going to set me up with a smart and beautiful Radcliffe student - my dream marriage partner. My sister introduced me to the president of her dormitory (I’d been dorm president at Caltech), an attractive math major who later won several computer science prizes and was a power woman in professional societies as well as on the Harvard board of overseers. Here she was! After a half dozen dates, I planned to say “we should take our relationship to the next level” but what came out was “well we don’t seem to be making great romantic progress, so let’s break it off.” I was absolutely amazed to hear myself say this! Later I realized it was the rejection of my then-planned future as an entrepreneur who would start high-tech companies - really the future my parents had planned for me. Meanwhile Andrea and I had been exchanging letters each week on all sorts of topics and I realized she was creative, deeply original in her thinking, athletic, amazingly quick to get what she was doing done, fun to do all sorts of projects with, thoroughly beautiful - in essence the woman of my soul’s dreams.
So I visited her over Christmas break, ate a lot of crow concerning my earlier “I’m going to marry a beautiful ‘Cliffie” remark and convinced her to come East for the summer - where my sister found her a job. Andy danced in the Summer thunderstorms, decided to switch grad schools from Claremont to BU, and found she loved Fall colors, Winter snowstorms that transform the world, and Spring flowers poking up through the snow.
But—we've been married for 52 years, enjoying one big project after another: artistically redecorated three houses, raised two sons, sailed our boat, dueled the con man who married Andy’s mom when she was getting Alzheimer’s, run the Abduction Study Conference, started an education company, etc. And we're really enjoying our forced time together due to COVID-19 quarantine.
Good. That’s a good thing.
Dinners are leisurely, and we've been watching a lot of movies we didn't have time for over the years, and just hanging out in bed for half an hour after the alarm clock while the music is playing, and talking about stuff.
I have to ask, Dave—so since you had a front-row seat both to Feynman’s style and to Schwinger’s style, and because so much has been made about their different approaches, I wonder what your takeaways were from learning how physics is done, given that you had such a close perspective on both of them.
Well, I would say two things. First of all, it’s a matter of taste. Schwinger is a mathematical genius, and he does things in a very formal, mathematical way. And if that’s what you like, that's your cup of tea. Feynman likes to do things in a much more intuitive way. I mean, it wasn’t he who figured out the justification for the Feynman diagrams; it was—oh, come on. The guy just died. [pause]
It’ll come to you.
He showed formally how you got from the formulation of Schwinger, the mathematical formulation, to the diagrams -- with Wick’s theorem and all that sort of stuff. But the real question is, what is science all about? It’s about habits of mind and evidence-based procedures that help you get to understand things. But it’s more, I would say, a social construct where you're constructing a community consensus about reality. I mean, I'm beginning to be much more a believer in the social construction of knowledge. And when you have an introvert with a capital “I” like Schwinger, he just doesn't have the social impact or the influence that a guy like Feynman does, who invents a method that’s easier to explain and that he can explain well to the vast majority of professional physicists.
The distinctions were one of style? In other words, you never detected a fundamental disagreement like Einstein and Niels Bohr had a fundamental disagreement?
No. I don’t think so. No, nothing on that order. I mean, that really was all about the philosophy of science and ontological significance of the wave function and what is a measurement, and all this kind of stuff. And Feynman and Schwinger were people who could understand each other’s work and accept it. At least, after Dyson showed that there was some mathematical justification for the Feynman diagrams. So there was the style of doing physics, and then there was the style of interacting with the rest of the community. And so obviously, Feynman had a much bigger influence on the Caltech physics department than Schwinger did on the Harvard physics department. To get to Schwinger’s office, you had to go through the secretary’s office. On the other hand, there’s Feynman out at the blackboard arguing with the other faculty members or usually with graduate students. And it’s just a different atmosphere.
Did you ever think to stay on at Caltech, or that was not advisable?
No, I think I philosophically…maybe I would almost say congenitally—an East Coaster. That was certainly the value system of my parents. On the other hand, our older son is congenitally a Californian. We talk about him and my father-in-law as having a California gene. The California gene worked like this for him. He graduated from Caltech with a PhD in physics, had an offer—this is in the Depression—he had an assistant professor offer from Georgia Tech, which is a pretty good place, and said, “No, the climate isn’t all that good there. I'm going to stay here and start a consulting company.”
Which he did, because he was a very competent and determined person. But that’s only one explanation for that – it is the California gene.
Right, right. So your education at Harvard—what was the divide between course work and lab work?
Well, you took courses for most of the first year and a half, and you worked maybe a little bit in the lab, and then you got into the lab essentially full time.
Whose lab were you working in?
I was working with Dan Kleppner.
And what was Kleppner working on, at the time?
Well, Kleppner was working on the hydrogen maser. They had just invented that. It grew out of his thesis, with Norman Ramsey. And then also, he was starting up an experiment where the idea was to polarize trapped ions with polarized atoms and do resonance frequency transitions on the ions, and monitor that transitions had occurred by looking at the scattered atoms. And as I was working on that experiment and studying for my oral exam, I realized the way they were doing the experiment was just making it harder, because they were just looking at the beam that went through the ion trap, and most of those atoms didn't hit the ions, and so they just had the same polarization as the original. And why did you want all that background? Why didn't you just move off a few degrees to where the collision is hard enough to have a good probability of exchanging the spins of the atom and the ion – thereby looking only at the atoms that had information about the ions? And then I started working out what was the interaction potential between atoms and ions. And I wrote a paper that was never published about it, and could solve a lot of the integrals, assuming that there was a small atom and smaller ion. And then I did one experiment at ten degrees with the apparatus that Dan had in the lab at that time, with another good graduate student, an older student named David Burnham, and showed that as my theory predicted, at ten degrees, there was maximal spin exchange. So you could see that atoms had got flipped, so it looked good for doing that experiment. But then I just got off on trying to understand the spin exchange process and building a large apparatus to make measurements about that. And that turned into my thesis.
Help me visualize—what was the instrumentation like in the lab? What kind of instruments were you working with?
So for my thesis, I built an apparatus that looked like a one-meter-diameter chowder pot, a great big pot about a foot and a half high with domes top and bottom. And then inside that was a source for a collimated beam of sodium or potassium which was on a carriage, so you could change the angle, and the source had additional motors to move the oven back and forth between the two sides of a Stern-Gerlach magnet to select atoms that had one spin or another. And it also had another motor that was an attenuator, and it had coils for liquid nitrogen cooling of the box around the oven. And then there was a straight tubular part sticking out of the side, which at first was a collision chamber where the ions, or later for my thesis, it was atoms, that came up from the bottom. And on the way down the straightaway to the detector, there was a velocity selector and another state-selecting magnet, so you could look to see the fraction of atoms that flipped their spin from up to down as a function of the scattering angle set by the angle of the course carriage.
Was Kleppner your advisor?
Yeah, he was my advisor.
And what was his style as a mentor? How involved was he with your research?
Well, he didn't work in the lab, but he was very up in discussing how the experiment would be done and should be done. So for instance, an important paper in German about the rainbow effect was found by Dan, and that was certainly an important part of the analysis. So he was using his time to think about the experiment while I was busy grunging, trying to figure out why it didn't work the last time, and the oven kept clogging up, and you know, fixing whatever other problems we had - and getting better data.
And at the time, what did you see as your primary contributions, both from a theoretical perspective and an experimental perspective?
Well, I really had mastered interatomic potentials and how you calculate/approximate them, and what they might look like, much better than Dan. So that was one of the areas that was of interest theoretically. And then the other one was that I planned out all the experiment and for instance decided that we needed better velocity resolution, and we could get that by building a rotating shaft with many stacked disks, each with ~ 360 teeth on the edge. I got the idea for making the disks photo-etched rather than machined to simplify the construction. The photo etching was a little gamble there, because the photo-etched material, beryllium copper, it comes in a roll, and these disks start off being curved, and then only when you get it up to about 3,000 RPM do the centrifugal forces flatten out the disks. But it worked like a champ. So then I built that and I came up with some other innovations of how you calibrate it.
When you said it worked like a champ, what exactly worked and how did you know it worked?
Oh. Well, first of all, we were able to rev it up to 60,000 RPM in a vacuum, which is a challenge for bearings, because bearings like to have oil in them, and vacuums don’t. [laugh] And so you wind up with silicon oil, and then you wind up floating the bearings so that the—something going that fast has to be either dynamically balanced, or it has to be allowed to rotate about its principal inertial axis. And we adopted the latter—a cheaper, quicker approach—but that meant the bearings had to float, so they didn't have good thermal conductivity. You know, you'd solve all those problems. And then I realized that with the thin disks, the velocity selector had different paths you could go through it—and you could spin it either way. And so I took advantage of that fact by installing it tilted to the beam line, and then maybe it would have a velocity resolution of—let’s see, what did I do? I had—it was 10% rotating one way. Going the other way, it was 20% with a resolution of 5% in second order. And then going to second order on the first one, it was 4%. And so I had a selector whose resolution you could change. And that hadn’t been done before. But also, when you put these selectors in there, the actual offset angle that the particle is going through the rotor at—the rotor is rotating, but even at 60,000 RPM—that’s a thousand revolutions a second—the tip velocity is substantially slower, than the atoms. And so the offset angle is very small. And so if you misalign it a little bit, it affects the velocity calibration. But by being able to run it both ways, you could measure the offset angle so that you could calibrate the selector much more accurately. So my selector had several advantages. And that was just one part of my thesis apparatus.
Did you think about any possible practical applications of this research?
No. I was really consumed with just doing the experiment, and understanding it. And I maybe should get back to childhood. My dad operated—if I said he operated in a praise-free zone, that would be a gross understatement. He operated in a regime of mental domination. So for instance, any project I did would result in criticism. I remember rigging up my U-control airplane so that it could be a bomber, and that it could drop a multitude of bombs, and that would get criticized because it looked as if I had structurally weakened the wing. And sure, it broke, and I fixed it. But the idea that I had actually done something new that was rather interesting never got complimented. In fact, looking back at it, I think—and I don’t quite understand the motivation, but I think there was a certain amount of mental cruelty involved.
This really became a problem for me when we moved from Milton, MA when I was 11, where I had a lot of friends and where I had been in a wonderful class where a bunch of smart kids did fourth, fifth, and sixth grades in two years—so we skipped ahead but we never skipped anything. We just had a good teacher and a small class of selected students, and we could just go faster, and it was a very, very congenial group of people. No one was saying, “Oh, geez, you're some sort of brain. You should tone it down.” We were all competing – more to innovate rather than with each other. And I didn't have another experience like that until I got to Caltech. And so that was a very happy time.
We moved to New Jersey, and for various reasons, the next youngest kid in my eighth grade class was a year and a half older than I. I had been one of the youngest kids in my year in Milton, plus I had the skipped year, and Haddonfield had a much older cutoff age for kindergarten. So here I am, 11 years old, and these other kids are 13, and it was pretty tough. So my dad decides around then that he won’t call me by my name anymore, and that when I did these projects, if he couldn't criticize the project, he would criticize the fact that I left the work bench a mess. And so generally speaking, he would be always on me for being a slob, and finally he drove this point home by creating a “Slob Army”. He made me into a—private, I think is the lowest rank—Private McGillicuddy—and I was a private in the slob army. And every time I spilled my soup or got up to do the dishes and the fork fell off the plate, or didn’t clean the workbench, I would get promoted. And so I would be promoted—you know, I moved rapidly up the ranks. I was soon lieutenant, captain, lieutenant colonel, colonel, brigadier general, first star general, two star—blah, blah, blah, blah, blah—supreme commander. But it doesn't do wonders for your self-esteem when your dad refuses to call you by your given name but rather it’s always “McGillicuddy” and just criticizes you for being a slob and solidifies that distinction by a rank in a slob army.
Did you learn how to take the criticism in a positive direction, ever?
It wasn’t possible to take his criticism in a positive direction. It was designed to belittle.
But I mean, in terms of your own motivation?
What I learned was that my motivation wasn’t getting some external reward for doing a good project; it was doing the project, and enjoying the process.
That sounds like despite your father, not because of him, though.
That’s right. On reflection I see that it has resulted in a couple of things. First of all, a reluctance to really try to convince people like Norman Ramsey or my department head that I'm actually doing pretty good stuff. I feel that if I go and try and tell them that, they're going to criticize and tell me to get back in my hole. Which is not true, of course. Because as a mentor or a department head, you want to promote your young guns. But this intellectual fact was completely emotionally covered up. And the second thing was, since I wasn’t doing this to get elected to the National Academy or to garner the esteem of my colleagues, I just did what I felt like doing! So often, it was something new, and sometimes it was something new that nobody was interested in. But as it turned out, often it was something new and lots of people became interested in it. But really the reward was in doing it and in figuring it out, and just getting the self-satisfaction from that.
Your last two years at Harvard, ’67 and ’68, of course was a time of great unrest on campuses all across the country. Harvard was no exception. And I wonder if you were involved at all in any of those activities or if it affected your life at all, or you were mostly aloof from all of that?
Well, first of all, at that time, Dan Kleppner had moved to MIT. And so I actually assembled my apparatus there. A lot of it was designed and pieces were built at Harvard, but it was assembled and the experiments were done at MIT. I sometimes joke that I got the best of two worlds—an MIT education and a Harvard degree.
I once turned that phrase on Norman Ramsey when he corrected the person introducing me at a colloquium who said I got my degree from MIT (It was from Harvard and Norman was my co-supervisor). He complimented me a couple years later, saying that’s “probably the best anybody ever got me in public”. But the unrest came to MIT a little bit later, when I was an instructor or a postdoc. And yeah, I was very sympathetic to a lot of it, but I was realistic enough to criticize the fact that MIT divested itself of the instrumentation labs too hastily. The instrumentation labs started because a professor named Doc Draper figured out how to make gyroscopes that ran off the vacuum of an airplane engine, and therefore he could make gyrocompasses and artificial horizons for small planes. But by the Vietnam War, the I-labs were making stabilization platforms for the guns on the helicopter gunships, and also inertial navigation systems for ICBM’s. So this wasn’t too popular with the demonstrators, and so MIT just let the I-Labs go. I thought we should have sold stock in it or taxed its future revenue stream as Stanford did with the Stanford Research Labs. [laugh] But by and large, my sympathies were with the revolutionaries, as any young person with red blood would have been.
Who was on your committee for your thesis?
Frank Pipkin, who was another of my mentors in graduate school; Norman; and Dan Kleppner.
And was your trajectory to stay in faculty life? Did you ever consider going out to industry, or that was always your direction?
Well, industry – specifically high tech electronic start-ups - had been an original direction, and I switched slowly. The break came when I decided, “OK, I will take this assistant professor offer from MIT,” I hadn’t gone out looking for assistant professor jobs. And that’s a bad mistake—not taking a postdoc in some other lab and not going out and trying to tell the physics community why what you're doing is important and they should therefore hire you.
So you went right into the tenure track line at MIT immediately?
Well, I was an instructor for two years, but the professorship was promised. In those days, MIT had made a deal with the devil, as it turned out, which was that the department would require one third of your 9 months salary to be paid on soft money (i.e. grants) in exchange for being able to expand by 50% the number of tenured professors. And since in those days—it was just after Sputnik—money was so easy to get—it was easy, but you wound up financing five out of the 11 months of your salary from your grant(s) including summer pay. And as things got tighter, which they did immediately when the Vietnam protests started, because the government insisted that military money be spent on military things, not on basic research—there was a big crunch. And in fact, I was the only one of seven assistant professors appointed in 1970 who got tenure in the Physics Department. Usually, we were up in the 50, 60, 70% range. And the thing that saved me was the laser. What I was doing wasn’t going to get me tenure, this atomic scattering, and when I realized that, fortunately I was able to get my hands on the second commercially made CW dye laser, and did a couple of pretty good experiments in the two years remaining on my tenure clock, and the tenure flowed from that.
So what would have been the value—I mean, counterfactual question, of course—but what would have been the value if you pursued a postdoc elsewhere, for you?
Well, the logical places to have gone would have been to Boulder, or to Stanford, and worked with people like Hänsch and Schawlow, or Jan Hall, or maybe some of the other guys at JIJLA. And you’re gonna learn a hell of a lot about lasers and precision optics if you do that, right? And talk to new people with different perspectives. Whereas I had to learn everything about optics myself.
But how did you get into optics and lasers originally? How did that develop?
When I said I was going to go into atomic physics, my graduate student classmates at Harvard would say, “Look, all the effort, all the excitement, all the interest, is in high-energy physics, is in nuclear physics. Atoms, we figured that out long ago. What’s making that exciting?” And I would say, “The tunable laser is going to make atomic physics incredibly exciting.”
What was your insight to make such an assertion at that time?
[pause] Well, one was the idea that you might be able to push atoms around with a laser. Second thing was that you're going to be able to do some sub-Doppler spectroscopy easily. Third one was that you could just do measurements of incredible precision that way. Maybe make much better frequency standards. There’s a fundamental limit in the accuracy of a frequency standard – it is how many cycles you can interrogate the atom for. And if your cycles are at ten to the fifteenth instead of ten to the ninth, with a given interrogation time you can therefore make a much better frequency/time standard.
And did you have partners in this endeavor, or did you feel like you were all out by yourself working on these things?
Oh. There were other groups at MIT that were interested in lasers, and I could get some guidance and help from them. But lasers were a new frontier in a relatively moribund field, and the “old guard” in Atomic physics had no advantage over us young guys – of which there weren’t so many then. But basically, as I indicated, my way of doing things was to do it myself, figure it out. And I should add one other thing, too. So at some point, I had decided I'm not going into industry; I'm going to be an academic. So what are my life goals? If you go into industry, it’s to make something revolutionary and get rich. But getting rich is only a way of keeping score. I mean, Steve Jobs didn't do things to get rich. He did things because he wanted to do cool, novel things. And that’s the kind of mentality and abilities that I have. And so I decided on a set of goals and standards: I want to get the love, the respect of my department colleagues, which will be reflected in a chair. I want to win a prize in my subfield, which is atomic physics, an APS prize. And I’d like to be sufficiently successful, known, whatever, in the physics community, to get elected to the National Academy.
That’s a tall order for assistant professor.
And although I didn't aim at those goals, and although a couple of them were held up by my flirtation with the alien abduction phenomenon (it was a fairly serious interest, I shouldn't say flirtation). But I think that the department and the National Academy, I know for a fact, didn't want to give me a chair or a membership if they thought I was going to wig out or generate bad publicity.
Why would they be concerned you would wig out?
Anyone studying alien abductions has got to be three quarters crazy to begin with!
[laugh] Did you ever think about keeping that interest private, for that very reason?
I did, to a certain extent. I didn't want—with the exception of one or two undergraduates, freshmen or sophomores who would help with some of the lab work—I wouldn't get anybody involved in it for a senior thesis or a graduate thesis. But I had to be a bit “out there” so investigators of the phenomenon would give me samples of allegedly alien material or artifacts. And occasionally I was “the voice of reason” on shows featuring alien abductions.
Because it’s a death sentence.
That would just not be the way to start anybody’s career off.
How did you get interested in this topic, to begin with?
In which topic?
Oh. You know, part of the desire to do something new is that you pick up and cultivate the ability to look at the arguments of the community and look at what they’re doing, and saying to yourself, “That doesn't really make a lot of sense.” The fact that there might be aliens is well accepted in the scientific community. I mean, every time we thought that we lived at the center of the universe or that our sun was the center of the universe, or that the Milky Way was in any sense central, or that EurAsia and Africa were the only places with people we got surprised. For example, a New World with other people on it. All these things have been major scientific revolutions. I would almost turn it around and say the majority of major scientific revolutions have changed our idea of where we fit in the universe. And that goes for Darwin’s revolution—"Hey, we're just like big monkeys, smart monkeys.” Or Freud—“We're not in charge of our mind, even though we seem to be.” And obviously if you look at the interest in radio SETI, if you look at the interest these days in extrasolar planets, it’s highly motivated by, “Geez, there might be other intelligent life out there.”
But to further refine that distinction, are you just saying that there’s the strong possibility that aliens exist, or that aliens exist and they're capable of visiting Earth, and they have?
Well, there’s a possibility that humans are not the first intelligent creatures to evolve, and if there are ET civilizations, a lot of them have five billion years of learning on us, and they have probably figured out how to get here, if they want.
Were you ever concerned, self-consciously, that you had to bound this interest within a proper physics perspective, or are you so naturally a physicist that there’s no other way for you to think about these things?
I’d say it’s more towards the latter; I’m a scientist with some expertise in physical evidence. At least that was my initial orientation.
So what are some basic physics concepts that you would bring to these questions?
Well, one is the question of experimental evidence. And that’s what I looked for, was artifactual evidence. I wrote a paper in one of these side conferences about what constitutes physical evidence for aliens. And I first of all said that performance is the strongest indicator - a pair of Radio Shack walkie-talkies would convince 17th century scientists that they were from the future or another planet but they’d lack the technology to convince themselves that a modern IC is anything special. Secondly, physical evidence is only in support of testimony. Finding the murder weapon does not convict anybody. Finding the murder weapon where a gas station attendant saw somebody with a certain license plate throw something over the bridge is pretty damning evidence if it turns out that the guy who owns the car with that license plate is a suspect. And so it’s not the gun; it’s the testimony and the gun. So my dream example was this. You see the aliens land, and it’s a nice day, and they get out on this big rock, and they sit there looking at the valley, and they eat their sandwiches. And after they leave, you go up there and you find a piece of self-adhesive thin film that wrapped the sandwiches. But it’s 100 times stronger than Kevlar. That’s a pretty elementary measurement to make. And it’s pretty probative—it has a pretty strong implication that this is not made by earthly technology. Because Kevlar is one of the strongest fibers we can make. A factor of two? Likely secret. A factor of a hundred? No. This is something—“What the hell is this?” And then we take it and we try to do analysis on it, find out its made of pure carbon fibers aligned or some exotic carbosilicate. Or whatever. That’s the kind of thing I was looking for. And I tried several avenues, and I didn't find it, and I quit to do education.
But, you know, I stand by my first piece of contrariness. Many serious people think it’s reasonable that we should look for radio signals that ETs are intentionally sending us. But we haven't intentionally sent a bunch of radio signals to lots of other stars. We sent probes to Mars. So why isn’t it at least as probable that those guys sent probes here? And don’t tell me they can’t do it, because they've got 50,000, 500,000, five million, 50 million years on us to figure out the science and technology – and are probably effectively immortal and hence not so bothered by a 10,000 year wait for data.
Figure stuff out that would make sense within fundamental concepts of physics? Or if your suppositions proved true, then that would overturn some fundamental laws of physics?
Aliens don’t need new physics to get here necessarily. You know, you can easily spread life, even intelligent robots—I mean, I actually wrote a paper which is a follow-up on a paper by Orgel and Crick, that suggested that we send spores to other planets so that when we get there, there will be something for us to eat. These days, I don’t think that would go down too well with people who have strong beliefs that we should not introduce invasive species into new habitats.
But I just did a rough calculation. I want to make a little bacterium that if it lands in the ocean, can collect up the elements and make a robot explorer that knows about chemistry, physics, biology, and geology - and that can send messages, maybe by using a pulsed laser, about what it’s finding. So I estimated the amount of information in a half dozen PhDs. And it turns out that most of the weight, if you package the information as DNA, is in the scientific information that it needs to study its new world. But nevertheless, my calculation gave a very simple result. If I take one ton of these bacteria into orbit and I blow it up, there is a 50% chance of populating a planet that’s within one light year of us. So if I have a little gun and I can just shoot it with one milliradian of accuracy, then I can hit any planet around a star whose position and proper motion I know about out to a thousand light years, with just one ton of this stuff, and so I just shoot it at the various solar systems. So that’s one way. And so possibly what we're seeing as aliens are these robots.
So at the end of the day, what is your feeling about the existence of aliens, after all of this research? Are you more sure, or less sure?
Well, first of all, my colleagues would come up to me in private, and first they’d have to concoct some reason that they were watching “Unexplained Mysteries” or a similar show where I was the one ‘credentialed voice of reason’. Then they would generally say one of one of two things—“I really admire your courage for getting out there and looking at this, in what is, since I know you, going to be an objective way.” Or they would say, “Dave, do you really believe this stuff?” And for those who said, “Do you really believe in this stuff?” my response was, “No, I believe in the scientific method. I’ll tell you why my hypothesis is reasonable, I think, and then the question is, what’s the evidence?” And I still feel that way, but I certainly feel that my work —and the absence of other convincing evidence —has raised the lower bound on alien clumsiness. [laugh]. It really seems that the answer to the Fermi paradox is elusive —you know, Fermi, as soon as he started considering aliens, said, “Well, how come they're not here?” And that’s a completely reasonable proposition, even if you have only chemical rockets, if you have 50,000 years or hundreds of thousands of years, millions of years you can spread across the galaxy, with chemical rockets, and then colonizing the next outpost. Some true believers claim that there are aliens but they just don’t want us to detect them (remember the Prime Directive of Star Trek). So they’re back to trying to prove the existence of God in a way.
Have you ever allowed your curiosity in such things to go beyond strictly scientific considerations, into for example, the spiritual realm?
That’s a very good question, and that’s the most important lesson I learned from investigating alien abductions. That there are numerous kinds of spiritual experience that people have. And I will define spiritual as some powerful experience that seems real enough to you that you question whether it is part of physical reality, however crazy that seems. You may indeed have some physical evidence which is convincing to you, but it’s very unlikely to be convincing to me. For example, people who allege alien abductions often have little white indented scars, where they claim the aliens scooped out skin samples. And that seems to be a systematic feature of alien medical exams—they don’t take blood samples; they take skin samples as John Miller’s paper at our Alien Abduction Conference showed. But until you get forensic pathologists to look at it and say, “That’s something strange - definitely not a cigar burn or a cigarette burn”—people get tortured that way a lot—it’s not very evidential to me! [laugh]. Now within the category of spiritual experiences, I would list near-death experiences, out-of-body experiences, past-lives experiences. If you've read some of Stevenson’s work, you either have to believe he’s a charlatan or that there’s some reality to past life remembrances with associated birthmarks in the right places. Also—satanic ritual abuse experiences, predictive visions of the future, personal visitations by God. I think carefully, instead of six, maybe I can come up with ten. And I think that a couple percent of the population typically has experienced each one of these. I think alien abductions might be experienced by one and a half percent of the people, if you judge by Roper Poll asking questions that seem to indicate alien abduction experiences. As I remember the Abduction Study Conference that we ran—in secret, I might add—the prevalence of these spiritual experiences was driven home to me by a scholar who was an expert on all kinds of religions. He came up to me at the coffee break the second or third day, and he said, “Dave, you know what this conference reminds me of?” And of course, I thought our conference was pretty unique, so I said, “No, what could this conference possibly remind you of?” He said, “A satanic ritual abuse conference that I went to, two years ago.”
He said, “It’s the same arguments.” There are many fairly consistent reports of people being strung up on a wooden cross with the bottle of blood on one side, and the vase of roses on the other, and there are scars from the alleged abuse, and there’s a heated debate about whether it really happens or not”. That’s pretty much where the alien abduction phenomenon was then. And still is.
Where does your interest in problems and paradoxes in physics play in with this? I mean, by focusing on unsolvable issues in physics, is that to suggest that there might be some very fundamental assumptions that we have that might just turn out to be wrong?
Oh. Well, that’s two questions in one. So the first one was, as a kid, I was always interested in these mathematical puzzles, and then I was always interested in considering various experiments. For example, I can breathe through a hose. Well, why can’t I just dive down under the water, and have the hose go up to the surface, and keep breathing? And then you realize, oh, I see, well, no, the pressure in the water actually is getting greater, and it’s going to compress all the air out of your lungs. But at first it doesn't seem unreasonable. So I always thought that a great way to learn was to be presented with something that you can look at it two sensible ways and reach different conclusions about what happens—well, it’s like Einstein’s gedanken experiments. You think you understand about bound states and quantum mechanics. You know, there’s always a bound state in one dimension, but there is a minimum size and depth of the potential for three dimensions. So if I have a spherical potential that’s not quite strong enough to bind; why can’t I imagine that I just chop off the spherical potential along x, y and z so I have a cubical potential? And now I attack the problem in cartesian coordinates x and y and z. I separate the variables, and for each one of them I assign the well depth one third of the depth of the spherical potential, and now I have a bound state in each of the three dimensions, and obviously the product of the three is a three-dimensional bound state. So now I've shown that there is a bound state in a potential that’s demonstrably smaller than the one that doesn’t have a bound state if considered in three dimensions.
Which tells you what?
Which tells you that something’s wrong. And where is it? I mean, they both seem like good arguments. And I've mentored really three and a half people who won Nobel Prizes. And in fact, I would always get a really insightful graduate student to co-teach my puzzles and paradoxes course with me, so that one of us could argue one position, and the other one could argue the other position. We would go back and forth, being as good lawyers as we could. A physicist lawyer. Using the physical principles we know to try to convince the audience that we're right, and that the other guy is full of crap. Two of my four co-teachers won Nobel prizes. Carl Wieman was one of my mentees, informally and would have been a co-teacher—except that was before I was teaching the puzzles and paradoxes course. But he paid me what I think is a wonderful compliment although he thought he was damning me. He said, “You are the only person that I've ever known who was able to convince me that I was wrong when I was actually right.”
[laugh] That’s great! [laugh] What was your response to that?
Well, my response was, “Well, I'm going to take that as a compliment, Carl.” You know? [laugh] Because what it means is that here’s a guy who’s a smart physicist, right, and you were able to argue from first principles in a convincing, logical way, that convinced him that what he correctly thought was right was in fact not correct. Now, to his credit, I mean, he’d go back and think about it and find the hole in my argument, and come back, and I would have to admit, “Oh, geez, I didn't think of that. Yeah, that’s right.” For example, to separate variables in x and y and z coordinates, you have to have the potential that’s a sum of functions of x, y, and z (instead of a product) and therefore the potential that has all these bound states extends all through the x plane and all through the y plane and all through the z plane. So it’s much bigger, much stronger than your little ball, and so you're off the hook. But you might not think of it that way unless you think really carefully, which is what the paradoxes force you to do.
I want to see if there’s a possible connection between something you said earlier and your interest in problems and paradoxes, and that is, later in your life, you've come to believe that knowledge is a social construct. And so I think the idea of knowledge being a social construct—I mean, if you really tear away all of the things that—you get right down to exactly what that means, it’s that essentially we're trapped in our minds, and that there’s an external reality or an external objectivity that’s inaccessible to us. So I wonder, then, if there are paradoxes or unsolvable problems in physics, what that really mirrors is a limitation of our own minds, and it doesn't necessarily mean that there are problems with how the universe works that are simply unsolvable with or without us.
Well, OK. So there has been a lot of research about how our minds relate to the world. And obviously we're just judging the world by what our sense organs send back to our central computer as observations, however biased, which we may not be able to figure out, et cetera, et cetera, et cetera. And of course a lot of this—I mean, there are the other kinds of puzzles and paradoxes in real physics, like how do you make quantum gravity, or what constitutes a measurement in quantum mechanics. And having made atom interferometers and interrogated the atoms by scattering light off them when they go through the interferometer, which is classic which-way experiments, I've been involved in—and very interested in, a lot of those kinds of considerations in fundamental quantum mechanics. I'm also interested in science as a religion. We’d like to think that science is diametrically opposed to religion. And Asimov gives a good exposition in a wonderful interview with Bill Moyers that you can look up.
I've read it.
About how, first of all, we don’t believe in just one source of truth like the Koran or the Bible. And secondly, we don’t really proselytize that you've got to believe in this, or you're going to go to hell, or whatever our version of this is. Except of course those are only true in first order. If you look at what people are saying now about how you're going to get the coronavirus unless you pay attention to the dictates of science, it sounds an awful lot like that, doesn't it?
And then you have to ask, “So what do we really take for granted?” And I think one is that there is an actual reality, that if you and I are careful enough in our observations, we can agree on. Secondly, that there is the hallmark of reproducibility and furthermore that the reproducibility comes because there are laws. And not only that God doesn't play dice with the universe; but that God never plays dice with the universe. And in addition to that, that we're smart enough to figure out some of those laws, and that we have a process—the scientific method—that will keep us on a track to figure out a better and better set of laws to explain the universe. But if something is happening in one out of ten to the tenth cases, because there really is an intelligence who occasionally messes with the universe, we're not going to find that out with conventional science.
Why not? Is it because conventional science is too rigid?
It’s just because of the detection problem for—like right now, there’s a debate—what’s the fraction of people who have coronavirus and who are asymptomatic? And so there’s a paper that I read—I didn't read it carefully—that suggests that it’s many times the number of known cases. And this is based on looking at flu-like symptoms and finding an excess of flu-like symptoms, and saying, well, that’s probably due to the coronavirus. Well, the point is that the coronavirus right now is well below one in a thousand. And now if the ratio is not one in a thousand but it’s one in a thousand plus there’s another hidden one in a thousand, how many tests do you have to do to find that other hidden one in a thousand? To get any kind of statistics on it, you have to do 100,000 tests. Now, if that probability weren’t one in a thousand but were one in ten to the tenth, then you'd have to do ten to the ten to the thirteenth tests to see the difference between 1 and 2 in ten to the tenth. I doubt that we’ve that many tests on anything! OK, that’s what I meant.
When did you develop your interest in physics and education? Were these things that you were considering even as an undergraduate, or did it take for you to become a professor charged with teaching physics to students that you really started to think about education in a systematic way?
Well, when I was in high school, several of my friends would think that I would become a judge or a teacher. In fact, even in grade school, some of my closer friends would call me Professor X. So I think that’s in the blood. And there’s another interesting consequence of being raised in a criticism-rich zone, which is if I do some new scientific thing, I’m worried, maybe subconsciously, that people are going to shoot at that and say, “Well, you didn't consider this. You didn't consider that. You went off a little bit half-cocked here, didn't you? Won’t you admit it?” Whereas if my protégé Wolfgang does it and wins the Nobel Prize, it’s just the glory that’s reflected on me, and I don’t want to be forced to defend some comment that he made in one of his papers - unless I was a coauthor. [laugh] And so in some ways it was psychologically very comfortable for me to be a mentor or a teacher.
And I think a lot of things combined in making an experimental physics career appealing. I like small teams, whether it’s sailboat racing or in the lab. I like the camaraderie that comes out in those kinds of small teams focusing on some goal and striving together and often reaching it. And I think it personally has to do with going back and being hunter-gatherers and the males going out on the hunt. And certainly the students who have worked together, when they come back to my reunion and they haven't seen each other for 15 years, it takes them about 100 feet between when one gets out of the car and greets the one who’s already there until they're walking arm in arm. It’s because the bond is so strong that you get doing this kind of stuff. So small team work has always been very rewarding for me.
Obviously any professor wants to be able to convey concepts in physics clearly, but it seems like you have gone above and beyond by really making a systematic discipline out of it. And so my question is, what are your motivations in working so hard in the area of physics education research? What goals are you seeking to achieve by taking a big chunk out of your time that could otherwise be spent on sort of more classically academic physics pursuits?
Well, first of all, it has turned into education research. And in my teaching it has brought out a little of the contrarian in me. My attitude to most of the teaching I encounter is well epitomized by a comment that David Hestenes, who was the guy who made the force concept inventory and invented the idea of modeling and developed the modeling curriculum that around 10% of all high school physics teachers in this country have now taken. I think he’s clearly the preeminent figure in physics education research. About his colleagues’ teaching he said, “This kind of behavior would be as disastrous in the laboratory as it is in the classroom. Why don’t they evaluate their teaching practices with the same critical standards they apply to scientific research?” I’m afraid that many of my colleagues seem to feel that “I [the professor] am blessed with knowing how to teach even though I've had no formal training, that the education research literature is all crap, and that I am the best judge of whether the students are learning anything.” I don’t run around asking all the people who are big wheels in the MIT edX and the MITx, “So what’s your evidence that all of this online education is actually teaching the students anything or in particular anything more than they learn the less expensive way we used to do it?” Because there’s virtually no evidence! I mean, maybe by now ~10 papers that show that people actually learn in MOOCs (Massively Open Online Courses). Our contribution to this showed that the amount that they learn when measured in a couple different ways, is independent of the level of ability which they come in with. Which is counter to what a lot of people say. A lot of people say, “Oh, the MOOCs are just for the people who already have college education. They don’t work for people in high school.” That’s not true. We found that people in high school who take them and finish the course get the same amount of normalized gain or they get the same amount of improvement on a z score (these are both technical measures of learning) as young professionals who take MOOCs.
But how can we improve education? And I had really hoped that online education—I thought it was going to be like the laser. It was going to allow us to have a lot of data, and to obtain data from A/B experiments, and to really figure out how we could teach better. And we've tried that, and it has worked to some extent. And in fact we’ve shown that MIT students learn from the Mastering Physics program that our son and I wrote and ultimately sold to Pearson, which is really a Socratic tutorial program. It’s tops in science and engineering in the English-speaking world.
If you look at the fact that two and a half million people pay to use this thing every year, and then you look at the fact that as a famous physicist I have 30,000 citations—30,000 lifetime versus 2.5 million times per year. So it certainly rates as highly impactful. But also I wrote some good problems with interesting answers and I hope some students become fascinated in physics and science when they see the power of scientific thinking on these.
So forgive me—let me just play devil’s advocate in terms of your fundamental critique to physics education. I'm a physics professor, I lecture, I make a test based on my lectures, the students take the test, some do well and some do not, and those who do well get what I taught them, and those who don’t, don’t get what I taught them. What’s the problem there? Tried and true method. What’s the problem with that approach?
The real problem—and I think Hestenes showed that with the force concept inventory—was lo and behold, you may have taught them to answer those kinds of problems that you give - and hand-graded word problems are typically the gold standard according to physics professors. What David did was develop the Force Concept Inventory—a number of elementary conceptual problems about forces and motion. If I run and smash into you and I'm heavier than you and I'm going faster, is the force that I put on you bigger or equal or smaller than the force you put on me? And those students will come into the class and they will say, “Obviously the big guy who’s running is going to put much more force on the other guy.” Or the truck on the sports car. And then they take the course—so at the beginning of the course, maybe 60% of the kids will get that question wrong; they will answer it as I just indicated. And then they go through your Newtonian Mechanics I, and then they do great on your problems. I suspect they’re using well-known methods like looking for equations connecting the givens and the unknowns (which you enable by always using), or by cancelling units in a numbercal question. Then you give them Hestenes’ test again, and the fraction who—of that 60% who got it wrong at the start, it turns out that less than only 25% will answer the question correctly after your course. Were you really teaching them Newton’s Third Law? Well, not really. You taught them to regurgitate his statements, but you didn't teach them to think that way with their intuition, and to approach either real-world situations or unfamiliar problems that way. And when you grade your exam, sometimes with your co-professors, you laugh at the ridiculous statements that some student made. I have a wonderful case of a guy who found the tension in the string using the perfect gas law (where T is the temperature rather than the tension). And he worked it through using the perfect gas law rearranged as T = pV/(Nk). It was a question about a block that had a certain mass and a certain velocity here, and a certain other velocity over there, and you're supposed to find the tension in the string that was pulling it and making it go faster. And so he had a p, which he assumed was momentum, so that was the mass times the velocity. And then there was another v, so pV must be the mass times the initial velocity times the final velocity. And then he had k, which he correctly wrote was 1.38 times 10 to the negative 23. But he couldn't find N, the number of atoms in the sample. But as far as he was concerned, he was proceeding in a totally valid way to solve that problem, and it just—just he was puzzled by how to get N. And I remember thinking, “My god, how does this guy miss the big picture by so much? If he passes, I have failed as a teacher.” And fortunately, he failed as a student – but not by much. Serious misconceptions appear all too often if you actually examine the papers of the students who got Cs. Is that satisfactory teaching? Well, they got 60% partial credit on your exam questions and passed. So that’s my response.
Given your emphasis on nurturing students’ ability to intuit, what are some of the hardest concepts in physics to teach, and what are some of the easiest concepts in physics to teach?
Hoo, I haven't thought about which individual concepts are hardest. And I want to cop out or side step it and say the main thing that we're trying to teach in a college education and in physics is to question things and to think from fundamentals. And in this context I’d cite my puzzles and paradoxes course that I used to teach in our Independent Activities Period. They're really good because there seem to be two different ways that give two different answers, or you just don’t see how to solve the problem at all. And you then have to go back and really think through, how do you get this knowledge? What is the knowledge that you have? And in doing that, you will find that you have to exercise these ideas, like superposition or what it really means to make a measurement in the Copenhagen interpretation. Or what justifies using the spherical Bessel function in the first place. And that’s the kind of thing that doesn't get taught. And there are numerous studies to show we teach too much about the trees and not enough about the forest. One of my colleagues at MIT in biology made a big, systematic, four-level compendium of all the topics in introductory biology—and then looked at standard biology tests. And biology tests will ask, “Which amino acids make up the generic code in DNA?” — not “What is a gene?” Or, “How many codons do you need to code for one amino acid?” These are third and fourth level things. And the second level is, “How does heredity work?” Darwin’s work was criticized by the following argument, which is a puzzle. Assume one proto-giraffe happens to get an extra-long neck. Now, his kids are going to have half that extra length, and his grandkids are going to have a quarter extra neck. And so this improvement is going to just disappear into the noise. So how does heredity actually make the fittest traits survive? And until you have the idea of genes that are units of heredity, you can’t understand that easily. Well, the giraffe is going to have two kids, and one is going to have a long neck, and the other is going to have a short neck, and long neck guy is more likely to survive, and he’s going to have more kids than the short-neck guy, because he can get more food. And then half of those kids are going to have tall necks, and blah blah blah blah blah. But the kids who can answer the third and fourth level question can’t answer the first two, can’t address the big picture. And that’s what happens all too often in physics.
Another key problem in teaching is that studies of experts show that if the expert solves the problem as fully as they can, that about two third of the requisite knowledge to solve the problem, they didn't even write down. “They don’t say, you have to start using an inertial coordinate system here, not the edge of the wheel” but start F=ma in a non-inertial coordinate system and follow with the resulting equations. And so we don’t give kids intuition, because we don’t start at the higher level. So a lot of these puzzles and paradoxes try to get at that, and so does the modeling approach to problem-solving. Energy and momentum and angular momentum and Newton’s laws are usually taught in different chapters with little concern for their interrelationships. They're all a manifestation of how force changes motion. And for each one, there’s a motion variable—it might be the momentum, it might be the kinetic energy, it might be the velocity of a single particle, it might be the angular momentum of a collection of particles.
And then there’s some aspect of the force that changes that. Once students understand this—this is what our “modeling applied to problem solving” pedagogy emphasizes, it makes it so that when they have a completely novel problem, that say, “I don’t know but answer, but I know how to start finding it.” So while the direct answer to your question about hard concepts is that “force changes motion” (rather than causing it) and Newton’s 3rd law are very difficult concepts. But what we really want to teach are habits of mind like figuring out what are the most fundamental ideas to start with and being critical of your answer (or your professor’s answer) and always checking if it makes sense.
Adjusting for the obvious differences in knowledge and maturity, do you take a basically similar or different approach in education towards undergraduates and graduates?
There’s a part of me that likes to be Mephistopheles. You know, “Oh, so you think this? Well, does that mean that—?” Or—like in our homework tutor—“You know, David, it’s 2:00, and you can’t start problem three so you’ll never get those 10 points. How would it be if I broke the problem into four parts, each of them worth only two points?” So you're sacrificing two points to get these four straightforward parts. Will you take the deal? And that’s what I mean. And that's the way I like to teach. Obviously, maybe the student isn’t up to it, so the student just has to be reminded, “OK, what are the various ways you can write down Newton’s second law?” Oh, well, you could write it F = ma, but you can also write it down with dp/dt. Now can you look at the system and figure out what the momentum is? So you try to lead them along with somewhat closed-ended questions. But I much prefer to ask open-ended questions, like “How do you jibe this with that?” So I wind up using more open ended questions with graduate students, and more leading closed ended ones with undergrads – sort of like helping them learn to swim before asking “how are you going to get over there given the intervening swamp?”
And yeah, being a mentor—well, there are a lot of things involved in that. At one point, I won an award for mentoring. And I had to give a talk at a conference for advisors and mentors in a meeting for alumni of an education school. And suddenly I realized I didn't know what the hell to say. So I sent open ended questions to some of my former mentees, and I did a couple rounds of that. And I found out that there were a number of things that I did that were really helpful, but the one that got the most praise was a thing that I call junior student lunch. I found that the students who were in the group for the first couple of years would come to the group meeting, and they would report how their project—their electronic project or their project in the shop -- was going. Usually, if we were talking about which experiment to do next, they wouldn't be up to speed. They wouldn't have read enough of the relevant papers and so on. So I wouldn't get an idea about how they really thought about physics - about their intellects. So I said, “OK, I'm going to collect up interesting open-ended questions and puzzles and paradoxes, and we're going to do/discuss them at lunch once a week. None of the senior students, none of the postdocs. I'm going to ask you these questions; you're going to figure them out.” And they said “That experience was so valuable because we knew we would be working on something we hadn’t thought about in our future research”. Or, “I learned to say something sensible, and why it might be sensible, or why possibly it wasn’t sensible, and then say what I don’t know. And to listen to other people when they did that, and to try to pick up on what was useful.” One of my students is president of a small university, and he says, “There’s nothing that has prepared me better for being a college president than Junior Student Lunch.” To be asked a question you hadn’t really thought about, say something sensible, and then say, “Well, I'll look into that.” And then the others said, “This was just great. It really made me more fearless in tackling hard problems, and gave more ability to just do a quick calculation that showed whether something was bullshit or not.”
Another frequent comment was that they perceived me as having two modes: a mode where we're brainstorming for ideas and anything goes and we'll pick up on that and try to carry it forward. Like, why isn’t this thing working; what might be a cause, what might be a remedy? And then there’s a much more rigorous mode, “You've got to really say what you did here, which you didn't do. And you've got to explain that there was some pre-processing of the data, which you didn't do. And then you've got to give an overview of the paper.” And there they felt that I was harsh but justified in that mode, but that these were two different modes. And a couple of them complimented me on having a sane attitude towards success, “Well—” [cough] Darn, I'm not used to talking this much, for this long!
They’d say, “So, when we got some new data or new result, that was great. And obviously you were pleased. And when we got the paper accepted, you were very pleased. But that didn't last very long.” I mean possibly at the end of the day, we’d go have a beer. But in general, they felt my reaction was, “OK, so that’s good. Where does it lead? What new problem can we attack next? What can we do with this?” And they found that attitude helpful. And learning to just enjoy the process of research for itself, not for the external rewards. I think my Dad’s influence is partly why that’s the way I am.
I want to ask you a very broad question about all of the different projects that you've worked on, and in all of the different subfields in physics you've worked on. The first part of that question is, it’s a process question. What’s the process by which you choose what project or what field in physics to work in, at any given time? And the second part of that question is, if you can answer retrospectively, as a narrative, you worked on this, and then you worked on this, and then you worked on this. Retrospectively and in real time, do you see those transitions more as island hopping, meaning that there’s this island that you worked on, and then you totally switched gears and you're unconnected, and now you're working on this project? Or is there a through line that connects all of the very different and diverse projects that you worked on?
Those are good questions. As an experimentalist, there tends to be a through line, based on expertise and equipment. So I’d been doing atomic collisions and knew I wanted to get into tunable lasers. So I got the laser and started to study collisions of excited state atoms. But I was also thinking “I have this tunable laser. What else am I going to do with it?” One night I woke up and started thinking about two-photon spectroscopy, kind of rederived Goeppert-Mayer’s thesis result, and as far as I could calculate in my bed decided that we could do that in Na. That quick experiment was probably the experiment that more than any other got me tenure. Because we beat Bloembergen/Levinson and the famous French spectroscopy group at Aime Cotton lab by two weeks, and furthermore we used a CW laser, which you wouldn't think would be the way to do two photons. You'd want to use pulse lasers so the signal, proportional to power squared is as big as possible. And as a consequence, we had much higher signal-to-noise data, and much narrower lines. So that’s the kind of home run that you need to get tenure at a place like MIT.
So after doing the excited state scattering experiments and finding that the theories were off by significant factors, we started thinking. Well, we've been doing these collision experiments that show that there are very weak Van der Waals potentials between, say, potassium and mercury, or even between sodium and argon, and maybe we could figure out how to make molecules of this stuff and do laser spectroscopy. So all of a sudden, you're in a pretty much different realm. Sometimes it’s long-standing interest in addressing certain problems. Like the idea that when we got the atom interferometer, one of the big payoffs would be that we could actually shine light on the particles into the interferometer, and turn all these gedanken experiments about interrogating the particle while it was traversing the interferometer into something real. So there’s some physics idea that motivates it. Sometimes it’s contrariness. Well, all these guys, these precision guys, they're all building clocks and measuring time. Why isn’t anybody trying—or only one group, Van Dyck’s—trying to measure mass very precisely? After all, with a laser, you can measure energies from a hundredth to a few eV using IR or visible light, but with mass, you measure MeV. Well, you know, a lot of interesting questions in nuclear physics are at that energy scale. So why don’t we try to measure mass well? And then you start thinking about that, and you say, “It’s just a question of measuring the cyclotron frequency. How do we do that?”
And so that one required a brand-new apparatus and quite a big gamble in terms of writing to NSF for support for a magnet, when I didn't have a detector sensitive enough to detect single ions. I wanted to detect the general ions, not just the light ones —whose high oscillation frequencies you can detect with FETs as van Dyck showed. And so, you know, you're gambling your reputation, a little bit. And so fortunately, we figured out how to make extremely high-Q circuits and developed new techniques to make these measurements, and then that led to a lot of pioneering work in mass spectrometry, which was completely different. Another example of contrariness is thinking, “Well, all these other guys are trying to suppress the Doppler shift in a gas and measure sub Doppler. What if you took advantage of the Doppler shift to pick out fast or slow atoms and then studied their collisions?” Or fast and slow molecules, and then you did experiments about how they transfer the excitation they have to other atoms or maybe just how molecules change vibration or rotation levels upon collision. And we did some really classic work in vibration rotation energy transfer in molecules. Sometimes it’s one thing leads to another.
In the case of forces on atoms, I refereed a paper, and I endorsed an explanation in the paper that I later realized was hogwash. And I said, “Wow, if neither the authors nor a person who is presumably one of the experts in this field (me) don't understand this light forces business when you crank up the intensity and have some spontaneous emission, that’s a fundamental knowledge gap and let’s see if we can start studying this.” So we started studying the scattering of atoms by standing waves of light and observed the long ago predicted results of Kapitza and Dirac (for electrons, not sodium atoms), and then Bragg scattering of atoms from the standing light wave. We observed momentum transfer in discrete units of photon momenta and these types of scattering have powered the development of more powerful atom beam splitters and atom interferometers.
Meanwhile, thinking about all the advances in precision measurements due to ion traps, I started wondering how to build an atom trap using laser light. The best thing to do is to use the scattering force of the light, not the fact that if you focus down like Ashkin did—if you focus down the beam to a very high intensity region - that’s just like a region of high field and it sucks the atom into it because the atom polarizes. But that’s just a very, very, very small volume trap. I mean, it has a lot of wonderful uses, especially in biology. But what about trying to use the spontaneous force? Well, Arthur Ashkin and J.P. Gordon had proved you couldn't do it. But if you really look at their proof—and my insight actually happened when I was explaining it to a student—you realize that they were talking about trapping glass beads. They weren’t talking about trapping atoms that have internal structure and therefore respond differently to different polarizations of the light. So maybe you can defeat their theorem, and we thought of several ways and tried several, finally getting a key idea from Jean Dallibard after challenging our colleagues in my talk in Finland. And that’s how the magneto-optic trap came about. And that’s probably the scientific contribution that I've made that has changed the world the most. I mean, the whole ultracold atom center at MIT and Harvard is just based on that trap, as is most of the BEC and cold fermions work: people start with a magneto-optic trap. And suddenly they have a fairly dense cloud of atoms, and they're at a quarter of a millikelvin instead of room temperature. I mean, that’s a really huge jump.
You discussed how you got new ideas that you pursued; you must get many ideas for new directions that you don’t pursue also. How then, do you pick the ideas to follow?
Well one way is an explicit way of ranking potential projects that some of my students call “Dave’s Formula”. It’s really cost benefit analysis. You have several possible directions and this formula gives the merit of each one:
Merit = (Scientific Impact) * (Likelihood we can be a pioneer)/(Effort)
– and if a couple of directions have comparable merit, then recompute with the impact squared. The likelihood term reflects our capabilities and the lead our anticipated competition has – essentially can we be one of the leading 2 or 3 groups times the probability that this idea will pan out. We don’t apply this rigorously, but as a guide to our thinking. When the playing field is open – for example when we had the world’s only atom interferometer which physically separated the atom wave on the two sides of the interferometer – we did this explicitly; sorting through ~20 ideas we had. The top three or four ideas were then each assigned a champion who would do more thinking about that experiment. But this formula is also my overall guide to which new area to get into.
Obviously the Scientific Impact is the hardest to evaluate – how do you do this?
Fundamentally I believe that science is about changing what other scientists think about, what other experimentalists do, and ultimately about changing how we think about reality and our place in it. For me, impact might mean that 3 years later there will be half a dozen posters at a conference that all build off the results or ideas in our paper. A higher level is that there have been entire conferences on things like atom interferometry or precision mass measurement with most of the papers referencing our pioneering work. Obviously developing something (like the maser, magnetron, or atom interferometric rotation sensor) that has useful applications is another avenue to impact.
Selecting research directions also involves matters of taste, belief, and personal factors. How do you fold these into your formulaic selection process?
Yes, these factors are always involved in such decisions – both consciously and unconsciously. My view of physics (and much of science) is that experiments decide what’s on the table in this era, and theory arranges it into simple patterns. I’ve been driven a lot by experimental possibilities: the tunable laser attracted me to atomic physics (even though it had not yet been demonstrated). That light could exert large forces on atoms guided my selection of atom optics and atom interferometry – plus interferometry has a wonderful history of precision measurement and truly fundamental experiments. A personal desire for novelty and contrariness, testing E=mc^2 much better, and other applications underlay learning how to measure mass (rather than time) precisely. Contrariness also influenced a couple of singularly unimpactful directions – combined electronic-vibrational energy transfer in atom-molecule collisions and longitudinal atom interferometry. The lack of interest and the frequently cavalier treatment of atomic collisions urged me to get into that also.
Have you ever experienced a truly dramatic eureka moment?
I mean, all of the contributions you've made and discoveries that you've been a part of—I understand that mostly it’s an incremental business, where you have to step back and look at it in total. That’s usually how it works. But there are dramatic eureka moments, and I'm curious if you've ever experienced one.
First of all, when I'm getting sick —just as I begin to get a cold but might not recognize it --- I get very depressed. And maybe I have a slight headache, but I haven't identified the fact that I have a virus yet. No. And so I'm lying there, and I'm thinking about my experiment, and I'm saying, “This is a bunch of crap. Why am I spending all this effort on this experiment? Is it really going to be all that great?” And what’s really wrong with this experiment is, we don’t have that good velocity resolution, and so all this quantum structure is going to wash out - all these quantum oscillations and cross sections –ugh! Oh, so maybe we need a velocity selector. Can we design and build a velocity selector, put that in there? I remember specifically—it was after dinner, and I had gone in my room, and I was lying down on the bed, and I was just cursing my fate as a graduate student working on this experiment, and that’s where that idea originated. And of course it made the experiment much, much better.
One eureka moment that I remember very dramatically concerned our excited state scattering experiment. Calculating potentials between sodium and argon, say, is very, very difficult, because the potential is a Van der Waals potential and it’s very, very weak. And if you're going to do a full-blown calculation, you're going to start from the ground, the bare nuclei at a certain distance. You're going to pile in all the electrons. And you're going to have thousands of—tens of thousands of electron volts total binding energy in this system. And now you want to know by how many millielectron volts it’s going to change if you change the separation between the sodium and the argon from ten atomic units down to nine atomic units. And it’s a very difficult calculation, and there are lots of approximations, like pseudopotentials, charge density, truncated bases, etc. Pseudopotentials was popular - where you somehow replaced the core of the atom with some fake potential whose one-electron solution mimics the valence electron’s behavior. And so we had decided we were going to test the calculated excited state potentials in the experiment I mentioned above. Exciting the sodium beam involved another clever trick, but I won’t get into that.
And so the optimum potential to observe was of sodium and argon and we were in the lab, and we got the whole experiment working. I was physically working in the lab, then. I was an untenured assistant professor. And the results started coming in. And it was bad—we thought that we should be seeing a big enhancement of the cross section when we turned the laser on at ten or 15 degrees, and we were seeing a factor of more than three reduction – there was almost nothing to study. And so we didn't have any interesting data to analyze. And I'm sitting there—I remember actually sweating. I mean, it was one of the few times that the connection between what I was doing in the lab and getting tenure or having to find another job was thrust upon me. And so I'm furiously redoing the calculation of how much extra momentum is given in this process of optically pumping the atoms and keeping them in the excited state. Is it possible that I had simply made a mistake in that calculation, and the recoil photons are imparting so much momentum they're throwing the atoms all over the place, and we're never going to see anything? I mean, that was a pretty scary thought, and it made it harder to be absolutely certain I had all the decimal points right. And I’m pretty nervous. And at this time, my graduate student, Gary Carter, said, “Well, this just isn’t working.” Gary was a guy who never believed anything would work.
And so he then said, “Well, I have a bottle of neon. Let’s try that instead of the Argon.” Well, you know, the neon potential wasn’t supposed to be deep enough to do any significant scattering! But we tried it, and lo and behold, we had a big signal with lots of angular structure. And so we analyzed that, and we said, you know, the pseudopotential calculation, and even Gallagher’s experiment that confirmed it, must be wrong. And it must be—there’s no way those atoms can get scattered out there at 15, 20 degrees, and we see all this structure that looks like the familiar rainbow scattering --- if there isn’t a much deeper potential by like a factor of 15 than the world believes. And that was a eureka moment. More emotional because it was preceded by a couple of hours of failure and panic.
[laugh] Well, had it not gone that way, I want to ask you—what is the scientific value of failure? If you're involved in an experiment and it just flops, or you just hit a wall and you can’t go anywhere, how do you make productive use out of that?
Well, failure can mean your experiment does not work – or that it does but the result is uninteresting. If it doesn’t work, you try to get out of the mess. And lots of times, something about the mess will suggest to you pretty quickly what the source of the trouble is. And then you go and fix that. OK. Now, sometimes if you're doing BEC, for instance, and you're a pioneer in that field. (Note, I always want to be a pioneer in whatever field I'm working in. I don’t like reading all the literature and making the incremental advance; I just want to do something new.) What if this supposed new BEC phenomenon doesn’t seem to be there or it's clearly a higher hill to demonstrate than we thought. But look, we have a large stable BEC so let’s try something else. There’s other predicted phenomena, possible experiments. There’s other things we can do here. Instead of trying to shake it, why don’t we try to stir it and make vortices instead of making pressure waves that travel back and forth? Contrariwise, when you're doing precision measurements, the goal for success in much narrower – i.e. measure the mass of Cs much better. So you have to overcome one roadblock after another. And so—I remember the worst time—well, there were two really tough times. I mean, we worked on that for 20 years. And the worst time was were when we had rebuilt a lot of the apparatus and we were getting lots of noise. We couldn't figure out where it was coming from. We had rebuilt the detector, and we had rebuilt the power supply and other electronics. There could have been a lot of explanations for where this noise was coming from.
And we finally decided—we couldn't tell if it was in our electronics or if it was outside the apparatus. And we bought a lot of copper screening at the hardware store. We wrapped the whole apparatus up, shielding it from outside. And we still saw the noise which threw us onto the track of thinking that it was our electronics. But in fact we had one wire that came from a frequency standard upstairs, down through the floor and the screen, and it was catching interference from a newly installed electronic switching power supply for an air conditioner in the upstairs lab. And so for the rest of that year, we were just stuck, and we went backwards, and tried reconstructing the apparatus the way it had been until we realized our mistake.
And another time was we were doing our two-ion experiment and we finally got two ions in the trap at once, so all the systematics due to the magnetic field changing when the subway started up or when people use the elevator or just because a truck drove by outside the lab --- these fluctuations were cancelled out because we were measuring the difference of the cyclotron frequencies of the two ions at the same time, and whenever the field changed, it affected both of them the same way. So that was wonderful. But then we started seeing jumps in the difference, and we didn't know if the jumps were coming because our frequency system was bad (we had ~ 7 synched frequency synthesizers), or because the ions’ motion in the trap was shifting between several different modes. And we went around this problem for most of a year.
And finally, we built an external field monitor, and the students came to our weekly meeting one day and said, “We've done correlation studies of the external field monitor with the fluctuations, and we’re pretty sure it’s the CO that’s jumping, and not the N2.” And I said, “Oh, yeah! The difference is that N2 doesn't have a dipole moment, and CO does have a dipole moment.” And—oh!, the dipole moment will give a large polarizability. A molecule, although it is an electrically charged arrow, it’s spinning around, and when it exists in a state of good rotational quantum numbers, you can show that its parity is definite, and it doesn't have a first-order Stark shift as it would if it were fixed in space, it’s only a second. But it can have a big second order term which is the polarizability. Indeed, it’s going to be much more polarizable than an atom because the field only has to mix the closely spaced lowest rotational states, not some electronically excited level. Years ago, when starting this experiment, I’d shown that the polarizability of an atom shifts its frequency, but that this effect was too small for us to ever observe. And then we realized that what we were seeing was the four-kelvin black body radiation in the apparatus driving the lowest rotational transitions in the CO molecule, and that it had different polarizabilities in each one of those states. We were able to show that this radiation was driving the transitions at the right rate and that the system obeyed Boltzmann’s law for the fraction of time spent in each state. We could then make the best measurement of the dipole moment of a charged molecule that had ever been made. And this was a general effect, and one implication was that it significantly shifted the measured proton-anti-proton mass ratio. But we were lost in the jungle for a year.
So sometimes you power it through and you discover something you didn't know. Sometimes you just don’t have a signal where you think you should have a signal, either because the apparatus just doesn't work and you ultimately can’t figure out why, or because the effect you think you're going to observe doesn't exist, which is a little safer situation, because at least then you can measure how close to zero it is. And sometimes you have to give up. And sometimes you get new experimental insights, and sometimes you realize, well, from where we are, we can do a closely related experiment, and so let’s do that instead. So that’s a long-winded answer, but I have no magic path to new areas of research – just keeping my mind open to possibilities that seem interesting and pursuing the most meritorious ones.
To what extent have you relied on advances in computational technology to further your own research?
In the Van der Waals molecule business, we worked closely with some people who were at IBM Research labs. Bowen Liu and Roberta Saxon had access to a lot of top-of-the-line IBM computer time, and they were able to do good Van der Waals potential calculations by figuring out how to go to 40,000 basis states or more. It was beyond the state of the art. So it was important for us to collaborate with them so that what we could measure well would be something they could calculate well. They might prefer a lighter system, and we might not be able to do the lighter system, so we’d have a discussion and, “Maybe that one is fine for us, but from what we know, it’s going to be way too shallow. It’s going to really push your computation too hard. So we'll do the molecule NaAr.” And then how did it turn out? Pretty well is the answer.
Back to computation. Well, the big, big, big difference is for experiments—how computers came into the lab. When I was doing my thesis, I knew there would be a lot of data, and so we realized that the counters we were using to count the particles—spin-up and spin-down atoms—would count for a few seconds the spin-up and spin-down with the target gas on, and then with the target gas off, and then with target gas on, and target gas off, and then we’d take a bunch of measurements, maybe a dozen, and then we’d do the subtractions to figure out what was due to the target gas. And then you'd be doing several of these for each angle. You're making a plot versus angle. So you have what was then lots of data—I can remember the teletype coming out with the data on paper—I guess it was the first good data for my thesis —and it was about 20 feet long. Dave Burnham and Dan Kleppner and I cut it in three parts and each took a third home; and we came back the next day, and I plotted it all up. But that wasn’t going to work as we got more and more data. The solution seems obvious: the teletype makes a paper tape—it’s an eight-hole paper tape, and you can take it over to the IBM tape reader and then put it onto a computer that puts the data on cards to be fed to the computer.
And at that point, you discover, oh, those bastards at IBM! They invented their own eight-hole paper tape code, and it’s not the same as the ASCII (American Standards Code for Information Interchange) that the teletype uses. And in fact, the “new line” symbol in the ASCII code means “end of message stop completely” in the IBM code, so when you put the tape into the reader [laugh] it just stops. Just keeps stopping. And so we made a pin board where you take the signals from IBM paper tape reader—switch the columns around and punch out some cards from the tape that have a different code, and then you code the cards back into a paper tape, and then you put that into the IBM computer. I mean, we were at this very, very, very ground level of analyzing lab data with computers. And now, I mean, there are ICs that are small computers that do the whole feedback loop for you. You know, the integral, the differential, and the value. They're called PID—proportional, integral, and differential—and these tune themselves up for the best feedback. I mean, stuff is easy! And then all kinds of analog to digital, digital to analog, little devices that plug right into a USB port.
And the result is – given a desire to do something new and significant - you just do much more complicated experiments. So all this enables experiments like those people are now doing—making the slow atoms, making the magneto-optic trap, making a dark spot magneto-optic trap, condensing it into a magnetic trap, doing some other thing in the magnetic trap, RF evaporation—all these steps that go into just making the BEC. And now that’s all automated. In fact, people are using machine learning algorithms to tweak the many variables to get the biggest condensate. And now you can get the BEC right away, and then you start spending your day doing something with that. Then you put it in the lattice, and then you do something with the lattice, and then you have this fancy way of reading it out and more computer controls, more lasers. My thesis apparatus looks as crude to today’s students as the diffusion pumps hand-made by glassblowers in the 30’s looked to me when I was doing my thesis.
I'm not sure if it’s a perfect binary, but can you explain—did you advance quantum theory through your work on atom optics, or did you advance atom optics through your work on quantum theory? How does that relationship work?
I think really that atom optics is quantum engineering.
What’s the difference between quantum engineering and quantum theory?
Quantum engineering is discovering and exploiting possibilities that exist in non-relativistic quantum mechanics. Say I’ve invented some refrigeration cycle that would make atoms cool off if they repeatedly went around it. We know different ways that I can switch atoms from one state to another. I can use straight resonance methods. I can use adiabatic rapid passage. I can do a stimulated Raman transition. And now I want to manipulate these atoms in my trap to realize this cycle - what methods do I use from this plethora of quantum phenomena that we understand? And so here’s that example fleshed out. If I have an atom in a magnetic trap, and it has two hyperfine states that have a different magnetic moments, then it’ll have one trapping potential that’s deeper and steeper than if the atom is in the other state. And now I start thinking about how can I use spontaneous emission to make it go around my cycle in a way that cools. This idea is like 30 years old; I have to think about it for a moment. I understand the Franck-Condon principle for traps, which is that the atom isn’t going to change position or momentum in the trap very much during a transition. So I want the atom to spontaneously decay.
Imagine the atoms start in the more deeply bound state. I adjust the RF frequency so only the atoms with the most thermal energy can get to the high value of potential where they are nearly stopped. These atoms have little kinetic energy and they are now stimulated to the more weakly trapped state, so they have far less potential energy. Then I use optical pumping to excite them back up to the more deeply trapped state, but closer to the center of the trap where this imparts less potential energy. Now the atoms are cooler and more tightly confined so they don’t get so far out in the trap, so I now slowly adjust the RF to excite the hottest of those atoms and transition them, resulting in more cooling. And now I'm going to repeat this whole process automatically by slowly lowering the RF frequency. This idea is also similar to Sisyphus cooling, which is a strange form of cooling. I’d consider regular Doppler cooling where the Doppler effect picks out the atoms that are moving towards you, and you whack them from the front with a photon, and they slow down that’s almost mechanical engineering. But if you experiment with the polarizations of two lasers, you can cool many times below the limit for Doppler cooling; now this is called Sisyphus cooling, and it’s an example of this cycle that I was just talking about, where the atom optically pumps itself to a state that’s the bottom of the potential.
And so then it’s like King Sisyphus myth; once the kinetic energy has been expended to push the rock uphill, it changes state and finds itself at the bottom of the valley in its new potential, and it has to do the whole thing again. The cyclic idea for traps didn’t work well, but we then suggested that one could use the RF to knock the hottest atoms out of the trap, leaving colder atoms behind – an idea called RF evaporation (just plain evaporation was previously demonstrated). This worked incredibly well, and most people who do Bose-Einstein condensation use now. All those ideas are quantum engineering. And it’s much like the people who are doing quantum information these days and coming up with new algorithms. I mean, in one sense, they're not adding to the very fundamentals of quantum mechanics. On the other hand, such ideas are destroying ideas that we have held dear for a long time, like that your precision is going to go as the square root of the number of atoms. If you do it really cleverly, you can put the atoms in quantum states such that the precision of the measurement should go as one over the number, not one over the square root the number. That would require essentially a perfect quantum computer operating on your n particle system. That’s a high order. But then you think of something that does that approximately and recovers most of the advantage even with a few errors. And so this kind of quantum engineering is an active field of research in this boundary of quantum mechanics and application to quantum computing, or to measurement, or to cooling atoms. One of my favorite Feynman quotes concerned Eddington’s claim that there were only three people who understood general relativity when he wrote a review of general relativity—Einstein, God, and Eddinngton.
And Feynman said, “That’s not true. Any good theoretical physicist who read Einstein’s paper understood general relativity. Quantum mechanics is different,” he went on. “No one understands quantum mechanics.” And what he meant was things like—like Berry’s phase, and hidden variables, and Bell’s theorem, and quantum cooling and computing. One of my friends from chemistry actually, Jim Kinsey—unfortunately deceased—said it well. He said, “To learn relativity, you have to gulp once. And then again for general relativity. To learn quantum mechanics, you have to gulp about three times – and then about every 20 years.” [laugh] You think “Oh, that—that can’t be true or that can’t work, but actually that’s what the theory allows if you’re creative.”
What is it about optics that has inspired such curiosity in you?
Well, I made and used telescopes as a kid. You're not making any discoveries with them, but there’s a satisfaction in seeing the moons of Jupiter or the craters in the moon or a globular cluster and all that sort of stuff. And there’s a satisfaction in making an optical instrument. And if you actually grind the mirror, which I only thought about, there’s the thought that using eighteenth and nineteenth century techniques, you can do work with precision of millionths of an inch. And using clever optical techniques, you can test whether you did it right. That’s pretty amazing. So I think optics has always been a field that’s attracted people, because of its intrinsic precision. Because it was the possibility of looking at the heavens or at the micro world in more detail. And it has impact —the whole field of biology is highly dependent on better microscopes. And now of course new imaging techniques like MRI and CAT scan and all of that and sub-diffraction microscopy, electron and ion microscopy, etc. So there’s a lot of appeal from that. But there’s also, when you get in the lab at night, and you have your brilliant green laser pumping your yellow dye laser, and where the focused pump hits the dye it sparkles like a little sun—you know, with lots of colors—and there’s the finely tuned orange light coming out of the dye laser—when it hits surfaces, sometimes you'll see speckle patterns on the wall, or you'll see images. I remember when I was doing the two-photon experiment, and I knew it was a very competitive experiment and also that the laser would likely fail soon, there was once when I almost took the camera off the oscilloscope and photographed one of those patterns that looked like a double eagle on the wall. So a lab with tunable lasers—it’s just beautiful. Optics is beautiful in many ways.
What’s your proudest moment as a mentor?
Well, I'm really proud of the mentoring three-plus Nobel Prize winners. Wolfgang giving me his medal, that’s really unusual. I mean, there are a lot of people who have won Nobel Prizes, and there aren’t very many mentors who have been given the gold medal in gratitude. It’s also very rewarding to mentor people starting off in their careers. I've mentored, as you know, four students who won national thesis awards. So I think there’s a lot of value there. Now, I also have won a couple of advising awards, and those have been for various things, but emotionally two stand out. Talking a guy who was so manic that he was pretty obviously a threat to himself—I mean, he might have believed he could jump off the frat roof and fly down the ground; he was that far out of it. And with Peter Fisher, we went to his frat, and I managed to talk him into going to the mental health center. And later the doctor called me up with thanks, and I said, “So, how long did it take you to decide to institutionalize him?” See, I can’t make the decision to commit him, but she can if she judges he’s a threat, and she did. And she said, “Between 20 and 30 seconds.” [laugh] Doing that, and thinking we may have saved him from self-destruction – and certainly got him help in time. That, and helping students get over deep hang-ups are emotionally high points of mentoring.
I had a student who was a religious minority - Baháʼí in Iran, who had escaped over the border on a white horse at night, even was shot at, and had then worked his way to the United States, and crushed a junior college for two years, and he got into MIT as a transfer student. So he came into MIT with sophomore standing, and he wanted to do a double major, and he planned also to take freshman courses. So I said, “Now, wait a minute. You have a three-year scholarship, it’s hard enough to get a double degree, and yet you are registering for these freshman courses for which you have advanced standing credit?” So we were at loggerheads, and he requested a different advisor because he wanted one who would sign off on his form. But the woman in the physics office said, “You've already got the best advisor. -- You just go back to him.” And so he did, and we started to work it out. The big puzzle is—why is this guy doing this? You can’t give him advice because he isn’t thinking rationally. And so how do you get around to the emotional truth? What is it that’s really motivating him, and how do you figure it out get him to see that? Our sessions got very emotional, and suddenly I said “Why do you really want to take Freshman courses?” and he cried out, “Because my government denied me the experience of being a freshman at a good college.” He only got into lowly-ranked colleges in spite of being the valedictorian of his class, which was what precipitated his decision to leave his home and homeland. And so “the freshman experience at a good college” had been his motivating goal through his tough and lonely three-year purgatory. Once he said that and laid it out on the table, then we made progress. “Well, this large sophomore course on differential equations, it’s taught the same way that freshman math is, and by an award-winning teacher beloved by the students - Maybe that can fulfill part of your need.” At that point, you can negotiate, can discuss, and you can advise. But the two of you must uncover his true motivation first.
But returning to your original question, my proudest moment remains being given Wolfgang’s gold medal – proud because it shows the deeply human side of mentoring, and also because Wolfgang arranged a nice ceremony. We were talking about physics at the table in my office, and he said “I had to rush around so much after the ceremony that I didn’t show you my medal.” Of course I wanted to see it and started to follow him through the interconnecting door to his office. But he pushed me back as he grabbed a paper bag from the bookcase by that door. Then he spread three gold medals on my table. “I got two gold-plated bronze replicated medals along with the gold one. Can you find the real one.” Of course he knows I cannot resist a physics puzzle, and obviously I know that gold is denser than bronze, so I quickly find the densest one and hold it out to him, “This is the real one.” “Yes,” and clasping his hand around mine so I can’t let go of the medal, he says “and I want you to have it.” I like this whole scenario in part because it exemplifies Wolfgang’s most unique ability – to find one solution that solves several problems at once. Elaborating:
1. Pushing me back into my office creates anticipation because this is not quite the “viewing” I’m expecting, which would be going into his office to see his medal.
2. If I know he has 2 replica medals, he’s showing me that his gift is the real one.
3. If I don’t know this I now see that he has the two replicas for his own use.
4. Of course my natural reaction was “I can’t accept this,” followed by giving it back but the hand holding gesture not only shows his deep desire to give it away, but it also prevents me from immediately giving it back.
The MIT Atomic, Molecular, and Optical Physics group is consistently ranked 1 or 2 in the US; do you think your success is due to good mentoring or good selection of appointees?
When Dan Kleppner came to MIT he didn’t have tenure and I was his graduate student - and we were the AMO group. So with 6 tenured profs now, I’d say it was largely recruiting - and having really high standards. Persistence helps also; we made an offer to Vladan Vuletic after Wolfgang and I took him to dinner and got into high level conversations about what’s next in AMO, what’s good to do, how to understand this or that phenomenon - and he was at a collegial level in the discussion (rare for assistant prof applicants). I was really really disappointed when he declined our offer for family reasons. Next year we had the same “hunting license” and we nearly made an offer to a candidate in quantum information, but decided to follow our rule , “Always remember that a bad hire is much worse than a good hire is wonderful” and didn’t make an offer. So a couple of years later I heard Vladan had applied for a job in Europe and quickly called him and we found a way to appoint him here at a associate prof level. Meanwhile the Media Lab was reluctant to consider Ike Chuang for tenure (for internal political reasons) but MIT was obligated to consider him for tenure someplace. I’d attended a high-level meeting on Quantum information at Stanford years ago and at first felt cheated when an important introductory talk was to be given by a graduate student. But Ike gave the talk with the broadest perspective and deepest insight of the entire conference - so I kept in contact with him.
So I and the rest of the AMO group managed to convince enough MIT brass (in particular Marc Kastner and John Allen) to give him space within our group area and a ½ time appointment in physics. So we got a pioneer and most-cited author in quantum information as a colleague - and it didn’t even cost us a “slot”. Once our appointees are here we have great collegiality - indeed I gave Wolfgang my/our cold atom apparatus in which BEC quickly became reality. Then he gave it to Martin Zwierlein. We have weekly lunches and a real sense of covering for each other and working together to improve our community. So while the support and mentoring of our AMO community is important to its success, I think the quality of the appointees is paramount, and also that simply running searches is not enough - we must all keep alert for all manner of potential appointees.
I don’t want to leave unstated my tremendous personal enjoyment of associating with the members of the MIT AMO group, and doing research jointly with Wolfgang in particular.
Are there concepts or ideas in physics that are as mysterious to you today as they were when you first came across them as a graduate student?
Well, I think that the whole wonderment about information swallowed by black holes and about inflation—there are a couple of examples of mysteries that keep getting more real if not yet resolved—I've also changed my attitude on how many angels could stand on the head of a pin.
OK? It’s touted as an extremely ridiculous and pointless discussion. But it’s not! If you're deeply into religion, then the physical properties of angels is an important issue. And the physical properties of angels, whether they take up space that excludes other angels from it, whether they're infinitely strong and infinitely deformable. For example, might they have little hooks that come in from different angles—you know, they're all stacked up above the pin but their deformable hooks all can fit on the pin? Or are they vaporous and they can intermix, all these things are brought to a head in this very specific example, “How many angels can stand on the head of a pin?”
I always took that question to represent mysteries that you couldn't address scientifically. And I put in this class Brans and Dicke’s suggestion that the interior of the sun rotated faster than the surface, and therefore the sun had more quadrupole moment than you would calculate from its exterior rotation rate—which is different at the poles and the equator, so that’s not a ridiculous supposition at all. It then gives the sun a quadrupole moment that screws up the agreement between regular relativity and Mercury’s perihelion precession, and leaves room for this extra scalar parameter of Brans-Dicke gravitational theory. But I always thought, how the hell are you going to test whether the sun rotates faster inside? I can imagine having a hollow tungsten float and an iridium sinker, and throwing this thing in the sun, and having it sink down to the point where the iridium sinker melts off, and then the tungsten float will bring it back up to the surface. And then you can see where it comes back up to the surface—I mean, I don’t know how—but maybe, maybe, in some way. But just basically I didn’t see how to measure that.
And then Ken Libbrecht at Caltech showed how you use seismic waves in the sun—what is called helioseismology—how to do it, and how to analyze it, such that you can answer that question. And similarly when I first knew about inflation—and Alan Guth is a colleague of mine, so that was before it was highly fashionable—well, how can you tell? Once you've made the big bang, you've made this incredibly hot stuff in equilibrium at a relativistic temperature; isn’t it going to erase all the information that could tell you anything about any previous inflation? So it turns out that’s wrong. It’s an experimentally testable model. And now the angle-angle intensity correlations from the Wilkinson satellite and others, gives this curve with all those wiggles? And wow, the inflation theory fits! I mean, it’s spectacular. So that’s sort of the power of science.
And I do keep getting impressed with it. The things you thought were outside the ken of the knowable are slowly subsumed. And then ideas like absolute space and time, and our conventional view of reality, just get rigorously and thoroughly blown out of the water. I think that’s the deepest of the mysteries – what sacred ideas will be replaced. On your original question about unsolved mysteries - we’re still wondering about the quantum measurement problem, and whether we need multiple universes to resolve it. I missed Alan’s colloquium on inflation and multiverses, and I'm just waiting for it to come online. But again, it seems like another thing that you can’t possibly experimentally test. [laugh]
And yet. And yet.
So I think that’s some of the grand wonder. But also, there’s small wonder. I’m into sailboat racing, and there’s a lot of physics in that, and actually some controversy about whether you can cover your hull with little surface irregularities that preserve the laminar flow, or at least cause a less lossy form of turbulent flow. And it has been used in the America’s Cup, and they said it made a difference. But I read several papers without real enlightenment, and maybe I should learn about it some more.
Well, Dave, I feel like you've taken me on a journey from the outer expanses of the universe to the atomic level. I think I have one more question for you, and that is—it’s a forward-looking question—and that is, personally what are you most excited about in the future? Where physics is headed, where your own research is headed. What are your motivations that keep you so curious and active and engaged and energetic?
I’d say it’s still education and the possibility that we can teach people to think like physicists, to do envelope calculations, to question what are the fundamental principles that they’re invoking to reach conclusions. And that we can do it better, ideally, with a combination of online instruction and understanding how the brain works - what’s currently being called learning engineering. You know, how do we apply the findings of science using the methods of engineering in a field like education? And damn it all, how can we get teachers to measure how well/much their students are actually learning? And how can we get teachers to sit down and think, “What do we really want the outcome of our four-year education to be? What do we really want the outcomes of freshman physics to be? Really justify—why do we teach the same old stuff pretty much the same old way?” I mean, should they be learning a lot more about energy and not angular momentum due to the energy/global warming crisis? I mean, let’s discuss whether that’s sensible in today’s world, because our graduates are going to be going out there giving advice about energy policy, not planetary orbits.
And then how can we help those teachers measure whether they are reaching the agreed-upon goals for their students?
And you're optimistic about these things?
Well, I'm less optimistic than I was, because I see so little real progress being made with the current system. [sigh] If there’s one central finding of physics education research, it is that people do not learn concepts or how to do problems well with what I call broadcast education, namely the lecture and the textbook, where the expert who writes them does not have page-by-page, point-by-point feedback on what’s being digested by the students - if anything. And I see the tremendous effort that people are putting into putting classes online in this coronavirus. But damn it, they're digitizing the dinosaur. I mean, they're making videos of lectures, notes like textbooks. Now, I think there’s some evidence that online lectures might be an improvement, because the person can stop it and they can back it up when confused, and they can play it fast when they're bored. You can mix questions in much easier and let the students take a variable amount of time answering them, than you can in a big lecture. But that’s a minimal level of improvement.
Most importantly, the current system lacks both a desire and a method to measure learning. (I discount final exams made by the professors - as I discussed earlier there’s good evidence that student success on these is not accompanied by much real understanding.) Without valid measurement, there is no error signal to guide improvement of the educational product, or to judge the effectiveness of our current instruction. At MIT we measure whether our theories fit the experimental data, whether our engineered designs perform up to specifications - why can’t we apply these highly successful procedures to our educational theories and course designs??
Well, Professor Pritchard, I want to thank you so much for your time.
[laugh] OK, fine, David. It was good to talk to you.
Thank you so much.