Hideki Yukawa

Wheeler:

This is an interview of John Wheeler with Hideki Yukawa at Kyoto on the l0th of July, 1962. Professor Yukawa, would you say anything about the work with (Ishiwara) and his influence on you?

Yukawa:

Yes. I really did not know him personally very well. I met him several times. But, I used to read his books on relativity and. quantum theory when I was in high school. He had a rather wide influence for popularizing theoretical physics, new theoretical physics, such as quantum theory and relativity. He was a professor of theoretical physics at Kyoto University, but be had then unfortunately resigned, and in the later years he became a kind of journalist and poet. He was very good in short poems, (Japanese poetry).

Wheeler:

Perhaps we could turn now to the more recent part of physics. One thing that occupied the minds of many people in the period 1933 to 1934 was the question whether quantum electrodynamics is good at the high energies. How could one understand the particles which penetrated through large amounts of lead? And some people doubted that the theory was good over 137 mc². What was your position at that time? What people do you remember discussing this issue with?

Yukawa:

About that time — it was only three years after I was graduated from Kyoto University — I was beginning to move from Kyoto University to Osaka University. And I was acquainted with Professor Nishina of Kyoto and also with Professor Kikuchi in Osaka. I remember that there was a great deal of discussion in Japan, just as in Europe and in the United States, about the applicability of quantum electrodynamics. And certainly there were two different opinions. But I think some of the physicists already tried to introduce some kind of cut off. I don’t know exactly; but some change in the high energy region of quantum electro dynamics but there was also some doubt about such things. And so, I myself was not sure about the break-down of quantum electrodynamics or whether it goes on at very high energy. However, I was very much interested in the structure of the atomic nucleus, and I was first interested in the behavior of electrons inside the nucleus. Of course that’s the question, whether the electron exists in a nucleus or not. Irrespective of whether it exists in the nucleus or not, the question remains whether the electron comes very close to the nucleus. So that first I tried to develop a theory of the electron which is very closely connected with the nucleus; not the orbital electrons but one which is more closely —.

Wheeler:

One which is inside —?

Yukawa:

It may be inside, or it may be created near the nucleus. So I tried to develop a kind of theory in which electron (fields, just as electromagnetic fields, are a mediator) of nuclear forces. This idea originated from the well-known Heisenberg theory of the atomic nucleus. His paper was published in 1932. I tried to develop his idea by explicitly considering the electron field. But then, of course, there were a great many difficulties connected with spin, statistics, conservation of energy, momentum and so on. But then in 1933 or ‘34 Fermi’s famous paper on beta decay could be very well accounted for by introducing neutrinos. But then immediately I began to think about the question of nuclear forces in connection with beta decay process. And I immediately noticed that the beta decay process is a very slow process, so that the electron-neutrino field would be too weak for strong nuclear interaction. But I didn’t try to draw quantitative conclusions. Then I saw in Nature two letters by (Tamm and Iwanenko) in which they gave the formula for a nuclear interaction produced from the electron neutrino field. As I expected, the nuclear interaction (in terms of) the beta decay process turned out to be much too small. So I was again encouraged to think farther for something entirely different from the beta decay process. Before that time, before I saw Fermi’s paper, as I already mentioned, I had been trying to develop an (electron field) theory which would also account for nuclear forces. And there were many troubles, but at that time Nishina suggested to me that there may be the possibility of thinking an electron has (Bose) statistics. But, I could not follow it up at that time because if there is an electron of different type, different statistics, then it could be immediately detected, it would be easy to detect by a (nuclear) process. So I have (pondered) for some time about this idea, but after Fermi’s paper I began to think it again. And gradually my ideas began to take shape, and finally sometime in September of 1931 it became clear to me that if we assume the mass of this new particle large enough, then there will be no difficulties with the unobservability of such a particle. And then I immediately undertook to calculate, to check the relation between the range of force and the mass of this new particle; so that my first paper was, at that time, almost there.

Wheeler:

Did you publish that very soon?

Yukawa:

Yes, very soon. So it was published early in 1935.

Wheeler:

Were you worried at that time about the connection with cosmic rays?

Yukawa:

Oh yes, now I must return to your question. I immediately suspected that this new particle, since it is much heavier than the electron, so it could be much more penetrating than the ordinary electron. So it may have some connection with the penetrating component of the cosmic rays. But at that time Anderson and others in the United States have been working very hard with this problem — the nature of the penetrating particles in cosmic rays. Of course there were two different theoretical interpretations of the penetrating component. One is that they are ordinary electrons and positrons but since they are very high energy they could be much more penetrating than the lower energy electrons, due to the breakdown of the quantum electrodynamics. But the other opinion of the nature of the penetrating components is different, maybe heavier. But of course the only heavy charged particle known at that time was a proton or as a nucleus. So, there is also the evidence that the penetrating particles (at sea level) mostly consist of particles lighter than protons, so that it worked very well with my idea of introducing the new particles of intermediate mass. So when I saw that in 1957 I expressed my opinion about the penetrating component of cosmic rays as consisting of these new particles which I introduced. And about that time Oppenheimer and (???) published a similar article which tried to identify the penetrating component with my new particle. Also I remember that, I was told by someone, that Stueckelberg, in Zurich, had a similar idea, maybe about the same time as myself. But he could not develop this idea because Pauli strongly objected to it. Also I remember that Wentzel had an idea, but his idea was somewhat different. He thought of a new particle which is heavier — even heavier than a proton — so that it is different. So of course we were in the dark. But the verification of the existence of the …(unreadable text)… of 1937 or during 1938, with Anderson and Neddermeyer (using a Wilson cloud chamber and also I think in Sweden, Stevenson, and also a little later Nishina with a cloud chamber). So that (the indications) became very clear —.

Wheeler:

Do you remember when you first heard the word of those experiments? Did it reach you by special letter, or in the journals?

Yukawa:

I think I have seen it in the Physica1 Review first. It is different from nowadays. Nowadays we hear mostly in the newspaper.

Wheeler:

When did you begin to think of the particles as (decay)?

Yukawa:

Then about that time, I think it was about the end of 1937 or early 1938 that Bhabha published a letter in Nature in which he discussed briefly about the spontaneous decay of such a new particle. So I looked at that letter and I noted that it must be true, and we wondered why we could not (recognize ???). So we immediately tried to calculate —.

Wheeler:

Who is we?

Yukawa:

I and (Sakata). So then, of course my idea was to connect the new particle, which now is called the meson, with nuclear forces on the one hand and. also the beta decay on the other hand. So, we can calculate the decay life time of meson by taking into account both the nuclear forces, and the beta decay. But we found that the life time of meson should be smaller — the theoretical life time of meson should be smaller by a factor of 10 or 100 than the life time of penetrating particles in cosmic rays.

Wheeler:

Do you remember any of the details of that discrepancy? When it first came to light?

Yukawa:

At that time the data on beta decay was not very convenient because what we knew about the matrix elements of beta decay was mostly for complicated nuclei. So we wanted to know the more elementary process, like with helium lithium, and so on. But these data came a little later so we (put in —. You know we were more naïve then). But the theoretical life time became shorter and shorter; the discrepancy between theory and experiment became larger and larger. And we concluded there is a discrepancy about a hundred. Nowadays we can say this discrepancy is (easily) larger, but at that time we had about a factor of 100. So how we can solve this? Then came another very disappointing thing, that the penetrating component, because of the smallness of radiative cross sections But at the same time the people working on cosmic rays found that the probability of occurrence of nuclear processes due to penetrating particles was very rare. So this was also very striking, very shocking for us, because such mesons should interact very strongly with nuclei so that the: nuclear process should occur much more frequently. So there also, as with the scattering cross section of mesons by nuclei, there was a discrepancy between theory and experiments of about a factor of 100. So these two things were very puzzling to us. Then came, about 1942 or so, from (Sakata) and. (Tanikawa) both working with me on the problem —.

Wheeler:

Was that in Kyoto or Osaka?

Yukawa:

Yes, this probably was in Kyoto. I already moved from Osaka to Kyoto because my teacher, Professor Tamaki, died in 1938. I succeeded him in 1939, and I came back to Kyoto. (Sakata and. Tanikawa) were there with me in Kyoto. We had been discussing these puzzling problems and then came to the …(unreadable text)… the particles as such, and let’s assume the existence of a new particle that is directly related to the penetrating component of cosmic rays. The penetrating components of cosmic rays consist of particles, a kind of meson which is not directly responsible for nuclear forces. But we had to abandon the original idea of mesons responsible for nuclear forces.

Wheeler:

Did this conclusion or position come before the experiments of Conversi and Pancini?

Yukawa:

Much earlier. Much earlier. During the war, much earlier. So the fact (Sakata) and (Tanikawa) must assume was the existence of two kinds of mesons. That was the only thing we could do if we altogether abandoned our original idea. But as for the nature of these two kinds of mesons, there were controversies among us. But this is controversy which was resolved after the war, after Conversi’s experiments.

Wheeler:

So it was possible to do some physics even during the war?

Yukawa:

Oh yes, it was possible. Of course the Japanese government tried to mobilize the scientists, but not really in a sweeping way. So, many of the scientists could continue their study. And also they didn’t think we (theoretical physicists) are very useful.

Wheeler:

Could I ask some questions about the background of your work, the state of physics in Japan? If we take the period from (1938) to 1939 because the history of quantum physics is being cut off in 1939. The physicists who were most active at that time, where had most of them been trained? The ones that you had associations with, had they all been associated with Nishina in one way or another?

Yukawa:

Yes, in some way or other, most of them are connected with Nishina. Nishina came back from Copenhagen about the time when we were — I and (Tomonaga) — graduated from Kyoto University. And he came to give lectures at Kyoto University.

Wheeler:

To Kyoto? From Tokyo?

Yukawa:

Yes. From Tokyo.

Wheeler:

What year would that be?

Yukawa:

I can check it. I think this is about 1931 or ‘32. But this was not the first. We were very fortunate. There were many European physicists who visited Japan and came to Kyoto University. (Laporte) when I was still a student, and he talked about new quantum mechanics. And then came Sommerfeld. And then came Heisenberg and Dirac too, and a few years later came Bohr.

Wheeler:

Who took the initiative in inviting these people to Japan?

Yukawa:

I think Professor Nagaoka was all powerful then among scientists. He was the president of Osaka University, but he was also the president of the Academy. He was the greatest boss among Japanese scientists. I think he was already over 60, but he was still very lively. And he had some very deep insight, although he did not work himself, he had —. I think we continue. I was talking about Nagaoka. Nagaoka was president of Osaka University when I moved from Kyoto to Osaka. But he was at the same time president of the Academy; he was the greatest boss among all the scientists in Japan. Although he was perhaps over sixty, he was mentally very young. And he was interested in new physics, and he tried to invite the first-rate theoretical physicists from abroad. (And of course Nishina cooperated with this.) Now Nishina was very (popular). (???) And Professor Kimura, (???) Kimura — his branch was spectroscopy, but he cooperated with Nagaoka, and he tried to invite these foremost physicists to Kyoto University too. So we were very fortunate to have the opportunity to listen to lectures by these first-rate, eminent theoretical physicists from abroad. And then, after that, we were also fortunate that Japanese physicists who had been studying in Europe, Professor Arakatsu, and Professor Sugiura, and (a little later) Professor Nishina, and some others, came back from Europe. They studied in Europe in Copenhagen or in Germany the new physics, the quantum mechanics, and we learned from them. So really we had a very good start. This was just the time when I was a student at Kyoto University, and I — [Four lines unintelligible] [N. seems to say he studied at a good time, when real problems were being solved.]

Wheeler:

And these people were the primary ones who had been trained in Europe, Nishina and Sugiura.

Yukawa:

Yes, they were graduated from Japanese universities, but they went to Europe to study further. And they were studying just at the time when quantum mechanics was beginning to rise. And then for myself, I was so fortunate that I learned many things in Japan and stayed in Japan.

Wheeler:

Did you regret that you had not gone to Europe?

Yukawa:

Since I could learn a great deal in Japan, no I didn’t regret. But this may be somewhat an exception. Tomonaga went to Germany to study — I think it was Leipzig — with Heisenberg.

Wheeler:

Are there any other features about the development of physics in Japan that occur to you as important and worth mentioning?

Yukawa:

Now, the one thing which we very often talk about in Japan is why Professor Nishina could be so influential in the development of (his school, in Tokyo, the theoretical physics department.) I think there are many reasons but one reason, certainly is the personality of Nishina. He was very open-minded and generous. His institute in Tokyo (is not a part of the University.) It is a kind of private institution, (???). His laboratory was very open, so we could really go there to talk with him and his staff about many things and in a very informal way, so that mostly it was quite different from the standard atmosphere of the university. But also I was very fortunate that I moved to Osaka finally, because at that time the faculty of science at Osaka had just begun to build. So that many people, many physicists, came from different universities, and some of them were rather young — younger than professors. I think the average age of professors in Osaka University at that time was probably much lower than the average age of professors in other universities. So it was very lively, and we could talk in a very informal way. And Professor Kikuchi was a very good (influence, he was still very young.)

Wheeler:

Was he the chairman of the department?

Yukawa:

No, not the chairman of the department. The chairman of the department was Professor (Yagi). Yagi is an electrical engineer. He is very famous for inventing the so-called Yagi antenna. He is not a pure physicist, but he took care of the organization of the physics department. Kikuchi was in charge of nuclear physics. Kikuchi could work very freely from Yagi. Also I think the personality of Kikuchi was very important. He was very frank. So I think in the Osaka University, among our group, (there was a most different atmosphere from other universities.)

Wheeler:

This was due to Nagaoka having asked new men, and new conditions.

Yukawa:

I think so. If we go back to the (???), then Professor Nagaoka was very influential in setting up the Osaka University faculty of science (in such a form as it was).

Wheeler:

Did Nishina’s group then lose its importance in physics in Japan?

Yukawa:

No, no! I went very often to talk with Nishina. So that Nishina’s group and our Osaka group were both important.

Wheeler:

You had informal discussions with him, or with his whole group?

Yukawa:

Yes, with his whole group. Tomonaga was already with Nishina in Tokyo. And also (Sakata) was there, (and he moved back to us at Osaka from Kyoto.) He was also with Nishina, then he later came back to Osaka from Kyoto. Also (???). It was very fortunate for (new students) that we had two nice centers, in Tokyo and Osaka, and also very good leaders, not only Nishina, but also (Nagaoka) and Yagi. And also I had a very good classmate, Tomonaga. And also, when I first gave a lecture on quantum mechanics in Kyoto University, Sakata, (Kobayashi) and (Taketani) were still students. But all of them were very good, so we could work together. So these are all very happy (places).

Wheeler:

I’ve just used up the hour which I promised to steal away from you. I hope that we can go on some other time.

Yukawa:

This may help at some point.

Wheeler:

Yes, wonderful.