by Gino Segrè
Bohr was known for his somewhat wooly delivery. His brother Harald, a distinguished mathematician and a famously lucid lecturer, once responded to the question why his brother was such an unclear speaker: “Simply because at each place in my lecture I speak only about those things which I have explained before, but Niels usually talks about things he means to explain later.”
Anderson had not understood much of what Bohr had said to him, but he perceived that it was weighty and Fermi should be informed. He rushed upstairs to the physics department to tell him what he had heard. As he later confessed, “Nothing like having a reason to talk to one of the really great guys who knew more about neutrons than anybody else in the world.” He soon found Fermi. Starting to recount the conversation with Bohr, he was quickly interrupted. As Anderson remembered:
So Fermi says ‘let me tell you about fission.’ Then he described it, and then I really understood it. With Bohr it didn’t make any sense to me at all. But when Fermi explained it, he really made it very clear. He knew, he’d already heard about it. With Fermi, all you have to know is to tell him what to think about and he knows how to take it from there and work everything out.
Fermi did more than succinctly explain to Anderson the conjecture that a uranium nucleus can be split. Fermi told him what would be needed in order to perform the key experiment. Anderson suddenly realized that he had all the necessary equipment in the physics department’s basement. Summoning up his courage, he suggested they do the experiment together. Fermi, still unaware that Frisch had already conducted the experiment, instantly agreed.
This was the beginning of both a working relationship and a close personal friendship that would last until Fermi’s death. As Laura Fermi wisely observed, their closeness was partially due to Anderson’s boldness and lack of need for praise. Fermi saw no reason to express appreciation for a job well done. It was to be expected. This bothered some people, but not Anderson.
Fermi and Anderson’s very first collaboration got off to a rough start because the basement cyclotron that was supposed to provide them with a neutron source wasn’t functioning properly. Fermi had to leave for Washington before it could be fixed. Undeterred, Anderson located some radon and beryllium in the department to make a source like the one Fermi had used in 1934. That night, having prepared it and now joined by Dunning, Anderson did the experiment. The two Columbia physicists were the first in the United States to see the gigantic pulses that fission causes. They immediately sent Fermi a telegram describing their success. Excitement was building.
The next day, January 26, the Washington meeting began at two in the afternoon. Following the revised schedule, Bohr spoke first and Fermi second, his diction accented but his meaning crystal clear. Richard Roberts, an experimental physicist who worked at the Carnegie Institution, remembered that Bohr “mumbled and rambled so there was little in his talk beyond the bare facts. Fermi then took over and gave his usual elegant presentation including all the implications.” With a jolt Roberts realized that he too had all the equipment necessary for doing the experiment. Roberts and a friend went right to work looking for splitters, the term Bohr was using, because he had not yet heard from Frisch the name that would be used ever after: fission.
By Saturday night, Roberts and colleagues alerted their Carnegie boss that they were ready to show anyone their results that evening. Fermi and Bohr, the latter smoking an after-dinner cigar, saw for the first time the giant pulses produced in the ionization chamber. And the whole world soon was informed about splitters because a reporter present at the Thursday afternoon talk filed a report for the Washington Evening Star. The Associated Press picked it up and so did the Sunday New York Times.
The exuberant press coverage added to Bohr’s anxiety because Frisch and Meitner’s role was not mentioned. What if the news item were to be picked up by the Scandinavian press? How might Frisch and Meitner feel if their scoop was not rightly credited? Still not having heard from Frisch, Bohr returned to Princeton on Sunday. There, to his great relief, he found a letter from one of his sons in Copenhagen that mentioned in an offhand way that Frisch had been successful in carrying out the experiment and had submitted letters to Nature.
Two days later, on January 31, Bohr received a long-awaited telegram from Frisch: “LINEAR AMPLIFIER DEMONSTRATES DENSELY IONIZING SPLIT NUCLEI BOTH URANIUM THORIUM DETAILED INFORMATION POSTED.” A detailed letter from Frisch arrived the next day. Bohr wrote back at once, “I need not say how extremely delighted I am by your most important discovery, on which I congratulate you most heartily.”
The personal letter to Bohr also explained why he would be using the term “fission” to describe the phenomenon: “It was suggested by the biochemist Dr. Arnold, who told me it was the usual term for the division of bacteria.” Bohr and others concluded that it was better to speak of “fission,” with its somewhat esoteric origin, than of “splitters,” a prosaic word with electrical associations.
Although Bohr was very relieved to hear from Frisch, he still worried about whether Frisch and Meitner would be appropriately credited. He was also feeling guilty that this might have been partially due to his not having warned Rosenfeld to remain silent. Bohr tried to remedy the situation as best he could by urging that any publication on fission wait to be submitted until the publication of Frisch’s letter to Nature. These efforts were not successful, and led to tensions between Bohr and some other physicists, including Fermi.
In Fermi’s first publication since coming to Columbia, a Physical Review letter entitled “The Fission of Uranium” written with Anderson, Dunning, and two other Columbia colleagues, the only mention of Frisch’s experiment is a paragraph included after the description of what Fermi and his colleagues had done. It says, “After this experiment had been performed, Professor Bohr received a cable from Dr. Frisch stating that he had obtained the same results some days before.”
As for Fermi’s point of view about the discovery of fission, it was stated at the beginning of the Columbia article:
The phenomenon was discovered by Hahn and Strassmann who were led by chemical evidence to suspect the possibility of the splitting of the uranium nucleus into two approximately equal parts. Through the kindness of Professor Bohr we were informed of these results some days before receiving them in published form, as well as the suggestion of Meitner and Frisch that the process should be connected with the release of energy of the order of 200 Mev.
In other words, Fermi was maintaining that Hahn and Strassmann had discovered fission and that Frisch and Meitner had only estimated the energy released during the process. Bohr felt this assessment was unfair. He suggested that an acknowledgment of Frisch and Meitner be more explicit in the article. Fermi replied in a March 1 letter that he would have been happy to accommodate Bohr, but it was too late to do so. The article was already in press.
Bohr’s opinion, as expressed in a response letter to Fermi, tried to explain again his position, namely that Frisch and Meitner’s “merit was to have grasped the fission idea so thoroughly and given so reasonable an explanation of the mechanism of energy release that it would appeal immediately to the interest of all physicists.”
This difference of opinion caused temporary friction but did not leave a lasting strain between Bohr and Fermi. Each greatly respected and appreciated the other’s strengths, even if they did not always see eye to eye. Bohr’s approach was certainly more generous, anxious to go beyond the basic physics to the human side of the equation. Fermi’s approach was based on facts directly laid before him. Fermi did not look for subtleties. Bohr was always searching for elusive truths. One of Bohr’s favorite aphorisms was that a great truth is one whose opposite is also a great truth. One cannot imagine Fermi voicing such a statement.
The discovery of fission had a huge impact on nuclear physics. Under the influence of the new understanding, nuclear physics was advancing at a dizzying pace. Groups everywhere were studying fission, a phenomenon that seemed unimaginable only a few weeks earl
ier. The nuclear physics community looked to its two leaders, Bohr and Fermi, one for his unparalleled knowledge of theory and the other as the greatest experimentalist and clearest expositor.
Bohr and Fermi were admired and loved by the physics community. Aside from his stature as a physicist, Bohr’s presence meant so much because, as written in a centenary volume commemorating him, “No great human concern left Bohr indifferent.” He was a humanist and a philosopher as well as a scientist. Fermi, by comparison, was admired and loved for his single-mindedness and the purity of his devotion to physics.
21
CHAIN REACTION
The world of nuclear physics had been transformed in a few short weeks. Everybody involved in fission and a host of newcomers were speaking of it, what it meant, and how they could participate in its auspicious potential. The anticipation was palpable. A few scientists were beginning to envisage with dread the possibility that this new source of energy, dwarfing in magnitude anything previously known, could make a weapon of unparalleled destructive power.
At the end of 1939 the two acknowledged leaders in studying fission, Bohr and Fermi, also saw their respective interests in the subject move in different directions within this broader context. Bohr’s focus was on anomalies in the different modes of fission and Fermi’s on commonalities in all modes of fission. Fermi postulated it was likely that extra neutrons would be released when fission occurred. He began speculating about what might happen if a neutron beam were aimed at a large number of tightly packed uranium nuclei. For example, if an initial fission were to produce two neutrons, those two would generate four neutrons by colliding with other uranium nuclei. From 2 to 4 to 8 to 16 to 32 to 64 to … It would be a chain reaction. And if each collision generated the amount of energy Frisch and Meitner had estimated a month earlier, the end product would be a mammoth source of power.
Following this idea to its logical conclusion, Fermi realized that such a chain reaction could result in a hugely powerful bomb. The challenge was whether the energy that was produced would blow the device apart too quickly for the chain reaction to be sustained. George Uhlenbeck, his friend from Rome, Leiden, and then Michigan, found Fermi contemplating this prospect in the spring of 1939. Visiting Columbia for the semester and sharing an office with Fermi, Uhlenbeck saw him one day staring out of their large office window at the view below him. Holding his hands cupped together as if there were a small metal sphere between them, Fermi turned toward Uhlenbeck. Looking first at the imagined sphere in his hands and then down at the Manhattan landscape he quietly said, “A little bomb like that and it would all disappear.” The bomb might be little in size, but its power would be earth-shattering.
Fermi put such notions out of his mind: it was too remote a possibility. For the moment his attention concentrated on the goal of producing a chain reaction, not its application. He was keen to get started. On returning from Washington, Fermi asked Anderson to join him in tackling the problem and showed him a list of experiments they would need to perform together. He had already formulated a plan of action.
Nor was Fermi the only physicist considering chain reactions in early February. On Monday morning, January 30, a young physicist was sitting in the barber’s chair at the Berkeley Student Union reading the day’s newspaper. Tall and blond, Luis Alvarez had grown up in San Francisco and gone to college and graduate school at the University of Chicago, demonstrating there his drive, originality, and technical wizardry. He would become a major developer of tools for both radar and the atom bomb, even flying over Hiroshima in a B-29 trailing the Enola Gay. Though he was only twenty-seven years old at the beginning of 1939, Alvarez had already established a reputation as a star among the experimental physicists in Ernest Lawrence’s cyclotron group.
That Monday in the San Francisco Chronicle’s second section, Alvarez came across a small item that said “some German chemists had found that the uranium atom split into two pieces when it was bombarded with neutrons.” Instructing the barber to stop cutting his hair, Alvarez pulled off the sheet clipped onto him and started running to his laboratory. On his way there he met the campus’s brilliant theoretical physicist, a thirty-four-year-old New Yorker named J. Robert Oppenheimer. Stopping for a minute, Alvarez told him what he had just read and how he was on his way to produce fission. Oppenheimer’s immediate response was that splitting the nucleus was impossible. Alvarez remembered Oppenheimer’s giving him “a lot of theoretical reasons why fission couldn’t really happen.”
That did not deter Alvarez. He rapidly succeeded in setting up and performing the experiment that revealed fission of uranium nuclei. He then invited Oppenheimer to come to his laboratory and observe results for himself:
When I invited him over to look at the oscilloscope later, when we saw the big pulses, I would say that in less than fifteen minutes Robert had decided that this was indeed a real effect and more importantly, he had decided that some neutrons would probably boil off in this reaction, and that you could make bombs and generate power, all inside of a few minutes … it was amazing to see how rapidly his mind worked and he came to the right conclusions.
Within a few years a much larger physics community would be commenting on the speed with which Oppenheimer seemed able to embrace all the implications of fission.
Though neither Fermi nor Oppenheimer knew it at this time, a visionary Hungarian physicist had been worrying for more than five years about the possibility of a nuclear chain reaction being used to construct a bomb. Until January 1939, he was unaware of an ingredient that made the prospect both more likely and more frightening: fission.
Forty-year-old Leo Szilard had extraordinarily acute political antennae. Jewish, he had fled Hungary as a twenty-one-year-old in 1919 to avoid persecution by the right-wing anti-Semitic government that had come to power. Moving to Berlin, he had rapidly distinguished himself through his insights in theoretical physics and his fondness for new technological instruments. He became a good friend of Albert Einstein, a man who shared both those interests with him; their partnership led to the joint filing of several patents, none of which became very profitable. Nonetheless the small amount Szilard received from his royalties subsidized the nomadic life he had chosen.
When Hitler came to power at the end of January 1933, Szilard—like Einstein—left Germany. This time he went to London, booked a small hotel room, and continued his previous lifestyle: a leisurely breakfast followed by two or three hours soaking in a tub. He would jot down on a pad ideas that had come to him while in the bath and then go about his business for the rest of the day.
In September 1933, Szilard had an insight that foreshadowed the work Fermi and his group carried out almost a year later and that contained nuggets of the problems that would fill the minds of physicists five years later. Szilard asked himself what might happen if neutrons instead of alpha particles were used as projectiles for the bombardment of nuclei. He proceeded to reflect on the prospect that this might produce extra neutrons. As Fermi and Oppenheimer had, he wondered what would be the result if this occurred amid nuclei packed together.
On the fifteenth of March 1934, coinciding with the time Fermi was writing his first paper on radioactivity induced by neutron bombardment, Szilard filed his patent for a chain reaction. It was his usual practice for concepts he found promising. Aside from his work with Einstein, he had already followed this procedure for the electron microscope and the cyclotron. Some accused him of doing this for personal gain rather than in the interest of advancing science, but that didn’t bother him.
Szilard was, however, irritated that nobody seemed willing to pursue the notion of a chain reaction. He needed help and funds but could not find anybody willing to assist him. Szilard’s fellow Hungarian Eugene Wigner was encouraging. He was a first-rate theoretical physicist with a mathematical bent, although his undergraduate degree from Budapest was in chemical engineering. He also had a practical side. However, Wigner, like Szilard, had no money and no laboratory. Chaim Weizmann, Zioni
st leader, chemist, and a friend, was intrigued but unable to raise the requisite funds.
In his memoir, Szilard mused on what might have happened had this not been the case. He concluded that it was a good thing:
I have often thought since that time that if Weizmann hadn’t failed me, almost certainly Germany would have won the last war. For though I was fully aware of the implications and determined to try what I could to keep the experiment a secret, it is almost certain that in prewar England it would not have been possible to keep such a discovery a secret, and that neither in England nor in America would this development have been pushed with determination in which it would have been pushed in Germany, which in 1935 was fully determined to rearm and go to war.
As was usually the case, Szilard weighed the political implications of scientific inquiry.
In 1934, becoming more and more troubled about the rise of Nazism in Germany, Szilard took several steps to keep his design for a chain reaction from falling into the hands of physicists who might cooperate with Hitler. He assigned the patent for a chain reaction to the British Admiralty with the proviso it be kept secret. He also initiated a campaign to persuade physicists outside Germany not to publish their research on neutrons. This request, coming from a relative unknown in the field, was deemed presumptuous and ignored.
Again thinking ahead, during a 1931 visit to the United States, Szilard had filled out preliminary papers for immigration in the eventuality of relocation. These became pertinent with Germany’s unopposed occupation of the demilitarized Rhineland in March 1936. England was not far enough from Germany for his comfort. On Christmas Day 1937, Szilard boarded the Franconia, the very same ocean liner the Fermis were to sail on a year later. And like them, he arrived in New York on January 2 and checked in to the King’s Crown Hotel on 116th Street. There were, however, notable differences between the welcomes accorded to him and to Fermi. Szilard didn’t have a Nobel Prize, had no obvious major accomplishments, and didn’t have a position or any likelihood of obtaining one soon. He seemed curiously unfazed by any of this.