They sat down in the cold on a tree trunk and started to calculate on scraps of paper. "The charge of a uranium nucleus," they found, "was indeed large enough to overcome the effect of surface tension almost completely; so the uranium nucleus might indeed resemble a very wobbly, unstable drop, ready to divide itself at the slightest provocation, (such as the impact of a single neutron) "Yet, as Frisch later recalled, there was a problem: "After separation, the two drops would be driven apart by their mutual electric repulsion and would acquire high speed and hence a very large energy, about 200 MeV [mega-election-volts] in all; where could that energy come from? Fortunately Meitner remembered the empirical formula for computing the masses of nuclei and worked out that the two nuclei formed by the division of a uranium nucleus together would be lighter than the original uranium nucleus, by about one-fifth the mass of a proton. [This was because] whenever mass disappears, energy is created, according to Einstein's formula E = mc2, and one-fifth of a proton mass was just equivalent to 200 MeV. So here was the source for that energy; it all fitted! "Yet the moment of realization was not entirely comfortable. Frisch felt as if he had "caught an elephant by the tail" without meaning to and now did not know what to do with it.
On 28 December Hahn wrote again. Prompted by Meitner's note of 21 December, he too was wondering whether the uranium might have split. He did not understand the full picture but recognized that, if true, it meant that the transuranics they had studied for four years did not exist. They were, instead, smaller, lighter nuclei—like barium—which formed when uranium was split. He asked her to look at a note that he and Fritz Strassmann proposed to publish in Naturwissenschaften. On New Year's Eve Meitner replied cautiously that perhaps it is energetically possible for such a heavy nucleus to break up." The next day she returned to Stockholm, where she immediately began reviewing the evidence for the transuranics and realized that they could, indeed, be light nuclei. It was a bittersweet discovery. She had wasted years of study on transuranics, but on the other hand she had provided the first theoretical interpretation of fission of uranium, showing how it produced radioactive elements and liberated large amounts of energy. On 3 January 1939 she wrote to Hahn, "I am now almost certain that you really do have a splitting to barium and I find that to be a really beautiful result." They were, she said, "wonderful findings."
· · ·
The most "wonderful" aspect of all, as Edward Teller wrote, was that "the secret of fission had eluded everybody for all those years." As Hahn put it, "None of us realized that we had done it." Fermi had failed to see it. So had Irene Joliot-Curie; her lanthanumlike substance had, in fact, been lanthanum, created by nuclear fission. Even the great Rutherford had been deceived. When an excited Otto Frisch returned to Copenhagen and broke the news to Bohr, the Dane "smote his forehead with his hand," exclaiming, "We were all fools."
Bohr urged Frisch to write a paper with Lise Meitner as soon as possible and promised to say nothing until it was published. By dint of long-distance telephone calls, aunt and nephew drafted a short note to the editor of the British journal Nature, describing the splitting of a nucleus and the theory underlying it. They also found a name for their new phenomenon. Frisch asked an American biologist, William A. Arnold, working in Bohr's institute, what he called the process by which single cells divide into two. He replied, "Fission."
However, before submitting the paper, Frisch wanted to be absolutely certain their conclusions were right. He conducted some experiments and became the first to provide experimental proof of the fission of a uranium atom when hit by a neutron. Having finally achieved the results he wanted, he went to bed at 3 a.m. on 13 January but four hours later "was knocked out of bed by the postman who brought a telegram to say that my father had been released from concentration camp." His parents had been granted visas to emigrate to Sweden. His happiness was complete.
Meanwhile, on 6 January 1939, Hahn and Strassmann's report appeared in Naturwissenschaften. It made no mention of Meitner's and Frisch's contribution, and could hardly have done so in the political climate in Germany. Nor was there any acknowledgment of Ida Noddack's earlier work. Piqued, she wrote a short article in Naturwissenschaften, pointing out that five years earlier she had suggested the splitting of the uranium atom. Paul Rosbaud, as editor, asked Hahn to comment, but he refused. Rosbaud therefore added a terse note beneath her article, stating that "Otto Hahn and Fritz Strassmann have informed us that they have neither the time nor the desire to answer the preceding note."
On 16 January Frisch at last mailed off his and Meitner's paper to Nature, together with a supplementary one reporting his experimental findings. To protect their friends, Meitner and Frisch took care to credit Hahn and Strassmann only for work already in the open literature. However, the articles did not attract the attention the authors deserved. Sadly, they were not finally published until 11 February, by which time the world knew all about fission, not only from Hahn and Strassmann but also from Niels Bohr.
On 7 January 1939 Bohr had sailed for the United States aboard the liner Drottningholm together with the Belgian physicist Leon Rosenfeld, to whom he confided, "I have in my pocket a paper that Frisch has given me which contains a tremendous new discovery, but I don't yet understand it. We must look at it." The two men spent the voyage in Bohr's stateroom going over and over the theory of fission until Bohr was convinced he had "got hold of the solution." As Rosenfeld observed, "It turned out to be extremely simple."
A group of scientists was waiting on the quayside to greet Bohr, including the American John Wheeler, who had worked with him in Copenhagen. Wheeler was staggered when, within moments of stepping on dry land, Bohr murmured, in the low voice he used when imparting information of the highest significance, that the uranium atom had been split. That night Wheeler took Rosenfeld off to Princeton, where the latter addressed the physics club. Unaware of Bohr's promise to keep the news quiet until Frisch and Meitner were in print, Rosenfeld announced the discovery, causing a sensation. A horrified Bohr tried to protect Frisch's and Meitner's primacy, but it was too late. All he could do was refrain from public comment himself. However, in late January the first copies of Hahn's and Strassmann's paper in Naturwissenschaften arrived in the United States, and Bohr felt free to reveal the physical discovery and theoretical explanation of nuclear fission.
The occasion was a conference at George Washington University on 26 January 1939. Some scientists did not even wait for Bohr to finish before rushing off to try the experiments for themselves. That evening Bohr was invited to watch the Carnegie Institution's accelerator in action. For the first time he saw the uranium atom splitting before his very eyes, the glowing green pulses on the screen of the oscilloscope leaping each time a uranium nucleus fissured. Leon Rosenfeld, by his side, recalled that "the state of excitement challenged description." By the end of January 1939 over a dozen laboratories worldwide had produced nuclear fission. At Berkeley, Robert Oppenheimer's initial reaction to the news of fission had been "that's impossible," but within days he changed his mind and was speculating that this "could make bombs."
· · ·
The new knowledge could scarcely have been revealed at a worse time. In October 1938 Nazi Germany had been allowed to annex the Sudeten German districts of Czechoslovakia under the Munich Agreement, which an optimistic Neville Chamberlain assured the British people guaranteed "peace in our time." Hitler promised once again that this was the end of his territorial ambitions, but many, especially those who had suffered personally at the hands of his regime, doubted this.
In the tense political climate, some scientists worried that nuclear fission was far too sensitive to be the subject of cross-border gossip. The old belief in a brotherhood of scientists, openly discussing and publicizing their findings from Cambridge to Columbia to California and from Liverpool to Leipzig to Leningrad, now seemed as naive as it was alarming. Colleagues and comrades would soon be competitors.
One of the first to grasp the danger was the Hungarian phy
sicist Leo Szilard, an eccentric, conceited man but, in the eyes of many contemporaries, "sparkling with intelligence and originality." Szilard had an uncanny prescience. He had been one of the quickest to grasp the peril facing European Jewry, arriving in England in the early months of 1933. Like his fellow Hungarian Edward Teller, he was the product of a liberal, cultured, middle-class Jewish Budapest family. Szilard had developed an early preoccupation with "saving the world." After the end of the First World War and the collapse of the Austro-Hungarian Empire, he was swept up in the fervor of Bela Kun's Soviet Republic, driving trucks draped with socialist slogans around Budapest. When Kun fled in the summer of 1919, Szilard found, like Teller, that the world had changed toward him. When he tried to enroll at the university, other students blocked his way, calling him a Jew. His protestations that he was a Calvinist (he had converted a few weeks earlier, believing it would be prudent) did him no good. They kicked him down the marble stairs.
Leo Szilard (left) and Edward Teller
A shaken Szilard had applied for a visa to study abroad. At first the government refused on the grounds that he had been a socialist agitator, but he applied again and with help from family friends just managed to get out. In Berlin he enrolled at the Technical Institute to study engineering but soon realized that physics was his true interest. In 1920 he boldly sought out Max Planck and announced that he only wanted to know the facts of physics. He would make up the theories himself. Life was hard. Szilard lived in shabby, rented rooms, and his family were too poor to send him food parcels. He survived on the most basic of rations and roamed Berlin's streets staring in shop windows at food he could not afford to buy.
But at least the intellectual life was satisfying. In 19 21 Szilard asked Max von Laue to supervise his thesis, which von Laue suggested should be on relativity theory. That same year, Szilard persuaded Einstein to tutor him and some friends, including fellow Hungarians John von Neumann and Eugene Wigner. Szilard's particular talent was for intense lateral thinking—teasing out patterns and then seeking ways of uniting them through a theory. His tools were statistics rather than experimental evidence. He applied this approach to a problem in thermodynamics and took his results to Einstein, who listened politely then said, "That's impossible. This is something that cannot be done." "Well, yes," responded Szilard, "but I did it."
After reaching England, Szilard had worked to get Jewish academics out of Germany, but then, convinced war was coming, he had moved on to the United States. He learned of the discovery of fission at Princeton while visiting Eugene Wigner, who was recovering from jaundice in the university infirmary on what Wigner considered a "miserably" un-Hungarian diet of "potatoes, beans and everything boiled in water." Szilard came to see him every day, and the two friends "discussed fission problems and this and that." One morning, Szilard said, "Wigner, now I think there will be a chain reaction."
As Szilard recognized, the possibility of creating a nuclear bomb depended on whether fission could be used to trigger a self-sustaining chain reaction. In other words, by using neutrons to bombard uranium atoms, was it possible not only to split the uranium nuclei but, in the process, to release enough further neutrons which, if they in turn hit other uranium nuclei, could trigger a self-sustaining chain reaction liberating colossal amounts of energy?
As early as September 1933 he had conceived the idea in theory, sparked by reading a newspaper report of Lord Rutherford's "moonshine" speech dismissing the idea that energy could be liberated from the atom. Szilard later described the Damascene moment. The article "sort of set me pondering as I was walking the streets of London, and I remember that I stopped for a red light at the intersection of Southampton Row. As I was waiting for the light to change and as the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbed one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction."
So alarmed was Szilard that such a process could be used to create an explosive device that in the spring of 1934 he had applied for a patent for the process he envisaged. He had assigned his patent to the British Admiralty for safekeeping but had done nothing further. Now that, four years later, fission had been shown to be a reality, Szilard wanted urgently to test his theory of chain reaction. Although he had no formal university appointment, he secured special permission to conduct experiments at Columbia University. He borrowed two thousand dollars from a friend, rented some radium, and, using some of the university's equipment, carefully set up his experiment. As he later described, "All we needed to do was to get a gram of radium, get a block of beryllium, expose a piece of uranium to the neutrons which come from beryllium," and then see whether neutrons were emitted in the process.
On 3 March 1939 "everything was ready and all we had to do was to turn an [electrical] switch, lean back, and watch the screen of a television tube. If flashes of light appeared on the screen, that would mean that neutrons were emitted in the fission process of uranium and this in turn would mean that the large-scale liberation of atomic energy was just around the corner. We turned the switch and we saw the flashes." The pulses of light proved that bombarding uranium with neutrons could indeed spark a chain reaction. The spectacle left Szilard with "very little doubt in my mind that the world was headed for grief." He was the first to perceive that a race would soon begin.
That night Szilard phoned Edward Teller in Washington, announced tersely in Hungarian, "I have found the neutrons," and hung up. Teller had been contentedly playing the piano when the phone rang. As he returned to the instrument the thought came that "the world might change in a radical manner. The prospect of harnessing nuclear energy seemed chillingly real."
EIGHT
"WE MAY SLEEP FAIRLY
COMFORTABLY IN OUR BEDS"
ADDRESSING DIGNITARIES at the Nobel Prize ceremony in Stockholm in 1905, Pierre Curie had posed a disturbing question: "One may imagine that in criminal hands radium might become very dangerous. . . . we may ask ourselves if humanity has anything to gain by learning the secrets of nature." He had not doubted the answer, adding reassuringly, "I am among those who think, with Nobel, that humanity will obtain more good than evil from the new discoveries." Thirty-four years later, scientists had greater knowledge and faced more difficult judgments.
As Leo Szilard had quickly grasped, the fact that nature's "secrets" might pose a risk to humanity implied new roles and responsibilities for scientists. Even before he had had a chance to conduct his own experiments at Columbia University confirming the viability of a nuclear chain reaction, Szilard launched a campaign to keep "nature's secrets" secret. The specter of an explosive atomic device in Nazi hands haunted him. It would be only too easy, he reasoned, for scientists in Nazi Germany to comb through the technical journals and piece together snippets of information. It was a standard technique of intelligence gathering, which in the current international climate might prove disastrous. The answer was to persuade scientists in the free world to adopt a policy of self-censorship.
Szilard correctly identified Enrico Fermi as one of the scientists most likely to solve the mysteries of a chain reaction and targeted him accordingly. Fermi had recently arrived in the United States with his wife, Laura, and their two children to take up a professorship at Columbia, one of six American universities eager to appoint him. Laura Fermi was the daughter of a Jewish naval officer, and they had decided it was not safe for the family to remain in Mussolini's Italy. Their opportunity to flee had come late in 1938 when Fermi was awarded the Nobel Prize for Physics for his identification of new radioactive elements and his discovery of how nuclear reactions were affected by slow neutrons. He was notified of the award the day that the Italian authorities announced that Jews were to be deprived of their rights of citizenship and their passports withdrawn.
By this time, the Italian authorities viewed the Nobel Prize with some disfavor. Their Ger
man allies had banned their citizens from accepting it after the 1935 Nobel Peace Prize was awarded to a German author and pacifist imprisoned as an enemy of the state. However, no one prevented the Fermis from traveling to Stockholm for the Nobel ceremony. Here they collected their prize money and never went home. Stepping onto American soil on 2 January 1939, Fermi declared, "We have founded the American branch of the Fermi family." Within days his wife was exploring what she called "the marvels of pudding powders" and of frozen food, just then appearing on the market. The process of Americanization was, she quickly realized, less tangible. It was "more than learning language and customs and setting one's self to do whatever Americans can do." It would take time to understand "New England pride" and "the long suffering of the South" and even longer, perhaps, to think of Shakespeare before Dante.
When Szilard first explained his concerns to Fermi in February 1939, the Italian was skeptical. He considered the likelihood of a chain reaction to be less than Szilard did, and regarded his censorship plans as alarmist and against the spirit of science. Fermi had seen intellectual freedom stamped out in Fascist Italy and was reluctant to participate in any scheme to suppress knowledge. Under the circumstances, as Szilard recalled, "Fermi thought that the conservative thing was to play down the possibility that this [a chain reaction] may happen, and I thought the conservative thing was to assume that it would happen and take the necessary precautions." Fermi and Szilard "had high regard for each other" but were "extremely different in personality, habits of work, outlook on life, and almost everything else" and "could scarcely work together on the same experiment," recalled Emilio Segre. One of the problems was, as another physicist described, that "Szilard's way of working on an experiment did not appeal to Fermi. Szilard was not willing to do his share of experimental work, either in the preparation or in the conduct of the measurements. He hired an assistant."
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