Interview with Hans Bethe by Charles Weiner and Jagdish Mehra at Cornell University October 27, 1966 Audio excerpt available

Transcript

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Mehra:

I would first of all like to greet Professor Bethe and congratulate him on the events and celebrations in which the whole physics community greets him as one of the most remarkable leaders in theoretical physics in the century.

Weiner:

I think that's a fitting beginning to the tape. I'd like you to respond.

Bethe:

Well, you are very kind and I am somewhat embarrassed and very pleased.

Weiner:

I'd like to note that today is October 27th, 1966, and we are sitting in Professor Bethe's office with J. Mehra and Charles Weiner listening and occasionally interrupting with clarification questions. Our intention is to discuss the development of nuclear physics as a field of inquiry.

Bethe:

Nuclear physics of course started a long time ago, and I know very little about the start. I know only what you read in the books, and so therefore it is not very useful. I think you should try to get other people who were present in the early days of nuclear physics to tell you about the excitement of those days. Nuclear physics in a way started with the discovery of radioactivity by Curie, then the exploration of radioactivity by Rutherford, which happened in the first two decades of the century. During this time one got an idea how radioactivity worked, that it was a transmutation of the elements, that alpha radioactivity decreases the atomic number by two and beta radioactivity increases it by one. Many people contributed to this, and in the course of time--especially in the '20s--the Rutherford school proceeded to quantitative experiments about the energies of radiations emitted from nuclei and thereby put it on a really scientific basis. They found, as you know, that alpha particles have very definite energies, and it was possible already in the '20s to make energy level schemes of nuclei corresponding to the various groups of alpha particles which are often emitted from the same nucleus. They found also--and that, I think, was discovered a little later--that beta rays are emitted in a continuous spectrum. This gave rise to a great deal of speculation in the early '30s--namely, the question whether the law of conservation of energy was violated in the case of beta radioactivity, and somebody as knowledgeable as Niels Bohr proposed that point of view. Others, particularly Pauli, opposed it; and I will later on tell a little about the way it was resolved. Then it was discovered that there are gamma rays, which were found to be X-rays of particularly high frequency, and these gamma rays, I think at a very early time, were attributed correctly to the transition of a nucleus between two energy levels. Now, the main question in these days was, of course, what the nucleus consisted of; and this question was asked very far back--I think certainly as far back as 1920--and the first idea of the workers in the field was that they consisted of protons and electrons, these being the only known fundamental particles at the time. So the more or less accepted theory in the 1920s was that the nucleus consists of protons, as many protons as its mass indicates, and then electrons to the extent necessary to give the right charge to the nucleus. Well, then came quantum mechanics, and with quantum mechanics came the first triumph of theory in explaining nuclear phenomena. This was the theory of Gamow and of Condon and Gurney of the alpha radioactivity. I forget the date. I could look it up if you want me to.

Weiner:

'28, wasn't it?

Bethe:

I think it was '28--something like that, it can be looked up very easily--in which they explained it in a very brilliant way as a penetration of a potential barrier. This explanation is still as correct as it was to begin with and gave the first confidence to physicists that the behavior of a nucleus could be explained theoretically on the basis of standard theory.

Mehra:

If I may ask you a question here, Professor

Bethe:

Would you say that the first real attempt to explain the nucleus in terms of the proton and the electron was really not the first theory, but the first theory of the nucleus was this alpha decay theory which tried to encompass perhaps saturation and isotopic structure, nucleus sizes, and so on. What would you say was the first theory of the nucleus?

Bethe:

I don't know. I think you would have to dig in the old literature. I have never tried to learn it. I just don't know. It is likely that you'll find something about it in the old book by Rutherford, Chadwick, and Ellis, but I don't know what they believed. Certainly they believed, as you suggest, that the alpha particle was an important constituent of the nucleus. I am pretty sure that Rutherford already believed that the alpha particle itself was composite and was composed originally of protons and electrons.

Mehra:

But that is prehistory.

Bethe:

That all is prehistory.

Mehra:

And in the modern sense, as you said, is the Gamow-Condon-Gurney theory.

Bethe:

The Gamow-Condon-Gurney theory was the first theory that explained something about the nucleus. It did not explain its structure. It did not explain its energy levels. It did not explain its stability. It explained only one thing--namely, the lifetime against alpha decay as a function of the energy of the alpha particles. And I think it has two great points. One is that it was the first theory, as I said, which gave people confidence that something useful could be said by theory about the nucleus; and because these other things we mentioned--protons and electrons and alpha particles, and protons and electrons--certainly were in no way quantitative and nobody could make anything of it which could be dignified by the name "theory." It was the first thing that was really a theory. It explained only a very small feature of the nuclear phenomena but it explained those quantitatively.

Weiner:

This was also, I gather, one of the first applications of quantum theory. When you referred to theory, you meant quantum theory?

Bethe:

No. By theory I mean anything which gives a quantitative and logical explanation of some phenomena. If this can be done by means of classical mechanics and electrodynamics, I don't object to it. I don't think any nuclear phenomenon has this feature, that it can be so explained. But if there were such a phenomenon, I would not object to the lack of quantum mechanics.

Weiner:

I just checked. All the papers--Gamow's paper and the Condon- Gurney paper were in 1928.

Bethe:

The second great importance of this theory is that it was later on found applicable to a lot of other nuclear phenomena--in fact, all cross sections of processes which involved charged particles involved this theory in some form. So that was the first great success. Then on the other side quantum mechanics made even greater trouble than people had had before with the phenomenon of beta decay because you could prove essentially without question that electrons simply could not be in the nucleus, the nucleus being much too small to contain electrons. Essentially one can use a loose form of the uncertainty principle and say that no particle can be contained in a stationary state in a space which is smaller than its Compton wavelength, which is h/mc, which for the electron is about 4 X l0- cm, whereas even the biggest nucleus is less than 10-¹²cm radius. So it was impossible to maintain the old theory, and just to make bad things worse, it was then found that also the statistics of nuclei and the spin of nuclei contradicted the idea of the constitution in terms of electrons and protons. In this respect it didn't make any difference whether you considered the alpha particle as a sub-unit or not. Electrons and protons give the same result as alpha particles, electrons, and protons, and in particular it was found that the nucleus nitrogen 14 has Bose statistics and has an integral value of the spin, whereas it should have--if it were composed of electrons and protons--Fermi statistics and should have a half integral value of the spin. So there were at least three independent proofs that electrons could not be in the nucleus, and if you want, you can add a fourth not quite so convincing, that it was impossible to construct a theory of the beta decay which was in any way similar to the theory of the alpha decay. So at this point I think physicists were at a complete loss, and it would have been entirely impossible to construct any logical theory of nuclear structure. This period ended in 1932 with the discovery of the neutron, and therefore I would like to call everything before 1932 the prehistory of nuclear physics, and from 1932 on the history of nuclear physics. And here perhaps is a good point to pause.

Mehra:

Here perhaps I could ask you a question. When and how did you happen to go into nuclear physics from atomic physics? I would like to know what prompted you to work in nuclear physics, especially since nuclear physics was hardly in a good or attractive state, or did you go into it because it was in a bad state?

Bethe:

I went into it only after 1932, and after that it was in a good state.

Weiner:

Was that a causal effect?

Bethe:

This was a causal effect. Not this way around, but the other way around. After the discovery of the neutron in 1932, it was in a general way clear what had to be done, and so after the discovery of the neutron, nuclear physics came essentially into the state in which quantum theory was after 1926. There was a fundamental theory and you could work with it. I think I am not in any way a genius. I cannot invent something out of nothing. Some people can. I think Schroedinger making wave mechanics, Heisenberg making quantum mechanics, Dirac making relativistic quantum mechanics--invented more or less something out of nothing, or out of very little. This I cannot do. The only thing I can do is to take a subject in which the foundations have already been laid and then try to exploit them.

Mehra:

So given the neutron you thought you could do nuclear physics.

Bethe:

That is correct.

Weiner:

Did you know it at the time? Was this a conscious decision? How did you react to the discovery?

Bethe:

I think I should probably describe the discovery of the neutron and surrounding events, and then I'll put my own decision in that framework. The discovery of the neutron has been described very well in a conference on the discovery of the neutron which was held at Cornell about five years ago.

Weiner:

In 1962.

Bethe:

And I think I cannot add anything to this--perhaps only a little bit of atmosphere. In early 1932 I was in Rome with Fermi, who was extremely puzzled by the experiments which had been done by various people--by Bothe in Germany and by Joliot in France and several other people, which showed that there apparently was a radiation coming from a bombardment of beryllium by alpha particles; there apparently was a radiation which had the most peculiar properties. It fitted in no way the properties of any known radiation. It clearly was not charged particles because it went through everything very easily. But it also clearly was not gamma rays because the absorption in some light material like paraffin was as strong or stronger than the absorption in the same thickness of lead; whereas, if it had been gamma rays, the difference would have been tremendous, with the lead absorbing much more. And also, this radiation seemed to be able to eject protons from nuclei, and that again didn't make any sense with gamma rays. Electrons want to be ejected by gamma rays and not protons. And so it was a most peculiar radiation. The existence of this radiation persuaded Fermi to go into nuclear physics, and in 1932, in the spring while I was there, Fermi determined that he would work experimentally on nuclear physics.

Mehra:

You were visiting him in Rome.

Bethe:

I was visiting him. I was a Fellow of the Rockefeller Foundation. It was my second visit to Rome. I had visited there a year earlier, at which time Fermi was completely a theoretical physicist without any question. But in 1932 while he was still doing theoretical work, he was determined to go into experimental nuclear physics. I knew, and had always known, that I have two left hands, and therefore did not decide to become an experimental physicist. The subject was at that time still too unclear to appeal to me. I think late in 1932 Chadwick found the right solution. I don't know exactly which month, but I'm sure this is known, and the right solution was the neutron. And he very soon did a series of really quite ingenious experiments showing beyond any question that this was a neutral particle of a mass very close to that of the proton. The final paper came out I think only in the spring of '33, and I remember this but not terribly well. I seem to remember that it was only in '33. What I remember is that I was to talk about this in the physics colloquium at Munich but was told a day or two ahead of time that I shouldn't because the Nazi students would demonstrate.

Weiner:

Against you personally.

Bethe:

Against me personally.

Weiner:

Had you had any inkling of this sort of reaction before that time?

Bethe:

Well, the anti-Jewish laws went into effect on the 1st of April and this was later, so I knew that there was something coming and I had been dismissed from my job in Tübingen where I was sort of assistant professor. So I knew something was coming. I knew that I had to leave the country, but I thought that a talk was still in order.

Mehra:

These demonstrations would have been from the members of the colloquium?

Bethe:

Yes, Nazi students.

Mehra:

And you had gone to Munich from Tübingen?

Bethe:

From Tübingen.

Weiner:

Did you hold a joint appointment that year?

Bethe:

No. When I was dismissed from Tübingen I went to Munich to my old teacher, Sommerfeld, who got some fellowship for me temporarily until he could find a job for me outside Germany.

Weiner:

What month was it that you were dismissed from Tübingen?

Bethe:

In April.

Weiner:

How did that come about?

Bethe:

Maybe we should come back to that, during the personal discussion.

Mehra:

Well, since you didn't talk about recapturing the atmosphere of those days, and it could not have been a very pleasant one, I would like to ask very briefly about the circumstances attending the development of nuclear physics in the places you had been--say at Tübingen and at Munich-- about these centers which had started prospering. Is that reasonable?

Bethe:

I think that's very reasonable. There was no nuclear physics in Tübingen. There was no nuclear physics in Munich. People were interested only in some news like the neutron as exciting news in an area neighboring to their main interest. There were only very few places in Germany where nuclear physics was seriously done. The most important places were Bothe's lab (I don't remember for sure where he was at that time) and then Meitner (Hahn and Strassmann got into this much later) so it was mainly Meitner. With Bothe was Gentner, who is now still practicing nuclear physics. These two--Bothe and Gentner--really did the only important work on nuclear physics in Germany at the time.

Weiner:

You mean post-1932. Or were they involved before 1932?

Bethe:

They were involved before 1932--already in the prehistory. This didn't essentially change until much later, as far as I know, and it changed essentially only because Heisenberg got interested in the theory, and then he inspired other people to go into experimentation.

Mehra:

It was Heisenberg who first proposed that the neutron was the constituent of a nucleus along with the proton, and I would like to ask you about the discussions that attended this new model of the nucleus in which perhaps you participated yourself, and also about the name "neutron." How did it come about? Was it labeled immediately by Chadwick, or by other people?

Bethe:

As far as I remember, it was labeled immediately by Chadwick; and I think it had been so labeled before it was born by Rutherford.

Weiner:

I think in 1920 in his Bakerian lecture he predicted this.

Bethe:

In 1920 Rutherford said it would be nice to have a neutral particle corresponding to the proton, and that would make the structure of nuclei much more symmetrical and much more understandable. But at that time of course there was no such particle. And I think he called it "neutron" already then.

Weiner:

I think this is covered in the Ithaca meeting. We can look that up.

Bethe:

I don't know, but I'm pretty sure that Chadwick immediately had that name. Now, I did not participate in the discussions leading up to Heisenberg's paper and I don't know about them. Heisenberg's paper, as far as I remember, still came in the same year--'32. As soon as that paper appeared everything became very clear, and it now was the question to get the right theory.

Weiner:

I just want to go back to give you an opportunity to answer the question that you started to answer, and that is about the centers of interest. You established the point that Germany was not such a center with certain exceptions, which you mentioned. This is no contradiction, but in 1932 you felt that this field became defined, at least for you personally. At that time you were between England and Italy. How was the situation in each of those countries?

Bethe:

The undisputed center was England, and the undisputed center in England was Cambridge.

Weiner:

And you were there?

Bethe:

Well, no, I was not there. I had been there in 1930, but at that time I was repelled rather than attracted by nuclear physics because it seemed to me really groping in the dark for energy levels and having not enough evidence and giving a quantum mechanical description of energy levels without really knowing what was going on. So in 1930 I did not work on nuclear physics while in Cambridge at all, and I had no idea that it ever would interest me. In fact, I began to be interested really only after Chadwick's big paper in 1933, plus Heisenberg's paper, plus emigrating to England in the fall of 1933. After that I was very closely associated with Peierls. We were both at Manchester University. And while Manchester itself was not terribly interested in nuclear physics, was very much in the air in England; and during this period of 1933 to '4 Peierls and I wrote our first paper about nuclear physics--namely about the disintegration of the deuteron by gamma rays, the photodisintegration. My chief interest at the time still was not nuclear physics but was divided between two things. One was solid state physics, in which I was influenced by Bragg, who was the professor at Manchester, and during that time I wrote a paper about order and disorder in alloys.

Mehra:

This would be Sir Lawrence Bragg.

Bethe:

That's right. Secondly, I was very much interested in the exploitation of relativistic quantum mechanics for phenomena involving radiation, and I mean now electromagnetic radiation--gamma rays--and the result of this was a paper with Heitler on the absorption of gamma rays in matter, their formation and gamma ray emission. At that time it wasn't clear whether this had anything to do with nuclear physics, although it was pretty clear that it didn't really have to do with nuclear physics, but was just a tool in nuclear physics and no more. So there were three different things which interested me. It is very likely in fact that I don't remember exactly when we wrote the paper about the photo-disintegration of the deuteron ...

Weiner:

That was published in 1934 in connection with the International Conference of Physics.

Bethe:

Yes. That's what I thought it was. And this means that we worked on it during the year '33 to '34 in Manchester.

Weiner:

You had worked with Peierls previously in Munich in '27. You marked papers together, he remarked. And he stated that he followed your example, though he thinks not consciously, by accepting a Rockefeller fellowship, going to Rome, studying under Fermi and then going to Cambridge. This implied that it was a rather close relationship.

Bethe:

Yes. We were quite close friends. In Manchester we lived together. He was married and had a child and therefore had a house and lived with them in that house.

Mehra:

Peierls had been a student of Heisenberg?

Bethe:

Yes.

Mehra:

And where did you meet?

Bethe:

In Munich. He was first a student of Sommerfeld, but didn't finally take his Ph.D. there but then went to Heisenberg to do his thesis.

Mehra:

To Leipzig,

Weiner:

Getting back for a minute to your visit to Fermi, you said that he became interested at the time while you were there; and I think it would be interesting to talk about that because you said your own interest really didn't get started in a strong way until about two years later. Do you remember the circumstances?

Bethe:

Well, one or two years later.

Weiner:

In '33 you did this work, and so therefore you must have been interested by then. Do you recall--how do you know this about Fermi getting interested? Was this the result of conversations?

Bethe:

We talked a lot. The lab in Rome was a very nice small place where all of the people working there, half a dozen or so, talked to each other constantly, so there were constant discussions. Fermi just was very free in telling everybody that this was what interested him and what he was proposing to do.

Weiner:

Laura Fermi in her book indicates that conversations, or at least one conversation between you and her husband, was in English because that was the best way to communicate.

Bethe:

Well, actually, when I was at Fermi's home we always spoke English because that was the only language which was common to the three of us. However, in the lab we mostly talked German. Fermi spoke German very well.

Mehra:

He had stayed at Göttingen previously?

Bethe:

Yes.

Mehra:

May I take up the discussion that you introduced in your opening statement about the development of nuclear physics that you weren't interested up to 1932 essentially? Would you again in broad strokes perhaps continue this development of nuclear physics and how it unfolded, again in the light of Charles Weiner's question about the centers, because that relates to the sociology and historiography of physics, and then also relating it to the various models and parents ... ?

Bethe:

Very good. As I said, in 1933 I think Cambridge was undisputedly the center of nuclear physics. There was a lot going on there, and I should mention at this point the building of accelerators because at the same time as the discovery of the neutron--just a few months later, I think--the first machine was built which was able to disintegrate the nucleus, and this was the Cockcroft-Walton generator which got up to about half a million volts and which was then the tool by which the Cambridge group--Cockcroft himself, but equally Rutherford and Oliphant, and then Chadwick--discovered one thing after the other about light nuclei. And the important thing at the time was to get some information about the simpler nuclei, about the light nuclei. I think, to come back to a point you mentioned earlier, Cockcroft certainly should be asked to give a good account of the invention of the Cockcroft-Walton machine and of the first work that was being done with it. Well, from this work we learned many things. One was that almost any nuclear reaction that you can think of which preserves the total number of neutrons and protons, almost any nuclear reaction will go, provided the two nuclei can come together; and whether the two nuclei come together of course is determined by their ability to penetrate the potential barrier. So we again get Gamow-Condon-Gurney. Secondly, we obtained very accurate data on the energy of the various nuclei. We obtained an accurate value of the binding energy of the deuteron, the simplest of all nuclei, and that came mainly from the work of Chadwick and Goldhaber on the photodisintegration. And this was the work which Peierls and I analyzed theoretically. Then we learned that the binding energy of the nuclei of mass 3, the triton and helium 3, was much bigger than that of the deuteron, and in turn the binding energy of the alpha particle, of mass 4, was much bigger than those two. The nuclei of mass 3 were formed for the first time in Rutherford's experiments. They had never been known before. We learned about the difference in mass between these two nuclei--helium-3 and hydrogen-3. We took a long time to find out which one was heavier, but that was enough for us to know that they were almost equal in mass; and then, since the neutron has greater mass than the proton, it followed that the hydrogen-3 was a little more strongly bound because it contains one more neutron; and this could be interpreted in terms of the Coulomb repulsion between the two protons which exist in the helium-3 nucleus. So from this we learned right away two tremendously important things. One is the size of the small nuclei, and the second was that the nuclear forces, the forces other than the electrostatic force, must be very accurately the same between two neutrons as between two protons, and that is now known as charge symmetry. So this came out of the very first experiments of the Cambridge school in 1935 or '34, I think.

Mehra:

May I ask here that it was at this stage that charge symmetry and charge independence ... ?

Bethe:

Not yet charge independence.

Mehra:

I see. It was only charge symmetry that came to be recognized as applicable to nuclear physics.

Bethe:

That is correct.

Mehra:

When would charge independence be recognized?

Bethe:

That was about '36 and I will come to that.

Weiner:

You mentioned--just before you go on here--that this very important work was based on the very early results coming from Cambridge, and at that time results from other places where accelerators were being built and this type of work was going on had not yet become significant.

Bethe:

Yes.

Weiner:

Although they were doing work, their work did not take effect as this work did?

Bethe:

Right. Then perhaps the most important work at the time was that concerned with the understanding of nuclear forces, and the two most important names in this are Heisenberg and Wigner. Heisenberg in his first paper suggested a model for nuclear forces, and he suggested in particular that they should be similar to the forces in molecules, that they should be exchange forces--exchange forces in which the proton changes into a neutron and vice versa. He didn't have it quite right. The man who got it completely right as far as exchange forces were concerned was Majorana, a very taciturn Italian physicist, who was extremely able but in contrast to other people, did not come to the lab very much. I think I saw him once or twice in the seven months I was there.

Weiner:

Was he a student in that period?

Bethe:

He had his doctor's degree. He must have talked at least during his examination. And he was, I think, about the same age as Segré and I.

Mehra:

Was he associated with Fermi, too?

Bethe:

In a vague way, yes--to the extent that he was associated with anybody. I think the only person to whom he talked a little bit was Segré. Somehow Segrè could draw him out. While Majorana studied the problem of intereaction closely and found that it was necessary to attribute a somewhat different property to the exchange forces—namely, that the two particles should exchange only their positions but keep their spins; otherwise you didn't get any bound deuteron. So this was one line. It should be mentioned that Heisenberg had the idea that there should be an interaction only between the neutron and the proton but there should be no interaction between two protons or two neutrons because no exchange could take place in this case. It was shown in 1935 and 36--and this is somewhat anticipating the history--by Tuve and Hafstad and others at the Department of Terrestrial Magnetism in Washington that there is a strong attractive force between two protons, and thereby this part of Heisenberg's idea was changed, had to be changed. There were forces between any pair of nucleons. Then these experiments of Tuve and Hafstad were evaluated by Breit, who has kept evaluating similar experiments to the present day. He showed that to quite good accuracy the force between two protons is the same as the force between a neutron and a proton in the same spin state--namely, when their spins are opposed to each other. This fact then gave rise to the idea of charge independence, and that I think is in a paper by Condon and Breit--there may be other people involved-- in about 1936 in which they postulate a charge independence plus the idea of how to formulate this cleverly--namely, by means of isotopic And the idea of isotopic spin goes mainly back to that paper, although Heisenberg already had a bit of that formalism in his original paper.

Mehra:

He had introduced isotopic spin.

Bethe:

Yes, he did.

Mehra:

May I ask you a question about exchange forces? In the first section of your three-part article on nuclear physics called the "Bethe Bible," under the heading "Saturation of Nuclear Forces," you argued that one had to look for an analogy with chemical forces. The popular one was the homopolar binding. This would lead to the saturation property. You said there, "It is interesting to note that it will also yield exchange forces." Was it in this article that the idea of exchange forces--at least as an important point--was first introduced? One is struck by the importance of the use of analogy in your article. I would like to know whether such an analogy was being discussed at the time, or did you just take it as an example, or had the idea of exchange forces already been established by Heisenberg and Majorana?

Bethe:

The latter. I think that Heisenberg and Majorana had established the idea of exchange forces, and I think that Heisenberg in his first paper--but I haven't read that for 34 years--already mentioned the analogy to chemical forces. Doesn't he?

Mehra:

Well, explicitly I have seen in yours.

Bethe:

But I'm sure he had it in mind and I think it is actually stated in his paper. Certainly, as I remember it, I got the idea from Heisenberg. Whether it's in his paper or not, I'm not sure. It was a current idea, and it was a current idea that it was analogous to molecules, and the idea of exchange forces certainly was discussed by the entire little group which worked on this--Peierls and Wigner and myself and Breit and so on.

Weiner:

This was a scattered group, though. Breit was here. Wigner was in Berlin at the time?

Bethe:

No, Wigner was already in Princeton. The discussions were partly in England, at that time mostly with Peierls and Goldhaber. Let's see who else was interested. There were some small meetings of a discussion group which was organized by Blackett which met maybe two or three times a year to discuss the importance of physics. I'm sure we discussed it in that discussion group. Then afterwards when I came to this country, which was in February '35, I discussed things with Wigner and later with Teller, with Teller also in England, and then with Breit, Condon and a few other people. But certainly the idea of exchange forces was entirely familiar to the group of theorists and also experimenters working in the field beginning in 1933.

Mehra:

Meson theory was formulated around 1935.

Bethe:

Yes.

Mehra:

In what practical manner did it affect nuclear theory?

Bethe:

Not at all until much later. I would say not until maybe '38 or so, which was really after the first big effort had been completed. It did not affect nuclear theory.

Weiner:

That was also after the mesotron was discovered in cosmic rays which was believed to have been Yukawa.

Bethe:

Yes, not until after the discovery, was it taken seriously by nuclear physicists.

Mehra:

And then it affected nuclear theory.

Bethe:

To some extent, not terribly much.

Mehra:

Would you say that that situation was perhaps like the relation today between the relativistic field theory and particle physics at the present time?

Bethe:

Somewhat similar, not terribly similar. There was the great difference that in nuclear physics we had a lot to work on and we got a lot of answers, I think many more than particle physics has got until maybe the last few months. It is now different. It is now getting somewhere also, I believe. But until a few months ago I think it was not getting very far. I think we did much better and had much more understanding of nuclear physics in the mid-1930s.

Mehra:

About 1935 you knew that the nucleus consisted of the neutron and the proton. You knew about charge symmetry and charge independence and you knew about exchange forces. And you knew that the neutron-neutron potential and the neutron-proton potential were the same ...

Bethe:

In the same spin state.

Mehra:

In the same spin state and to explain the experiments that fitted the data.

Bethe:

Yes. I should now mention two other points which still belong in the same period and were very essential. One point was Wigner's analysis showing that the nuclear forces must be very short range. He did this on the basis of the facts which I mentioned previously about the binding energies of the nuclei containing two, three, and four particles, which binding energy increases very rapidly with the number of particles. On this he concluded that the deuteron was only just barely bound. That is to say, the potential must be very, very deep compared to the binding energy of the deuteron. The binding energy of the deuteron is about 2 MeV, and he concluded from fairly simple calculations that the depth of the potential must be 20 MeV, maybe 40 MeV or so. This was necessary in order to explain the very large binding energy of the alpha particle, about 8 MeV per particle. When Wigner did his first work, he did not know about exchange forces. Luckily, it didn't make any difference because these four light nuclei have a symmetric wave function, and therefore it doesn't make any difference whether you exchange them or don't. The wave function stays the same and therefore you get just about the same result with exchange forces and with ordinary forces. Because he used ordinary forces in this, these are very often referred to as "Wigner forces;" and I think it was during a visit which he paid to England--and I'm not sure what time this was; it was probably early '34--Peierls and I had a long conversation with him in which we convinced him that there are exchange forces and in which he convinced us of his theory. So this was one important point. The second important point about general nuclear theory, were the measurements of the cross-section, for the scattering of slow neutrons by protons. And for this purpose it was of course necessary first to have slow neutrons. They were produced in the experiments of Fermi and his collaborators in Rome, and once they were produced everybody could imitate them and could experiment with slow neutrons. I'll talk about these experiments later on in a different connection. They are logically not important in the present connection. However, the measurement of the cross section of slow neutrons and scattering from protons gave a result which was a complete surprise--namely, Peierls and I had thought that we had calculated that cross section in our paper which I mentioned before and that we had calculated it to be about 2 barns. It turned out 20 barns after some corrections for molecular binding of hydrogen which were only interesting because they led physicists astray for a year or so. Anyway the cross section turned out about ten times the size that we had calculated, and we felt pretty sure of our calculations. This difficulty was resolved again by Wigner, who pointed out--I remember it almost in detail; the conversation took place in the subway going from Columbia University to Penn Station--and in spite of the not very inspiring surroundings, Wigner pointed out that there could be an influence of the spin because if you had a proton and a neutron, clearly you could make of this an object of total spin 1 or of total spin 0. Now, the deuteron was total spin 1, as it had been known for quite some time. And therefore the theory of Peierls and me referred to this case of total spin 1, and therefore all we had calculated was the scattering for total spin 1. We couldn't know anything about spin 0 and so Wigner suggested in the subway that it was the spin 0 which probably gave the large cross section. This must have been in '35 still, and it solved everything. And much much later experiments were carried out which demonstrated that he was right. At the time it was only a conjecture. So Wigner in this way discovered the spin dependence of nuclear forces, and in those days one described it as a mixture of Majorana and Heisenberg forces. It was mostly Majorana and a little Heisenberg. And then it was this spin 0 state which had to be compared to the proton-proton scattering because two protons, when they have 0 orbital momentum can only have spin 0. So when I said before that proton-proton forces and proton- neutron forces are equal, they are equal when you use the neutron- proton forces in the spin 0 state. In the spin 1 state, the neutron- proton force is somewhat stronger. So I think this completed pretty much the knowledge of nuclear forces which could be obtained in those days and very little additional knowledge, only confirmation, could be obtained from then on until about 1948 when the first high energy accelerator came into being.

Weiner:

Just to pin this date down, you described a subway ride. Was this prior to his publication?

Bethe:

I don't think he ever published this.

Weiner:

How widely diffused was this knowledge then?

Bethe:

It was never published by Wigner. It was published by me in the first article in Reviews of Modern Physics, giving credit to Wigner. That's the only publication it ever had.

Weiner:

And so in '35 there was this discussion. And the first publication of the three-part article was in '36. You're saying that there was a period of 12 years--'36 to '48--where the situation in that aspect of it remained pretty much the same. And this takes into account everything that happened in '35--Yukawa's statements ...

Bethe:

Yes. I have to modify it in a few points. Go on.

Weiner:

I just wanted to pinpoint some events. The other point that I was going to make is that the work in cosmic rays didn't seem to materially affect anything in this period.

Bethe:

No. The cosmic ray work never was accurate enough or gave enough data to tell us anything. However, I want to modify my statement in three ways. One is about the detailed description of the nuclear forces, and particularly the forces between two protons. There were lots of good experiments in the late '30s and early '40s on the scattering of protons by protons at energies ranging up to about 14 MeV. All of them were analyzed by Breit and collaborators. The collaborators changed. The subject didn't. And he got a very good knowledge of the potential energy as a function of distance that was acceptable. He found that many forms of the distance dependence were acceptable. He used a square well and a Gaussian and an exponential and a Yukawa function; and they all represented the data about equally well. This result was understood later on after the war by work of Breit himself, of Landau and Smorodinsky, of Blatt and Jackson, and of myself, which is now summarized in the theory of the effective range, which made it possible to put all of these results under one heading and to explain why the results were so insensitive to the shape of the potential assumed. Each potential is described by two parameters--roughly speaking, its range and its depth--and two parameters are sufficient to describe the whole scattering. Well, this was one important additional piece of information. The second, which has already been mentioned, is the use of the Yukawa potential. The Yukawa potential was one of the potentials which Breit could use to fit his experimental data. And of course the Yukawa potential is based on meson theory. This was very satisfactory because it showed that something which could be derived from theory could also explain the experimental data. This is all it did, but this was quite a lot. Third, and this is perhaps even more important--the quadrupole moment of the deuteron was discovered--I forget at exactly what time but I think it was after I had published all my articles. I think it was '38 probably--I don't remember for sure--but it may have been '37. This showed us that we had been still too naive about describing the forces between nuclear particles. We had assumed that they depend only on the distance and then on the relative direction of the two spins. But now we were shown that it also depends on direction and we had to introduce what is called "tensor forces." As you know, and as I'll discuss later on, it finally became still more complicated.

Mehra:

Spin orbit interaction.

Bethe:

Spin orbin interaction. But the tensor forces tied in very nicely with the Yukawa idea--namely, if one took a vector meson or a pseudo-scalar meson, then one could explain these tensor forces and they came out of the theory. I wrote a long and learned paper I think in 1939 or thereabouts on the way this could be used to explain the quadrupole moment of the deuteron. It was much too detailed because the information at that time really didn't warrant such detailed studies. So these are the three additional points.

Mehra:

I would like to ask you about your three-part article on nuclear physics in Reviews of Modern Physics. What prompted you to write these articles because that was not just a compilation. There was something new in them. I would also like to isolate what was new.

Bethe:

The second part may be hard to do. The first part is easy. As I said, in February '35, I came to this country. I found in this country a big community of physicists all eager to do nuclear physics and all eager to learn all about the theory.

Weiner:

Had you expected this to be the case?

Bethe:

I had expected some of that. When I was offered my position here, I was told that a small cyclotron was being built here by Livingston, and that the department expected to go into nuclear physics. I had known of course about the big cyclotron which had been built by Lawrence in Berkeley--big on the scale of those days. I think it gave something close to 10 million volts. Livingston had been associated with Lawrence in this construction. I knew, therefore, that lots of nuclear physics was going on in Berkeley and hopefully at Cornell. But there was much more. Almost everybody seemed interested.

Weiner:

In this country?

Bethe:

In this country. And this was one of the most stimulating experiences really, to come here ...

Mehra:

Was one of the excitements to go to America?

Bethe:

Yes.

Mehra:

Weisskopf was very excited and interested to come to work in the United States.

Bethe:

Yes.

Mehra:

Were you prompted for approximately the same reasons?

Bethe:

No. My reasons were as simple as could be. It was the only position which was offered to me which promised any permanence. In England I had obtained , and probably could continue to obtain, yearly appointments with no security and with very doubtful prospects for promotion. Here I was offered an acting assistant professorship, which wasn't very wonderful perhaps on an absolute scale, but on a relative scale it seemed splendid. I had every confidence that once I was here I would get promotions, and even the beginning salary was extremely high on the standards of a refugee in those days, namely, it was $3000 a year. So I came purely for the purpose of finding a secure position.

Mehra:

Now to go back to your article.

Bethe:

I will come to the article very quickly. Now when I came here I found great interest in nuclear physics, and I found, on the other hand, that many of the things which had been common knowledge in the very small circle in England in which I had moved were unknown here, and I also knew that a lot of the things we had done could be much extended by a little bit of theoretical work, and this would be worth doing.

Weiner:

What were the sorts of things you had known about in your circle in England that were not known here? Theoretical? Experimental?

Bethe:

Mostly theoretical and very largely the things which I described to you in the last hour. And I was asked to give many talks. I was asked many questions when people came to my office, and I thought it would simplify my life greatly if I wrote it down once and for all and thereby answered all the possible questions in one big article. So therefore I proposed to Mr. Tate, the editor of the Physical Review and Reviews of Modern Physics that I would write one article about nuclear physics. He said, "That's fine. I'll take it." And then I began writing. After a short time I wrote him, "I have to write two articles, one on the stationary states and one on nuclear reactions." And after a while I wrote him again and said, "No, I have to write three articles." And that's how it came about.

Weiner:

This is 35 that you started?

Bethe:

I think I must have started in the fall of 35. So I began writing this. I had two important advantages. One was that Livingston had a card file of every paper which had been written on nuclear physics-- something that I could never manage to get together--but he made this available to me and so this was most useful. The other was that I don't remember exactly from what time on, I had two very able collaborators-- Konopinski and M. E. Rose, now at Indiana and Virginia, respectively, who were very eager to do calculations about nuclear physics. Among other things, they did some of the necessary calculations for the articles. They had also quite a lot of out of publications of their own. And so in this manner we got it often. Now you ask what was new. It's a little bit difficult to tell, but I tried at every point to fill in the gaps. That is, I took what was in the literature and then I wondered just how much more was necessary for a coherent presentation of the subject. So in this manner, when something new was necessary, I sat down and did it. I don't think you want me to go through it in detail, do you?

Mehra:

I want the highlights, I'm looking for the crystallization of concepts. You mentioned a while ago about the discussion with Wigner in the subway. Perhaps it formed part of the scheme.

Bethe:

It formed part of the scheme.

Weiner:

We have the table of contents and the bibliography. I Xeroxed them from a bound volume.

Bethe:

I would rather get the articles. [Glancing through articles.] The first chapter of article A is just putting together known things, but they had never been put together in this manner, never so briefly and I think never so completely. The second chapter on the qualitative arguments is a simple statement of the things I have been talking about and a couple more. No, actually the second on qualitative arguments which goes mainly into the evidence from heavier nuclei except for Section 9. Now, in the third chapter the Section 13 on excited states of the deuteron had never been done before; 14, the scattering of neutrons by protons cross-section contains the Wigner idea; 15, the angular distribution of the scattering, was common knowledge, again, but had never been stated and lots of people had said stupid things about it; then 16, photoelectric disintegration is essentially the paper with Peierls somewhat elaborated; 17, the capture of neutrons by protons think is mostly, again, Peierls and myself, plus some ideas which I think were due to Fermi on capture due to magnetic transmission; 18, scattering of protons by protons is more or less what Breit said.

Mehra:

That was a new "deuteronomy."

Bethe:

Now, in IV, the theory of the three-body and four-body objects, Section 20 was, I think, essentially new, and, in 21, I think, I greatly simplified the theory of Feenberg. Section 22, I think, was again more or less common knowledge but put in a quantitative form-- namely, getting the radius of the three-body systems from the difference in binding energy.

Mehra:

So three-body potentials have been discussed already at that time?

Bethe:

Three-body potentials were not discussed, no--only three-body systems with two-body forces.

Mehra:

I see. In the formulation of Feenberg's equivalent two-body problem, you were trying to discuss the three-body systems essentially in terms of two-body forces?

Bethe:

In terms not only of two-body forces but also two-body wave functions.

Mehra:

Yes. Was Feenberg working with you?

Bethe:

No, I don't remember where he was but he was not here. #23, excited states of the alpha particle, I think was new. The next chapter V on statistical theory is nothing terribly original but some of the things are new. For instance, the section 30 on "Weizsäcker's Semi-empirical Formulae"--that section is much simpler than his theory and I corrected a mistake that he had made. It's more straightforward than what he had done. Section 27, the "Disproof of Ordinary Forces," is somewhat interesting. I showed in that section--I'm sure I was not the first one to think of it, but perhaps it's the first time that it is put down on paper in a coherent way--that if you had ordinary forces between the nucleons, then the nucleus would collapse or the nucleons would go into a clump of the size of the nuclear forces and the binding energy would be proportional to the square of the number of particles rather than proportional to the first power, similar to what I told you at lunch about Dyson's theory of ordinary matter.

Mehra:

This is like collapse in the theory of gases.

Bethe:

Yes, just so. So I think it's put down coherently here for the first time and it is one of the things that people have forgotten-- namely, there are now, 30 years later, some people who still think that they can use ordinary forces between nucleons without a repulsive core and can get saturation. That this is impossible was proved back then. I think it's very interesting and somewhat sad how some things that science used to know just get out of the consciousness of people working in the field.

Weiner:

But you're not implying that this was ignored when it was presented. It was learned, learned well perhaps, but ...

Bethe:

It was learned very well at that time and forgotten in the course of 30 years.

Mehra:

But it seems to have been forgotten again and perhaps reselected afterwards in the theory of condensation. There's no theory available yet, but to try to explain condensation on the basis of entirely attractive forces and the collapse of the volume is the same type of thing afterwards.

Bethe:

Yes.

Mehra:

The main question there again is whether it is possible to explain condensation either by means of entirely repulsive potentials or entirely attractive potentials. It seems that neither works.

Bethe:

Neither works. You have to have a combination. In this article I made a big mistake which Feynman is very fond of pointing out--somewhere (and I don't remember in which section) I say that you need either exchange forces or you need a repulsive core. The repulsive core had been proposed by Feshbach and Morse in 1935. And I said at that time that it is much too complicated. Forces couldn't possibly be like that. Nowadays we all believe in the repulsive core, and Feynman just loves to rub my nose in this. Now, in the "More Detailed Theory of Heavy Nuclei" [Chapter VI], talk in Section 32 about "shells" of nucleons, which of course is long before the shell model. This was not original. It had been previously suggested by others, especially by Elsasser and it is a critical discussion, but the things which I contributed to this are, first of all, that I took seriously the Hartree model and did a couple of calculations using this shell idea, and this was soon afterwards continued by Wigner and Feenberg in a much more extensive way. But more important--and that was one thing which I was especially fond of in those days--I pointed out experimental evidence for shells. Experimental evidence until then had been very loose and very qualitative, and I had worked quite extensively on obtaining actual values of nuclear masses. And in the process of this, I had better numbers, let's say, for the binding energy of oxygen-16 and so on; and on the basis of these numbers I could show that, indeed, oxygen-16 was more strongly bound than its neighbors, carbon-12 and neon-20, all of which are alpha particle nuclei , and that therefore one could very well claim that in oxygen-16 a shell was closed. And I think this is one new point.

Mehra:

Was it a recognition of the Pauli principle somehow?

Bethe:

The Pauli principle is in this very strongly, and of course the Pauli principle comes in very quickly when you go beyond helium-4. Namely, you have to say: Because of the Pauli principle, you have to start a new shell, and I think we already had the right idea that the next shell will be a P shell rather than an S shell. And because of this, the binding of the next nucleon, nucleon number 5, is extremely weak. In fact, it's so weak that there are no stable nuclei with mass 5. In one of the other articles we go into quite a lot of discussion showing that nuclei of mass 5 are in fact unstable. Some people had claimed to have discovered them.

Mehra:

So the shell model was already in vogue at that time.

Bethe:

It was indeed, but we were only able at that time to get up to mass 40. We could show that calcium 40 should be a closed shell nucleus, and one of the sections [25] here says: "energy of oxygen-16 and cal- cium-40," these being two closed-shell nuclei. Beyond calcium-40 we got into hopeless contradictions, and at that time we simply had to say that we didn't know how to continue the shells beyond 20 nucleons of each kind.

Mehra:

Wasn't the exclusion principle regarded?

Bethe:

The exclusion principle was fine, but we did not know what angular momenta to attribute to the next nucleon states, and this was only solved in '49 by the introduction of the spin orbin coupling.

Mehra:

And what was your feeling about the shell model before it was in vogue and later on--during this long gap between its original vogue and its resurrection?

Bethe:

Well, I'm just trying to remember. I think I was always confident that indeed the shell model would some day be established. I was always confident that we had the right answer up to calcium-40, but I certainly didn't know how to do it correctly beyond that.

Mehra:

And this was long before Jensen and Goeppert-Mayer?

Bethe:

Long before Jensen and Mayer. But I didn't know how to do right. They did. They knew. Then the second feeling I had--and I was going to talk about this-- is that we learned a great deal about nuclear structure in the late '30s which was different from the shell model, if I may say--namely, the Wigner model which talks about the symmetry of the wave function more than the shells occupied by the nucleons and then the compound nucleus. And so in the second half of the '30s these features seemed much more important than the shell-model feature. Nowadays I think it really has become different, and I think the shell model I would now put as the most important feature of nuclear structures everything else being obtained as a modification from this.

Mehra:

It has had a very stormy history.

Bethe:

It has indeed.

Mehra:

I would like you to recapitulate some of the controversies that surrounded the shell model from the beginning.

Bethe:

That would co think better when I progress in history to 1949.

Weiner:

I think we're doing very well systematically going down section by section, and we're only on the first installment. But I think this is very productive. As these things come to you, you're helping us answer the question that you didn't believe was possible to answer--to demonstrate what was new and what was changing.

Mehra:

And indeed the shell model has had a place right here.

Bethe:

The prehistory of the shell model is here.

Weiner:

Let's make a note to follow this up later.

Bethe:

Very good. Now, Chapter VII is about the beta disintegration. By that time there was a theory of beta disintegration, and I haven't talked about that at all because this really went quite independently of the general development of nuclear theory, and it is very independent of the general theory because we are dealing with a weak interaction, whereas inside the nucleus we have a strong interaction. The ratio of the strengths of these interactions is something like (I don't know) 10-40 or some nice number like that. So one can certainly discuss nuclear forces without knowing anything about beta decay, and one can discuss beta decay without much knowledge of nuclear forces. I left beta decay in the year 1930 with the controversy between Bohr on one side and Pauli on the other side: whether or not energy was conserved in the beta decay. In subsequent years--and especially after the accelerators came into operation--we got a lot of evidence on the energies of nuclear states, both ground states and excited states, and we got, for instance, very accurate information on the difference in energy between the ground state of nitrogen 13 and carbon 13 from the study of disintegrations leading to these nuclei. And, in fact, some of this book, Part III, is concerned with the obtaining of good values for these energies. So we knew the energy difference between these two nuclei, and, on the other hand, we knew the spectrum of beta rays emerging from nitrogen 13 in its decay; and from this we could conclude that it was the maximum energy of the beta rays which was equal to the energy releases--that is, to the difference between nitrogen-13 and carbon-13. And this answered experimentally the question raised by these two people in favor of Pauli--namely, energy was conserved but it was not just conserved on the average (Bohr had wanted to conserve it on the average Energy was conserved, but then something had to disappear and this gave rise to the neutrino hypothesis of Pauli. I don't know where Pauli's hypothesis was published by Pauli, and I'm not terribly familiar with the theory of beta decay altogether. The man who probably knows most about it in this country is Konopinski, who has written a book about it. The book probably contains it, but also it might be useful to interview him personally. Now, Pauli's theory was of course put into explicit mathematical form by Fermi, and Fermi's theory in turn is valid to the present day.

Weiner:

This was about l934.

Bethe:

34.

Weiner:

In the same year you published a paper on the neutrino.

Bethe:

Yes, that was a very unimportant paper. The question was raised: could you detect a neutrino after it had been emitted? And the question was: what properties would it have? It obviously had no charge, so it wouldn't make ions. But maybe it might have a magnetic moment, and so this unimportant paper deals with the ionization which one might expect if it had a magnetic moment. Of course no ionization was ever found, and one couldn't set a limit on the magnetic moment, which was very low, and already at that time I think everybody, nearly everybody, believed that the neutrino could be found only in one way--namely, by making an inverse beta process. Peierls and I wrote a paper--and that is more important, although very simple--in which we calculated the cross- section which would be expected for such a process, and that estimate of course later on turned out to be correct. The cross-section was indeed that low, 10-42r so square cm. In spite of this, the neutrino was found, as you know, in the 1950s by Reines, Cowan and collaborators. And nobody in the 1930s would have thought it possible that such a small cross-section could ever be discovered.

Weiner:

You referred to a subsequent paper that you wrote of more importance on this subject.

Bethe:

No, that's probably earlier than this.

Weiner:

I see. The cross-section paper is the early one.

Bethe:

Bethe and Peierls in Nature.

Weiner:

The Nature one is called "The Neutrino."

Bethe:

That's the more important one. There's another one in the Proceedings of the Cambridge Philosophical Society which is "The Magnetic Moment of the Neutrino" or something like that, and that's unimportant.

Mehra:

You mentioned about beta decay. Would you recall the circum- stances in which Fermi's theory of beta decay came to be accepted? I would also like to ask: What ruled out Yukawa's hypothesis of mesons being the mediating agency for beta decay.

Bethe:

Well, this is quite a deep question. Point one: Fermi's theory in principle I think was accepted immediately because it was obviously a sensible theory, a sensible explanation of the phenomenon. In detail, it was modified at one time by Konopinski and Uhlenbeck, who introduced the gradient into the theory, and this indeed is discussed in this Chapter VII of my article. At the time when this was written, this was the accepted theory, but in fact this was due merely to the experiments being bad. Nobody could do good experiments on the spectrum of the electrons in beta decay, and with bad experiments, it seemed to fit the Konopinski and Uhlenbeck theory. With good experiments, which came out I think before l940, the Fermi theory was restored to favor. Then the Fermi theory itself was somewhat supplemented by the theory of Gamow and Teller, and with the combination of these, I think we had a satisfactory theory as far as it went. It was only a short while ago in '62 or so that much real progress was made by the theory of Gell-Mann and Feynman. Now, you asked about Yukawa's theory of the beta decay. Yukawa's idea was that the meson--he had only one, which we have to identify with the pi meson--that the meson disintegrates spontaneously into an electron and a neutrino. We know it doesn't. It does so only very, very rarely. It disintegrates into a mu and a neutrino--and a different neutrino from the one which comes in beta decay. And the disintegration into electron and neutrino is exceedingly slow and is just not sufficient to explain the beta decay of nuclei. It is too small be a factor like 108 or something like that. So the meson, the pi meson, is not a sufficient mediator to explain the beta decay. Everybody nowadays believes that pi mesons are the main term in nuclear forces, and this is well established, but the pi meson doesn't give enough beta decay and therefore one needs some other source of beta decay to explain nuclear beta decay. All right. So then you might look for some other meson to do this. At various times people have suggested that there could be another meson, now called the W meson, which would do this. CERN, in particular, has searched for such W mesons, with their 30 GEV accelerator.

Mehra:

That would be intermediate bosons?

Bethe:

Intermediate bosons, yes.

Mehra:

And this is what Lee and Yang have proposed.

Bethe:

Correct.

Mehra:

So it seems, in the Yukawa case, that the meson was to be the intermediary agency for nuclear physics, and Lee and Yang have proposed the intermediate boson, but nature doesn't seem to work that way.

Bethe:

It has not so far worked this way. I don't think a negative answer here is conclusive. All that has been proved is that the intermediate boson does not have a mass less than one and a half times the proton mass, but it could very well have a mass three times the proton mass and no existing experiment will contradict this. The trouble with the intermediate boson is, apart from its not having been discovered, that it is very difficult to construct a theory of the electromagnetic interactions of a charged vector particle, a charged particle of spin 1. This gives rise to all sorts of the most horrible divergences. Lee and Yang have tried to evade those, I think not very successfully; and so therefore this vector boson is not a very happy particle.

Weiner:

To bring us back, you got on the subject of beta decay by looking through Chapter VII and then you decided to recapitulate this since in your earlier account you had left out the story of beta decay. And I think you had come to the point where you had brought it up to the time of 1936 when you dealt with it here. But you mentioned, though, that there was another thing that you had left out. I wasn't sure whether you had covered it. Maybe that was the idea of the neutrino.

Bethe:

When I talked about nuclear forces, I mentioned three things. That is not what you mean.

Weiner:

No, I meant here when you said there were two things. The beta decay section here reminded you that there were two things you left out. One of them was the history of the beta decay. I wonder-- the positron and pair creation thing is not what you meant?

Bethe:

This is not nuclear physics.

Weiner:

So that's not what you had in mind certainly.

Bethe:

I don't remember what we had in mind, so I guess we better erase that.

Weiner:

May I suggest that we continue through chapters VII and VIII and then perhaps we can take a break and see where we are?

Bethe:

All right. Let me say one more thing about Chapter VII. In those days there was no meson, or at least I didn't take account of the meson. And there were accordingly attempts to explain such things as the magnetic moment of proton and neutron by interaction with electrons. That was stupid, and of course the right answer is that such things are explained by interaction with mesons, and in this respect--that is, in the theory of the nucleons of the elementary particles--meson theory really is important. What we did back in 1936 was just not very sensible. In fact, I think I said here in this article that it isn't very sensible and that you can prove anything about the magnetic moment as long as you would take electrons and neutrinos as the responsible agents for the moment.

Weiner:

Was that one of the areas in this paper that you tried your hand at before coming to a conclusion? Had you worked at that a while?

Bethe:

No. There I just examined the proposals and criticized them and that's all. Now, Chapter VIII was written by Bacher. It's on nuclear moments and the evidence on nuclear moments, their values, quadrupole moments and the evidence from spectroscopy and so on. It is not directly nuclear physics, but it is a tool for the investigation of nuclear physics.

Weiner:

This is the work that Bacher was doing here at the time?

Bethe:

Right.

Weiner:

He came after you were here?

Bethe:

He came half a year after me, and I think a year after that, approximately, Livingston left to go to MIT and then Bacher took over from Livingston. He took over the cyclotron and its running. I'm not sure anymore what year this was. It may have been as late as 39.

Weiner:

I talked with him this summer, by the way, on a very short- range interview, and we covered some of the background of that. I think that now we've reached the end of the table of contents or the first installment of your three-part paper, and I would suggest it would be a good time for a break, and we'll find out how much longer you want to continue today.

Bethe:

I'd like to continue till six, if that's all right with you.

Weiner:

Fine. I find this is good to do this. I don't know if you've ever had the opportunity to do it systematically.

Bethe:

I never did, no.

Mehra:

I think it's very nice. This also gives the possibility of examining what were the new elements that were introduced here. [Reel 1, Side 2 of tape; continued after pause of about five minutes.]

Mehra:

Since we have been talking about beta decay, Professor Bethe, may I ask you something about the nature of beta and gamma decay? Was it always assumed that the nucleus acted as a collection of particles, each subject both to beta and gamma transitions? I would like to know who told us that when nucleons are bound in a nucleus, they still exhibit their characteristic interaction?

Bethe:

I guess that was just assumed, and perhaps assumed as the simplest possible assumption. It is of course still one of the important points to verify, that is, to verify that, and to what extent nucleons in the nucleus are the same as they are outside the nucleus. This has lots of ramifications. The most important question perhaps is whether the forces inside the nucleus are the same as you have between two colliding nucleons; and this is a question which we are now only beginning to answer. We are now beginning to come to a point where we can say that the forces, while not exactly the same perhaps, are sufficiently closely the same so that it is sensible to start from the forces of interacting nucleons. In the case of gamma decay, gamma emission, I think the case is the simplest--namely, we have a system of charged particles; and since we have charged particles, you'll have an electric and a magnetic moment associated with any transition. Now, the electric moment one would imagine can be derived from the distribution of charges just the same way as in an atom. So one would expect that one should get from the general theory of the nucleus a description of the wave function of the nucleus, giving the position of the charges, and from that one could calculate the electric moments, dipole moment and higher moments. We have every reason to believe that this is a good description, and all the experiments which have been made in great profusion since the war on electric dipole and quadrupole moments of transition, and also higher moments, seem to bear out the assumption that the distribution of the protons--that is, the wave function of the nucleus as a function of the proton coordinate-- will give us properly the electric moments of various orders. For the magnetic moments already exist in the normal state of the nucleus, in the stationary state, not only in transitions; and there are also transitions made by magnetic moments. The moments in the ground state of the nucleus have been measured many times and very accurately, and here the accuracy of the measurements is such that we know that the magnetic moments are not always simply those of the free nucleons, but there is a change of the nucleon when it enters the nucleus. However, the greatest change is in the nuclei of mass 3, the triton and helium 3, where the moments of the odd nucleon are enhanced by some 10% over the value which they have for the free nucleons. This is considered to be a meson exchange effect and an effect which nobody has yet understood in any quantitative detail. When you come to heavier nuclei, it really looks rather better. The magnetic moments of heavier nuclei seem to be closer to what you might expect from the free nucleons. But half of part of this is that we just don't know the theory so well for the heavier nuclei, and so maybe there is more discrepancy than we see. But I think the conclusion is--and this is just a conclusion from shell model and experiments-- that magnetic moments are preserved to within a few percent when a nucleon enters the nucleus. We can't say it any better than a few percent. Finally the beta decay. Quite some volumes have been written, and, again, Konopinski is the expert on this, on the probability of beta decay, that is, on the matrix elements of the transitions. In this case, since beta decay is a very weak interaction, the proper thing to do is essentially the same as for electromagnetic transitions; namely, one wants to have the wave function of the system before beta decay and after beta decay and get the matrix element connecting the two. In the case of beta decay, in fact it is better than in the case of electromagnetic interaction just because of the troubles which I mentioned before-- namely, just because Yukawa's theory of beta decay didn't work. We know that the mesons which are transmitted between nucleons as part of the nuclear force do not give much contribution to beta decay. The meson decay in fact is weaker than the nucleon decay. So if we get beta decay, we get it from the nucleons in their natural state, and we don't get it when they are dissolved into a meson and a nucleon. Whereas, in the case of charge, this is not so. The mesons have the same charge as a proton, plus or minus, and therefore electromagnetic currents of the mesons are by no means negligible, and these electromagnetic currents are believed to explain the deviations which I mentioned in the magnetic moments of the nuclei of mass 3. But for beta decay we have no reason to expect such deviation. We have every reason to believe that you should just take the matrix elements between the nuclear wave functions before and after the decay. Of course to get these matrix elements is quite difficult and I will not describe this, but I think so far there is every confirmation by experiments that this picture is correct--namely, that one nucleon in the nucleus decays and decays into the other type nucleon with emission of an electron and a neutrino governed by the same beta decay interaction as we have for free nucleons, but one has to take into account the wave functions.

Mehra:

If I could take an analogy of raw cucumbers and pickled cucumbers, it seems that the nucleons behave the same whether they are pickled in the nuclear jar or outside in a dish of their own.

Bethe:

Yes. Very good.

Mehra:

Would you remember something about the discussions, perhaps in the '40s, about the earlier ideas on the universality of the beta interaction?

Bethe:

I don't remember very much about this. I really didn't participate in these discussions. I've heard about that occasionally, but I think you should ask other people to tell you about this. I found after the war that my time was very limited and so I restricted myself to smaller and smaller areas of physics: in the late '40s, mostly to high-energy physics, and then since about '55 to the theory of nuclear matter.

Weiner:

think perhaps this is the point to get back to the project ...

Bethe:

think I would like to take up one more point here.

Mehra:

The point about the history of the universal vector axial-vector interaction. For the record, is it historically correct that Marshak and Sudarshan were the first to analyze the data and show that the only consistent picture would result if the interaction were chosen as vector axial-vector interaction? There were, as you will remember, at that time several experiments, notably the apparent absence of the electron decay mode of the pion. Marshak and Sudarshan suggested that these be redone, and they were redone and found wrong. Feynman and Gell-Mann wrote about it at almost the same time, but did not make the same exhaustive analysis of the data. Sakurai also did that later. Were you involved in these discussions; and if so, I would like to have your comments.

Bethe:

I was not involved. I think the account which you gave is correct historically. I think Marshak and Sudarshan did it first. Not only did they do it first, but they used as a theoretical argument one that is still current and still used and is very useful--namely, the argument that the interaction should be invariant against the insertion of an extra operator gamma 5. They used this. Feynman and Gell-Mann did not use it, to my recollection, but used a different argument, which is obsolete. So on all these counts I think Marshak and Sundarshan should be given credit. I am told--but this I only know from being told by the author--by Marshak that he gave a paper about this at a conference in Padua in the fall before the publication of the Feynman-Gell-Mann paper. Gell-Mann apparently was in the audience. So were many other people. Most of them thought he was crazy, especially in his statement that the interaction should be axial-vector and vector rather than scalar and tensor which had been commonly accepted. The experimenters, in particular, objected to his suggestion that their experiments -night be wrong. Unfortunately, the only manner in which these remarks were published was in the proceedings of that conference. Unfortunately, Marshak and Sudarshan did not write it up as a paper for a normal journal. And I think this has a lot to do with the fact that it is forgotten, that they have been forgotten. Gell-Mann and Feynman wrote it up for the Physical Review, so everybody read it, and the other was just in the conference. Most of the participants in the conference didn't believe it when they heard it. Nobody ever bothers to read the proceedings of a conference afterwards, and I think this was just bad luck.

Weiner:

What year was this?

Mehra:

It was in the '50s.

Bethe:

It was late '50s--I think '57 and '58. I think Marshak and Sudarshan must have been fall of '57, and Feynman and Gell-Mann were early '58. I believe that's right. It could have been a year later.

Mehra:

Professor Bethe, to go back to the Bethe Bible, I would like to ask one question. You had worked on the group theoretic computation of this splitting of energy levels in a crystal. Why were such techniques not applied to the nucleus by you or other people--for example, Hund, Wigner or Racah?

Bethe:

They were applied by Wigner, and this was precisely the next thing I was going to talk about. They're not in the "Bethe Bible" largely because it was done after the first testament was written and it didn't fit into the two other volumes. But Wigner, I think in '36 or `37, did apply the group theory methods to the structure of nuclei, and in fact his papers were exceedingly important and exceedingly good. As a matter of fact, some of this work was very recently studied again in connection with elementary particles. What Wigner observed was that there are two quantum numbers of nucleons, which are the isotopic spin and the spin, which play a very fundamental role in nuclear physics. Each of these two can have two values, plus or minus, so you get four different combinations. Since you have four different combinations, you can put four nucleons into one spatial orbital; whereas, in the case of atoms only two electrons can go into one spatial orbital. Consequently, the first shell is completed at atomic weight 4, the alpha particle, and the same structure can be followed to higher atomic weight. Well, Wigner did this in a really admirable way, using very powerful group theory. You should really get Wigner himself to describe this. But he was able to predict from the symmetry arguments the special stability of nuclei which are multiples of the alpha particle without using an alpha-particle model. It just comes out of the group theory. It is the most symmetrical wave function which you can form of the nucleons. When you can put the nucleons into a wave function which is as symmetrical as possible with regard to interchange of the spatial positions of the nucleons, then you get particularly close correlation between the nucleons and particularly strong attraction by the very short-range forces. Now, on the basis of this, he was able to predict the ground-state energies of nuclei up to about atomic weight 40 or so, and he was able to predict some nuclei which had not been found. For instance, he said, "Sulphur-36 should be a very stable nucleus. It should be stable against beta decay, and it should therefore exist in nature." It had never been found. The mass spectroscopists looked again at sulphur and promptly found it. There is sulphur-36. And I think he predicted two or three other isotopes which had never been found before. Technically his paper (his papers really) are also particularly important because it was, I think, the first time in atomic physics that two groups were put together, so to speak, multiplied--namely, the group of isotopic spin and the group of ordinary spin. And he showed that the nucleus could be characterized by three quantum numbers--the total isospin, the total spin, and then a third quantum number which I think he called Y, which had something to do with the product of ordinary spin and isospin. And he could show that to each state in the nucleus one could assign these three quantum numbers. I think his papers were the first papers in which the importance of the total isotopic spin was stressed. And the total isotopic spin is different from the charge, from the difference between the number of neutrons and protons. 0f course, the basis of Wigner's paper was the observation which I discussed previously of Breit and Condon of the charge independence of nuclear forces. This made it possible to classify states just by total isotopic spin. Now then, partly still in the very late '30s, but mostly in the late '40s and in the '50s, people have found the concept of the total isospin of a nucleus exceedingly fruitful and they have been able to find states in neighboring nuclei which correspond to each other which have the same total isotopic spin. Wigner calls these "analog states. Such pairs or multiplets of states were first found in the lighter nuclei. For instance, carbon-14, nitrogen-14, oxygen-14 are three nuclei having the same total number of particles. Carbon and oxygen each have different numbers of neutrons and protons and need an isotopic spin of at least 1. Nitrogen-14 can exist either in a state of isotopic spin 0 or isotopic spin 1, the ground state is isospin O. There are definite states in nitrogen-14 which correspond exactly to the states of carbon-14 and oxygen-14, and these corresponding states have been traced in great detail, especially by the Caltech group of Tom Lauritsen, and all these predictions have been beautifully borne out. These states are very important. For instance, in beta decay they can go into each other with the greatest of ease with highly allowed transitions. And Wigner took this beta-decay transition problem into account when he wrote his first papers. From the decay of oxygen-14 into the analogous state of nitrogen-14 one derives the best number for one of the beta decay couplings, namely the axial-vector coupling. Now, Wigner in recent years, together with experimenters at Princeton, is following this up to much higher atomic numbers, and he is following it into the continuous spectrum, and this is really what he calls the analog states; namely, you can take a quite heavy nucleus-- let's call it barium--and you can change one of its protons into a neutron by a nuclear reaction. Thereby you get an isotope of cesium. And preferentially this transformation goes through an analog state of cesium -- namely, to a state which has the same isospin as the barium state from which you started. This state of cesium lies in the continuous spectrum of cesium. Therefore, you get not a bound state but you get a tremendous enhancement of transition probability to an unbound state of the final nucleus. While the example which I gave I think has nothing to do with reality, it has been possible to show that the idea of isospin carries up to very high atomic number in spite of the force. Wigner thought originally that it should stop at about atomic weight 60, but now they are working with weights of 120 and more and it still works. And this classification of states according to the symmetry, according to isospin, ordinary spin and the Y quantum number was extremely powerful and was one of the things which I mentioned previously when we talked about the shell model. I said that in the late '30s and until l949, the shell model was not very much emphasized because it was shown by Wigner that the symmetry considerations based on group theory were really much more important to determine the energy of medium Wight nuclei than the shell structure. So that was the application of group theory, which was eminently successful.

Weiner:

That's quite a clear, coherent explanation. I don't want to press you, but would you now like to continue ...?

Bethe:

I would like to go into Part B.

Weiner:

I would like to ask a question as it relates to that. I notice that there's a year gap between publication of Part A and Part B. The question is this: Did you submit all three pieces of the manuscript simultaneously?

Bethe:

No. I was writing it as it was being published. Part A was published as soon as it was written. Part B and C were written more or less simultaneously because it was a little difficult to separate them, and that's why there is a big gap between A and the other two. And I think B was submitted a little earlier than C, but the writing was done at approximately the same time.

Weiner:

I see. And B was the only one in which you don't share authorship with the others.

Bethe:

Right.

Weiner:

Are there any general comments you'd like to make about that before we go on ...?

Bethe:

A general comment is that on Part C, which is the experimental part, Livingston did a good deal of the work. He was an experimenter. He had studied in detail some of the experiments. And many of the experimental chapters were written by him rather than by me. I'll come to that when I come to Part C. Well, specifically, the Chapter XV on "Experimental Methods;" Chapter XVII on "Results of Disintegration Experiments," were Livingston's work. Chapters XVI and XVIII were mostly mine. And that's all there is. So the two longer chapters were his work.

Weiner:

And that also accounts for the reversal of the name order in this one?

Bethe:

That's right.

Weiner:

Because of his larger role in

Bethe:

Yes,

Weiner:

Not larger than yours necessarily, but relatively larger role.

Bethe:

No, I think it was larger than mine. I helped a little in the interpretation in Chapter XVII. I didn't help at all in XV. He helped some in XVI and in the Auxiliary Data, and some in Section 108, "Masses from Disintegration Data," 109, "The Excited Energy Levels of Nuclei. So I think it was more his work than mine.

Weiner:

Now, getting back to B, "Nuclear Dynamics, Theoretical."

Bethe:

Here I think the main thing I should say is now the second point which I mentioned in connection with the shell model--namely, the model of the compound nucleus. And now I can resume the story of Fermi. In 1934 Fermi experimented with slow neutrons. Artificial radioactivity had been discovered by Curie and Joliot in '33. This was an excellent tool for investigating the result of a nuclear disintegration. It was used by many people but particularly by the group at Rome, who used neutrons for bombarding nuclei. I think this story is a different story which I shouldn't talk about but which should be told by one of the participants. I think Segrè has written quite a lot about it and knows a great deal and I think is very good in talking about it. Amaldi is another. Rasetti at Johns Hopkins is another of the participants in this early work, which must have been terribly exciting. Well, in this work they discovered at a fairly early time that the effect of neutrons was enhanced when they were transmitted by paraffin, and I think this might never have been discovered if Italy were not rich in marble. Namely, a marble table gave different results from a wooden table. If it had been done here, it would all have been done on a wooden table and people would never have found out.

Weiner:

Sounds like geographic determinism.

Bethe:

Anyway they found out that slow neutrons were much more effective than fast neutrons and they found out very quickly after that that among the slow neutrons there were specific types of neutrons which did specific things. There were some neutrons which activated silver, and others which activated cadmium and others which activated indium and so on and so forth. Well, on the basis of this everybody got the idea. I believe it was not published by the Rome group, but the idea occurred in many places. I think in Copenhagen Bohr got this idea, and here Wigner and Breit, and I got the idea while I was moving from England to this country, that these were resonance levels of the nucleus. And I tried in the first paper I published in this country to give an account of this in terms of the interaction of one neutron with the nucleus. And, sure enough, I found resonances, but I was absolutely wrong in explaining Fermi's resonances in this way. The resonances which I then postulated theoretically, later on came back, in the 1950s, in the theory of Weisskopf, Porter and Feshbach of the one particle resonances of a nucleus. And this was brought back at that time to explain certain experiments by Barschall on the scattering of neutrons in the hundred-kilovolt region, to explain chose experiments in which they found very striking maxima and minima, both as a function of energy and as a function of atomic number. So the theory which I produced then in '35 was entirely premature. There was no experiment which would fit the theory, but these experiments only came I believe around '50, '52 or thereabouts.

Weiner:

The paper that you refer to as the first one in this country was "The Theory of Disintegration of Nuclei by Neutrons." This is a Physical Review paper in 1935.

Bethe:

That

Weiner:

Prior to that time all of your publications were in European journals.

Bethe:

That's right.

Weiner:

At a later time, I will ask you why you waited until you came to this country to publish in the Physical Review.

Bethe:

Well, the answer is short and simple. I think I published wherever I was, and it happened that my early publications, were all in German journals. At that time I think the center of gravity of physics was indeed in Germany. Then in 1933 or so with the Rutherford school publishing the nuclear physics papers, and nuclear physics coming into prominence in the whole world, the center of gravity shifted to England. And in 1935 I had the good fortune to come to America just about the time when the center of nuclear physics also shifted to America. The small accelerators which they had in Cambridge were very good for the exploration of the lightest nuclei, but when it came to heavier nuclei you needed the bigger accelerators, the first of which was Lawrence's cyclotron. And I think 1935 was just about the time when significant experiments were begun on the Lawrence cyclotron. So this was just a piece of luck. Now as I said, my theory was inapplicable to the resonances which were found in those days which were resonances at a few electron volts. And the correct theory was given by Bohr: it was the theory of the compound nucleus--namely, the idea that when you add the neutron to the nucleus you form a new nucleus, the compound nucleus, which is highly excited because you have added the 8 MeV binding energy. You are 8 MeV or so above the ground state. And because it is highly excited, you have many levels at very close spacing. Bohr I think formulated this theory most clearly and before everybody else. Breit and Wigner very soon afterwards developed the dispersion formula for cross sections with closely–spaced nuclei. I think they didn't have the full Bohr picture in mind, but a somewhat simpler, if you want, picture where they talked about two or three nucleons rather than all the nucleons acting together to make the energy levels. But they were the first to give that resonance formula. So, accordingly, in Part B, the first few sections are devoted to Bohr's theory of the compound nucleus and to the dispersion formula.

Weiner:

May I interrupt at this point to ask you to clarify something, not that you said but that somebody else said? It seems that Frisch-- and I'm not sure where this quotation comes from; I think it's probably in connection with the quantum physics interviews and conversation with Tom Kuhn--talks of 1936 when Bohr gave a lecture to the Copenhagen Academy, and that you were giving a talk and Bohr was thinking during the talk of some of these ideas that led him to the compound nucleus. Here is the quotation that I have: "Frisch feels Bohr was during Bethe's talk realizing that it was absurd to think of the nucleus as bound together by interparticle forces." And not long after, the compound nucleus idea was fully worked out. Do you remember that particular occasion and do you remember if Bohr did ask questions ...?

Bethe:

I remember it vaguely. I remember that I gave a talk about the structure of nuclei and this may have been something like the shell model and things like that. I don't remember what year it was. In a way I would be surprised if it had been '35 or '36.

Weiner:

You were here then.

Bethe:

I was here. I visited Europe during those years. However, I didn't go to Copenhagen for very personal reasons. Namely, I had been engaged to a young lady at the Copenhagen Institute in 1934 and I broke the engagement, and so I didn't think I should show myself for several years. In fact, I didn't until I think '51. I saw Bohr in between. So I'm pretty sure I was not in Copenhagen later than the summer of '34. It is very likely that in '34 I gave a talk. It is very likely that it was about nuclear physics. It is very likely also that it was very much based on the particle idea, on specific wave functions, on individual quantum states for each nucleon.

Mehra:

Independent particle model.

Bethe:

Independent particle model. Whether it was shell model at that time, I don't know. I think I got interested in the shell model only after that, but certainly independent particle model. And I remember vaguely that Bohr asked a number of questions, and the way that he operated was that his questions were very tentative and seemingly very vague. I think it is quite likely that Frisch's story is correct and that indeed he thought of that idea in opposition to my independent particle model talk.

Weiner:

His paper on it came out in '36, and I think the reference was that Bohr himself gave a lecture to the Copenhagen Academy in '36 and perhaps that was the paper that was published in Nature. This followed the seminar paper that you gave. If your seminar paper was 34, it still would fit in.

Bethe:

It must have been August of '34 approximately--maybe September.

Weiner:

And of course, as you said before I interrupted you, the Bohr compound nucleus idea was the first idea in Part B.

Bethe:

Yes. Now, as I remember it, I heard about the Bohr compound nucleus earlier than '36, and it may well be that people in Copenhagen talked about it already in '35, and then I probably heard about it from Placzek possibly in the summer of '35 or some such time.

Weiner:

In this country?

Bethe:

Possibly in Europe. I visited Europe every summer and it's very likely that I saw Placzek during this time, or it is possible that Placzek came to this country on a visit. I'm pretty sure I got it from Placzek. I'm pretty sure I got it as early as 35, and it was then still in development in Bohr's mind.

Weiner:

Did your presentation of it amount to an advancement of the idea rather than just a public restatement?

Bethe:

You mean back in 34?

Weiner:

No, I meant in '37.

Bethe:

Yes.

Weiner:

You had thought about it and reworked it somewhat?

Bethe:

I had, yes.

Weiner:

In what way?

Bethe:

Particularly in the sections after the first two--well, 51 is just very general, representing Bohr's idea; Section 52 is Wigner and Breit. But now in 53 and following--and this is essentially original work--I tried to be more quantitative; and, in particular, I derived the distribution of nuclear energy levels--how many energy levels per million electron volts or what-have-you. And what I did at that time turned out to be really quite good, with one exception. Unfortunately, I thought that the nucleus was very big, almost twice the radius which later on turned out to be true, so the numbers in here are all wrong, but the ideas are quite good and the formulae are quite right. So I calculated the distribution of energy levels and got the right order of magnitude of the number of levels per hundred volts or so. I calculated the width of the nuclear levels. Bohr had made a general model, the evaporation model, and I now tried to get the width of the energy level from this general theory. And, again, I think this is mostly original work, al- though some of the work was done by Weisskopf at the time.

Weiner:

Excuse me, I don't understand. Done independently by Weisskopf or done in connection with this?

Bethe:

We talked together but I don't remember.

Weiner:

Weisskopf would come and visit with you. He was close by, wasn't he?

Bethe:

Yes. I think it was done independently. I think he did it a little better than I and then I incorporated it in here. I think I had done some of it. Section 55, "Derivation of the Dispersion Formula." A little simpler than Breit and Wigner had done it. Then 56 is essentially original work, partly done with Placzek, where I tried to get numbers useful to the experimenters from the theory and where also I tried to go from the low-energy theory, which had been experimentally established, to a higher energy theory, where the resonance levels overlap and where you want to get cross sections averaged over large energy intervals. So that is what Section 56 is devoted to, and part of this was also published separately with Placzek. The next chapter [X] is about neutrons, which were, after all, the cause of it all and were the instrument by which all these regularities had been discovered. It has a lot of discussion of the Fermi papers. Some of the sections are about interpretation of experiments, especially Section 59, "The Diffusion of Neutrons." There I relied quite a lot on Fermi's previous work. It is essentially a reproduction of Fermi's work in somewhat different language. Section 60, "The Neutron Resonance Energies," getting these energies from observation. I think much of this was developed here by Placzek and myself, especially the "boron absorption method." It had been used previously, but I think we did it a little better. The boron absorption method is a way to determine the energy of the neutrons in a resonance-- namely, you observe that, let's say, silver is activated by neutrons, and then you put in a boron absorber between the source and the silver. Boron fortunately has a cross-section which goes exactly as one over the velocity and the constant is known in this formula, and therefore you can determine the velocity of the neutrons which are absorbed by the amount of absorption in boron. This was our method in those days to, determine neutron velocities and that's how we knew the resonance levels. It had been done by a number of other people, and many of the experiments were from places other than Cornell. It was just put together here. So the levels were obtained in this manner, and this is the first compilation of all the resonance levels observed. Then we discussed the width of the neutron levels. It's difficult to tell just what was new and what not. It was again one of those examples where everybody talks about the subject and similar work had been done at Columbia in particular and in England by Moon and several other people and by the Italian group. But I think this was again the first time in which it was put together in a coherent fashion.

Mehra:

Is that P. B. Moon who had himself a stormy history?

Bethe:

Did he have a stormy history?

Mehra:

I mean politically perhaps.

Bethe:

Politically? I didn't think so. It must be a different Moon. This Moon is in Birmingham and I think is quite conservative and had no storms.

Weiner:

Real conservatives are allowed to have storms, too.

Mehra:

But not in England.

Bethe:

So this became a compendium of what to do when one did neutron experiments. Then in Section 62, "Neutron Width [and the Absolute Cross Section as Derived from the Experiments," this is the quantity about which I talked in Section 54, so this was now a connection with the theory and theoretical conclusions were drawn from it. Then 63, "Scattering of Slow Neutrons," again has to do with the question of getting the neutron width, and also in this I point out that there is not only a contribution from the compound nucleus, but also a contribution, so to speak, from the rough shape of the nucleus. That is, the nucleus doesn't let the neutrons in and therefore it has a cross section equal to its geometric cross section, as I then thought. Later on a paper by Weisskopf, Feshbach and Porter, which I mentioned previously, corrected this. It's not as simple as this, but there you now have to take into account those one-particle resonances which I had talked about in early '35. I didn't know that in 37. Then 64, "Disintegration by Slow Neutrons with Emission of Charged Particles" and some processes on fast neutrons--more or less just putting things together so that experimenters could work with it. The next chapter [XL], "Alpha Radioactivity" is just a repetition of what people knew. However, I think I somewhat simplified the theory compared to Gamow and company. (Just today I gave a lecture on this to my senior class, and I found that in the textbook I'm using my theory is used because it is simpler.) But this chapter otherwise didn't contain much new. Now Chapter XII, "The Scattering of Charged Particles by Nuclei," and XI)I, "The Disintegrations Produced by Charged Particles," contained perhaps the most work, particularly the most work of my collaborators, Rose and Konopinski. We tried to put everything in good order. We calculated potential barrier penetrations to discuss the nuclear reaction. We combined the idea of potential barrier with the idea of the compound nucleus, and I think this work in these two chapters is mostly original except where I quote other people. It isn't anywhere a profound theory. It's just putting together these two things which most people knew, but I think it was useful to the experimenters because it gave them very simple easy-to-use formulae for the interpretation of their experiments. I don't think I need to go into detail. Perhaps one point that is interesting is Section 83, "The Selection Rules," saying that not every reaction will take place which is energetically allowed, but many more selection rules have been found subsequently, partly in connection with the point I discussed previously about Wigner's theory of the isospin and other quantum numbers and there are a few more. We tried as much as possible--and this is in Section 84--to make it quantitative, to give absolute cross-sections, not just relative cross-sections. Then the last chapter of this part, Chapter XIV, is about "Gamma Rays." This is related to the subject which you brought up earlier-- namely, can you explain gamma ray probabilities on the assumption that nucleons are nucleons in the nucleus and what's the transition probability? So transitions were classified, multiple transitions. An important point which had been discovered, and I think suggested, I don't remember by whom, was the idea of the metastable level. I'll look it up here and find out who suggested it because it had been observed that certain nuclear levels (Weizsäcker was the person) take hours or days or even years to decay by gamma ray emission; whereas on simple theoretical considerations you should expect that it takes about 10 seconds, maybe 10-¹², for lower energy gamma rays, but certainly a very short time. And Weizsäcker suggested that this probably was due to high angular momenta of the excited state. Then you might have a transition from a nucleus of angular momentum 5, say, to the ground state of angular momentum 0, and therefore it cannot be done by dipole or quadrupole but it requires a 32 pole to do this, and these transitions are very unlikely by general laws of quantum mechanics. So this idea of Weizsäcker explained these metastable states, and later on that same idea was very important in the establishment of the shell model. Namely, these meta- stable states of high angular momentum occur mainly in certain regions of the periodic system. And these are just the regions where nucleons of very high angular momentum are being added to the nucleus, and so the shell model got immediate confirmation by pointing out that just where very high angular momenta should be added, according to the shell model, these very high angular momenta do occur in the metastable states. 0f course in 1937 I didn't know about the good shell model, but I did know about metastable states, and these are discussed here. Other things discussed are that transition probabilities are generally much lower than you would expect for dipole transitions. This was explained only in the '50s by the so-called giant dipole resonance. At this time it was just recorded as a posit. Quadrupole transitions are, however, as strong as the theory would predict. So that's one of the sections in this chapter. Mostly these actual estimates of quantitative transitional probabilities I think were original. And that's just about the end of Chapter XIV. We discussed the processes which can follow gamma ray absorption. That's just putting gamma-ray absorption together with the compound-nucleus model. Unfortunately, there is not terribly much to be said about Part C.

Weiner:

Part C, as you explained, was the real experimental review, and this was made possible by the large card catalogue that Livingston had and he made use of it in this one. How did you make use of it, though, in the other sections--in A and B?

Bethe:

For examples. I got my examples from .

Weiner:

Was this a bibliography or were these notes on the articles?

Bethe:

Notes on the articles. For each article he had a little card with a short paragraph of notes.

Weiner:

Did he explain to you how come he happened to have them?

Bethe:

Well, he was working in nuclear physics. He thought there was awfully much literature. He couldn't keep track of it by keeping it in his mind. There were hundreds of papers already in that card catalogue.

Weiner:

I saw him for a few minutes, not for a formal interview, and I think he mentioned a number in the thousands--close to 3000 or something like that. He said they are all destroyed now.

Bethe:

Too bad.

Weiner:

And he said the reason he did it, was because he felt that at Berkeley he didn't have a full picture and one of the things that was a compelling thing to him when he came to Cornell was to get this full picture just as soon as possible. And so he did this and had it on hand when you arrived on the scene.

Bethe:

Yes. Well, the number here used is something like 600.

Mehra:

Already a phenomenal number to absorb.

Bethe:

Yes. We obviously didn't quote them all.

Weiner:

We have a little remaining time, so ...

Mehra:

May I just quickly ask one or two questions that still pertain to this? At what stage did people recognize the parity quantum number? When did people get convinced about it being conserved, and I would like to know whether it was just a matter of educational time?

Bethe:

Parity certainly was known from atomic physics, and it's back in the old atomic physics papers--papers like Wigner and von Neumann and Jordan. Around '28 when people emphasized the parity number, it was emphasized in atomic physics because it explained La Porte's Rule. La Porte's Rule is that the energy levels of a given atom can be classified into two groups which we may call even and odd, and strong spectral transitions can only occur from even to odd or from odd to even but never between two even states or between two odd states. And La Porte's Rule, which had been derived empirically from the spectra before quantum mechanics, was explained in quantum mechanics by parity--namely, that it was the wave function which was odd or even with respect to taking the mirror image at the origin. So naturally when nuclear physics came, we all knew about parity. And when the quadrupole moment of the deuteron was discovered, in particular, one of the main points in setting up the tensor forces was that parity should be conserved, that is to say, that the tensor force ought to be invariant with respect to the parity trans- formation. Everybody assumed this as a matter of course, and it wasn't questioned. I think Wigner was one of the first to write down this tensor interaction, and nobody questioned that you had to take a Hamiltonian of even parity.

Weiner:

Before you get into another technical question, maybe we can defer the technical questions until tomorrow because it's getting close to the hour that you said you wanted to leave. I would like to ask some easy questions, some sociological questions, that do fit in while you're in the mood or even while you're tired of talking of this three-part paper. One is a very simple one, I think, about the time that you put into it, about how long it took you really working on it.

Bethe:

It took me the better part of two years. I started working on it probably in the summer of '35--I can't date it very accurately; it may have been late spring. It was finished in something like April or May of '37. During this time I worked mostly on this. As I worked on it, I got some fall-out. Certain things became clear to me which seemed suitable for a separate paper to be published in the Physical Review, and so I published a lot of papers in the Physical Review during that time.

Weiner:

That's what I was getting at because there seems to have been a great deal of achievement.

Bethe:

All connected to the same subject. Whenever I found something while writing this ...

Mehra:

So when you were getting independent ideas to complete the picture there, you were publishing those ideas elsewhere at the same time.

Bethe:

Yes, at the same time.

Weiner:

The bibliography reflected that, and it shows that you were collaborating, as you indicated, with Rose quite a bit during this period.

Bethe:

And with Konopinski.

Weiner:

Yes, but most of the papers where another author is involved involved Rose rather than Konopinski. As a matter of fact, he's not involved in any of them.

Bethe:

Oh, he isn't?

Weiner:

Not in the 35-'37 period.

Bethe:

When is the first paper with Rose?

Weiner:

Rose comes in on a paper entitled, "The Maximum Energy Obtainable from the Cyclotron," which is another paper that we'll talk about some time. That was 1937, and then the others follow--"Nuclear Spins," and so forth. Rose might have been earlier, but I don't see him.

Bethe:

Nothing earlier? I am a little bit surprised, but maybe isn't entirely complete.

Weiner:

My point in asking this is that you were doing this and you apparently had teaching responsibilities as well. About how heavy a load did you have during this period?

Bethe:

About one course each term. I think occasionally I had two. 0ccasionally I had a lecture course and a reading course. The reading course took very little time--just telling people what to read and then giving them a short oral exam to find out whether they had read it.

Weiner:

Was the lecture course a graduate course?

Bethe:

Always a graduate course with one exception. I taught one section of sophomore engineers who had flunked the course before. This was the most miserable teaching I ever did. It was a most depressing class. Most of them tried to flunk it again.

Weiner:

Then you were spending your summers in Europe.

Bethe:

Yes.

Weiner:

But you didn't do any writing in Europe.

Bethe:

I did a little writing in Europe. In fact, usually during my summers I would spend two weeks maybe visiting other physicists, mostly in England. I would spend a month at my mother's in Germany and at my father's, and during the time with my mother I worked and wrote.

Mehra:

That was in Frankfurt.

Bethe:

My mother was in Baden-Baden; my father was 1n Frankfurt.

Weiner:

They were divorced?

Bethe:

Yes.

Weiner:

How was it going back to Germany at such a time?

Bethe:

Let's do that tomorrow. So I did work during the summers. Then we had usually several weeks in Switzerland on vacation. During that time again I did not work. So this was the set-up. I am surprised that I did not have papers with Konopinski. It may well be that this is incomplete. I have the impression that I worked about as much with each of them.

Mehra:

Since we still have a couple of minutes, could I still ask about nuclear physics a little?

Weiner:

Let's give Professor Bethe his choice. The question that I wanted to ask ...

Mehra:

I don't want to interfere with that.

Weiner:

The question is this: the evaluation of this entire series in two respects--that is, your evaluation in terms of the impact this had on the field, including the immediate and long-range response, and the second is what it meant to you, how you felt about it--that is, in terms of your total work, in terms of the time it represented and what it meant in your own development and in your subsequent work.

Bethe:

Well, for myself it was very much worthwhile. I really went through this subject, and I believe I understood it all at that time. I don't any longer. I connected all the parts of it, and I had the feeling that by this work I really had made things clear to myself so that after this I could work in this field without needing to read anything because I had written it all down. And so I had the feeling that it helped me tremendously, and that it stimulated me to work during this period and after this period, and really made it easier to work because I had a basis from which I could then find the interesting problems and go after the interesting problems without much effort. The impact on other physicists, I think, was quite great even if I say it myself. I think it was considered as the standard work. It was for many years I think the most comprehensive treatment of the subject, and I think remained so until Blatt and Weisskopf's book came out. And so in the intervening 13 years this is where people went for information, and certain parts of it were not reproduced in Blatt and Weisskopf and were used even after '52--not many but a few. It came of course at a time when nuclear physics was just bursting out everywhere in this country and in England, and to a lesser extent in other European countries, and so it was, I think, very useful for the practicing nuclear physicists, both theoretical and experimental, to have one book to which they could go to get the first information and one book which would tell them what was already known so that they could then work on from there.

Weiner:

This suggests--and the remarks of other physicists back it up-- that the very existence of these articles served to stimulate the field, that it wasn't merely a reflection of interest in the field, but the very existence and pulling together of all that was known into this type of presentation helped to attract people to the field and to stimulate the growth of the field.

Bethe:

That is certainly true.

Weiner:

Was the type of contribution you had made in this generally recognized? You told us here how you had rewritten certain things, how you had reformulated and how much new material was in it, but was this recognized as a review article or as original work?

Bethe:

I think the experts knew what was already learned and what was new, and also we had profuse quotations, so that I think it is clear in every section. In some sections there are dozens of quotations and so this is clearly review. 0ther sections have no quotations or one or two. So it is clear that those are original work.

Weiner:

A final question related to our lunchtime conversation about the function of review articles in absorbing past literature. How useful do you think the bibliography that you published with the article actually was? In other words, do you think people went to that bibliography and used it or were apt to just rely on the article alone?

Bethe:

I think sometimes one and sometimes the other. I think in many cases they just relied on the article alone. I would imagine that most people who consulted it for actual research certainly in the first ten years of its existence probably went and read the appropriate section here and then looked up two or three of the references. So I think the references were useful. I think the article without the references would also have been useful. But I think it's good to be able to dig into the original references when necessary.

Weiner:

That was your reason then for including them.

Bethe:

Well, there is also the reason that you want to give people credit. I think these two reasons were about equally important.

Weiner:

In a sense we're not talking about references but about bibliography. The references occurred in the text and you did give people credit. This was sort of an appendix. It's six o'clock. Just to show that we're honorable men, I going to turn off the tape recorder now to be resumed tomorrow.

Weiner:

This lovely Indian summer day is October 28th and it's in the morning, and we're resuming our discussion with Professor Bethe. I think the point that we left off on yesterday was the period in 1936 and '37 with the three-part article in Reviews of Modern Phy