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In footnotes or endnotes please cite AIP interviews like this:
Interview of Willis Lamb by Joan Bromberg on 1985 May 10,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Some of the topics discussed include: the development of his career along with the problems of the simultaneous development of his wife Ursula Lamb's career in academia; radiation; megnetron oscillators; laser theory; quantum theory of lasers; government funding of research. Some of his associates mentioned are: Arnold Nordsieck, George Uhlenbeck, Van der Pol, Charles Townes, Leonel Menegozzi, William Wing, Marlan Scully, E. T. James, William Bennett, Richard Fork, Sargent, among others.
Now, two things that I thought belong on that tape that you’ve mentioned while we were off tape were, first of all, this feeling that you had that you wanted to reconcile the kind of electromagnetic theory that you had found in Stratton’s text and the kind that you find in Heitler, as one source of thinking about this. Is that a correct formulation?
I certainly said something like that.
Another thing that I thought was very interesting that you mentioned at lunch was the relation of your theory to experiments in the field, that working from the framework of your theory would really require a kind of dedication to one kind of setup and tests• on it, whereas it was so very easy for the experimentalist to quickly change into other kinds of experimental setups. There I think you need to make this a little bit more precise.
Yes, well, obviously when I was in the middle sixties, the computers that were available were fed by punched cards, which is a very inefficient way to go. Now, one has tremendous possibilities in the computer and it’s much more convenient, but in general, the — working out the laser theory requires the algebraic material, it requires the ability to compute the values and compute the values of the functions that appear in the theory, and it requires the ability to integrate the differential equations that have to be solved to find out what happens, and this takes a fairly elaborate set of computer programs, so probably Fortran is the language in which the programs are written, and if the output is in the form of numbers, that can be put on paper or on the screen, it is very very difficult to find out what is going on. It’s practically essential that one should have the ability to show graphically what the result is, and this has to be done in a fully interactive mode, so that the computer will quickly display a graph and then one can easily change the parameters of the problem and display another graph. Ideally one would like to have a situation in which the computer would be simulating the experimental facility that the experimental person has, namely, everything starting with the massive table on which laser experiments are elected. And well, I think that there are big advancements in the direction of having convenient, quick, interactive programs, but it’s been very hard to get university computer centers with their main set of computers to provide that kind of facility. I think the new generation of personal computers, like the LPC-AT with a fully occupied memory and its potentially very high address space will increasingly make it possible to imitate the experimental behavior.
Is any of this kind of work being done in the last years on?
— I have colleagues in Tucson who are certainly in several respects getting close to the situation which I have outlined. Although they haven’t fully developed the system and right at the moment I can’t do what I want to do. Partly this is just because I don’t know enough about computer programming, but I’m about to learn.
So you yourself will be doing or at least working cooperatively with these experiments.
Well, the experiments in Tucson are not likely to be involving a test of multi-vibrations. It’s just finding out what to work on first. I suspect that if such tests were to be made, they wouldn’t represent what a topnotch experimental physicist would want to do any more. As a matter of fact, I am hopeful that I can make calculations that would show interesting effects and possibly get some of them verified, if they are sufficiently interesting to attract the qualified experimental interest.
Now, there is, however, at least some cases in which you did work on this theory very closely with experimentalists. For example, when you were working to show the Lamb shift with (Ali) Javan at MIT, McFarlane at Bell, and then later on with the pressure broadening, I guess you worked with (Richard) Fork. Weren’t those situations in which there was a fairly close working between theory and experiment?
Well, letters were written or conversations were made, but the — with the case of the tuning dip, the theory said it was there and the experimental people at first didn’t find it, and then did. And so I think you can say that’s a matter of working together with people. But what I was talking about was having the computer do, with the help of a theorist, what the experimentalist would do.
And preferably the two would actually be it was carried on be not too far away from each other.
OK, then I misunderstood a little what you were saying and this clarifies it. I think it would be, I don’t think we talked so much last time about that interaction with Javan and MacFarlane, nor do I think we talked about the later work with Fork at Bell. It might be worthwhile to see if you have any recollections worth putting in her on those. You say you were writing and talking to the people. Do you remember how?
Well, my relations with people at Bell had several stages. At the first stage, Ali Javan was spending a while at Bell. I think it was during the time at Bell, six months or so, I was at Columbia, ‘61.
That’s right, he was there then.
And at that time, I didn’t have the prediction that — of the tuning dip — That connection with Bell was I think initiated perhaps at his request but certainly made possible by Sidney Millman, who was something like head of the physics department. And —
— so Javan came to you and wanted to know more about lasers?
I can’t remember in detail what — but we had many conversations. The most spectacular thing that I remember was flat, I think I had visited him at West Street, and he drove me up to Columbia where I was staying in an apartment, and there seemed to be no lights on.
It was the time of the big blackout. The time when it was thought by some that nine months later there was an enormous increase in the birth rate, because many people were caught in elevators and had nothing to do for several hours. But not everybody agreed with that theory. Anyway, Javan got me to Columbia safely.
Sort of like driving through a rural neighborhood.
That’s a famous bit of history, something like the Blizzard of ‘88 year. Well, then I went back to Oxford, and I think that I was beginning to see signs of it but I didn’t believe it, before I went to Oxford, but when I got there I checked all the algebra with Peter then, and pretty soon had to believe it. And I’m almost sure I can find letters that I did write to Javan maybe at his request.
Those would be special.
And to (William) Bennett also. Then when I got back, to, when I got to New Haven, in the middle of ‘62, no one indicated that they wished to continue the relationship, and my principal contacts there, as far as laser theory was concerned, was Richard Fork. I didn’t have much to do with McFarlane except there were general conversations in which he took part. And there were, such people were pretty aware of what the theory was like.
Fork is a name that, although I know his name, I’ve never really had any sense of his participation, the quality of his participation, the character of it in laser work. What was he working on, do you happen to remember that? What were you and he talking about?
Well, we talked about things, it’s too long ago to know what, except that I’m sure the effect of a magnetic field on laser was one, and that led in due course to the thesis that Murray Sargent wrote on Zaiman lasers.
I see, so already that early time you were —
Well, this began in ‘62 and continued for perhaps half a dozen years. Actually, the person to whom I was “assigned” was not Fork, but P.K. Tien. And we talked about lasers, but — and I know there was some discussion about an electron bombardment laser, and I think Bell Labs even did something about getting a patent on that work. I know that I had to sign some papers. I expected to receive a dollar for my contribution, because that seemed to be a Bell Labs stunt, but since I was only a fractional time consultant, it’s possible it may have been overlooked.
They didn’t even give you a quarter?
No. Bromberg Well, Tien was a kind of department head, I think.
Yes, he was. And he has been very much involved in the propagation of optical waves along surfaces, which would be an important contribution for optical communication, but that would be many years later.
Then you worked with Fork later on. Didn’t you work with him on pressure broadened (?), and there was some question about the computer in ‘67 at Bell?
No, the computer I may have talked to you before about was at Perkin-Elmer. Bell had computers, I knew, but nothing that I did at Bell had got far into a computer in any way that I had anything to do with.
I’m sorry, so I’m getting confused, it’s really the papers on the Zaiman laser where you worked with both Sargent and Fork.
And you did a computer calculation, but that computer was at Perkin-Elmer?
The computer at Perkin Elmer which was a Sigma 7 was, if it had been used, if it could have been used, it would have been used for ring laser theory.
OK, so what computer did you use for the Zaiman effect?
Oh, that would have been the Yale computer, or Murray Sargent made the calculation that he made with the Yale computer which meant punched cards, but he was a very good programmer so he didn’t bother him as much as it bothered me. He did also have such skills as being able to get the computer to draw things on a CRR and they had a facility for transferring the image to a television screen that was photographed, and if you could get the pictures side by side on a strip of film, then Murray found out that there was at Bell Labs a program that could convert such a series of pictures into a moving picture. That is, instead of having the pictures lined up at right angles to the camera, they were turned 90 degrees and transmitted as successive images, in the way that a moving picture camera would make. So some of the work was dealt with — that was really being done at the time of the work on the quantum theory of the laser was being done.
So you were working with Sargent on this very complex and semi-classical theory, and at the same time with Scully on the quantum theory?
Oh, the quantum theory was being worked on by Scully, but Sargent was around Yale, and when it came to computer problems would help us out. Murray went on to spend a couple of years at Murray Hill at Bell Labs when it was at Murray Hill, and there he got even more deeply involved with computers, but he was already very knowledgeable before he went to —
I had mentioned to you at one point that I really didn’t understand — well, let me put it this way. You told me that there was a lot of practical interest in Zaiman lasers in this period. Something that I had just not known. In fact I hadn’t really known why you attempted this very complicated Zaiman laser —
— well, a magnetic field concerning frequency of a laser, and change the moding properties and certainly amplify, certainly amplitude modulate lasers, so that one of the things one might think of is making the laser, which by itself is a fairly monochromatic device, into a useful tool for communications. Of course, it isn’t done that way. It might have been done that way.
I see, so this would —
At the time there wasn’t any conspicuously better way.
I didn’t understand that at all. I didn’t realize that. So I would guess that Bell would have been quite interested in that piece of work.
Well, Bell Labs is a large organization.
But I mean, it fits in with communications nicely.
Well, if Pork was working on Zaiman lasers, that’s sufficient indication to me that there was some potential. But not necessarily a very direct connection. Fork was at Murray Hill at a certain time. The division in which he was located was moved to Holmdel. And my consulting relations with Bell were modified, in that instead of going to Murray Hill I was supposed to go to Holmdel, an extra half hour’s drive perhaps. Fork I think did not come, I think he remained at Murray Hill so that I had contact with other people at Holmdel.
Well, now I just want to go through papers and pick up what we have not really talked about. And I thought, we talked about the general theory paper. We didn’t talk at all about the second in that series on spectro-profiles, in which you discussed a case where the density matrix is diagonal. I don’t know what exactly — did you have any particular reason for putting that material in the second paper rather than in the first?
All of this is quite dim in my memory. I haven’t looked at the second paper for some time. I thought that one thing done there was to find the frequency spectrum of a laser which is oscillating under certain conditions. It’s not absolutely non-chromatic. It’s (?) and I would think that it would be off diagonal almost to the density matrix that would be, would have to be looked at.
That’s quite possible. You know, I may have misunderstood what I read. It happens frequently.
A laser in which you are concerned only with the diagonal elements of the density matrix or the radiation field is — any laser, unless you’re talking about entirely in terms of the number of photons, numbers of photons, there isn’t a well defined number of photons, there’s a range of numbers possible, but you’re not at all concerned with the electric field of the laser. The off diagonal elements of the density matrix are needed in order to calculate the electric field, and to know that the laser has a frequency nu so that the electric field varies with time, like cosign of nu T plus some phase sign, or perhaps it requires that you have off diagonal elements of the density matrix. The equations for those are of the same general field as the ones for the diagonal elements, but they are more complicated by the appearance of square roots, and the algebra isn’t so simple to get the steady state solution. If you had a completely monochromatic phenomenon, and you took the product of the amplitude for that phenomenon at one time, and then at another time, a time tau later, and then took the average of that power over all possibilities of what might be happening, you would get something which, if the signal were really monochromatic, would be a completely periodic function that would go on regularly forever, but if you do that for a very nearly monochromatic laser, you find that the resulting expression falls of exponentially with the time, like e to the minus t over something like t1 or t2, losing notation that occurs in other theories. And this means that the, it’s a way of expressing a non, the departure from completely monochromatic behavior, and in general the explanation of it is that the radiation field can be changed by the laser through processes of stimulated emission, and spontaneous emission. The spontaneous emission has a certain random or stochastic element about it, and is able to effectively make the phase angle of the monochromatic signal vary with time in an erratic fashion.
Now, I’m going to leave that, and get back to some of the historical work. Something we didn’t talk about at all, for example, last Thursday was a paper you did with Stenholm where you found fine structure in the dip. The semi-classical paper in which you took the theory out to the fifth order.
Well, we took it more than that. We took it to arbitrarily higher.
How did that come about? What made you go into that development?
Well, the earlier forms of the theory had been carried out as in a perturbation theory in which you first start with the an atom in the upper state, and then in first order perturbation theory, there’s some radiation and the atom is in a lower state, and in a higher order perturbation theory, your radiation… (off tape)
Let’s start with the last sentence.
OK. When you go beyond, the lowest approximation would describe things like the emission of radiation. But in order to work out the theory of the laser, you have to go beyond that. One of your approximations, you go up to at least the fourth order, and that’s what I did in the ‘64 paper. However, the calculations still could be made to the fifth and the seventh and the ninth and the eleventh orders, except that it becomes very complicated. And I think there were a few efforts on the part of a Japanese physicist to go to the fifth order, and he found that the tuning dip was still there. But of course, you could only believe it was still there when you had the laser at a low enough intensity that you didn’t need to go to a higher order. So that there was always the question, how much the phenomenon of that the third order theory gives would remain in a higher order theory, and better yet in a theory in which you can have arbitrarily strong fields.
Now, in the first paragraph of that paper, you mention experimental results by, I don’t know how you pronounce his name, a Dutch physicist, Bolwinn, deviations from theory, and I was curious on reading that, and others also, how much that development there with Stenholm was inspired by these experimental deviations and how much was inspired in a more general sense, that you just wanted to see how to work with higher orders.
Well, the need to go to higher orders was obvious, both from purely theoretical considerations, but the indication that there might be some experimental need for it would certainly have a definite incentive. And —
— it led me of course to wonder —
When Stenholm came to Yale, the time was ready to do these calculations, and together we figured out how to make them.
It led me to wonder how much, I mean I’m sure this varies between papers, but how much of the impetus for a piece of research would be experimental results, and how much would be sort of theoretical, I don’t know if you can generalize it at all, but — how much new theory comes in and makes you say, well, we really have to look at this again. I’m sorry, new results. New experimental results are motivating you.
Well, in earlier times, and even still, the theoretical physicists make calculations of scattering processes, and if they haven’t anything better to do, they use first order or first approximations, and very likely that’s as much of a calculation as they have the time or energy to make. But sometimes it’s important to know why a stronger degree of interaction would lead to. If you tried to work out the theory of electron scattering, which is too often used to learn about the properties of matter, the various nuclei in the solid can scatter radiation. And the theory is only simple if you make a lower perturbation calculation. But with things like electron microscopes very definitely need to have available a stronger interaction calculation.
OK, so it’s the needs of the —
— a high power laser, ideally you’d like to have arbitrarily high intensities just in case you might be able to get it. This was before the days of Star Wars, but —
I guess in ‘68 you were just beginning to get stuff like gas dynamic lasers.
Well, lasers were pretty intense; even when the tuning dip was seen in the first instance, it was probably seen under conditions which exceeded the limits that the third order perturbations could be trusted.
I see. And these fine structures — that just fell out of the theory without any reason to expect it they’d come out?
Well, the tuning dip was found, but there was some little bump in the bottom of the dip, I don’t think should be taken very seriously, because if the excitation of the laser medium is sufficiently intense, it won’t be sufficient to consider only a single mode oscillation. The work with Stenholm did insist on having only a single mode oscillation. But what still remains to be have a theory that doesn’t limit you in any way as to how strong the signal is, and doesn’t limit you in any way to the number of modes that may be extended.
Was that theory picked up very much or was it just sort of held in reserve?
Well, it was picked up to some extent because Michael Feld at MIT, and I can’t remember his colleagues, did find that we had made a, that some of the curves we showed had strange features which we wouldn’t have had if we had made the calculations in a better way.
Was there any reply to that? Did you go back and forth?
No, we didn’t need to reply to it. He was right, and we didn’t feel that it really mattered very much, because I can go into more detail about what was really going on. We were solving some recursion relations. Similar relations occur in the theory of Bessel functions. And there is some question about what happens when you get to very large failures of N where N is the parameter that tells you many recursions you make, and we had the computer making the calculations for us, and at a certain point the computer began to notice that it didn’t have enough precision, so it began indicating that something was happening which in fact shouldn’t have been happening if it hadn’t been fully rounded off in truncation errors. And, but, in the limiting case of large N, the recursion relations that were being solved were enough like those for Bessel functions that Feld and colleagues were able to see what the problem was.
I see. Is there more that we should be talking about in that whole subject, any —
I doubt it, because of the — if the laser excitation is high enough, there’ll be a number of things that are bound to be important, such as the possibility of more than one mode, such as the possibility of collision effects, and you’re trying to put as much excitation in the medium as you can. That means there are going to be lots of, you have a high density of atoms and you have collisions and probably plenty of ions around to produce things like Stark effect, and it becomes a difficult matter to decide what you can neglect and what you can’t.
Well, the paper right after that, with the man whose name is unpronounceable, Aslam Icsevgi?
Well, I was always led to think that it was Icsevgi.
What is he?
Turkish. Aslam Icsevgi.
Now, I had heard at one point that Scully was very actively in communication with DeMaria and very interested in ultra-short pulses, and I wonder if that had anything to do whatsoever with your taking up this unified treatment of —
Well, Scully was pursuing other activity on essentially the same problem at the same time. Hopf, Frederick Hopf and Scully wrote a paper on pulse propagation. That was probably more nearly stimulated by the United Aircraft connection. Scully had such a connection. I didn’t have one.
Now, where was Scully at this point?
Where was Scully then? He might have been at MIT. He — when he got his PhD from Yale, he became, he moved to MIT and was, he got at least to be an associate professor. He may have gone there as an instructor.
How did you come to decide to take up this particular topic?
Well, it was there.
But there were lots of things that were there.
It was not even a new paper, but interesting things happen when you set courses?
Of course, the very short pulses were rather new, and I would guess they might have been very exciting.
Yes, well, they were. And there was theoretical work, and I knew there was a paper by (N.G.) Basov, the Russian, and there were some things about the paper that I didn’t like, and I guess that’s what led me to think about how the same problem would be treated using the form of laser theory that I knew.
I noticed when I did research on years before that one of the things that almost always set him off was reading an analysis he thought was wrong. Is that something that — I say that Bohr, you find again and again, he is set off on some particular line of research by reading an analysis he considers to be wrong. Is that something that happens with you too, that you’re very stimulated by an incorrect theory?
Well, the Basov paper on pulse propagation, I can’t remember the details at all, but in spirit I — He didn’t use a form of laser theory that I liked. (Sorry, I just noticed note, transcribe A only, going to next tape)
OK, there is —
I’m still a member. I no longer subscribe to the journals. I kept on with one of the sections for years and years, until I ran out of book shelf.
Well, you know, really, there are two kinds of scientists — people who get very very much involved in the activities of the society, and people who just do what they need to do once in a while.
Well, I haven’t been to meetings of the APS for a long long time.
Did you ever go as a matter of, were they ever of importance to you?
Well, I gave a number of invited lectures. Four or maybe six, I’m not sure. That didn’t take very much. All one had to do was Darryl or Havens know that one had such material. That was all it took to get to be invited, so it didn’t mean much.
Was that an important forum for you, for finding things out, or you prefer to read or talk to people individually? Were meetings?
Oh, it was fairly important. In the Columbia days, the meetings, some meetings were at Columbia but some were — there was an annual meeting in Washington. I went to those. Then after ‘5l, when I was at Stanford, I don’t think — I might have gone to a meeting of the APS at Berkeley or Pasadena, a time or two. When I was a member of the National Academy I went to a few meetings there. They were held the same week as the APS meetings. It was usually the first days of the week and APS met the last days of the week, and I guess by the time I was in the NAS, if I went to their meetings, which was a very infrequent thing, I did not stay the rest of the week to go to the APS. Maybe I did on one occasion.
Would that reflect the fact that you were just too busy working to put in all that time at meetings, or?
Oh, it just didn’t seem, a combination of expense and time. Other things were getting too big.
I see, so it was partly just the postwar expansion of physics, is that correct?
Well, plus increasing age on my part.
You must have been pretty young in the fifties.
A month or so ago, I went to a regional meeting of the National Academy which was held in La Jolla. They have now regional meetings, and each year they seem to go to Berkeley, to Pasadena, to La Jolla, to Chicago, to Boston, and the scientific content is zero. I did see a number of people I hadn’t seen for years.
Something else I would like to ask you about, because I think that in the not completely unlikely event that someone attempts a biography, they would be glad to know, is something about your patterns of work. Are you one of these seven day a week people? Are you night and day or do you work in a more well defined way? At some point in the day? Do you like to work alone or do you like to work at a blackboard with other people? That kind of thing. That may have changed in the course of your —
Yes, it’s changed. I mean, it depends on who the people are, what the convenience of things are, so what I would say would be different if I told what it was in the earliest days, or middle or later years.
Why don’t we talk just a little bit about major periods, pre-war, then post-war, immediate post-war — say, the Oxford years —
When there was a suitable blackboard and somebody likely to be interested, talking and writing on the blackboard was an important thing. I don’t have a blackboard in my office anymore because, you see, in my physics office you were in, the blackboard is very small. The reason for that is that there are so many books that the book shelves took over, so the book shelves remain, the blackboard disappeared. And in general blackboards, chalk blackboards shouldn’t be in a room with a computer.
Oh. That’s something I didn’t think of.
Well, if you want the blackboard model, you want the computer, then of course, —
I know for example Dirac never works on Sunday, I guess. Neither does Townes.
Yes, they are people who only will work six days a week and not seven. Is that something you do too, or you just work whenever you feel like it?
What’s, what’s today? Here we are working busily on Sunday.
I don’t know what you do on Sunday, but —
I work. I have no rhythm whatsoever, in life.
Now, I want Monday as a (?). I told you I have a lecture on Christmas Day in (?). I guess that’s the only time. Usually the university doesn’t have classes on Christmas Day.
There is a paper which I didn’t read, but was briefly described in the textbook LASER PHYSICS, and found very interesting. That’s the one with Lang and Scully, “Why is the Laser Line So Narrow?” in which you talk about universe modes. And I was very interested in that, how that came to be written. That seems to be far out.
Well, the title was sheer undiluted Scully. And I didn’t really do much about the calculations of that paper. I think I did the things that were needed, such as the model for dealing with the, let’s see, the ways for communicating with the universe.
Whose idea was that, to make such a general theory as that? How did that even come up, such a —?
Well, I really can’t put it together very well. The ideal form in laser is, has some kind of cavity resonator, and this ideal form might be the universe, in which you would work — but a laser to be of any use has to have a window, and the radiation has to get out. That means that having the closed cavity resonator is oversimplifying the problem. Well, what you really have is some, somewhat leaky resonator embedded in a much larger region.
I’ve never seen anyone else tackle the problem in that way. Is that because I haven’t read enough?
Well, if I said — you know, in a sense, the answer is yes, but…
— that’s all right, I’m very conscious of my lack of laser physics. But it did strike me as absolutely novel.
No, it isn’t all that novel. It’s just that the whole thing came up earlier. During the war years, John C. Slater at MIT worked at Bell Labs on microwave devices, and he wrote articles and he wrote books, several books, dealing with microwave oscillators, and in that, he had a discussion of a microwave resonator, one might as well say an optical resonator, it wouldn’t make much difference, and — which had a window, and which meant that it was connected to a transmission line, which particularly would go off some place and be terminated with a load. The load could either be one that would make a matched load, which means that signals sent down the transmission line don’t get back, or it could be unmatched, which means the signals would get sent back. It could be a horn that would radiate out into space, which in effect would never get back, until later, many years later, and anyway he developed the theory for that kind of microwave device, and I had some occasion to read it through the years, and when the optical problems came up, I somewhat imitated what he had done. And this, to some degree the imitation was done in at least three places. One was in this paper about why the laser line was so narrow. There were two papers that Martin Spencer wrote, his thesis problems at Yale. One of those was called “Theory of a Laser with a Window,” or maybe just “Laser With a Window”, and “The Theory of Two Coupled Lasers.” Well, —
About 1971, ‘72?
Yes. Those, when we sent those papers in to be published, an unknown referee wrote back and said we should have paid more attention to what Slater had one. Well, we did have a reference to Slater, but nevertheless he did ask a question about why it was necessary to re—invent the wheel. Something like that, the way he put it. But the optical situation is sufficiently different that — also, Slater was really very, he didn’t have to be very specific about what the microwave oscillator was. He usually represented that by a nonlinear negative resistor of some kind. And whereas our laser had atoms in it, and it wasn’t representative — it was representative in a kind of oversimplified way, but it wasn’t quite as oversimplified as Slater was using. And we represented the window in a way which was appropriate for the optical devices, and Skater represented the coupling to the oscillator in the language of electrical engineering, mainly impedance in the transmission line. Then there’s a paper by Gyorffy on ring lasers, has a paragraph, a section that deals with a discussion of what it means to have a ring laser cavity embedded in the universe. Embedding of the laser in the universe, I would say, was done first in some form by Slater, but pretty independently, except I didn’t know about what Slater had done, but we couldn’t use it, just had done, variable variability. The way we represented a window was to imagine that we had essentially a uniform — we then, the dialectric bump, the dialectric pump will reflect waves. If the bump becomes very very strong, dialectric pump, that is its induction refraction becomes enormous; then the pump acts more and more like a mirror, a metallic minor, but radiation leaks through. Well, the Spencer paper is pretty explicit about that model, which is a good model, but it’s not like real windows, it’s just — a real window is more complicated. You have a thickness. We tended to take a very thin dialectric discontinuity.
And in applying this whole idea to way of looking at things, to the question of the narrowness of the laser lines, is that a general difficulty that was widespread, the understanding of the reason for the narrowness?
Well, let’s see — even if you had no window on the laser at all, the radiation was completely confined, the spectrum would not be monochromatic because of what we call the interquantum spontaneous emission properties, but you have a leaky window, then there is that full universe full of modes. And so the question arises, how can you get away from treating the problem as a single mode problem, when you have an infinite number of modes? That’s the basic question. And the only answer to it can be found by as at least I could only find an answer by trying to make a model that described the new complications, of hoping to find in the end that it didn’t make very much difference. It still effectively had the kind of oscillation you would have had if you’d had a single mode. But the openness of the structure does some things.
What questions would you have asked if you were the questioners?... I said, what questions would you have asked of yourself if you were asking the questions instead of me? Just another way of saying, what should we be talking about that I’ve omitted?
Well, I don’t know.
If you were asking yourself about your work or about any of the antecedent circumstances or style of working, those things. Were there any particular people you always turned to, to discuss your ideas with, sort of central core people who were especially useful to you for clarifying ideas?
Well, there have been such people, but they haven’t always available to ask. Even when there was no geographical barrier, sometimes their patience was finite, and — I can give you the names of the people of whom I have had many chances to ask questions, and from whom I’ve learned a lot.
Well, Oppenheimer would be among the first in many ways on the list. Arnold Nordsieck. Albert Serber(?). Felix Bloch. George Uhlenbeck. Enrico Fermi.
Were you ever working, nearby Fermi?
Not for long, but it was noticeably effective for me. He was at Columbia when he left Italy. He came to Columbia. But I had a chance to come to know him a little bit. He spent a summer at Stanford. What summer would it have been? ‘46? It wouldn’t have been ‘35, I went to Ann Arbor. ‘46, (Victor) Weisskopf, no, ‘36, (George) Gamow was a summer visitor at Stanford. ‘37, Fermi was the summer visitor. ‘38, I.I. Rabi was the summer visitor. ‘39, van Vleck, I think. ‘40, I went to Ann Arbor again. And I undoubtedly wasn’t –- I had a son in ‘41, and maybe that was van Vleck. So I might — anyway, so, Fermi was there. I didn’t really get to talk to him very much, but I got to talk some. In the Columbia period there was more talk. He was there about a year, I think. And he was very helpful in answering questions and some of the things I did came from talking and suggestions of his. Uhlenbeck was also at Columbia for a while, and he, Nordsieck and I used to meet in the seminar room an hour or so a week, and one paper came out of that. I didn’t have much to do with the writing of the paper but my name was put on it because I was in the discussions group.
Was that a group which had some particular formal arrangement? Or you just walked in and talked?
No. Uhlenbeck wanted to work up a certain problem, and he was willing to do it in this way, having a regular meeting to discuss it. The problem involved the theory of fluctuations in cosmic ray showers.
That sounds like a very high-powered session. And then, in the Stanford or Oxford period?
Well, let’s see, I didn’t finish all the names that I should have put. I certainly had many helpful discussions with Leonard Schiff, and Gregory Breit was at Yale. I didn’t have very many discussions with him but did have some. And I had many discussions with Scully, while he was a student, and afterwards. I had many discussions with Peter Franken. He spent a term at Oxford, and just about the time he was, he hadn’t, this was before the frequency doubling. This was at a time when he was embedding level crossing spectroscopy.
Was Dirac in England while you were at Oxford?
Dirac was at Cambridge.
Is that a situation where one doesn’t go very much between the two?
Well, I think I did it three times in the six years. … and then, I think I met him for the first it was very early during the Oxford time, ‘56, ‘57, I think I went to Cambridge. I’m pretty sure I met him at that time. And my wife met him, and I think he took her on a walk through St. Johns College grounds. There was the (?) Triennial Conference of Nobel Prize Winners in Landau. You know what Landau is?
Landau is a city on Lake Constance. It’s in Bavaria. It’s on Lake Constance which the Germans call Bodensee. And that’s a city which, during the occupation after the Second World War, was occupied by the French, and the French allowed the citizens of Landau to open the casino, and at the other end of Lake Constance, is an island called Mainau, and Mainau has somehow come into the possession of a member of the Swedish royal family who married a commoner. He therefore wasn’t in the line of succession to the throne. This was a Count Leonard Bernadette. Count Bernadotte somehow inherited the island of Mainau, which he proceeded to turn into a place of great beauty, gardens, trees, — he has a zoo. He has a business which brings in several million dollars a year, in fees from tourists. And somehow or another, he got the city of Landau to engage in an annual conference of which Nobel Prize winners come and give lectures, and the lectures are given to students who are brought from all over Europe. And this, every third year it will be physics, except now they’ve brought in economics, so it’s physics and economics. And then the next time it will be chemistry, the next time it will be medicine. They don’t have the Peace Prize or literature. Bernadotte seemed to feel that that was going too far. Well, he presides over this annual event. I’ve been to quite a few of the physics meetings. I’ve never been to any of the others. And Dirac has been to more than I have. There’s one this summer; he won’t be there. But he would have been. Well, during one of those conferences, I had a long walk around through the garden with him. But I really never was able to talk about physics with him.
That was really what I was getting at.
I was too inhibited for that. And he probably was too inhibited for that. I suspect. — question about the hydrogen fine structure work. He told me that he had read about it, somebody sent him a clipping from the front page of the NEW YORK TIMES, and perhaps he felt I should have told him more directly, but he didn’t say so, but one read between the lines that perhaps that’s what he thought. He asked me, did I enjoy it? And I said, yes, I enjoyed it, but I would rather have discovered the Dirac equation for the electron. He said, “Things were simpler then.”
That was a very gracious reply. So, there in Oxford, were there any other important —
— I’m not quite through with Dirac. After he was forced to retire from Cambridge, for age, he went to the University of Miami in Coral Gables. He was a professor there in a so-called Center for Theoretical Physics, which is presided over by an old Turkish physicist called Kursunoglu. He was a man of great charm and few inhibitions. So I’ve been to Miami a number of times. Well, that article about the laser history was, at the time — Dirac was there at the time that was written. But we didn’t talk much about laser theory. I let him know that I knew he’d been an electrical engineer before he became a physicist. But he didn’t really want to talk about it very much. I think probably (Jagdish) Mehra got more out of Dirac than most people. There is some transcript of conversations that Mehra had with Dirac. Well, I assume that this letter this is (?) is in reply to a letter that Ursula wrote. This is probably a letter from Margaret Dirac, Dirac’s wife. So we’re old friends, just because we’ve been friends for so long, but we really haven’t been — now, on the other hand, Mrs. Dirac is quite different. She’s not at all timid about talking to people.
I was going to say that my — well, I actually…