The Charms of a Physicist

In the fall of 1950, when I was a senior at Harvard, I decided that I wanted to study the quantum theory. This was only moderately unreasonable. I was a mathematics major and had taken a solid course in freshman physics given by the theoretical physicist Wendell Furry. I had also read some popular books and had a great many philosophical talks about the quantum theory with my teacher Philipp Frank. While I knew some higher mathematics, when it came to physics I knew next to nothing. Nonetheless, I decided to enroll in the first-year graduate course in quantum theory taught by the then reigning genius in theoretical physics at Harvard, Julian Schwinger.

To say that Schwinger’s lectures were both brilliant and impenetrable would be an understatement. They were very brilliant and impenetrable. Schwinger was, it turned out, trying out an entirely new formulation of the theory on us—the old one would have been hard enough—and since he lectured from memory questions were discouraged. (Years later Robert Oppenheimer once said to me of Schwinger that when most people discuss a problem they show you how to solve it, but when Schwinger discusses a problem he shows you that only he can solve it.) After a few weeks I was lost. I was commiserating with a friend, a physical chemist, who was also taking Schwinger’s course. He said, what you should do is come with me to MIT and audit “Viki.” Viki, I learned, was Victor Weisskopf, who was Schwinger’s analogue—I am tempted to say antiparticle—at MIT.

My visits to Viki’s class in quantum mechanics at MIT were, in every way, a culture shock. The class and the classroom were both huge—at least a hundred students. Weisskopf was also huge, at least he was tall compared to the diminutive Schwinger. I do not think he wore a jacket, or if he did, it must have been rumpled. Schwinger was what we used to call a spiffy dresser. Weisskopf’s first remark on entering the classroom, was “Boys [there were no women in the class], I just had a wonderful night!” There were raucous catcalls of “Yeah Viki!” along with assorted outbursts of applause. When things had quieted down Weisskopf said, “No, no it’s not what you think. Last night, for the first time, I really understood the Born approximation.” This was a reference to an important approximation method in quantum mechanics that had been invented in the late 1920s by the German physicist Max Born, with whom Weisskopf studied in Göttingen. Weisskopf then proceeded to derive the principal formulas of the Born approximation, using notes that looked as if they had been written on the back of an envelope. Along the way, he got nearly every factor of two and pi wrong. At each of these mistakes there would be a general outcry from the class; at the end of the process, a correct formula emerged, along with the sense, perhaps illusory, that we were participating in a scientific discovery rather than an intellectual entertainment. Weisskopf also had wonderful insights into what each term in the formula meant for understanding physics. We were, in short, in the hands of a master teacher.

After I got my degree in physics in 1955 I had the chance to see Weisskopf in a somewhat different role—as a sounding board for scientific ideas. By this time I was a postdoctoral fellow working in the Harvard Cyclotron Laboratory. Only a few theoretical physicists then could be found on the staffs of Harvard and MIT. We used to meet once a week for lunch and during these lunches Schwinger would try out his new ideas on Weisskopf. The styles of the two men could not have been more different. Schwinger always built extraordinary mathematical castles in the air and Weisskopf had a wonderful way of trying to anchor them to the ground. One of the things I learned from those sessions was not to be afraid to say, “I don’t understand,” even in the most exalted company, something that stood me in very good stead when I left Cambridge for the Institute for Advanced Study at Princeton.

I also got from these discussions with Weisskopf a sense that physics was a culture with a history full of colorful, larger-than-life characters. Weisskopf, who had been born in 1908, only three years after Einstein created the theory of relativity and invented the quantum, had worked with most of the major figures in twentieth century physics, including Niels Bohr, Werner Heisenberg, Wolfgang Pauli, and Oppenheimer. He was, for me, a precious link to these people. For all these reasons, when I heard a few years ago that he was writing an autobiography I was very pleased, thinking I would soon be able to read in detail about many of the subjects I had heard him talk about in passing over the years. The autobiography, The Joy of Insight, is, as I expected, charming, and it is sometimes revealing, but it falls short of what I had hoped it would be. A clue to why this is so may be in a brief statement that Weisskopf makes about himself. He writes, “I am aware that I am not a good psychologist. Often in my life, I have overlooked the negative qualities of a person and seen only the positive side.” This quality no doubt makes for a more agreeable life, but it tends to make an autobiography less than satisfying.

Weisskopf was born in Vienna to a well-to-do and highly assimilated Jewish family. A great uncle had changed his name from Cohn to Colbert. The household included a cook, a chambermaid, and a nurse for the children, and the family was highly musical. Weisskopf learned to play the piano and has had a lifelong passion for classical music. Indeed, at some point he had to choose between going to a conservatory to study to be a conductor and continuing his studies in physics. The Christmas and summer holidays were spent at the Weisskopf country house in the mountains in Alt Ausee. Weisskopf was an excellent student, athletic enough to enjoy skiing, and with enough of a social conscience to become a socialist. He recalls writing songs and sketches for a political cabaret.

Weisskopf entered the University of Vienna in 1926 just at the time the new quantum mechanics was being formulated. After a two-year course, he was advised by his professor, Hans Thirring, to leave Austria since the universities there could no longer teach him anything useful. He chose Göttingen. As he writes, he might have gotten a better education, especially in mathematics, if he had gone to Munich to study with Arnold Sommerfeld, whose pupils included Pauli, Heisenberg, and, later, Hans Bethe. In particular, Weisskopf thinks he would have become a better mathematician. People often think that all theoretical physicists are mathematicians. Some, like Freeman Dyson, are, but most are not. Of course theoretical physicists use mathematics, but a physicist like Weisskopf, who is not a mathematician, uses as little as possible. Theoretical physics is a science of approximation. What counts is the ability to see what is “big” and what is “small”—which effects are important and which can be neglected. Many mathematicians who try to do physics don’t have a feeling for this at all. They think of physics as an abstract logical structure, which it isn’t. Weisskopf’s special ability lies in his physical intuition, which enables him to pick out the features of a problem that are important; mathematics are secondary.

Weisskopf had intended to study with Born. But Born was, under the best of circumstances, aloof, and to make matters worse, he suffered a stroke, from which he eventually recovered. Weisskopf found himself on his own; but he soon hit on a problem involving the radiation emitted when an electron in an atom makes a quantum jump from one excited state to another. In his book, Weisskopf does not hesitate to describe his scientific work, and he does so in a way that an interested layperson should be able to follow without much difficulty. In the problem he invented for himself he needed more mathematical skills than he had.

It was at this point that Eugene Wigner came to the rescue. If there ever was a mathematical physicist it was Wigner, although he had been trained as a chemical engineer. The two young men, Weisskopf was twenty-two and Wigner a bit older, published their paper in 1930, and it established Weisskopf as a physicist of promise. Fortunately Weisskopf’s family was willing to pay for another year of study with Heisenberg in Leipzig, where he went after taking his degree in Göttingen in 1931. But after only six months he received an invitation from Erwin Schrödinger, in Berlin, to come to Berlin to be a paid assistant.

Thus, within a year, he had a chance to work with the two men who did the most to invent the quantum theory. This was followed by a Rockefeller grant which he used to go to Copenhagen to study with Niels Bohr. In 1933 he was invited to come to Zurich to be Wolfgang Pauli’s assistant. No better education in physics could be imagined and Weisskopf had the intelligence and self-confidence to make the most of it. Pauli, with whom Weisskopf did important work on the quantum theory of fields, was known for his almost childlike honesty as well as his brilliance. Of a young physicist he once said, “So young and already so unknown.” Oppenheimer said that he was almost identical with his caricature, and he was. A reader of Weisskopf’s chapter on Pauli will come away with a sense of his genius and integrity. It was for me a little sad to find that Weisskopf refers to Pauli, in the present tense, as the conscience of physics. He died in 1958 and my own students, for example, know nothing about him apart from references to him in textbooks.

In 1932, while still in Copenhagen, Weisskopf married his Danish wife, Ellen, whom he met while working with Bohr. She died in the summer of 1989, and the book is dedicated to her. (There was a theorem at Bohr’s institute that no young man could stay in Copenhagen for more than two years without marrying a Danish girl.) She went with him to Zurich and, in 1936, to the Soviet Union, where Weisskopf thought he might accept a permanent job. One year there was enough. The Stalin terror was much in evidence and it affected many of the scientists he knew. In 1937 he had an offer from the University of Rochester where he remained until he went to Los Alamos in 1943. It was Weisskopf’s good fortune that he was able to bring his mother, brother, and sister to the US from Austria. Indeed, one is struck again and again by the combination of good luck and common sense that enabled him to stay clear of the political disasters that surrounded him. No member of his or his wife’s immediate family was killed by the Nazis. But for Weisskopf, like so many of the others at Los Alamos, the effects of the Nazi aggression changed his life.

Physics before the war was, at least as outsiders perceived it to be, a marginal field practiced by unpractical eccentrics. The atomic bomb changed all that. An unworldly physicist like Robert Oppenheimer suddenly acquired the glamour of a movie star. Weisskopf did important work at Los Alamos, and by 1943 he had switched his field to nuclear physics, which suited very well his intuitive style. He had an almost oracular sense of how nuclear reactions would work. The excitement of the work propelled him and the other participants forward as if they were riding a great wave. He writes,

Today I am not quite sure whether my decision to participate in this awesome—and awful—enterprise was solely based on the fear of the Nazis beating us to it. It may have been more simply an urge to participate in the important work my friends and colleagues were doing. There was certainly a feeling of pride in being part of a unique and sensational enterprise. Also, this was a chance to show the world how powerful, important, and pragmatic the esoteric science of nuclear physics could be.

One effect of physics’ new prosperity was the build-up of physics departments in universities in this country after the war. In particular, MIT decided to develop its program in nuclear physics and Weisskopf was an obvious choice. He moved there soon after the war and has, with some extended leaves of absence, been there ever since. For five years, however, ending in 1966, he was the director of CERN (Centre Européenne pour la Recherche Nucléaire), which, despite its name, does not carry on research in nuclear physics but rather is concerned with the fundamental physics of elementary particles. In many ways, the center was conceived by the late I.I. Rabi, who saw it as a way of re-building European science which had been shattered by the war. By 1952, there were groups of European scientists and engineers studying the construction of particle accelerators in the newly chosen site of Geneva. The laboratory now spills over the border into France and the huge recently functioning LEP (Large Electron Project) accelerator orbits electrons back and forth across the border. Although Weisskopf was neither an experimenter nor an elementary particle physicist, he turned out to be the ideal choice for the laboratory’s third director. He was, after all, a European by origin. He had enough scientific prestige to more than hold his own in the debates in scientific policy and he had a taste for the kind of Machiavellian infighting that went on when one dealt with some twelve diverse member states with their various nationalistic egos.

I saw a good deal of Weisskopf during this period when I was a regular summer visitor, and interviewed him extensively when I wrote an article on CERN for The New Yorker, I was surprised when, in 1965, he told me that he was thinking of leaving. As he says in his book, part of the reason was that his wife never felt entirely happy in Geneva, a town whose inner, French-Swiss social circle foreigners seldom come to know well. Weisskopf also felt that he would lose his ties with American science. He told me that if he returned to MIT he might have a chance to influence the style of American science. I am not sure to what extent he has succeeded in doing this, but he was certainly instrumental in maintaining MIT’s physics department as one of the strongest in the country. Since his return he has used his influence to promote the public understanding of science and particularly to warn against the consequences of nuclear war.

When a biographer, or an autobiographer, states, as Weisskopf does, that “often in my life, I have overlooked the negative qualities of a person and seen only the positive side,” the reviewer must ask how this affects his work. In Weisskopf’s case the result has been what I would call an ineluctable gemüchtlichkeit—a rosy good feeling—which at least, in my view, has caused some distortions and omissions. I will give three examples based on his accounts of King Christian of Denmark, Wendell Furry, and Werner Heisenberg.

Christian X was the septuagenarian Danish monarch during the German occupation. I believe he was a decent man. From what I have read of the behavior of the Danes during the occupation they have much to be proud of as compared, say, to the French or the Dutch. But Weisskopf’s statement that the “king wore a yellow armband with the Jewish Star of David on it whenever he rode through the city on horseback” is a myth. Various published historical accounts, including several in these pages, have made it clear that Christian X did not wear a Jewish star. In fact the only Jews in Denmark to wear a star were the few hundred who were deported.¹ He might have looked more closely at the sources of this misleading fable.

As I have mentioned, Wendell Furry first appeared in my life when, as a sophomore at Harvard, he taught me freshman physics. In 1953, when I switched from mathematics to physics for good he reappeared. My shortcomings in physics were so glaring that the then chairman of the Harvard physics department, Kenneth Bainbridge, suggested I take a special reading course with Furry on electromagnatic theory. This was, in principle, an excellent idea except that during the course Furry’s mind was elsewhere. He was under investigation by Senator Joseph McCarthy.² Furry had been a member of the Communist party for some years after 1938. By 1953, he said, he was no longer a member and I believe him. On November 4, 1953, he was called before McCarthy’s committee and refused to answer any questions.

The university took a position that made Furry’s position impossible. It decided that he had to testify about his activities, but that he need not name names. I have no idea how much thought went into this decision, but it should have been clear that if Furry had complied, he would have had to give up the protection of the Fifth Amendment and therefore risked prison if he did not name names. In fact, McCarthy understood this immediately, and on February 15, 1954, he held an open hearing in Boston, which I attended and of which I still have the official transcript. It was a nightmare that ended with McCarthy saying of Furry,

Because of men like this who have refused to give the government the information which they have in their own minds about Communists who are working on our secret work [all of this was, of course, nonsense], many young men have died in the past, and if we lose a war in the future it will be the result of the lack of loyalty, complete [immorality] of these individuals who continue to protect the conspirators.

On the way out of the hearing room a woman spat on Furry. Weisskopf seems to have forgotten all of this. He characterizes Furry as having withstood what he describes as an “unpleasant grilling.” He never considers that Harvard’s position on the matter put Furry in jeopardy. Until he appeared before the committee he had broken no law, but he was then indicted for contempt. The case was eventually dismissed by Judge Bailey Aldrich after Furry had spent many thousands of dollars, which he did not have, on his defense. He had some help from the members of the Harvard physics department who voted to turn over to him a small percentage of their salaries; but it was an experience that broke Furry’s heart.

Weisskopf compares Furry’s case with that of the MIT mathematician Dirk Struik, whose situation, for some unstated reason, he calls “more serious.” Struik had left-wing sympathies and appears, although Weisskopf does not make this clear, to have refused to sign the absurd loyalty oath to the state of Massachusetts that all teachers in the state were required to sign. The MIT authorities then suspended Struik, although they continued to pay his salary.

Weisskopf joined a faculty group that protested the university’s policy. “We were careful to express our opinion in a way that was loyal to MIT [he does not explain what this odd phrase means] while reflecting our sense of the injustice of this action.” The petition failed, though Struik was allowed to use the library. Five years later, he too won his court case. In retrospect it seems that the only form of protest that might have been effective would have been a unanimous or nearly unanimous refusal by the Harvard and MIT faculties to sign the loyalty oath; and in the atmosphere of the time, this was as likely as joint self-levitation by the same professors. I signed the oath and so did Furry. Only poor Struik had the courage of our convictions. It might be mentioned, although Weisskopf doesn’t, that at about this time the Berkeley physics department was badly damaged when several of its most gifted faculty members refused to sign a similar California oath. I admired those people then and I admire them now. While I was reading this section of Weisskopf’s book I kept thinking of something that his Viennese compatriot Karl Kraus once said, “If I have to choose the lesser of two evils, I will choose neither.”

Barring some new revelations, it is unlikely, I believe, that we shall ever be able to fully sort out Werner Heisenberg’s wartime activities. I do not think Weisskopf’s treatment of him is of much help in clarifying what is known. What is universally agreed on about Heisenberg is that he was one of the greatest physicists of this century. In some sense, his was the first truly quantum mechanical mind. He came to the subject—he invented it—with no intellectual debts to classical physics. Bohr, who had such debts, was, nonetheless, able to adapt to the new way of thinking and served as Heisenberg’s philosophical guide. The second point on which there is universal agreement is that Heisenberg was never a member of the Nazi party. He was a blond and nature-loving German nationalist—a “boy scout,” Weisskopf calls him—and saw himself as a good German. He began working on a German atomic bomb with enthusiasm in 1939 just after the discovery of nuclear fission in 1938 by Otto Hahn, Fritz Strassman, and Lise Meitner. By the time Hahn and Strassman had observed what turned out to be nuclear fission, Meitner had escaped to Sweden from Germany, and it was she who correctly interpreted their experiments. Her nephew, the physicist Otto Frisch, who was with her, coined the name “fission.”

What is still at issue is why the Germans failed to make the bomb and what part Heisenberg had in this failure. Did he have, as some have claimed, moral reservations about creating such a weapon for Hitler? Or did he fail because of technical incompetence? By far the best discussion I have seen of these matters was sent to me by Professor Paul Lawrence Rose, a historian at the University of Newcastle in New South Wales, Australia. It is a manuscript of a lecture he gave in June 1984 at the Hebrew University in Jerusalem, entitled “Heisenberg, German Morality and the Atomic Bomb, 1939–1945.” I do not know if it has ever been published.

As Professor Rose points out, Heisenberg kept altering his version of events as the years went on. The closest we can get to what Heisenberg really thought at the time, in my opinion, are the parts of the so-called Farm Hall transcripts that have found their way into books written by some of the few people who had access to them. Farm Hall was an estate outside Cambridge where the leading German nuclear scientists, including Hahn and Heisenberg, were held after they were captured in 1945. The Dutch-born physicist Samuel Goudsmit had led a mission called Alsos—“Groves” in Greek, after General Leslie Groves, who headed the US atomic bomb project—to investigate the progress the Germans had made in building an atom bomb. The scientists had been rounded up as part of that mission. While they were at Farm Hall, their conversations were secretly taped, including the table talk on the evening of August 6, 1945, when the Germans first heard of the attack on Hiroshima. Groves, Goudsmit, and R. V. Jones, the British intelligence officer whose idea it was to monitor these conversations, have all published parts of the tapes. The full transcript has never been published, it is said, because of fears of offending German sensibilities. This seems a pity. Nearly fifty years have passed since they were made and it is time the tapes were made available to historians.

In any event, we know from the tapes that were released, which are not mentioned by Weisskopf, that Heisenberg’s first reaction to the news of Hiroshima was disbelief. He said, “I am willing to believe that it is a high pressure bomb and I don’t believe it has anything to do with uranium.” Then he added,

There is a great deal of difference between discoveries and inventions. With discoveries one can always be sceptical and many surprises can take place. In the case of inventions, surprise can really only occur for people who had not anything to do with it. It’s a bit odd after we have been working on it for five years.³

It would help if one could see these somewhat obscure remarks in the original German, but the sense seems clear enough: What was dropped on Hiroshima couldn’t be a uranium bomb since we tried to make one for five years and couldn’t. This statement stands in contrast to what he later said. After 1945 Heisenberg never was willing to acknowledge that it was incompetence that kept him from making the bomb. Gradually the reason evolved into the “moral” one of keeping the weapon from the Nazis.⁴ I never believed this since I had been told by people I trust that during the war Heisenberg had said that it was important for Germany to win the war. As Goudsmit has written, Heisenberg visited a colleague in Switzerland and told him, “How fine would it have been if we had won this war.” A few years ago, when I discussed these matters with Hans Bethe he told me,

In 1942 [Heisenberg] had come to the conclusion that the Germans should win the war. That struck me as a very naive statement. He said that he knew that the Germans had committed terrible atrocities against the populations on the Eastern Front—in Poland and Russia—and to some extent in the West as well. He concluded that the Allies would never forgive this and would destroy Germany as a nation—that they would treat Germany about the way the Romans had treated Carthage. This, he said to himself, should not happen; therefore Germany should win the war, and then the good Germans would take care of the Nazis. It is unbelievable that a man who has made some of the greatest contributions to modern physics should have been that naive. But he seems to have said similar things to people during the war. He never mentioned anything to me about the morality of making a nuclear weapon. In 1939, though, Heisenberg was certainly anti-Nazi. He had been attacked by the Nazi press for being too friendly to Jews and for teaching “Jewish physics”—including Einstein’s theory of relativity.⁵

About Heisenberg’s intentions, Weisskopf has only this to say:

Obviously, by remaining in Germany, he soon became involved in the efforts of the Nazi regime to exploit nuclear energy, which began as soon as the possibility of a bomb became evident. The scientists on the Allied side and those on the German side, specifically those who were more or less opposed to the Hitler regime, faced vastly different situations. The Allied scientists were eager to work on the bomb because they trusted the policies of their leaders, Churchill and Roosevelt, and wanted to prevent the other side from developing the bomb. Those German scientists who, like Heisenberg, were not in favor of the Nazis would have been in a terrible situation if they had been called on to devise this awesome weapon.

I believe myself, for reasons I have tried to make clear, that Heisenberg’s failure to make a nuclear weapon had nothing at all to do with lack of “eagerness.” The documents that have survived the war—many of Heisenberg’s private documents seem to have disappeared—show that the German scientists were as eager as the Allies to make a bomb. Why then did they fail?

The clearest analysis of this question that I have seen is in the paper of Professor Rose I have cited. The type of nuclear fission that is relevant to bombs, or reactors, occurs when a heavy nucleus like that of uranium absorbs a relatively light neutron. The neutron and proton are the constituent particles of the nucleus. The neutron, unlike the proton, carries no electric charge and is readily absorbed by the heavy nucleus. When this happens, the newly formed nucleus finds itself in a highly excited state which is unstable. It then splits, or fissions, into lighter nuclei, quanta of electromagnetic radiation, and additional neutrons. These additional neutrons can then cause other nuclei to fission, thus establishing a chain reaction.

This process works because more neutrons are produced in a fission than the single neutron that caused it. In the fission of uranium 238 (U238), the common isotope of uranium, an average of 2.5 neutrons is released for each neutron absorbed. This does not mean that sometimes half a neutron is released, but rather that sometimes there are two or three neutrons, or whatever, averaging to 2.5. These neutrons are very energetic and are often referred to as “fast” neutrons.

It turns out that the effectiveness of the capture of neutrons by the U238 nuclei increases as the neutrons are slowed. This is why reactors whose fuel is largely U238 contain “moderators” like carbon or heavy water—deuterium—to slow the neutrons down.

However, in September 1939 Niels Bohr and John Wheeler published an article arguing that the rare isotope of uranium U235—“natural” uranium is 99.275 percent U238 and 0.720 percent U235—could be fissioned by fast, as well as slow, neutrons. What kept the Germans from getting the atomic bomb was that Heisenberg, and the other German scientists, never understood the implications of this finding for making a nuclear weapon. It is clear that Heisenberg did not understand this until August 14, 1945, when, after having stated a week earlier that a uranium bomb was impossible, Heisenberg gave a lecture to his colleagues at Farm Hall to explain how it had been done.

When he began working on the bomb in 1939 Heisenberg’s idea was to use the slow neutron fission of U235. This led him to overestimate the amount of uranium needed to make a bomb by a factor of many thousands. This was the fatal error and we can all be grateful he made it. The reason for Heisenberg’s error is that it takes a slow neutron much more time than a fast neutron to find the next nucleus to fission. If we think of the uranium fuel being shaped into a sphere, then as the fissions proceeded this sphere is blown apart. One of the things that is blowing the sphere apart is the pressure from electromagnetic radiation produced in the fissions. This pressure will blow the sphere apart more quickly than the slow neutrons can cause the chain reaction to run away and this has the effect of stopping the nuclear explosion. The only way to counter this with slow neutrons is to use a larger sphere and hence more uranium—several tons of it.

The first person to realize that this problem could be surmounted by using the fast neutrons, without slowing them down, to fission U235 was Lise Meitner’s nephew Otto Frisch. This was early in 1940 by which time Frisch had moved to England. He and Sir Rudolf Peierls submitted a short memorandum to the British government, which had all but abandoned the bomb project, that argued that if fast neutrons were used the critical mass required for fission would be reduced to only 600 grams, a little over a pound. It was this calculation that began the nuclear age, not Einstein’s letter to Roosevelt. It was inconceivable to these anti-Nazi scientists that Heisenberg had not made the same calculation and come to the same conclusion. If they had known how misguided he was, there might not have been, one may speculate, any atomic bomb project. Not knowing, they felt themselves in a desperate race to make the bomb.

Meanwhile, Heisenberg had decided to make a nuclear reactor which he seems—and here the facts are murky—to have wanted to use as an explosive device. In 1942 he met with Albert Speer. No notes of this meeting survive, but Heisenberg seems to have told Speer about the practical impossibility of making a bomb using slow neutrons. Speer gave his blessing and highest priority to the reactor project which, as it turned out, Heisenberg was never able to complete. I agree with the general tenor of Weisskopf’s remarks about this part of Heisenberg’s story although Weisskopf is fuzzy about the details. In his account Speer has somehow become an admiral in the German navy and none of Heisenberg’s difficulties in making the bomb is explained.

There is an equal fuzziness in Weisskopf’s account of one of the strangest episodes in Heisenberg’s wartime career—his meeting with Bohr in Copenhagen. This took place in October 1941 after Copenhagen had been occupied by the Germans. Weisskopf says it was in 1942, and he leaves out some very important details. Heisenberg was accompanied by his colleague, the physicist C. F. von Weizäcker. To this day no one, Bohr included, has ever understood the reason for this meeting. Professor Rose’s inquiries lead him to the conclusion that one of Heisenberg’s motives was to protect Bohr’s and his institute by making so prominent a German Aryan as Weizäcker its director.

In 1943 Bohr and his family escaped to Sweden in a fishing boat. Bohr was then flown to England and came to Los Alamos soon afterward. One of the things that Heisenberg brought to the meeting with Bohr in October 1941—this is not mentioned by Weisskopf—was the drawing of some sort of nuclear device. The drawing was sent to Los Alamos by Bohr, who had become convinced that the Germans were on the verge of making a bomb. When Bethe told me about this, he said,

“Later on, at Los Alamos, this drawing was transmitted to us by Bohr. It was clearly a drawing of a reactor, but when we saw it our conclusion was that these Germans were totally crazy—did they want to throw a reactor down on London?”

Weisskopf, who was Bethe’s deputy, must surely have seen this drawing. All he tells us is that Heisenberg on this occasion “expressed himself vaguely, fearing that any direct statement about Germany’s nuclear effort, or any doubt of a German victory, would put him and his family in mortal danger.” But, in this case, why did Heisenberg come to Copenhagen to see Bohr at all? Was it really, as Weisskopf claims, to talk “about the problems that the development of nuclear explosives had created for humankind and, in particular for the community of scientists.” I doubt it.

In the introduction to his book Weisskopf informs us that he has written largely from memory, without diaries, letters, or documents. The problem is that much of his book is written as if it were history, and, in view of his eminence, people may well take it for history. What the examples I have given suggest is that what he has written is not history, but the charming memoir of an admirable man. I wish it were more.