Szilard linked these findings in his mind by asking himself the question (while on a walk in London and waiting for a light to change, as he later reported{19}): What if an energy-releasing nuclear reaction were triggered not by a proton, as in the Cockcroft-Walton experiment, but by a neutron, and what if, from the reaction, two neutrons emerged? These released neutrons could stimulate more reactions of the same kind, and one would have a nuclear chain reaction, potentially releasing vast energy. The idea of a chain reaction existed already in chemistry, and could potentially be explosive, but, as Szilard knew, a nuclear chain reaction, if it were to occur, might outdo the chemical chain reaction a million-fold.
Szilard did not imagine nuclear fission. That came more than five years later—and was a total surprise when it did come. He was thinking instead of a reaction like the one achieved by Cockcroft and Walton, but with the incident particle being a neutron instead of a proton, and the emitted particles being two neutrons instead of two alpha particles. To be sure, the Cockcroft-Walton experiment released nuclear energy, but the nuclear energy that came out if it was far less than the energy put into the accelerator that supplied the protons used to bombard the lithium. In a speech delivered shortly before Szilard’s walk in London, Rutherford (by then Lord Rutherford), aware of this imbalance between the energy put into the machinery and the energy released in a nuclear reaction, had said “anyone who looked for a source of power in the transformation of the atoms was talking moonshine.”{20} This remark, duly reported in The Times of London on September 12, 1933—the very day of Szilard’s walk—bothered Szilard and contributed to his invention of the idea of a nuclear chain reaction. As he said later, “Pronouncements of experts to the effect that something cannot be done have always irritated me.”{21}
A bit more entrepreneurial than most scientists, Szilard applied for and, in 1934, was granted a patent on the idea of a nuclear chain reaction, a patent that he soon assigned to the British Admiralty as a way to keep it secret. His application to conduct experiments in search of a nuclear chain reaction at Rutherford’s laboratory in Cambridge was turned down, but he managed to conduct some experiments, first at St. Bartholomew’s Hospital in London, and then, in late 1938, in Rochester, New York (he had just moved to the United States and, in typical Szilard fashion, was bouncing around among labs). In neither place did he find any evidence for such a reaction.{22}, [29] It is hardly surprising that when the news of nuclear fission reached New York in January 1939, Szilard was among the first to see its possibilities for generating a chain reaction and for providing a weapon of surpassing power.{24}
The discovery of fission is an oft-told story.{25} In brief: In Berlin in 1938, the German chemists Otto Hahn and Fritz Strassmann, who had been bombarding uranium with neutrons to see if heavier elements might be formed, found evidence of the element barium being created. This was totally puzzling to them, yet, after the most careful checks and cross checks, the barium would not go away. Barium is element number 56, while uranium is number 92. The atomic weight of barium is 137, not much more than half of uranium’s atomic weight of 238. Where was the barium coming from? Hahn sent off a letter to his former physicist colleague Lise Meitner to see if she might have an explanation. Meitner, a Jew, had had to flee Germany, and was then in Sweden. As it happened, her nephew Otto Frisch, also a physicist, and then working not so far away at Niels Bohr’s institute in Copenhagen, came to spend the Christmas 1938 holiday with his Aunt Lise, and was there when Hahn’s letter arrived. On a snowy trek through the woods, Frisch (on skis) and Meitner (keeping up on foot) pondered the matter and asked themselves: Could uranium nuclei, stimulated by neutrons, be splitting apart into smaller nuclear fragments (which could include barium nuclei)? Excited by the idea, they sat down on a tree trunk, pulled out some scraps of paper, and started to calculate, working from a formula that Meitner had in her head, the so-called Weizsäcker mass formula. This was a “semi-empirical” formula advanced by Carl Friedrich von Weizsäcker in 1935{26} (and later refined) that provided the masses of nuclei to good approximation across the whole periodic table. Their conclusion: Breaking a uranium nucleus apart into two large fragments would release energy, a lot of energy. Their estimate was 200 MeV, which proved to be right on the mark.{27}
Once Frisch was back in Copenhagen, he hastened to Niels Bohr to report his and Meitner’s “speculations” about the breakup of the uranium nucleus.[30] Bohr, set to leave for America in a few days, immediately accepted the idea, exclaiming, according to Frisch, “Oh what idiots we all have been! Oh but this is wonderful! This is just as it must be!”{29} By the thirteenth of January, while Bohr was en route to America, Frisch had conducted experiments that directly confirmed the reality of nuclear fission.
On his week aboard the MS Drottningholm, Bohr convinced himself that indeed the process made great sense, and he had no hesitation in reporting it as real when he reached New York. Nevertheless, he limited his discussion of fission at first to a few colleagues at Columbia and Princeton Universities—not from any sense of the military potential of fission, but to give time for Meitner and Frisch to prepare a paper for publication and to get the credit they deserved. As it happened, the Meitner-Frisch paper was published with lightning speed. It appeared as a “letter” in the journal Nature on the very day Bohr reached New York.{30} Ten days later, on January 26, Bohr gave a public report at a conference in Washington, D.C., after which the news spread quickly across the country. (Probably on January 30, the physicist Luis Alvarez came across a newspaper report of the discovery of fission while getting his hair cut in a Berkeley, California barber shop. He reportedly leaped from his chair without waiting for the barber to finish, and hurried to his lab. By the next day, he and his student Phil Abelson had verified the reality of nuclear fission.){31}
The mass spectrometer, invented by Francis William Aston in 1919,{32} made it possible to measure the masses of individual atoms[31]—at first with enough precision to clearly distinguish different isotopes of the same element, later with the greater precision needed to establish that the total mass of nuclei after a nuclear reaction need not be exactly the same as the total mass before. For example, Cockcroft and Walton, in their 1932 experiment, knew that the masses of the proton and lithium-7 nucleus added to more than the mass of two alpha particles. The mass difference, they found, was accounted for by the energy of the emitted alpha particles. The books were balanced, not on mass alone, but on mass-energy.
29
On December 21, 1938, after the negative results in Rochester, Szilard wrote to the British Admiralty asking that his chain-reaction patent be withdrawn. Five weeks later, on January 26, 1939, with the news of fission in hand, he sent a telegram to the Admiralty saying KINDLY DISREGARD MY RECENT LETTER.{23}
30
I don’t know if Frisch used the term “fission” in this meeting with Bohr. If not, he must have introduced it within a few days thereafter, for it is a term that Bohr brought with him to New York on January 16, 1939, after a nine-day crossing of the Atlantic. Frisch got the term from an American biologist visiting in Copenhagen, William Arnold, whom he asked what cell division is called. “Fission” was the answer.{28}
31
The mass spectrometer actually measures the mass of an ion (a charged atom). From the ion’s mass it is easy to infer the mass of the uncharged atom and also the mass of its nucleus, since the electron’s mass is known.