Thinking No Pedestrian Thoughts
Eccentric Hungarian physicist Leo Szilard spent many hours thinking in the bathtub. But perhaps his most significant scientific insight occurred while he was crossing the street in London in 1933. As told here by Richard Rhodes, Szilard had just stepped off the curb when he realized the possibility of a nuclear chain reaction.
From The Making of the Atomic Bomb
BY RICHARD RHODES
In London, where Southampton Row passes Russell Square, across from the British Museum in Bloomsbury, Leo Szilard waited irritably one gray Depression morning for the stoplight to change. A trace of rain had fallen during the night; Tuesday, September 12, 1933, dawned cool, humid and dull. Drizzling rain would begin again in early afternoon. When Szilard told the story later he never mentioned his destination that morning. He may have had none; he often walked to think. In any case another destination intervened. The stoplight changed to green. Szilard stepped off the curb. As he crossed the street time cracked open before him and he saw a way to the future, death into the world and all our woe, the shape of things to come.
Szilard was not the first to realize that the neutron might slip past the positive electrical barrier of the nucleus; that realization had come to other physicists as well. But he was the first to imagine a mechanism whereby more energy might be released in the neutron’s bombardment of the nucleus than the neutron itself supplied.
There was an analogous process in chemistry. Polanyi had studied it. A comparatively small number of active particles—oxygen atoms, for example—admitted into a chemically unstable system, worked like leaven to elicit a chemical reaction at temperatures much lower than the temperature that the reaction normally required. Chain reaction, the process was called. One center of chemical reaction produces thousands of product molecules. One center occasionally has an especially favorable encounter with a reactant and instead of forming only one new center, it forms two or more, each of which is capable in turn of propagating a reaction chain.
Chemical chain reactions are self-limiting. Were they not, they would run away in geometric progression: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304, 8388608, 16777216, 33554432, 67108868, 134217736…
“As the light changed to green and I crossed the street,” Szilard recalls, “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 absorbs one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction.”
Drawn by Robert Grossman; courtesy of William Lanouette
Leo Szilard spent hours in the bathtub, his favorite location for deep thinking. This cartoon also features a dolphin in reference to his allegory, “The Voice of the Dolphins.”
“I didn’t see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. In certain circumstances it might be possible to set up a nuclear chain reaction, liberate energy on an industrial scale, and construct atomic bombs.”
Leo Szilard stepped up onto the sidewalk. Behind him the light changed to red.
Reprinted by permission of G.T. Labs ©2001 Jim Ottaviani and Janine Johnston
These pages from the graphic novel Fallout visualize Szilard’s initial conception of nuclear fission while crossing the street in London in 1933.
“The atomic bombs burst in their fumbling hands”
In 1914, H. G. Wells published a science fiction novel that envisioned an atomic bomb for the first time. Reading The World Set Free decades before the Manhattan Project, Leo Szilard became captivated by the possibility of a nuclear chain reaction. This selection from The World Set Free highlighted seemingly limitless possibilities that would accompany such control of nuclear energy. Wells’s fictional bomb differed from the real one, but the moral and ethical concerns he posed anticipated those that mankind soon had to confront.
From The World Set Free
BY H. G. WELLS
“Given that knowledge,” he said, “mark what we should be able to do! We should not only be able to use this uranium and thorium; not only should we have a source of power so potent that a man might carry in his hand the energy to light a city for a year, fight a fleet of battleships, or drive one of our giant liners across the Atlantic; but we should also have a clue that would enable us at last to quicken the process of disintegration in all the other elements, where decay is still so slow as to escape our finest measurements. Every scrap of solid matter in the world would become an available reservoir of concentrated force. Do you realize, ladies and gentlemen, what these things would mean for us?”
The scrub head nodded. “Oh! Go on. Go on.”
“It would mean a change in human conditions that I can only compare to the discovery of fire, that first discovery that lifted man above the brute.”
The gaunt face hardened to grimness, and with both hands the bomb-thrower lifted the big atomic bomb from the box and steadied it against the side. It was a black sphere two feet in diameter. Between its handles was a little celluloid stud, and to this he bent his head until his lips touched it. Then he had to bite in order to let the air in upon the inductive. Sure of its accessibility, he craned his neck over the side of the aeroplane and judged his pace and distance. Then very quickly he bent forward, bit the stud, and hoisted the bomb over the side.
“Round,” he whispered inaudibly.
The bomb flashed blinding scarlet in mid-air, and fell, a descending column of blaze eddying spirally in the midst of a whirlwind. Both the aeroplanes were tossed like shuttlecocks, hurled high and sideways and the steersman, with gleaming eyes and set teeth, fought in great banking curves for a balance. The gaunt man clung tight with hand and knees; his nostrils dilated, his teeth biting his lips. He was firmly strapped.…
When he could look down again it was like looking down upon the crater of a small volcano. In the open garden before the Imperial castle a shuddering star of evil splendor spurted and poured up smoke and flame towards them like an accusation. They were too high to distinguish people clearly, or mark the bomb’s effect upon the building until suddenly the facade tottered and crumbled before the flare as sugar dissolves in water. The man stared for a moment, showed all his long teeth, and then staggered into the cramped standing position his straps permitted, hoisted out and bit another bomb, and sent it down after its fellow.
The explosion came this time more directly underneath the aeroplane and shot it upward edgeways. The bomb box tipped to the point of disgorgement, and the bomb-thrower was pitched forward upon the third bomb with his face close to its celluloid stud. He clutched its handles, and with a sudden gust of determination that the thing should not escape him, bit its stud. Before he could hurl it over, the monoplane was slipping sideways. Everything was falling sideways. Instinctively he gave himself up to gripping, his body holding the bomb in its place.
Then that bomb had exploded also, and steersman, thrower, and aeroplane were just flying rags and splinters of metal and drops of moisture in the air, and a third column of fire rushed eddying down upon the doomed buildings below.…
Such was the crowning triumph of military science, the ultimate explosive that was to give the “decisive touch” to war.…
A recent historical writer has described the world of that time as one that “believed in established words and was invincibly blind to the obvious in things.” Certainly it seems now that nothing could have been more obvious to the people of the early twentieth century than the rapidity with which war was becoming impossible. And as certainly they did not see it. They did not see it until the atomic bombs burst in their fumbling hands.
“SOME OTHER MAN WOULD BE DOING THIS…”
One of Wells’ fictional atomic bomb inventors describes a haunting sense of the inevitability of atomic weapons as he writes about the papers on which the plans for a bomb wer
e outlined:
“It is not for me to reach out to consequences I cannot foresee. I am a part, not a whole; I am a little instrument in the armory of Change. If I were to burn all these papers, before a score of years have passed, some other man would be doing this…”
“If only we had been clever enough”
Austrian scientist Lise Meitner and her physicist nephew Otto Frisch conceived of nuclear “fission” while on a walk through the woods in Sweden in December 1938. In this excerpt, their story is told by one of the leading female scientists of the Manhattan Project, Leona Woods Marshall Libby, who worked closely with Enrico Fermi at the Chicago Metallurgical Laboratory.
From The Uranium People
BY LEONA MARSHALL LIBBY
In the spring of 1938, Lise Meitner, being a Jew, had to leave Berlin and went to a job offered her by Manne Siegbahn in the Nobel Institute of Stockholm. Being Austrian, she had not up until then been seriously affected by Hitler’s persecution of Jews; however, in the spring of 1938, Austria was annexed by Hitler and she had to get out of the country. Dutch colleagues smuggled her into Holland without a visa and thence to Sweden. The German team would have to carry on without her.
[Otto] Hahn and [Fritz] Strassmann continued working together and found they had to assume, from the production of so many different half-lives, that the uranium atom broke into several smaller pieces, belonging probably to elements in the region of platinum, which they thought they could fit to the chemical characteristics of the “transuranium” activities. They wrote this conclusion to Lise Meitner before their results were published in 1938.
Lise Meitner was lonely in Sweden. Her nephew, Otto Frisch, was working in Copenhagen, and he went to visit her at Christmas in 1938. He found her at breakfast, in a small hotel near Göteborg, brooding over a letter from Hahn. The letter said that barium was one of the fragments formed by neutron irradiation of uranium. Frisch remembers that “we walked up and down in the snow, I on skis and she on foot (she said and proved that she could get on just as fast that way), and gradually the idea took shape that this was no chipping nor cracking of the nucleus but rather a process to be explained by Bohr’s idea that the nucleus was like a liquid drop; such a drop might elongate and divide itself.”
Frisch wanted to discuss his plan for his next experiment, so he suggested that Hahn’s results were wrong. Lise shook her head and said that Hahn was too good a chemist to be wrong; his results must be correct, “But how can one get a nucleus of barium from one of uranium?” Frisch remembers, “We walked up and down in the snow trying to think of some explanation. Could it be that the nucleus got cleaved right across with a chisel? It seemed impossible that a neutron could act like a chisel, and anyhow, the idea of a nucleus as a solid object that could be cleaved was all wrong; a nucleus was much more like a liquid drop. Here we stopped and looked at each other.” They remembered the already classical liquid-drop model of the nucleus and imagined that a drop might get pulled out into a dumbbell shape with a waist in the middle, and then elongate more until the waist was so thin that the drop might break into two pieces. At first, they thought that the surface tension would keep on pulling it back into round, but then they sat down on a log and began to calculate from the liquid-drop model of a nucleus how much was the surface tension of a uranium nucleus containing 92 protons. Because all the protons were repelling each other by reason of their positive electric charges, they realized that the surface tension was canceled out by this electrical repulsion. The drop, necked out, would consist of two pieces that would soon begin to repel each other as elongation increased to the point of division into two separate pieces, say, barium and krypton (charges 56 and 36), or perhaps rubidium and cesium (37 and 55), as chance might have it, or zirconium and tellurium (40 and 52), and so on. Here was a plausible explanation why neutron irradiation of uranium produced so many radioactive species; namely, although the charges of the separated drops would be correct for nuclei of barium, krypton, rubidium, cesium, zirconium, tellurium, and so on, the drops would have an excess of neutrons and so would be unstable against beta-ray emissions or other radioactivity until they attained the neutron-proton ratio of stable nuclei in the periodic system. Frisch goes on to remark, “It could have been foreseen if only we had been clever enough.”
More or less, in their words, a classical picture of these new disintegration processes suggests itself. On account of their close packing and strong energy exchange, the particles in a heavy nucleus would be expected to move in a collective way that has some resemblance to the movement of a liquid drop. If the movement is made sufficiently violent by adding energy, such a drop may wiggle about until it divides itself into two smaller drops. It therefore seems possible that the uranium nucleus after neutron capture may divide itself into two nuclei of roughly equal size, the ratio of the sizes depending partly on chance; energy from the difference in stability between uranium and elements around barium would be released in an amount estimated at about 200 million electron volts (compared with natural alpha particle energies of about 5 million electron volts).
Frisch estimated how the split of electric charge would decrease the surface tension of the drop, allowing it to divide, and Meitner calculated that the energy emitted by division would be about 200 million electron volts. They spent the Christmas holidays getting the explanation straight, and then Frisch returned to Bohr’s Institute at Copenhagen and told Bohr the result just as Bohr was about to catch a ship to New York. He recalls, “I had hardly begun to tell him about Hahn’s experiments and the conclusions Lise Meitner and I had come to when he struck his forehead with his hand and exclaimed, ‘Oh, what idiots we have been! We could have foreseen it all! This is just as it must be!’ And yet even he, perhaps the greatest physicist of his time, had not foreseen it.”
How should one name this new kind of nuclear reaction? Frisch asked a biologist at the Bohr Institute what the word was for bacterial division and was told it was called fission, whereupon he took that word for the splitting of uranium upon neutron irradiation. He wrote the article about fission of uranium as deduced from Hahn’s results and read it over the telephone to Lise Meitner. “It was an expensive phone call, all the way to Sweden (from Copenhagen, about 300 miles), and it took quite a while because she had her own suggestions on how we should put matters. But in the end, we agreed about everything, and I got the article typed, ready to be sent off to the editor of Nature.”
Frisch, a refugee from Austria, had learned to read Italian so that he could follow the papers of the Fermi group closely. These papers were coming out at a rate of almost one per week in the Italian and British journals. Frisch had repeated the Italian measurements that demonstrated the slowing down of fast neutrons to room temperature by rattling around with atoms of water until, like billiard balls, they became sluggish in their movements. Considering his Christmas visit with his aunt, Lise Meitner, during the week when they figured out the theory of fission of uranium, it is interesting that in the 40 years elapsed since then, the theory of fission has advanced very little beyond what they put together that week. He says it was much a matter of chance that he was there to help her figure it out. Curiously, 3 years earlier, it was also a matter of chance that he, a refugee from the Nazis, had been present at the seminar when Niels Bohr conceived of the theory of the compound nucleus and the liquid-drop model of the nucleus. He considers it “good luck that I was there when the news came from Berlin. And it wasn’t my contribution alone, it was the result of a discussion into which Lise Meitner more or less forced me—I would much rather have talked about my own work. Also at that moment it was clear that lots of other people would have had the same idea; it wasn’t really a particularly bright idea, I feel.” And he did not follow it up in all its ramifications. Instead, “all I did was to do a simple measurement—into which I was prodded—to show that (after fission) the barium nuclei did move off as fast as expected.” He didn’t go beyond that measurement because “I had the feeling that whateve
r I started, I wouldn’t be able to finish a very difficult project such as finding out how many neutrons are created in fission—I didn’t even know how to tackle that.”
Instead, Frisch found a job in England at the Birmingham laboratory of Mark Oliphant to escape from the war that was fast enveloping the Continent; however, Germany declared war on England soon after Frisch reached Birmingham. He began to work on the problem of separating two isotopes of uranium—uranium-235 and uranium-238—by thermal diffusion, using a vertical tube with a hot central wire containing uranium hexafluoride gas. He and another émigré, Rudolf Peierls, computed from the success of this separation that it was entirely possible, with 100,000 such tubes, to separate a few pounds of uranium-235, enough for a bomb.
U.S. Department of Energy
Lise Meitner and Otto Hahn were pivotal to the discovery of nuclear fission in uranium.
“What wasn’t expected wasn’t seen!”
The news of fission spread quickly. A conference in Washington, D.C. in January 1939 introduced the subject to physicists, both American and foreign, who were in town for a regularly scheduled conference. As remembered by physicist Edward Teller, fission stole the show. Looking back on the timing of the discovery of fission, Teller ponders how the history of nuclear weapons and the world could easily have been very different.
From Memoirs: A Twentieth-Century Journey in Science and Politics
BY EDWARD TELLER
As 1939 began, I was looking forward to seeing Fermi at the fifth theoretical conference at George Washington University, scheduled for January 19–20. Much to Geo’s [George Gamow’s] and my pleasure, Niels Bohr, who had just arrived from Copenhagen to work for a few weeks at Princeton, was also going to participate in the program.
The Manhattan Project Page 3