Oral History Transcript — Dr. John Wheeler
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Interview with Dr. John Wheeler
This is one of 22 sessions of oral history interviews with John Archibald Wheeler conducted by Kenneth W. Ford between December 6, 1993 and May 18, 1995. They represent research material for Wheeler’s autobiography, Geons, Black Holes, and Quantum Foam: A Life in Physics (Norton, 1998).
RETURN TO THE UNITED STATES AFTER COPENHAGEN
The most stupid thing I did in my life was not getting married when I went to Copenhagen. At any rate, that meant that I didn't stay a second year there. It meant also I had to get a job back in the States at a time when job conditions were very tight. I've written about job conditions in my piece for Roger Stuewer, "Men and Moments." At any rate, I had a little old, battered typewriter, and I batted out typewritten notes of application. As best I can remember, it was to Harvard and Chicago, and perhaps other places that I wrote. But I didn't get any payoff.
The place I finally did go to, North Carolina, was initiated, if I remember correctly, not from my end but from North Carolina's end. Arthur Ruark had heard me give a talk at the American Physical Society meeting in Washington in the spring of 1934 about deducing the law of force between particles from phase shifts of the wave function, and deducing phase shifts of the wave function from scattering. It sounded like a golden highway to the future. I guess it caught his imagination.
INTERACTION BETWEEN ALPHA PARTICLES
This subject kept its hold on my imagination, not least because at the London meeting in October of 1934 I heard about the experiments on the scattering of alpha particles in helium. And I undertook to analyze that scattering in terms of phase shifts. One of the features of that analysis was the fact that it presupposed that there is such a thing as a phase shift, that there is such a thing as a wave function to describe the separation of the coordinates of two alpha particles, or an alpha particle and a helium nucleus.
The story at this point branches. One side of the story is taking the idea of phase shift for granted and deducing as much as possible about the interaction between the two helium nuclei from the dependence of phase shift on energy. Heinz Barschall and I later wrote a paper on that subject, and there is a subsequent commentary by Barschall about what that paper of ours foresaw and did not foresee. As I recall—now I have to get myself straightened out— the work with Heinz Barschall was not on interaction of two helium nuclei, but interaction of a neutron or a proton with a helium nucleus. And in that was the spin-orbit coupling, a revelation of it that, if we had been foresighted enough, would have allowed us to work out the shell-model picture of Mayer and Jensen, because that shell model depended in an important way on spin- orbit coupling, and the spin-orbit coupling was revealed in this work on the scattering of neutrons in helium.
Coming back to the actual process, the scattering of alpha particles in helium, what justification was there for a wave function at all and a phase shift at all? I had undertaken to give a foundation for that approach by setting up a variational wave function for the two alpha particles in terms of individual neutron and proton states and extremizing the energy with respect to the choice of wave function for the separation of the centers of mass of the two groups of nucleons. This gave a wave equation out of this variational principle, but the wave equation had one difference from the usual wave equation, that the interaction was not described by a simple potential but by a kernel—kernel of an operator, a kernel deduced from the individual neutron and proton wave functions. One could be skeptical about those neutron and proton wave functions, not least because we knew so little about the interaction between the nucleons at that time. But the idea that in the end one comes out, hell or high water, with a wave equation with an integral operator looked like something, a conclusion that rose above all special details or approximations.
INTERACTION BETWEEN NUCLEONS
This integral operator then required an interpretation. It became clear it was really the way to describe an interaction between two alpha particles that depends not only on the separation between their center-of-mass coordinates but also depends on the speed, the relative speed. Such an integral operator had a foundation, but what about the possibility that the interaction between elementary nucleons themselves is described by such an operator? What about the possibility that it is velocity dependent? My first graduate student at North Carolina, Katherine Way, and I worked out a comparison of such velocity- dependent forces and their consequences with the type of forces that had already been under consideration.
Was there at this time any evidence for a finite size of the proton or neutron? Scattering experiments had been done, and from them one could deduce phase shifts. But what do these phase shifts say about any force? It was still up in the air.
It seems to me that about this time Eugene Wigner was developing his theory of the interaction between nucleons, describing the distinction between a neutron and a proton by a spin-like coordinate, isotopic spin, analogous to the angular-momentum spin that one already knew about. Wigner described the interaction between particles in terms of a potential, dependent not only on the spin coordinates of the two nucleons but dependent also on another spin-like quantity he called isotopic spin, which served to distinguish between the neutron and the proton. This was an improvement on Heisenberg's theory of interaction between nucleons.
Heisenberg had written a couple of papers on the constitution of atomic nuclei, which were great objects of study in that day, many of the ideas producing payoff year after year from then on. But one idea did not work, and that was the proposition that the force between nucleons arises from exchange of neutrinos between the particles. Probably that had some resonance with Yukawa's thinking at that time. Perhaps it helped stimulate Yukawa to come up with his idea that the force between nucleons arises from the exchange of mesons between them. These mesons, it took time to recognize, were not the mesons that so predominate in cosmic rays, the ones we today call mu mesons. These were mesons of the type that today we call pi mesons.
NUCLEAR INTERACTIONS, MODELS, AND STRUCTURE
Katherine Way I called on in mid-December at the retirement community, so- called Carolina Meadows, in Chapel Hill, North Carolina, where she lives now, 91 years old. She was in the infirmary because she had broken her hip about four or five months before, but she seemed in good spirits and sharp as ever.
We, Katherine Way and I, worked on the cross-section of the deuteron for the absorption of gamma rays. In this time of ferment in the field of nuclear physics, it was interesting to consider the analogy between nuclear structure and molecular structure. I understand that Wigner was upset that work on the compound nucleus and compound-nucleus model of nuclear reactions of Bohr did not refer to Michael Polanyi, who had done a great deal of work on describing reactions between molecules in just such a language, a compound system. Chemistry, theoretical chemistry, at this time was undergoing a rapid development in the hands of Linus Pauling and others who saw how to use the new quantum mechanical methods. One of the key considerations there was the idea of a molecule resonating, as the word was, between one binding structure, which might be one neutral atom interacting with another neutral atom, and another binding structure, which might be the atoms now in the state of ions, one positive and one negative—with the wave function for the whole system described as a linear combination of these two configurations. Ed Condon, while he was still at Princeton—I had visited Princeton with Eugene Wigner for the Einstein lecture—Ed Condon, hearing about these considerations of mine on using a mixture of components for a wave function where one had alpha particles and individual nucleons, components in the wave function, suggested using the word "resonating group structure" for that, and I did.
My present-day colleague here at Princeton now, Frank Calaprice, in nuclear physics, told me a few months ago about a nucleus whose energy levels can only be properly understood by recognizing it as a rod-like shape, effectively a group of four or five alpha particles in a row.
Wilson, at Rice University, had purported to recognize rotational energy levels in nuclei. The pattern of spacings of low-lying levels seemed to him to indicate that nuclei do rotate. Edward Teller and I—I can't recall being specially motivated by that work of Wilson—took up the idea of a collective motion of the nucleus which was rotation and wrote a paper on the sorts of things one has to expect, many of which were later discovered.
K: Was that the paper in which certain spins were excluded by symmetry considerations?
Yes, certain spins excluded by symmetry considerations.
THE S MATRIX
In my paper on molecular viewpoints in nuclear structure, I introduced—if I remember right, that was the place—the so-called scattering matrix, which was later introduced by Heisenberg for describing interaction between elementary particles. I know I had been in correspondence with Wigner about this scattering matrix, and he had given me a helpful suggestion about its symmetry with respect to interchange of rows and columns.
When I was two weeks ago in Santiago, Chile, Susskind of Stanford was there. He told the group about his own encounter with the scattering matrix. He had been graduated from City College of New York—CCNY, as it's abbreviated— and had applied to enter Princeton as a graduate student. He was assigned to me as somebody to interview him. He mentioned in his remarks that I spent about three hours talking with him, reviewing a wide range of interesting problems. He was just enchanted. But he wasn't admitted to Princeton. He went to Cornell and he worked with Bethe. Bethe had assigned him to work on the scattering matrix. One day Bethe was going away on a trip and he got Susskind in to review with him what to do while he was gone, and he described further work to be done on this scattering matrix. Susskind said he was bored as could be with the scattering matrix. "Damn it all, I would like to do something else. Why can't I work on something interesting, like one of those problems John Wheeler was talking to me about at Princeton?" "Young man," Bethe told him, "you should know that this S matrix was invented by John Wheeler."
In connection with nuclear models that came so much to the fore at the time of trying to understand the results of the Rome group on cross sections for nuclear reactions, it was a big shock to many people to find that the nucleus was not the open type of system that many people had expected by analogy with atoms and planetary structure of the solar system. It was instead a much more compact system.
NUCLEAR MODELS AND REACTIONS
I've been told that the liquid drop model of the nucleus appeared as one of many ideas in an early edition of Gamow's book on nuclear physics, but I don't remember ever having looked that up myself. I know that Bohr had taken up the liquid drop model with Fritz Kalckar from the fall of 1935 onward, and they were doing an extended article on the applications of the compound nucleus and the liquid drop model to understanding nuclear transformations.
Was it Kalckar—who is unfortunately no longer living—was it Kalckar who spoke to me of the later article that Bethe wrote on the theory of nuclear reactions? As simply a derivative of the Bohr-Kalckar paper. At any rate, Bethe did a marvelously complete treatment using the compound nucleus and many other ideas to produce an article that was popularly called from then on the "Bethe Bible."
I have a memory of a visit by Bohr to the United States some time in the period of the 30s, maybe 1936. I recall talking to him on the stage after he finished his talk, privately, talking about the alpha particle model of nuclei, and he listened attentively. But I can't recall any special comment he made. That could conceivably have been at a conference held at the Carnegie Institution for Terrestrial Magnetism in Washington under the sponsorship of Merle Tuve and Gregory Breit. Or it could have been during some lecture of Bohr in North Carolina, if he came there during the time I was there. He might have lectured either at Chapel Hill, where I was, or at Duke, which was not many miles away.
LIFE IN NORTH CAROLINA
Now about North Carolina: I was at Chapel Hill for three years. I was promoted there in the last year from assistant professor to associate professor, and they had been good enough to give me a leave of absence for a semester so I could go to the Institute for Advanced Study in Princeton. We liked the place and we liked the people. Janette cried when we left. Two of our children were born in the Duke Hospital there, at Durham, a few miles from Chapel Hill: Letitia on [July 30, 1936], and Jamie on [May 5, 1938]. Later in life Jamie was interviewing—I've forgotten whether it was for admission to medical school or at a later stage of his career, for an internship—but at the medical school of Duke University, he called on and talked with Dr. Carter, who was the doctor who delivered him as a baby.
On my visit to North Carolina in December of '93 I had a chance, thanks to Jimmy York, to visit the outside of the house at 416 Pittsboro Street, where we had lived in Chapel Hill. We had no idea of making or building or buying a greenhouse, much though Janette liked green plants. But I did make for her something like a small-scale greenhouse, one that would fit in the window—a tray, a zinc tray about 3 feet square and about 7 inches deep that would hold gravel and soil but not leak water. This was then surrounded by glass sides and back and sloping glass roof in the out-of-doors portion. This zinc tray was half indoors and half out of doors, and the glass only encompassed the half that was outside. That window box was no longer there at the time of my December 1993 visit, but at two rooms further back in the house I saw projecting something that could have been store-bought window boxes.
In the last year or so of our stay in North Carolina we had a Japanese student living in one of the rooms in the house. He was learning English. We weren't sure how far his religious background was responsible for burned candles, because they certainly were not needed for light. There was good light. It would be great to find out what happened to him in the war.
We had a young black girl helping Janette around the house. She, I guess, must have walked from her home to get to work with us, came every day, $6 a week. But when we had two children and Janette raised her wage to $7 a week, some of the neighbors expressed their discontent with our breaking the wage scale. Her name was Tammon May Hopson. Once Janette and I came home to our hotel room at Swannonoa near Asheville at the time of thunder and lightning. We found Tammon May had taken the children into the bed with her; she was under the covers, terrified.
MOVING TO PRINCETON
It was a long drive from Chapel Hill to the summer place where Janette's parents had kindly invited us in Salisbury Cove, Maine, on Mt. Desert Island. On the way we went through Princeton, and I recall Jamie [Tita?] as a baby jumping up and down in the back seat as we drove down Nassau Street. With a real estate agent we lined up a house that we could rent, but the same day the same house was rented a little earlier by a different agent, so we had to bargain later on by mail for another one.
With Bob Robertson leaving Princeton to become a director of research at the Westinghouse Laboratories in Pittsburgh, a position became available at Princeton. I don't know whether that was the position that I ultimately got; I doubt if there's any easy way to tag a position with that degree of identity. During my visit to Princeton from Chapel Hill I had been invited to give some lectures on nuclear physics. Princeton at this time was eager, as so many other places were, to build up a status in this then coming field. Princeton invited Milton White to come from Berkeley to build a cyclotron. We didn't have at that time the faculty lunch center that Princeton has now, but it seems to me it was to give the chairman of the physics department a chance to meet and talk with me that Eugene Wigner invited Smyth and me to lunch at the restaurant that still exists at Princeton on Witherspoon Street, the restaurant called Lahière's. I think Smyth must have hinted at the possibility of this theoretical position. I did not especially like the "house theorist" label that some may have applied to this proposed appointment, but I did think of Princeton as a splendid place to be and work.
K: Was Wigner already back from Wisconsin at the time you came?
Yes, Wigner had come back from Wisconsin. He had been for a year [two years] at Wisconsin with Breit.
At this time Johns Hopkins was making an offer to me to come there as an associate professor, whereas the Princeton offer, if it developed into a real offer, would be only an assistant professor. Nevertheless I felt that Princeton had the more potential and that the name of the position didn't matter so much. I should add that the dollar in those days was different from the dollar today. The salary I had in the beginning at North Carolina was, if I remember correctly, $2,300 a year. In the same year that I began, Janette had been offered a teaching position, because she had been teaching at the Rye Country Day School, and her proposed salary would be $2,400 a year. So it was quite an act on her part to stay with some low-paid guy like me.
ROLE OF THE INSTITUTE FOR ADVANCED STUDY
It may sound rather mixed up, my account, because it's some g of Princeton University and something of the Institute for Advanced Study. Actually the Institute had no building of its own when I was giving my Princeton University lectures and making my Institute for Advanced Study visit. I think that was the spring of 1937. The university had offered its hospitality to the Institute as a way to get the Institute's show on the road. I can well believe as part of the negotiations it would be a real plus to the university to have the Institute in the same community—although it would have been perhaps more natural for the Institute for Advanced Study to move to and set itself up in Baltimore in association with Johns Hopkins University, because Johns Hopkins was, after all, the first genuine research university in America, operating on the European model and developed under Daniel Coit Gilman.
JOHNS HOPKINS UNIVERSITY
There is a wonderful story about Gilman identifying leading people in the various fields of knowledge—Gildersleeve in classics and Rowland in physics and others in other fields—and calling this group together—I don't know whether it was eight people or twelve people—-for some period of days to contemplate the idea of a real research university. As the discussions proceeded, more and more enthusiasm developed among the group of people, and Daniel Coit Gilman had the human perception to tap this enthusiasm. He said to Professor Gildersleeve, "And if such a university is set up on such a model, would you be willing to come?" [laughs] Gildersleeve said yes. So Gilman got the show going.
The gasoline that made the engine [of the Institute] turn around was a from the Bamberger and Fuld families. They had been attracted to Flexner as an advisor because of the ideas he expressed in his book Universities: American, English and European, where he pointed out that American universities had become too much like shopping malls, offering bits of this and bits of that rather than having broad currents of leadership. The president of Johns Hopkins at this time was the geographer Isaiah Bowman. He was president at a time when Hopkins was having heavy financial problems. Much of its original investment had been in the form of stock in the Baltimore and Ohio Railroad, and railroads at the time we're talking about were having a difficult time.
I can recall later when I was a trustee of the Battelle Memorial Institute of Columbus, Ohio and we had so much of Xerox stock—their way of paying Battelle a debt for developing the xerography invention of Chester Carlson. I persuaded my fellow trustees to get rid of the Xerox stock and diversify, which we did.
Bowman was handicapped not only by money problems, but by lack of a big enough vision. There was a very promising research person, Maria Mayer, at Hopkins, well known to my own research professor, Karl Herzfeld. Maria Mayer and Herzfeld together ran a seminar course on quantum mechanics in which we used the then new book of Max Born on quantum mechanics. Maria Mayer's husband was a professor in the chemistry department, a live wire person. Maria Mayer and her husband Joe Mayer had given a party for Janette and me when I came back from Copenhagen three days before we were married. So I always felt grateful to them for that. Maria Mayer had done work of a kind that I very much admired on statistical treatment of atoms, which was a wonderful background for her later statistical treatment of atomic nuclei and their energy levels. But Bowman wouldn't even give her a faculty appointment. She's the one that got the Nobel Prize and not her husband.
BOWMAN AT HOPKINS
Karl Herzfeld was so upset about the failure of the Johns Hopkins University under Bowman to back up good research people that he nursed along an offer that was made to him by the Catholic University of America in Washington to go there as a professor. And he threatened to go. Bowman called the threat, and Herzfeld went. Another loss to Hopkins.
INSTITUTE FOR ADVANCED STUDY, AND FINE HALL
The correspondence about putting the Institute for Advanced Study at Hopkins I have never seen. At any rate, it went to Princeton, and thanks to Princeton's hospitality, the initial emphasis could be on people rather than buildings. An example: Abraham Flexner talked with Einstein and wrote to Einstein and got Einstein to come to the Institute, even though Robert Millikan thought he had Einstein committed to go to Caltech. Einstein had, in an earlier visit to the United States, gone to Pasadena, but he was not committed.
I think that probably arrangements for going to the Institute at that time were as they are today. One applies for admission as a visiting member. At any rate, I did apply and I was admitted.
Among permanent members at this time were Heimann Weyl, John von Neumann, and Kurt Gödel. The mathematics members of the Institute are the ones that I used to see. Their offices were in Princeton's so-called Old Fine Hall, named after Dean Fine. My office during that visit from Chapel Hill was the library, as I think was customary for other visitors; I had a drawer for my papers.
The central institution of Fine Hall was the Tea Room, where people in mathematics and physics met for tea every day. As Oppenheimer later on put it, "Tea is where we explain to each other what we do not understand."
Oswald Veblen had been a catalytic agent in promoting the development of the Institute in its mathematical lines. He had been a professor of mathematics in the University, but he became a member of the Institute on its coming to Princeton. I think he had had a lot to do in earlier years with the design of Fine Hall—the library on the top floor, the lecture room on the main floor, and, on the intermediate floor, a passageway going all around the building so that one could keep talking and walking until the conversation or the problem was finished. The fireplace in the professors' room on that intermediate floor bears, chiseled in stone over it, over the metal place [?], the words of Einstein: "Raffiniert ist der Herrgott, aber boshaft ist Er nicht." (God is clever, but He's not malicious.) In other words, there's hope of finding things out.
Weyl, though he had come as a member of the Institute, early on arranged to give lectures at the University. I was fortunate enough to learn about these lectures, and go to them, on what today would be called mathematical forms. He told me at one point that he had to give lectures in order to see mathematics in its length and breadth and spot where there were gaps, things missing that need to be worked out. But I won't try to replicate my chapter on Weyl in my new book At Home in the Universe.
GETTING SETTLED IN PRINCETON
In the fall of 1938 the little Wheeler family was established in Princeton on a street called Murray Place. We had no guarantee that my appointment would go on more than a standard three years of an assistant professorship, but we nevertheless felt that we ought to get a better living accommodation, and we proceeded to plan a house. To get property, we were able to take advantage of Abraham Flexner's good will and helpfulness. He said, "You can buy a lot from the Institute, from some of the Institute property, because we think it's a good idea to have a close relation between the Institute and the University, and if you lived in that part of town, that would help to establish such a relation." Well actually, the lot that we got was across the street from our good friends the Ten Broecks, whom we had met at Salisbury Cove in Maine and who had let us live in their house for a period of something like a week while we were getting established in the fall of 1938. The lot that we got was across the street from them, next door to Panofsky, the art historian, and close to the house that Hermann Weyl was then building, and to the house of Vladimir Zworykin, the Radio Corporation of America man who was popularly regarded as the inventor of television. And we were one block further down the length of Battle Road than Veblen was. Veblen's had been the first house on Battle Road, the road that led to the battlefield of the Battle of Princeton, whose date I am ashamed to say I can't give you exactly.
Negotiations to get the land had taken some time in the beginning of the academic year, 1938-1939, and the next thing was to work out plans for the house and get the construction under way. Abraham Flexner had already said to us, "The fool builds. The wise man buys." But we were going to go ahead and be fools. Our new house was ready on September 1, 1939, the same day as the start of the war in Europe and the appearance in the Physical Review of the Bohr-Wheeler and Oppenheimer-Snyder papers.
BOHR'S VISIT, 1939
For Niels Bohr, the central theme of his life and work was the quantum—what it meant. He relied very much on discussions with colleagues to develop his ideas. Since Einstein was the number one opponent of Bohr's way of looking at things, he was glad to have a chance to have heart-to-heart talks with Einstein in the spring semester of 1939. Bohr had received an invitation from the Institute to make such a visit.
I knew that he was coming on the steamship Drottningholm, and I could look up in the newspaper and find out when and at what pier the ship would arrive. I went to New York by train, walked across to the west side to the pier, and met Niels Bohr and his son Erik and Leon Rosenfeld when they disembarked. I can't remember anything boisterous about our meeting. It wasn't the habit in those days to have the boisterous bear hugs that the Russians have taught us.
While I was waiting, who should turn up there but Enrico Fermi and his wife Laura. As the story jokingly puts it, on his receiving the Nobel Prize in Stockholm in December, he lost his way back to Italy and by mistake ended up in New York. At any rate, his arrival had been eagerly awaited by the faculty at Columbia University, and he was already established there. As I think is widely known, the motive for Fermi's coming was not anti-Semitism, which would have no application to him personally, but anti-Semitism as it applied to his wife, Laura. But I suspect that the story is told much better, more fully, and authoritatively in Laura Fermi's own famous book, Atoms in the Family. I would have to go back to the New York Times of the next day to see if there was a New York Times reporter there at the pier, but I have no memory of any such.
Bohr did not mention fission upon his arrival. He stayed a day or two in New York before he came to Princeton. He didn't mention it—fission—to Fermi. He was trying to preserve the priority of Frisch so that Frisch would get the credit when the announcement of the idea came out. Bohr worked even harder at getting recognition for some of his associates than they worked at it themselves, because Frisch didn't seem to realize how pressing it was to get his paper out in Nature.
But Rosenfeld had not been advised by Bohr to keep quiet, so he told me about fission on the train from New York back to Princeton. I asked him to report at the 7:30 p.m. Journal Club that every evening, which he did. This upset Bohr a bit when he heard of it, but following that announcement, the news of fission spread rapidly.
WORK ON THE THEORY OF FISSION
Rosenfeld's scientific contributions would be called for in the intended interaction with Einstein, so he would not be somebody Bohr could naturally call on to help on understanding fission, but I had been involved in nuclear physics, and so it was natural that he invited me to work with him on the subject. I can recall the very first thing we did was to rush up to the library and see if there wasn't a better word than "fission," because how can you use fission as a verb? You can't say "fish," "the nucleus fishes." Anyway, all the possibilities we tried in the dictionary were dropped in the end in favor of fission.
If Bohr was giving less attention to the quantum during his Princeton visit than he had intended, I was giving less attention to the subject I was deeply interested in at that time: the idea of action at a distance, which had seemed for a long time to me to be a much simpler description of electromagnetism than field theory. I was especially interested in the idea of action at a distance. Both Bohr and I were interested in fission as a new variety of nuclear reaction, and hoped it would shed light on the whole theory and enlarge the whole theory of nuclear reactions and nuclear stability.
I ought to say, for the sake of completeness, that I felt especially stupid not to have realized, long before, the possibility of fission. Stupid because my student Katherine Way had—I think it was for her thesis—investigated the magnetic moment of the nucleus on the liquid-drop model. The idea was if you have a droplet of fluid with a certain ratio of electric charge to mass given by the ratio of protons to neutrons, and then have that nucleus endowed with a certain angular momentum, then automatically it has a certain magnetic moment. And does that magnetic moment agree with the moment that's observed? Well, it didn't, and we sort of recognized that important lead to the individual particle model that Maria Mayer and Jensen later developed. Katherine Way found that if the nucleus rotated too fast, there was no solution of the equations. In other words, the nucleus is unstable and explodes. Well, if it will explode on rotation, it's natural to think of other ways it might explode, and that would have been a natural line of reasoning to lead to fission.
WORK ON THE THEORY OF FISSION (continued)
In the attic of the physics laboratory at Princeton there was an accelerator which would accelerate deuterons, and when they hit the appropriate target give out neutrons of essentially controlled energy. In this way it was possible for Heinz Barschall and Rudolph Ladenburg and Morton Tanner to measure the target area of uranium for undergoing fission when bombarded by neutrons of this, that, or the other energy. It turned out that this target area, or cross-section, was substantial for neutrons of a few million electron volts but then fell off. For low-energy neutrons, there was again a substantial target area for fission. What kind of a crazy thing is this, and how can you reconcile this with your ideas of nuclear reactions? That was essentially the issue put up before Bohr by George Placzek, a wonderful person for physical insight and for questioning and for raising doubts. Placzek raised his question at breakfast time at the Nassau Club, the place where the Bohrs and Rosenfeld were staying during their Princeton sojourn in the spring of 1939.
Bohr and Rosenfeld talked about Placzek's question as they walked across the campus from the Nassau Club to Fine Hall, where Bohr's office was and where he would shortly be meeting to talk with me. Suddenly, part way across the way, Bohr said, "Now we have it." He said the substantial target area at high energies must be due to the abundant isotope uranium 238 and the substantial fission cross-section of uranium at low energies must be due to that rare constituent of uranium, the U-235 that's present only three-quarters of one percent abundance. When Bohr arrived at my office, we went over again the picture of nuclear reactions as we had it, and then Bohr fit these new thoughts into it. They fitted beautifully.
Mathematically it was difficult to analyze how much energy it would take to jiggle a nucleus enough to cause it to undergo fission. The very simplest mathematical approach employed a power series with nuclear properties near the critical value, analyzing how these nuclear properties would change for departures from the critical value, critical value being that amount of charge on the nucleus that would cause it to break up spontaneously. How much energy would it take to disturb this, that, or the other nucleus enough to cause it to undergo fission? This power series gave a rough way of making predictions for heavy nuclei, and, among other things, allowed one to predict that the nucleus of plutonium 239, which had not even been seen, would be fissile.
PRACTICAL IMPLICATIONS OF FISSION
Neither of us recognized how important this prediction [the fissionability of Pu 239] was in suggesting a way to get fission material without the almost impossibly difficult business of separating one isotope from another, uranium 235 from U 238. It opened the door to a chemical separation of plutonium, if plutonium could be made. Louis Turner, my Princeton colleague, recognized the practical importance of that possibility when he was doing his great review of the papers on the physics of fission. He worked on it, I believe it was the summer of 1939, at Woods Hole. [JAW said 1938, but it could not have been earlier than 1939. The RMP paper was published in 1940. KWF]
I don't think Turner was at the discussion in Wigner's office, which was then Einstein's office. Two months to the day after Bohr's landing, Wigner, Szilard, me, I've forgotten who else was there—Bohr of course—went about the possibilities to make a bomb, and those possibilities at that time all rested on getting uranium 235. As Wigner said, "Yes, it would be possible to make a bomb, but it would take the entire efforts of a nation to do so."
To short circuit this isotope separation, the problem that led Wigner to say such a thing, by making plutonium, and making plutonium by making a chain reaction, that idea came to Louis Turner in this country and came to Carl Friedrich von Weizsäcker in Germany. I suppose that it didn't produce a big lunge forward in the German program because already German energies and money and people were absolutely overloaded with keeping the war going. Germany was having such a frightful time on the Russian front.
In his book about the making of the atomic bomb, Richard Rhodes begins with Szilard, Szilard's dream of finding the nuclear reaction that would lead to a chain reaction, and Szilard's mistaken idea at the beginning that beryllium would admit such a reaction. I believe Szilard even patented the idea in Britain. Szilard was mentally prepared then to capitalize on uranium to get something going.
Szilard lived at the King's Crown Hotel in New York, and Wigner would often visit him there, but Szilard would often come to Princeton, and he and Wigner would walk back and forth on the street talking. The idea of a chain reaction picked up resonance with Wigner. Wigner worked out the theory of the multiplication factor. Now it depends on the dimension of the lattice. Wigner and Szilard were the ones keen on this idea, but they spoke mainly in Hungarian. So I often overheard them talking, but did not know what they were talking about.
If Weizsäcker had had that plutonium idea earlier in the game, the Germans might well have got their project going in a big way early on. But a more likely way the Germans could have got going earlier on would be for Fermi to have been the one to discover fission, and he could have, if he had paid attention to a paper of Ida Noddack, who had suggested that the rich stable [1 of activities that resulted when uranium was irradiated by neutrons arose from fission products. That was at least three years, if I'm not mistaken, before the discovery of Hahn and Strassmann.
K: She was reacting to experimental work with Fermi and his group.
K: Where was she? Was she in Berlin?
I don't know where Ida Noddack was. Did we look up her up?
K: No, we didn't find her.
We didn't find her. We could look down in the library in the Biographische Literarische Handwörterbuch.
[Note: Later JAW and KWF found Noddack's 1934 and 1939 papers, in Angewandte Chemie and Die Naturwissenschaften, respectively. She was in Berlin. JAW later confirmed that neither he nor Bohr were aware of her 1934 paper when they were working on fission theory. One must assume that Fermi, following his 1934 work on neutron bombardment of uranium (in which he postulated that he had produced element 93), was also unaware of Noddack's work. Had he known of her work, he would, almost surely, have performed chemical tests for lighter elements among the radioactive species produced. JAW notes that had Fermi discovered fission in 1934 or 1935, as h e could easily have done, it might have put Germany on a track toward a bomb and conceivably changed the outcome of World War II.]
Had I known more movers and shakers and had more experience in influencing policy, I might have joined in trying to get uranium work started in the national interest. But I didn't. Bill Golden is a good ex a pie of the kind of person who would have been effective if he had been activated. It's clear that pushers are needed to make things happen. Consider Oppenheimer for Los Alamos, Teller and York for Livermore, Speer in Germany.
After the war, some of the movie people wanted to do a film with Lise Meitner in it, and she somehow was advised—I suppose by Bohr—to ask me to be her agent in deciding whether the text that was being produced was acceptable. I remember the conclusion in the end was No.
K: You mentioned yesterday the Meitner-Hupfeld effect, but I forgot what that is.
Yes. I had got interested through my attendance at the London conference in the back scattering of gamma rays by lead, the experiments by Gray and Tarrant reported at London, which went beyond experiments that had already been done by Jacobsen at Copenhagen and by Lise Meitner and Max Delbrück at Berlin. I still hope that some student will come down the pike who will get interested in doing all the slogging number work that's needed to treat that process as what I call a "cosmic ray mini-shower," with the photon coming in producing electrons in pairs, which then get heavily scattered in the lead and turned around backward, re-emitting radiation going in the new direction. It has nothing to do with the fourth-order quantum electrodynamics effect that Delbrück was working at so hard with Heisenberg.
MORE ON BOHR'S 1939 VISIT TO PRINCETON
K: I think you said that when Bohr spoke and Einstein was in the audience, Einstein reacted in a very, very mild fashion, and that Veblen was surprised that it wasn't more animated.
Yes, yes, that's right. I got the impression that our friends at the Institute who had invited Bohr rather hoped for something like a gladiatorial combat in the lecture room at Fine Hall, a gladiatorial combat on this quantum business. So there was Bohr speaking, and diagrams already put on the board in advance by Rosenfeld at Bohr's request, but Einstein sat quietly in the audience. He was not a person for jumping up and down and pounding his fist. So I think Veblen and the others concerned in the invitation to Bohr to come must have felt a little bit let down. It can be that Pais will tell something about this in his biography of Bohr, because he was around at that time. I can remember doing what I could to invite him to Princeton.
Was Bob Robertson still around? I think so. I don't think Robertson left until after the war. He found that, after the war, his wartime commitments had left such an entanglement with Washington he was always going to Washington. He thought he'd get away from it all if he accepted the invitation to go to Caltech, but then he found himself taking the Red-Eye Special all the time between Los Angeles and Washington. It was great to have Bob Robertson around, because he was so interested in relativity. Relativity—I mean general relativity—at this time didn't bulk large in the scheme of things, or cosmology. Why didn't it?
There were people around working with Einstein whom I heard described by some disparaging colleagues as "one-legged people"—all they knew was general relativity, and nothing else in physics. That was not the best setup for getting relativity on the road. Pauli and Einstein, I think, about this time wrote a paper, I think indirectly connected with the problem of gravitational collapse. Could a collection of particles go around and around and not fall in? I think they concluded, Yes. So this was anti-black holes before there was ever a black hole to be anti.