History Home | Book Catalog | International Catalog of Sources | Visual Archives | Contact Us

Oral History Transcript — Felix Bloch

This transcript may not be quoted, reproduced or redistributed in whole or in part by any means except with the written permission of the American Institute of Physics.

This transcript is based on a tape-recorded interview deposited at the Center for History of Physics of the American Institute of Physics. The AIP's interviews have generally been transcribed from tape, edited by the interviewer for clarity, and then further edited by the interviewee. If this interview is important to you, you should consult earlier versions of the transcript or listen to the original tape. For many interviews, the AIP retains substantial files with further information about the interviewee and the interview itself. Please contact us for information about accessing these materials.

Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event. Disclaimer: This transcript was scanned from a typescript, introducing occasional spelling errors. The original typescript is available.

Access form   |   Project support   |   How to cite   |   Print this page


See the catalog record for this interview and search for other interviews in our collection


Interview with Felix Bloch
by Charles Weiner at Varian Physics Building
Stanford University, California
August 15, 1968

Listen to Bloch on learning from war work

Oral history interviewee photo

Transcript

Weiner:

This is a tape-recorded interview with Professor Felix Bloch. We are sitting in his office in the Varian Physics Building at Stanford. We mentioned in our earlier discussion that in your interview with Tom Kuhn conducted in 1964, a good deal of your early background, your early work in Europe, your work on electron conduction generally, and your thesis work with Heisenberg had been covered. He had taken you through about 1933. There are a few general questions I would like to ask about that period. It seems to me that there was a great deal of mobility in your career, because you were able to hop around on one fellowship or another recommended by some of the leading physicists in Europe. And in the course of this you were exposed to different styles, different traditions of research. Now, on the surface it seems that this might be true—that they were different traditions—but in fact were there clear divisions?

Bloch:

No. You see, the physics of that time was very different from what it is now. There were only relatively few people, at least better known people. They were rather small in number. And it was really a family, and everybody knew everybody else; so therefore there was a great deal of unity. And although there were, of course, great individual differences, I think there was a common purpose all over Europe at that time. And I'm sure it's due to the fact that the number was so small that everybody knew everybody else personally—his family and so forth. It was a very close relationship, much closer than it is now with the great number of physicists.

Weiner:

Do you think this was due as well to the short distances between one center of research and another?

Bloch:

Well, of course, the distances were shorter than they are now, but, on the other hand, travel was not as simple as it is today. To go from Leipzig to Copenhagen took a full day at least. But it was nothing that people at that time considered as prohibitive. There was a good deal of visiting all the time.

Weiner:

This was the normal course then of immediate post-doctoral education?

Bloch:

Well, if you refer to my having worked at different places, I would say yes. This was what one hoped and expected to do after one's doctorate—to work at different places and get in contact with the different leading physicists. I considered this perhaps the most important time of my education as a scientist—this period of traveling in Europe to the various centers before I came here.

Weiner:

What did you have in mind to do when this was over?

Bloch:

Well, that was a question which one did not really too seriously consider. The hope was that eventually one would associate oneself with a university and join one of the universities. It was by no means clear where that university would be or which it would be. Nor could I say that it really concerned us very much. I think I can truthfully say that we were really so engrossed in our work that the question of where it would lead to materially was, in a certain sense, a secondary question. Of course you do remember or perhaps you know that I established myself at the so-called habilitation as a privat-docent, which is the first stage of the academic career, in Leipzig in 1932. So already before I left Europe I had some kind of a university association, but that would by no means mean that I would know where or when I would get a professorship.

Weiner:

You had first gone into engineering, and apparently you had some doubts or other people had doubts about careers in physics itself. Now, by the time you got involved in theoretical work, obviously this was not an issue anymore. You were doing physics and you were doing it happily.

Bloch:

That is correct, yes.

Weiner:

But did you have any general feeling about the opportunities to pursue this kind of work? That is, in regard to other countries—did you think the situation in Germany provided more opportunity or less?

Bloch:

Well, of course, it so happened that due to the presence of several of the leading physicists in Germany, Germany was probably, I would say— maybe next to England, though I'm not sure—the leading country in theoretical physics. Besides it was a large country with many universities, so a priori the chances of joining a university were probably greatest in Germany; but I cannot say that I had any fixed ideas about that. Switzerland, which is my home country, was certainly another possibility. Besides I must also say that the idea of eventually going to America was by no means a remote or fearful idea at all. I had discussed that often with my friends, some of whom, as you may know, went already to America quite early. So this was also another possibility. But, as I say, it was not a matter of prime concern really at that time. We had the confidence somehow or other, rightly or wrongly, that sooner or later we would find a position all right, and we were really in no hurry. We were also very young.

Weiner:

Good things come to good physicists.

Bloch:

Well, at least we hoped so.

Weiner:

In your discussion with friends about the remote possibility of coming to America, what was raised as the advantage or disadvantage of doing such a thing?

Bloch:

Well, there was, of course, a certain spirit of adventure behind it in any event—to see new parts of the world. I had this adventurous spirit quite a lot and several of my friends had it, too: There was also, of course, the fact that we knew that the possibilities, the openings, in America were more numerous than they were in Europe. And perhaps somewhere way back, although I can't say for sure, there was also perhaps the feeling (though I may be constructing things backwards) that it might not be a bad idea to go into a country where the tradition is not so highly developed and stand on one's own feet. I don't believe you should overestimate the importance of these remarks. It looks as if I had had prophetic foresight, which I did not have. I'm sure that the main idea was that, indeed, we were Europeans, and it seemed natural to stay in Europe; but, as I say, the idea of going to America was not a strange idea at all.

Weiner:

Well, you were at Leipzig; you had the beginnings of an academic career. But to get a professorship—although you weren't consciously worried about it at the time—you would have had to wait for some opening because there was generally not more than one theoretical physics professor at a university. But since you were in a circle involving the leading people, you didn't worry too much about this.

Bloch:

No, not too much.

Weiner:

In '32 you'd gone back to Leipzig, I guess, starting in the fall-

Bloch:

No, in the spring of '32 I went back to Leipzig.

Weiner:

So then you were beginning on the academic career there. Now, what happened to change things? Let's go back a bit. Were you aware of the changing political situation?

Bloch:

Oh, yes, indeed. One could not help being aware of that. Of course, Nazism was growing in Europe. It had already been growing for several years. The University of Leipzig was a particularly good point of observation because the students of the University were among the early enthusiasts for Hitler. So we noticed that very strongly. There was nothing violent about it, but it was quite clear that these ideas were gaining ground. It was quite obvious to me, in any event, that fearful changes were on the horizon. Nevertheless, I was happy at that time. I had a very close relationship to Heisenberg, and I was happy to do my first teaching at the University of Leipzig. I don't quite remember now, but I think it was already in '32 that I applied, or maybe Heisenberg applied for me, for a Rockefeller fellowship; and it was very clear to me at that time that this might come in very handy in the sense that my staying in Germany would be limited. I'm talking about '32, especially about the fall of '32, when it didn't take great foresight to see that things were coming to an end.

Weiner:

You say that the students were enthusiasts and were taking up with the Nazis. How did this manifest itself? How did you become aware of it?

Bloch:

Well, I had contact with students in places where I lived. I rented a room or two in a house with other students together and we talked about these things. There was nothing aggressive in their attitude at that time. But they felt that as an outsider, as a Swiss, as they considered me, I was entitled to some education; and they gave it freely to me. I did not hide the fact that I did not share their opinion, and they accepted that as one of the facts; but it was very clear that they were taken in by this. It, of course, was the time of the depression. And so the atmosphere was very awesome.

Weiner:

How about anti-Semitism? Was there evidence of this?

Bloch:

Well, it was a strange thing in Germany. You see, anti-Semitism was not a social phenomenon as it is in many other countries. It was almost more of a theoretical attitude. That is to say, I knew many Jews and Jewish families in Leipzig, and I don't hardly recall that they experienced violent anti-Semitism in the sense of beating or things like that, which occurred, as you may have known, in Hungary and Austria. That did not exist in Germany. But it was a dogmatic philosophical anti-Semitism, which at that time existed side by side with otherwise perfectly normal relations.

Weiner:

How did it affect one's university career?

Bloch:

At that time it was just starting. I don't think that my own habilitation—of course thanks to Heisenberg's support and authority— made any serious difficulties. But the first dismissals of Jewish professors started already at that time.

Weiner:

In '33?

Bloch:

In '32 even. In '33 they were practically all dismissed. In '32 there was the case of Lessing and other people. The official anti-Semitism made itself felt.

Weiner:

I was thinking of the official laws that came out, I guess in March, indicating who could and who could not be retained and so on.

Bloch:

I'm talking about preceding that. Of course, as soon as Hitler came to power in March '33, then the picture changed abruptly, and dismissals occurred right and left.

Weiner:

Did Heisenberg have any reaction to this? Did he discuss this with you?

Bloch:

Yes, of course, we discussed it a great deal. Probably Heisenberg felt perhaps that my fears were somewhat exaggerated, but it was not that he considered them groundless.

Weiner:

Perhaps then, as you suggested, the application for the Rockefeller fellowship, which had to be initiated by him-

Bloch:

Yes. I think it was initiated by him—not quite sure.

Weiner:

Well, someone has to do this according to their rules.

Bloch:

Yes. He may have done that, and also Debye may have been in on it.

Weiner:

By the way, I can check that because the listing of the Fellows shows who did the recommending*, It could have been either of them— that's right; they were from the same institution. Why did you pick Rome? *The listing of the Fellows shows who did the recommending only before October 1930, and does not cover Bloch's fellowship in 1933.

 

Bloch:

I've tried to remember the exact details. Well, I think it was partly again perhaps a spirit of adventure. I wanted to go to an environment quite different from the one I knew in the northern countries of Europe. I had met Fermi, of course, before and had great respect for him. I think it was also partly the cultural attraction: it was an interesting historical city. But I think probably the main reason was the presence of Fermi. I don't believe I would have gone to Rome without Fermi being there. You may recall that I split the Rockefeller fellowship into two parts—half of it to be spent in Rome and half of it to be spent in England in Cambridge.

Weiner:

But you didn't consummate the second half.

Bloch:

No, I went to this country.

Weiner:

Well, the reason I brought up the fellowship now is because we were talking about 1932, and that's when you made application apparently. When did it become clear to you that you no longer could remain at Leipzig? Were you dismissed?

Bloch:

No, I was not dismissed. I simply quit. I went home to my parents in Switzerland. I received, in fact, later, a letter from the dean of the University of Leipzig begging me to come back and teach my course, which I had announced, and rather ironically promising that I could get guards in my lecture room to prevent any troubles which might arise. That was rather ironical, because the only people who would have made trouble were the guards, because the few people who sat in my lectures were friends and they certainly wouldn't have made any trouble. I didn't even answer that letter.

Weiner:

When was all of this—in the spring?

Bloch:

Yes, that was in the spring of '33. You see, I left Leipzig in March of '33 and never went back, but this letter may have come—I don't know—in April or May. I'm sorry to say that Heisenberg, rather naively at that time, also felt that I should certainly return. He saw no particular reason why I couldn't go back. And I may say that from the spring until the fall of '33 I had no job. My Rockefeller fellowship started then, and so Heisenberg evidently felt: "Well, if he's free anyhow, why can he not at least give us a lecture while he is here?" I mean my very strong feelings about what was going on in Germany he apparently did not quite understand.

Weiner:

And so you actually physically left just after the Nazi laws came out at the beginning of March and you went home.

Bloch:

Right.

Weiner:

There's something interesting here, because according to the rules of the Rockefeller fellowship, if I remember correctly, you had to have a permanent position, a position to which you could return before you could go on their fellowship. I may be confusing it. There were two sources of funds. One was the Rockefeller Foundation and one was the International Education Board. It's really the same thing, but the rules were somewhat different.

Bloch:

It was in fact the International Education Board. Outside of America it was not called Rockefeller. It was called International Education Board.

Weiner:

But what you're saying is that perhaps because the application was made in 1932-

Bloch:

At the time that I made the application, I certainly had a position as Assistant and Privat-Docent both at the University of Leipzig. Now whether they afterwards considered this fact to be very serious or not, I do not know. As I say, I never officially resigned, nor did I get a letter of dismissal from the University of Leipzig. I simply left and that was it. But the people at the International Education Board undoubtedly—in fact, I'm quite sure—knew what was happening, and if they had such rules, I suppose they simply disregarded them. I think it was probably true that from a sheerly legal point of view I could have gone back to Leipzig. After all, I was not a German. As I say, I never went. I'm sure nobody at the International Education Board expected me to go back to Leipzig.

 

Weiner:

Who were you in touch with during this nearly six-month period before you went to Rome?

Bloch:

I spent part of it in Zurich, and of course in Zurich there were Pauli and Wentzel. Zurich was not a minor part of physics itself. In fact, I did some work there on quantum mechanics; as a matter of fact, I wrote a paper there during the time I was in Zurich. Then I was invited, to my very great pleasure, to give lectures at the Institut Poincare in Paris, where I spent two or three weeks. It was a very wonderful time. I stayed at the house of Langevin. I knew his daughter and his son-in-law quite well. I cannot say that I was at all unhappy, nor did I lack occupation during that summer. Then after that I went to Holland to visit Kramers again, with whom I had been before. And there again it was a fruitful period. We discussed physics and various ideas. And then I knew anyway that I was going to go to Rome in the fall, so the summer went by very interestingly. Of course there was also a great deal of political discussions about what was happening in Germany at that time.

 

Weiner:

Was there much feeling that this would blow over; that it couldn't last?

Bloch:

Well, I don't think too many people outside of Germany believed in the thousand years of Goebbels. But certainly I did not think that it was something that was going to disappear soon.

Weiner:

How about others in Europe in that position? Were you in touch with them during this period?

Bloch:

Physicists?

Weiner:

Yes, other physicists who were either dismissed or resigned or felt that their future in the German university system was limited. Of course you weren't in Germany?

Bloch:

I was not in Germany during that time? I don't recall whether any one of the German physicists visited at that time. Whether I saw them even in Zurich, I don't recall at this moment, though I remember at one point I saw Weyl. Weyl came to Zurich during the summer of '33 and told us about all that was happening in Germany. But of course we knew it anyway.

Weiner:

It's very interesting: Von Neumann was in Germany visiting: He spent half his time in Princeton and half his time in Europe in '33, and he wrote a letter on June 19th. He had just been through Berlin and through Gottingen, and he was writing it from Budapest. He told of the situation there, and then he gave a list of people who were in need of assistance, who were dismissed or who soon would be? And he used the term, "beurlaubungen," forced to leave, and your name was on the list. There were others—all of them prominent theoretical physicists. He listed them. He said, "Here's a list of leading theoretical physicists whose future in German universities will be impossible?" So apparently others knew of your plight, and he was sending this letter to let it be known so that in case positions came up—

Bloch:

Do you know to whom he directed that letter?

Weiner:

Yes, to Oswald Veblen, a Princeton mathematician.

Bloch:

I see, because when I received this offer to Stanford, I realized that it was known and in fact there was a list of scholars and I do not know if it was the same. Von Neumann circulated it at that time, and my name was on that list.

Weiner:

This list was from the Academic Assistance Council in England, was sent over to this country to an American group called the Emergency Committee in Aid of Displaced Foreign Scholars.

Bloch:

I see. That's very interesting. I didn't know that.

Weiner:

In two weeks I'll be working with their files in England. I can check it out and see if your name is on the list. [Bloch's name is on the November 1934, Academic Assistance Council list of "Displaced German Scholars Available for Academic Positions?"]

Bloch:

Well, I was often really wondering to whom I'm indebted for having received this offer to Stanford very early, because it was undoubtedly on the basis of this being known.

Weiner:

The way it had to be done according to the regulations was that the offer had to be made by the university, by the department; and then they could apply for funds to one or another of the groups to consummate the appointment. The point is that it wasn't a question of just the placement bureau as such.

Bloch:

No. They got the salary for the first two years.

Weiner:

Right. It had to be initiated locally.

Bloch:

Yes. But nevertheless the fact that they knew that I was away, that I must owe to somebody, I've sometimes thought it might be Oppenheimer, who knew me also and was at Berkeley. They had received my name from someplace. I thought maybe it was this list that was circulated.

Weiner:

It could very well have been. They were widely circulated. The reason I ask that is that in this interim period you knew you were going to Rome, and I would think that then you would wonder what would happen after Rome, or after Cambridge which would have been the other half of your fellowship.

Bloch:

Yes. Well, again, as I recall it, I was not too greatly concerned about that. I should say that I think it was still in the fall of '33 before I went to Rome that I received the offer to come to Stanford, and I must say that I was very happy about that. So it was certainly true that I would have to go someplace. I felt maybe at that time if I went to England, perhaps I could stay in England—at least for a while. But when the offer from Stanford came, and after some thinking, it was clear to me that that was what I should do. But I don't want to give any false impression by any chance that I was, for example, looking frantically for a job at that time. I was young, and for the next year I knew what was going to happen to me, and one did not look too much into the future.

 

Weiner:

Well, according to something—again maybe it's what you told Kuhn in the interview—you got the specific offer for Stanford when you were in Copenhagen.

Bloch:

Yes, that's right.

Weiner:

Now, was this after the first six months at Rome that you visited Copenhagen?

Bloch:

No, this was before I went to Rome. As I recall it may have been in August or September. Then I went to Rome.

Weiner:

Well, that's another stop that you didn't mention. You mentioned Paris and Holland.

Bloch:

And I also went to Copenhagen. That's right.

Weiner:

And what were you doing there that time?

Bloch:

Well, I often visited Copenhagen after I'd been there before in '31 and '32. I visited occasionally, and that was just one of the visits. I was completely free that summer, and so I went there to see my friends and see Niels Bohr. It just happened by the sheerest coincidence that the telegram arrived while I was there. I think the people here [at Stanford] probably didn't know that I had left Copenhagen and thought I was still there.

Weiner:

What was your reaction when you got the telegram? Was there any other thing before that? Was there a letter or anything feeling you out?

Bloch:

No, not at all. It was just a telegram signed by Webster, chairman of the department; and I must confess to my great shame that I didn't even know where Stanford was, and only the fact that the salary was mentioned in dollars made me suspect that it probably was in the United States. There's a rather amusing story there. Heisenberg was also in Copenhagen at that time, and I went to him and asked him. I knew he had been around the world, so I asked him whether he knew something about Stanford, and he said he only remembered it vaguely. He said, "It's somewhere on the west coast and nearby is another university, the name of which I've forgotten," and he told me, "They steal each other's axe." Now, you may not appreciate this, but this was a sort of a game with students. Before the big football game, Stanford has a symbol, an Indian axe, and the Berkeley team stole that. This incident was the only thing that Heisenberg remembered about Stanford. Also the name of Webster, I'm ashamed to say, didn't mean anything, either to me or to him.

But then I went to Niels Bohr, and Niels Bohr did indeed know the place, and he advised me. He said, "It's a very fine place." He advised me strongly to accept it.

 

Weiner:

Bohr was active at that time in trying to find what the situation was in Europe, sort of taking an inventory, of who was in need of jobs. He was one of the people who contributed names.

Bloch:

In this case it was most direct. Because I was in Copenhagen, I went with the telegram and showed it to him and said, "What do you think?" And then he told me about Stanford and said, "If I were you, I would take it." I not only took it, but I wrote to the Education Board and said, could I please go to Stanford in the spring of '34 rather than going the end of June. The offer for Stanford was, in fact, for the same fall of '33, but I did want to take my fellowship at least in Rome.

Weiner:

Did they approve that? In other words, did you come over here for the second semester of your fellowship as a fellow?

Bloch:

No. When I came here I simply relinquished the fellowship, with their approval.

Weiner:

You had to notify them.

Bloch:

oh, yes.

Weiner:

You mentioned that you had had some friends who came over earlier. Who were they?

Bloch:

Well, there was Wigner and Von Neumann particularly.

Weiner:

And you had known them in Europe.

Bloch:

Yes. But then I had also heard about America from Ehrenfest. He came to Ann Arbor quite often, and I knew him, too, and he was very enthusiastic about America; so I had heard about the American scene before from the Europeans. Also I had met, of course, American physicists in Europe before I came here.

Weiner:

Who were they?

Bloch:

Van Vleck, particularly, and Houston.

Weiner:

That is, people in your field.

Bloch:

Yes, of course. I met them as colleagues. Van Vleck I think I met in Holland first and Houston I believe in Leipzig. He was at that time with Sommerfeld. There were some others, too, but I think these are the most important.

Weiner:

Yes, Van Vleck went to that Solvay conference in 1930—the one on magnetism.

Bloch:

Yes, but I don't believe I met him at the Solvay Congress. I met him in Holland on one of his visits. He came to Holland quite frequently.

Weiner:

When Bohr told you about Stanford and you began putting an image together of the place, what physics did you associate with the institution?

Bloch:

Well, I must confess again to my shame that I did not know too much what was being done here. I learned, of course, later when I came here that they did very fine work in X-Ray physics, but I did not know that at that time. However, I must also say that I had met Oppenheimer before, and I knew that Oppenheimer was in Berkeley, and so therefore I knew that there would be at least some theoretical physicist with whom I could discuss my work. Although I did realize that I was going rather far away from the centers, the fact that I was not all by myself, that there was a fellow theorist in the neighborhood, certainly also had its attraction. But otherwise I must confess that if you ask me whether I imagined Stanford the way it was, I must frankly say no; I had no way of knowing what I would find.

Weiner:

You were adventurous.

Bloch:

Yes, I was adventurous.

Weiner:

And you came on Bohr's advice, too. Do you think that was the most important factor, that Bohr sort of verified that this was a reasonably good institution?

Bloch:

Well, I would say that at least unless I had had some assurance that it was a reasonable institution, I might have hesitated, although the idea itself of going to America—and particularly to California—was very appealing to me. But, yes, I would say that Bohr's recommendation, when he told me that this is a good place, decided me.

Weiner:

When you went to Fermi in Rome—Fermi who had had experience here—did you have any opportunity to discuss your future in this country?

Bloch:

I don't believe we talked much about that. Fermi, of course, knew that I was going to America and I just accepted that as a fact. I talked with Emilio Segre about it at that time; and he sensed already that things in Italy were not going to be too good either. In a certain sense he envied me for going to America.

Weiner:

Although at that time he had no particular plans.

Bloch:

No. He himself had no plans. That something that one finds perhaps difficult to understand nowadays, but of course at that time the inertia—the feeling that after all Segre himself felt—he belongs to Italy, and the feeling that people belonged to the country where they came from—was still quite strong in spite of everything.

Weiner:

I would understand that for Italy and for France and perhaps for England. Would you say this would be true for Germany?

Bloch:

Well, I would say of course it was made so absolutely clear particularly to the Jews who left Germany that they were not wanted there anymore, that they could not very well have any serious thoughts of coming back. On the other hand, I know very well that many of them, to put it very mildly, deeply regretted the fact that they had to leave Germany. They felt that there were their roots and that they were torn out of them, and I suppose some of them hoped that they would return. They hoped that this would blow over and that they would return at perhaps not too late a time. Now, I must say that for me Germany was out and I think I can safely say forever at that time. I would not have considered going back. But of course it was a different matter because I was not raised in Germany.

 

Weiner:

Well, then in Italy you worked. You described some of this work in the earlier interview when you told particularly of how your discussion with Fermi may have started him on the path to the neutrino.

Bloch:

It's possible. But that sounds much too good really. It was more a technicality. We talked at that time about what was called second quantization. I had just begun to understand it. It actually went back originally to Wigner and Jordan, I believe. But it was considered rather highbrow at that time. I made some application. I wrote a paper in Rome and made use of that. And I think it was more that Fermi, hearing me talk about this sort of thing and probably having also heard other people talk about it, felt perhaps that it might be time to get acquainted with it. I think the neutrino was almost an exercise for Fermi. Once he understood that he realized that the theory of the neutrino as he proposed it really essentially required the second quantization. So I don't want to say, by any means, that I suggested to Fermi the theory of the neutrino. It was only that I mentioned to him a certain tool that he was clearly not acquainted with at that time. He was rather reluctant at that time. He felt that it was probably a lot of highbrow nonsense, which one didn't need. But then in typical Fermi fashion, without saying anything, he went home and studied it.

 

Weiner:

And then once he knew, he might as well use it.

Bloch:

That's it exactly. Fermi, of course, once he understood something he really understood it. There was no such thing as half understanding.

Weiner:

Before we get you to the United States, I wanted to get back to this general business of having been in contact with Pauli, Heisenberg, Bohr and in a different sense with Kramers and then in the more recent period with Fermi-

Bloch:

Not in a different sense with Kramers. I spent half a year before that in Utrecht and worked very closely with him.

Weiner:

Kramers and then Fermi. You described how Bohr had given you a taste for something that really was not natural for you and that is the epistemological approach, which still isn't your own approach but helped you clarify your thinking on certain questions. It's very hard to identify elements of style and where these elements come from. But did you pick up anything else, do you think, that you can identify as clearly as that?

Bloch:

I don't quite understand your question. Do you mean in the sense of epistemology?

Weiner:

No. From Bohr you picked up a particular approach. Can you identify what you may have gained from Fermi or from Pauli and so forth? Now, the answer may be "no," that you can't identify it because it's intangible.

Bloch:

It is largely intangible, although on some questions, of course, it was quite direct. It was not that they told certain problems to me. Of course, I heard them talk about various things. I think I followed pretty much my own interests, which were at that time still in solid-state physics. But, of course, whenever I felt that there was something worth discussing I had discussions with these people, and very often of course got very valuable and important criticisms or even some suggestions about which way to work on them. These are things one cannot say. Even though they are not so easy to describe in exact words, the influence and the contact with these people was of enormous importance. Then, as you say, their style of thinking: the fact that one can approach physics from very different angles was a great revelation to me.

 

Weiner:

It makes one tolerant.

Bloch:

It's not so much a question of tolerance as a question of widening one's horizons. Especially as a young man and having worked in a very special field and being primarily subjected only to a very constant and very important contact with one physicist, namely Heisenberg. Nevertheless I realized as I went around that Heisenberg's particular style, if you want to call it that, was not the only one.

Weiner:

Well, all of this is at the age of 29 with all of this background. And then you did come to the United States after Rome? Did you leave directly from Rome?

Bloch:

I wasn't even 29: I was 28 when I came here.

Weiner:

It seems that we're talking about a whole life story and yet all of that was within this brief period of time.

Bloch:

Yes. These years between 1927, when I went to Leipzig, and '33. When I left Europe, were probably the most important ones in my entire development.

Weiner:

With that you came here. Did you go directly from Rome?

Bloch:

Yes. Well, I think I stopped once more and visited my parents in Zurich, but then I came directly to the United States.

Weiner:

I assume you docked in New York. Did you proceed directly to the west coast?

Bloch:

No, I think I spent a day or two in New York. Breit, whom I had known in Zurich before, was very kind and met me at the dock and showed me a bit of New York at that time. But, as far as I remember, I spent only a day or two in New York. I remember especially warm feelings toward Breit. When you arrive at the shores of a foreign country and you meet somebody whom you know and is friendly, this makes a great deal of difference.

Weiner:

Knowing his love for walking, I wonder if you walked all of Manhattan.

Bloch:

I also remember that I still did not feel very good because the boat had been shaking and I had this peculiar feeling of land sickness. I was rather depressed during those two days in New York, I remember, but that was partly fatigue.

Weiner:

Was it mostly social with him—I mean seeing the city? You didn't visit NYU or Columbia?

Bloch:

Certainly not Columbia. I don't quite recall at that point whether I visited his lab, whether he showed it to me or not. I certainly saw it at a later stage, but I'm not quite sure whether it was at that occasion. I think that it was not more than two days that I spent in New York.

Weiner:

And then you took the cross-country train?

Bloch:

That's right.

Weiner:

Directly to California?

Bloch:

Yes.

Weiner:

I can't help wanting to know what your impression was at seeing this country, 3000 miles worth, from a train window.

Bloch:

Well, of course, the enormity of it, especially at that time when you went by train, was I suppose the most impressive thing for me. Otherwise you don't see too much from a window of a train. But these endless plains and endless mountains and so on. I was not used to such large scales coming from Europe.

Weiner:

How about your command of English?

Bloch:

Oh, I think it was quite acceptable. In fact, I don't think it has improved that much since I am here. Once I knew enough English to get around, I don't think I improved—I learned English, of course, in school and had spoken it before. So that was not a difficulty.

Weiner:

And when you got here, whom did you check in with? Who took charge of you?

Bloch:

Well, I'm sure it was Dr. Webster, who was chairman of the department. I don't quite recall, but I believe he met me at the train station. And then the first days are simply lost in a fog in my memory. I suppose I was so busy absorbing these impressions one after the other that I don't have an orderly, clear recollection of my first few days. All I can say is that people were exceedingly friendly here. I had a wonderful feeling of being really wanted, which after what had happened in Europe was not a minor aspect. Obviously they were clearly glad to have me here, so that was a very fine start.

Weiner:

This was in the summer or the fall?

Bloch:

The spring of '34. I think it was early April. I started to teach immediately.

Weiner:

That's right. You explained that before. What kind of teaching responsibility did they give you at first?

Bloch:

Well, I was not told in any dictatorial way what I should do, but it was clear that I was to teach the regular theoretical courses, which I proceeded to do from then on. I was free to choose what I wanted. I think I taught over the whole gamut. There were very few students here, and so I taught one course after another—electrodynamics, mechanics, thermodynamics, quantum theory—everything.

Weiner:

What level was this?

Bloch:

I think it was probably mostly graduate. I didn't teach any other courses. I had a very small audience.

Weiner:

And who else then was doing this? Was anyone else doing this work?

Bloch:

Well, Oppenheimer was doing it in Berkeley, of course.

Weiner:

But I mean here.

Bloch:

Nobody else.

Weiner:

Were there any theoretical physicists on the staff?

Bloch:

No, I don't think so. I was the first here: Of course, I must say that Webster, particularly Webster, knew a great deal of theory, amazingly so according to my feeling compared to an experimentalist in Europe. The distinction, the division, between experimental and theoretical physics was much sharper than it was here. But it may have been, of course, partly because the professional theoretical physicist did not exist. I think Oppenheimer was the very first in California altogether. I don't think that existed before.

Weiner:

There was Tolman.

Bloch:

Well, Tolman also was an experimentalist. I don't think he was the pure theoretical physicist as it was bred in Europe and then came here. I think first was Oppenheimer, and I was probably the second here in California. So, therefore, the experimentalists here, by need or by desire, indeed were theoretically much better equipped than the experimentalists in Europe. This I found out. Nevertheless, of course, the refinements were not known here—particularly in quantum mechanics, which was really a new science at that time. Except in its rudiments, it was not known here. People knew the principle of it. I was the one who was to preach the gospel.

Weiner:

At Stanford, you mean.

Bloch:

At Stanford.

Weiner:

Certainly there were Van Vleck and Slater and others in the East.

Bloch:

I'm talking about Stanford. And I'm sure in Berkeley it was Oppenheimer, and then Oppenheimer went occasionally to Caltech. He more or less commuted.

Weiner:

This question of the lack of distinction between theoretical and experimental outlook within one physicist in this country as compared to the sharper divisions in Europe—was this a consequence of the training of physicists in Europe? Did an experimentalist get much theory in his work?

Bloch:

Well, he could get it. Of course, the whole system of education in Europe was much freer than it was here. You entered the University as a student and took any courses you damn pleased, or you didn't take them—nobody cared. And, of course, the experimentalists had the opportunity—and some of them availed themselves of it—of taking theoretical courses. But it was perfectly clear to anybody who aimed at being an experimentalist that that was something that he should not spend too much time on. He had to get into experimental work if he was to be an experimental physicist. On the other hand, the theorist sometimes never went to a laboratory.

Weiner:

He didn't have to. In other words, it was not required. That's what I'm trying to get.

Bloch:

Nothing was required. There were no requirements at all ever.

Weiner:

It would be interesting to compare and really do a good study of what was required in this country in the education of a physicist, because if he's forced to take that much theory, we know at least he's getting some background. It depends, of course, on who's teaching it.

Bloch:

Well, I mean the fact that there was no enforcement, I'm sure, helped this strong division. As you say, nobody forced him to do that. So therefore, once you decided, "I'm only interested in theory," well, that's what you did and simply didn't care about anything else. I remember it came in my case quite early in my student days. Quite abruptly at some point I simply decided: "No, I'm more interested in theory than experiment," and then I dropped experiment cold.

Weiner:

I guess it depends also on what stage physics is in, because there are some stages in the development of theory where you desperately need to be in very close touch with experiment, and there are other stages when it's not so important.

Bloch:

Well, of course, this sharp distinction, which I think was particularly outspoken in Germany, was not of such a very old date either. I would say that it started around 1900 at the earliest, because the people before that—Hertz, Helmholtz, people like that—certainly did both. I'm not even sure whether Planck did not still do experiments as a young man. He probably did. Einstein, of course, was among the first absolutely pure theorists. I would say that this tradition of sharp separation was not that old in Europe either. As you say, it does depend upon the development, and this is very much conditioned by regional conditions.

Weiner:

It also depends on the personalities of individuals at a specific institution. At Gottingen where you had Born and Franck together, then you'd expect there to be collaboration, because the two of them were interested in it.

Bloch:

Oh, collaboration existed. That did not mean that the experimentalists and theorists do not talk to one another. Oh, they did that. But each felt that he had one technique and the other had the other technique.

Weiner:

But in some institutions there was less talking than in others. Let's put it that way. At Munich there would be less than at Gottingen:

Bloch:

That is quite true, yes.

Weiner:

But as the work developed here—you said you went into teaching— how did that compare with your teaching experiences in Europe?

Bloch:

Well, it was quite different because in Europe I had only taught two courses, two semester courses in Leipzig. I started in the spring of '32 and left in the spring of '33. And these were highly specialized courses. I remember I taught one course in general relativity. So these were entirely specialized. The regular larger courses were given by Heisenberg. That is, in Germany the senior professors taught the more elementary courses, and the special courses were left to the young professor with an audience of two or three people or something like that. Well, when I came here that was a different story. Then I really had to teach what I considered the basis of theoretical physics. Besides, hardly any of my students had any intention of becoming theorists. I doubt that any became theoretical physicists. That was just part of their general training.

 

Weiner:

The ones here.

Bloch:

Yes.

Weiner:

It would be very hard then for you to compare preparation with the preparation of your students in Germany, since you were really teaching different kinds of courses.

Bloch:

Oh, entirely. I had a different caliber of students. As I say, in Germany in my very specialized courses I had only a few very advanced students, whereas here I had younger students and not very well trained?

Weiner:

What about the style of teaching? Were there significant differences in that?

Bloch:

Well, generally speaking, there was a great deal of difference. That is to say, the formality of teaching was far greater in Europe than it was here. Now, in my own special case the difference was not too great, because, as I said, the courses which I taught there were also taught before a small audience, and so it was more a matter of conversation; and that's how I conducted my courses here first, too. I knew all the students. I called them all by their first name, and the moment they didn't understand something they would ask and we would have discussion. But one can say, generally speaking, that I felt also that the larger courses were much less formal here than they were in Europe. There was much more close association between the teacher and the students than in Europe where the professor was on one plane and the student was on another plane, and very little did they meet.

 

Weiner:

I'd like to get back to teaching later. I was just asking now about first impressions. When did you first get in touch with other physicists beyond Stanford?

Bloch:

You mean here in America?

Weiner:

Yes.

Bloch:

Oh, I think quite soon. Well, of course, Oppenheimer I knew already, and we saw each other constantly.

Weiner:

Did you go to Berkeley?

Bloch:

Yes, we had joint seminars. Either I went to Berkeley or they came to Stanford, so we were very close at that time. Then I believe Oppenheimer took me for the first time (I don't know whether I went with him or alone) down to Caltech, and I met other people there. I think I met Millikan for the first time there. So these were my first contacts in California. And, again, I don't quite remember when I went to the first meetings in the East. It wasn't the same year but I'm sure it wasn't later than '35 or '36 that I met most of the then theoretical physicists and many of the experimental physicists of this country: Again, there were not very many. At a meeting of the American Physical Society there may have been 40, 50 people, of which I probably knew one-third already and the second third of them I knew very soon.

Weiner:

The first one you attended was the Washington meeting?

Bloch:

I don't remember, but I believe it was at the Bureau of Standards in Washington. I believe so.

Weiner:

When did the seminars get started—the colloquia, the meetings with Oppenheimer and his students? How long after you arrived did you get in contact with them?

Bloch:

Oh, very soon. You see, Oppenheimer was the only person on the west coast whom I knew from before. I don't remember. I'm almost sure it was not more than a few days after I arrived that I went either to Berkeley or that he came to see me here; so that was very direct.

Weiner:

I'd like the full story, if you recall, of the seminars themselves. I've heard so many other people mention it as being a feature of the scientific environment here, the joint discussions and the fact that they got pretty lively at times, and they often had debates between you and Oppenheimer.

Bloch:

Yes. Oh, yes.

Weiner:

On one evening from quantum electrodynamics to—I mean loads of things were mixed in.

Bloch:

Yes, yes.

Weiner:

I'd start at the logical point of how the idea came about for such joint seminars and then what form it took.

Bloch:

I don't remember. It was probably Oppenheimer who suggested that, but it was a very natural thing. Again, these seminars—there was nothing formal about it. Sometimes we met here or in Berkeley and sometimes we met someplace else-down in Carmel or anywhere. The attendance was half a dozen people, all of whom were known. So you shouldn't think that these seminars were in any way formalized. Whether we met actually regularly a day a week, I don't know, but I think more or less they were about every week. We ran them the way a seminar is run nowadays. One of us would go up and tell about something he had thought about and read about, and then there would be discussions. It was very stimulating for me. I did not feel quite as isolated as I would have felt otherwise.

Weiner:

When it was in Berkeley, did your students come along, some of them?

Bloch:

I believe some of them did, although, as say, there was none who was particularly interested in theory.

Weiner:

The basic group would have been the Berkeley group with you as an addition?

Bloch:

Practically, yes. Besides Oppenheimer, there were not many: I think Melba Phillips was there at that time, a few of the old-timers, you know.

Weiner:

Was Serber there?

Bloch:

Serber was not there yet, no. We're talking now about '34:

Weiner:

Already in '34. I see.

Bloch:

Oh, they started right away. We had our meetings already in the summer of '34. Then, of course, later on, the seminar grew more and more.

Weiner:

But at first there was this small nucleus...

Bloch:

Oh, it was quite informal and quite private.

Weiner:

No money involved? No funds required?

Bloch:

No, of course not. We paid our own gasoline and that's all. More than that you didn't need. In fact, gasoline was cheap, too, at that time.

Weiner:

What seemed to be the recurring theme? It's true you take up something that interests you, and he does, too, and others in the group may-

Bloch:

Well, I don't think we even particularly collaborated at that time. I remember at one point I gave a seminar in which I reported about the then new theory of superconductivity of Fritz London. Well, it happened that this was something that hadn't been thought about before, and I told it to the people there. I don't know how much impression it made. It was just this sort of thing, that whenever you thought you had understood something, you told it to the others and they would ask you questions. I don't quite remember what Oppenheimer did at that time. I think he was already interested in the theory of showers. His interest was definitely in the direction of cosmic rays and high-energy phenomena. I think that that's what he worked on. Now I knew about it, but I did not work in that field at all.

 

Weiner:

But you would participate.

Bloch:

Oh, yes, of course.

Weiner:

In the discussion.

Bloch:

Yes.

Weiner:

It was a theoretical seminar, though. Were there any experimentalists involved?

Bloch:

No, not in that particular instance. That was pure theory.

Weiner:

I know that at Berkeley they had a Monday evening journal club, which did bring together experimentalists and the theoreticians. Did you have anything of that type here?

Bloch:

Yes. We had a weekly journal club here too. But that was, I would say, on a much more elementary level. This seminar with Oppenheimer was really a high-level discussion and quite technical. Here the colloquium was aimed at a broader group. It was called Journal Club, as it is now.

Weiner:

Was there any parallel to that in Europe?

Bloch:

Oh, yes. Colloquia in Europe was very important.

Weiner:

I mean in the same sense as the Journal Club?

Bloch:

Well, I don't really quite know what the difference between Journal Club and colloquium is. Journal Club meant, of course, that people should tell what they have read, what they read in the journals and tell it to others; but this of course happened often in Europe, too. That is, if somebody talked to a colloquium, he would either talk about his own work or he would talk about what he read, and I think the same was true here.

Weiner:

I see. Because sometimes it's confusing: you think there's a whole new social institution being developed when in fact it's the same thing with a different name.

Bloch:

It's the same thing: We renamed our Journal Club, "Colloquium" some ten or 15 years ago. We somehow felt it was more appropriate.

Weiner:

The seminar started off with just a handful of people and grew, you said, in later years. Apparently you mean up until the time of the war. About how large did it get?

Bloch:

Oh, I would say the last seminars had at least 20 people. This was still not very large, not by today's standards. But I would say there were probably about 20 people. I mean at the early seminars it was a matter of course that after a seminar we would go to a restaurant and eat together. One would just ask for a not-too-small table and that was it. Later on when Oppenheimer occasionally took the seminar as a whole, that was a big affair.

Weiner:

He treated them generously?

Bloch:

He treated them very generously. He was a well-to-do man. But then it required a large table.

Weiner:

This was in San Francisco?

Bloch:

Yes.

Weiner:

I think Serber in his remarks at the memorial ceremony for Oppenheimer recalled that.

Bloch:

Yes, we went to San Francisco. We went to a fish place down in the harbor. Oppenheimer had all kinds of exotic restaurants that he knew and liked to go to.

Weiner:

Getting back now to Stanford, when did the work on the cyclotron here start and how did you become involved?

Bloch:

Now that's much, much later. You see, in my previous discussion I still had more or less the time '34-'35 in mind. By the way, I was also in Europe in between several times. If you want to go to the cyclotron, you're making a jump of at least five years.

Weiner:

I don't want to do that? I want to go chronologically. I asked you about how you began to get in touch with other people. We talked about contacts with Berkeley, and we talked a little bit about Pasadena, and then we talked about going east for the meeting. What about other people from Europe coming here, having come to the U. S: either to live or to visit.

Bloch:

Well, I certainly remember later that we did receive visitors. When we got our first visitor from Europe here I don't quite recall. I recall, but that was much later, a visit from Niels Bohr here in 1939, but that stands out in my memory because it was Niels Bohr. I'm sure we had other visitors here before. I think I would like to tell you a little bit now about the years between '34 and approximately '39, because it was a rather important development for myself. In the first place, of course, I immediately became interested, because I was in this environment, in quite different problems than I had worked on before. My first paper, in fact, here was written on X-rays. It had something to do with the work, which was done here, because people discussed things and I felt I had the theoretical answer to that. Then, at the same time, and very shortly afterwards, I started to do experimental work myself. Now, this, I'm sure, is a development which would not, or hardly that early, have happened to me in Europe. I told you, and, in fact, thought it was a good thing that the distinction between experiment and theory was not as sharp here as it was in Europe.

Of course, in Europe, Fermi was the only outstanding, shining example of a man who did both experiment and theory, and I remember one of these casual remarks that Fermi once in a while made. He said to me in Rome once: "You know, you should sometimes do some experiments: It's really a lot of fun." That little remark of Fermi's stuck somehow in the back of my mind. When I came to this country then I realized that I wanted to do experiments: Let me put it a little bit clearer. I was here from '34 until the summer of '35 practically without interruption. Then in '35 I went to the summer school in Ann Arbor, where Fermi was again, and from there went to Europe really to visit my parents and, in fact, didn't come back until the winter of '35. I had the summer and then the first quarter of the fall, so I had a half a year free.

 

Weiner:

Why did you have it free?

Bloch:

Well, the summer was free anyway, and the people just were generous here and said: "You don't have to teach the first quarter." Things were quite informal here. So I spent half a year away. Then when I was in Europe, I thought of course what one should do in physics, what was the interesting field; and neutron physics was the sort of thing that interested me at that time. Fermi had just started his important work. So when I came back I approached one of my colleagues, Bradbury, who is now director of Los Alamos, and I suggested to him that we should build a neutron source. We had no other equipment except X-ray equipment, which provided 200,000 volts: Now, 200,000 volts was a marginal but possible case in which one could produce neutrons by accelerating deuterons rather than electrons. So at least we had the voltage for that. And I suggested at that point when I came back that we should try to produce some neutrons, do something with neutrons. I thought that was a new and interesting field. But before we could do that, we had to build an ion source to produce ionized deuterium. Bradbury was an expert in gas discharges; he had studied with Loeb. So I said to him: "Let's get together and build an ion source." I had never, except in my early student days when I did some elementary physics lab, had any experimental experience. But it was a great experience then to work with Bradbury and do some of the really low-brow work, soldering and whatever is necessary; and so I did acquire at that point—and I must say with great joy—the elements of experimental physics. And we did build a neutron source. We installed our source, our atom source, which eventually was built in one of the X-ray tubes, and by very primitive means convinced ourselves that we indeed saw some neutrons. Now, of course, that wasn't a great achievement, but nevertheless it was very exhilarating.

Now, that had a good deal of influence on my work. During that time, there were several things which I did. One of them turned out rather important later on, and that was during a visit of Nordsieck, Arnold Nordsieck. I don't know whether the name means anything to you.

 

Weiner:

Yes. You published some things with him.

Bloch:

He went to Leipzig after I left, and then from Leipzig he came here. And we worked together at that time on a rather strange difficulty of electrodynamics and solved it. There is a still famous paper of Nordsieck's and mine, which we wrote at that time. At the same time I was doing the neutron work. I began to think what one should do with neutrons when one had them. Then I had also rather an important idea at that point, and that was that one could produce polarized neutrons: I don't know whether you know what that means.

Weiner:

Yes, this is '36, your paper in '36.

Bloch:

I wrote a paper at that time; you're quite right. In fact, the Nordsieck paper and the neutron paper were my most important papers at that time. They came practically at the same time.

Weiner:

I think I have reference to that. Had he come as a staff member here?

Bloch:

No. He came with a fellowship—I don't remember which fellowship.

Weiner:

That paper came later with Nordsieck, in 1937.

Bloch:

'37, right.

Weiner:

And your paper on polarization was in 1936?

Bloch:

Yes. That came right when I came back. When I came back from Europe in early '36 I wrote this paper, and also I started to think about the other problem. I wrote this paper on neutron polarization, and we started to work together—Nordsieck and I—on that other problem. He came here and said: "This is a scandal. There is this problem, and one should be able to solve it." So he convinced me, and we worked together on that. Now, about visits, I would like to say the following: It so happened that there was money for a visitor at Stanford during the summer. The summer was free, and nevertheless there was some money available. So I suggested to the people that they should use that money to invite some rather outstanding people to spend the summer here, knowing that it is a very nice place. The first man whom I brought here was Gamow. Gamow was here the summer of '36 and was of course a very stimulating, interesting man who brought a breath of fresh air into it. We were very close; we saw each other all the time.

Then the next visitor was Fermi, who was here the summer of '37. He came from Ann Arbor upon my invitation. He spent, I think, about four weeks here: And of course that was marvelous. I think I got more out of Fermi than I ever got in Rome, because he was here for no other purpose than talking, and we went together to the Sierras and to the coast and so on.

 

Weiner:

Was his wife with him on that trip?

Bloch:

No. It was at that time that I sensed that Fermi was moving away from Fascism and Italy. Although I have no proof of that, I'm pretty sure that at that time it was pretty well decided that he was not going to stay.

Weiner:

I recall somewhere the story about the Burma Shave signs. I wonder where I heard that.

Bloch:

Probably in Mrs. Fermi's book someplace. Yes, Atoms in the Family. Yes, she was quite shocked about that.

Weiner:

I don't recall how that went. I remember vaguely.

Bloch:

Fermi was here in the summer of '37. He went back to New York; I went also to New York but via the Canal. Then we met in New York, Fermi and I, and we traveled together on an Italian boat to Naples. Then Mrs. Fermi met him in Naples, and I proceeded on my own to Rome. But then we drove from Rome to Florence together. I drove with the Fermis. And there were these signs which said "Mussolini is always right" and "Believe, Obey, Fight" and all these slogans, you see. And Fermi read them and always said, afterwards: "Burma Shave" because he had seen these signs. It was clear that his respect for the regime was gone by that time. But to Mrs. Fermi it was a surprise. I'm sure this happened to him during the time he spent in America. Well, there was the Abyssinian war, which Fermi already didn't like, and he really moved away from it then.

Weiner:

Then racial laws came out in Italy, too, about that time.

Bloch:

Yes, they began to come out. Strangely, although Fermi was not of Jewish descent whereas Mrs. Fermi was, he was more sensitive to that than she was.

Weiner:

There are a couple of things that you touched on. I want to get back to your main line of research. But before we do, you opened up some interesting things that I'd like to get to because they're chronologically appropriate. You went in '35 and spent a good many months in Europe?

Bloch:

First I was in Ann Arbor, Michigan.

Weiner:

That was for the summer school.

Bloch:

The summer school. I think I went to Europe for about four months.

Weiner:

I first wanted to ask about the summer school, if you have any special impressions of that. I know the atmosphere generally was very informal.

Bloch:

Oh, that was a great institution. That was a gathering place for the best people—partly from Europe, partly from here. It must have been due to the presence of Uhlenbeck in Ann Arbor that they had arranged that mostly. And they had people coming there. Fermi, of course, was always a mainstay of interest, and Segre was there again, and we were there together at some time. I think Willis Lamb was there. I'm not quite sure. But there were many people there. They were most interesting.

Weiner:

Was there anything like that in Europe?

Bloch:

No, not that I recall except of course for the meetings in Copenhagen, yes. The Cophenhagen meetings were much the same, the ones that Niels Bohr convened. Then there were usually certain special topics that were discussed at that time. In fact, the Copenhagen meetings probably historically were much more important that the Michigan meetings, but they were similar.

Weiner:

But the Michigan meeting lasted longer generally because it was a summer session, six weeks or something.

Bloch:

Yes, four weeks, six weeks, something like that.

Weiner:

And from there you proceeded to Europe. Where did you go?

Bloch:

Well, are you speaking now about that first visit?

Weiner:

That's right.

Bloch:

In '35. Oh, as far as I remember, I went directly to Switzerland.

Weiner:

To see your family.

Bloch:

Yes. I spent that time in Switzerland.

Weiner:

You didn't go to Germany at all?

Bloch:

No, no. I may have visited Austria. I probably did because my grandmother lived there.

Weiner:

You mentioned, though, that during this period you became more and more interested in neutron work, which seemed to be occupying many people at that time—for good reason. How did that come about? Did it come about through conversations with individuals in Europe?

Bloch:

Yes, I believe so. I have the distinct notion that when I came back from Europe I had it formulated in my mind that that was the thing to do. But I cannot pinpoint it now, just who it was with whom I talked. Wait a minute. In 1935 I was in Copenhagen. Yes, now I remember. For the 50th anniversary of Niels Bohr we went to Copenhagen. There was a meeting. That was during that summer. At that time, probably, it was mainly through Bohr. I'm sure that this idea of neutron work originated on that occasion.

Weiner:

Do you recall anything about that 50th anniversary celebration?

Bloch:

Well, not in detail. I mean it was one of these wonderfully friendly things. We met at Niels Bohr's Institute. There were many people there. I saw my old friends in Copenhagen. Do you mean from the point of view of physics?

Weiner:

Yes.

Bloch:

I'm not quite sure whether I heard there for the first time about, for example, the experiment of Cockcroft and Walton. I'm sorry—I should have studied the date.

Weiner:

That '32. Artificial radioactivity came in 1934.

Bloch:

I'm sorry; I can't recall in detail what felt—

Weiner:

Your main contacts then were family and then Copenhagen.

Bloch:

Well, then, I'm sure I saw Pauli. Yes, I know I saw Pauli in Zurich, too. He was in Zurich. So I did meet some of the physicists.

Weiner:

Did you get any impression of change in the general social-political situation compared to the year earlier, more of a sense of despair and desperation?

Bloch:

Surely the awareness of the seriousness of what was happening in Germany had more deeply penetrated. You see, the problem is that when I came back to Europe I was, so to say, a different person myself. That is, I came from America. I had already spent a year and a half in America, and so probably when I saw things in a different light it may partly have been my own reaction to them, and it may partly have been objective. I can't say that with too great certainty.

Weiner:

But it was sort of a catching-your-breath period after...

Bloch:

I don't think I had the feeling in '35 that this was going to probably be my last visit to Europe. It wasn't either. But by my second visit I was pretty sure that was just about the end. Of course, on the other hand, I do remember I think I came back with the feeling this was the last. This was not going to last much longer—the sort of normal way as you met it outside of Germany. Pauli was great in trying to imagine that these were all not such important events that life would go on as usual.

Weiner:

When you came back and started on the neutron work did you have your old theme of magnetism in mind? What were you trying to produce neutrons for?

Bloch:

Well, you see, the idea of neutron polarization was done by magnetic means. But the reason you can polarize neutrons is because it was assumed and pretty well experimentally supported that neutrons had a magnetic moment, and, in fact, the polarization was done by passing them through magnetized iron. So my early work on ferromagnetism was directly linked with this. In fact, I might not have had this idea of polarizing neutrons that way if I had not known so much about ferromagnetism from my earlier work. So there was a strange interplay between my solid-state work and my neutron work.

Weiner:

That's interesting. What I meant, though, was when you decided to produce neutrons, it wasn't just for the sake of producing them; it was for this...

Bloch:

No. At that point I just simply wanted to produce neutrons.

Weiner:

You wanted to make neutrons.

Bloch:

First make neutrons and then once one has a neutron source, then one can see. Well, I took pretty much the attitude that people took here towards X-rays. I mean you have to have X-rays. You wanted to do X-ray work, so you had to have X-rays. Then you were able to do various things with X-rays. No, I was not too clear about it at that point. I mean it was clear it was a big field. There was much to do. I was pretty sure that once we had neutrons we would do something with them. Now, it so happened that this was a rather original idea—on neutron polarization. I was very pleased. I felt that the justification for starting with neutrons would come later. Incidentally, that experiment to polarize neutrons was never done here. Our neutron source was never worth much. To me it served only as a stimulus to ideas. It was much too weak.

Weiner:

It was done by the people at Columbia and other places.

Bloch:

Right.

Weiner:

I'll talk to you later about the so-called Dunning effect.

Bloch:

I don't know whether you want me to fill up this gap now.

Weiner:

I do want you to fill it up, but II also wanted to know how the polarization ideas came about and how you developed it. It's one thing to say what the ideas are about and what form they finally took. But did it come in stages?

Bloch:

Well, you know, the exact circumstances on how an idea occurs is never too clear. I may say that the idea that the neutron may have a magnetic moment was in my mind and was quite old. In fact, I had already mentioned it to Heisenberg in Leipzig, and Heisenberg thought it was something worth pursuing. In fact, I had a very erroneous argument at that time, which made me think that the sign of the neutron moment would be opposite to that of the proton. But then, of course, experiments were done by Stern when he was still in Hamburg which showed that both the proton and the deuteron had a magnetic moment, and from that, one could conclude with some certainty, since the deuteron consisted clearly of a proton and neutron, that the neutron would have a magnetic moment. So that pleased me because I had sort of dreamt before that it might have one. Now, I am sure that we did not start neutrons here because of this idea of the magnetic moment of the neutron, which I had before. But once we started to do that, then I started to think that maybe we could do something more about this interesting aspect of neutrons here—the magnetic moment. And then at that point I think somehow this idea occurred.

Weiner:

You suggested how it could be.

Bloch:

Oh, yes, I wrote a paper on it.

Weiner:

The 1936 paper.

Bloch:

Right.

Weiner:

Although the work itself wasn't done. In the course of writing the paper were you at the same time trying to work out your prediction?

Bloch:

Yes, I was hoping we could do it here. But I don't know whether we had already our ion source functioning at that time. We were way behind. Probably if I had known that we could do the experiment the next week, I might have just done the experiment and then published the paper.

Weiner:

Published the theory and the experiment.

Bloch:

Yes. But realizing that the experiments were far in the remote future here, I published it anyway. And I wasn't sorry that somebody else did it—not at all.

Weiner:

What was the reaction to the paper? Were you in contact with anyone?

Bloch:

Oh, yes. Well, it was noticed at that time. In fact, Schwinger quoted it in his very first paper. As a very young man he wrote a commentary to my paper pointing out that there was something which was not quite correct in my paper, and he was right.

Weiner:

Was this the one he did with Rarita?

Bloch:

No. That was Schwinger alone. It was his first paper. He was 18 years old at that time. He noticed that the way I had treated interaction between neutrons and magnetism, the magnetic shell, was not quite correct, so he did it in a different manner and came to a slightly different result. He was very proud of it and rightly so because he was very young.

Weiner:

Did he get in touch with you personally?

Bloch:

Yes, of course and with Rabi. I spoke to Rabi. I knew Rabi also by that time and saw a great deal of him in New York. That paper was well known. It may have been Rabi or Schwinger or somebody who went to Dunning and said: "Why don't you try it? You have a neutron source." I don't know. As I say, physicists knew each other? The group was very small, and everybody knew what everybody else was thinking.

Weiner:

Once this paper was published, once you had completed the paper, was it clear then that your next piece of work would be to follow this up and to try first of all to do it and then to...?

Bloch:

Yes. I don't quite recall the chronology. Yes, I'm sure. I told that to Bradbury. I said, "By the way, once we get our neutrons going I have a nice experiment we could do with them." He was of course delighted. And that was clearly our intent. But then Dunning did it before. But then during the visit of Fermi in 1937, I had another idea. In fact, it was practically simultaneous with Rabi's idea. I'm afraid it's getting too technical.

 

Weiner:

Don't worry.

Bloch:

You see; the polarization of neutrons demonstrated that indeed free neutrons had a magnetic moment and approximately gave how big it is. But then I felt that one should make an exact measurement of the neutron moment. And that idea occurred to me either shortly before or during the time when Fermi was here in '37. I sort of remember that I mentioned it to him orally. That was a much more complicated arrangement that had to do with polarizing and analyzing a neutron beam with a constant magnetic field and a radio frequency field in between. It is closely related to the technique that Rabi then used for molecular beams, and in fact Rabi wrote his first paper in 1937 pointing out that a combination of a constant magnetic field and an oscillating magnetic field could be used to measure magnetic moments of nuclei. That's what he was interested in, you see. And I had this same idea independently, but I didn't publish it.

Weiner:

For neutrons?

Bloch:

For neutrons. So at that time Fermi knew certainly that I was out then not anymore for the scattering experiments, which I believe by that time had been done anyway—the polarization experiments had been done—but for exact measurement. And that, as you remember, I did later in 1939 with Alvarez: It was clear that our source was not sufficient. We tried first with our artificial source, then tried with a radium source, which was also still too weak. That brings me eventually to the cyclotron. But let me just finish with the summer visitors because you asked me. They were quite a regular feature. So in 1937, as I told you, I went to Europe with Fermi. Then I stayed there until the spring of '38.

 

Weiner:

Again one of these long trips. What did you do then in that whole period in Europe?

Bloch:

What did I do? I'm not sure that I was very productive at that time.

Weiner:

In '37 of course there was the paper with Nordsieck that was done here.

Bloch:

That was already finished. One thing I do remember I did in '37— I went to the Galvani Conference in Bologna. There was a big meeting. The King of Italy was there and so on. I met lots of people there. I saw Heisenberg also there for the last time. It was long before the war, and I tried to persuade him to leave Germany. It seemed at that time quite possible that he might. He unfortunately decided not to do it anyway.

Weiner:

Did he give the basis of his reluctance?

Bloch:

Yes, he had difficulties with the Nazis at that time, partly because of his former Jewish collaborators, and he was very angry at them. He needed a little push. But I think Debye was really the one who persuaded him that he should not leave, although Debye himself then left. Anyway these are matters, which I don't want to go into too much. Well, that's one thing I remember I did at this time—visit this conference in Bologna. Otherwise, I don't remember what I did in detail. I think I was mostly at home with my parents and went to the colloquia at the university in Zurich.

Weiner:

Pauli was right there.

Bloch:

Pauli was there all the time. I saw him a great deal.

Weiner:

It's misleading then when one looks at your biography and your resume to assume the European contacts ceased, because here you were very close to Pauli in '35 and '37.

Bloch:

Yes.

Weiner:

Do you recall, though, what work you were doing then?

Bloch:

Frankly I don't. I think I must have been in some kind of transition period. I must have been still thinking of this neutron work, because I remember that shortly afterwards when I came back we started to do the experiments here—the measurement of the neutron. That was in '38. During that summer Rabi was here. I mentioned to you Gamow first, then Fermi, and in '38 Rabi was here. Then, of course, we talked a great deal about this kind of thing. In fact, we had that experiment going and Rabi came every morning and saw how this was going.

Weiner:

Was this the Alvarez experiment?

Bloch:

No, no, we did it here. We had a radium source and did it with very primitive means and hoped that we could do it and found out that it wasn't possible.

Weiner:

These summer visitors—was there any formal summer school established?

Bloch:

No, they gave some lectures. I don't know what Fermi talked about. Rabi talked about molecular beams. But then people came from Berkeley. In fact, I should say that these sessions here became rather well known. Willis Lamb spent several summers here. He came here for this occasion when Fermi was here and when Rabi was here and so forth:

Weiner:

Didn't Weisskopf come?

Bloch:

Yes, I wanted to say that. Weisskopf came in '39. Weisskopf was also here as a visitor before, but we invited Weisskopf in '39, I believe, and then Bethe: That was in '40. But we had a little summer school, so to say, going on, although it was always centered around one single visitor.

Weiner:

That clarifies it, because people have mentioned to me: "Well, I went out there for the Stanford summer school." And I didn't know what this meant. They were using the same kind of term that was used at Michigan, but they didn't mean the same thing.

Bloch:

It wasn't a real summer school. Gradually it grew as more people came, but it was always centered around the one man whom we invited. It was a godsend. The people here didn't care much about that, but when I found there was money available for inviting a man for the summer, I said, "Let's do it."

Weiner:

What about other people from Europe who were in need of positions? I know that Marcel Schein came out once.

Bloch:

Yes. In fact, I drove with him through the country once, and he was looking for some sort of job at that time. Then another man I brought here (he was also a student of Heisenberg's) was Arnold Siegert. He is now at Northwestern. He's quite well known, especially for his work in statistics. He came here as an assistant. That was all we could offer him, and he was very happy to come here as an assistant. He was a bachelor; had the minimum salary. That was the same time that Nordsieck was here. That must have been '36 that he came. Then, of course, gradually I did meet other people who came over here. As I said, Weisskopf. There was Gamow. I'd count Gamow among these people. He came from Russia—fled Russia. Gamow spent the summer here in '36.

 

Weiner:

He came in '34 for the Michigan summer school and then came out to Berkeley because he thought Lawrence could get him something. I'm just wondering if it was on that trip. That would have been '34.

Bloch:

No, that was before. He was here later?

Weiner:

He had already become established at Washington.

Bloch:

Yes, he was already at Washington.

Weiner:

I talked with him in April. One other point on this before we get back to the main line of research and the new thing, the cyclotron. I came across this letter that you had written where apparently you were concerned about the deteriorating situation in Europe for Jewish scientists especially. This is a copy to Tolman. He then sent this copy to Millikan, asking for Millikan's help. The date is '38.

Bloch:

It's strange—I had entirely forgotten that letter. I wonder; did Millikan do something about this?

Weiner:

Yes. This is another project I've been tracing.

Bloch:

Do you have the answer from him?

Weiner:

The answer to you? No, I have Tolman's covering letter, where he says: "I'm sending you herewith a copy of Schrodinger's letter," which I have here. He said, "I'm also enclosing a copy of a letter from Bloch of Stanford which is self-explanatory. He asks me in the letter to approach you with regard to sponsoring some action as to the persecution of Jews in Germany. But I see no reason why I should not do this best by showing you Bloch's actual letter." Now, I don't know what role this played in it, but later that year Millikan was involved with a number of American university presidents.

Bloch:

Was that handwritten?

Weiner:

Apparently it was, because this is a typed copy: The only reason he would copy it would be if the original was handwritten. Then this would be easier to read, assuming that Millikan is very busy and so forth. Then there's also a copy of the letter from Schrodinger, which later on I'll check...

Bloch:

To Millikan?

Weiner:

To Tolman, a very interesting letter about what was going on in Europe.

Bloch:

That was written from England?

Weiner:

Yes, from Oxford. Anyway, so apparently you were involved in that. You don't recall the letter, but does it stimulate any other recollections of what prompted you to do this?

Bloch:

No, I'm ashamed to say I don't remember details. I remember my concern about what was happening in Europe. I dreamt about it, thought about it.

Weiner:

Of course you had been back in '37, so you had even more chance to see.

Bloch:

Oh, yes. The impending doom was clear. And I think when I left in '38 then I had still visited once more my grandmother in Vienna. It was just shortly before the Nazis took over. I remember I left Vienna with great depression, trying to tell my relatives there to take anything they can and leave, and they did not want to. They left eventually. This letter is earlier. I see, November '38. Of course, that's right. I came back from Europe in the spring of '38. At that time Austria was already overrun, too.

Weiner:

Things had really reached big proportions. It wasn't a question of a few people, but of thousands. Getting back to the work underway here, you mentioned that when Rabi came out in '38 he was watching with interest the development of the experiment. How did that proceed from that point?

 

Bloch:

Well, then I realized, and it may well have been Rabi's advice, that our source would never do, and that the right thing would be to do it on the Berkeley cyclotron. So I think it was in the fall of '38 that I spoke to Lawrence, and he was very kind and understanding and also felt this would be an important experiment. I don't know whether he asked Alvarez but Alvarez said he would like to do it with me. So that started my collaboration with Alvarez. I think it started in the fall of '38.

Weiner:

And this was with the 37-inch cyclotron.

Bloch:

That was with the 37-inch cyclotron in Berkeley. And then I commuted, practically, between Stanford and Berkeley. Once in a while I spent a night up at Segre's in Berkeley, but usually I came back and gave my lectures. But then I did not pursue the experimental work here anymore. I worked with Alvarez.

Weiner:

In a sense it was very fortunate, because he was probably the only one around who had a neutron beam.

Bloch:

Oh, surely, of course. That was practically the only working cyclotron in the world. It didn't work too well either. Once in a while it worked.

Weiner:

What about the work on it? Of course, there must have been interruptions because of the fact it didn't work, and it was used for other purposes.

Bloch:

Oh, yes. There were frequent interruptions. In fact, once we had our apparatus built, I remember for weeks and weeks what we did mostly was sit around waiting until the cyclotron beam was on, which sometimes happened once a day. Sometimes it didn't happen for three days. Sometimes it lasted five minutes, and sometimes it lasted three hours. Whenever it was ready we had to work?

Weiner:

I talked with Alvarez, you know, and got some of the background. He explained that you'd have to wait when the machine was being used for cancer treatments.

Bloch:

Yes. Besides the machine wasn't really working very well at that time.

Weiner:

We also have the notebooks, by the way. He gave us the notebooks of the experiments, which we have preserved in our archives. There was a photograph on the cover of you sleeping on... I don't know what. We looked carefully at the photograph, and it looks as if it's on a boat.

Bloch:

It might have been the ferryboat between Oakland and San Francisco.

Weiner:

It could be. There's supposed to be something symbolic about the picture. I never asked him why that particular picture. And then the data in the book is in your hand and in his as well.

Bloch:

Yes. So that went on through all of the spring of '39 and then also in the summer of '39. That's when we finally finished the experiment and got our results.

Weiner:

Did somebody named Laslett work with you, Jackson Laslett?

Bloch:

Yes. Oh, there were a lot of people there. I don't think Jackson was particularly close to our experiment. He was one of the people. I mean there was a whole crew running the cyclotron.

Weiner:

Was this your first experience with such large-scale research facilities?

Bloch:

Yes.

Weiner:

Well, your experience was limited anyway.

Bloch:

Yes, of course. Yes, it was. It wouldn't be considered large-scale nowadays, but at that time it was, yes. Lawrence was most generous. I think he gave us as much time as could be made available at that time. He really felt that this was one of the really good experiments to be done with the cyclotron. There were not too many at that time.

Weiner:

Did you think that, given a working machine and given access to it, it would have taken just a short time to do? Did you think that this was a relatively simple matter?

Bloch:

Well, I don't know. I probably underestimated the difficulties when we started, but then gradually it was clear that it was not so simple. This is always true in research. I think very few people can make a proper forecast of how long it will take; because you never know what difficulties you will run into. But then we had, of course, always our little encouragements and discouragements and so on.

Weiner:

In the course of this, Alvarez mentions the attempt to find the effect that Dunning and others had reported. It had been reported in three places, based on your 1936 paper, and yet you had difficulty in finding it.

Bloch:

First of all, we had difficulties in finding it at all, really. We didn't quite realize, but we suspected that the iron had to be very thoroughly magnetized before the effect would show up at all. The first thing was, as you say, and as Luis remembered correctly, to simply reestablish the Dunning effect, so to say. That is, to find that there is indeed an effect upon the transmission of neutrons by the magnetization of iron. Then once we had that, I think that from then on it was not so hard anymore. But we had to do that first; that's right.

Weiner:

You were convinced there was, because you were the one who had written about it in the first place. But apparently you weren't convinced that Dunning had seen it.

Bloch:

I was not quite sure, and I'm not quite sure to this day whether he had seen it. I'm sure he thought he had seen it. It was marginal, and in our case it was marginal, but it was clear enough that finally we were convinced that there was an effect.

Weiner:

And then you could proceed.

Bloch:

And then we could proceed.

Weiner:

So that added to the time involved in the experiment.

Bloch:

And then, of course, after that experiment was finished in Berkeley, then I realized that there was more to be done along those same lines; and I could not keep on commuting to Berkeley back and forth. Therefore, we then decided to build our own cyclotron.

Weiner:

You mean the pressure for the cyclotron here, at least from your point of view, was because you had a specific research need in mind.

Bloch:

Of course. Definitely.

Weiner:

It's quite different than how it developed in other places.

Bloch:

Quite definitely. Yes, yes. I spoke to Lawrence about it, and he said, "Yes, I think you should have your own cyclotron. We'll give you all the help you need."

Weiner:

Was there anybody else here who felt that way about it?

Bloch:

Yes, particularly Staub. He is a Swiss. He came here in '38. He was at Caltech and then came here, and he and I were very close collaborators. We built that cyclotron together. That was one of the first things we did when he came here. He's also mentioned in Mrs. Fermi's book.

Weiner:

Did he come as a refugee?

Bloch:

No. He was a Swiss and not a Jew. He came here on some sort of an exchange fellowship or so, spent a year in Caltech at Pasadena, and probably even thought at that time he would go back. But then I told him: "Why don't you come here?" and so he stayed here until 1949.

Weiner:

Yes, there's a great picture—I don't know whether you're on it or not; I think you may be—at Los Alamos with Bethe and Staub and perhaps you and Weisskopf on top of a mountain skiing.* <I>(*Picture deposited in the Archives of the Center for History and Philosophy of Physics by Emilio Segre shows the following group skiing at Los Alamos in 1943: Segre, Fermi, Bethe, Staub, Weisskopf, Mrs. Staub and Mrs. Segre thermodynamics.)</I>

 

Bloch:

It could be. I didn't stay in Los Alamos very long.

Weiner:

This was very early:

Bloch:

It could have been. Yes, Staub and I went up there together.

Weiner:

So you built it together. First of all, how does one go about convincing the university that they need a cyclotron?

Bloch:

Oh, well, the only thing is to get the money. Now, in that respect I had the help of Rabi—I think it was in the fall of '39—who took me to see Dr. Weaver, who later was president of the Rockefeller Foundation. Rabi knew him. I'm sure it was due to Rabi's support that they gave me $4000. At that time the Rockefeller Foundation almost entirely sponsored medical work, but nevertheless, they gave me that. So then I went back to Stanford and saw the president and said: "I want to build a cyclotron." He said: "Who will raise the money?" I said, "I've just raised $4000, but I don't think it will do." We made a very close estimate and we thought it would run over $5000. He said, "I'm sorry—I don't have any money." This was Ray Lyman Wilbur. So I said to him: "Do you think I should start anyhow?" And sort of with a twinkle in his eye he said: "Well, if I were you, I would start anyway." Then somehow we financed it. We didn't get a real gift from the University, but we got a little bit of money here and a little bit there. We scraped together the last thousand dollars. There was no need to convince the University. You could do anything you wanted as long as it didn't cost any money.

 

Weiner:

What argument did you use with the Rockefeller Foundation? Did you use the medical argument at all?

Bloch:

No, no.

Weiner:

You could Berkeley used it.

Bloch:

No. Rabi knew exactly what it was about, and Rabi was very much interested in this experiment. No, we told them exactly the truth. That's what it was for. And I had done the experiment already before, so it was clear that there was something in it.

 

Weiner:

But you don't build a cyclotron just for one experiment or one series of experiments.

Bloch:

Well, it was not one experiment. There were ideas. In fact, they came up later and were also used later for using the magnetic scattering altogether for determining structure in iron. This is work which is done at Oak Ridge even now. Of course, they have much better neutron sources. It was not this one experiment. I considered that more or less finished except that I thought it could be done better. It was clear there was a whole new field.

Weiner:

But basically you would say that this cyclotron was designed as a producer of neutron sources. This is the thing you had in mind.

Bloch:

As a producer of neutron sources for the purposes of magnetic study of them. I was quite specific. The Rockefeller people knew that.

Weiner:

Did that have anything to do with any special design considerations?

Bloch:

Not particularly, no. We just wanted, for the money which we had, to get the strongest neutron source we could, and it was practically a copy of one of the earlier cyclotrons.

Weiner:

How did Lawrence help you on it?

Bloch:

Well, he helped, of course, with advice. He also let us have one of their old tanks. It didn't help us much because it leaked so very much we had to build our own anyhow: He did whatever he could, but I would say the most important thing was the moral encouragement. In material things, although he meant well, the things we got from him were not too excellent.

Weiner:

You had your own magnets.

Bloch:

We had to build them, build them with our own hands. We bought steel, the cheapest kind of steel.

Weiner:

I didn't know that you were involved in that day-to-day work.

Bloch:

Oh, very much so.

Weiner:

And who else?

Bloch:

Staub and Stevens, who is now at Pennsylvania.

Weiner:

Was he a student at the time?

Bloch:

No, he was an instructor. I think we had occasional help from students also.

Weiner:

What was its size?

Bloch:

Well, it was about 20 inch or so. We accelerated deuterons to about 2 million volts. It was the simplest design we could find. It was air-cooled. That was an innovation because our water was full of chlorine and it would corrode the cooling pipes, so we had an air-cooling system:

Weiner:

I'd like to find out when you started construction. You told me about getting the money, but it wasn't clear as to the date.

Bloch:

In '39, late '39.

Weiner:

I see. And then you started construction right away.

Bloch:

Pretty soon, yes. I suppose we started then designing, started to buy material, and so forth. It took a long time: It was finished, I believe, in early '41.

Weiner:

So you didn't have much time to use it once it was finished.

Bloch:

No, not much. [Pause in recording]

 

Weiner:

We're resuming now after a break for lunch and a rest. When we left off, we were talking about the work with the cyclotron, which you said just got started before the war. We agreed that it would be good to stop at that point, and then to pick it up and to see if there was anything else we missed in the late '30s and early '40s, and if not, then to take the next step.

Bloch:

Well, I just wanted to tell you there was some work we could do with the cyclotron before we started war work. Is that what you wanted?

Weiner:

Yes, I wanted to find out what happened once it was working.

Bloch:

Well, the first thing we did was to very greatly improve the polarization effect: You recall I told you this morning that Alvarez and I had great difficulties in the beginning reestablishing even what Dunning had done, the very small effect. But we worked on that anyhow. Well, when our cyclotron was functioning in '41, the first thing we did was to go back and investigate what it was that caused these rather poor polarization effects. And we found, although there was some theoretical guidance on that that one had to push the saturation of the iron very, very close to completeness. At that point then we built special magnets for that, and then we achieved effects of the order of 20% instead of one or two percent: That was a systematic investigation we did, so that we knew from then on one could get much higher polarization effects? But then the Manhattan Project got underway, and I was in this early group with Bethe and Teller and Oppenheimer. Oppenheimer called me up one day and told me I should come and see him in Berkeley. I realized then that things had completely changed, and we were told then the first problems about the atom bomb.

 

Weiner:

Before that, how did you receive the news about the discovery of fission?

Bloch:

Oh, that was very interesting. Again, I got a telephone call from Oppenheimer. Wait a minute. The news of fission I think I read in Naturwissenschaften. You mean Hahn's paper?

Weiner:

Hahn and then subsequently Frisch and Meitner.

Bloch:

That I'm not quite sure whether I saw, but I saw my first evidence in Berkeley. Then I got a telephone call from Oppenheimer that I should come up and see something. Gentner was there at that time from Germany, and they had gotten in emulsions these beautiful recoils of fission fragments simply by neutron irradiation.

Weiner:

Did they call them splinters then?

Bloch:

I don't remember what they were called. Anyhow that was the first time I actually saw the fission process with my eyes.

Weiner:

This would have been in '39.

Bloch:

That was in '39 or perhaps in early '40.

Weiner:

Had you attended any of the Washington theoretical meetings, the ones that Gamow and Teller were involved in?

Bloch:

Yes, I was in Washington, too, at some of the meetings. That was earlier, though. That was not on the atom bomb.

Weiner:

No, they were the ones at George Washington University and Carnegie Institution.

Bloch:

I'm pretty sure that I was at least at one of them, yes.

Weiner:

But you apparently were not at the one, I guess, in the early part of '39 where this was discussed?

Bloch:

No, I wasn't. But to come back to that now, Bethe was here in the summer of '40. Then he came back in the summer of '41. And it was Bethe and Teller and someone else and myself. We learned then about this so-called Manhattan work going on, and we were all asked to join it and do whatever we could. And it was assigned to us here at Stanford to determine the energy distribution of the neutrons emitted in the fission process, which was a rather important question for the chain reaction. This is what we did. The laboratory — down in a light well under a glass roof—was closed. It was done under strict security regulations.

Weiner:

By this time you were a citizen?

Bloch:

I became a citizen in '39, yes.

Weiner:

You got married then.

Bloch:

I got married in '40.

Weiner:

And your wife was a physicist.

Bloch:

Yes, that's right. She was doing research at MIT before we married.

Weiner:

Had she come from Germany, too?

Bloch:

Yes. In fact, she's from Gottingen. She got her doctor's with Victor Moritz Goldschmidt. He was a geophysicist.

Weiner:

When was this?

Bloch:

She got her degree in '38, shortly before Goldschmidt left Germany. He went then to Norway.

Weiner:

And then she left at about this time?

Bloch:

Then she left at that time. Then she was also in Copenhagen, and then she was in Geneva, but we didn't meet at that time. We met over here.

Weiner:

You were on a trip to MIT when you met her?

Bloch:

I met her actually at the house of friends of ours in New York.

Weiner:

Well, getting back to the beginning of the work on campus, the war work.

Bloch:

We investigated that. Already at that time we had shipped some rather strategic materials—very, very pure graphite blocks which we needed. Some of them were used for the plant in Chicago. Then we got the first enriched sample of Uranium 235 from Berkeley, very rare, too, but we needed that. Well, we finished that work, and we got some quite interesting results out of it. It turned out that the fission spectrum extended rather to higher energies than one thought. It was pretty well the Maxwell distribution, but extended to something like 5 or 6 million volts, whereas people felt perhaps 2 million volts would be the results. So I think it was not without significance. But of course that was classified and was not published. Already before that was finished, the Los Alamos project got underway. We were asked to go there. I must say I had some hesitations before going there. I wasn't sure, first of all, what I would do there and whether I could really live in this military atmosphere. But nevertheless I felt it was my duty at least to try. And Staub went with me, too.

 

Weiner:

Who was the one that recruited you? Was it Oppenheimer?

Bloch:

It was Oppenheimer. So then I did go to Los Alamos and started early. I found then that the nuclear physics that was going on there, special theoretical work, really didn't interest me. I didn't fit in there. That was Bethe's group, and he had his people compute it. I found that it was not what I wanted to do. I got interested in the implosion, in Neddermeyer's group: Neddermeyer initiated this idea.

Weiner:

This is the Neddermeyer who was at Caltech earlier?

Bloch:

Seth Neddermeyer, right. And he had worked, I think, at the Naval Ordnance Laboratory and knew about what explosives could do, and had done already preliminary experiments on implosion. I did some work on that, both theoretical and experimental, to show—people were not convinced of it; I must say I had very little doubt about it—that the velocities and pressures which one would get were indeed those which Neddermeyer said, and everyone said. But they needed experimental proof, and I did that. Then I thought that my usefulness at Los Alamos was over. Besides I just could not live under this atmosphere. It was a military atmosphere. Letters were opened and one was under constant surveillance and so forth. Maybe that's a rationalization. I came to Los Alamos in the summer of '43 and left in November 1943. The only reason I joined the project, like many of us, was the fear that the Germans might develop it and would be ahead of us. I had no real evidence for that. But I felt that this was not very likely to happen. If it had happened, I probably would have felt very badly, but it did not happen. So I left, somewhat to the annoyance of some of my friends, in particular Oppenheimer.

I left Los Alamos and joined then a group at Harvard at the Radio Research Laboratory, which worked on defense against radar. The Germans started to develop their radar, and our job was to find out what one could do against it. This work was almost entirely theoretical. I did some experiments with microwaves to find out the reflectivity of certain materials. It was pretty straightforward work and nothing really terribly profound. I don't know whether it was of great use. Van Vleck was the head of that group—an old friend of mine. In fact, after the war, we wrote a paper together on the radiation of antennae.

 

Weiner:

By the time you got there it was well organized, though. Quite different from Los Alamos.

Bloch:

Oh, quite. It was well organized. We lived in the city, and we had a civilian life. I must say it was a great relief to me. I found it much more congenial there.

Weiner:

You were a newlywed, too.

Bloch:

Oh, well, no. We were three years married.

Weiner:

Had your wife gone to Los Alamos, too?

Bloch:

Oh, yes, with our firstborn children. My wife didn't mind Los Alamos. But of course since I was unhappy, she was perfectly glad to leave. It was a good time I had in Cambridge. And actually didn't realize that what I did there was going to be much more useful for my post-war work than what I had done and learned in Los Alamos. Of course, in 1945, in early '45, the end of the war was very much in sight. At that time I began to think about what one could do after the war. And Bill Hansen, whom I mentioned to you this morning, came quite often to Cambridge. He gave lectures at the Radiation Laboratory. He was a great expert. He was, of course, one of the early developers of the klystron and a great expert on microwaves. I also talked with Rabi quite a lot at that time. The Rabis lived only two blocks from us in Cambridge, so we saw each other quite often. I told him I would like to go back to the neutron work. I knew we could polarize iron—this much I knew already. And there was one problem that I felt. I think what limited our approach to a great extent was simply the measurement of the magnetic field. One had to measure the magnetic field and then one had to measure frequency. That's all. The frequency was easy to measure, but the magnetic field is not so easy to measure, and Rabi knew this well from his molecular beam work; and we had all sorts of ideas that perhaps we could ship a permanent magnet to Columbia for calibration with molecular beams and then it would be shipped back to us. It sounded extravagant, as one would never know what happened to the magnet in shipment. At that time war work was not so urgent anymore—early '45. I began to think whether one could not do it in some other way: And then I found out that one doesn't really need molecular beams to study the nuclear magnetic resonances—Rabi did only molecular beams—but that one should be able to do it in condensed matter, not in a vacuum, in liquids in fact. I thought almost exclusively of water from the beginning.

So I had this idea then of what I called nuclear magnetic induction.

In fact, now the term nuclear magnetic resonance is applied almost exclusively to this field, because it has become practically much more important. But strictly speaking, I invented the phrase nuclear induction because I wanted to state that nuclear magnetic resonance was not our invention. That had been done by Rabi and his people before. But the fact that the signal was received by induction, in the Faraday sense induction in a coil, is why I called it nuclear induction. However, the word didn't stick? It is now used in a very special sense. I called my first paper "Nuclear Induction."

 

Weiner:

You were consistent.

Bloch:

mp3 It is, of course, a technical point. Anyway, so I was all ready: I did my work mostly in the evening, my calculations, convincing myself that this should at least be possible; that the size of the signals were big enough. At that point I had learned something about radio techniques, and I knew what a receiver and an antenna was, and I knew what noise was. That was all-important in our game. That was of great importance to the work I was planning then. That's why I said before that I learned more, for me, useful things at the Radio Research Laboratory than in Los Alamos, because I knew very little about radio techniques before I joined this Harvard laboratory. I may interject that that is probably a rather common experience. People felt that through their war work they came in contact with other fields of science, which they had not been acquainted with before. And I believe at least part of this sudden quick development of physics after the war was due to this phenomenon. It was certainly eminently true in my case, but I'm quite sure other people had the same experience. They had worked in too narrow a field, and now they were forced to go into other things, which they thought they would never use. They did so only because the war required it, and later it turned out it was very important.

Weiner:

I know that's true in the case of radio frequency techniques, because at the stage of development of nuclear physics, in order to produce higher energies you needed these kinds of techniques. I know that came out of it, but I don't know too many other instances. In your case the microwave technology again tied in. But I wonder if it was true for other aspects of war work. I know people learned something about nuclear reactions—that's true.

Bloch:

Well, you see, you mustn't forget, of course, in each field there were experts who may not have learned much, but people who were taken in from other groups who did not know anything about it and then learned it— I'm quite sure that there were many people in Los Alamos who knew very little about nuclear physics altogether, let alone fission, and learned it there. And then, of course, the technology and metallurgy of uranium; these were all things that people didn't know about and learned. How terribly important they were in the long run, I cannot say: But I think it started a general stirring up of interest that I think was very beneficial.

Weiner:

Would you say that this also involved people? In other words, you were speaking of coming in touch with new fields, but you also came in touch with people.

Bloch:

With new people. That's certainly true. Now, in my case this was not particularly true, because of course my association with Bill Hansen was very old. In fact, the early work on the klystron—I was in on that and knew all about it. And we met each other frequently. He was down in Garden City with the Sperry Company. They worked on klystron problems at that time. But he came often to MIT and we met. I remember very well how we walked over the Charles River to a restaurant to have lunch and I told Bill that this was something I wanted to do when I came back to Stanford. We knew it would come pretty soon. He was very much interested and promised his collaboration. Even though I knew something about radio techniques, I needed somebody, and Bill Hansen was a great expert, and he immediately thought of ways of how that could be put into practice.

He also told me on the same occasion that he also had thought about something, and that was in essence what is now the linear accelerator. His early interest in microwaves came from the problem of accelerating electrons. That's how he started. But again the war, and of course the tremendous gain in power that people achieved in the meantime, made him feel that one could build actually a larger electron accelerator than this one we have here, the 300-foot accelerator. He died before it was finished, but it was very clearly his child. The big accelerator is only an offspring of that.

So then two really great projects were brewing for Stanford. When Bill and I went back we both had our hands full.

I came back here on a very modest scale. We had very little money at that time, by the way. The affluence of money came only somewhat later. Our whole first experiment on nuclear induction cost, I think, $450, of which you had to spend I think $300 on cathode-ray oscillography because even that didn't exist. We used an old demonstration magnet, a lecture demonstration magnet, which was not too good. Later on we realized that the best type of magnet was just barely good enough. We managed somehow. It's amazing how much one could do with little money if one needed to.

 

Weiner:

When your expectations aren't high, as far as getting funds, then you do what you can.

Bloch:

They were practically nil. There was one little thing. Bill had already gotten some money from the Sperry Company, and he had one machinist whom he put at our disposal. We had other machinists, but there was one man who was really then put on the work. Everything was handmade.

Weiner:

Where did the funds come from? The $450?

Bloch:

Oh, that came from the University.

Weiner:

From the departmental budget?

Bloch:

The departmental budget; that much was allowed.

Weiner:

Let me ask a question getting back to the cyclotron. How long did you run it before you shut it down for the war?

Bloch:

Until '43. It must have been almost two years. The first year or at least half year was spent on this work I told you about—improving and studying polarization—and the rest spent on fission neutrons.

Weiner:

So it was involved in the war work.

Bloch:

Oh, yes. This work on the energy distribution from fission was done here.

Weiner:

It wasn't clear to me that it was the same machine, but then again you wouldn't go out and build another one. Whatever happened to that?

Bloch:

Oh, that was in use a long time. That finally went, I think, about six years ago. Meyerhof joined us after the war came and used it for a long time. I think in 1960 only, I believe, it was finally given as a present to a small university somewhere in St. Louis or some such place. I think per dollar there is no high-energy machine in the world that has served better.

Weiner:

Your giving away small cyclotrons is like the old practice of giving away small telescopes. The observatories, as they graduated to bigger and bigger ones, would give them away. There's a trickle-down effect.

Bloch:

I don't know whether that cyclotron is still running. If so, it's probably just used for graduate students to get a little experience. It's really a toy.

Weiner:

You did some useful work with it.

Bloch:

Yes, we did.

Weiner:

You started, though, on the new work. You mentioned $450. But actually what had to be done?

Bloch:

Well, of course, all the radio equipment had to be built. That we assigned to a graduate student, Packard, who is now with Varian Associates. He built the radio equipment according to the design of Bill Hansen. I myself was most involved with the magnet itself and its properties. We had another student, Manning, but he dropped out. So there was just the three of us—myself, Hansen and Packard. We went rather slowly at it. We got the first results early in January. I came back in August or September of '45. In January of '46 we had the first positive results. I heard just about at that time from Professor Stern in Berkeley that Purcell had a similar idea and had developed something. I didn't quite know what it was, but it was clear to me that it was in principle the same idea.

Weiner:

Had you known Purcell at all before?

Bloch:

No, I'd never met him before.

Weiner:

At none of these laboratories or meetings.

Bloch:

No, strangely enough, not. He was at the Radiation Laboratory, I believe, in Cambridge, but we never met. I met him later. Well, we both then presented our results at the spring meeting of the Physical Society in Cambridge, and that's where I met him the first time. We compared our notes, and we realized we were speaking an entirely different language but realized very quickly that it was basically the same thing.

Weiner:

You presented your results. This was one of your short papers.

Bloch:

Ours was only a letter to the editor. And so was Purcell's. It had appeared in the number just preceding ours. I think ours was January '46, and his must have been December '45.

Weiner:

You have one on October 1st and 15th, 1946.

Bloch:

That's a more extended one. This was a letter.

Weiner:

As a matter of fact, I have a copy here.

Bloch:

Is this my bibliography?

Weiner:

It's not a full bibliography. I have a '48 one.

Bloch:

Here: Physical Review, No. 70, page 474, 1946. Then I wrote an extended paper. No, it's the earlier one: Physical Review, 69, 1946, page 127—a letter; that's what it was, a letter to the Physical Review. And what you are referring to is, I think, my extended paper. That appeared later, and then was followed by a joint paper with Hansen.

Weiner:

You have one by yourself and Condit and Staub.

Bloch:

Yes, that is the one on the polarization I mentioned before, which is the last one we did before we went into the Manhattan work.

Weiner:

So the one with Staub in '46 was the publication of the results of the pre-war work?

Bloch:

Well, there is one on neutron polarization and ferromagnetic saturation. That must be the one—in 1942 and '43 with Hamermesh-Hamermesh and Staub and myself: "Neutron Polarization and Ferromagnetic Saturation." And '46 with Condit—I remember now—it was this way. The one with Hamermesh and Staub was to study the magnetic effects accompanying neutron polarization. We established an experimental law of how the polarization increased with the approach to saturation. Our magnet was not strong enough to go higher, but we did know by extrapolation that if we could get a stronger magnet, we could do it. Now I remember: The 20% was not achieved until this paper with Condit after the war. But it was based on our earlier knowledge. Then we had money, and could build strong magnets. It's very interesting how one forgets.

 

Weiner:

Actually, you have to reread the papers to be sure. That was following up the polarization work, but then we were talking about the nuclear induction work.

Bloch:

Yes, after the war. Well, we did both. I mean I did both simultaneously. I worked with Staub and I worked with Hansen, although Hansen dropped out of it very soon because he turned all of his attention then to the accelerator work.

Weiner:

At this meeting in Cambridge you presented your results in a brief paper.

Bloch:

And so did Purcell.

Weiner:

Did anyone know to bring you two together or did you just find yourselves...?

Bloch:

Yes. We had known about this in the meantime. I knew of Purcell's work.

Weiner:

You say Stern had told you about it.

Bloch:

Stern told me only in a preliminary way. But then his paper appeared already in December, and I had read it? I don't know whether I wrote to him at that time. It wasn't necessary. So we certainly knew about each other in detail. I think Bill Hansen went to see him in Cambridge, I think in February before we met, and they discussed this.

Weiner:

This is '46 now.

Bloch:

'46. But I didn't meet Purcell until that meeting.

Weiner:

And subsequently has there been the need for collaboration?

Bloch:

Well, I think it was a very useful thing: Purcell and I, so to say, split up the field. We said: "You do this and I'll do that." But the Cambridge group was more interested in work on crystals, I think, perhaps largely because of Van Vleck's influence and interest, and they did some very fine work there. Pound was there and Bloembergen was there, some very good people. Now, we specialized much more in liquids. I'm talking now about what happened after 1946. We did two major things, I think, here. One was after we had established the whole effect and studied it in detail on protons in water, simply in water. Then we proceeded to use it for precision measurement of many, many nuclei. This was primarily Proctor, but that was later. We started already at that time. Proctor built special equipment. We had several machines going then in the basement. And that was one line, which we followed through, simply the precision measurement of every possible nucleus: Everyone posed some problem of its own because we always wanted to have them in liquids. It was a question of solutions in which they could be dissolved and so forth. But once that was done, it was usually very simple to measure the moments with accuracy and determine signs.

 

Weiner:

Other than people who were pursuing the same line of work, who was most interested from other fields in your results? Because it would have applications in solid state, in particles, in nuclear physics.

Bloch:

Yes. Well, I think the measurement of the magnetic moments, which I told you about, was of interest to the nuclear people, because at that time the shell model had started to develop and of course the magnetic moments of nuclei were a test of the shell model. So I think that was the interest which the nuclear physics people took. As far as solid state, it was done at Harvard. They did some on crystals. I don't recall details now. Of course at that time I think ours were the only two really active groups in the field. Other people started, but it had not proceeded yet to the point where it could be used as a tool for many applications. There was just too much to be learned about the technique itself and what one could learn from it. Speaking about our own work, already in the very early work I could say I was always fascinated by the question of accuracy. That is to say, I don't want to become too technical, but if you don't mind: We reported already at the very first meeting in Cambridge about something called the relaxation time—that is, the time which it takes for these nuclei in water, for example, to realign themselves and establish equilibrium. We measured that and found it to be about two seconds. I think later measurements gave it somewhat more accurately at something like three seconds. But the amazing thing is that it was so long. Now, because of the indeterminacy relation, if you want, the relaxation time could have a direct connection with a frequency width. That is to say, if the relaxation time is two seconds, and nothing else complicates matters, then one should be able to measure frequencies down to one-half of a cycle per second approximately. That was, of course, extraordinary because our frequencies at which we worked were about 30 megacycles; so it corresponded in accuracy of about one part in ten to the eighth. That, of course, fascinated me. But we never observed such sharp lines, because our magnets were not homogeneous enough. They were broadening simply because the magnetic field varies over the sample; and if it varies only by a part in a million and very probably more than that, it obscured already the effect.

Therefore, it was our goal then to go and see whether one could actually establish frequencies to such accuracies, get lines of such extraordinary sharpness, much sharper in proportion than any lines in spectroscopy one had had before.

That was the other main line, which we pursued, which turned out then to be practically a question of magnet design. It just simply meant that the pole faces had to be polished and very accurately aligned and so on and step-by-step...

 

Weiner:

This was done here.

Bloch:

That was done here, yes. This is what one calls now high-resolution work. I think I may claim that was altogether our project. I mean some people thought: "Oh, well, if you have it to a part in a million, why do you want it to a part in a hundred million?" But to me it was a sort of fascinating question. And, of course, my paper had dealt a great deal with the theory of relaxation time, and I really wanted to understand what was going on there. Then, out of this, later in 1960 or so, grew probably the most important application of all this.

There is perhaps one thing more I should tell you before. It must have been about 1950—in fact, my name may not even be on the paper— that there was a paper by Packard, Arnold and Dharmatti. It was a rather important paper. It dealt with the structure in alcohol.*

<I>(*Arnold, Dharmatti and Packard, Chemical effects on nuclear induction signals from organic compounds, Letter in J. Chem. Phys., 19, 507 (April 1951).)</I>

But I want to tell you something else before that if you don't mind, because I want to finish up this question of the magnetic moment of the neutron, which, as you see, was always my companion since the very early days. I mentioned that in my Nobel lecture.

 

Weiner:

I remember the theme.

Bloch:

Because then in 1947, after we had established these high polarization effects with Condit, and the nuclear induction was working, it began to become a tool. In fact, one of the earliest applications which we realized right away was that it was the ideal magnetometer because if you put a proton sample, water let us say, into a magnetic field, then all you had to do is to measure the resonance frequency and you knew the field to a very great accuracy. But we did even better than that. Packard built a magnet stabilizer. That is to say, you can use the signal, which comes out to readjust the field automatically electronically. And since we needed not only a very homogenous field but also a very constant field for the neutron experiment, we combined this. That is to say, we measured the field with great accuracy. We also used a nuclear induction stabilizer. Then we did the experiment for the neutron moment once more, and did it with very great accuracy.

Weiner:

In a sense you knew exactly what you had and you could maintain it at that same level.

Bloch:

Right, right. One could always hear. You had an earphone or radio equipment and could hear from the tone how much you were off.

Weiner:

You learned to recognize noise in your war work.

Bloch:

Oh, yes. Well, that was not noise; this is a control device. You simply beat the frequency, which you have against a constant frequency and you know you have to stay on it. You slip a little bit off, and an audible tone comes out. You turn back and it goes "oohhmm," and then disappears. And then you know you're all right. That's just gadgeteering.

Weiner:

But the way one feeds back into the other is very interesting. Now, all of this occupied how much space here?

Bloch:

We were not in this building. We were down in the old building, and we were down in the basement. We had a laboratory, I'd say, about twice as big as this.

Weiner:

Twice as big as this large office you mean.

Bloch:

Maybe a little bigger.

Weiner:

It's still small-scale physics.

Bloch:

At that time it was. At that time it was normal physics. Nowadays it would be very small-scale. At times I think we had as many as three or four simultaneous magnets and probably six or eight people working in the laboratory, sometimes two together doing something.

Weiner:

And these people got their degrees with you, generally.

Bloch:

Yes.

Weiner:

Meanwhile, what was Rabi doing in terms of this? Was Rabi at all involved in this?

Bloch:

Well, Rabi, of course, had done his work earlier; and then his group continued. After the War Rabi was not so very active anymore. It was Kusch primarily. They did not use this method. In fact, in the beginning Kusch thought this was cheating; that was not the right way to do it. Later on he persuaded himself that this method could even be used usefully in conjunction with molecular beams. Molecular beams have their own right of existence, and they did, of course, this wonderful work about measuring the magnetic moment of the electron, which then tied up with Schwinger's work. That was Kusch. So this group pursued their own interests, sticking to their molecular beams.

Weiner:

So the ones closest to you would have been the Harvard group then?

Bloch:

Oh, definitely, definitely. We had a constant exchange with them, yes. As a matter of fact, I know it was at a meeting that this business of pushing the line width down to a minimum was decided, a meeting together with Purcell. We felt that some sort of a standard moment should be used in conjunction with measurements of the Bureau of Standards. And for a standard, you know, you need very great accuracy. So it was at least partly that. That was at least the rationale besides my theoretical interest—the drive toward the accurate standard developing in these high-resolution techniques. And then out of it came also, not quite by coincidence, something else connected with the work of [Erwin] Hahn. He came from Illinois, a very original man; and he had developed something called the spin echoes. That was his own version or really a variation, a very interesting variation of our game where, instead of applying the radio frequency at a constant waveform, he put it on pulses and observed what came out from that. He developed it very ingeniously. He also, when he came here, started to work on substances other than water and found some peculiar structures, which he interpreted as chemical effects. Well, we wanted to see those with our own eyes when he sort of predicted that one might see some kind of a structure. And then these three people—Dharmatti, Packard and Arnold, collaborators of mine—established for the first time in alcohol these three separate lines, which have become very famous in the meantime, for those three groups of hydrogen. That is to say, that the protons in the same molecule need not all have the same frequency; but they are only slightly different, and one could not see that until one went to high resolution. But if their frequency was about at 30 megacycles their separation was about 100 cycles; that still didn't require the separation of half a cycle, but one had to come pretty close to it. Then the further we went with the resolutions, the finer structures and details showed up. I believe the alcohol spectrum now, any time you see it resolved, is something like 48 lines or so, and some of them indeed separated by just a few cycles.

 

Weiner:

So it wasn't just an idle pursuit.

Bloch:

It was not an idle pursuit.

Weiner:

The better you did, the more knowledge you had.

Bloch:

But it sounds like foresight. We didn't have that. The chemists think nowadays we developed this exclusively for their Purpose, but we had no idea of it when we started?

Weiner:

In other words, these applications of the structures, for example, only came after you had established that.

Bloch:

Yes. That did not really get going until the middle of the '50s. The earliest work was achieved about 1950 or so, but that was just laboratory work, just seeing three broad lines in alcohol. And then, of course, come all the refinements and so forth. It took several years and there were other tricks, which we also had to invent: the fact that if you spin the sample rather than keep it stationary, you narrow the lines by averaging all the fields. These are technical tricks but very important practices. But, of course, it could not be used as a practical tool until all these methods were developed. And then the last major work which I did in 1954 ... Well, there was the spinning. That was a contribution that was done in '54 or in '53 maybe. And then Arnold and Anderson, two students of mine, did the first basic work, you may say, on alcohol particularly. Anderson then started to look at other molecules to measure the structure, to explain it, to understand all about it really.

 

Weiner:

But still there was that central theme of your—that you apparently started back in the '30s when you were concerned with the magnetic moment of the neutron—that you pursued in one form or another.

Bloch:

Except, of course, by that time in 1950 the magnetic moment as such was well known. It became a tool. I visited Copenhagen frequently after the war. At one point gave a talk in Copenhagen, and then afterwards we met with Bjerrum. Bjerrum was a chemist and a great friend of Niels Bohr, and I was there, and Bjerrum was interested in what I told him. And Bohr said to him: "You know, what these people do is really very clever. They put little spies into the molecules and send radio signals to them, and they have to radio back what they are seeing." I thought that was a very nice way of formulating it. That was exactly how they were used. It was not anymore the protons as such. But from the way they reacted you wanted to know in what kind of environment they are, just like spies that you send out. That was a nice formulation.

Well, anyway, I think that was almost the end of my main contributions to nuclear magnetic resonance—in 1954 before I went to Geneva.

 

Weiner:

Now, all during this time other things were developing in other branches of physics, and yet you stayed in a field that you were doing major work in, and that field didn't grow very rapidly compared to other parts of physics? Or am I wrong in that? Relatively, it didn't grow as large, for example, as the whole rush into particle physics and higher-energy physics.

Bloch:

Well, you mustn't forget that all these fields have a relatively quiet initial period where you don't hear much about it. And those who work in the field realize the progress, which has been made. Then at some point when it gets ready, it booms out and spreads out. Particle physics also had its very slow beginnings. Until not too long ago, all one knew of particle physics was only from cosmic rays.

Weiner:

The late '40s anyway.

Bloch:

Well, I mean until the big machines really started to work. So there were also many years in between when people said: "What's the use of these big machines? They never yet have taught us anything new." It may look a bit as if this all came overnight—it didn't. And although our field is of course not as fundamentally important as that of particle physics, I would not say that its development was untypical. It is not slow to take ten years for a field to develop, apart from the fact that of course before these refinements were done, it was already being used by solid-state physicists. So it had become useful, too, before that.

Weiner:

Do you think that since about 1950 it has emerged as a full-grown, strong, growing specialty or that...?

Bloch:

Yes.

Weiner:

Or rather...? You see, the other alternative is that because it's so successful, it's sort of taken up by other fields and used.

Bloch:

I would say mainly the latter, yes: It became a tool. That is to say [my own intention was to use it] to measure magnetic moments, and that job was simply done by that time. And then it was taken up by other fields, partly in physics and partly not in physics. As I say, chemistry has become a very important user of the technique. But it's true that the art as such was practically finished.

Weiner:

Is that why you turned your attention somewhat to other things after that? For example, the theory of superconductivity.

Bloch:

Well, that came after I came back from Geneva, but I may say— and this is, I think, as far as you want to go today—that [explains] the fact that I considered to take the position at CERN at all, I think I can say psychologically it was due to the feeling that my necessity of staying with nuclear induction was not so great anymore. At least my own heydays were over.

Weiner:

That's interesting, because in this case the completion of the work, apparently the satisfactory completion of it, coincided with the recognition—in this case the Nobel Prize.

Bloch:

That came earlier. It came in '52. I still continued after that—until '54.

Weiner:

I understand that. By that time, you really had carried through this program of research that you had established for yourself many years earlier.

Bloch:

Well, I don't think we would have received the Nobel Prize if we hadn't done that. Not only did we have to develop the technique, we also had to show that something could be done with it.

Weiner:

But what I mean is: there are two things. One is setting yourself a goal and accomplishing it, and the other thing is that your colleagues recognize this.

Bloch:

Oh, I don't think it needed the Nobel Prize. My colleagues realized that before. That was only the publicity for the while.

Weiner:

So you felt that when you had completed it and it was good and it was done and people knew about it...

Bloch:

And I was quite confident that other people would take it up, and that was indeed what happened.

Weiner:

This puts you in the early '50s.

Bloch:

It is true that nevertheless I still did take some equipment to Geneva. Arnold packed it before I went to Geneva and actually did go with me to Geneva, because I didn't want to be entirely separated from nuclear induction work. They set up a laboratory at Geneva at the University. What little my time allowed me then, I still came and was interested in this.

Weiner:

Let me ask you about the prize. When you had done the work, had it occurred to you that this might be of Nobel Prize caliber?

Bloch:

Well, I don't know. I think we were quite aware of the importance of it. What gets a Nobel Prize and what doesn't, is not so sure to say. I had some inklings a few weeks before I received the prize but not before that. And then you know there is always the usual gossip among physicists to which one better pay no attention to what they say: "I'm sure you'll get the Nobel Prize next year" and so on. No, I think quite largely it was a pleasant surprise. I must say in all modesty that it didn't come entirely out of the blue sky.

Weiner:

What was your reaction when you did get the word?

Bloch:

Well, I woke up early in the morning. They called me from Associated Press at 4 o'clock in the morning, I think, and I got up and I was so sleepy. When they told me that, my reaction was "Nonsense, I must be dreaming." But then very soon the telegrams started coming in. Well, as I say, I was not quite without warning. In fact, the Stanford Press had heard something about it, and a man from the information office came to me the day before and told me he had heard such rumors and said to me that I had better hold myself ready for the next day. "Well," I said, "I don't know what's involved." My wife and I sat in the evening before we went to bed, and I said to her, "You know, of course, it may all be a false rumor. But even to think that it is possible is a very nice thought."

Weiner:

Yes. I'm sure that your colleagues here were overjoyed, along with you.

Bloch:

Oh, yes.

Weiner:

I think you were about to suggest that we talk about CERN.

Bloch:

That's right. Well, now, the first idea bout CERN came up in the fall of '53. I received a letter—I believe it was from Bohr—telling me about the importance of this new laboratory and asking me whether I would consider the directorship. I went through great struggles at that time, first feeling: "This is sheer nonsense. I shouldn't go into this kind of thing. I'm not made for administrative work." Then thinking that perhaps it wasn't altogether administrative. Well, finally in the spring of '54, upon the insistence of Bohr, I went to Copenhagen and talked it over with him. He urged me very strongly at that time to take the directorship of the laboratory: He felt that it was important that an active physicist should head the laboratory, and he believed and gave me the impression that as director still my main function would be scientific. And although many of my friends warned me and said, "Don't believe it—it's not going to be that simple," nevertheless I felt at that point that I should try it. I was not sure that this was really the kind of work I wanted, but I felt I wanted to try it. I think I came to a decision some time in the spring of '54 when it was offered to me, and I made it a condition then that under no conditions would I stay more than two years. I had my doubts about the job right from the beginning. But anyhow I did go there then in '54. Heisenberg also was very eager. I think it was mainly Heisenberg and Bohr who wanted very much that I should take that job.

 

Weiner:

Well, they were on the council. Bohr was head of the theoretical side.

Bloch:

Yes, they were on the council. Heisenberg I think represented Germany on the council.

Weiner:

But October '54 is when the council announced your appointment.

Bloch:

That's when it was official.

Weiner:

That's when they made it official. When did you move there?

Bloch:

I think I went there in early October. It was probably the first council meeting.

Weiner:

Actually you made a public statement then, too, right after that, so you must have been on the scene.

Bloch:

Yes, yes. Oh, yes. Certainly. All the details were settled before that. I was in Paris I think after I was in Copenhagen. I was in Geneva. I met Amaldi there, and then I went to Paris where the council was meeting. Then at that point, I think it was that I said that if they offered it to me I would try it.

Weiner:

What was the situation in European physics at the time after the war?

Bloch:

Well, of course, they had fallen very much behind in the war in two very important aspects. The first was electronics, because radar techniques were far more developed here and of course in England, too. England had a head start on that, but it was then taken up with greater intensity here. And the second thing was clearly atomic energy. On both these things the Europeans were badly behind. I think it was Rabi who first suggested to them that they should get going on their own and have this large laboratory built in Geneva. It started officially when I went there—I was the first director—but all the groundwork had been done by Amaldi.

Weiner:

I have this report that Amaldi gave, and he covers four years, from 1950 to 1954, showing the different stages, mostly administrative.

Bloch:

Right? But actually when I joined it there was already some work going on, on a very small scale. It was at the airport in Geneva.

Weiner:

Did it catch on in terms of being regarded as a prestigious place in which to work?

Bloch:

Well, of course, in the beginning it was not so much yet. That is, during the time I was there it was very much in its initial stages. We had, in fact, quite some difficulties recruiting people. People did not want to go to Geneva very much in the beginning, because you know how it is in Europe. They have their positions, their association with the university; they don't easily give that up. We got two theoreticians from France. That's something I insisted upon: And there was a very good experimental group under Adams, but they were mostly engineers. They were concerned with design and construction of the large machine. When I was there, there was practically no equipment. There was a little work going on. The cyclotron was beginning to be built. And my time, unfortunately, at that time was indeed almost all taken up with administrative work.

 

Weiner:

What sorts of problems seemed to be the most difficult?

Bloch:

Well, I think the personnel problems were probably the ones that concerned me most, and then, of course, I was the connecting link between the staff and the council. I had to present to the council the wishes of the laboratory and vice versa. We had these council meetings, and they had to be prepared. There were also minor things. I had a British civil servant, and he was in charge of the purely technical administrative work, and I had great help from him. But there were questions about personnel insurance, and I mean all the problems that come up are new problems. And in spite of the fact that I was not considered an expert, after all, being the director I had to acquaint myself with these things. I must confess I was rather unhappy. That is to say it became quite clear to me after a relatively short time that that was not the kind of thing—that my scientific interests were practically unused or were used to a very limited extent. Perhaps it could be that the people were very nice to me out of respect, because of my prestige. So maybe in that respect it helped. But I decided already in the spring of '55 that I was going to go to the end of the year and find a successor, but go no further than that. It caused in the beginning some consternation, but I think it was forgiven. So I left. I went back in '55. In retrospect I must say it was an interesting, enjoyable experience, but I'm certainly glad I did not carry it further.

 

Weiner:

You felt you weren't able to contribute anything or very much as a physicist except in terms of your general knowledge and maturity and wisdom. But in terms of a director responsible for policy, the policy that was needed at that time didn't involve very much physics.

Bloch:

Very little. I mean the building of a machine was the main goal and that was already going.

Weiner:

Was this ever in dispute, by the way—the fact that this should be basically a high-energy physics laboratory?

Bloch:

No, that's what it was meant for from the very beginning.

Weiner:

And it could have gone in other ways?

Bloch:

No, no, the proton-synchrotron was already the goal. That finally started to function in 1960, I believe.

Weiner:

The thing that I would like to dig into with other people who were involved in the early policy discussions, is how that decision was reached.

Bloch:

That you must read in Amaldi's report. By the time I came that decision was already reached.

Weiner:

Amaldi gives very little on that, but Kowarski is writing a history of the early stages.

Bloch:

By the time I became director that was already decided. In fact, the group under Adams was already very active.

Weiner:

I have a press release that was issued on February 25th, 1955. "Professor F. Bloch has informed the council of his desire to resign his appointment to take effect from 31st August." So in other words, you got there in October. It was less than five months when it became clear to you.

Bloch:

Yes. In fact, it was clear to me before that. Because between the official announcement and the decision of acceptance by the leading people, there was also a lapse of time.

Weiner:

That's true, because in the same release they mention your successor.

Bloch:

That had already been decided.

Weiner:

So they must have had time to do that.

Bloch:

No, no, no. It didn't take me very long. I felt I could not afford sticking my head into the sand. I realized from the moment I personally decided I didn't want to go on, that until I could really leave it would take several months, many months anyway.

Weiner:

Have you maintained contact in a policy-making or advisory position in any way?

Bloch:

No, I visit CERN whenever I am in Geneva, but I have not taken any role.

Weiner:

We have a couple of minutes. Let me ask a general question. And that is: in summing up and thinking about the work you've done through the years, which would you say has given you the most personal satisfaction?

Bloch:

Well, that is a difficult question to answer because there are several types of satisfaction. That is, for the immediate satisfaction, one of the greatest joys that a scientist can have is when a good idea hits him. That is, when headaches, which have been brewing in his mind for some time, come together to a solution: That happened to me a few times. But those are moments almost. Those are moments of elation, which might last for a day or a week or two, or something like that.

Weiner:

Which instances?

Bloch:

There was a paper with Nordsieck where we stewed and worried for a long time, and suddenly I realized: "???" Ah, this is the crux of the matter. That gave me great pleasure. Well, I'm sure that when I first realized this idea of the nuclear magnetic resonance—just on paper it came, in checking and rechecking and saying, "Yes, by God, I haven't made a mistake. I'm not fooling myself. It should really be possible." That was also a moment of great joy.

Then, of course, there are also long-range satisfactions—that is to say, the development of a certain line, like this resonance work after the war, which extended over many years: It was not a period of constant happiness. I mean there were ups and downs, but by and large it was a time of great satisfaction.

Weiner:

How about in the other sense of the importance of the work as judged by your colleagues—that is, the long-range importance? If you had to identify a single piece or discovery, what would you select?

Bloch:

Well, I would say probably two. The one goes way back to my thesis. That was my work on conduction in metals. That certainly has had a great impact. And the second was, I think, the discovery of induction.

Weiner:

Nuclear induction.

Bloch:

Yes. These, I think, are the two contributions, which have had the greatest impact.

Weiner:

Of course I ask the question in terms of long-range. Would the same answer be true of immediate impact on the field, as well, for both of them?

Bloch:

Well, there's always a certain starting time, you see—from the time that a new discovery is made until its full importance can be objectively evaluated takes a certain time. I think one has a certain feeling of instinct right away whether something is at all important or hardly important at all. That's easy to say. But it's not easy to say whether something which may sound terribly important today may not be completely old-fashioned in three years or whether it will go on, and I must say I have had good luck with these two things. I mean the work based, I may say, on my early work on electron conduction is still going on. It's very important in solid-state physics. And nuclear induct on has become a tool which is made use of widely.

Weiner:

That's quite different from discoveries as they occur in experimental physics. If you discover a new particle, you know, that's something that remains important.

Bloch:

Not necessarily.

Weiner:

Well, maybe recently things have changed. But I know that the neutron has. I mean a particle in that sense, a real particle.

Bloch:

Well, of course, I may say that my discoveries have never had the glare of the novelty of, let us say, the discovery of the neutron—I mean something totally unexpected almost. None of them were of that type. There was always some background that was growing in that direction. This supreme joy of hitting something really terribly important that nobody ever suspected before I have not had. Very few people have had that.

Weiner:

But you've had the parallel to it in cracking a tough problem.

Bloch:

Well, cracking a tough problem and then in some minor way realizing something which was expected before, I mean like, let us say, the fact that electrons can indeed run through a metal despite all the ions being present—that is also a very nice insight.

Weiner:

And you knew that, you had predicted it, even though there was a year or so before there was any experimental verification of that. I'm referring to the idea in '36, your paper in '36.

Bloch:

Yes. I was certainly pleased. I felt it was a very original idea. Nobody had thought before of combining ferromagnetism and neutrons and getting something useful out of it. This whole idea of magnetic scattering was a novel idea. What would grow out of it? No, I don't think I foresaw that at all.

Weiner:

I think we've covered a lot of ground here, a lot of things I probably will want to think through and come back better prepared on specific technical things. But for the things that we said we'd do, I think we've done very well. I think you've done very well.