Robert Bacher

Weiner:

I'd like to start off by asking you to recount, if you can, how you first became interested in science.

Bacher:

Well, I was born in Loudonville, Ohio, and when I was about five, my family moved to Ann Arbor, Michigan, and I went to grade school, high school, and to the university there in Ann Arbor. I think that as soon as I went to high school, I began to realize that I was interested in scientific things, and when I was about a junior in high school, I saw that this was along lines of chemistry and possibly physics. At that time, I didn't know very much what physics was. About the time I was a junior in high school, my family bought a summer place at a lake nearby, and it developed that the family who owned the house two doors away was the family of Professor Harrison Randall, who was the head of the physics department at the University of Michigan.

One day when, I guess, I was perhaps a junior or so in high school, I went over to talk to him. I knew his sons very well. We used to swim together and so on. I went to talk to him and told him some of the things that I found interesting, which I thought at that time were chemistry. He said, "Well, the things you're interested in are in atomic physics." I thought, well, that's interesting. I didn't have any idea really what physics was. But this began to become clearer, as I went into my senior year and started a course in physics—which, incidentally, had nothing whatever to do with the things that I found interesting. This doesn't mean that the introductory course in physics was not an interesting course in physics. As a matter of fact, it was a very interesting course in physics, and was taught by a very unusual man called H. N. Chute, Horatio Nelson Chute.

He had taught physics in the Ann Arbor High School for nearly 50 years, and was the author, in conjunction with a man named Carhart, who was professor of physics at the University of Michigan, of a book called, Carhart and Chute. Also I think he'd written himself a book for high school physics. In fact, I think I still have it here somewhere in my library. If it isn't here, it's at home. He was a quite unusual man, very much of a stickler for doing things in a very precise way, but you did learn something about the principles of classical physics from him, in a no-nonsense sort of way. happened to him now; his name was Joseph Grant—who was a year ahead of me in high school, and who had gone on to college at the university at that time. Incidentally, one of my fellow students in high school During that same period, I had a friend who—I don't know what was Harry Spedding. Both of us used to run on the Ann Arbor High School track team.

Harry Spedding is now a member of the Academy and director of the laboratory at Ames, and a very good chemist. But at any rate, either though Grant or Harry Spedding, I learned where the chemistry library of the university was, and by putting on a raincoat or some sort of a coat I found I could get in and pose under the guise of being a student, at least enough to look around on the book shelves and spend a few hours there browsing over the books, which were supposed to be for university students. There on that book shelf I ran into the first edition of Aston's book on isotopes, and the moment I saw that, I knew this was it.

This was the stuff I was interested in, and from then on it was relatively easy sailing. At least I knew what direction I wanted to go, and with the very expert guidance of Professor Randall, I proceeded to go to the university. I had what for then was the somewhat unusual idea that I wanted to get a general education before I started studying physics, so for two years I studied mathematics, but mostly studied history and languages and other things, and didn't really start any physics until the end of two years in college. This was not a particularly good idea, but anyway, it's what I did.

Weiner:

Do you remember the type of physics courses you took?

Bacher:

Well, I took introductory physics at the university, and took the arts college course in physics. I signed up for what was called the problem section. It turned out you had to join the engineers—had to go over to the engineering school to take that. So, though I was an arts college student, I did that, and reported in once every week for this problems session over there. It wasn't very hard, as a matter of fact, after my introductory work in physics in high school, and I'm afraid I was interested in a lot of other things at that time, and I don't think I did much more than all right in the course.

I think I got an A all right, but there were a lot of things I didn't learn that I might have. I remember very well the people who handled my—what were called recitation sections then—because they made a great impression on me, and in later later years were very helpful to me in many ways, as I started in on a more advanced career. One of them, the first one I had, was Professor Walter Colby, and he was then professor of physics. It was a rather unusual thing—I don't remember the precise circumstances under which he was teaching such a class, but at any rate, he was.

Another was Professor E. F. Barker, Ernest Barker, from whom I later took a course in atomic structure as an undergraduate, which made quite an impression on me. I might say subsequently I took courses as a graduate student from Walter Colby, who is now one of my very good friends. When I graduated there at Michigan, it seemed a good idea that I go away to school for at least a year. So during one year I went to Harvard as a graduate student. This was quite a different sort of a thing from what I'd had. I think most of the students had better pre- paration than I did.

But on the whole it didn't go too badly. I had courses with Kemble, with Slater, with Pierce, and two courses in mathema- tics, one a course in mechanics from, I think it was Remington. I'm not quite certain of that name but I think it was. And a course in mathematics from Groustein. Toward the end of that year, my father was taken ill, and it seemed unwise for me to have the unusual expenditures that I might have if I were that far away. Also, at that same time, during the year I had been away, Otto LaPorte came to Michigan as the first of a series of theoretical physics professors.

Weiner:

What year was this?

Bacher:

This is now in 1926. I graduated from Michigan an-I went to Ann Arbor as a professor of theoretical physics. And I think also that Cambridge in the year 1926-27, and during that year Otto LaPorte came to David Dennison came back to Michigan at that time. In the fall of '27, I then decided I would go back to Ann Arbor, because the opportunities for things I wanted to study seemed awfully good there, and also this was a much less expensive thing to do, which was a rather important consideration to me. Though it wasn't until considerably later that I took an assistantship there.

Assistantships weren't so easy to come by, and anyway I didn't have very great expenses. The tuition was low and I could live at home. At any rate, I went back. That fall, Goudsmit and Uhlenbeck came to Ann Arbor; I can still remember the first time seeing them, as vividly as though it were yesterday, when Dr. Randall brought them into the library, showing them around, and here was this one tall, very thin young man, and a very short one, and I thought, "My, I wonder what this is going to be like."

But I started taking courses from both of them, early that fall. Of course, they had grown up in the Ehrenfest School and were simply wonderful teachers. I took courses subsequently from Colby, from La Porte, and started to work with Goudsmit. In fact, I was Goudsmit's first graduate student in this country. During that period, we estab- lished a friendship which has lasted all these years. We've written a good many papers together, and wrote a book together at one time.

Weiner:

During the year at Harvard, who footed the bill?

Bacher:

It was all private. Relatively few students starting into graduate work at that time had teaching assistantships. A few did, but not very many. My expenses were, I think, if I remember correctly, all paid by my family.

Weiner:

Then, when you returned, the situation at Michigan was apparently changing with an influx of theoreticians—not necessarily theoreticians, but of European physicists, including theoreticians. How do you account for this change? From interviews with Randall and Dennison, you get the feeling that it was a self-conscious attempt to change the situation.

Bacher:

Well, I don't think there's any question about that. It was a self-conscious attempt that had gone back a good many years before. I think this was largely motivated, among the most senior members of the department, by Professor Randall and Professor Colby. I think that Professor Randall leaned very heavily on Professor Colby for his judg- ment about theoretical physicists, and I think Colby is the one who sort of picked out the people that they would get there. During about that same time, they had made some effort towards a summer session, but it was only really after Goudsmit, Uhlenbeck, La Porte, and Dennison all came there that they really started the summer session, that, at least in that period, was quite unusual among places in the United States, and became very well known. People such as Dirac and Fermi and Oppenheimer and so on were all there, which you can read about in all the books.

Weiner:

It was sort of a mecca for theoreticians.

Bacher:

It was a mecca, and this was a simply wonderful place for me. I mean, I couldn't have been luckier. I just couldn't have been luckier. I had the opportunity of working individually with these people, because the number of students was very small, and I worked some individually with La Porte. Then I found that the sorts of thing that Goudsmit was particularly interested in were things that particularly appealed to me, and I worked with him really quite intensively during that period, and learned a great deal from him. I think, to give you some idea of how this was done—Goudsmit always worked by the Socratic method.

I mean, if you wanted to talk to him about something, it was customary to go out and walk around the town, walk around or at least sit down for a while but pretty soon you'd go on for two or three hours at a time and sometimes when you got tired, digressing off in some other direction, or coming back where you had a blackboard to work at. But this was simply a wonderful experience for me. Goudsmit was just full of all of the things that were known in Europe about atomic spectra. He had a terrific encyclopedic knowledge of atomic spectra. And this was just a very fine thing for me. find yourself wandering around somewhere. These conversation would often

Weiner:

Were you conscious, during your undergraduate period, of the vast changes that were beginning to take place in quantum theory?

Bacher:

Well, you see, in fact most of those did not really occur until I was about a senior in college. Because remember I graduated in 1926, and those things were done in 1925 and '26. In fact, they didn't really begin to get into courses until '27 and so on.

Weiner:

Who picked them up at Michigan? Of course, the new people, imagine.

Bacher:

All the new people picked them up. La Porte picked them up immediately. The quantum theory course taught at Harvard in the fall of 1926-27 was still almost exclusively along the traditional lines—the course taught by Kemble. This is not a reflection on Kemble because there wasn't any mechanism really to get into this yet. But during that spring, Schroedinger came to give a series of lectures down at MIT, and all of us went down to hear him. Also during that spring, during the spring of '27, I took a reading course with Slater. Gerald Almy (at the University of Illinois) and I took a reading course with Slater, and I learned an enormous amount from that. This was an extraordinary stimulating thing. In fact, it was probably the most stimulating thing I had during that whole year. I enjoyed it enormously.

Weiner:

The reading course covered what?

Bacher:

We read that little book of Born's on quantum mechanics; a little red book.

Weiner:

Kemble and Slater of course were making contributions of their own—were among the few Americans then working in quantum theory.

Bacher:

That's right.

Weiner:

But in Kemble's case, this didn't yet reflect in the classroom?

Bacher:

Well, I think this was probably a conscious judgment on Kemble's part, that a certain amount of background and the more traditional Bohr approach involving the sorts of things that had been worked at for a good many years was an essential part of moving from there into the more advanced things. As a matter of fact, that kind of approach to quantum mechanics actually persisted for quite a long time, and it was only subsequently that one moved away from it. So this may look peculiar in retrospect, but I don't think it was at all peculiar at that particular time. Now, whether it was the right thing to do or not is another matter.

Weiner:

I think Kemble then wrote a book on quantum mechanics, and probably after that came out, he was in a better position to give the students ...

Bacher:

I think that was at a much later date. I don't know when that book of Kemble's came out, but my feeling is, that's quite a bit later. Yes, I think that's quite a bit later.

Weiner:

Well, getting back to Michigan (we can check the date of the book and put that point to rest, if we find it)—I had just asked you about Michigan once again. After the Harvard interlude, you returned to Michigan.

Bacher:

That's right.

Weiner:

I wanted to get your impression of these summer sessions on theoretical physics.

Bacher:

Well, in the first place, they started out in a quite modest way, during that first summer I was back there, which was the summer of 1927. One of the principal things I did was take a course in quantum theory from La Porte, and this was a very interesting course. This was before Uhlenbeck and Goudsmit arrived. This was very interesting, and, as I say, during the next three years there I had a simply magnificent time. The summer lectures got started—actually, I guess, it must have been started before, and then Professor Randall got some small amount of money from the university.

Actually, it was a very small amount of money, $5000 a year or something of that sort. It was largely on his initiative and with the advice of the people he then had on his faculty, and the connections they had in Europe, that we started getting a significant number of younger very able people coming there. It just included everyone, and this was invaluable to me. I had a particular joy about it because, I think I remarked earlier, my family had a modest summer place out at a lake nearby. As I was there, both at that time and sub- sequently, as a postdoctoral fellow, we often used to have them all come out to swim on Sunday afternoon. This was very enjoyable, and I got to know people in a way in which I wouldn't possibly have known them otherwise.

I always felt some responsibility for them, when they came out, and we had a couple of close calls. Mrs. Bohr fell off the life raft out there and the thing tipped over. Robert Oppenheimer was stung by a very large wasp at one time, and I can still remember the first time that Fermi went out into the lake and started to swim. He just swam out, and I thought, "Somebody's got to go along," so I swam after him, and this ended up by our swimming across the lake and back again. It was three-quarters of a mile across.

Weiner:

I understand he liked to take long swims, and considered practically a race.

Bacher:

Oh, yes. I was very surprised. I didn't know at that time how well he could swim, and I found out, but I managed to keep up with him. I'd been swimming quite a lot out there.

Weiner:

How did the sessions themselves actually go? Were there lectures for large groups, the entire assembled group, then discussion, or was it somewhat informal?

Bacher:

Well, first, they varied with the lecturer. But on the whole, they were given as lectures, with an opportunity to ask questions, and then there were seminars, and subsequently people had an opportunity to ask questions then. But there was usually an opportunity to ask questions. In fact, it was more or less traditional that the classes at Ann Arbor were relatively small when someone was lecturing. I mean, while we speak about this as "this well-known summer session," my guess is that the total number of places to sit down in the room couldn't have been more than 40.

Weiner:

I have pictures ...

Bacher:

Of that room?

Weiner:

Sam Goudsmit gave us—no, not of the room, of the group. He gave us pictures. The group itself varies from 25 to maybe 40.

Bacher:

I have some—yes—that's right.

Weiner:

A small group but a very significant one. And it's interesting to get your reactions as a student. We have it usually from the other point of view.

Bacher:

Yes. Well, these were while I was a student there, and then subsequently—you see, even when I was away, I came back in the summer time because that was my home. And I was there during the summer of '26, and I was there during the summer of '27, '28, '29 and '30. Then I was away, out here as a matter of fact, as a National Research Council Fellow in 1930 and '31, and then I was away I guess also in '32 ...

Weiner:

At MIT ...

Bacher:

'31, '32, that's right. Then from '32 to '34, I was back in Ann Arbor. Then I went East, but we still used to spend part of our summers out in Ann Arbor. So I was there during quite a lot of the mid- thirties as well, sometimes working, sometimes there for just a shorter period.

Weiner:

What do you think was the accomplishment of these summer sessions? Do you see a change in time over the years in the character of the sessions?

Bacher:

Well, I think the biggest impact that these summer sessions had was on the people who could come there from universities not too far away, who had had relatively little contact with anything that was going on in the most modern things that were really happening then in quantum theory. To get an opportunity to come there and to hear these things from the people who were working at them, made, I think, a simply enor- mous impact. It was much more difficult, and relatively more costly to travel at that time, and so I think this was invaluable to the people in Ann Arbor. It was invaluable to the people not too far away. I think it made an enormous impact on the whole Midwest.

Weiner:

I know the professors came from other universities. Did the students?

Bacher:

Yes, sometimes they did. For example, I first met the son of Cady, Sr.—whose first name—

Weiner:

There's one that passed away a couple of years ago.

Bacher:

It's longer than that. That's Willoughby Cady. Well, his father is still alive. He's 90 some odd. He worked on piezo-electric effect, and I know him perfectly well. Anyway, his son came there. Walter Cady is the father. His son came there. I think he came there while he was a student at Harvard, or a young postdoctoral fellow. In fact, I met him there. This was some time in the early thirties. I think by that time, a fair number of postdoctoral people, or at least young people in physics, used to come there. Phil Morse came and lec- tured one summer, when he—shortly after he'd taken his doctor's degree. Condon came there. Kramers came, Bohr came, Ehrenfest was there.

Weiner:

Then of course the California people—Lawrence came—

Bacher:

Later Lawrence came. That is in the mid-thirties.

Weiner:

'34, '35.

Bacher:

Something along in there, yes. I guess I'd met Lawrence before then.

Weiner:

I'd like to ask about your dissertation work, the type of problem you tackled, why, who suggested it, and who you worked with.

Bacher:

Well, as I mentioned before, I worked with Goudsmit, and the problem I worked on had to do with Zeeman effect in intermediate fields. It turns out, in atomic spectra, that most of the multiple structure is of such size that you usually get what's known as weak field Zeeman effect, but there is an effect known as the Paschen-Back effect which essentially predicts what you do as you go toward very strong fields, and this is of some importance in atomic structure. It turned out that in hyperfine structure, this same multiplet system persisted. It was due to the interaction of the nuclear magnetic moment with the electron moment. And here, because of the size of the structure and the fact that a given magnetic field would give a much stronger effect, one could study in much greater detail the intermediate stages of the magnetic field.

And my thesis was essentially on the question of working out the theory of the Zeeman effect in intermediate field strengths, in relative- ly simple cases, and actually making a comparison, comparing these with theory in the case of thalium and bismuth, which were the things that Back was working on. I can still remember, and I believe I have—it reminds me, I promised Sam Goudsmit I would send him the copy of the cable- gram that we got from Back that gave us the structure that he had ob- served in the thalium line, and which we had calculated and put down on a piece of paper ahead of time.

When we got the result, it was absolutely miraculous to us, because when we put in—Well, we knew the field, we knew everything, and the numbers agreed, all the lines were present as they should be, and the pattern agreed to a fraction of a percent, or about one percent.

Weiner:

He mentioned to me to remind you to send him those.

Bacher:

Well, I will make a note and find that thing, because some- where in my stuff at home, I'm pretty sure I still have that paper.

Weiner:

That should be very interesting. He told me, "I'll bet you'll never believe we did physics by cable." There was a publication, I know, in 1929, with Goudsmit. Of course, you weren't through with your thesis yet, so that was something else?

Bacher:

No.

Weiner:

That was something else; you were already publishing?

Bacher:

That's right. Yes. I had published two or three things with Goudsmit by the time I came up for thesis work. Actually, the things that I did in my thesis proper were published in the Zeitschrift fur Physic, because Back had done the experimental things in Germany, so we thought it was nice to publish there. But I had written a couple of things with Goudsmit by that time, and we wrote a number of papers together subsequently.

Weiner:

You went to MIT in 1931-32. Was that also as a National Re- search Council fellow?

Bacher:

Yes. Well, I was married the end of May, 1930, and we came out here that year, had a very nice year here. I spent a good part of that year working on the manuscript and doing a lot of the spadework on the book that Goudsmit and I published together, which is over here. This was essentially a compilation. I would say, as of that date, it was a semi-critical compilation of what was known about atomic energy states. There had been no compilation of atomic energy states up to that time, and Goudsmit and I had started this during the previous year, and discovered what an enormous job it was, since it had never been done before, and we put it aside while I finished my work for my thesis.

Then when I came out here, I used to work at other things part of the time, but as I got deeper into this, I realized I was never going to get it done unless I just worked at it all the time. So I was sending off every three days new stacks of stuff for Goudsmit. I had, fortunately, a good library available here, both in Bridge Laboratory and particular- ly at that time up in the Mt. Wilson Laboratory at Santa Barbara Street. In fact, I used to work up there a good bit of the time. But the manu- script of that book was all sent off from out here, and I think ! finished it some time along in March or so.

Weiner:

It was published in 1932.

Bacher:

That's about the right lead time. I know that during the year I was at MIT, I was reading proof on it.

Weiner:

Now, why did you pick Caltech for your National Research Council fellowship year?

Bacher:

Well, that's an interesting thing. I'm not sure I can answer that question properly. In the first place, it was new, it was some place completely away, in another part of the country. They had done very good work in spectra here. Some of the most interesting work in the classification of spectra had been done by Henry Norris Russell, in collaboration with people at the Mt. Wilson observatory, where they had very fine spectroscopic equipment. While that wasn't my particular in- terest, I knew there were people that worked in that field. Also, Houston was here.

He was doing some work that I was interested in. It developed after I got here that Houston was going to be away during the spring quarter of the year. So, as a matter of fact, I taught one of the sections of one of his courses while he was gone. And this was good practice for me. I had a very good year here. I worked a great deal by myself during that year, but on the other hand, I had an oppor- tunity to work with some people that I got to know in subsequent years. Dr. Meggers from the Bureau of Standards was out here part of that year, and I got to know him up at Mt. Wilson.

He was interested in hyperfine structure, and we had some very interesting conversations together. A couple of my friends from Michigan were also here. John Strong was here, and also Hugh Wolfe from the American Institute of Physics. He took his degree. We were in the same class at Michigan. He took his degree one year ahead of me, and came out here, and was working I guess with Houston and with Oppenheimer. Oppenheimer was spending part of his time here, and of course was a very considerable attraction too.

Weiner:

Already at that time was Oppenheimer's reputation sufficient to be an attraction?

Bacher:

Oh, yes. Oh, yes.

Weiner:

Did you have much contact with Oppenheimer? Did you sit in on any of his lectures?

Bacher:

Well, he lectured here during the spring term, and I got to know him a bit. But I was not working in theoretical physics in Oppen- heimer's sense at that time. As a matter of fact, I was doing at that time a little collaborative work with one young man at the Institute here, whom I got interested in experimental work in hyperfine structure, and I was trying to learn some things about experimental work from him at that time.

Weiner:

Who was that?

Bacher:

A young man named Campbell, Stuart Campbell, who was subse- quently at the University of Rochester. He's dead now.

Weiner:

Did you have contact with the other work that was going on? It seems to me there were at least three other groups—Lauritsen and Anderson and Bowen and Millikan.

Bacher:

Well, I knew Bowen, of course, and I knew Dr. Millikan. On the other hand, I must certainly have met Lauritsen at that time, but this came at a time before I'd become interested in problems having to do with nuclear physics. I think, as a matter of fact, they were not yet set up to do nuclear physics work, but were still working on the development of their machine over here.

Weiner:

It was only after the Cockcroft-Walton experiment that they began to use the machine as an accelerator.

Bacher:

Yes. Yes.

Weiner:

That was '32. I imagine you were present when Anderson was doing some of the work that led to the positron, but I imagine it was too early to hear any talk about it?

Bacher:

That's right. I remember that to start work in this direction was something that Millikan had urged very much, and Millikan was pretty strong at saying, "Now, let's go do this," and being so persuasive that persuasive man. I don't know, but I think that he must have been the one who persuaded Carl Anderson to go off in this direction, which, at that time, was a quite unusual type of thing to do. It was actually the beginning of the use of cosmic rays as a source of energetic particles, he had people moving in this direction before long, even if they might which was a really extraordinary thing.

Weiner:

Yes. It was not only a source of energetic particles, but not think this was the most important thing to do. He was an extraordinarily particularly a source of energetic particles for the purpose of probing the nucleus.

Bacher:

Yes, that's right. Well, a source of energetic particles for probing whatever one might have. This was really the beginning of that. And that was just beginning at that time. I think it must have been during that year of 1930-31 or thereabouts that Anderson got started on this experiment. I remember vaguely.

Weiner:

He started in the summer of 1931. Of course, he was building his equipment before that.

Bacher:

Well, that fits in, because I don't remember it being started during that year. I left here in early October of '31. We stayed during that summer and then went east at the end of the summer.

Weiner:

The letter I showed you was dated November 3, 1931.

Bacher:

Oh, is that so?

Weiner:

And it discussed the results he'd been getting. Bechar: Well, before I left here, I'd never heard anything about this, and it was only subsequently that I heard about it. It fits in with the time picture very well.

Weiner:

Let me take you back and ask you when you got interested in the nucleus?

Bacher:

Let me go on just a little bit, in a more logical way or at least what seems more logical to me. In 1931-32, I was at MIT, and I had stopped for an extended visit in Princeton on the way up. I'd stopped at Columbia and met Rabi for the first time. Rabi was starting to work on nuclear moments and so on, and I had been working on the hyperfine structure end of it from spectra, and this was extraordinarily illuminat- ing to me, to hear what they were doing there. I was very much impressed by this.

We then went up to MIT, where I worked with Slater. I learned a great deal working with him. That year with Slater formed a background for quite a lot of things that I did subsequently. On the other hand, I also had a lot of work to do on getting that book out, because, as you can see, the reading of proof on that was quite a big job. I stayed at MIT during the summer of 1932. And I was just remembering today, because I was writing a letter that's to be included in a volume that's to be presented to Chadwick on his 75th birthday—I was just remembering that seminar during that summer. This was a seminar— there were a few students—but mostly the people were either postdoctoral fellows or members of the faculty. Condon was there during the summer. As a matter of fact, during that summer, Condon and I wrote a short paper on the spin of the neutron, which the moment the neutron had been discovered, a discovery paper of Chadwick came out that summer.

Weiner:

Had you seen the paper?

Bacher:

This is part of the story. Certainly I'd seen the paper. This is part of the story, but at any rate, people who were in that seminar were Morse, Allis, Ned Frank, I'm sure Jay Stratton was there, Ed Condon, Manuel Vallarta, I don't know, quite a number of other people whom I don't remember right at the moment. At any rate, these papers came out, and this seminar we had was partly reporting on new things that people were doing, and partly was reporting on new things that were coming out in the literature. These papers were spotted, and somebody said, "We must hear about these."

There's probably nothing in them but they must be reported here." So they were assigned to me, and I set to work at them. Well, I hadn't spent very long at them before I realized that these were really extremely important, and that the paper of Chadwick was just essentially revolutionary. Not only was it a revolutionary paper, but it was most unusual among papers of that sort in the sense that it was an extremely tight paper, in that it was all right there. It was really a beautiful thing. I can still remember going into that journal club that afternoon. I had a group of people, and I don't think there were two people in the room that had looked at the papers or studied them carefully, and I don't think there was anybody believed them.

When I started giving the results of these things, and laying it out—which was very hard for me to do, with all these people who were considerably my senior there— I did succeed, by the end of the afternoon, in having most of them per- suaded that these papers were right. Those papers practically got worn out in the next few days. But they really were a masterpiece. Here was the point. The things I had worked at in atomic spectra were related to nuclear things, and this was something that was related to nuclear physics. I was an atomic spectroscopist, and this was not really my field that I was reporting on. But I went back to Michigan on a postdoctoral fellowship for a year, and then I spent a year in Michigan without a job, because this was right down in the bottom of the Depression.

Weiner:

Was it difficult then in general for physicists to get academic jobs?

Bacher:

Sure. If I had taken a job instead of a National Research Council fellowship in 1930, I would have had a nice job in 1933-34. But instead I took a National Research Council fellowship, and as a consequence, when the fellowships ran out and these things died out, there weren't new jobs to speak of. Very early in the year 1933-34, I had gotten a job for the next year, because things were beginning to get a little bit better, and I got a job as an instructor at Columbia for the following year. But I spent a year at Michigan without a job. It was a very good year, I might add, and I have absolutely no regrets. One of the best years I ever had.

Weiner:

What about the year at MIT? Was that on a fellowship?

Bacher:

Yes. I was there on the second year of a National Research Council fellowship.

Weiner:

What did you do in the year back in Michigan?

Bacher:

Well, when I went back to Michigan, during the year 1933-34— no, '32-'33 and '33-'34—I did two things. In the first place, with the general encouragement of Ralph Sawyer, who was extraordinarily good to me, both when I was a graduate student at Michigan and particularly sub- sequently when I was a postdoctoral fellow there—I guess I should say that during the late spring of the year I took my degree, I needed a place to hang my hat, and Sawyer invited me in, and I shared his office with him.

When I returned to Michigan as a young postdoctoral fellow, he was then one of the more senior among the younger people in the department there, and interested in experimental spectroscopy, and he in- vited me to share his office again, and I shared an office with him for two years, as his guest. It was his office. In addition, I used his laboratory, worked in his laboratory, used his equipment. He taught me. He had some interferometer plates there which hadn't been used for visible interferometry work, and he taught me some things about them, and I learned some things in the school of hard knocks, and learned how to take spectra and get some pretty high resolution work, which I guess was the only interferometer work that had been done in the visible at Michigan up to that time. But I learned a lot, and I learned a lot of it from Ralph Sawyer, who was extraordinarily kind and also very helpful to me.

Weiner:

You did a paper with him in 1934, as a matter of fact. this one.

Bacher:

Yes. Well, that was one of the first things that I worked at. During 1932-33, I worked with Goudsmit, and we had some really good ideas. This was quite an interesting line of work. Goudsmit and I wrote a paper on this atomic energy relation, and that was quite an interesting bit of work. As a matter of fact, there are parts of that that have never, never been completed even, which Goudsmit and I never have really finished up, because I got off and got interested in experimental things at that time. And, as you can see from that, the first thing was that isotope shift in magnesium. I was interested in the interpretation of this, and I think Sawyer had been interested in the experimental end of it. Subsequently I did some other things there, and worked at them. But this was very useful to me, and it gave me a sort of a start in experimental things and made it possible, so that subsequently I could either work on the experimental end of the thing or to some extent on the theoretical end of it. During the following year, when I was at Ann Arbor, I went back to doing some of this theoretical work, because actually those two things are in reverse order, I think. But in any event, there must be some things that are lacking from that, and I don't remember.

Weiner:

This is just a random bibliography.

Bacher:

Weiner:

This was a real coming of age for you, I realize, at Michigan, I'm not quite sure that I can remember all of them at this time. leisure to—

Bacher:

It was very good. I had a very good year there. I didn't have this year when, after your several postdoctoral fellow ships, you got the any obligations at all, and I worked like mad. See, one of the problems that people today don't remember, in looking back then—and this goes back to the time when I was completing a degree—at the time I was com- pleting a degree with Goudsmit, I knew I could complete a degree all right, but it was by no means clear, even when I got a Ph.D. degree, that I could make a living as a physicist. I wasn't so sure. I mean, students then had to make a decision that you might go on and complete your work for a doctor's degree, and you might have to go do something else. Because you weren't entirely sure you could make a living at it. There just weren't that many possibilities at that time, and particularly when the Depression got going, why, then it got pretty rough.

Weiner:

Had you always considered an academic career in physics as your aim?

Bacher:

Well, at that time there were relatively few jobs in what one might call basic science, or particularly in the fields I was interested in, in industry. So one naturally gravitated toward the academic institutions, because that's where most of these things were done. It wasn't until subsequently that industry was really very much interested in them.

Weiner:

After the year at Columbia, you went to Cornell.

Bacher:

Yes. The year at Columbia I spent in part teaching and in part trying to work on some things in connection with this theoretical work I'd worked on with Goudsmit, which in fact turned out to be really pretty frustrating. The year in Columbia was sort of a hard one, in many ways. But I enjoyed association with Rabi's people enormously, and gave a course during the spring term that a lot of his advanced graduate students went to—some people who were his postdoctoral fellows.

Weiner:

How did the transition from Columbia to Cornell come about?

Bacher:

Well, the transition from Columbia to Cornell came about in part because my wife and I find New York City a pretty hard place for us to live in. My wife very much wanted to have a baby during that year. So we decided, when we saw an opportunity that we could get away from New York City, that we thought this was a fine thing to do. This was also, I think very largely stimulated by the fact that Hans Bethe came to Cornell in January, 1935, and came down to Columbia to give some lectures. I had never met Bethe before, and I was very much taken by him, very much interested in the things he was working at. I thought this was a wonderful thing. It turned out that there was a little possi- bility that they might have a job opening up there, and in the spring they invited me to give a seminar. I went up and I just fell in love with the place. It was arranged that I leave Columbia and go up there for the fall. So we went back to Ann Arbor during that summer, and then went on to Ithaca that fall, and I was there until I went away on war work.

Weiner:

Well, now that we're into Cornell, it opens up a whole new chapter, a most interesting one.

Bacher:

Yes. One of the reasons I wanted to go to Cornell was that it offered to me an opportunity to get into experimental nuclear physics, because the arrangement that Gibbs promised me when I went there—Gibbs was then the chairman of the department, and a fine man—Gibbs was a spectros- copist, and I had known him somewhat for some time. When I went there, I should say that they had quite a lot of high resolution spectroscopic equipment which wasn't being used. I went there with the idea that I would first get some experimental work going in spectroscopy, and after I had that going and perhaps some people working with me, then I'd get an opportunity to go and do some work myself over in nuclear physics. And that's exactly what happened. In the meantime, Livingston went there and was well engaged in building that small cyclotron which was built there. Subsequently, then, I started learning a little bit about how to do experiments in nuclear physics.

Weiner:

You had a good place for it.

Bacher:

Yes, I had a good place for .

Weiner:

How did the work with Bethe then come about, leading to the Reviews of Modern Physics article?

Bacher:

Well, of course, this was Bethe's idea. I would say that practically all the credit for those review articles ought to go to Bethe because while some of the rest of us worked at it, they never would have been done without Bethe. And it was Bethe who, by his extraordinary force of intellect, ingenuity, and tremendous energy, managed to pull all of this stuff together, rework it where it had holes in it, and get it out in the form of a Review article.

Weiner:

Get all the stuff together—what do you mean by that?

Bacher:

Well, I meant that after all, this was a Review article. It involved looking into all sorts of things, including the re-interpreta- tion of experiments. And when Bethe looked at experimental work and found that they didn't seem to have made certain corrections, we went right back to the raw data and started working it up again. Or, if he went to a theory and he got into it and found out that certain things in the theory had been neglected, he reworked the whole theory. So that parts of that book, particularly the things that Bethe worked on in there, are really simply very much more than simply summarizing what's in the litera- ture. My contributions were, I would say, somewhat minimal, compared to Hans Bethe's.

Weiner:

Did you work as a group, Livingston and Bethe and yourself? Or did each of you work separately with Bethe?

Bacher:

I think the way it started was that Bethe laid out the plan of this, and at first it was going to be a very much shorter thing than it subsequently turned out to be. He first, I think decided—sort of laid out what the first part would be, and he had certain parts of the first volume that he thought I might work with him on, particularly the part on nuclear moments, and I think some other parts, but essentially the part on nuclear moments. Then he had all the second volume that had to do with various problems about kinematics and so on, and then the third part which was all experimental. Well, he got Livingston to work with him on the third part, that had to do with the experimental things, and I worked with him on the first one, and he did the second one all by himself. I think it's fair to say that, at least in the parts with which I'm familiar, the bulk of the credit for that goes to Hans Bethe.

Weiner:

Over how long a period of time was this work done?

Bacher:

I wish I knew the answer to that. Bethe was an extraordinary worker. I think—well, the things are dated, as I recall, over a period of only about a year and a half, something like that. I'm not quite sure. My volume of that is at home. I know it's not here. Hold on a minute and we can look. The first section was published in April, 1936, so let's see— April, '36. I didn't go to Cornell until the fall of '35, so Bethe must have been at work on this at the time I went there. I hadn't really realized that this was the case, but that must be it, and this must be one of the things that we worked at right away. This, incidentally, was something that helped to get me interested still further in nuclear physics. The part that Bethe wrote, which came out a little later that same year, Nuclear Dynamics, it was called—this came out in April, 1937.

Weiner:

Let's see, the first one was April, '36. I see.

Bacher:

Wait a minute. I'm not quite sure it's April, '36. The way this is done, the way this is bound together, I'm not quite sure that that's when it came out.

Weiner:

My reference here says that.

Bacher:

It does? Yes, that's right. The other one is April '37.

Weiner:

The third was the one he— That's the second piece. The third piece is still—beyond this.

Bacher:

—did with Livingston, yes.

Weiner:

Did Livingston have a tremendous bibliography?

Bacher:

Here it is. The other one was July, '37, you see—here. They then started reprinting these things. Here is an advertisement for it, back here.

Weiner:

I know where I can find , then.

Bacher:

I guess the answer to that is that the work must have been completed by the summer of '37. I would say Bethe must have worked— I mean, this work must have been done over a period of about two years, then, a little less.

Weiner:

That means that you were very directly concerned, then, with this new field of nuclear physics.

Bacher:

Well, yes.

Weiner:

For example, another thing happened at this time, and that was the discovery of the mesotron, as it was known then, or the mu meson, as it's been known more recently. What was the reaction to this at Cornell?

Bacher:

It didn't make a major impact on me, because I was still at that time—what year was that? Was that '36?

Weiner:

Well, it was '36, but it was really announced in an effective way in 1937.

Bacher:

Yes. Well, at that time I was just beginning to get concerned with nuclear physics. See, this was then tied up very much with cosmic rays, and I didn't know very much about cosmic rays. I hadn't had very much connection with them. I was interested in nuclear physics. Remember that when your maximum ene rgy is of the rde of r a million or a million and a half volts, and this is the range in which nuclear reactions are taking place, I mean, trying to think of the relation to cosmic rays, which today seems sort of a, you know, a gap that's been bridged, it was almost just another world.

Weiner:

It was another field of physics.

Bacher:

It was essentially another field of physics at that time. The way I got interested in this was, very largely, trying to do some simple- minded experiments with neutrons. And this was one of the reasons that I went to Cornell, because there was an opportunity to do this, and I guess that's the best way to describe them—I started doing some simple- minded experiments that were sort of going back and learning some of the things that Fermi had done. Then, shortly thereafter, we started finding out some things that were interesting. In 1938, Livingston left Cornell to go to MIT, to build a big cyclotron there, and at that time, they put me in charge of the small cyclotron. One of the last developments that Livingston worked on before he went to MIT was in collaboration with a young graduate student named Charles Baker.

He developed an arc source for that cyclotron. And while this sounds like a sort of a plebian thing at this late date, that made one of the biggest differences in cyclotron development that had been worked up. It immediately increased the current of our cyclotron until it was larger than any other of the cyclotrons at that time, and really put us in business. What was more particularly the case, as far as I was concerned, was that I read during the following year of some work that Luis Alvarez had done at—I guess this was probably now 1938—at Berkeley, in modulating the Dee voltages on the 37-inch cyclotron at Berkeley, and doing some time-of-flight experiments with neutrons. This was done with a great amount of labor. I began to read this stuff.

This paper of Luis'—for whom I have great admiration—said among other things that this was a fine method but wouldn't work above the thermal neutron range. I began to think about that and wonder why that was the case. It suddenly hit me that that wasn't true, if you could modulate fast enough and really do it, and I started figuring about the times that it took neutrons to slow down and so on, and realized that the time to slow down into the epithermal range was very short, and that if I could ever manage by tech- nical means to get over this thermal range, that we could get up into the resonance range by this time-of-flight method. Together with Charlie Baker, we started working at this, and succeeded in doing it. Just before, in the period 1938-'39-'40, we succeeded in getting up and measuring, for the first time, with the time-of-flight method, the shape of the first neutron resonance. And that was done at Cornell.

The first one was cadmium. We then started doing silver, indium, rhodium and so on, up the line, and as we moved on up to the resonances, we dis- covered, to our horror, that the things that we measured did not agree, and did not agree by quite a sizeable amount, with the values that Fermi had measured by the so-called boron absorption method. This was just where the work was when I was called away to go to the Radiation Laboratory, just after Christmas, in 1940. I was called up by Lee DuBridge who said, "Won't you come up after the meeting in Philadelphia?" or wherever it was, and I said, "Sure, but do you want me or do you just want somebody?" I went up, and when I saw what they were working at and what the problems were and got some of the feeling for it, I said I'd come.

I said I had some interesting work going in the laboratory at home, could I arrange with them to spend three days every three weeks to go back over a weekend to keep my lab going for a while? Well, we managed to keep the lab going, and we found the origin of this difficulty. When we found out what it was, we realized that all of the neutron resonances were wrong by about 60 percent. In other words, they were at a lower energy than had been anticipated, if I remember the sign of this right. I could look it up but think that's right. We began to think about this, and first I wanted to make sure that— I had such an enormous respect for Fermi that I stopped, the next time I could, going through New York, and went out to show him what we had.

Weiner:

When had you first met Fermi?

Bacher:

Back in Ann Arbor, swimming across the lake. I'd known him for a long time. This was now eight years after that, so I'd known Fermi for quite a long time. I showed him this, and I said, "I want you to look at this, because it's in complete disagreement with the things that have been done. Also, I don't know whether it has an impact on this uranium business or not." I said, "After all, they have discovered fission. After all, there are some resonances." So I stopped off to see Fermi. After all, I knew Fermi had been working on uranium, and I knew also—we'd done a little about fission, but had not really worked at this problem.

But the question was that if the energy scale on the resonances in general were off, then the resonances also, whatever one knew about the energy spectrum in uranium, were off, and I didn't know whether it was a good idea to publish this. So I went to Fermi for guidance on two points: 1) Did he think the work was right? It took him about one minute to conclude that the work was right. He took a look at what we had and he said there wasn't any doubt about that. Also he said that we had pinpointed it in the right place as to where the error came in.

Well, we had a pretty conclusive picture on it, because you don't contradict something Fermi does without looking into it very thoroughly. But the second thing was, he said, "Well, this is an important piece of work, and you ought to get it out and get it into the literature. We sent it in to the Physical Review, but after it was in, I began to get cold feet on it. So I wrote to them and asked them to hold it until the end of the war, and it was published in 1946. In fact, the next I saw of some of these things, they were in the files of the Metallurgical Laboratory in Several papers that we then subsequently wrote did not get published until the end of the war. Chicago, marked Secret.

Weiner:

This paper you published with Baker and McDaniel—

Bacher:

No, it was earlier than that.

Weiner:

I see. Well, I don't have the complete list. Oh, here's one— "Experiments with a Slow Neutron Velocity Spectrometer."

Bacher:

That's probably it.

Weiner:

have to check.

Bacher:

I'm not sure just which one this was. We had published work on this before, but finding out that the resonances in general were at the wrong energy was not something that we'd known before, and it took us some little time to find this out. And even after we found it out, it took us some little time to believe it.

Weiner:

After going to the Radiation Lab at Cambridge, I don't imagine the initial commuting back to your laboratory kept up for very long.

Bacher:

I kept that up, I guess, for about a year and a half.

Weiner:

Oh, that long?

Bacher:

Yes, or a year and a quarter, something like that, because the laboratory in Ithaca—let's see, we closed the laboratory in Ithaca down probably in the spring of '42. Yes. I was in the laboratory on a Sunday when Pearl Harbor happened.

Weiner:

In the little time that we have left, I'd like to ask a couple of general questions. There's a lot more to talk about, and perhaps we can schedule another session, but for this session—

Bacher:

afraid I've talked on.

Weiner:

No, I expected that this would go longer than two hours, be- cause there are a lot of interesting things, once a person gets started. As it is, you've left out, I'm sure, many details, which if I were more knowledgeable, I would have been able to probe. But just let me ask a general question. I think it's appropriate anyway. As far as the effect of the war on the developments in nuclear physics, other than the fission question—I'm trying to determine whether people feel the war was a question of putting things away and then just picking them up where they left off? Or was there a certain new approach to nuclear physics?

Bacher:

Well, I think there was a certain new approach, among other things. Certainly, in the early days of the war, certain things in nuclear physics didn't get done that would otherwise have been done, because a lot of nuclear physicists were working on radar. You see, in the spring of 1943, Rabi and I went to Los Alamos for the conference, at the time that was set up, and really there were two reasons for getting some help from the Radiation Laboratory: one, a lot of nuclear physicists had gone into the Radiation Laboratory and were there; secondly, a lot of electronic things had been developed at the Radiation Labora- tory which were applicable to nuclear physics.

After all, one of the reasons I think I got sort of requisitioned to go into radar work in the first place was, in this time-of-flight work, we had been measuring times-of-flight down to a few microseconds, which is exactly the same thing that you do in radar. In radar you're measuring the time of an electromagnetic impulse to travel to some distance away—the time required for an echo of an object about a mile away, if I remember correctly, is about 10 microseconds. Well, you see, it's in the same general category. The time-of-flight of a thermal neutron for one meter is 450 microseconds. The time-of-flight of a 1 volt neutron is, I've forgotten, 60 microseconds or something like that.

Weiner:

Now, so in the early stage, a lot of the people in nuclear physics were not at their home universities, but were doing work in radar. Then, of course, just about everybody was doing work at the Los Alamos project, whether there or at related institutions. But then, after the war, there seems to be a whole new development. What were the certain new approaches that were involved?

Bacher:

Well, in the first place, one of the things that I think is interesting in this is that during the war, a great deal of fundamental work in nuclear physics was done at Los Alamos and at some of the other laboratories. To be sure, the bulk of this was neutron physics, but whole new techniques were developed for getting at some of these problems. Also, I think that people, by the end of the war, came to be very greatly interested in these higher energy phenomena. For example, in the summer of 1945, a rumor went around that they had, at the General Electric Company produced mesons artificially in the synchrotron they were developing there. And this was in the summer of l945.

Weiner:

As early as that?

Bacher:

Yes. Well, I'll tell you how I know about that, because in August of 1945, I went with Robert Oppenheimer to Washington, and we were on the train going East on VJ Day, and the purpose of our trip to Washington was to talk with General Groves about what did we do with the Los Alamos Laboratory. Groves of course was overjoyed at the way things were going, and said, Is there something that we would like to do in the East? We said, "There certainly is. We'd like to go up to the General Electric Company and see what's going on up there, from this rumor we had heard." They got a plane and flew us up there, and we went up there and spent some time looking at that and a few other things. And this was in August—this was VJ Day, 1945.

Weiner:

But in fact they hadn't found mesons and their pictures were wrong.

Bacher:

No, that's right—in fact they had not found mesons and their pictures were wrong. But on the other hand, the reason for telling you this was not that this was the discovery of mesons created by this method, but it was, I think, an indication that people were extraor- dinarily interested in this at that time.

Weiner:

By the way, when you went up there, did you look at the plates?

Bacher:

Yes.

Weiner:

And what was your reaction at the time? Was there enough there so that you could make a judgment?

Bacher:

No, you couldn't tell, It's hard to capture something like that in retrospect, but my memory was that, trying to divorce myself from what I know to be subsequently the situation, was that it was interesting and might be true, but it wasn't something like Carl Anderson's picture of the positron, by a long shot.

Weiner:

Here they are developing the synchrotron at GE even before the war is over, really. Why is this? Because they thought they wanted to get into the atomic energy field?

Bacher:

I haven't the faintest notion. I don't know what the background of that is. I just don't know.

Weiner:

Guy Suits would be the person to ask.

Bacher:

Yes, Guy probably can tell you about that. I don't know why they were doing this. It may well be that this was part of some other development, and it's very likely that with the slowdown of the war in Europe, which you see had taken place some time before, some of their advanced development people may quite a long time before that have gone back to work on other things. I just plain don't know. I don't remember the circumstances of that.

Weiner:

What other symptoms were there of things to come, at the end of the war or shortly thereafter? Were you aware of any other attempts to build higher energy machines?

Bacher:

Oh, yes. Hans Bethe and I both went back to Cornell at the end of the war, and right away we set about getting permission at the uni- versity there to build the High Energy Laboratory that now exists, up on the hill there, and to go ahead with the construction of a 300 million volt electron synchrotron. We started right after that, and started developing the designs. That was started during the fall—well, I didn't get back there until the winter of 1945-46, but we started right ahead with that. Now, I didn't stay there even long enough to see the building built, because I was pulled off to Washington, when the first Atomic Energy Commission was set up.

Weiner:

That was for a three year period, wasn't it?

Bacher:

Yes.

Weiner:

Feynman came to Cornell right from Los Alamos.

Bacher:

We hired Feynman for Cornell at Los Alamos. He was one of the very bright young theoretical physicists there, and we persuaded him that it was a good thing to go to Cornell. One of the other very bright young theoretical physicists there was Christy, who is here at Caltech and who works in astrophysics.

Weiner:

There are other developments. Someone told me that at the University of Illinois—by the end of the war, there was a very large machine ready to go into operation, and then Berkeley had plans, constant plans for increasing energies.

Bacher:

Well, Berkeley was involved in building the 180-inch cyclotron before the war, and then it was converted and used for this electro- magnetic separation work. Then at the end of the war, they started converting it to this. But don't forget that the thing that really put the impetus on the machines was the work that McMillan did at Los Alamos, and McMillan started this wo at Los Alamos really before the first bomb went off. I mean he had A idea for this , and went off and was, working on this in spare times, and as quickly as he could be spared from the work with which he was immediately charged, he immediately went to work on this thing.

I don't know what the date on that paper is, but it must have been early in the fall of l945. But that paper was written in Gamma Building, because Ed McMillan was one of the group leaders in the division for which I was responsible at Los Alamos. When he started to work on this, it was perfectly clear that this was a really stupendous idea, and it was this—well, together with the work of Veksler's, which of course was independent—this was really what gave the high energy machines their impetus. It was an idea how, either by changing the frequency or the field or both, you could get over the catastrophe of the relativistic effects that came in, and at the same time get a stable beam that could be accelerated through this period and get up into the really relativistic ranges.

Weiner:

During the earlier period in the 30's, when did you feel that you were really in nuclear physics, and what would you have called yourself, if someone had asked you what your specialty was? What would you have said?

Bacher:

Well, usually when people have asked me, over my life, what my field was, I've said I was a physicist. you working on?"

Bacher:

Well, I would say, during that period along in the early 30's,

Weiner:

But suppose this was one of your colleages who said, "What are I would have said I was interested in atomic spectra. And if you'd asked me after the mid-30's I would have said, I guess, that I was a nuclear physicist, or becoming a nuclear physicist.

Weiner:

I ask that particular question because it's interesting to note when a field becomes clearly defined in the minds of the people that are doing the work.

Bacher:

But on the other hand, remember, that's when I went into the field, so the field was well defined before then. But I think, to come back to one point that you spoke about before, there was not probably, until the end of the war, when the possibilities of making really high energy machines came forward, the beginnings of an identification between the work that had been carried on with cosmic rays on elementary particles and so on, and the field of nuclear physics. And in fact, there was a certain amount of feeling on the part of people, and I've heard some very good theorists say (I won't mention any names) in 1945, that if they knew the proton-proton scattering really accurately up to 4 million volts, they'd know all about nuclear forces. Well, in retrospect that looks pretty silly.

Weiner:

A lot of things happened in a few years right after that.

Bacher:

That's right, and nobody should be blamed for making a statement like that, but it gives you a little bit of the perspective that everybody had at that time, of focusing on these low energy nuclear reactions as the only course of getting at some of these problems having to do with nuclear forces and so on. Well, it's turned out of course to be a fan- tastically much more complicated problem.

Weiner:

What made the change in people's minds? The discovery of the pion? Was that the thing?

Bacher:

Well, yes. I mean, don't forget that the Yukawa paper had been written before. It had been predicted that a meson might be associated with this. I think it took a long time for people to realize that this was not just something connected with the mechanism of the force, but also something that was really involved with it. But it was only really when there came to be the possibility that you could do experiments in this region that people began to sort of—you know—break out into a real white heat about it. Of course, during the period immediately after the war, practically all of the significant results (I shouldn't say practically all, but a large fraction of the important results) in particle physics came out of cosmic rays. But as soon as the big machines got going, why, this changed quite a bit.

Weiner:

In trying to establish some guidelines for historical documentation, I find that it's relatively easy up until you get to the mid-40's, and then the situation changes. You just don't know where to look because things go off in so many directions. And yet, there must have been some themes in this period, in terms of major developments—some way to organize and describe all that was going on.

Bacher:

Well, I don't know quite how to answer your question. First of all, a good bit of the work in low energy nuclear physics was con- cerned with the detailed study of nuclear reactions—finding out precisely what the cross-section for a given reaction was, what alternate reactions there might be, and what the cross-section was as a function of energy. And with the techniques available at that time, this was an extraordin- arily laborious thing to do. And yet, that work was done. It was done in the Kellogg Laboratory here at Caltech. Livingston and his students did it—Livingston, Holloway and Baker and others did it at Cornell.

Tuve and others did it at DTM, and various other people did it at various other institutions. Also, of course, there was the work that got started on the neu- trons, and this was very largely stimulated, first, by the extraordinary discovery of the neutron, and second, by this fantastic stuff that Fermi discovered about the unusual properties of slow neutrons. And this happened all right along about the same time. I mean, those first papers of Fermi's about the properties of slow neutrons and the effect of thermal neutrons I think was published in 1934 or '35. Well, if you want to get some feeling for something like the amount of experimental information that was known in that field of nuclear physics, pick up the latest survey of this by Tom Lauritsen and Fay Ajzenberg-Selove, and put it down beside that Review article that we were just referring to a little while ago. This would be quite a startling thing.

Weiner:

Well, now it's a question of feeding results into a computer in order to even make sense of them.

Bacher:

Yes. You find out more in one afternoon than you could find out in a year, before.

Weiner:

But what date would you say—and it's really unfair to ask a person to describe a process with a specific date, but anyway, a period of time that characterizes the beginning of this new era.

Bacher:

Which new era are you speaking about? The beginning of nuclear physics, or what? Because that goes back still further.

Weiner:

The change in the level of activity, and the breaking out of high energy physics as a separate field.

Bacher:

Well, I think actually that most of that occurred really after the war. On the other hand, Anderson and Neddermeyer and a few other souls had been in this up to their ears, on the cosmic ray side, for years. And Powell did, of course, some extraordinary work, during the war, and his work with photographic plates, done during the war, is an epic of what can be achieved by a really clever man without very much equipment to work with. But I'm not sure it's easy to say it that way.

I think what really happened might better be described this way, that it was always recognized in nuclear physics that there were fundamental problems of nuclear forces, and that these were somehow involved with large energies, but one could not really hope that one could carry out large numbers of quantitative experiments at these energies, because one didn't know any way of getting up there. And really, it was the combination of the extraordinary pioneering work of the people who worked with cosmic rays, on the one hand, showing that not only were there pions and mu mesons, but also other unstable particles which they only partly got classified; together with, at the same time, the extraordinary ingenuity that was used in development.