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In footnotes or endnotes please cite AIP interviews like this:
Interview of Emilio Segrè by Thomas S. Kuhn on 1964 May 18,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with circa 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Niels Henrik David Bohr, O. M. Corbino, Enrico Fermi, Otto Robert Frisch, Ettore Majorana, Chandrasekhara Venkata Raman, Arnold Sommerfeld, Otto Stern, Pieter Zeeman; Como Conference, Rome Conference, Rome Institute of Engineering, and Universität Hamburg.
Let's start out, if you will, biographically. . . . I ask: Who were you? How did you get interested in science? Our interest is partly in biography as such, but we also concerned with you as excellent example, and an illustrious one, of the new generation in physics.
I was born in 1905. I vas born in a family which was definitely a family of intellectuals. Although my father was an industrialist and had some paper mills, I also had two uncle: one was a distinguished geologist; one was a distinguished professor of Roman law. Both people with international reputation and so on. Actually I am the third in the Italian Accademia dei Lincei—the two uncles and then me. On my mother's side also they were pretty intellectual people, I would say—business, and engineers, and so on. I had an uncle whom I barely remember, but he was a schoolmate of Volterra. Volterra always used to say that he was the best mathematician he had ever known. He died relatively young; he became an engineer in the railroads and then died. I myself was probably—well, I was interested in machines, in apparatus and physics and so on. The thing was, so to speak, injected in me—on fertile ground—but it was certainly injected.
By the family?
By the family essentially.
You say intellectual, and you document this admirably, but that doesn't mean interested in machines, necessarily. Was it your father's paper machines?
No, no. My father was the only one who did not go to the university, you see, and the reason why is this: My two other uncles went to the Collegia (Gislieri) which is the Lombardy counterpart of the Scuola Normale in Pisa, which was because they were not rich. They were not extremely poor, but certainly not rich. Since two brothers could not be in the college at the same time, my father, who was the intermediate brother, didn't get in. You see, the oldest brother got in, and then when he came out the other [could go], you see. So it was that my father made the money, and the other two became the scientists or the intellectuals. However, although my father had only a secondary education, he had learned languages,he knew history well, and he was extremely well-read both in Italian and other languages. He was a man of not superficial culture, and so on.I was interested in science, I presume, because around 1910 science was the natural thing to be interested in. Remember, the previous generation grew up at the time of Spencer, of the late 1800's, and so on. I read, I can tell you quite well, I read I Maestri della Scienza and Gli Eroi del Lavoro by (Di Santieri) I think it was, which were serious biographies of "Masters of Science" and "Heroes of Work." This was fed to me when I just barely knew how to read and write. It made on me a tremendous impression, and I found it all natural. However, it was on fertile ground because—and on this I have documentary evidence; this memory is not concocted afterwards—when I was six years old, because it happens to be dated, I have a school book, a notebook, in which it says "Physics experiments by Emilio Segrè." I wrote all the little experiments that I was doing, which I read in books more or less for children, but I liked to do them myself. I had the spectrum of the sun, the atmospheric pressure—all my little experiments. So that's very, very early.
...Yes. That is very early and very interesting. You say that this was the thing to be interested in in Italy, but in that period there were not very many people interested in it. You belong to the first generation of significance of physicists in Italy.
Well there were the mathematicians.
But that's different isn't it?
One of my uncles established the first testing laboratory, sort of a Bureau of Standards, for the Italian railroads. Anyway, I was interested in this, I can't tell you why, but I was. And I read all these books. Then I read a book of physics of Ganot.
In what language?
In Italian. And I read the edition of 1865 or '63 or something like this, which had belonged to my uncle, in which it always said "the experiments of Mr. Faraday in London" who had done this and that, you see. Faraday was still alive—I don't know when he died, but he was always called 'Mr. Faraday' in this edition I had, you see. Then, since I was so interested in this, when I had my tenth birthday my uncle gave me a new edition of Ganot of 1914, in French—I knew French quite well; I had no difficulty reading French—in which he said be gave me this with the wish that physics would always serve the arts of peace. This was in 1915; there was the First World War going on. I still have this book, so it can be seen and read. Then I went to secondary school. In secondary school I got interested more, quite a bit in the mathematics. I had done quite a number of physics experiments by myself, somewhat on the playful side—a mixture—some were real physics and many were just playing, making discharges in gases, and so on. I had the Rumkorff inductor and things of this type, you see, I enjoyed very much.
No, no, in vacuum. You could buy little tubes with a poor vacuum; probably there was a tenth of a millimeter of pressure, something like that. Of course, my main point was to take light bulbs, which at that time were evacuated; they didn't have gas inside. You could put a capacity outside and make the discharge inside the light bulb, the old light bulb, and these were my great pleasure of gas discharges. I made these things just for the fun of making them. Then I made an Atwood machine, . . . and I made a thing that would write complicated harmonics on paper, all kinds of things of this type. Then my brother, who was eleven years older than I am, who was studying engineering, had to prepare the exam of analytical chemistry. He bought a bunch of chemicals and made all the experiments at home. This was my great—I would spend all the time seeing these things. This was when I was a child, say, before puberty, so to speak, when I was really quite a child. Then I went to high school in Rome, and there I had to study a lot of Latin, and so on. I had rather poor professors of science—well, not quite poor, but I had good professors in literary matter, and so I got interested and learned languages; and I learned quite a bit of mathematics. Very unfortunately, I didn't learn the mathematics I should have learned. I learned number theory, Euclidean geometry, things of this type, instead of learning algebra and analysis. This was a very unhappy thing.
Did you have any consciousness of this unhappiness at the time—?
No, I didn't know. More or less, I thought that was the mathematics I should learn. . . . There was a very bad thing that my brother did to me. Namely, he took away all the books of mathematics because he wanted to be an important man; he said it was bad for my health to study mathematics. Complete nonsense. So I ended by studying useless mathematics instead of studying what I should have studied.
The books he took away were books of the more useful sort of mathematics?
Yes, because they were books of calculus, books of analytic geometry, and so on.
These were books you'd gotten to study by yourself? They were not in your classes—
No, no. Books that were at home. All the books—we had a library at home of various kinds; there was everything in it. I studied chemistry, the chemistry of Dumas, of 1850 or something like this. I went to school with Pasteur, [so to speak], studied chemistry under the same professor as Pasteur. And so on, you see, all old stuff. Then later I went to the university.
I don't understand about your brother's taking these mathematics books away. What is that really about?
Well, he took them away.
He thought you would ruin your health?
That's right. Probably he wanted to be important; he wanted to be the only one to know them, I don't know. Anyway that's what he did.
What was he doing? What was his—?
He was studying engineering. He was eleven years older than I am, you see.
Was he your only brother?
No, I had two, one still older. The other one was not at home. But then, I could find, for instance, I found also the books of my uncle;I found the algebra of Tod Hunter of 1870 or something like that. I was not really a good mathematician, but I could have learned a little more. Well, anyway, then in the secondary schools I learned mainly literary things, and I learned quite well my mathematics which was taught there, and my physics. I also read, apart from what I was told at school, I read by myself quite a bit. In physics, for instance, I read Maxwell's theory of heat, with great effort. I had a tremendous difficulty in understanding it, and I didn't understand it, to say the truth. But I read part of it.
Did you read it in English?
In English. I knew languages; I mean I had learned language by that time. And I read Glazebrook's book on astronomy, and things of this type. And when I was—oh, probably, I would say 18 or 19, at the very end of high school, I read Fritz Reiche's Quantumtheorie, which I still have. This was probably when I was 19 or 20 or 21, something like this. That made me a very deep impression. That book impressed me tremendously. I didn't understand it all, understood it only in part. But it made me a very clear idea that there were terrific things going on in physics.
Was this the first time you had gotten the impression of big novelties that were now developing in physics?
That's right. I knew nothing before that. I knew that there were gas discharges, electrons, and so on; maybe I knew of radium, I don't know. But somehow it didn't impress me very much. Because I thought—I mean, all the physics I knew was the physics of Ganot, a little glorified, but essentially that, you see. Strictly classical. I knew about spectral lines, I knew diffraction; I knew this then because I had made experiments, I remember. . . . I remember very well one thing that struck me with this: that out of Ganot I knew that everything was proportional to something else, or maybe proportional to the square. I don't think I arrived ever to the exponential. Maybe. I don't know. It had been a great puzzle for me: 'Well, they have invented so many functions in mathematics; why don't they show up in physics?' And in Reiche was thefirst time I saw formulae like, say, the black-body formula, integrals,and so on, all showing up in physics. I was very impressed by that. Then, more or less, you see, when I came out of high school in Italy, I could have become a lawyer, a doctor, or an engineer. There were only these three professions, essentially. Nobody in his mind would have become a professor of Latin and Greek, or a philosopher, or anythinglike that. Now there was the paper mill of my father, and the best preparation for it would be to be an engineer. So I registered in engineering.
Intending to go on in engineering, or intending to go to the paper mill?
I can't tell now what I really had in my mind. Probably intending to go into the paper mill, I don't know. I really don't know.
...You did a tremendous amount of science by yourself, reading andexperimenting and so on. This, I think, must have been quite unusual. Did you have friends you did this with, or who were interested in it?
No, I did all this alone.
Did this isolate you from contemporaries?
Not much. No, not very—I would do this in my spare time. I had things of this type though. For instance, in school, all the Middle School— what we would call high school—for many years, I had a sort of tacit agreement with the professor. I was getting good grades, I was always third or fourth of my class—but I had this tacit agreement that he would never bother me and I would be an excellent student, would never bother anybody, would sit at my place, would not talk, end so on. I would read whatever interested me, and he would do whatever interested him, you see. So I brought a book of physics, and he was explaining, say Dante, and if I was not interested in Dante I would read physics. Provided I knew at the end, it was none of his business, and they were quite nice about it. It was quite easy to get along in this way.
Did you have friends among your contemporaries?
I had, yes, I had friends. Well, I mean, I had a friend who became quite influential in many ways, and that was Enriques, who was a son of the mathematician Enriques, you see. And he introduced me to his father, and then to Levi-Civita, and then to Castelnuovo, to all the mathematicians in Rome. So I got to know them all. Although these guys didn't care about children, they were interested only—they spoke among themselves, and they sent all the children to speak between themselves, which was tremendously tedious. But, of course, my parents helped me in a certain way. They certainly gave me books; they never bothered me in any way because I was doing well at school, d why should they? Then, they had a high regard for science; they never thought that I would become a scientist, because the only scientist in sight was my uncle, the geologist. But then I went to the university, and at the university in the first two years, it was quite interesting, because I came in contact with very top-notch people: mathematicians—Seven, Castelnuovo, Levi-Civita; Corbino as professor of physics. There was only a professor of chemistry who was an ignoramus, who was Parravano; he just didn't know anything. . . All the others were quite good, and they gave very good lectures. We had good books and so on, although we had no personal contact with these professors, never talked to them, essentially. But you had the opportunity of learning a lot.
In spite of the amount you had done by yourself, there was much to be learned from Corbino?
Oh, yes. Yes, sure. Because I didn't know—I mean you see, I was completely self-taught as far as it went, and not being too smart I didn'tknow it systematically, and I didn't know the important things, and so on. I would know quite a few pieces of work Corbino was telling here and there, but not as a connected thing. And also all the mathematics—I knew only little pieces here and there. Then I went to the engineering school, and then there a let-down.
This was the third year?
Yes. The third year started to be engineering. . .
How much physics, and other things had you had in the first two years?
Well, I had had algebra; calculus; lots of geometry—projective geometry, analytic geometry, descriptive geometry—and then physics, limited toelectricity; and mechanics from Levi-Civita, very much from a mathematical point of view.
Was the electricity from a relatively non-mathematical point of view?
Electricity, more from the point of view—It really was a sort of first volume of Abrah and Becker, a little simplified.
Even that's a fairly mathematical treatment. Vectors, for instance?
Yes, sure, sure. Well, I mean people were assumed to have a certain maturity in mathematics.
It interests me that it was from Levi-Civita that you took the chanics; this means, for one thing, that mechanics was taught in mathematics, not in physics.
Yes, that's right.
Now that separation between the things that were mathematics and the things that were physics—I don't, know to what extent this is true in Italy, but it is true in many places—creates a very real division which makes it very hard to do theoretical physics.
But there was very little physics in Italy at that time; it was primarily mathematics. I don't know. I took the exams there; I did always very well. When I took the exam from Levi-Civita—it must have been my third year, probably—I had summa cum laude, I don't know what, and I got gold link-cuffs [sic] from my uncle. At that time I was going in the mountains. Now, the third and fourth year of the university were a let-down; because except for Levi-Civita, all the rest—and there was also a good professor of technical physics, where I learned thermodynanics—but except this, the engineering school was just three steps below in respect to the university. Electricity remained under the aspect of electrotechnics; it was no good.
What sort of engineering would it have been?
Electrical. Electrical was my specialization—chiefly sort of generical engineering. But just the people were not the quality of the people at the university; [they were] much below. That brings us to '27, which was quite a crucial year for all these things. Namely, several things happened at once. My parents bought me a car, so I acquired a car. Rasetti came to Rome; and Giovanni Enriques, who was the son of the mathematician, decided to bring me and Rasetti, because he wanted to explore some mountains in central Italy—I was the only one with a car, so he said, "All right, let's go with a car, with Emilio's car"—and so we went, all three. And in this way I got acquainted with Rasetti, who was a young physicist—(Telephone interruption]At that time Fermi had come to Rome, the year before.
Were you conscious of this at all?
Oh, yes, I was quite conscious because when I was in the first or second year at the university, I had heard—I used to go to mathematics seminars for older students, I just went there—and I heard quite a few of the famous mathematicians in the world at that place. Very often I didn't understand a thing. I remember I heard dau speaking of number theory; I mean, it was fantastic. And I heard all kinds of other things. But then once I heard Fermi speaking about quantum theory, you see. And it was—it didn't take much, I an—first of all, it was intelligible. Secondly, it was absolutely plain that here there was a man who knew whathe was talking about, beyond normal. It was absolutely plain.
When would this have been?
. . . Must have been after '22, because I went to the university in '22. Now Fermi was in Rome, was in Pisa before '24. So it must have been in the interim period while Fermi was not yet at Florence but had left Pisa, so it must have been '24. And I think I can still remember what he spoke of and probably if I look in the collected papers of Fermi it will be even possible to find out what was his subject, because probably he printed what he said. It was a sort of popular talk. But this was my only contact with Fermi at that time.
How was it received by others? Was there discussion? How did people feel about quantum theory?
There was a sharp division. Are you speaking now of quantum theory or a personal estimate of Fermi? Two things.
Let us have both—I agree that they are quite separate things.
Quantum theory—I would say that except Corbino, there wasn't anybody in Italy who realized its import ce or really saw anything out of it. Not even the mathematicians. The mathematicians were very conscious of relativity; they understood—well, Levi-Civita, after all, had worked actively in relativity, so these people knew, relativity quite well and appreciated it. Now, quantum theory was beyond what they knew, was just not a thing for a mathematician. Corbino certainly knew quantum physics, to a certain extent. He certainly knew it was the most important thing that there was; there's no doubt about it. The rest—there were quite a number of mathematicians who either were not interested or despised or hated mathematics—I mean relativity—and almost all the physicists plain thought it was wrong—it just was wrong. And the situation was different because the mathematicians did understand it, at least the mathematics of it; for instance, Volterra certainly knew relativity and understood it, had no difficulty, has written papers on relativity, but was somehow not interested in it. So the only mathematicians who were really interested in it were Castelnuovo, Enriques, Levi-Civita, well certainly Marcolongo because he wrote a book on it, and a few more. Among the physicists there was, let's see, Majorana, not Ettore Majorana but the uncle, and [M.] La Rosa—they were always trying to revive the ballistic theory of Ritz or to do something to destroy relativity. Quantum theory was not even ted of.
Then how was this account by Fermi received?
It was a seminar of mathematicians, and certainly—well, let me put it so—Corbino, for sure, I don't know whether he was there at this speech, but Corbino was already a friend of Fermi; he knew him privately. And certainly Enriques and Castelnuovo and Levi-Civita and so on all knew that this was a sort of wonder boy, and they were listening. Whether they were very interested, I don't know. But certainly they were listening, and they certainly—well, they invited him to speak, although it was not mathematics. In '27 then, after this, during the summer, I went in the Alps and climbed the Matterhorn with Rasetti and other friends of mine, the son of Enriques, and Amaldi. No, I can't remember whether Amaldi was there. ay, it was some of us. At that time had already started strongly to decide to go to physics. My uncle Claudio who was the geologist didn't like very much to hear that I should quit engineering.
Because he wanted me to have a solid profession in my hands, so to speak. Plus he was an engineer.
He, as a geologist, was an engineer?
Yes. He was in engineering.
Was he a mining geologist?
No, he was, he took—you see, he was born in 1853. He was probably a civil engineer who had specialized in geology later. He was an engineer, and he was a geologist, but apart from this he had founded and was the director of this experi ntal laboratory of the railroads, which I mentioned. So he was—for instance, he knew quite well Poincaré 8; he knew Le Châtelier; he knew all these people, because he had met them at international conferences. When there were these international conferences around the turn of the century of some kind or another, the Italian government sent him to represent Italy or the Italian railroads, or I don't know what. Anyway I also was not quite so sure, because a career in physics in Italy at that time was nothing so easy or rosy; it was pretty much just getting out of the window.
How did your family feel about it?
It's not quite clear. I don't think they were very enthusiastic. What they said is "Well, all right, you have now made four years of engineering, you are in your fourth year; why don't you do both?" And then I saw that it was impossible to do both; I started to do both, and then I saw that I just couldn't do this. I didn't have any time nor the force to do both. But I don't think they obstructed it in any way. They had enough money so that really there was no problem in my earning a living, you see. So that was not a consideration. It took out difficulties [i.e., my obligation to my father was reduced] probably, because my older brother who had studied engineering had gone in the paper mill; he was a very jealous type, a very narrow-minded man who didn't want to have anybody else; he wanted to be the only one. So my father would have been in a sort of quandary with two people in the paper mill. He saw that one didn't to do it, all right. Then I came to meet Fermi.
In the summer of '27. Early summer. There I have not a very clear recollection, but I have a recollection of some things that Fermi taught me. We went to the seashore together several times, swimming and bathing and so on. Then he would ask to me, "Well, how would you do this?" And the only thing I can remember is that he had a heavy, dangling rope, attached at one end; and I had to study the vibration. This took Bessel functions, and I didn't know the Bessel functions, and this one was beyond me. I didn't know how one could do a thing like this. Then he told me, "Well,you know, this is done with Bessel functions." "Well, all right, so I'll go and look at the Bessel functions." I mean, I could write the equation, but I wouldn't know how to solve it. But this is one of the few things I remember. Then, during the summer, I went—
Tell me, when he asked you a question like that, what was his purpose? Was he being a teacher?
Oh, he wanted to see whether—I'm sure that he wanted to see whether he had found a man to bring in, in physics; you see, he was looking [for] people. Somebody had told him, in all probability Giovanni Enriques or Rasetti—somebody had told him, "Now, look, this boy is doing quite wellat school; he's at the top of his class. He is very interested in physics;maybe this is what you want." And so I came to talk to him; I was also smelling him, re or less; it was a reciprocal process. I was convinced, as far as I'm concerned, so he was convincing himself. He asked me, "Do you know this? Do you know that?" Not that he gave me an examination, but it was probably sort of—-he would make a note of what I knew and what I didn't know, and whether I answered intelligently or not, or something. Then we went to the Alps. I went with Rasetti and Amaldi and Pontecorvo's brother and a few more people, always with this car. We climbed lots of mountains in the Valais; it was a tremendous season. And then after that there was a Como Conference, and I went to the Como Conference. There I found again Fermi. At the Como Conference there were all these famous people. Plus the booklets of K. K. Darrow explaining modern physics in the Bell Laboratory, which the Bell Company had sent there, you see. I got all these booklets which were distributed there; and would see the people and say, "This is Franck," and then I would look in the booklet at the Franck-Hertz experiment. And so I learned atomic physics essentially, because although I knew something, all the physics I knew, except for this thing of Reiche, was completely limited to the black body and specific heat and so on. I knew nothing of spectroscopy and all that.
You had never looked at Sommerfeld's Atombau?
No, I didn't know that Sommerfeld existed. I didn't have it. When I went back to Rome in the fall of '27—
Did you attend the lectures regularly at Como? Do you remember any of them?
Yes, I attended lectures, but I didn't understand much of what was said. I remember the lecture when Sommerfeld spoke, because everybody went to hear this great Sommerfeld and he mentioned Fermi, and everybody was very interested because he was practically the only Italian physicist mentioned at the whole conference. I mean the other Italian physicistswho were there, except Corbino, were not much of ything. I remember the speech of Bohr, which was not ch de tood, but there was lots of discussion about it.
There was a lot of discussion about it?
There was a lot of discussion, but what was said I don't know.
How about the Heisenberg paper?
I don't know; I don't remember. I know that there was a tremendous pow-wow between Pauli and Heisenberg and Fermi who were there and talked together a lot. What they said I have not the slightest idea. But they met considerably. And then I went back to Rome in the fall. In the fall I registered in physics, and then I saw that I just couldn't do both things. So soon thereafter I quit engineering. And then I brought in Majorana, because I knew Marjorana; Majorana was my schoolmate in engineering. So I wentto Majorana after a few weeks and said, "Now, look, with your brain, you'd better come to this other part of the world; it's better used." And he came there.
It was easy to persuade him.
Oh, yes. Quite easy.
Was his brilliance seen immediately by Fermi.
Oh, yes. I mean you could see. it because you could ask Majorana things that you couldn't ask anybody else in the world. He was a prodigy. At least, I mean, one couldn't know whether Majorana would become a second Newton, but you could get him to integrate, I don't know, a very complicated integral and so on; he would look at it and tell you the answer without even writing it. He could do feats of this type, plus numerical feats, but the numerical feats were more of a kind of theatrical show. . .
He also could conceptualize in non-mathematical ways, couldn't he?
Oh, well, yes, he could do pretty ch everything. But during that winter, then, and the foil rig spring my le died; hevery old, and died.
This is the geologist?
The geologist, yes. And I got involved more and more in physics. Fermi taught us privately. What he taught I can tell because I still have notes that I wrote at that time; I have them.
When you say 'privately' what do you mean?
He gave a course in which he explained exactly what is contained in Introduzione della fisica atomica, which one can get out of the library and see exactly what he taught; it was exactly that.
Whose 'Introduction to Atomic Physics' is that?
Fermi's. He wrote one in '27. He taught exactly that. Then, I would say three times a week, in the afternoon at five o'clock or something like this, he would give us a private lecture. To Amaldi, to me, to Rasetti, and Major a, and occasionally Corbino would come. Not always everybody was there, but I was always there and Amaldi was always there; Majorana, yes, but very often he would say, "Well, it's beneath my dignity. Why should I learn these things? You are doing it in a childish way; it should be done this way."
Did he say this, or did he just act as if he thought this?
Oh, he said it. Even showed it.
How did Fermi react to that?
Nothing. He didn't react to that. Except a year or two later he decided that we were not worthy to be present at the interviews [when] Majorana[was there], and then they would closet themselves together, you see, because they went very fast in very difficult theory and so on.
And what about those notes of yours on these lectures that Fermi gave?
I have them. I have them at home. . . One thing I'm sorry that I have lost . . . is a little notebook of the very, very, first of these lectures when he told me how you make a vibrating rope with discrete points and then you pass to a continuum—I mean, things very classical, very standard. And he made me make diffraction problems and so on. Actually Duane-Hunt quantum arent, the grating, and the periodicity of the grating, and so on—I mean, all kinds of things of this type. This was lost; I don't know where it is gone. But the bulk of it I have.
Do you know how he felt about that, the Duane theory and grating periodicity at that point?
It was a very nice thing. He said, "Well, you see how it comes out." I remember this was '27; it was after Como, so already "Que la luc' è un corpo ed un' onda . . . " was already acquired, so to speak.
Were any of these lectures on matrix mechanics or on the Schrödinger equation?
Oh, sure. Yes. They were on everything. They were on capillarity, relativity, fluctuation, statistical mechanics, and then quite a bit on Schrödinger equation; [also] Dirac metrices with continuous indexes, discrete indexes, and so on. They were all what we would call here, at Berkeley now, upper-division course level. They were not graduate.
No. Not now, in 1964. It's all stuff that we would teach now in upper division.
In 1944 it was graduate some of it.
Maybe, but not now. No more. It's all stuff that is taught in upper division now.
What, at this stagee of the game seemed to be pressing problems? You were now a candidate for the degree in physics?
Yes, I got the degree in July of '28.
Did you have to do a thesis for that?
Yes, I made an experimental thesis, and I saw with great pleasure that in the last bulletin of the American Physical Society somebody has measured the same things that I measured then. This is the state of physics now, you see. [I'll have] to write, 'A former student of mine in Los Alamos has re-measured these things just because the literature is so'—he doesn't know it. He needed these numbers, the . . . oscillator strength in the principal series of alkalis. So he has re-measured them. I have still to write him a letter and ought to write him, but I haven't done it.
How did that problem get picked?
It was started by Rochdestwensky in 1907, or something like this. Then Puccianti had made it in Pisa. Puccianti had been Rasetti's professor. Rasetti made his thesis on it, you see and then I had to make a thesis all right so I chose to make a thesis on that also. Instead of measuring cesium, measure lithium; and instead of asuring it in the visible, measure it in the ultraviolet. Among other things the thing was quite fashionable and was still not completely senseless, I mean [wasn't] just a routine thing, because there was this question that the s—> p combination, instead of having the ratio of intensity 1:2 as due to statistical weights, have in cesium I think 1:4 or something like this, have quite an anomaly. And instead—in sodium, they're regular. And this at that time was a great puzzle in quantum theory, because the statistical weights were—. And it was only probably around 1933 or '34 that finally it was a problem that persecuted everybody around for years—that Fermi finally solved it, found out what was ing the anomaly. There was a perturbation and so on; he wrote a ape you can see the very thing in his collected papers.
And that goes back that.
Oh, yes, that goes back to Puccianti. The fact—
Yes, the fact, but it wasn't an anomaly with Puccianti yet.
For Puccianti it was not an anomaly, but certainly it was an anomaly withRasetti in 1922. Well, there are also already papers of Fermi in his collected papers on statistical weights of qauntum states where these things come out. Intensities of spectral lines—Holland with Ornstein and Dorgelo and those people, so he knew that kind of thing.
Could you have considered doing a theoretical thesis at this time?
Oh, yes. Yes. Actually—
You point out that Fermi could scarcely have considered doing a theoretical thesis, in the biography. You talk about his doing the X-ray thesis because physics was really thought to be experimental.
Yes, but there was a professor of theoretical physics Fermi. He was the first professor of theoretical physics.
Why did you then do an experimental thesis?
A little by tradition, a little—I don't know; wanted to do something experimental.
Did you think of yourself as experimental rather than theoretical, or as theoretical rather than experimental, or—?
No, no. Actually everybody who was connected with Fermi was considered as a theoretical physicist, so much so that somebody in Bologna said "He has made an experiment in theoretical physics." (Laughter). This was the general reaction. Any experiments which had to do with spectroscopy were experiments in theoretical physics.
Oh, yes, sure.
That's terribly different from the situation in Germany. Paschen was not doing experiments in theoretical physics.
No, that's right.
When Rasetti did his experiments for Puccianti, were these experiments in theoretical physics because they were in spectroscopy?
No, no, because this was an application of an interferometric method, you see. And, after all, it was still classical physics because you found the number of dispersion electrons which was something that H. A. Lorentz had done. There was no h constant; there was no quantum and so on, so it was legitimate.
Then at what point—this is a subtle point, but it's a very lovely one— at what point did it become an experiment in theoretical physics?
Essentially when h entered the formula. Well, in my thesis I wrote down an explanation of the Zeeman effect by Bohr magnetons and so on, the thing that one teaches now; and of course, that contained. It disappeared at the end, but it still had h in it. I think that once it had h, it not right, it was 'dirty.'
But it wasn't simply spectroscopy as such then; it was where the analytical elements entered?
Yes, it was spectroscopy done by people who knew s, p, d, and so on; we knew the angular momentum, we knew the vector model, and so on. I had read Hund's Quantentheorie der Linienspektren, or whatever it is.
As soon as you did the thesis, in '28,—
I went immediately to the Army. . . for 18 months six of which I was in Officers' Training School. Fermi came to visit me there. I think I was the only officer in the Italian army who was a super-snob. We were in Officers' Training School, and I don't know whether you can remember the barracks, the nolicing and so on. I had there Courant-Hilbert, Methoden der Mathematischen Physik, in German of course, and The Portrait of Dorian Grey of Oscar Wilde, in English. By that system everybody looked at me like a sort of funny guy. But the officers were quite smart and nice, and we had to point our guns, and to make our triangulation, to use artillery, and so on. Immediately, I mean it didn't take long, the general came for an inspection. By chance I had always to point the guns; (Laughter] they always called me. . . . But not on horseback, because I fell when the general arrived. [Laughter] I fell from the horse at the feet of the general.
This went on with you for a year and a half? Did everybody go through this?
Yes, well yes. No, but after six months I went back to Rome, you see. Since I came out at the top of the school, and they didn't have the limitation of the enrollment and that kind of thing. If you came out at the top of the school, you went where you wanted. So there was really effort to come out on top. So I came out on top of my course, or at least high enough, so that I could go to Rome, where I wanted to go. And in Rome I was, you know, in the Italian Army; all that I had to do was to go there, went to the barracks and took with me Polare Molekeln of Debye, the rest of my Courant-Hilbert, and a little more physics of various kinds. I read that, and I learned to play cards with the captain. I was there five or six hours a day, and then I went to the laboratory, in uniform.
What did you work on in the laboratory? What was going on in the laboratory?
Raman effect. Raman effect had been discovered and—
Was everybody doing Reman effect at that point?
I was doing Raman effect, d quite a few people had been doing effect. Then one day Raman showed up in person, and he came in on the birthday of the queen, by chance, so I was in high uniform, with a blue thing, dull gold, and all that. They phoned me at the barracks in a hurry; "Come here; there is a guy who doesn't speak Italian; he speaks English, and apparently he's Raman, but we're not sure. Rush down!" So I came down, and I arrived, and Raman was very touched that I had put on all my gold braid, and he—
He had showed up to see you at the barracks, had he?
No, no. Down at the physics department. He had come to see Rasetti, because Rasetti had been doing Raman effect in gases in Pasadena. And we were at that time pretty active in Raman effect, and so he was there. Fermi was away, I don't know. I was the only one. [We were doing] Rama effect and molecular spectroscopy at that time.
. . . What did you hope to learn by doing this sort of work in 1929 and'30? To what extent did one feel it was all already over?
On this there is very good documentary evidence. You should look because—this is written at the time—there is a speech by Corbino, pronounced by Corbino but in fact written certainly with intimate consultation with Fermi, on the status of physics, in 1931 or '32 it must be. This is in Acta della societa italiana al progresso della scienza which we have, and which I consulted recently. He says essentially that with the systematization of quantum mechanics, spectroscopy was finished, and that there were two outstanding problems: nuclear physics and big systems, probably living cells. This was the future.
You say this is perhaps '31 or even '32. . . . But at the time when you actually were still working on Raman effect and on molecular spectroscopy... when did one get the feeling that these were through? When did you think they were through?
In '31. Around '31 or '32.
Why? What did you hope to get from them earlier than that? In what sense was it not clear already that—what was clear in '31 that had. not been clear in '29? What made the difference?
In '27, '28, and '29 one didn't know whether quantum mechanics was true or whether it was a freak or what the hell it was.
Well, that may be right. That would be true in '27 perhaps almost everywhere; in '28 I don't think it would be still true.
Well, it depends how hard-headed you are, you see. . .One was very eager at this time to find phenomena which could be explained by the new quantum mechanics and not by the old. Now, Raman effect had, at least in appearance—now we know that it isn't true, that one can explain it also semi-classically—but at that time, for instance, I wrote a paper which at that time was considered a very good paper, pointing out that if two levels combined in Raman effect, the transition between them cannot appear optically. At that time, it was a mystery. I mean now stupidly, but it was a mystery, because one said, "Well, look; these frequencies are very strong in infra-red, and they don't appear in Raman effect; whereas these frequencies which don't appear in infra-red appear in Raman effect." Now of course [if] one considered it with the literal question of the quantum, one said 'the absorbed fraction of a quantum;' then if thissition is easy optically, why shouldn't you just absorb that? Well, then I wrote the formula for the Raman effect in quantum mechanics, and there are the matrix elements which contain q-ik q-kl Now, if these two are different from zero, each one, it means that the parity must change twice, and so between the initial and the final they have the same parity, and this makes them forbidden. Now this was a great discovery at that time; it was incidentally a discovery made in many places at the same time by many physicists, but I was one of those, and it was considered, well, you see—. This comes ct with quantum mechanics and doesn't come out if we go by the old quantum mechanics. . . . When people were persuaded, I don't know, but they were certainly persuaded by '31 or '32.
What about the sort of puzzle that hung around with Rasetti's work and other work on hyperfine structure d the fact that the parity didn't seem to be right?
Here there are two things. For instance, the hyperfine structure on which Fermi and I worked—we were quite interested in this because we thought that there might be a mystery, new forces not electromagnetic and so on. The question of Rasetti, the one on the parity of the number of particles contained in a nucleus, that was a puzzle. It was felt very strongly as a puzzle. This was also around 1930. Now, one also doesn't [always just] stop to do something [else]—you see, we were also experimental physicists; we were limited to the techniques we knew. And furthermore there were all kinds of elegant things that I enjoyed no end of doing—my forbidden lines, quadrupole radiation of forbidden lines. I liked that tremendously. It's true; it was already in the books, if you read the quantum mechanics right; but you still had to find it. After all, if you make a gyroscope, it's all written in Newtonian mechanics, F = ma, but still you like to see a gyroscope; it makes funny things. So there was a little of that character. And Ferri and Corbino—well, Fermi wrote this article and Corbino made the speech, and he was jumped on terrifically by all the physicists in Italy.
Oh, yes. So much so that—
Not also by others in the Fermi group?
No, no. By all the other ones. So then he had to write another article—it's in the library here, but I didn't go to dig it [out]—to try to smooth up the waters a little; but anyway the cat was out of the bag with the first one.
When he said that these fields were now finished, I take it he talked about fields that for most of the physicists in Italy had never existed.
Yes, that's even true, even true, yes. On the other hand, I must tellyou one thing: that at that time (Palucha) who was a physicist in Italy—he's old and retired now—[and] who was a violent opposer, made fun and persecuted all these new people. But he said that the real future was in solid-state and specifically in the semi-conductors, in copper oxide. We said, "That's dirty stuff; you shouldn't touch it." The cuprous oxide rectifier we used to make fun [of], because he said a cuprous oxide rectifier was a thing to be studied. He was more right than I knew.
You speak in your biography about this decision and about the arguments that went on about it. Tell me about those—the decision, the arguments, who was on what side, what sort of thing did they—
Fermi was for going to nuclear physics. The argument was essentially: 'We have spectroscopes, we know how to [do] spectroscopy; and [furthermore) how do we start doing nuclear physics if we don't know anything about it? Also, let's not exaggerate; spectroscopy's not finished. There are still lots of things to be found.'
Who said these things?
Me. And almost everybody, except Fermi. Well, and then there was back and forth: 'We have no radium, we have no radioactive sources; what do we do? We start here d what happens?' Really it was a mixture of practical things and—'we have to write a certain number of papers to become professors; you are a professor but I am not, and I can't writea paper on radioactivity [but] I can write another one in spectroscopy.' And things of this type, you see.
What is it about Fermi that accounts for his going so directly, at this point, into a field which so far as one knew at that point was pretty strictly an experimental field? Here he is the professor in Rome of theoretical physics [going into a field which] doesn't stay so, but which looks at that point [to be essentially experimental].
Fermi has never considered himself a theoretical physicist, really, oran experimental physicist. You know what he wanted. He always did this: When he was among the theoreticians he would say, "Well, I am an experimentalist, and my theoretical colleagues tell me [such and such]; can you explain?" And when he went to the experimentalists, he would say, "Well, I a theoretical physicist; I know nothing, but as a theoretical physicist I would say 'so'.” He always did that one. He was really both.
Yes. He certainly was. But it would certainly have looked, I think, to most people in '31—by '33 again, it's different—as though making the transition to nuclear physics is a transition to an experimental subject from a theoretical subject. Wouldn't it?
Well, he would have done 'experiments in theoretical physics,' as I was saying before. Also, you see, what he tried to tackle at first—it's quite clear—he tried to go to [the] nucleus. The first thing he did was to try to go by hyperfine structure, which was a link to something he knew. Then he tried to make gamma-ray spectra, gamma-ray spectroscopy, find nuclear levels—I mean these are published papers, but they're not important; they're not well known, except the one on the hyperfine structure. And then it was the discovery of artificial radioactivity wherehe said, "Well, now we're all equal, because on this one we have the source, and we start."
Now I take it that it was as a part of this transition that you went to Hamburg?
No. My going to Hamburg was before that, was in '29. I got a fellowship in '29; I went in '30.
I thought it was '31-'32 that you were in Hamburg.
'30-'31. . . . I'm not quite sure. I can look it up. [See below] But at the time I went to Hamburg, Rasetti went to Lise Meitner, Amaldi went to Debye; amd the purpose was that we all would go to a place where you learn a new experimental technique, and bring them all back.
Then it was really with an eye to nuclear work, wasn't it?
With an eye of enlarging our fields. We were even considering at a certain moment building a cyclotron. We had heard about—Stern had spoken about Lawrence. We knew that one had to learn vacuum technique; we couldn't make a vacuum of our own, between all of us put together. And so they said to me, "Go there; you have to make a molecular beam; you learn how to make a molecular beam; you learn how to make a vacuum." And. Rasetti went there to learn the nuclear techniques. Amaldi went there to learn X-rays, and burned his hand. And so then we would have more variety, more freedom.
What had been done to make it possible for all of you to hang around in Rome? How were you being paid?
Oh, those were Corbino's finaglings. Well, it's simple. Fermi was prfessor of theoretical physics. Rasetti was Corbino's Aiuto, which means first assistant. I was assistant to the chair of theoretical physics. Amaldi was still a student. Then Rasetti became professor of spectroscopy in Rome; they made another chair. Then I became the first—
Was this another chair? He did not replace Lo Surdo or something like that?
No, no. Lo Surdo was still there.Over the dead body of Lo Surdo.
I can believe that.
They just made another chair, you see; and Rasetti got it. And I became then the first assistant of Corbino, the Aiuto; and Amaldi took my place. Then I got a fellowship, Rasetti got another fellowship, and so on.
And it was the fellowship that took you to Hamburg, was it?
Yes. But first I went to Zeeman. You see, I got the fellowship in two steps. First, I made in Rome some work on quadrupole radiation. This work was quite nice. I liked it, but I wanted to do it with better resolving power. So I wrote to Paschen, I wrote to Back, I wrote to (Konen), and I wrote to Zeeman. 'Can any of you take me into his institute and give me a grating, and so on and so forth?' My father had the money; he would pay for the trip; there was no problem there. Everybody said—well, two said no. One was an s.o.b. and said, "Oh, I just had a Doctor and towhom I gave this work a few weeks ago." Zee said "come". And so I went to Zeeman, and I did this work there. And while I was there, I had applied for a Rockefeller fellowship, and then I told Zeeman "I have applied for this Rockefeller fellowship and nothing happens." And Zeeman said, "Oh, well, good that you told me. I'm going to write to Paris." And then I got it within a week, you see, because probably Zeeman wrote to Paris and said, "Yes, I have this man in my laboratory; he's an able person; give him the fellowship." Anyway the fellowship came.
You used it there at first and then went on to Hamburg?
I used it in Hamburg and I went back to Amsterdam quite a few times.
What was going on in Hamburg? Tell me about Stern and what was going on there when you were there.
Stern was measuring two things. Hamburg was organized so that everybody had a room and apparatus and was making an experiment. Stern would go around and would look pretty close at what was happening. And Stern himself was working with Frisch. They made two experiments: the magnetic moment of the proton, and the diffraction of helium atoms. Frisch is a lazy bum, but Stern kept him to work with an iron fist. He's very capable, Frisch, but lazy—just doesn't work by himself. Really Stern was a pretty iron-fisted man; he kept him working. And in both experiments two things happened. Then I myself was making this experiment on the turning of the space quantization. On all three it happened that the theory had to come from Rome.
Yes. Because I kept on writing, you see, back and forth, on the troubles and what was going on and what there was mysterious and so on. In my turning around, the theory was made by Majorana. There was a previous theory by Pauli. in Gottingen which was really not applicable because I changed the experiment. So the old theory wouldn't go and there was a singularity; a funny thing happened, and Majorana disentangled it. On the magnetic moment of the proton there was a diamagnetic term in the molecule the way Stern did it, and he understood vaguely what was wrong or what was going on, but he couldn't disentangle it. And then I came to Rome for a Christmas vacation or something. I told Fermi, and Fermi of course immediately said, "Oh, yes, that's the two-electron slip; the one turns this and the other turns this way, and you see they slip, they lose phase," and I don't know what. Then he told me, "You calculate it. You have understood what it is; now make a gadget. The molecule is too difficult, but you can make something which will show the property and so on." So that was the second. The third one: when Stern reflected the beams, he found that for a certain special velocity, the beam was just lost, disappeared; it was a very mysterious thing. And I saw this experiment and so on, and then I came across in the Annalen der Physik that somebody in England, or I don't know what, had found in optical gratings an anomaly due to the periodic surface propagation of a wave on a grating, but not by reflection but on surface waves—something akin to the old Sommerfeld theory of diffraction—. It's quite a difficult thing, but anyway the figure was clear; there were these intensities and then holes. So I told Fano—you know Ugo Fano who is now at the Bureau of Standards?
I know him just as a name.
He was taking his doctor's degree in Rome [I said] "I have a thesis for you. Show that this thing of Stern is this," and indeed it was, came out to be.
What about the magnetic moment of the proton? Was this result surprising to people?
Yes, yes, it was.
How did this surprise develop? How did people take it?
Everybody knew that the magnetic moment of the proton had to be one nuclear magneton; that was a foregone conclusion. [But it] turned out to be 2.7. At first people wouldn't believe it, so they tried all, the corrections, one thing and another. And then they resi ed to it. That's about all I know. And deuterium was discovered during that year. I remember that.
Were there discussions about the whole problem?
Why the neutron also must have been discovered around that time!
Yes. Well, you see this is '31-'32.
Yes, that's right. Oh, yes, it's '31-'32 then. . .[Segre checks his files] '31-'32, that's right. And I remember in colloquium Minkowski was there, you see. And I remember very clearly when this discovery of the neutron occurred, and of deuterium —pretty much about the same time. That paper would talk about it.
For that group w the neutron more or less an anticipated thing?
I don't think so.
Rutherford had expected it . . .
Rutherford may have expected it. I mean, people were chiefly spectrascopists, so they really hadn't thought very much about this neutron.
There is this story of Majorana saying of the early neutron exper that it wasn't—
That was as soon as he saw it, but he had seen the experiment. He had seen the recoil of protons. That's quite true.
But this group in Hamburg, were they skeptical?
No, I wouldn't say so.
It was just a very surprising thing?
Yes. . . . No, because you see they were in close connection with Lise Meitner, and immediately at that time Rasetti was with Lise Meitner; at least he had made recoils and so on. The thing was in the center of interest, and . . . people were convinced about it. I mean it was not a thing that took long to be convinced of, that there were neutrons.
What about the positron, which comes just after this?
All I can remember personally is that I was shown this picture of this thing, and it was explained to me; and I had a hell of a time in understanding why, turning around the whole argument, that the increase of ionization showed you the direction which it goes and the increase of curvature and all that. I was not familiar with all these things. I was a spectroscopist; I knew nothing that particles increase in specific ionization when they are going in one direction; and then they told me at the end, "This is the positive electron," and I said, "Well, OK, let it be."
This was the Anderson picture?
Yes. That was the only thing that was done yet.
Was this taken immediately seriously in Rome?
Because there were many, many places where it was not.
No, I would say it was taken seriously. They worried about it; they went through all the arguments and so on and they said, "Well, it looks like it." But at that time there were already people—I was not—but there were people very close to cosmic rays, you see. Rossi, for instance, was already quite active. And he probably had already seen the showers trigger coincidences in a line, and things of this type. But I was not very deep in cosmic rays; I was pretty much of a spectroscopist.
Were you worrying, for example, as Fermi must have been, about the problem of the electron in the nucleus?
I was taught about these problems, but I didn't worry about them. You see, I was taught that there is this difficulty, all right, more or less like a student who is told, "There is this difficulty; we don't know the answer."
Were you at the conference in Rome in '31?
That was sort of the inauguration of the Rome program in nuclear physics, wasn't it?
That's right. That's why it was laid down so; it w made for that purpose, to have an opportunity of talking to all these people and see them in their face.
Do you remember the discussions there?
Not much. Mostly it was stuff that I didn't know. You see, Madame Curiewould make big discussions on the embranchement du thorium, and I know what she was talking about.
Were you in, at all, on the background of Fermi's beta-ray theory was that the sort of thing that he did by him self?
No, I know pretty well—you see, Fermi had made the quantum theory of radiation, the paper in Reviews of Modern Physics. And he could never understand second quantization, these creation and destruction operators. He went crazy on these. He tried to explain it to us many times, and without much success, because he had difficulty himself, very much, to understand these things. And finally he said, "Now I understand the creation and destruction operators, and I want to make an exercise to really see that I know them." And then he wrote the theory of beta decay. It was pretty much—it appeared very much as an exercise to demonstrate that he really understood the creation and destruction operators.
Which somehow or other nobody had ever thought of in quite that way before anyway. Well, I suppose they had. Some people speak of the creation and destruction operators as being Fermi's own formulation of the theory, rather than as being what distinguished it from the earlier one. It's hard thing to say.
... There were no other theories of beta decay except Fermi's.
No, no, excuse me. But I mean there's the whole notion that what you had in quantum electrodynamics or in second quantization was the creation and destruction operators. That, in a sense, is Fermi's way of putting that. I think other people would not have spoken of creation and destruction operators.
I think that Jordan and Wigner and those people spoke of—
Well, they had the operations . . but I don't think they called them creation and destruction operators. I'm not sure whether that's original with Fermi; there's a lot of literature in this field, and I know very little of it. I have read, at least, of Wigner-Klein and the Jordan-Wigner and the Jordan-Klein work—
Well, he had read those papers, and really had a very, very hard time with them. And I bet you that if you'd look in his Pisa notebooks, if they're preserved, there might be pages full of that kind of stuff, because he just couldn't understand how it worked and what they did and so on. Finally, he said, "Now, I really understand them." And he made the theory of beta decay.
Do you have any idea why that gave him such particular difficulty? It isn't that it's so very much—
Creation and destruction operators—they give me difficulty right now.
But, you know, you can see these are simply transformations of the Schrodinger equation. Did you people read, the transformation theory papers?
Yes. Those are no trouble.
Well, you can get a lot of this on straight transformation theory.
Not quite, because they don't keep the number of particles constant, you see, and that was the great difficulty. Any way, those a and a* and so on were really black sheep in the thing.
Obviously a lot of people felt that way. I talked to Bloch a few days ago and he made one remark to me that I don't understand. He said, he'd spent some time with Fermi in Rome, and he said, "I liked him very much, I had a good time with him, but he didn't like to talk physics." Do you know what that was?
Why shouldn't he like to talk physics?
I don't know. I think he meant he did not like to talk about what he was thinking about himself in physics at the time. But I'm not sure, and nobody has—
What year was it?
I think that was '33 or '34.
What was Bloch doing? I remember when he was in Rome. I don't know what he was doing.
He was on his way out; I think he had a Rockefeller fellowship and was there in fall and then went to Stanford in the spring.
We were told to read the paper of Bloch of '28 on—I remember Fermi told me to read that paper, and I read it at that time. But I don't know; I don't know why he shouldn't like to talk physics. Unless maybe the same reason why Teller didn't like to talk physics with Fermi, namely that he started with something and before he knew it Fermi had already written it on the blackboard, finished it, and had done it better. You see, apparently this was disagreeable to many persons. Could be.
It could be so, though I'd expect that less with Bloch than with Teller. I don't know. [We'll have to] let it go. I may also have misunderstood the remark. I would like it if you'd be willing to talk some now about this question of Fermi and Bohr. . .This all came to me first in a remark that somebody made about Fermi's attitude toward Bohr later in life, but you indicated to me that it went back, you thought, very far.
Yes...I think that what must have happened is this. In—very early—1924 or so thing like this, Fermi wrote a paper which he considered as very important and one of the best things he ever did. Certainly he would have put it among his very best things. Indeed it is also a good thing. And then, shortly the hereafter in the Physikalische Zeitschrift or in something—
It's [24b of the Fermi bibliography, Uber die Theorie des Stosses zwischen Atomen und elektrisch geladenen Teilchen," Z. Physik, 29, (1924)] that paper, I think.
All right. You saw [then] what Bohr wrote. "Unter diesen Umständen kann es kaum als eine Stütze für die von Fermi benutzten Annahmen betrachtet werden, dass eine auf der Forderung der Energiebilanz gegründete Schätzung der Bremswirkung Resultate gibt, die angenähert mit den Versuchen übereinstimmen." [Z. f. Physik 34, 1925]. Actually, if you read the paper, he speaks even worse than that. And then he probably was very mad. I mean, if you think it was 1924, Fermi was just struggling to get somewhere, to get a fellowship, to get a professorship, to get something. And he got the big boss of all theoretical physics who says, "What you wrote is no good." Fermi was a man with a very long memeory, and I think this was it. Plus there is also an intellectual difference. Fermi was a man always, for many, many years, who liked to do concrete problems. He didn't like generalities, didn't like principles, and so on.
Do you remember whether he said anything as early as Como about this? . . . Como is the place where Bohr delivers his papers in which he's really got complementarity in it, duality. Were these distasteful to Fermi? How did he feel about this sort of discourse?
I don't know. I know that when he taught qu mechanics and so on,he always taught as little as possible of the uncertainty principle and all that. He mentioned it and so on, but he never put it as the—you see in his presentation of quantum mechanics, even in Chicago as late as '53, he starts always with de Broglie, the wave, and the Schrödingerequation and the analogy with optics. He may have done that for pedagogical reasons, but—
He may have done it partly for historic reasons; he learned it that way.
Well, maybe. Whatever it was, he did it so.
. . . You said to me when I raised this with you once before that Fermi had said to you at some point, I take it quite late in life, that he really was very uncertain that Bohr had the right answer. Do you remember that?
Yes. Well, I think that Fermi was—this I don't know, because I have no precise documentation, and he never said anything extremely precise to me—but I have the vaguee idea that he expected that sooner or later, quite certainly the formulae of quantum mechanics will stick, but that the interpretation may change in some way or another. He certainly didn't know how; otherwise he would have said it.
Did he worry about how? This was the sort of problem—
Did he think it would u e a difference if the interpretation changed?
Oh yes, yes, yes. And certainly in 1930 he tried to explain the uncertainty principle, complementarity, and so on, and to base it on very concrete examples. And there is a paper written by him which originated from a discussion which we had in the thematics seminar, in which he says he wants to explain really what is an observable, what is compatibility and what is not, and he makes an example which is very nice. You can find it in his papers. . .He asked himself this, "I want to know at time t with infinite precision the coordinates of a particle. This I can do. I don't want the momentum; I want the coordinates. What observable should I look at, at time zero?" And he says, "x - vt. This is an observable. I can measure it with infinite precision. If I find, that x - vt is equal to so and so, then I know at time t what will be the coordinates for a free particle." And he proves this by quantum chanics.