John Clarke Slater - Session I

Slater:

One thing I might say right at the beginning. I have had vague thoughts every once in a while that I might write some memoirs some day. I don’t know whether I will or not. That’s one reason why I wasn’t too anxious to go in for this thing, but I thought it would be worthwhile for you to know that I might at some future time, if I get time to amplify some of this stuff.

Kuhn:

Let me say two things that may be relevant to this. One of them is that if somebody who when he stops being a data collector, has by himself every intention of working on history of quantum physics, I immensely hope you will expand a series of memoirs. The other thing is that, so far as we’re concerned, this material we’re collecting now is not for publication. It is for an archive that, I hope, a lot of people over a long period of years will use and find useful.

Furthermore, it is my understanding that this is going to be accessible not only to scholars, but the control of the material should rest in the hands of the people who gave it. In this sense, the libraries will tell anybody coming to them that they’re welcome to look over this material, but the quotations and so forth should be cleared by the person writing with the man who owns the material.

I don’t think this is quite ultimately a foolproof guarantee. I don’t know where short of the courts you finally get that. But it is to be standard procedure that anybody given access to the material must first show bona fides, and having shown these, must also be informed that permission to look at it is not permission to use it. I think this is particularly relevant to a person like yourself who is thinking of writing a memoir, and for my part, I very much hope you will.

Slater:

It’s all a question of how energetic I feel when I get through with the other writing projects I’ve got.

Kuhn:

It isn’t something that leads me to say, “oh, if you’re going to do a memoir we can just forget it" because I know too many people who have told me they were going to. Of course, Pauli had said at one time that he might very well do this, and what a difference it would make to the subsequent history if he had! So I can’t stop because you said you may do this, but I very, very much hope you will.

Slater:

Now let’s see: you start out by asking about my bringing up, background and so on, which I guess Van [Vleck] knows fairly, My father [John Rothwell Slater] was professor of English at the University of Rochester for many years. He’s still alive, he’s 91, and he’s still very energetic. So I grew up in an academic enviroment. My father is not a scientist, but I think he has a great deal of general interest in it. He’s all the time trying to get me more interested in science than I am. Practically every week in his letter he’ll enclose some clipping from the Scientific American or something like that about some item that I might have missed. So I think I come by it naturally.

I sort of fell into doing sort of scientific things completely naturally. Even in grade school, certainly in high school, I was puttering around, we had a little work bench and so on, and I found myself getting books out on how to do this and that, and making electric motors, and trying out and making incandescent lamps, and all kinds of stuff like that, so that I was making gadgets at a very early age. Then I also quite independently got interested in mathematics and was doing mathematics on my own from an early stage. My father had studied some of his mathematics in a night school—he didn’t do all of his work in school—and he gave me some of his own texts. I had a text on trigonometry, and a text on analytical geometry, and so on, which I read with great fascination, and I played around with all kinds of things. I had fancy little problems that I worked out, I remember, in my early high school days.

One, for example, was the following: this really introduced me into transcendental equations. My mother would make an omelet; she threw it over so that it was a semi-circular omelet. We had four people in the family, and my father would obviously cut it in two, and then he wanted to cut each half in two. And I said, “well now, this would be a nice little problem,” given this quadrant, where you put a straight line parallel to one side so the two sides are equal to each other. And I set up the equation for that and found that that had X's on one side and tangents or something on the other, and I read enough in these books to convince myself that there was no analytic solution, so I worked out a numerical solution, and it came out fine. A number of other things like that just sort of intrigued me, and I played with these. So, in other words, I was playing along in mathematical things as well as in practical things before I ever had any courses in a thing like this.

Kuhn:

Were your father’s interests also practical? Was it his shop in which you played around?

Slater:

Well be set it up for me. He’s practical in that he likes to make things. He had done a good deal of that, so it was set up. for both of us. Well, I took physics in high school. I didn’t— (I was fairly standing) out then, because I remember the lab instructor played tricks on me. I remember once we had an experiment we were supposed to measure the voltage of the current in an incandescent lamp with a voltmeter, and ammeter and the watts and so on. I remember playing around with that. Everybody else was having luck, but I couldn’t get anywhere at all.

I had volts on the thing, it was shining, but there were no amps. The instructor Mr. Beard—or whatever his name was—came around and said, “why do you suppose that is?” I said, “the only thing I can figure out is that it’s an AC meter instead of a DC.” Sure enough, he’d switched an AC meter in to see if I could figure out what was what. Well, that reminds me when I look back that I must have been not the standard physics student. Also I knew in my math courses in high school I was trying to trisect angles and so on to see if I could fool the professor into thinking I had done it, knowing that I couldn’t.

So in high school I was clearly interested in these things, but I didn’t know just what I was interested in. In fact, the first time I ever heard I was interested in physics was before I took physics in high school. We had a college girl who was helping with the chores, sort of part-time maid. This was before I was in high school; I guess I was in grade school. She saw all these things I was interested in, she said “well, you’re interested in physics.” And I said, “what’s physics?” She was taking the physics course, and she explained what physics was. Then I knew that I was interested in physics. I went through high school rather young, I was sort of accelerating my education all through, so that I actually was in college when I was 16, at the University of Rochester.

I would probably have gone to Harvard— my father had gone to Harvard—and he wanted me to go there, and I would have, except that the First World War was coming on and things were very complicated and it seemed better just to stay at home. What I did was to stay home and speed up my college course—did that in three years—and got to Harvard when I was 19, I guess, as a graduate student, and I learned most of my stuff there. In other words, I was being a graduate student at Harvard at an age when I probably should have been an undergraduate.

Kuhn:

I’d like to take you back a little bit, and ask you just a few other questions before we get you even into college. Clearly you must have been putting a good deal of time into your math and scientific interests—?

Slater:

Quite a bit. This was one of my various hobbies.

Kuhn:

I’d be interested in knowing what the other ones were, and to what extent there were other career alternatives that you considered.

Slater:

Well, I’ll tell you. I‘ve never been anywhere on sports. I wasn’t interested in that at all. But I was quite interested in watercolor drawing and that kind of thing. I went far enough to take lessons in it, and go out sketching, etc. I was quite interested in architecture. That was enough so that actually I took a term out from my high school period—the schedule starting college didn’t work out right and it looked as if I conveniently could.

I had relatives out in Texas, and one of them was instructing a girls’ boarding school: this was my aunt, who incidentally taught mathematics too, so there was another mathenatical side to the family. And my folks suggested, why don’t I go out and spend a few months there in the architect’s office that was designing the building for them. They realized that I was almost as interested in architecture as I was in science. So I’ve always been sort of interested in that, played around with it a little bit, but never to the extent of doing anything with it. Well, I helped design the (terials Center) Building they’re having out here. So that was another hobby.

Kuhn:

Did you read a lot?

Slater:

I read a lot. Of all kinds of things. I read a great, deal.

Kuhn:

Literature as well as science?

Slater:

Yes; and I still do. I read a great deal of history and biography and all kinds of things. I don’t draw anything anymore, but I do a lot of photography. Then I was somewhat interested in music. I took music lessons, but I realized I never was very good at that, so I didn’t try to follow it any further. But as I say, I never was any good at any sports, so that never attracted me very much. I did Boy Scout things, hiking, and riding on my bicycle all over the countryside, etc. So maybe that will give you more or less or an idea.

Kuhn:

As of the time you got to college, how much had you had in physics, math, the sciences, both in the curriculum and by yourself?

Slater:

In physics I had just about covered the standard high school physics course. I don’t think I was ahead of that. In math I had done quite a bit on my own. When I entered college I took a standard freshman math course which was just telling you about what the graph of X^n was and things like that, and I realized that I was way ahead of that. As a matter of fact, I never took a course in calculus. I really did the calculus all by myself, and right in the second year I took a course in differential equations. I guess I was reading calculus by myself at the same time I was taking freshman math. I don’t think I had done this in high school as I remember it.

Van Vleck:

Didn’t you even have a course in calculus of several variables?

Slater:

No, I don’t think I ever had any course in calculus, as far as I remember. I just felt that I knew it all. I don’t think I did, I think I would have got some stuff. But I read over not only elementary calculus, but Wilson’s Advanced Calculus and so on, so that I covered the material well enough so that I could take more advanced courses. I may well have missed items that I would have got had I taken the courses. But anyway that’s what I did. In college I was not ahead of the game on physics. I just had these various hobbies.

Kuhn:

Had you read any books that had particularly impressed you? I‘d ask you also in particular whether you had read any popular accounts of modern physics? Were you aware of relativity and quantum problems?

Slater:

No, I don’t think that I had at that time. I had read some of these rather old-fashioned books on mathematical recreations and that kind of thing where they tell you about magic squares and stuff of that sort. But I had not had any books on modern physics. I got immediately into physics in college, partly for a practical reason. I wanted to be earning a little money. Father, of course, being on the faculty, knew the other faculty members, and the physics professor was a man who lived around the corner from us and whom we’d known for many years. He went in the summertime to a place in which we bought a little cottage, and we went out there too, so that we knew him very well in a personal way—Professor Henry E. Lawrence.

This was long before the days when. Rochester had any physics to amount to anything. We all realized that he was a stodgy old fellow who knew no physics—his main training was in electrical engineering. He saw I was a bright boy, and he needed some lab assistant, so he took me on as a freshman as a lab. assistant. I was just assisting in the elementary lab., a teaching laboratory. I had one strange experience. I assisted in a co-ed. Section and (had to handle that) situation, which I managed. Another hobby that I forgot to mention. My father always had a little interest in the publishing and printing business. He had been an editor for a couple of years before he went in for teaching. His father had done newspaper writing.

He thought that a boy ought to know something about printing, so he bought a fairly good whole printing outfit, that is, you could set type and print things. He put that up in the attic, and I earned a fair amount of money in high school and in the beginning of college by printing things up there. I did a little bit of a printing business. I remember in a room in the attic there we had a war map of World War I posted up in front of the place where we'd set type, and we put pins in to show what the front was doing on that particular day, then I'd go print my cards or whatever I had to do for my little job.

It turned out that they wanted to have some lab. folders with all the spaces for putting the formulas or the results in and description of how to do the experiment with the mathematics in and so on and pass these out every week. I volunteered to print these things, so I got an old set of mathematical type with square roots and things, and I set the whole business up. That gave me some good practice in how one sets up type. That’s one reason why, when Van tells me about the problems of the Review in setting up type, I know something about it.

Van Vleck:

This was by hand?

Slater:

This was completely by hand. So I could set just about anything by hand in the way of formulas. That I was doing the first couple of years. I spent only three years in college so that, I guess, the last year I perhaps had some kind of scholarship or something, not that I absolutely needed it but my father liked me to have a little bit of earning money. During the college period I took essentially all the physics courses they gave, and also several chemistry courses; this was the war period. We were having war training and so on, and everybody was supposed to be doing something for his country, and they had a chemical warfare service course there, so that you had to go in for one thing or another in the way of useful courses, and that was the closest to physics.

So doing that for a year during that period, I was taking qualitative and quantitative analysis. I could see that I was very much interested in the chemistry as well as in the physics, and I also took most of the courses in mathematics available; having got the start and skipped calculus, I could go on to take the more advanced math courses. So I had about equal amounts of physics, chemistry and mathematics as an undergraduate, and I was just considering which one to go in for. I saw that I was equally interested in all of them, and as I look back and see the reasons why I made a decision, I think I had made the decision to go on to physics for fairly adult reasons.

I figured that I was interested in the mathematical side as well as the practical side. This decided me against chemistry. I was interested in the subject matter of chemistry almost more than the physics. I was interested in how atoms and molecules hold together, I was fascinated with quantitative analysis, for example, because we heard all about inorganic compounds, but I felt that there was more chance of getting the mathematical side in physics than in chemistry. I also felt that the mathematicians were much too abstract for my field of interest, and I wanted to steer clear of those, so I felt that physics was certainly the choice, and I had definitely decided it by the time I was a senior.

Kuhn:

This interests me a good deal, and relates to another question I wanted to ask you. I have a strong impression from a number of people that in this country, very often until one got fairly advanced, the notion of physics was very definitely the notion of experimental physics.

Slater:

Well, it was not with me. I had just as much interest as an undergraduate even, with the terrible teaching they had there and so on, in the theoretical side as in the experimental side. For example, I remember one specific thing that I played with. I remember that when we were having a course in electricity and magnetism and in getting hysteresis curves and magnetization curves. I said to myself, “now can’t I figure out a model with the bar magnets and so on placed together which would reproduce this?” So I started inventing a theory of hysteresis curves. Well, I didn’t know that people had done this for years back. Anyhow, it interested me enough so that I wanted to play with the theoretical side as well as the experimental side.

Then another thing that shows this quite clearly: there was an annual prize that someone had given for the senior who would write the best special thesis or paper on some topic in physics. Since I was the only senior studying physics, I thought it was a fairly safe bet that I could get that if I tried for it. But I decided that I‘d do a real piece of research, so I got Professor lawrence to agree to let me have a little room and all the apparatus lying around that I wanted, and I decided to study the hydrogen spectrum. He said, “well, if you want to do something in spectroscopy, you’d better read up these papers that Mr. Bohr has been writing about atoms. In other words, this professor, even though he was no modern physics type, still knew about the Bohr atom. So I got the Phil. Mag. out and I read Bohr’s papers, and I was perfectly familiar with those.

I did not get as far as correspondence principle as an undergraduate—that I missed. I should have had it because what I was trying to do was to excite the hydrogen spectrum just by a simple arc discharge and then change pressure and change the amount of power I was putting through and so on and see how the relative intensity of the different Balmer lines changed. I rigged up a spectrophotometer out of various pieces that I could find around, and I rigged up a pump so that I could change the pressure and so on. So I was taking the different lines and measuring their intensity with my home-made spectrophotometer as a function of these various quantities. Then I said, “now let’s see what we can do in the way of setting up a theory as to what ought to be the intensities.” The only thing I could figure, not having any thing giving the probability of transition,of course that I’d missed in correspondence principle—was to assume the probability of transition for each jump was the same; but obviously if you have a lot of excited states, and the transitions down from s’s to p’s and so on—.

We weren’t looking at them in that way then, but for any of the transitions coming down from one of them to another, there’d be some transitions that could be carried out in more ways than others, and therefore I figured those might be more intense than the others, and I made a theory. It worked fine: it reduced the results very successfully. At any rate, this was no fancy theory, but it showed that I was interested in theory, and it showed that I had read some of the Bohr atom as a senior. Obviously I got my prize. When I started applying to Harvard I sent that along as one document, and that apparently helped.

Kuhn:

In fact, Kemble remembers it.

Slater:

He remembers it, that’s amusing.

Kuhn:

I don’t know as he remembers anything like as much about it as you just told me, but he does remember that there was a paper that came along with your application. Although I think that this tells me something I very badly need to know, it doesn’t quite answer the questions I meant to be putting. Let me simply contrast your own case with Ted Kemble’s. Having started out at Case, at least insofar as scientific education was concerned, he went all the way through in physics, he was interested in mathematics and therefore was doing some mathematics, he practically had, I would say, to invent mathematical physics for himself. The education he was getting was almost exclusively experimental education. You clearly recognized earlier than he did, and to some extent could work quite naturally into a mathematical physics undergraduate career, and I wonder what characteristics of Rochester or of your own reading may have made that possible?

Slater:

I’m sure no characteristics of Rochester. In other words, the present good qualities of Rochester had not started at that time. No, the one good thing I can say about the physics department at Rochester when I was there was that they had sense enough to leave me alone. In other words, they gave me the run of the place; I could use any apparatus I wanted to, they had not a bad library, I could read what I wanted to and so on. But I got nothing out of the courses there, that is, nothing beyond very elementary stuff. As a matter of fact, I should not be quite so categorical about it, because they had some good second and third year courses in physical measurements and so on, much better than we have now at MIT. I learned how to use potentiometers, cathetometers and a variety of things, so that I really was at home in a laboratory in a way that our present undergraduates don’t get to be at all. In that way I think I had a good training.

Kuhn:

Can you give me some notion, as of the time you finished at Rochester, really what level of work in math and physics and the various parts of physics you had gotten, and perhaps something about any books or other articles like the Bohr articles that you remember having read?

Slater:

I don’t think I had read any other articles. I had read nothing on modern physics; this can’t literally be true, because, of course, I knew something about radioactivity and that sort of thing. I’m trying to think about what texts we used. I know we used a text by Edward L. Nichols of Cornell for this physical measurements course. I know we used elementary texts of Millikan: I know that I had read, I guess, the oldest edition of Millikan’s book on the electron. I know that I had read Michelson’s little book on light waves and their uses: that’s a very nice little book on interference and diffraction and so on. I was quite familiar with that sort of thing. I suspect that was about the extent of it. I had no courses on modern physics.

Kuhn:

Just so far as the standard subject matters are concerned, how far do you suppose you’d gone with mechanics, how far with electricity and magnetism?

Slater:

All of them exceedingly elementary. I had never heard of Hamilton’s equation, Maxwell’s equations, Lagrange’s equations, none of that stuff.

Kuhn:

How about any taste of statistical mechanics, kinetic theory, that sort of thing?

Slater:

I certainly had had none in formal training. I might have read a little bit about that, I think I probably had. Enough to know what the gas theory was about. This, if so, was some outside reading, and I’ve forgotten what book it was in. But I have a vague feeling that I was not unfamiliar with that.

Kuhn:

What about mathematics?

Slater:

In mathematics I knew, as I say, some advanced calculus and some differential equations, and some fairly good trigonometry and simple functions of complex variables and so on. But a good deal of this I’d got by myself. In other words, I found when I went to Harvard that I was way behind what I wanted to catch up to.

Kuhn:

Would you say that you were behind at all as compared with other people coming into the graduate school?

Slater:

I don’t think so. I just realized that there was an awful lot that I could learn.

Kuhn:

Up to the time you came to Harvard, had you gotten any sense of the mess that modern physics was in, the sense of unsolved problems which might even be unsolved in principle as well as in practice that differentiated it from the previous century?

Slater:

I wouldn’t say so. Except, as I say, I tried to play with intensities in the hydrogen spectrum, and I saw there was no rule for finding the intensities in the hydrogen spectrum. In that sense I realize that there were unsolved problems. I think I certainly realized that the whole atomic ideas were just beginning to be developed. I don’t think I needed to be told that there were a lot of things that hadn’t been done.

Kuhn:

I take it that at all periods in the development of physics there have been problems that remained to be solved. There was that particular sense, which at least in Europe by the 1920’s was awful urgent, that not only are these problems not solved but nobody’s got any idea with respect to certain of the problems how the devil you would go about solving them. That is a different situation, I think, from the sense of the existence of puzzles.

Slater:

It really is, and, I think, I was not very conscious of that in Rochester, but I was immediately as soon as I got to Harvard. I was just conscious of the fact that I was interested in these things when I was at Rochester. I realized more from the chemistry than I did from anything else that there must be a great deal of theory behind how these atoms all combined into molecules and so on, which I knew would interest me if I got to find out how it worked.

Kuhn:

When you came to Harvard, was this decision largely dictated still by the fact that you would have gone there as an undergraduate?

Slater:

That’s right. My father came from a family which had no college background. He hunted all over, read all the catalogues and so on. He had to earn his own money because his parents both died when be was in high school, and so he worked, went to nigh school and earned his money. He read the catalogues and decided Harvard was the best university in the country and he went there. So it was always assumed that I would go to Harvard. As I say, it was not practical for an undergraduate, so it was just clear that I would go there for graduate work. Not that it was dictated, but it seemed like the obvious thing to do.

Kuhn:

You yourself did not look at catalogues?

Slater:

I myself did not look at catalogues. I just took this more or less for granted. If I had gone on somewhere else it would certainly have been Chicago. My father had done his graduate work at Chicago, and we were actually living there when I was born. I had read enough about Michelson. and Millikan to know that Chicago had a fine physics department and I didn’t know a thing about the Harvard physics department. Nevertheless I think that I had just rather assumed that I’d go to Harvard.

Kuhn:

In retrospect, are there any other places you might have gone to that might have been as good or better for you as a physicist?

Slater:

I definitely don’t think so. No, I feel that it was a good piece of luck that I went to Harvard.

Van Vleck:

That’s my expression about myself.

Slater:

No, I never had any reason for wishing that I’d gone anywhere else. I think that G.W. Pierce was the one who really had a piece of luck. You know the story of G.W. Pierce when he came to Harvard.

Kuhn:

No, I don’t.

Slater:

G.W. Pierce, you know, was rather older than the normal graduate student when he came to Harvard. He had been down in Texas. The story goes that he was sort of a cowboy down in Texas but he decided that he was interested in physics. He didn’t have any money to go east, so, being in that business, he thought he could go on a cattle train, take care of the cattle, and get free transportation that way. He was going to Chicago to learn all about physics from Michelson and Millikan.

He got on the train, and he found that he got on the wrong train, or maybe there was no train going to Chicago that day, but there was one going to Boston. So he said he believed there was a college called Harvard at Boston, he might as well take that cattle train as the other one. That’s how he came to be in Boston. Did you hear that story, Van?

Van Vleck:

I heard a slight variant, that when he got to St. louis, the train going to Chicago was a sheep train and it was below the dignity of any cattle man to take care of sheep. I think it’s apocryphal, I didn’t rely much on that.

Slater:

No, but apparently there was some element of truth in this. So he got there completely by accident. [Interruption]

Kuhn:

It’s the Harvard experience more than anything else that becomes central for us. I’d like terribly to know, both in personal terms how it hit you to come here and see what was going on in physics, but then simultaneously as much about the people, the curriculum, the things that were exciting and that weren’t so exciting, just as much of this as you can tell.

Slater:

Of course, the first point was that I applied for an assistantship or a fellowship and was given an assistantahip assisting Bridgman. So that’s how I got in with Bridgman. The first year I was their lab assistant. He had a very interesting way of running things with his assistants. He felt that the really difficult problem in his laboratory was making the equipment and making the samples rather than taking the readings. So he would be spending his time in the shop making the gadgets, etc., and I’d take readings with him. So I had experience right from the beginning on his high pressure work.

He was just starting in to work on his compressibility gadget at that time so I was in on the design of that. He was starting in on methods of producing single crystals. He also gave me a couple of jobs which I did on my own essentially, taking readings on my own. One was just the change of resistance of wires of various things with tension. But these weren’t simple wires because they were bismuth and antimony and things like that and if you try extruding a wire out of bismuth and antimony and hang weight on it, you find that it’s one of the most brittle things in the world. So that developed a considerable facility with handling materials of that kind.

Another was an experiment on trying to find the deviations from’ Ohm’s law at very high current densities. He had a scheme for doing that. Well, this gave me a variety of experiments as well as the standard high-pressure things which I got very familiar with. So I was spending part of my time in there, and part of my time taking courses. My first reaction to being around Harvard was that this was a whole new world. I'd been thinking about these atoms as something that I'd read about in the Phil. Mag. and here people draw pictures of them on the board, they’re natural things to think about, you know just how big they are and things like that. In other words, I was very much impressed with how lively the whole field was.

 

Kuhn:

How many people were there at Harvard who were drawing pictures of atoms on the board?

Slater:

Well, I can’t tell on that. What I know is that the way I got to see what was going on was the weekly colloquium, and everybody who talked in the weekly colloquium was drawing pictures of atoms on the board, and the weekly colloquium was an extremely lively place. While you may say that there was only Kemble who was interested in quantum theory then, this was really not at all true because most of the experimentalists were interested in this too. The more I got into the field, as I took Kemble‘s course and read Sommerfeld’s book and so on, the more I realized that here was a real center of things.

One impression that I want to give you very strongly was that in those days Harvard was not on the outskirts looking in, Harvard was one of the places that really were doing modern physics. There you had Lyman who had discovered the Lyman series, one of the things Bohr had used. There you had Duane who had done a lot of the X-ray measurements on the limit of the X-ray spectrum; he’d done the best measurements of h; he had his 100,000 volt battery upstairs in the attic which was an extraordinary thing to see.

He had a number of other interests before I got through Harvard, but those things were going on in those days. There was Saunders who was working With Henry Norris Russell on spectra. Before I got through Harvard, Saunders’ coupling was coming out. There was old Professor Hall who had done his Hall effect and would talk about that every once in a while, and I think everybody realized that this was out of fashion at that time, but would surely come back in again. Anyhow, I had that very definite impression. And a number of other people. Of course, Bridgman was doing his high pressure work which very few people paid any attention to but which everybody realized was quite unique.

So you would read in Sommerfeld about what was going on, you’d see footnotes referring to the people who were sitting in the colloquia. And they were proceeding to talk about atoms as if they knew about them, whether they were theorists or experimentalists. So I had a very definite impression that practically everybody around, except the people in Cruft, were really working on atoms. Of course, G.W. Pierce was also working on atoms. He had a wonderful version of the Thomson atom that he was interested in at that time. He wouldn’t read Bohr but he had what be called the “onion skin” atom. Do you remember the “onion skin” atom, Van?

Van Vleck:

Vaguely.

Slater:

Of course, the Thomson atom will give you one vibrational frequency, but Pierce said be thought he could invent one that could give you every-thing in the Balmer series. He made first a sphere, then he had a shell around that, and another shell around that, and another shell around that, and so on. Inside each one he had the potential arranged so that there was a potential minimum inside each shell. An electron inside any one could oscillate back and forth and he had them all fixed up so that the spectrum was the Balmer spectrum. Nobody took him seriously, but he always was a good show when he gave a colloquium.

Kuhn:

He, I take it, did take this seriously?

Slater:

Oh, he took it seriously, He took everything very seriously with a twinkle in his eye. No matter what he was doing, he was always the same way on everything.

Kuhn:

But would you say that he felt strongly that whether this was the right way to do it or not, that there was no question that something as classical as this was to be the way out?

Slater:

Pierce always did everything with a twinkle in his eye; he never took anything very seriously. So I wouldn’t say whether he believed this, or whether he was just doing it for a joke, I really couldn’t tell. Did you know Pierce?

Kuhn:

No.

Slater:

He was the most amusing man in the department in many ways. Well, there were others around, I don’t remember who-all. Oh, Professor [Philip] Franklin [of M.I.T.] from Tufts or some one of the colleges around, used to come to the colloquia, he was always in the character of the incredulous fellow who can’t understand all this modern stuff. He actually understood it very well, but he’d sit in the front row and ask the dumb kind of questions that people ask just in order to get the speaker to explain really what he's talking about. But they were very striking colloquia. I felt that I was really being “initiated” into modern physics at a great rate.

Kuhn:

Do you remember any particular colloquium that made a particular impression on you or reports on things—?

Slater:

I do remember a few specific things. There were a few invited speakers. I remember that we had H.A. Lorentz giving a colloquium once and talking about the virials. This was the first time I ever heard about the virials; this was an absolutely wonderful colloquium. And a few things like that. But also they had a series of connected things called the Physics Conference. These would be in the evening, we’d have four or five lectures on one topic. I remember that Harvey Davis gave a very good one on thermodynamic potentials. I really learned the thermodynamic potentials from that. I also remember that Julian Lowell Coolidge gave one on “probability” he couldn’t pronounce his “r’ s”. He would appear for each one of these in his black tie and tux, for a lecture in Cruft.

The first time there were 30 people there, the second time there were 3 people there-I was one-the third time, by some mistake, we were locked out, and we had to look for some room where we could get in. We finally found that we could get into the astronomical laboratory and he gave his lecture still. There were two or three of us at that one, and he lectured to us just as formally in his tux, as he lectured to the 30. At that point the Physics Conference died. That was the last time it ever met. So I do remember that most memorably.

Kuhn:

What was the pattern in the colloquium itself? Was it reports on work being done?

Slater:

I would say it’s very similar to what it is now reports on work being done, reports on outside work, just depending on what was the most convenient thing for that particular time.

Kuhn:

There were some quite interesting things that happened in this period. I mean, I think particularly now right in the middle of your graduate training, the Stern-Gerlach effect.

Slater:

I don’t doubt we had a colloquium on that. At least I know I was familiar with it—you have something about this—I don’t remember very specifically just when I heard about it, whether it was in Kemble’s course or whether it was in a colloquium. I surely know that I heard about it while I was at Harvard, and I suspect it was just that Kemble worked it into his course. Kemble kept very well up-to-date on things, and so did the rest of us. I think that we followed the literature much more carefully than one does now. Of course, it was possible because there was a small enough amount of it. But I know that as soon as the Zeitschrift would appear or the Annalen on the shelf in the Jefferson room, we‘d all run for it and read it right away, so that all the papers of the 1920’s I read as fast as they came out.

Kuhn:

What journals, would you say,you expect to have kept up with every issue of as a matter of course?

Slater:

Primarily the Zeitschrift. Then the Annalen, the Phil. Mag., Physical Review, Royal Society, I guess about that number and Nature. Nature was more important then than it is now, and National Academy was more important then than it is now. I’d say that was about the set of journals that were really very active, and we tried to look into them as they came. So I don’t think we missed things very much.

Kuhn:

Van, I asked you a similar question, and you felt probably in your own case you had not followed things quite as regularly and as closely as that.

Van Vleck:

A little later when things were breaking, I read the Zeitschrift fuer Physik.

Slater:

I think I must have been doing this as a graduate student, because, for example, I remember reading Bohr’s paper in the Zeitschrift on the periodic system of the elements before the next issue had come out. In other words, I saw that within a couple of weeks of the time that it came; similarly, most of Bohr’s stuff, and most of Sommerfeld’s stuff, and so on. You asked in here about whether any body was following the Zeeman effect, etc. Well, we were following it in great detail. Of course, we’d all buy each edition of Sommer- feld’s book as it came out, and that was a very good way to keep up on literature. I don’t feel that we were lagging behind at all appreciably in knowing what was going on in those days.

Kuhn:

Stepping back a minute to the actual course work preparations, what subjects did you take and how far did they go, and what books, if you remember them?

Slater:

I don’t remember all the details, but I remember, in the first place, Kemble’s book in quantum theory, and he used Sommerfeld, as I remember, at least we all were reading Sommerfeld.

Kuhn:

Had you all had enough German so that you could do that without the language being a handicap?

Slater:

Oh, I certainly had. I had had enough French and German in college so that I had no trouble with it. And I think most people managed all right; it was just assumed that they would. Of course, we had German and French exams for the Ph.D., which I don’t remember people having any great amount of trouble with. In other words, I think the language preparation was better then than it is now, or than it has been at some intermediate time; I think it’s getting good again.

That was the text we used in quantum theory. Bridgman gave a course in electromagnetic theory using Jeans and Abraham both. Also he gave a great deal of original notes. Bridgman's lectures I had both in electromagnetic theory and in thermodynamics, and, I think, on the whole they were the best courses I had. I think that Bridgman was an extremely good teacher in classical theoretical physics. I don’t think that people realize this. He was very stimulating and had all kinds of thoughts about the ins and outs of things, what they really meant. He had done a great deal of thinking and a great deal of reading about it. Of course, he knew no statistical mechanics. I had statistical mechanics, I guess, from Kemble—somebody did anyway—but I did a good deal of that myself too.

Now on mechanics—I wonder—I had one with [Wm. F.] Osgood, and that’s probably what I had. I don’t believe there was a mechanics course in the physics department. I know that I got hold of Whittaker and I studied Whittaker pretty carefully by myself, but I don’t think that was part of the course, I think I was just interested in it so I went through that pretty thoroughly. Then in optics and light, I guess (Simon) had a course, didn’t he?

Van Vleck:

We both had a course with Kemble.

Slater:

Oh, that’s it, Kemble eectro-mag. theory. That’s what I was thinking of using Jeans and Abraham I guess rather than Bridgman—. No, we had courses in electromagnetic theory both with Bridgman and with Kemble. Kemble was more specifically electromagnetic theory of light— that was it—which was done out of notes.

The general situation with the courses then was that they were not following any text very carefully, they were done from notes, and I thonght the notes were a lot better than the texts were. In other words, I got a very definite impression there that the available texts were not very good. And I think this is one reason why I've gone in for book writing as much as I have. I just felt that there was an enormous need for more up-to-date better books than we had.

Kuhn:

When you say “done from notes”, does this mean that the notes were mimeographed or something of the sort?

Slater:

No, no, nothing of the sort. lecture notes from class, and I‘ve still got some of these, I took them in quite a lot of detail so that I could almost reproduce some of these lectures. No, I did this with all of my courses. Everybody else did: it was the standard thing to do. Well I ‘m sure that there were other courses that we had.

There were some courses just in more elementary heat, I guess, as well as thermodynamics. Maybe I ‘m thinking of a course in college, I know I had a course at some time in using the very elementary stuff with heat. And I took a lot of math courses. I had courses with Osgood and Birkhoff and Graustein and so on.

Kuhn:

How far did that take you in math?

Slater:

That did a pretty good job on function theory. I had a course with Graustein in differential geometry. I had a course with—who was this man who was a visitor for awhile?- -some other face—I know his name very well but I can’t think of it at the moment—on methods of mathematical physics or something like that. The Fourier series and harmonics, etc. Kellog, yes [O.D.] Keilog’s course. Several courses with Birkhoff.

Van Vleck:

Relativity?

Slater:

I had relativity with someone, I’d thought it was Bridgman; Bridgman gave a course in relativity. It was very good, thorough training in special relativity and not very much general; but relativity theory of electromagnetic transformations and all that stuff, as well as the mechanics. No, I’d forgotten that, that was a good course. Bridgman always gave out assignments of writing a term paper in his courses instead of having an examination, and I remember doing quite an elaborate job in my relativity course for a term paper.

I tried to demonstrate that all the transformations really would work with real clocks and real meter sticks and so on, on the basis of setting up interatomic forces, etc. which were relativisticaily invariant. In other words, if your forces between atoms really obeyed relativity then automatically when you set a thing in motion, it will shrink according to the Lorentz contraction. It was more complicated to show that automatically clocks would set themselves because you had to move the clocks around.

Therefore you had to accelerate them in the process, therefore you had to put in the general relativistic correction on that. I convinced myself that this would automatically set the clocks properly too, and so on. I did quite an elaborate job. I don’t think Bridnan ever believed it, but I did, and I‘m not sure it wasn’t right. In other words, that the Lorentz transformation was not just a formal thing but represented what would happen to real objects, real clocks and real meter-sticks if you tried to use them for measuring. So those are some of the courses that I took.

Kuhn:

Would you say you took rather more mathematics than most of the people took?

Slater:

Well, more than some of them. There were a number of others. Of course, Van was taking just as much as I was.

Van Vleck:

I don’t think I took much.

Slater:

Is that so? Maybe I was taking more than most of them. But there were an awful lot of gaps in the mathematics. In other words, I feel that my mathematical training was much more inadequate than my physics training was in many respects. Of course, I didn’t learn group theory. I was saying to Van the other day I don’t think there was a course in group theory.

Kuhn:

No, I don’t think anybody learned group theory except for some very abstract mathematicians until Wigner came out. Now that’s something to come back to.

Slater:

In other words, there were lots of mathematics that were taught to European students that we didn’t get.

Kuhn:

But I don’t think the European students—?

Slater:

Any of them at that period—?

Kuhn:

I don’t mean that I don’t think any of them did, but, by and large, the people doing physics knew nothing about group theory.

Slater:

No, that’s probably true.

Kuhn:

I know from people I’ve talked to that Wigner’s paper came as a total shock to them, and a rather unpalatable one.

Slater:

I didn’t too much like the teaching of the math department. I felt they were too abstract, too much interested in rigor rather than understanding what it was all about. I never was really fond of Birkhoff and his methods and so on.

Kuhn:

Well, he was just a plain bad teacher, wasn‘t he? He certainly was by the time I had him.

Slater:

Well, he was that. But I would have made allowances for that if I thought he really understood it, then I would have been interested in the mathematics that I wanted to learn. I wasn’t even convinced of that, though this is saying it a little too hard, because Birkhoff was a good mathematician. But anyway I did not feel that I was getting the mathematical training I should have, and perhaps to try to make up for it, I took about as much as I possibly could because I knew I wanted it.

I didn’t take any chemistry as a graduate student. I probably would have done well to have taken some. I got a little contact with the chemical people, because [Theodore William] Richards had a student who was also doing compressibility-you see I did my thesis on compressibility of the alkali halides. Richards had a student doing that, so I saw a little of them. But I really had very little contact with chemistry. One big gap is that I never studied organic chemistry either as an undergraduate or a graduate student. I wish I’d had it. But that gives a general idea of what I had as a graduate student. Of course, as a graduate student, I did a good deal of outside reading.

I was reading quite a variety of things, including literature, etc. So I was pretty well up on the history of quantum theory and what had gone on in all the various branches of physics since 1900 by the time I finished. My graduate work, even though I don’t think I was required to do this for any of the courses or examinations or anything.

Kuhn:

Do you remember in addition to reading in the literature any books that you read that were of particular interest or use?

Slater:

No, I really don’t. I had relatively few texts, and probably the few texts that I used are still here [in the office]. Here’s Abraham, Byerly’s Fourier Series I found a very useful thing. I think Kellog used that in his course. I did some reading in the history of science. I did not have Dampier in those days. I read some in these little monographs—Ostwald’s Klassiker and some other similar things. I’m trying to remember which texts I had as a graduate student and which ones I got afterwards.

Jeans, of course, both Gases and Magnetism I knew as a graduate student. Here’s (this old crystal theory) that I had in some courses. I believe (Robert Stuart Mannings) I might have used some then, I’m not sure of it, I think later Sommerfeld in all the various editions, I knew a lot of Sommerfeld’s. But I don’t think very many others. There’s one physical optics-I had that way back, and I still have that. But I think most of the rest of these things are from a later period. In other words, when I came back from my year abroad and started as an instructor, as assistant professor, I was teaching a variety of courses. Then I did a lot more.

Kuhn:

I look here at my outline and notice this question on the spirit of physics; you really talked about this one very well already. You’ve got a very real sense of very rapid progress on fundamental questions on all of this.

Slater:

I very definitely did. And when we get to my experiences at Copenhagen and Cambridge, I will tell you that I found them no more up-to-date and no more lively than Harvard was. I felt I was just as much at the center of things at Harvard as I was either at Cambridge or at Copenhagen, absolutely. Well, I felt just as much so as I did at Cambridge, and rather more up-to-date than at Copenhagen. I thought that Harvard was more lively than Copenhagen.

Kuhn:

I want to postpone Copenhagen a little. This evidence I’m sure is right, but I may say that it is not, on the whole, I think, the impression I’ve had either from Van or from Ted Kemble, and one thing that occurs to me it does sound to me as though you were doing a good deal more reading by yourself than other people were.

Slater:

I very likely was. So what I say may be that I created a spirit of being alive there for myself, rather than getting it from the outside.

Kuhn:

I don’t mean to say that I get the impression from other people that it was dead, or anything of this sort. There was obviously a lot of very good and fundamental work going on, but I don’t get the impression that people in general were really so up-to-date, without saying that they were laggingly bad as this, and I wonder to what extent that may have been your own individual experience?

Slater:

It may have been. It’s hard to say. I certainly didn’t find, that I was talking these things over with very many other people around the place.

Van Vleck:

Well, that could be. After all, Kemble said You and I and Breit were here, (???) nucleus.

Slater:

Yes. We didn’t talk physics very much as I remember, Van, you and I, did we?

Van Vleck:

Oh, I think we did some.

Slater:

Some. But I’m sure that we would have done more than any other combination because we lived right across the hall from each other in Conant Hall, and yet I’m sure a very great many of the things I was reading I just didn’t talk to anybody about. Oh, I may have talked to Kemble about them some. I certainly did talk to him quite a bit. But, of course, I didn’t take my thesis with Kemble as Van did, so though I saw him a good deal, I wasn’t in as close contact as Van was. And Bridgman didn’t follow these things very much.

Kuhn:

How did you happen to do your thesis with Bridgman?

Slater:

Oh well, of course, I’d been assisting Bridgman for the first two years. The third year I had to look around and decide-I guess I had started my thesis even before the third year-whom to work with, and I just convinced myself by seeing him in action that he was the best person in the department, and that I’d rather work with him. I was very much impressed with him as a person, general qualities and so on, and I’d learned the techniques, so I thought this was the very obvious thing to do.

Kuhn:

Did you think of yourself at that time as very probably going on in that direction of experimental work, or were you—?

Slater:

I was on the fence. I could have been interested in either one or both. During the year when I was finishing up my thesis, as a matter of fact, Bridgman made the suggestion; he said, "why don’t you go in for theoretical physics?" He had seen that I was doing a lot more theoretical physics than most of the graduate students were doing. The way I tell the story was that the suggestion was made the day after I broke a particularly large piece of equipment. I’m sure this was not on Bridgman’s mind, but it makes a nice story.

I didn’t break an abnormal amount in the lab, and actually I knew lab. work perfectly well. I was doing laboratory work all those 3 years, and I’ve done it since at times. But I realized that I could do the theoretical things better than most of the people around, whereas in the experimental side, I realized there were plenty of others who could do it too. So that I decided then that he was probably right, that I’d better concentrate on theoretical things.

Kuhn:

This was really when your thesis was very nearly finished?

Slater:

It was during that third year.

Kuhn:

Was the three-year-degree a pretty standard one? Now Van, you and Ted Kemble both went through a little more quickly than that, which is just astounding when one thinks of the way it is now?

Slater:

Yes, I ‘m amazed at that. But I remember Van was awful speedy.

Van Vleck:

Well, you did it in 2 years when you figure that you were a teaching assistant.

Slater:

That’s right. You see, I was really assisting half-time. This was not a sinecure, I was spending half of each day really taking readings for Bridgman, so it really was equivalent to 2 years.

Kuhn:

None of this was to say, “weren’t you an awful slow-poke?” What I was concerned with is the rather fast rate at which people, particularly considering that most of them had had remarkably little as undergraduates, did go through.

Slater:

No, I think people went through fast. And don’t forget that even though I took three years, I was 19 when I got my doctor’s [Bachelor’s] degree, so I don‘t think that my parents would have stood for my trying to do it any quicker; they thought I ought to hang around longer and delay getting my degree.

Van Vleck:

There was less to learn than there is today certainly.

Slater:

There was less to learn, but still there was plenty. There were a lot of things to learn then that one doesn’t worry about now. Another book that I forgot to point to there was a Mr. Woldemar Voigt’s Lehrbuch der Krystallographie or whatever it was and all that stuff. I bought that in a spirit of ambition when I knew I was going to work with Bridgman on crystals, thinking that I’d learn all about the crystaline properties, thermoelectricity and all that stuff. I should have learned it but I never did. I’ve had it on my shelf ever since. But that was an illustration of the kind of thing that one would try to learn then more than one does now.

Van Vleck:

Did you take Bridgman’s course in elasticity?

Slater:

Yes. I had a course in elasticity, in hydrodynamics, and in various things which I forgot to list. So there were a lot of things then that I learned that one just does not learn very much now. A great deal more optics, crystal optics, and that kind of thing.

Kuhn:

When you undertook this thesis, did you initially conceive of it largely to get the experimental values?

Slater:

Oh no, the object was just what it turned out to be, to try to tie the experimental side in with the theory. You see, Bridgman had just worked up his technique for doing compressibility. I was naturally reading the old literature of Born and von Karman and so on, Lande etc. on the lattice theory of elasticity and observed that they quoted results on about three crystals, the only ones in which compressibility had been measured. And it just seemed to me, “well, here’s a thesis just waiting to be done to make as many alkali halide crystals as one can, and do the compressibilities, and then see how they fitted in with the existing theory.” And I realized from the beginning that with the high pressures you had you could get change in compressibility with pressure and get more information then Born and Iande had been able to use.

Kuhn:

You also tied in, I think in the published article about the thesis, to the Bohr theory of the periodic table. Was that also in your mind at the start to design this so that you could go through a period and watch it?

Slater:

Sure, obviously. The whole set of alkali halides fit together and you see what the periods are doing. I thought this was the one most obvious example where you could get some tying in of the properties of solids with the periodic table.

Kuhn:

What sort of a role in this discussion of models did the Langmuir work as against the Bohr, Born, Lande plan?

Slater:

We all felt that Langmuir had got something, which was surely not right, but which was very suggestive. I never believed this Lande business of things going around the corners of a cube, I thought this was silly. I thought that Langmuir’s cubes were silly, but I thought that the shell of 8 obviously made sense. Of course, we didn’t know at the time where it came from, but as soon as Bohr’s periodic table came out, we saw that in a general way that they tied in together. And I think we realized that there must be some three-dimensional picture which Bohr had not got at that period, which would give a slight suggestion of the cubical things that Lewis and Langmuir were playing With. In other words, I don’t think anybody took Lewis and Langmuir seriously except as suggestive things of the way the theory might develop.

Kuhn:

Was there any resistance in this period to this sort of model building at all? It wouldn’t have been so very long before this that people would have thrown up their hands and said that that wasn’t physics.

Slater:

No, I don’t think there was then. I think that that kind of prejudice and the kind of prejudice that Ostwald had and wondered if atoms existed and so on had completely gone by then, at least around Harvard. Everybody was making models and, as I say, drawing pictures on the board of what the atoms were like and so on, so that everybody was ready for it.

Kuhn:

One terribly minor thing that I’d asked in the outline simply about the publication of your thesis. The experimental part comes out really several years after the initial article.

Slater:

This is just the very obvious thing that a graduate doesn’t like to take time off to get his stuff in shape to publish. In other words, I was so interested in getting on with other things, that I delayed on this. It’s conceivable that there were one or two bits of calculation I wanted to finish up or something just to have it ready, and that’s a slow journal to publish in, so that there was no fundamental reason whatever about that. I essentially had that material ready at the same time I had the Phys. Rev. paper ready, but it just got pushed back, it did not get speeded up.

Kuhn:

I think we’re very shortly going to get you into traveling fellowships, Cambridge and Copenhagen, but before doing that, let me simply fish once more in this general area of particularly exciting developments that occurred in this period. I know one thing I'm quite sure you will be able to help with, and maybe we should go straight to it. Let me ask simply first, whether you remember any particular things coming out in this period that were particularly closely followed or that were sources of controversy, remarks in seminars, debates? Obviously the thing I lead to immediately that was a big issue at Harvard, is the Compton effect, but before we get to that—

Slater:

That was somewhat later, though. That was in 1921., I think, wasn’t it?

Kuhn:

1923, I think really. I think that would have been before you took off.

Slater:

Perhaps, could have been.

Van Vleck:

I remember your telling me, “The Compton effect is so.”

Kuhn:

That I think starts quite early in ‘23.

Slater:

Yes, I guess you’re right. But I don’t think there was any particular argument about it then, I think it was later that Duane got going on it and so on. That was probably after I came back from my year abroad, is when my recollection is on that business.

Kuhn:

I think you’re wrong; I think it would have been before. I’d particularly like to pin it down, because, somehow or other, I feel the Compton effect, this strong evidence on the photon in particular, is very likely to have had some relation to your own work in Europe.

Slater:

I think you’re right, I’m just trying to think of the Duane controversy.

Kuhn:

I think that that was quite soon.

Slater:

That was in that same year.

Kuhn:

Yes. I could be wrong here.

Slater:

I remember the controversy pretty well, but I just didn’t place it as exactly when it was. As far as that controversy is concerned with Duane, I think we all said “Duane is a nice man, he’s done many fine things, we will just keep quiet.” I never believed that Duane was right, I’d always assumed that, of course, Compton was right, and I think the most of us assumed the same thing. I don’t know. As I say, we didn’t talk about it very much, because of politeness to Duane.

Kuhn:

Van tells me that he has the decided recollection, though no background to go with it, that the issue was somewhat open, that you came back from somewhere one night and said to him “Compton’s right.”

Slater:

I don’t remember this at all, and I don’t remember that I had any serious question about it. In other words, I just rather assumed from the beginning that Compton was right.

Kuhn:

What had been previous attitudes toward the photon? In much of Europe the photon wasn’t taken a bit seriously.

Slater:

I think it was not taken too seriously at Harvard either. Until the Compton effect, I don’t think I took it very seriously. Not that there was any very good reason for this, I think we just didn’t think too much about it.

Van Vleck:

How did you come to believe in the Compton effect without experimental evidence, or did you just believe that Compton’s experimental evidence was good evidence?

Slater:

I thought it was good evidence. I gather that later it turned out it was. No, I wasn’t critical enough about the experiments to realize that there could be real questions about it.

Kuhn:

Still, this was an area in which Duane had also done first-rate work.

Slater:

That’s right. But I think I took the point of view, and I think I would again, that is, between a man who’s found a new effect and another man of an older period who says that he can’t see it, I'm much more inclined to believe the man who does see it than the man who doesn’t. I know it’s easy enough to miss something. I never had any expectation that Duane would turn out to be right.

Kuhn:

Tell me then how this opportunity to go to Europe arose, and how you felt about this.

Slater:

As far as that’s concerned, I simply saw that most of these things were going on in Europe, I realized that it had been rather the thing to go to Europe for a while, not just in the last few years, but way back, before 1900. I thought that this was an interesting thing to do, I had been in Europe, I had seen the sights before, but-

Kuhn:

Had you done that often before?

Slater:

No. Once, when I was in grade school, my family went over and we spent the summer traveling around, so I knew what it looked like before the First World War. But I felt that a year there would be a good thing. Harvard had these Sheldon traveling fellowships, so I applied for one and felt that by golly, I‘d go. As for the question where to go, I think I felt rather inclined toward Bohr because I liked his papers. I liked the way in which he would go straight to the physical side of things instead of wrapping it up in a great deal of mathematics. I felt that he must understand.

The physics of it quite well. I noticed that very few Americans had gone to Bohr, and I thought that this would be a good chance. So actually I applied, I think, not only for the Harvard fellowship but also for a Scandinavian-American fellowship or something—I’d have to look up records to find out exactly how this was. I seem to remember that it was a case of at first not having anything, and then suddenly getting two and then having to decide which one to take, but, I think, I took the Harvard fellowship, as a matter of fact.

I was planning to spend a year in Copenhagen, and then after I got the fellowship, it was all arranged, I learned that Bohr was going to spend the first half year in the United States. That’s the reason why I went to Cambridge. So I couldn't go to Copenhagen immediately. As for another place, I wasn’t particularly looking for science. I knew, of course, that Rutherford and various other people were there, that there were interesting things that had been done in Cambridge, but (I’d been to Cambridge, I liked the look of it), I thought it would be pleasant to spend a couple of months in Cambridge and to see what the Cavendish was like. I’d read about the Cavendish and its history, etc.

Kuhn:

Did you consider Munich or Goettingen at all?

Slater:

I didn’t consider Germany at all. I just never did. I don’t quite know why, because we were pretty much brought up on Sommerfeld’s book, and plenty of other Americans were going to Munich, but not from Harvard. Apparently there was no very direct connection, so that there was nothing that suggested this to me directly. So that was the explanation as to why I went where I did when I did. Coming back to your question about interesting developments during the three years I was a graduate student, the Compton effect I didn’t place right, but it was one.

I think we were all following, that is the ones of us who were in Kemble’s course and so on, very closely, the development of the inner quantum number and that kind of thing, all the work on the development of atomic spectra, and also molecular spectra. We tried to understand all this, and to see how it held together. And also, I think, it would be worthwhile staying back in the period when I was a graduate student a little bit more—

Kuhn:

I’d be delighted. I would always rather stay back if—

Slater:

You asked in here whether I did or did not realize that there were things that had to be done to physics and so on and how I came to be interested in going in for such things as quantum theory when I’d been doing compressibilities, etc. Well, these all hung together very straightforwardly. I felt that it was obvious that we did not have a theory which could explain the alkali halides, or any other kind of solid state problem. You asked why I didn’t like Mr. Lande's cubes and the cubical model.

My straightforward objection to these static cubes was so simple that I was ashamed to say it, namely Earnshaw’s theorem in electrostatics that you can’t keep a rigid collection of charges at equilibrium, you turn this in another direction and instead of repelling they would have attracted, and I thought that it was so absurd of Lande not to have realized this that I wouldn’t even write a paper mentioning it. I thought it was an insult to Mr. Lande to tell him this.

But it was obvious that you couldn’t have anything in the form of a static atom. It was obvious that Bohr’s atoms didn’t give anything in the way of inter-atomic forces. Van and Kemble were trying to make models of the hydrogen molecules and they didn’t work, and it was clear that they weren’t going to. It was clear that the whole of quantum mechanics was based on conditionally or multiply periodic systems which were a very special case in that you couldn’t expect this to hold in general.

So I just had a complete conviction that the Bohr theory was a very temporary stage, that we were going to get something else before long. Furthermore, I had a very clear conviction that what we were going to get was going to give us the inter-atomic forces as well as the explanation of single atoms and of radiation.

Kuhn:

Can you date at all the growth of that conviction?

Slater:

Oh, I think I was ceratinly convinced of this the whole last year I was a graduate student. And I was also convinced that the same development, whatever it might be that was going to tell you how to do radiation properly, was also going to tell you how to do inter-atomic forces properly. In other words, I obviously did not foresee wave mechanics, but I foresaw something like wave mechanics that would tie together what had been done in Bohr theory and what was being done in the work in the quantum numbers inside the atom, and all these things like inter-atomic forces that hadn’t been touched. I was perfectly sure then that I wanted to continue with the study of solids and matter in general. I thought this was the most direct route to get to it.

Kuhn:

Do you remember discussing this conviction with anybody around Harvard, that is, before you went to Europe?

Slater:

I don't think so. No, I don’t think there was anybody who was particularly interested in this kind of point of view.

Kuhn:

I have the feeling, I‘m not sure about Van, though when I asked him this same train of questions, he thought he was concerned with solving particular problems and that this sort of larger point of view about the state of the field passed him by. I think Ted Kemble didn’t have it.

Slater:

I don’t think he had. With respect to my various friends and colleagues I didn’t find anybody particularly to talk with about those things.

Kuhn:

This issue is particularly the growth of this sense that something fundamental as against solution of problems with existing techniques has to be found.

Slater:

For example, I was perfectly convinced that something fundamental was missing when Bohr gave his theory of the periodic system, because this was based on electrons which were plowing into the inner shells and then coming out again, and the whole current theory of a single electron problem worked all right if it were in a force field, a central field; but obviously it would not work with the interelectronic interactions that you had there. It was clear that they had to act on each other in some sort of an averaged way. I was sure that this was going to come and I just wanted to be in on it when it did. So I actually was working pretty hard, even during that last year as a graduate student, trying to think what could be the directions in which quantum theory could change.

Kuhn:

Do you remember any particular thoughts that occurred to you as to directions then?

Slater:

I think that the most straightforward one was the one that I went to Copenhagen with, namely, the relation between the length of the finite wave train and the breadth of the spectrum. This was obviously something that we'd got in courses, we had courses where we’d done Fourier analysis and so on, so I was perfectly familiar with this, and I just said all this business about an instantaneous transition from one state to another is perfectly silly. That nobody in his right mind will throw away the whole relation between breadth of spectrum lines and the finite wave train, as Bohr has thrown it away, and that you had to have some long period in which radiation was being emitted in order to (get lengths) relating to the breadth of the spectrum lines, and that the only long period you had was while you were in a stationary state. In other words, I had that idea in my mind quite definitely before I went away from Harvard. And I felt that this was a fruitful direction to work in, something where there were paradoxes, where it was obvious that people were not thinking along those lines, and where I thought I could introduce something new.

Kuhn:

Did this tie in at all for you With the problem of interelectron forces in the complex atom, which is clearly another area?

Slater:

Not directly, but I was sure that whatever would really explain radiation would also explain the interatomic forces. That is, I knew enongh straight electromagnetic theory, classical theory, to know that there's no clear distinction between the radiation field and the field close up to an atom or to a radiating electron. There’s just different parts of the same field. The same field that gives you your static effects also is giving you your radiation. Therefore, I felt that the radiation was perhaps a place that one could attack the problem, where you could make some progress, but I was sure that whatever was going to do this would also give us something on the inter-atomic forces.

And I was trying to clear up—as you point out, I was quite convinced of photons by then, the Compton effect had come out; I’m sorry I was mixed up on that chronology there for a minute, because I was trying to tie in the photons with the radiation field, and trying to work toward a straight statistical connection between them, in other words, a radiation field to guide the photons. I felt that this would be a thing where one could make some progress and I said, "well, we have this, we’re going to have diffraction effects, and we’re going to have these finite width of spectrum line effects. The radiation field has to be emitted for a finite length of time in order to give the sharp spectrum line, and therefore, it must have been emitted during the stationary state." So that I went to Cambridge as well as to Copenhagen with that idea.

Kuhn:

Tell me, do you have any notion where the concern with the breadth of the 320 spectrum line itself came from as a fundamental puzzle for you?

Slater:

I don’t think I remember in detail. It just seemed to me as soon as I knew the mathematics behind it that this was such an obvious breakdown the way things were being looked at. I saw plenty of other things that looked equally ridiculous to me, but they didn’t seem like anything that you could put your finger on quite so well and might hope to make some progress on. I was trying to look for many different aspects of the theory that didn’t make sense. Of course, all the business of the inner quantum number and so on with the Atom Rumpf etc., didn’t make sense either, and that didn’t start making sense until the spinning electron came along. But that was a different thing and that was later.

Kuhn:

I’m interested in your saying this, because although it’s perfectly clear that the Rumpf model didn’t solve all the problems there was a period of a couple of years in which it seemed to be solving an awful lot of things.

Slater:

It was solving an awful lot of things, I felt it was solving things, but it wasn’t making sense. What were all those quantum numbers doing there?

Kuhn:

That I don’t understand. What do you mean?

Slater:

Well, an inner quantum number was a vector sum of the azimuthal or orbital quantum number and something else. This made no sense to me to think that that came from the inner part of the atom.

Kuhn:

Why?

Slater:

If it did, then a single closed shell ought to have an angular momentum momentum and spin, (and there was no) evidence that it did. I don’t remember all the arguments I went through, but I just wouldn’t swallow that one, it didn’t seem to make sense.

Kuhn:

Later, in a very important paper we will come to, you point out certain particulars, but this is really after the Pauli exclusion principle. By this time a good many questions had been raised. But you found this a pretty implausible notion from the start?

Slater:

I felt it pretty implausible from the start, as far as I can remember it. In other words, the whole thing seemed to me like a lot of formalism which I thought was exceedingly useful. It was obviously tying things together, but I did not think that Sommerfeld had gotten to the bottom of it. So this was just another place where I was convinced that more was to come. Then I was also convinced that more was to come in the direction of the correspondence principle. Here was a principle that gave you some approximations, but didn’t give you an exact way to calculate intensities. In other words, there were four or five different aspects of quantum theory at that period, all of which made me completely convinced that we were on our way to something better.

Kuhn:

Let me ask you one question in connection with the correspondence principle that will inevitably take us a little ahead. I had the feeling, looking at some of the literature at this period in the United States, that at least as compared with both Goettingen and Copenhagen, not Munich, the correspondence principle here in America was less central to people's thinking about quantum mechanics.

Slater:

I think that probably is true.

Van Vleck:

We took Kramers’ papers pretty seriously.

Slater:

Oh, we took them pretty seriously. We studied the correspondence principle all right, but I think that if you’d asked me then, and you asked me now, what the most fundamental thing, I would have said the quantum conditions in giving the energy levels as being more central than the correspondence principle.

Kuhn:

But remember by 1923 at least, the correspondence principle in both Goettingen and Copenhagen, people talking about extensions of the correspondence principle, and about something much more general than the frequency principle and the intensity theorem or whatever, and there was obviously in some sense a mystique on this score, often a rather fruitful one. Was there any such larger sense of the asymptotic convergence of classical and quantum to be used as a guiding brpothesis in all sorts of ways?

Slater:

Well, I can’t tell you what the general thinking was. But I say, I didn’t talk very much to people. I Imow what my thinking was on the subject. My thinking in general was that I do not like "mystiques”; I like to be definite. I felt the correspondence principle was a good suggestion which would probably lead to a more definite theory that would come later, that I would rather have looked at it from that point of view rather than build on more and more indefinite vague things as the people who were interested in the correspondence were intending to do. In other words, I always looked for an exact thing with which the correspondence principle would be an approximation.

Kuhn:

Now is there anything more you’d like to say about this period really before you get to—.

Slater:

No.

Kuhn:

I would of course be interested in any particular episodes that may come to you, and clearly we are not going to finish today, which delights me, because if all this had to be done today it means it wasn’t a good interview. So things may occur to you as you go along, so I hope you’ll jot them down as you get them next time.

Slater:

I don’t believe there’s anything.

Kuhn:

Right. Well then, what about Cambridge?

Slater:

I was quite interested in being in Cambridge a little while. Of course, I didn’t get the real student life, because being there a short time I had to stay at a boarding house and so on, but I saw what it was like.

Kuhn:

Let me check, when did you get there? How long were you there?

Slater:

I was there 3 or 4 months. I spent the summer traveling and got there September or October, so I spent the term until Christmas. Just before Christmas I went to Copenhagen. So I was there essentially October, November, December. I took several courses, I went to colloquiums. [R.H.] Fowler sort of took me under his wing and I talked with him and so on. I saw what the general life of the place was like.

I formed a definite impression, as I said, that while it was a very lively and interesting place, it was no better than Harvard. The colloquium was quite similar. Of course, it was obviously a fine thing to have Rutherford and Aston and J .J. Thomson and so on around, but it was also a very fine thing to have Saunders and Duane around. It was very similar, and the talks were very similar.

Kuhn:

This was the Cavendish colloquium?

Slater:

This was the Cavendish colloquium. The people were very friendly. They invited me quite a bit. I got to know almost all these people in a personal way. I went to a couple of courses which seemed to me very much on the level of the Harvard courses given in very similar ways in spite of all the talk about how completely different the method of instruction is. One that interested me very much was run by a man named [E.] Cunningham on electromagnetic theory. He had written a book which I had read carefully.

He was thinking rather out of the ordinary electromagnetic theory, and it happened that what be was thinking was tying in with what I was interested in, because I was trying my best to tie together the photons and the waves; and obviously the first thing that you try when you want to do this is to see if you can’t define, from the electromagnetic wave, a vector, which would be the vector velocity of the photon. Cunningham in his book actually had a vector which had some of those properties. I played around with his vector quite a good deal to see if it would work, and it would not.

People since then have all discovered the fact that you cannot define a quantity that transforms like a vector out of Poynting’s vector and the energy density. Well, I found this too, and so I gave this up and decided that you could not set up a real vector field that was guiding the photons. But that’s what I was playing with at that time. I wanted to see how definite one could get in tying together the fields and the photons. So it just happened that what he was talking about gave me some electromagnetic theory that was rather different from what I‘d had at Harvard, and tied in with this.

Kuhn:

Did you talk with him about your own idea?

Slater:

Not particularly, no. Then I had the elementary atomic structure course just to get it. Rutherford was giving it, and I wanted to see how Rutherford would give it, and he was giving it, all the experimental discoveries and things like that; I was interested to see him in action. And I talked over my ideas on breadth of energy spectra, etc. with Fowler, and he was perfectly polite, and made absolutely no contributions.

I just got nothing whatever out of Fowler. I thanked Fowler kindly in a later paper because I felt I ought to thank Fowler whenever I had a chance to do it, but I got nothing scientific from the fellow. He was a nice man. On one occasion I lost a filling out of a tooth and I asked him where to find a dentist, and his cousin was a dentist, and I got referred to him, so he was very useful to me. I always liked Fowler, but he was not interested in the kind of thinking I was doing.

Van Vleck:

Darwin even less so, I suppose.

Slater:

I‘m not sure Darwin was around; I think Darwin wasn’t there just then. No, Darwin actually was much interested in these things. You see, Darwin was writing papers on radiation theory and so on about the time that I got to doing it after I was in Copenhagen. In fact, Darwin and I had some slight arguments about some points on this physics, so that I did talk some with Darwin later, but not then.

Van Vleck:

Fowler was doing his statistical mechanics and so on.

Slater:

Fowler was doing statistical mechanics. No, I didn’t blame Fowler at all. He was doing his job nicely and so on. Anyway, I didn’t find many people to talk with.

Kuhn:

Did you find anybody in Cambridge at the time you were there who was really doing quantum theoretical physics?

Slater:

Not anybody whom I could talk with. Dirac pointed out to me many years later that he’d seen me on the other side of the room in Cunningham’s course. He was taking it at the same time I was, but we didn’t meet each other. I didn’t really get to know the students.

Kuhn:

Were there any other Americans there when you were there?

Slater:

There could have been, but I don’t remember. As I say, I was not living in one of the colleges so that I just got on the bicycle, rode home after class and studied and worked in my room, so I didn’t get a chance to meet people very much.

Kuhn:

By the time you left Cambridge and went on to Copenhagen, how developed was the idea as it comes out not very much later than this in your Nature note, then in the Bohr-Kramers-Slater paper?

Slater:

The idea was perfectly developed and it was not what came out in the Nature note and in the paper. What I wanted to do was perfectly straightforwardly to have a wave, to have photons. And by then I decided you could have only a statistical connection between them because you couldn’t set up a vector to represent the velocity of the photon, that you would have to have the intensity of the wave governing the probability of finding the photon there; and I wanted to have the wave emitted during the stationary state so as to get it emitted over a long enough period so that it would have a suitable spectral distribution.

Kuhn:

You were also perfectly clear that you had to violate energy conservation at the microscopic level and make it statistical.

Slater:

I was all in favor of letting the energy be the energy of the photons, saying that the wave did not carry energy, that the energy was conserved and it was conserved because photons carried it when they went from one — when you ejected a photon from an atom, it was conservation. If you had slightly different frequencies at different times the atom would come off with slightly different energies and therefore the breadth of its energy band and the photons would come out slightly differently. No, I was in favor of exact conservation. Now we will come to the point which is that those papers were dictated by Bohr and Kramers very much against my wishes. I fought with them so seriously that I‘ve never had any respect for those people since. I had a horrible time in Copenhagen.

Kuhn:

Will you tell me more about how that developed from the beginning?

Slater:

I went there. Bohr was very nice, he invited me to Christmas dinner, I told him about my ideas, he felt these were fine, “But, you see, they’re much too definite." Now we cannot have this exact conservation. We must not think too specifically about the photons. We don’t have photons like that.” In other words, he wanted to make the whole thing just as vague as he could. Kramers was always Bohr‘s “yes-man” and wanted to do exactly the same thing. He said “This is a fine idea, if we will modify it in such and such ways.” That was the last I saw of it.

Bohr and Kramers wrote the paper, they invited me to sign it, the letter to Nature was the first paragraph out of the paper, they invited me to sign it, take it or leave it. This was my experience with Mr. Bohr and Mr. Kramers. Since then, it has developed in a very interesting way, namely, that I was right and they were wrong. They didn’t realize this until Mr. Bothe came along with his experiment showing that the photons were really there. So I completely failed to make connection with Bohr. I could have made connections with Kramers if it hadn’t been for Bohr, but Kramers was completely playing Bohr’s game.

Kuhn:

I realize this is a sensitive subject; I’d like to get all of this development just as circumstantially and in as much detail as we can. I have a feeling, but not a lot of evidence for it, that until perhaps the Compton effect—I’m not sure that the Compton effect single-handedly did it or what—that in the Copenhagen tradition there had been very little use for the photon. And it also seems to me likely that ‘by the time you got there it was being taken seriously, otherwise, if you’ll excuse me, I don’t think Bohr and Kramers would have bought any (stock) of this idea as quickly as they did. Even before we get into the reaction, anything you can give me that may be helpful on this whole question of the attitude towards the photon itself, in Copenhagen—

Slater:

Now let’s see, do we see any photons in the Bohr-Kramers-Slater paper?

Kuhn:

Let me put it this way, in a very important sense, we don’t, at least we don’t see them in any very literal sense. On the other hand, what we do see is a deep concern with the problem of the photon, and it’s that concern as against the photon itself that I‘m particularly concerned in isolating.

Slater:

Right. But I think in that paper, Bohr and Kramers were still trying to fight against it as hard as they could. They were trying to do the Compton effect by the moving mirror scheme and so on, which of course, is the proper way to do it, but I think that they cut the photon out just as thoroughly as they could. In other words, I don’t think they still believed in the photon.

Van Vleck:

What you say explains a lot. I got an impression from that paper of trying to eat your cake and have it too. What you said about the origin of the paper explains a lot of that feeling that I had.

Slater:

No, no, they wrote it. They kept me in the back room—or they kept me in the front room while they went in the back room—and they wrote it and kept on changing it, and they would ask me, would I agree with the change; I would agree with the change all right. You notice that the paper was sent in at the end of January. I got there at the end of December. Presumably it was several days after I got there that I told my ideas to them. Bohr imnediately liked some parts of it so much that he gave a talk, I guess it was in Danish in his local seminar about it. He and Kramers sat right down to write it, and they got it done during January. As I say, the changes they made I didn’t like, but I didn’t see that I could fight against them.

Kuhn:

Did you try?

Slater:

I didn’t try very hard, I simply said I don’t like this. I don’t know how definitely I said this. I agreed to go in for it, but as the time in Copenhagen went on I got more and more against them, and finally, I don’t know whether to come to this yet or not I just left. In other words, I did not stay in Copenhagen.

Kuhn:

I didn’t realize that.

Slater:

I didn’t stay very long.

Kuhn:

How long were you actually there?

Slater:

The thing that I did was the following. I was still there a couple of months after the paper went in, but I was staying—Kramers had arranged it—with a very nice old lady, a widow, Mrs. Maar who was a wealthy lady who liked to take one foreign student in at a time. I had a room and meals with her, and was really part of the family. She saw what was going on. She had a nice summer place on the North shore of Zealand near Elsinore.

She wanted to go out there anyway, so she said “why don‘t you just come out here with me, and you work by yourself for the rest of your period at Copenhagen?” We went there—I’d have to look up my diary to find out when, but it was in the middle of the spring some time—and I spent the rest of the time there. I didn’t see any more of Bohr. Kramers came out once to prowl (I would have no part of it) I just couldn’t stand it.

Kuhn:

Did you get the impression from Mrs. Maar’s attitude towards this that this was something she had seen happen before?

Slater:

I don’t know. No, I didn’t particularly get that impression. I think she was probably an understanding lady, and I probably let on enough so that she saw how I felt about things. No, you see, the thing that I wanted to do was—. Well, I was perfectly willing, I agreed that we didn’t have any good evidence on the existence of photons, I was willing to knock the photons, I didn’t care too much. It was really the Bothe experiment and things like that that made it perfectly clear that I was right originally in wanting to have them. But I was willing to drop that, but I wanted to put these ideas together and get something more definite about it. That was when I was working on this later paper on the model of radiation, or whatever I called it, radiation and emission in—

Kuhn:

This was the one you published in the Review shortly after you got back?

Slater:

Yes, that’s right. “A Quantum Theory of Optical Phenomena” [Physical Review, 25 (1925), 395-428]. I wanted to start working on that. I always approach a problem in the sense of wanting to be able to make it definite and work out the details. I feel that if you can’t work out the details you can’t be sure it’s right. I have a great distrust of the hand-waving approach to anything. I had supposed, when I went to Copenhagen, that although Bohr’s papers looked like hand-waving, they were just covering up all the mathematics and careful thought that had gone on underneath.

The thing I convinced myself of after a month, was that there was nothing underneath. It was all just hand waving. I just said, “I’m not going to content myself with this, I’m going to go ahead and see if I can’t work out a physical picture without much thought that it was the correct one, which at least would show that these ideas can be made to hang together logically.” For instance, the simple point that to fit in with Kirchhoff’s law the breadth of the spectrum line must depend on the initial state and the final state symmetrically. Well, how are we going to work that in? Obviously it can’t be just the photon is emitted during the stationary state the thing is in to start with, it must have some relation to the one that it goes to, etc.

So I tried to see if any set of hypotheses could be hung together that would be somewhat logical. I think that the final result was that I could do this. Well, I was working on that. Bohr was contemptuous of it. He would have nothing whatever to do with it. He said he wasn’t interested in looking at it or anything like it. Same way with Kramers. They just had no use whatever for this. So I decided I had no use whatever for being around them, I went away. And I’ve never had any respect for Mr. Bohr since.

Kuhn:

I mean you speak of Kramers as having served here as Bohr’s “Yes -man.” I wonder though whether you have any more detailed sense of differences between them?

Slater:

No, I don‘t think so. I think Kramers was very faithful to Bohr. But I say that Kramers is quite a different person. I’d say it because, of course, Kramers was a man who did many good analytical jobs, who had a very logical mind and so on, so that I think that he was favoring Bohr and everything because that was his job.

Van Vleck:

He liked to do explicit things.

Slater:

Yes, he liked to do explicit things rather than more general things. No, I liked Kramers quite well. I continued to like him, and I knew him all his life, and I have a great deal of respect for the things that he did. But in this respect, he was being Bohr’s man. He was trying to act like the wise papa who was telling the little boy how he has to know how to handle the great man, or how to behave towards the great man. Oh it was very much the case of the great man and the little boy (in the corner). I wasn’t used to this. Nobody at Harvard had ever acted that way.

Kuhn:

You’ve said just a little bit about Bohr’s attitude, what he explicated, why he objected to your more concrete version. Can you tell me more about that?

Slater:

Bohr always would always go in for this remark, “You cannot really explain it in the framework of space and time.” By God, I was determined I was going to explain it in the framework of space and time. In other words, that was Bohr’s point of view on everything, and that was the fundamental difference of opinion between us.

Kuhn:

How much of it, do you think, was that, how much of it was the photon itself, or getting rid of the photon?

Slater:

No, I think it was that. The fact that Bohr was fundamentally of a mystical turn of mind and I’m fundamentally of a matter-of-fact turn of mind. I just think that our temperaments could never have fitted in. Unfortunately, I didn’t realize that when I went there. If I had, I would have gone to Göttingen or Munich. I’m sure I would have made out very much better there.

Kuhn:

There is, as you say, this outline, this report Kramers gave up on this theory even before the Compton.

Slater:

This I don’t remember. I don’t think Kramers talked it over with me.

Kuhn:

Did you have any contact with them by letter or otherwise after those experiments came out?

Slater:

Practically none. I sent in that later letter to Nature by myself as I remember it. I can’t swear that I had no contact, but I don’t think so.

Kuhn:

There‘s something I‘m very curious about, about those papers and their historical role. Clearly, however they came into being, they had a very important historical role in the development of the whole theory. Very promptly, or relatively promptly after those papers had come out, you’ve got experimental disproof, not of your version of it, but of the version that comes out.

Slater:

It certainly set people working on the thing. Everybody who believed in photons immediately wanted to do an experiment to prove that photons existed. So this was very useful, and probably would not have happened if the paper had been written the other way, assuming that photons did exist. It might have, but it might not.

Kuhn:

But one thing that carries on clearly from that paper—even after energy conservation is reinstated—is why, unless people were following you, do people go right on using the correspondence principle in the sense of the virtual oscillators? Now, in part, my question really is, to what extent did that come from that paper, to what extent does it really go back to the Ladenburg, and Ladenburg-Reiche?

There’s a sense, you see, in which the extended correspondence principle in terms of the use of virtual oscillators could have grown out Ladenburg and Ladenburg-Reiche papers, yet my impression from the literature is that there was little done with that until after the Bohr-Kramers-Slater paper.

Slater:

I think that’s true. Of course, I was very familiar with the Ladenburg-Reiche things, so was Bohr. I think that we helped popularize it in a sense. Of course, this also came at the same time, approximately, that Kramers was working on his dispersion formula. That again is operating with things very much like the virtual oscillator, so they all seem to hang together, and I think it was a combination of the oscillators from our paper, from the Ladenburg-Reiche, and the Heisenberg-Kramers dispersion that really set them in operation. It so obviously fitted in with the correspondence principle. Of course, the correspondence principle from the beginning was thinking about the existence of oscillators.

Kuhn:

In a sense not from the beginning unless you choose to look back at it that way, and say, “yes, that’s what we’ve been doing all the time,” but—

Slater:

No, this is another of those cases where one would read Bohr’s very hazy papers, and say, of course, Bohr had done all this mathematics and he’s just not writing it down. I wondered afterwards, maybe he hadn’t. I’d assumed that Bohr had done all this. But it’s not clear.

Kuhn:

Do you have any notion about how this work relates to the Kramers dispersion formula? The Kramers dispersion formula does come a bit later; it‘s clearly worked out in terms of this virtual oscillator.

Slater:

Kramers was working on it at the time I was there, and Kramers, I’m pretty sure, while I was in Copenhagen, gave me, on a piece of paper, essentially the formulas which he had worked out. I don’t know whether he had really started working with Heisenberg. Heisenberg came to Copenhagen shortly before I left, and he actually stayed in the same place with mrs. Maar that I had. So he came into it a little later. I think that Kramers had got the main ideas while I was there.

Kuhn:

Do you have any notion that he’d already been working on this problem before you got there?

Slater:

Yes. He had been doing it before I came, I’m quite sure. I’m quite sure that he had those ready, and that was one of the reasons why the general idea appealed to him. He felt that that would tie in with his oscillators that he wanted for the disperison formula.

Kuhn:

I’d like to pin this down. Because it’s generally taken for granted, and I haven’t had any counter-evidence, that the genesis of the dispersion formula is later than the genesis of the Bohr-Kramers- Slater paper.

Slater:

I think this is not true. I don’t know whether I could even find the papers that I had in those days, but

Kuhn:

I’d be terribly grateful if you’d look, because here’s an area in which it could be quite important.

Slater:

I‘m sure that I had a paper from Kramers in his handwriting in which the dispersion formula was written down, and I’m sure that I had that available to me when I was writing that paper on quantum theory of optical phenomena.

Kuhn:

Yes, but that wouldn’t necessarily make it earlier than your own arrival in Copenhagen!

Slater:

No, that paper I was working on—. Let’s see on the dates of that thing. This was published in 1925, but it was sent in December 1922. That was just the winter after I came back from Copenhagen. This is the thing I was working on when I went out in the country with Mrs. Maar in Copenhagen. I had had most of it done in Copenhagen before I came back to Harvard so that I started this paper within a month or two after the Bohr-Kramers-Slater paper was sent in. Before I started this, I had a copy of this dispersion formula from Kramers, and I gathered from him and Bohr, that this was not brand new, that he’d been thinking about it probably for some months. In other words, I think that he had this before I arrived on the scene. I don’t think it was suggested by me in any sense.

Van Vleck:

When did you make your calculation on the correspondence principle for absorption? You had ail that calculation but never published it.

Slater:

No, there’s some of that in some of these papers here, isnt there? Oh yes, on the correspondence principle. Well, there’s a lof of that in this “Quantum Theory of Optical Phenomena.”

Van Vleck:

You and I had made very similar calculations.

Slater:

I think we’d done that when I was still a graduate student, hadn’t we?

Van Vleck:

No.

Slater:

Was that afterwards?

Kuhn:

This work Van did after he got to Minnesota.

Van Vleck:

You were in Copenhagen at the time.

Slater:

I was in Copenhagen at the time. I know I was working on these things just during this Copenhagen period. In other words, this quantum theory of optical phenomena paper was one in which I was perfectly familiar with how you set up an oscillator to do the radiation, with the A and the B and so on, because they were all in there, and the way in which this ties in with the Kramers-Heisenberg dispersion formula, because the dispersion terms were all in there. So I was working on it then, and as I say, I did not invent the dispersion thing, that was from Kramers.

Kuhn:

And you think not only was it from Kramers, but it was from Kramers before your virtual oscillators entered the picture?

Slater:

I think so. I think that he shoved me this formula within that first month that I was in Copenhagen. I think it was one of the first things that he showed me. When my idea was produced, he got out his little piece of paper, and wrote down his formula, to show that he’d been thinking along those lines. In that sense, he really put things into it.

Kuhn:

That’s terribly interesting, and also I think terribly important, because it certainly reverses what’s a very general impression about the order of development in a key set of developments. Is there any chance that if that paper’s around, it has a date on it?

Slater:

This would be very remote.

Kuhn:

This is useful as far as it goes. I needn’t tell you that whenever there’s a chance to find a document, we’d trust it more than we’d ever trust memory. This opens up a whole new point.

Slater:

I would, have to look pretty hard in my very old papers, because while my papers since about 1927 or 28 are in good order, the ones before that are in very poor order if I have them. So I don’t know whether I have it or not. I could look. it would be in some boxes of old papers from that period, it would be interesting and I shouldn’t be at all surprised if I couldn’t produce it. I’m sure it would not have a date, but I ‘in sure it would be in with other things which would date it.

But, you see, the evidence I can give is perfectly straightforward. Namely, that I had that piece of paper from Kramers, it had the dispersion formula on it, he gave it to me when I was in Copenhagen. I was in Copenhagen from Christmas until I got disgusted and left in the middle of the spring, and by then I had not only had it but I had been playing with it for weeks. So I think he must have given it to me in January l924. And he gave it to me as something that he had been working on, for probably a few weeks, something he hadn’t just thought about that day.

Kuhn:

There is this other puzzle raised by Van’s question. In Van’s paper, in which he does a theory for absorption, not simply as it previously had been done, I think, for the rotator and the linear oscillator, but for the general case of a multiply periodic system, he adds a note in proof, saying that he has recently heard that you had been through this whole calculation yourself independently. Now none of the things you speak of, and I think nothing that is explicit in your paper on optical phenomena, would really seem to refer to that work.

Slater:

I'm afraid I don’t remember this.

Van Vleck:

I remember very definitely some correspondence with Kramers. I knew he wrote me that you’d made this calculation, and he said he wasn’t certain you’d determined the partial of rho with respect to nu; he wanted the more interesting terms. And I asked you if you had it, and you said yes, you did.

Slater:

I see. Well, that’s a little more definite then. That was relating derivatives to differences and that kind of thing. Oh yes, that is a somewhat later period, as far as I’m concerned. In other words, not as part of this same business, but as an effort to tie in those derivatives and the differences and get something out of that. I was trying to work on the trail of what evidently Heisenberg was working on the trail of too, in his matrix mechanics; in other words, I was playing around with those things. I never published anything on that.

Van Vleck:

But that was in the spring of 1921 because, I think, that’s when my paper was submitted.

Slater:

In the spring of 1921. Well, that’s when I was in Copenhagen.

Van Vleck:

I thought that probably you and Kramers had talked that particular calculation over.

Slater:

Kramers, of course, as I say, had got his dispersion formula, he had got the relation between derivatives and differences and all those things. I suspect that this was something Kramers had done that he talked over with me rather than what I’d done, because I don’t think I had added very much to Kramers at that period.

Van Vleck:

(Kramers had done the absorption [sic] calculation, and he had done the asymptotic connection of the Fourier series for the refractive index, and that you did it for absorption.)

Slater:

This could well be, but in that case then I just disagree with you that it’s not in this quantum theory of optical phenomena, because that is doing absorption along lines which fits in with Kramers’ dispersion. This one here. It’s got absorption calculations.

Kuhn:

I don’t think the difference here was the question whether there’s absorption in this paper, but whether there is the general parallelism for computation for the full multiply periodic system.

Slater:

That I certainly did not put in there, but I think that I was familiar with it. I think that I was playing with it, I was also doing absorption, but I didn’t put the absorption in here using that notation. That’s probably the answer.

Kuhn:

There are an awful lot of ways we could go at this point. You begin to work on quite a large number of things, and I’m not sure of the most strategic way to continue. I think perhaps what I’d like to ask you, even though it will violate chronological order a bit is, can you follow through for me the development of your attitude toward your own virtual oscillator theory? You go right on with that, not only when you come back with this paper, but again in another paper that’s after the Bothe-Geiger experiment, and then even a paper shortly after Schroedinger, in which you reformulate it for the wave equation; then it seems to drop out. I would be interested both in the development of your own thinking, and I guess what one had better call the final abandonment of this view, except to the extent—

Slater:

There I disagree with you, I don’t think I ever abandoned it. In other words, I think the virtual oscillators are simply just another way of describing the oscillating charge that comes into Schroedinger theory. I didn’t say anything more about it because it seemed to me there it was, it was derived from Schroedinger theory. So I just felt that my general point of view was justified that a theory which will describe this oscillating charge, will also describe inter-atomic forces. At that point I went back to helium atoms and their interaction, because I felt that we’d now got what I was looking for.

Kuhn:

O.K. That seems to be right. What sort of reactions were there here or abroad, both in your own experience to the original papers which you didn’t like, and then to your subsequent more exact work on it?

Slater:

I think actually I got very few reactions of any sort. I think that obviously publishing a paper with my name and Bohr’s name made my name known. In that sense it was good advertising. I certainly didn’t do it for that reason. As for reactions that really meant anything, I got almost nothing.

Kuhn:

What about your subsequent versions of it, how were those received?

Slater:

Also there were almost no reactions. This has been very commonly true; in other words, I got so disgusted along there because nobody seemed to be paying attention to my papers, that I jumped from one thing to another hoping something would interest somebody.

Kuhn:

Not only no positive reactions, but no negative reactions either?

Slater:

No. Also I switched around some, from one thing to another, because I found I was doing the same thing that Dirac was doing. I had followed along with this business of derivatives versus differentials and so on, so that as soon as the Heisenberg stuff was coming along I was right playing with those things. As a matter of fact, I had a paper all written—here’s another document that might be interesting to resurrect if one could find it—in which I was pointing out—and it had never been done before—the relation between the Poisson brackets in classical mechanics and the commutator in quantum mechanics. I had this all written down, ready to send the thing into Phys. Rev. the next day. The next day Dirac‘s paper came doing the same thing. No, I had that all worked out. That was Dirac's first paper, I had never heard of Dirac.

Kuhn:

I must say there were damm few people who had at that point.

Slater:

But at that point I decided here’s somebody who’s going to run me a race, and then I proceeded a year or so later to do this absorption business with Schroedinger’s equation which did the absorption all right, but the radiation didn’t work, and Dirac’s paper on that came out at just the same time.

Van Vleck:

Weren’t you the first one on the absorption?

Slater:

I think I probably was. I think I may have been a bit before Dirac, but clearly we were running a race, and clearly he was a smart guy, and I decided I’d better shift to something else. That’s when I shifted to doing helium. I just decided that obviously our thoughts were running so much along the same line that if I kept on without shifting I’d just find every paper I wrote was written by him first.

Kuhn:

Somebody else got caught in that trap and couldn’t decide for some time what to shift to: that’s Jordan, who really, one paper after another, he and Dirac, Dirac always a little earlier, and almost always a little neater.

Slater:

Yes, that’s right. I decided that this was no race to be in.

Kuhn:

I’d love to see that manuscript.

Slater:

There again I don’t know whether I’ve even got that. But I think it would be interesting to look back and see if I could find this. I didn’t go nearly as far as Dirac, but I had some of the same things.

Kuhn:

It’s again no issue of who had it, the question is just how close to the surface that was is very much illuminated by simultaneities of this sort.

Slater:

I’d be interested if I can find my own calculations of those dates to find just what I was thinking about, because I know that I was really working toward quantum mechanics before quantum mechanics came out. I’m sure if it had been delayed a year or so more, I would have got it before the others did. In other words, there were a number of people so close that it was bound to be found. I was working along the lines from the Kramers-Heisenberg dispersion formula to try to formulate general quantum mechanics rules that would handle other problems. And I saw that you had to get meanings for Poisson’s brackets and that they were commutators and so on. In other words, I was very close to the Heisenberg formulation before he had it, but I didn’t get there.

Kuhn:

Your Poisson bracket expression was actually not only simultaneous with Dirac’s, but it was independent of Heisenberg’s.

Slater:

I would have to look back to find out just how this was standing.

Kuhn:

Dirac definitely takes off from the Heisenberg paper.

Slater:

I guess probably I was taking off from the Heisenberg paper too, but also I was thinking along the same lines as Heisenberg before the Heisenberg paper. In other words, I was thinking along these lines from the time I came back from Copenhagen. The dispersion formula which I was thinking about in Copenhagen started me on these lines, as it started Heisenberg on these lines.

Kuhn:

Had the dispersion problem itself existed for you as an important problem before you got involved with it through Kramers?

Slater:

Oh, I would say so, yes. I had read the Iadenburg and Reiche stuff with much interest before I went to Copenhagen.

Kuhn:

Had you worked at the dispersion problem, so far as you know, in that period?

Slater:

I hadn’t really looked to see what kind of formulas you’d had. No, the dispersion formula struck me as something beautiful and new, that I had never thought of. I had no pretense of having thought of that.

Kuhn:

Of course, there was a good deal that appeared to most physicists as pretty totally ad hoc about the Reiche-Ladenburg work, and the whole question as to why it was the transition frequencies that occurred in the denominator rather than the orbital frequencies and so on.

Slater:

This seemed to me perfectly obvious. You Know you’ve got Kirchhoff’s law, you know that you must have absorption and emission behaving the same way, and if you’re going to have emission and are going to try to describe it in terms of a wave pattern, you’re going to have to have oscillators of that sort, and therefore you’re bound to come out with something like this for the dispersion. So that I was all ready for the dispersion formula, and I thought it was a beautiful thing when I saw it. In other words, that was quite a lift, as far as I was concerned.

Kuhn:

Do you suppose that this piece of paper as you describe it that Kramers gave you had only the results on it?

Slater:

That’s all.

Kuhn:

It‘s one of the remarkable things about the formula itself, that until the Kramers-Heisenberg paper, which is a good deal later, Kramers never gives the derivation. He gives a couple of hints

Slater:

Oh well, had some derivation because he more or less told me how he got at it.

Van Vleck:

He wrote a second letter to Nature to really explain how he derived it although he didn’t give a derivation.

Slater:

No, he got at it all right. But all he wanted to do to me was to write it down so that I’d have it to think about it.