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Interview of Robert Dicke by Paul Forman and Joan L. Bromberg on 1983 May 2,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
World War II work at the MIT Radiation Laboratory. Early postwar years at Princeton University. Research orientations; application of microwave techniques to determination of fundamental atomic constants. Background to paper on super-radiance. Government committee work to about the mid-1950s. Princeton Applied Research, a company organized by Dicke & others. Contact with Charles H. Townes. Consulting for Radio Corporation of America; patent obligations to RCA. Sources of financial support for research; Signal Corps support. Methods for choosing student thesis topics. Signal Corps meetings; contact with colleagues. Technical support at the Princeton Physics Department. Graduate students; financial support; his style of super vision; modes of communication; comments on some specific students. Dicke's habits of documentation.
We're going to speak with Professor Robert H. Dicke today on May 2. I'm Joan Bromberg, and Dr. Paul Forman of the Smithsonian is here also. And it is 1983. We would like to talk a little bit about the Radiation Laboratory; what group you worked in and what kinds of experiences you had there. And perhaps we'll ask some questions that lead from there also.
You had mentioned in your memoir that it was contact with [Lee] DuBridge that drew you into the Radiation Laboratory.
Right. That contact with DuBridge actually went back to the time when I was freshman at Rochester. I spent two years there as an undergraduate, and then transferred to Princeton for two years. I learned my elementary physics from DuBridge, who gave marvelous demonstration lectures to a rather small class. When my thesis was nearly done, DuBridge asked me to be an instructor at Rochester the following academic year. This was in the spring of 1941. I imagine it was actually early spring, perhaps January; somewhere in there. And then he mysteriously disappeared. Nobody knew where he was. Then I got a mysterious telephone call from him one day, saying that war was coming and I should forget about this instructorship, and come and join him at MIT. He told me practically nothing about what would be involved. So I did it. It's hard to believe. I arrived—I believe it was September, 1941—the thesis having all been written but the final oral not having been taken yet. My thesis was on nuclear physics. It was a joint experiment with another student, John Marshall, whom you may know from the Manhattan Project.
Is he the Marshall who's now at Los Alamos?
He's the one, yes. He married Leona, now Leona Libby. He and I heard a talk one day that fascinated us, and we thought, gee, that's an experiment we could do. It was one of the first experiments on inelastic scattering of protons. It wasn't suggested by any professor, so we didn't have any supervisor. We went off and did this on our own, and with some encouragement from Vicky Weisskopf, who was there on the faculty.
So he was the nearest thing you had to a supervisor?
That's right. We had no supervisor on the experimental part at all. And then we took the data and divided it in two sets. He wrote up one half of it. I wrote up the other half. We both got degrees out of it.
And then wrote a joint paper?
We wrote a joint paper. Yes, it was a joint paper. He was drafted into going to Los Alamos, about the same time I was drafted to go to MIT. I remember ever so well, arriving one Saturday in September at MIT, having been told, I think, that I would be in the so-called Advanced Development Group, or something like that. The head of that group was Ed Purcell, Harvard, and it was in Rabi's division. So on a Saturday afternoon, Purcell welcomed me there, said, "Dicke, let's not lose any time. I'll take you right up to the roof and show you the new radar we've just developed, and how it works." This was a three centimeter radar, one of the very first ones. We spent the whole afternoon trying to tune that thing up and make it go, and it wouldn't. The klystron was so complicated, with three cavities that had to be tuned.
Then did you start in working on the three centimeter radar, or was it practically finished?
I can't recall what I started doing immediately. Later I was asked to be concerned about crystal rectifiers and receivers, and I was actually jointly in the receiver group and this group for the last year of the war. But before that, I can't really recall what I was doing.
How would you estimate your knowledge of electronics at the point at which you entered the Rad Lab?
I knew sort of standard electronics, low frequency amplifier tube type stuff. A little bit of electronic experience, not a great deal. But in that way, I was just like all the rest. It's surprising, the extent to which the Rad Laboratory was manned by scientists without a real background in the field. Electrical engineers didn't play as important a role as the scientists did. But they rather quickly learned. Microwaves was such a foreign business—even electronic engineers knew zilch about it. So we all had to learn.
How did you go about learning?
We had tremendous help from Bill Hansen of Stanford. He came in one day a week to do a lecture, and had thought through these things. Then, much of the theory that was later on written up in the Radiation Laboratory Series didn't exist at that time. We had a theoretical group—George Uhlenbeck was in it, and the theorist at Harvard....
Well, Van Vleck was not in the laboratory very much, as I recall. He came in occasionally. But the one I was thinking of was this young fellow who was at Columbia with Rabi—Schwinger, Julian Schwinger. Schwinger invented a lot of the theory. There were some other theorists who contributed quite a bit. So the theory for the microwave electronics was gradually developed. When the war was over, a group of us stayed on and wrote up much of this stuff. A lot of it had not even been put into practice. That is the Radiation Laboratory Series. The way that laboratory was scattered and broken up afterwards, it's a good thing this was done. This information would have been forgotten in very short order. It wasn't organized at all, until we started to write this series of books. I helped with one of them.
In fact, I helped with several of them. There's one other thing I might say, because this leads naturally into the way I got into this research. One of the things I did was to, not invent, but for the first time make use of the lock in amplifier there, in connection with some of the measurements we made on antennas. This suggested a way of making a very sensitive radiometer, for microwave frequencies, which is a radiometer that later on was incorporated in the radio telescopes. We built one of these, just used it to discover that the trouble with K-band radar was the water absorption in the atmosphere. We had three of these radiometers all done and used, de-bugged and everything else, when the war was over, and I took one of these to Princeton when I came here. I tried to interest our astronomy department in a joint effort for some radio astronomy in the K-band. I couldn't get any interest over there, so I cannibalized this into some parts for starting research here. That's how I got into some microwave spectroscopy here.
In your little sketch, you mentioned that there were a couple of radar forms you invented that weren't put into practice until later. I think you mentioned a monopulse and chirp radars?
Yes, that's right.
Did you get involved with those getting into practice afterwards?
No. Nothing but the invention of them, the patents.
So they didn't go back to you, consult you, and say "You invented this, now help us make it"?
Chirp radar, for example—the name "Chirp" isn't mine. It's the fellow who re-invented it later, who put that name on it. My patent was classified, so the examiners, who should have had access to it, apparently missed it. There was a parallel patent issued later on, that reads just like it. The US government for years has been paying enormous royalties to this fellow—I guess it was to GE, because he worked for GE. And then they found my patent, and wondered why they'd been paying all these royalties. They tried to find some notebooks of mine, or other information. But my notebooks have been left to MIT. I didn't have anything to help them.
And what about some of the other radars? Some of the other inventions you made at the Radiation Lab?
Well, the radiometer was one. That series of—
That was all done when you finished?
Yes, that was published properly. There was never any patent application connected with that, although there could have been. There are some wave guide junctions of various kinds. They all are described, not all, but, probably you'll find some Radiation Laboratory patents in here. [Dicke hefts a bound volume of his patents.]
How do you decide when you want to patent something and when you just want to publish?
Well, if it has some commercial value—you have to guess at that, and you usually guess wrong. This is a so called "Magic T." Now, there's an interesting story about that. That was a widely used junction which has very interesting properties. Well, I invented that. But I didn't realize it. The reason was that somebody came in one day and showed me a picture that had appeared in some abstract—I think it was from the Bell System Technical Journal, or some such. And the picture looked just like the magic tee. Well, it turned out, the key part was missing. So Bob Pound was the one to take out a patent—not the broad invention, but a particular way of matching the tee by putting a post in back here. [Pointing to the figure] That's the only patent that exists on this thing. It's very restricted, when it could be much broader.
Why don't we go on then and get you to Princeton?
One of the interesting things about this is that it really hadn't occurred to me, either as a graduate student, or later on at the Radiation Laboratory, that I might end up as a member of a faculty. I had thought in terms of going into industrial research, or something like that. I was rather surprised to discover, when the war was just about over, that universities were interested in getting me to join their faculties. And I had a number of offers at that time. That was the first time the thought ever occurred to me that I might go into teaching.
Does that have anything to do with coming from an engineer's family? Or?
No. Well, maybe in part it might. It's hard to say. But you have to remember that, at the time I started going to college, the average man in the street didn't know what a physicist was. And there clearly weren't a tremendous number of openings for teacher types. At least it didn't seem that way. But of course, physics expanded very rapidly after the war, and there were more openings; but during the Depression, the thirties, why, it just never would have occurred to me that there might be possibilities in that field.
When you came in here, what was it like? Where did they put you? What kind of laboratory space? What were the students like?
First of all, we were in the old Palmer Laboratory up the hill a ways. The department was very small at that time, compared with what it is now. There might have been a dozen of us altogether. Graduate students—we might have had thirty or forty. Undergraduates majoring in physics—perhaps six to ten—half the number we have now.
Were there other people coming to Princeton at the same time with this microwave interest?
Well, one of the reasons I ended up here was that Milton White, a member of the faculty here, had been at Radiation Laboratory, and was in fact one of the division heads there. He knew me from my undergraduate days here. It was he who thought of bringing me in here. There was another member of our faculty, Frank Shoemaker, who came from there. There might have been another. I can't remember now.
How did you make the acquaintance of D. R. Hamilton?
Oh, yes, he's another one that came. He had not worked at the Radiation Laboratory, although he was associated with it. He had been with Sperry during the war working on klystrons. Earlier he had been at Harvard, and before that had been an undergraduate at Princeton as well. Anyway, when he found out that we were also going to Princeton, (he had been at Princeton as an undergraduate as well), they invited us over there one night. It wasn't until then, a few months before we came up here, that we became acquainted.
Then you were here together at the beginning of your work.
Yes. In fact we occupied the same office here together, and we lived next door in town for a good many years. But we didn't really collaborate in the research, although we had research interests that were somewhat similar. His research turned naturally to his work at Columbia, where he'd been a graduate student, and he went into atomic beams, as applied to radioactive nuclei. What I did was to try to turn the microwave techniques to making what I though were important measurements, but not involving beams as such. Actually, the first experiment I did here was to try a new kind of counter that later on became quite important, a counter for highly energetic charged particles. That's the Cerenkov radiation counter. I have a patent on that. It's in that book. Then I turned to what I thought would be an interesting experiment; namely to look at the fine structure in the excited state of hydrogen, to see whether everything was in order or not there.
Your Cerenkov radiation foray didn't really take advantage of your Rad Lab microwave background.
No, that's right.
Do you recall what sent you off in that direction?
First of all, I remember when I first read about Cerenkov radiation I was rather fascinated by it. It seemed like photomultipliers were available and you could detect the light, and it looked like an interesting way of detecting the path of a moving particle, because you could get at the velocity of the particle this way.
Were you working alone on that at this point?
When did you start to acquire graduate students?
Not until I started doing some work on atomic spectroscopy, that sort of thing. Well, to come back to my first work on the fine structure of hydrogen. Rabi came in one day, and talked to me quite a while about what I was doing, from him I found out then that Willis Lamb at Columbia was working on this same problem. So I stopped that. Then I got involved in a long series of experiments, all of which failed. I was trying to measure the magnetic moment of the free electron. Horrible series of difficult things that didn't quite pan out. You'll find lots and lots of notes in my files there on that. That's the experiment that was later on done successfully by the chap at Seattle. Dehmelt successfully did this, after he was able to trap an individual electron. I had a letter from Dehmelt a few weeks ago. He was interested in knowing what I had done about this, so I sent him a copy of some of my old scribbles.
Then I got interested in the fact that, from the standpoint of highly precise measurement, the usual natural broadening of spectral lines was completely insignificant. Rather, one of the things that would be limiting you in all such measurements would be the Doppler breadth of the lines. So I started looking at some special ways for dealing with that. And my first graduate student, George Newell, who's been dead for many years, did the first experiment aiming at the elimination of the Doppler effect. He did it very successfully. Let me just describe the physical.... [See Newell and Dicke, Physical Review, 83:1064-5 (1951).] You have a big box, filled with a microwave-resonant gas, such as ammonia. The box contains a series of equally spaced rectangular frames with many parallel wires stretched across them to form grids. Between these grids are electric fields. Then, by causing these fields to oscillate, you can effectively set up propagating waves that move this way and that way.
The effective wave length of this wave is given by the wire spacing, and the frequency of the oscillation determines the velocity at which they run. You can make this velocity be the average velocity of the molecule. Thus there'll be a fairly substantial class of molecules that move right along with these waves. Well, that class, [is] stimulated through the Stark Effect to shift its microwave lines, and it's locked right in. So when you send a plane microwave through the box, perpendicular to the grids, that class constructively reflects at the stark-shifted frequency, and you can get the spectral line unbroadened. And that's Newell's thesis, which you'll find up there. [indicates his bookshelf]
I looked at the brief paper you published in the Physical Review on that, but wasn't able, before you clear explanation, to make out just what was—
It's probably written in "Physical Reviewese", special language, which is meant to obscure things.
There were a number of people in the country who were, in one way or another, trying to attack the problem of narrowing the lines. I'd be very interested to get your view, as you now look back on it, on what the world of microwave spectroscopy looked like to you then, and whom you are most in touch with. Have you any recollections of what one might call the style of the different groups, as perhaps it contrasted with the style of the work you were building up in Princeton?
I wasn't really all that close to most of the microwave spectroscopy at that time, Gordy, for instance.
Woody Strandberg at MIT, whom I knew at Radiation Laboratory. I knew Woody quite well, while there. But all those people were working on molecular spectroscopy, the structure of molecules. I was really not that interested in molecules. What I wanted to do was to try to pick out some important fundamental atomic numbers that played a fundamental role in physics, and try to make an accurate measurement of those. I wasn't terribly successful at some of these, but it was a good idea. I don't recall, in the early days, anyone being terribly concerned about the line breadth problem. For purposes of the molecular spectroscopy, they didn't need it. They had adequate line breadths for their purposes. And their equipment usually made use of the Stark modulation cell, or the long cell.
Would you say that you were interested in the reduction of the Doppler width with an eye to its practical applications primarily or because you were chiefly intrigued by the art for the art's sake?
I think it was mainly the art for the art's sake, initially. It was only later on, when it looked like it might be an interesting approach to the atomic clock problem, that I became interested in .... [Giving examples of practical applications of the technique in his laboratory:] My student, Jim Wittke, worked on the hyperfine structure of the ground state of hydrogen, [Physical Review, 96: 530-31 (1954)] This work made use of the line narrowing associated with trapping a gas by means of a buffer gas. Another example, is the technique that my student Ed Lambe used in looking at the g factor of the electron, in the hydrogen atom. Here, again, the line narrowing technique was used.
I was struck, in the reprint you gave us on the Jubilee of Kastler, on how little there seemed to be of real contact with Europe, compared to today. I wonder if you remember whom you were seeing, say from France or from Europe in general, in those days, who the visitors might have been, what the conferences might have been that you actually went to. There seemed to be so much going in parallel. I'm just curious, what kinds of relations you had with Europe at that point?
We had none with Kastler's group, until we had a visit from Brosselor Baterre. One of them came through. A few years later, we had a visitor from Kastler's group who spent a year with us. A female physicist, I think she worked primarily with Tom Carver [M. A. Guiochon, perhaps]
So a few years later, we're talking about the middle fifties?
Probably the middle fifties, maybe even late fifties.
These contacts began probably in the spring of 1953.
I think they probably didn't begin until we were familiar with Kastler's work and he was familiar with our.
You mentioned Brossel who had been at MIT in 1949, 1950.
Do you think you were aware of his work there, with Bitter, at that time?
I was certainly aware of it at some stage, but I can't tell you when. I think I remember an unsuccessful experiment they tried on mercury or something like that. But when that occurred, I can't say.
That was in 1949, 1950.
It certainly wasn't at the time; I think I was aware of the publication later. They published an attempt to see some kind of affect there. Probably this led to their successful experiments later in Paris, another idea of optical pumping. The way I got into this optical pumping was sort of peculiar. I had been working for some time on this problem of trapping the electron, to get at its factor, and tried several different means of polarizing the electrons, and then analyzing them later on. One of the analyzing schemes I used, for example, was to evaporate iron onto a thin substrate, and magnetize the iron. Then I shot the electrons through there at high energy, and I looked at the transmission as a function of magnetic field, or rather, as a function of spin orientation. One of the things I tried was this. [Draws on blackboard] I had a sodium resonance lamp and a rotating wheel driven by a motor at 30 cycles per second, this wheel was made up of polaroid in six sectors, with half of the wheel polarized radially, and half polarized transversely. As this wheel rotated, the light coming through would have its polarization switching back and forth.
Then I put a quarter wave plate in the path so that light that went through this would be circularly polarized, first this way, then that way, as this wheel rotated. So the light was being switched. Then the circularly polarized light went into a vacuum system, where I had an oven evaporating sodium to form a sodium beam going up, like so. The light was absorbed by the sodium atom, which went from the ground state up to an excited state. I should say there was a magnetic field in the structure, a very weak magnetic field oriented parallel to the beam of resonance radiation. Then, as the circular polarization switched back and forth, you excited one set of levels, or another set compatible with that circular polarization.
Now, look at what the implication of circular polarization is: when circularly polarized this way, the beam of resonance radiation tends to populate those levels where the electron spin is spinning parallel to it. And then, when your circular polarization switches that way, the electrons are spinning primarily antiparallel. At the same time, there was ultraviolet light coming in here, to excite you out of that state, and free the electron. So, when you photo—ionize, you have free electrons that are circularly polarized one way or the other. Not 100 percent polarized, but with a fairly good ratio.
Was this written up?
No. All this stuff was unsuccessful, so nothing was ever published. You will find discussion of it in my files there, I think. Anyway, that's what led then into the idea that you can affect the populations of these upper levels by the polarization of light that's used.
There's a style question that I had in mind. When I look at your papers, I'm struck by the fact that you seem to be going back to analysis of the basic physical mechanism, whereas many of the papers I've been reading—well, for example, papers on lasers—discuss things in terms of rate equations or such. I was interested in this method of yours, to go back and analyze the basic processes. Did you also conceive of that as rather different from other work? Also, what was the source of this very great interest in work with the fundamental interactions of light and matter? Was it perhaps textbooks you read?
I don't know. It might have come out of my interest in quantum mechanics. As an undergraduate, paradoxes of quantum mechanics interested me a great deal. I worked hard to try to understand them. Now, although this is really a complete diversion, I must tell you anyway. When I was an undergraduate, at Princeton, one of the paradoxes that bothered me—I couldn't straighten it out—involved a very strange problem in measurement. In the usual discussion, the Heisenberg microscope, you look to see where a particle is an electron, say, by scattering a photon off it. In the process you transfer momentum to the electron and you make its momentum uncertain, so there's a connection between uncertainty in position and momentum.
What bothered me was this: I could conceive of a situation where I don't ask precisely where the electron is, but ask, "Is the electron there, under the microscope, or not there?" I look to see whether it is there by shining a great burst of light through the microscope. If I discover it's not there, I have not interacted with it. But in the process of not interacting with it, I've determined it's not there. If it's not there, it must be over here. So I've obtained information about the electron's position without interacting with it. And this seemed to me to be a terrible paradox. Well, this bothered me—not very much, because I didn't have time to be bothered much as a graduate student. But I did discuss it with Vicky Weisskopf at Rochester, and though he couldn't help a great deal, he did make one useful point.
Then I kind of forgot about it for years, and came back to it many years later. I talked to Wigner about it, who also didn't quite see a way out of it. Then I finally discovered what the resolution of the paradox is. That was published last October [American Journal of Physics, 49:925-930 (1981)], and this paper deserves a prize. Not for quality or anything like that, but for length of time in preparation, because I worked on it for some 40 years! Unfortunately, the resolution is not a simple one. I haven't been able to find a simple way of describing it.
You know, that leads me on to this 1953 paper, because this paper on the super radiance [occurs] at the same time that you've got an experimental, practical program for getting line narrowing. I just wondered how the interest in the quantum mechanics of the correlated spontaneous emission, and the work on the experiment, fed into each other?
Well, I think one has to go back further on this. Nuclear magnetic resonance was discovered by Purcell and his group, and independently by Felix Bloch. The Purcell group looked at this from the standpoint of the absorption energy. But Felix Bloch had the following way of looking at it. Starting with a nucleus, that's either in this level or that level, you excite the system into a superposition energy state, treating the radiation classically. With the nuclei in the superposition state you can't talk about what the precise energy is of this one and that. But what you can do is consider an ensemble, a large number of nuclei, and get the net polarization of the gas, and see how the polarization varies with time.
When I got interested in the microwave transition business, while the[other] people were looking at the molecular transitions and molecular spectroscopy from the standpoint of the Purcell group, as the energy that's absorbed and the reduction of the wave that goes through, I was interested in the parallel thing, which is to excite the gas into superposition states, and then cause it to radiate, so you pulse excite it, and then you get a train of signals out. I found, incidentally, in my files there, some of the old photographs I took of the radiation by ammonia gas, that was pulse excited, and you can see the interference beats and the signal coming out. After using these techniques of pulse exciting, and using this idea—it's a classical description of radiation process, not proper quantum mechanics—it occurred to me one day that there ought to be a way of describing this in a proper quantum mechanical way. And that's how I got into this, super-radiant states and things of that kind.
Is this pulse excitation technique in the microwave region, does this go back to your Radiation Laboratory...?
In a way, yes, because we were so familiar with using pulse techniques there, that I knew how to generate pulses and that type of thing.
I don't want to jump too far ahead, but you continued then to use this kind of technique when you began to think about a sodium clock, and how you might find the signal after the excitation?
The thing that particularly appealed to me about the pulse technique was that you so cleanly separated the excitation process. You may need a very large signal to excite a certain transition, and [with] that very large signal, in the CW process, you would just blanket everything, or else have the very difficult job to balance things so carefully that it wouldn't get into the receiver. If it got into the receiver it would cause no end of trouble. But the pulse technique was very natural, because you suddenly turned on the excitation. You didn't try to observe at all while the excitation was on. Then you turned it off, and, when everything's nice and quiet, you let it quiet down, then your receiver can function without all that interference. That was the basic improvement that we made, by going to pulses. It's really similar to radar that way.
And then the idea of a coherent spontaneous radiation, from this excited system, followed upon considering it as a single excited—
—yes. I think one would have to say this. That didn't appear until I looked at the thing quantum mechanically, in the super-radiant paper. In the usual approach, when you have a whole collection of atoms, let's say where you looked at only two levels—where you had a superposition state of these two energy states—and the whole system is radiating cooperatively, you had maximum radiation when you had these two levels excited to the same amplitude and they were all phased relative to each other in the same way. Then you had [a] cooperative radiation pro cess. You got the maximum radiation out. But I looked at it in a proper quantum mechanical way, and saw that in describing the cooperative states of a large number of atoms, one got a whole long series of equal spaced energy levels. [Drawing on blackboard] At the top energy level, here, was the one where they were all in the excited state, and this was where they were all in a ground state. You put[the system] here, and just let it trickle down and make transitions. It goes into cooperative states where the maximum radiation comes out here, in the middle. But in order to get there, you have to start up here; what should I do but generate this cooperative state right off with a pulse. By contrast, with the usual approach, you didn't need a pulse if you started here, because it just naturally developed this cooperative radiating state. And I gave a talk to the Physical Society, I remember, on this business and I talked about this as an "optical bomb" as descriptive of the way this generates cooperative states.
There is one more thing I'd like to ask about. In a paper you gave in 1963, at one of the Quantum Electronics Conferences, you mentioned that it was already very well known that you could look at the nucleus, nuclear structure, a little bit, by looking at the correlations between successive photons. I was wondering when I read that whether that was something you'd known for years before you started?
I think, I'm almost certain I got that from my colleague, Don Hamilton, because he as a graduate student had written up the theory. I don't think he'd done any experiments. But he had written a paper in which he examined the correlation of successive gamma rays. One gamma ray would come out, and then its direction would determine the structure of the energy levels that had been excited. And the next gamma ray that appeared would have its direction correlated with the first gamma ray. There were experiments done on that kind of correlation process later. When they were done, I'm not quite sure. But in any case, I think that's the origin of this cooperative radiation process.
You know, one thing that I would like very much to find out, I guess it's really a matter of what was in your peripheral vision, that whether in addition to the problems you were working on most, you were also doing some consulting or working on government committees that would have brought in other problems that might have just been in your mind—just to get an idea of the total landscape that you had in your head in those days.
Well, I was on quite a few government committees. When this all started, I can't remember. I was chairman of one of the subpanels of the selection committee for Fulbright awards. That was one of the first government jobs I had. About the same time, I was on the physics panel for the Bureau of Standards. Now, these were probably early fifties. Then I got into a series of NASA jobs of various kinds.
Did that early Bureau of Standards work involve clocks, or were you already involved in clocks at that point?
No, I don't think that did. I think at that time there was already some interest. I think [Jerrold] Zacharias at MIT had developed an interest in atomic clocks and the use of beams for the purpose. I remember, he was also concerned about line narrowing; one very interesting idea involved throwing atoms up over a transom and letting them fall down on the other side.
Was this something he talked about a meeting?
No, he tried hard to make it work, I think. It was around that period of time, I think, that people in a number of places became interested in the question of whether you could build a good atomic clock.
You were on sabbatical at Harvard, you wrote in your autobiographical sketch, in 1954-1955. And this was in fact just the time when Zacharias's first atomic beam clock was successfully operating. Do you recall any contact there over this question?
No. I certainly was aware of it. I guess I saw Zach at meetings. I knew him. We talked about things.
You were even closer that year because you were both at Cambridge.
Yes. As a matter of fact, we both were on a study for the government at that time, Project Lampblack or Lightblack or White Light or Lamplight or something. The first study that I was on was Project Hartwell. That was in the forties, I believe. It was a sort of prototype for these governmental studies, these special things. That came rather shortly after the war.
What was that one studying?
That was concerned with undersea warfare, mainly problems of underwater sound, the difficulties there. Oh, one of the things they were doing was using magnetic fields to detect submarines. Trying to see what the limits were there. I remember, I made a calculation on the effect of the waves on the magnetic field of the earth. But Edward Purcell was there, and Zacharias, I remember. I can't remember who the others were. I have an amusing recollection of the Lamplight study. I was there only two weeks and we had a kind of seminar in the early afternoon, the last day I was there. I was just getting ready to leave, and I thought of an idea on a counter measure problem, of how you avoid jamming troubles, for radar. And I scribbled this down and handed it to Zach, as I was leaving the room. I think maybe I mentioned it very briefly, what I thought this idea might be, and he said, "Scribble it down, write it down, "so I wrote it on a thing about this big and handed it to him, and I disappeared. And I never heard any more about it. Years later, I was on a plane one day, working, had some notes out, and the chap next to me glanced over and he saw my name, Dicke. He said, "You aren't by any possibility the Dicke of the Dicke Fix?" I said, "What's the Dicke Fix?" He described it to me, and of course, it was that invention!
So you got full credit.
I got full credit. Apparently, the Russians had known about it too.
Was Lamplight, do you recall, early in 1954 or early in 1953.
It was while I was at Harvard. That's all I know. So that would fix it.
It was a kind of tutorial, for people in industry or military who were being introduced to—
I really don't remember that much about it, to tell you the truth. I think the particular group I was with was concerned about countermeasure involvement—how do you avoid jamming of radars etc.
So your recollection of it was that it was not pedagogic.
It was not pedagogic. I think we were supposed to get some real research done.
Could we go back to the 30 cycles again at the Rad Lab?
Right. Why 30 cycles? You didn't want 60, obviously, because you'd have 60 cycle interference. On the other hand, you had a synchronous motor, so you're phase locked. So that means that whatever interference you had isn't going to be beating. Any other frequency that's low, unless you choose it very carefully, is apt to be beating with 60 cycles, and giving 60 cycle wiggles on everything, beat frequency wiggles.
But now these 30 cycles are kind of a red thread through your experiments.
Yes—well, you know, you build an apparatus that works. And that's another long interesting story, the 30 cycles, because Purcell and his colleagues used this locking amplifier technique, the radiometer technique, in the discovery of hydrogen in space, and the 21 centimeter line. And that, plus the work that we were doing and publishing, almost everything of which involved a locking amplifier somehow or other, produced a lot of interest. People wanted to use this technique. But there wasn't any locking amplifier they could buy. So I got no end of requests for circuit diagrams, for circuits, which I mailed out. Later on, a group of us here organized a little company, Princeton Applied Research, just to build electronics. We started out building power supplies. That business didn't work very well.
Almost everything we built didn't go very well. Finally we thought of this locking amplifier, and I had a little bit of a job convincing my colleagues that it might be something to try. They said, "Well, how many of these could we sell?" "Just on the basis of the number of circuit diagrams I mailed out, I think I can guarantee roughly 100 potential customers. But I don't know how many of those will actually buy—you might only sell ten." On the basis of this slender recommendation, this thing was designed. The chap that designed the first one that was marketed was my graduate student, Jim Brault (whose thesis is up there) because he had changed the basic circuit, and designed a variation of it, for his experiment. So that was incorporated, and it received the code name "JB-1," for Jim Brault. But in the first year, we sold far more than that hundred potential customers that I talked about, and it continued to be a very good seller, year after year. Thousands of these are made and sold.
This company is still going on?
Oh, the company was bought out by EG&G about five, six years ago. Now it exists as a division of EG&G.
Did Brault join the firm?
No, he was working there part time. He was a graduate student, and when he finished here, he went to Kitt Peak.
Did you find other things that you did, came out of your inventing for that firm?
No. Not really. There's one other device they manufactured, that I had some input on. That carries the initials, HR. I forget what it was. But there were three of us involved and each one of our final names was one of the letters in the code.
On the maser—I'm just curious, when Townes's maser did come out in 1954, 1955, whether you found yourself talking to Townes or to other people about how to interpret this, since you'd been thinking about—
Townes's first thoughts along this line must have been about the same time I wrote that paper on super radiance, because I remember he invited me out to Columbia to talk about it. He showed me around the lab, and he mentioned something that they were doing at the time, trying to make the maser work, a gaseous maser. So the time must have been very nearly the same. We were completely independent, of course. And of course, his way of looking at it was very different. He used the idea of stimulated emission, with an excess population and the rate equation, and I was looking at it from the standpoint of these coherent states.
Is there something we could find in the files? Did you correspond? Would you be likely to correspond on this kind of thing?
You might find something, but I rather doubt it. I don't recall any contact with him on this, except the conversation I had with him one night at Columbia.
Is this a situation where, for example, we might expect you to have been in closer contact with people like the Stanford people, like Bloch's group, than with Townes' group?
I can't recall any correspondence with the Stanford group at all. Another uh, kind of interesting story connected with, this time, the laser — of course, as soon as the maser was developed, it was clear that there was a possibility of doing this optically — and um, I remember a Physical Society meeting in Washington, I can't say when, but it was certainly after my patent was issued, because I saw Charlie [Townes] and I asked, "How are things going?" He mentioned that he had a good way of building a resonator for this, I don't know whether he called it "laser" or not, but the maser to operate with light waves. He said, "All you have to do is put a couple of mirrors on it." I said, "That's great, Charlie, but it's not new, because I've got it in the patent." I think that patent was already issued at that time. I had the reflecting plates on it.
Do you remember anything about the origin of that idea of using reflecting plates? I also wanted to know why you were putting infrared radiation in there, whether you had some specific idea in mind?
I was consulting with RCA at the time, and it seemed like a kind of cute idea. I didn't have anything specific in mind to do with it. It was hard for me to convince RCA that this was an important invention. I actually wrote up three separate patent disclosures on various aspects of this thing, and they thought, well, it might be worth patenting if they would combine all three in one. So, the result is [that] the patent application's a great mess, because they put too many things in it. It would have been much better writing them up separately.
What was the consulting on? Did it have some specific problems?
Well, it was generally the idea of using microwaves in atomic systems. You'll find that best understood by just looking at the patents that came out in that period of time. I had a patent arrangement with[RCA] where I had title to the patent and they had license but they also had the right to sub-license to others. I think the reason that [the]laser patent never got anywhere was because they got in trouble with the anti-trust boys, signed a consent decree, where anybody can get a license by paying two dollars or something like that, and my patent was thrown in this pool. So if anybody can get a license under it, it's worth nothing at all.
I guess that goes back a little bit to this question I was asking about how one decides when to patent something and when not to. It sounds as if one also decides in terms of one's relations with, in this case, with RCA. Is that true?
That's fair, I think. If you're on a retainer, you like to feel that you're doing something for them, so when you think of something you take the trouble to write it up. Otherwise, you wouldn't bother. That is, I think, an important factor.
Is it a lot of trouble to write up a patent?
Yes, it's trouble. It detracts from what you really should be doing.
Does it inspire anything that you really should be doing? Do you trace any of your interest back to the work you did to elaborate a patent?
Well, some inventions are fun, and so it's nice to carry them through, by making up a reasonable patent disclosure. If it's not fun, it's just tedious. I got very little out of these patents financially, and so, some 15 years or so ago, deliberately decided that I wasn't going to bother writing up any more patent applications. Then I published a paper, and I had some Navy support at the time, and somebody in the Navy saw this and said, "Where's the patent disclosure? This has to be patented." In fact, my contractual relations were such that I was supposed to disclose useful inventions to them. So I ended up with another patent I didn't want.
What you were doing with the Signal Corps?
The Signal Corps was my first financial support of the research here. You were asking about what things were like when I first came. It's interesting how little we had to get by on. The department had a tiny little budget that supported building apparatus and so on, and there was no federal support initially. Rather soon afterwards, Milt White arranged to get a contact from ONR to support the rejuvenation of the cyclotron there, which had been built on a shoe string during the thirties. So that was rebuilt that way. And Milt found some justification for giving me a bit of support under that. I can't remember now, whether that came after my Signal Corps or before. I think it was probably before. My only support initially I think was via that. Very little. Then, later on, Marcel Golay, of the Golay cell, in infra-red spectroscopy, stopped in to see me one day. He'd read a paper of mine, and he was chief scientific officer for the Signal Corps or something like that, and he wanted to know whether I'd like some support from them. So after that I had a contract with them. It's very interesting, though. He said, "I'm not sure we can do this. It's new." They'd never done anything like this before. He said, "It's going to be an experiment." He had to devise some way they could legally get some money for me. And it turned out that their contract officers didn't know how to cope with this thing. They came up with a very simple expedient. I had to deliver so many copies four times a year [of a] report, free on board, specified to satisfy salt spray tests and all the rest of the Navy requirements.
It had to be salt spray—what?
No, I'm just kidding on that. This was a means—they were buying something.
And this contract really was just a grant, for you to do whatever you wanted?
Well, it turned out to be a grant, but they had to masquerade it in such a form that it didn't appear to be a grant. So it turned out that the contract was written, it's all spelled out, that I had to supply so many copies of this report four times a year. That's what they were buying. They didn't tell what had to be in the report.
And they didn't say at all what areas they were interested in?
No. Well, maybe. I can't say that for sure. But I think you're probably familiar with one of those reports. They were a real pain, but one shouldn't begrudge reports. I think they help. They pull your ideas together and force you to write when you otherwise might not be writing at all.
That one was the 8th quarterly report, for the period that ended in September 1957, so that series would [have] begun in the fall of 1955, but probably there were earlier ones.
I think there might have been earlier reports, but again, you'll have to go to the files to find out.
Might I ask about the relation of the experiments that you were doing in conjunction with your graduate students, and the ideas that you were thinking about. Was it always that you had an idea, and then you thought about an experiment that a graduate student might do which would incorporate this idea, and give them a good thesis, good training, or did it sometimes work the other way around?
Well, it's hard for me to remember now, to tell you the truth. Whether the idea was stimulated by a student coming and saying that he needed a problem, did I have anything to suggest? Or whether I thought of something and had it in the back of my mind, and the student appeared. But I think it was motivated always by the value of the idea, not by the suitableness for a graduate thesis problem. In fact, some of the experiments I did, I thought were so unsuitable for thesis experiments that I wouldn't put a student on them. One of these was the solar oblateness in 1966. I had lots of graduate students working as research assistants on that, over a period of time, but I felt that was so iffy that I didn't want to see a student committed to it. I've changed my mind on that now. We've got the machine running again, and it's on Mt. Wilson, and this time I do have a student on it.
What about this little work you did with Griffiths on coherent light sources? How did that come into existence?
Well, I guess this was at the general time I was interested in this problem of coherence, and again, the problem of line narrowing and getting rid of the Doppler effect? In this particular case, if you have an atomic beam, with the atoms all going in one direction, the normal way of making use of this to get a narrow line would do it in absorption. But note, associated with any absorption there's also [an] emission process. When an atom absorbs a wave, it absorbs it by sending out a wave that cancels out the original wave and interferes with it. So you had available to you not only the loss of energy in the original wave but you also had available to you the wave that was scattered. You can do the whole thing in emission if you want to. In the process, you get rid of the huge background.
There was a lot going on in coherence. I guess Wolf and Mandelin the fifties, were working on coherence, from one angle. And then there was a man, Senitzky—at—
—at Fort Monmouth, right. You must have done a lot of reading.
Not really. But I was wondering, were you in contact with these people?
I must have had a little correspondence with Senitzky. I can't remember, on what. I used to see Senitzky occasionally. We had these meetings, connected with our support by the Signal Corps group. There was a frequency control conference every year that all of us who received support from the Signal Corps were expected to go to as a matter of duty. Once a year we would go to this. It always met at Atlantic City in those days. In one of the hotels. Charlie Townes would go—all of us who had some support from—
Was Senitzky a Rabi student?
I don't know.
There's a I. R. Senitzky and a Benjamin Senitzky and I have not figured this out.
I can tell you which one it is, because he made a remark on my paper at the Paris quantum electronics conference—Page 35. This is B. Senitzky, TRG, Inc. He's the one. This fellow was the one that I knew of as Senitzky.
Well, Atlantic City must have been a nice place to meet your colleagues.
Yes. In fact, that's the only place I saw many of them. I didn't do that much traveling.
Didn't go to the APS?
I went to APS meetings pretty regularly, then I guess, I used to see them there, and Atlantic City.
What about EEE? Somebody like Joseph Weber, for example, would be going to both the Microwave Spectroscopy and the IEEE. Was that your circle?
I don't remember what Joe Weber was doing at that time, whether he went to the Signal Corps meetings or not. I can't remember. I met Joe before he got into the gravity wave business. I met him in connection with something involving atomic resonance, but I can't remember what.
He was doing microwave spectroscopy, and he had this early idea of stimulated emission. Could it be either of those things?
Yes, it must have been connected with that. I think I remember a conversation in an elevator. He mentioned something about stimulated emission or something like that. I can't remember what it was. Or again, did I even hear him give a talk on this? I'm sorry, my memory is....
Then there's one other thing that I want to bring up, at some point. When you get a little bit later on, you have rather a controversy developing in the coherence world—for example, Glauber versus the Rochester people—
And I just wondered if you had any part in that, maybe as a referee or whether this little contretemps was something that you—
No, I can't recall. Oh, what was the conflict—between the Rochester group and—?
The conflict was really over whether and how much quantum mechanics you needed in discussing coherence, so it might have been something that might have interested you, with Glauber saying that you really can't use semiclassical methods, and a man named E. Sudarshan figuring in, so—
Sudarshan came from Rochester.
I think so.
Yes. No, I guess I probably was following it at the time, but I can't recall now, what issues were in that.
We were speaking a bit about Palmer Research facilities [at lunch]—about the technical staff—you mentioned a glass blower, [and] the secret recipe for mixing up red wax. But on the other hand, there were probably some skills that were available then which are not so readily available today.
We no longer have a glass blower in the department, physics has changed so much, but when I was an undergraduate at Princeton, there was a glass blower who was kept busy all day long. His name was Lee Harris; he was remarkable for his even temperament. He never got riled. Almost unique among glass blowers. All the other glass blowers I've ever seen were very unstable and would break everything if things did not turn outright! We had a machine shop, with, at that time, perhaps two or three machinists. There was no contract support. We had a laboratory technician who took care of such matters as mixing up the very special ingredients that went into the production of red wax. It was rumored that you could only do this when the moon was right. I'm not sure that was completely correct. But it was general purpose red wax, used for all sorts of things around the laboratory.
You mean that the contract, for example, with the Signal Corps wouldn't incorporate technicians?
That came later. At the time I first arrived here, 1946, we didn't have contract support yet. I can't remember when it came. Maybe 1948-1949. And then, first with the Office of Naval Research support. There was a little bit of grant money from foundations occasionally, to support special things.
Well, when the cyclotron was originally built here, I think there was a little foundation money in that.
Who supported the graduate students then? They all just went on teaching, or what?
They were supported by teaching, as I was as a graduate student. You taught perhaps a recitation section, then a couple of laboratories a week.
When did your graduate students start getting supported on a contract, do you remember that?
At first support, as I recall the matter, was only for buying apparatus, things of that kind. There wasn't any salary support. Salary support for graduate students came a little later, and then finally summer support for faculty, and also, in the case of junior faculty, part time support. This might have come a decade after the war, mid-fifties, something like that. I must say, my memory is very faulty on this. I'm not sure this is right.
People like Lambe and Pond, they were probably also teaching?
How did you organize the work of the graduate students? Would they be doing the actual laboratory work, or did you go in for part of it?
When they were working on a thesis?
Yes, your people, when they were working.
Of course, you run into different styles. My style tended to be to leave them on their own until they had trouble. When something didn't work, they usually came to me and said, "This doesn't seem to work," and I'd go and look at it, and usually could cure it with a laying on of hands process of some kind.
Did they work in the same laboratory that you were using for your own work?
Yes. In many cases, when I had a lot of graduate students, I had trouble doing anything of my own. All I could do was help my students.
Another thing I'm interested in is how much the other faculty people would be involved with your students? For example, I've heard from one man that when he worked at Columbia, Rabi knew what all the students were doing. I just wondered if there were contacts among the Princeton faculty that would be—
I don't think so. I think Rabi was probably unique that way. I don't recall that much here. There was some common interest, for example, between the beam work and ours. Certainly the students knew what each was doing. But there wasn't that much direct influence of the various faculty members on each other. You mentioned before, the matter of the congestion in the Palmer Laboratory. I found it rather surprising, that when we moved into our new larger building here, the degree to which the interactions fell off, and the bad effect of this on the laboratory. It was great in many ways to have the extra room, but you also, I think, missed the elbow rubbing that went on in the Palmer Laboratory. A bit later, when I started getting interested in the gravitation type research, the whole group used to meet every week, one night a week, and that was great; we knew what everyone else was doing.
You didn't have such meetings in the microwave group?
No, I don't recall our having organized meetings, as such, then. The students all knew—we were so close together that we knew everything that was going on.
Seminars and so on? Journal clubs?
In the department, yes. The department had journal clubs. But our particular research group did not, in any systematic way, as we did later on, when the group was larger. When I first started being interested in gravitation, I really had two groups going at once. I still had the hold-over from my earlier years, plus the new students. So we had a very large group.
You wanted to ask when clocks were—
Well, yes. Could we go back to your g-value of the free electron? This involved techniques which presumably were not as familiar to you as the microwave electronics, involved quite a mixture of experimental techniques. Do you recall what kind of apparatus it was? Do you recall in general how you documented your experimental work?
I can tell you about documentation. I was never able to document anything well, so the documentation is always poor.
Yes and no. I've been keeping a notebook in connection with my recent interests. I have it around here somewhere. But I don't put everything in it. When I do, I write in a looseleaf—loose sheets of papers that go in folders. When I get something developed I think I might want to be able to recall later in a clearer way, I rewrite the thing in a notebook. But the great bulk of stuff is just lying around, not in a very complete form.
And when you make sketches for apparatus, and the like, that's also done on these sheets?
Yes, in loose sheets, and I guess I had a notebook there with some sketches, for things we built last year. On the subject of apparatus, new things and so on, I was rather inclined to try my own ideas to see if they worked, rather than try to go to the best source for how to do it. You learn that way. You also learn how not to do it.
Bruce Hawkins came in as a graduate student, and took over this apparatus.
Practically the same one I had used for this, this g-factor thing. That front end. To try out this scheme of optical pumping.
Do you recall how you came by him as a graduate student? Do you know how that worked?
No. He probably came to me to see if I had anything to work on.
The theoretical side of this dissertation is also impressive. His calculation of the optical pumping matrices.
Well, that, I think he had a lot of help on. That probably was given to him on a platter.
Well, it's certainly nice to have it recorded somewhere so it can be cited. Wittke's experiment on the hyperfine structure—did you conceive that experiment directly as a test of your buffer gas collision narrowing?
No, it was conceived as an important experiment to be done. Then it became clear—I guess the two halves of it came hand in hand. But what's not at all clear in the publication record is that that represents the first accurate measurement of that. What happened was that Polykarp Kusch, had made a beam experiment, that had a value which was wrong. He had some mistake in it. And we made this accurate measurement. Instead of doing what many people would have done, which is to rush a letter off to the Editor, we called up Polykarp and said, "We've got this number.... It doesn't agree with yours." And Polykarp came up and looked at our stuff, went back, and in one week he re-measured that thing, he found his problem, got a new value, and got a letter off. So his publication was before ours. But he did it right. He found his problem.
Do you remember, with Bender, of course, it was a real buffer gas study. His dissertation was exactly to study the dependence on buffer gas pressure—
Yes, right. Was he before Wittke or after?
He was after Wittke, I believe.
Wittke's was the first use of a buffer gas for line narrowings but Wittke's experiment was just a transition. Bender's experiment was the first in optical pumping using a buffer gas.
Now, Carver joined you in the summer of 1954. And it was in the autumn of 1954 that you went off to Harvard.
For a sabbatical. Now, do you remember the circumstances under which Carver came and what his role was? Was he substituting in a sense for you?
There was no intention of his substituting, but he was certainly a big help with the students while I was gone. I came in about once a month, to check with my graduate students. I think having Carver there was a big help. Now, we certainly didn't bring him in for that year. The normal intention, in bringing in junior faculty, would be for at least three years, unless he just didn't work out or something.
When Carver was hired, did you anticipate this close collaboration with him?
Yes. He came with the understanding that he was joining our research group. I found him, and that was the understanding.
And did the idea of an atomic clock, the idea of carrying this basic physics in a more technical direction, was that clear to you at that point?
At that point? No.
There was a meeting of the IEEE or the Institute of Radio Engineers, as it was then called, in the spring of 1955, a famous meeting at which Zacharias and you and Townes all presented papers on atomic clocks, in effect.
The fall of 1954, you say?
No, this was the spring of 1955.
So Carver was already here then, had been here two years.
No—unless I'm wrong. I don't know definitely, but it appears that Carver arrived in the summer of 1954.
I would think that's probably right.
And you were then spending that following academic year at Harvard, so this would have been towards the latter part of your academic year at Harvard that there was this meeting in New York in the spring. And Zacharias and Townes and their successes, their experimental successes to report on, and you were reporting on an idea. You were proposing, in someway, against their devices, another device.
You know a lot more about this than I do!
This is all that's there in the published record. I wondered, whether you had any recollection of how the idea of going for a clock came up; what its connection with Carver's arrival and presence might have been.
I certainly remember being interested in this general problem of the clock, and I remember being interested in Zacharias's "over the transom" experiment, things of that kind. It was obvious, in the context of our line narrowing, and we'd adopted that, that this was a good approach. But I think maybe the best answer to your question is to be found in those Quarterly Reports of the Signal Corps. I may have almost a complete set in there. I don't know. I can't remember, though, to tell you the truth. I remember that [because of] the purpose of those annual Signal Corps frequency control conferences, I was trying to think of something that might be of interest.
Were those Signal Corps conferences quite dedicated to frequency control?
Yes, they were called frequency control. They went back much further than this. They were organized to try to deal with the irregularities that appeared in the quartz oscillator type frequency control, and as long as I went to them, they continued to have most of the meeting devoted to this. I don't remember how many days were involved in these. Three or four days, something like that; then there might be one day devoted to some other ideas, not quartz.
I wonder if I might just ask you to, very briefly, what the Signal Corps needed very precise time for at this point? Was that for radar?
I can't really tell you why they were that much concerned about it. This was before Sputnik, so it couldn't have been the rocket control thing.
Of course, they were certainly working on missiles right along, in this period, even though not with the prominence.
No, I can't really say. If it was the Navy, I think one might have thought they were interested in some of their navigation problems, knowing where their ships were. Presumably the Signal Corps would know where its trucks were.
Was this also a meeting of just physicists, or did you get a good complement of engineers?
Oh, those meetings were mainly engineers. Mainly quartz oscillator engineers.
In the event, you wouldn't really be having much interaction with them, or did you also talk with them?
I talked very little with them. I think that's true of the other physicists, too. Those of us who came to talk about atomic clocks, things of that kind, talked to each other.
I would like to pick up on a few things that we've just mentioned, partly in our walk to the lunch room. You mentioned briefly that there was some material that you patented that had to do with what your father was interested in.
Well, I think you'll find a couple of joint patent applications there [indicates his bound book of patents], by my father and me. There may be one in there that's in a field he had done some inventing, where I thought of something in connection with something he did.
What was his field?
Well, his pile of patents is maybe 50 percent greater than that, and largely train control circuits and things of that kind. He was patent attorney for the General Railway Signal Co. at Rochester. But he also got interested in clock systems and clocks of various kinds. I believe there are a couple of clock inventions in there.
This was way before the atomic clock era?
Yes. Mechanical clocks.
There's another question I'd like to pick up on. You mentioned that at RCA, here in Princeton, there were some seminars, some of which you talked at, some of which you heard, and I have not got a notation of who it was—do either of you remember, when we were talking about—
We were talking about Joe Weber in that context, weren't we?
I'm not absolutely sure that I heard Joe Weber there. I'm very uncertain about this. I remember either reading about his ideas, or hearing it from him. One or the other.
Well, we've recorded for about two hours, and you may want to bring it to the end for the moment.