The Making of the Atomic Bomb: 25th Anniversary Edition

by Richard Rhodes

New nuclear powers are a threat, we are warned; old nuclear powers keep the peace.

In that way it has contributed to a general understanding of the paradox of nuclear weapons. I don’t mean the paradox of deterrence, which partakes of the fetish object delusion that Harrington de Santana delineates. I mean the paradox which the great Danish physicist Niels Bohr first articulated: that, though nuclear weapons are the property of individual nation-states, which claim the right to hold and to use them in defense of national sovereignty, in their indiscriminate destructiveness they are a common danger to all, like an epidemic disease, and like an epidemic disease they transcend national borders, disputes, and ideologies.

In 2008, some of the scientists who modeled the original 1983 nuclear winter scenario investigated the likely result of a theoretical regional nuclear war between India and Pakistan, a war they postulated to involve only 100 Hiroshima-scale nuclear weapons, yielding a total of only 1.5 megatons—no more than the yield of some single warheads in the U.S. and Russian arsenals. They were shocked to discover that because such an exchange would inevitably be targeted on cities filled with combustible materials, the resulting firestorms would inject massive volumes of black smoke into the upper atmosphere which would spread around the world, cooling the earth long enough and sufficiently to produce worldwide agricultural collapse. Twenty million prompt deaths from blast, fire, and radiation, Alan Robock and Owen Brian Toon projected, and another billion deaths in the months that followed from mass starvation—from a mere 1.5-megaton regional nuclear war.

In its most succinct form, the axiom of proliferation asserts that As long as any state has nuclear weapons, others will seek to acquire them. A member of the commission, the Australian ambassador-at-large for nuclear disarmament, Richard Butler, told me, “The basic reason for this assertion is that justice, which most human beings interpret essentially as fairness, is demonstrably a concept of the deepest importance to people all over the world. Relating this to the axiom of proliferation, it is manifestly the case that the attempts over the years of those who own nuclear weapons to assert that their security justifies having those nuclear weapons while the security of others does not, has been an abject failure.”

Just then, in 1932, Szilard found or took up for the first time that appealing orphan among H. G. Wells’ books that he had failed to discover before: The World Set Free.63 Despite its title, it was not a tract like The Open Conspiracy. It was a prophetic novel, published in 1914, before the beginning of the Great War. Thirty years later Szilard could still summarize The World Set Free in accurate detail. Wells describes, he says: . . . the liberation of atomic energy on a large scale for industrial purposes, the development of atomic bombs, and a world war which was apparently fought by an alliance of England, France, and perhaps including America, against Germany and Austria, the powers located in the central part of Europe. He places this war in the year 1956, and in this war the major cities of the world are all destroyed by atomic bombs.64

This just goes to show that if you want to succeed in this world you don’t have to be much cleverer than other people, you just have to be one day earlier.”

Polanyi’s model of an open republic of science where each scientist judges the work of his peers against mutually agreed upon and mutually supported standards explains why the atom found such precarious lodging in nineteenth-century physics. It was plausible; it had considerable scientific value, especially in systematic importance; but no one had yet made any surprising discoveries about it. None, at least, sufficient to convince the network of only about one thousand men and women throughout the world in 1895 who called themselves physicists and the larger, associated network of chemists.110

It was a hard and healthy childhood. Rutherford capped it by winning scholarships, first to modest Nelson College in nearby Nelson, South Island, then to the University of New Zealand, where he earned an M.A. with double firsts in mathematics and physical science at twenty-two. He was sturdy, enthusiastic and smart, qualities he would need to carry him from rural New Zealand to the leadership of British science. Another, more subtle quality, a braiding of country-boy acuity with a profound frontier innocence, was crucial to his unmatched lifetime record of physical discovery. As his protégé James Chadwick said, Rutherford’s ultimate distinction was “his genius to be astonished.” He preserved that quality against every assault of success and despite a well-hidden but sometimes sickening insecurity, the stiff scar of his colonial birth.111, 112

Out of the prospering but vulnerable Hungarian Jewish middle class came no fewer than seven of the twentieth century’s most exceptional scientists: in order of birth, Theodor von Kármán, George de Hevesy, Michael Polanyi, Leo Szilard, Eugene Wigner, John von Neumann and Edward Teller. All seven left Hungary as young men; all seven proved unusually versatile as well as talented and made major contributions to science and technology; two among them, de Hevesy and Wigner, eventually won Nobel Prizes.

When tall and birdlike Chadwick finished speaking he looked over the assembly and announced abruptly, “Now I want to be chloroformed and put to bed for a fortnight.”601 He deserved his rest. He had discovered a new elementary particle, the third basic constituent of matter. It was this neutral mass that compounded the weight of the elements without adding electrical charge. Two protons and 2 neutrons made a helium nucleus; 7 protons and 7 neutrons a nitrogen; 47 protons and 60 neutrons a silver; 56 protons and 81 neutrons a barium; 92 protons and 146 (or 143) neutrons a uranium. And because the neutron was as massive as a proton but carried no electrical charge, it was hardly affected by the shell of electrons around a nucleus; nor did the electrical barrier of the nucleus itself block its way. It would therefore serve as a new nuclear probe of surpassing power of penetration. “A beam of thermal neutrons,” writes the American theoretical physicist Philip Morrison, “moving at about the speed of sound, which corresponds to a kinetic energy of only about a fortieth of an electron volt, produces nuclear reactions in many materials much more easily than a beam of protons of millions of volts energy, traveling thousands of times faster.”602 Ernest Lawrence’s cyclotron, spiraling protons to million-volt energies for the first time the same month that Chadwick made his fateful discovery, fortunately proved to be adaptable to the production of neutrons. More than any other developm...

He would look for the something which objects hid, though his particular genius was to discover that there was nothing behind them to hide; that objects, as matter and as energy, were all; that even space and time were not the invisible matrices of the material world but its attributes. “If you will not take the answer too seriously,” he told a clamorous crowd of reporters in New York in 1921 who asked him for a short explanation of relativity, “and consider it only as a kind of joke, then I can explain it as follows. It was formerly believed that if all material things disappeared out of the universe, time and space would be left. According to the relativity theory, however, time and space disappear together with the things.”625

“Relativity” was a misnomer. Einstein worked his way to a new physics by demanding consistency and greater objectivity of the old. If the speed of light is a constant, then something else must serve as the elastic between two systems at motion in relation to one another—even if that something else is time. If a body gives off an amount E of energy its mass minutely diminishes. But if energy has mass, then mass must have energy: the two must be equivalent: E = mc2, E/c2 = m.632 (I.e., an amount of energy E in joules is equal to an amount of mass m in kilograms multiplied by the square of the speed of light, an enormous number, 3 × 108 meters per second times 3 × 108 m/s = 9 × 1016 or 90,000,000,000,000,000 joules per kilogram. Dividing E by c2 demonstrates how large an amount of energy is contained within even a small mass.)

In brief, the Elders have stage-managed the invention and dissemination of modern ideas—of the modern world. Everything more recent than the Russian imperial system of czar, landed nobility and serfs is part and parcel of their diabolical work. Which helps explain how so obscure a study as physics came in Germany in the 1920s to be counted part of the Jewish conspiracy.

Longstanding anti-Semitic discrimination in academic appointments weighted the civil service law dismissals in favor of the natural sciences, fields of study that had evolved more recently than the older disciplines of the liberal arts, that German scholarship had looked down upon as “materialistic” and that had therefore proved less impenetrable to Jews.688 Medicine incurred 423 dismissals, physics 106, mathematics 60—in the physical and biological sciences other than medicine, an immediate total of 406 scientists. The University of Berlin and the University of Frankfurt each lost a third of its faculty.

When Stanislaw Ulam arrived in Princeton in 1935 he found von Neumann comfortably ensconced in a “large and impressive house. A black servant let me in.” The von Neumanns gave two or three parties a week. “These were not completely carefree,” Ulam notes; “the shadow of coming world events pervaded the social atmosphere.”730 Ulam’s own enthusiasm for America, formulated a few years later when he was a Junior Fellow at Harvard, was tempered with a criticism of the extreme weather: “I used to tell my friends that the United States was like the little child in a fairy tale, at whose birth all the good fairies came bearing gifts, and only one failed to come. It was the one bringing the climate.”731

Before it is science and career, before it is livelihood, before even it is family or love, freedom is sound sleep and safety to notice the play of morning sun.

On August 2, 1932, working with a carefully prepared cloud chamber, an American experimentalist at Caltech named Carl Anderson had discovered a new particle in a shower of cosmic rays. The particle was an electron with a positive instead of a negative charge, a “positron,” the first indication that the universe consists not only of matter but of antimatter as well. (Its discovery earned Anderson the 1936 Nobel Prize.)

Both Fermi’s biographers—his wife Laura and his protégé and fellow Nobel laureate Emilio Segrè—assign the beginning of his commitment to physics to the period of psychological trauma following the death of his older brother Giulio when Fermi was fourteen years old, in the winter of 1915.764 Only a year apart in age, the two boys had been inseparable; Giulio’s death during minor surgery for a throat abscess left Enrico suddenly bereft. That same winter young Enrico browsed on market day among the stalls of Rome’s Campo dei Fiori, where a statue commemorates the philosopher Giordano Bruno, Copernicus’ defender, who was burned at the stake there in 1600 by the Inquisition. Fermi found two used volumes in Latin, Elementorum physicae mathematicae, the work of a Jesuit physicist, published in 1840. The desolate boy used his allowance to buy the physics textbooks and carried them home. They excited him enough that he read them straight through. When he was finished he told his older sister Maria he had not even noticed they were written in Latin. “Fermi must have studied the treatise very thoroughly,” Segrè would decide, looking through the old volumes many years later, “because it contains marginal notes, corrections of errors, and several scraps of paper with notes in Fermi’s handwriting.”765

On October 18 they started a systematic investigation, a series of measurements made inside and outside a lead housing. By October 22 they were prepared to measure what might happen when only a lead wedge separated the neutron source from its target. But the experimenters had to give student examinations that morning and Fermi decided to go ahead on his own. He described the historic moment late in life to a colleague curious about the process of discovery in physics: I will tell you how I came to make the discovery which I suppose is the most important one I have made. We were working very hard on the neutron-induced radioactivity and the results we were obtaining made no sense. One day, as I came into the laboratory, it occurred to me that I should examine the effect of placing a piece of lead before the incident neutrons. Instead of my usual custom, I took great pains to have the piece of lead precisely machined. I was clearly dissatisfied with something: I tried every excuse to postpone putting the piece of lead in its place. When finally, with some reluctance, I was going to put it in its place, I said to myself: “No, I do not want this piece of lead here; what I want is a piece of paraffin.” It was just like that with no advance warning, no conscious prior reasoning. I immediately took some odd piece of paraffin and placed it where the piece of lead was to have been.818 The extraordinary result of substituting paraffin wax for a heavy element like lead was a dramatic...

Amaldi and Segrè had not been wrong about aluminum. They had simply irradiated different samples of the element on different tables. The hydrogen in the wooden table had slowed down some of the neutrons and enhanced the almost-three-minute activity. As Hans Bethe once noted wittily, the efficiency of slow neutrons “might never have been discovered if Italy were not rich in marble. . . . A marble table gave different results from a wooden table.826 If it had been done [in America], it all would have been done on a wooden table and people would never have found out.”

Thus by the mid-1930s the three most original living physicists had each spoken to the question of harnessing nuclear energy. Rutherford had dismissed it as moonshine; Einstein had compared it to shooting in the dark at scarce birds; Bohr thought it remote in direct proportion to understanding. If they seem less perceptive in their skepticism than Szilard, they also had a better grasp of the odds. The essential future is always unforeseen. They were experienced enough not to long for it.

Bohr went on to say that the common aim of all science was “the gradual removal of prejudices,” a complementary restorative to the usual pious characterization of science as a quest for incontrovertible truth.922 To a greater extent than any other scientist of the twentieth century Bohr perceived the institution of science to which he dedicated his life to be a profoundly political force in the world. The purpose of science, he believed, was to set men free. Totalitarianism, in Hannah Arendt’s powerful image, drove toward “destroying all space between men and pressing men against each other.”923 It was entirely in character that Bohr, at a time of increasing danger, publicly opposed that drive with the individualistic and enriching discretions of complementarity.

Frisch: “I remember that I immediately at that instant thought of the fact that electric charge diminishes surface tension.”988 The liquid drop is held together by surface tension, the nucleus by the analogous strong force. But the electrical repulsion of the protons in the nucleus works against the strong force, and the heavier the element, the more intense the repulsion. Frisch continues: And so I promptly started to work out by how much the surface tension of a nucleus would be reduced. I don’t know where we got all our numbers from, but I think I must have had a certain feeling for the binding energies and could make an estimate of the surface tension. Of course we knew the charge and the size reasonably well. And so, as an order of magnitude, the result was that at a charge [i.e., an atomic number] of approximately 100 the surface tension of the nucleus disappears; and therefore uranium at 92 must be pretty close to that instability. They had discovered the reason no elements beyond uranium exist naturally in the world: the two forces working against each other in the nucleus eventually cancel each other out.

They pictured the uranium nucleus as a liquid drop gone wobbly with the looseness of its confinement and imagined it hit by even a barely energetic slow neutron. The neutron would add its energy to the whole. The nucleus would oscillate. In one of its many random modes of oscillation it might elongate. Since the strong force operates only over extremely short distances, the electric force repelling the two bulbs of an elongated drop would gain advantage. The two bulbs would push farther apart. A waist would form between them. The strong force would begin to regain the advantage within each of the two bulbs. It would work like surface tension to pull them into spheres. The electric repulsion would work at the same time to push the two separating spheres even farther apart. Eventually the waist would give way. Two smaller nuclei would appear where one large nucleus had been before—barium and krypton,

“Then,” Frisch recalls, “Lise Meitner was saying that if you really do form two such fragments they would be pushed apart with great energy.”989 They would be pushed apart by the mutual repulsion of their gathered protons at one-thirtieth the speed of light. Meitner or Frisch calculated that energy to be about 200 MeV: 200 million electron volts. An electron volt is the energy necessary to accelerate an electron through a potential difference of one volt. Two hundred million electron volts is not a large amount of energy, but it is an extremely large amount of energy from one atom. The most energetic chemical reactions release about 5 eV per atom.

When she was thirty-one, in 1909, Meitner had met Albert Einstein for the first time at a scientific conference in Salzburg. He “gave a lecture on the development of our views regarding the nature of radiation.990 At that time I certainly did not yet realize the full implications of his theory of relativity.” She listened eagerly. In the course of the lecture Einstein used the theory of relativity to derive his equation E = mc2, with which Meitner was then unfamiliar. Einstein showed thereby how to calculate the conversion of mass into energy. “These two facts,” she reminisced in 1964, “were so overwhelmingly new and surprising that, to this day, I remember the lecture very well.” She remembered it in 1938, on the day before Christmas. She also “had the packing fractions in her head,” says Frisch—she had memorized Francis Aston’s numbers for the mass defects of nuclei.991 If the large uranium nucleus split into two smaller nuclei, the smaller nuclei would weigh less in total than their common parent. How much less? That was a calculation she could easily work: about one-fifth the mass of a proton less. Process one-fifth of the mass of a proton through E = mc2. “One fifth of a proton mass,” Frisch exclaims, “was just equivalent to 200 MeV. So here was the source for that energy; it all fitted!”992

Alvarez wired Gamow for details, learned of the Frisch experiment, then tracked down Oppenheimer: I remember telling Robert Oppenheimer that we were going to look for [ionization pulses from fission] and he said, “That’s impossible” and gave a lot of theoretical reasons why fission couldn’t really happen. 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 . . . he had decided that some neutrons would probably boil off in the 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.1071

One of Oppenheimer’s students, the American theoretical physicist Philip Morrison, recalls that “when fission was discovered, within perhaps a week there was on the blackboard in Robert Oppenheimer’s office a drawing—a very bad, an execrable drawing—of a bomb.”1074

In 1935, using a more powerful instrument, physicist Arthur Jeffrey Dempster of the University of Chicago detected a second, lighter isotope. “It was found,” Dempster announced in a lecture, “that a few seconds’ exposure was sufficient for the main component at 238 reported by Dr. Aston, but on long exposures a faint companion of mass number 235 was also present.”1091 Three years later a gifted Harvard postdoctoral fellow named Alfred Otto Carl Nier, the son of working-class German emigrants to Minnesota, measured the ratio of U235 to U238 in natural uranium as 1:139, which meant that U235 was present to the extent of about 0.7 percent.1092 By contrast, thorium in its natural form is essentially all one isotope, Th232. And that natural difference in the composition of the two elements was the clue that set Bohr off.

Thorium was lighter than U235, U238 heavier, but the middle isotope differed more significantly in another important regard. When Th232 absorbed a neutron it became a nucleus of odd mass number, Th233. When U238 absorbed a neutron it also became a nucleus of odd mass number, U239. But when U235 absorbed a neutron it became a nucleus of even mass number, U236. And the vicissitudes of nuclear rearrangement are such, as Fermi would explain one day in a lecture, that “changing from an odd number of neutrons to an even number of neutrons released one or two MeV.”1093 Which meant that U235 had an inherent energetic advantage over its two competitors: it accrued energy toward fission simply by virtue of its change of mass; they did not.

Bohr’s third graph demonstrated: the probably continuous fission cross section of U235. From slow neutrons on the left only a fraction of an electron volt above zero energy, to fast neutrons on the right above 1 MeV that would also fission U238, any neutron an atom of U235 encountered would agitate it to fission.

Bohr also considered U235’s behavior under fast-neutron bombardment. “For fast neutrons,” he wrote near the end of the paper, “ . . . because of the scarcity of the isotope concerned, the fission yields will be much smaller than those obtained from neutron impacts on the abundant isotope.”1098 The statement implies but does not ask a pregnant question: what would the yields be for fast neutrons if U235 could be separated from U238?

More ominously, two initiatives originated simultaneously in Germany as a result of the French report.1139 A physicist at Göttingen alerted the Reich Ministry of Education. That led to a secret conference in Berlin on April 29, which led in turn to a research program, a ban on uranium exports and provision for supplies of radium from the Czechoslovakian mines at Joachimsthal. (Otto Hahn was invited to the conference but arranged to be elsewhere.) The same week a young physicist working at Hamburg, Paul Harteck, wrote a letter jointly with his assistant to the German War Office: We take the liberty of calling to your attention the newest development in nuclear physics, which, in our opinion, will probably make it possible to produce an explosive many orders of magnitude more powerful than the conventional ones. . . . That country which first makes use of it has an unsurpassable advantage over the others.1140 The Harteck letter reached Kurt Diebner, a competent nuclear physicist stuck unhappily in the Wehrmacht’s ordnance department studying high explosives. Diebner carried it to Hans Geiger. Geiger recommended pursuing the research. The War Office agreed.

Atoms on the surface of a mass of uranium are exposed to neutrons more efficiently than atoms deeper inside. Fermi and Szilard therefore decided not to bulk their five hundred pounds of uranium oxide into one large container but to distribute it throughout the tank by packing it into fifty-two cans as tall and narrow as lengths of pipe—two inches in diameter and two feet long.

Szilard thought first about an alternative to water. The next common material up the periodic table that might work—that had a capture cross section considerably smaller than hydrogen’s, that was cheap, that would be thermally and chemically stable—was carbon. The mineral form of carbon, chemically identical to diamond but the product of a different structure of crystallization, is graphite, a black, greasy, opaque, lustrous material that is the essential component of pencil lead. Although carbon slows neutrons much less rapidly than hydrogen, even that difference might be put to advantage by careful design.

von Weizsäcker remembers realizing in discussions with a friend “that this discovery could not fail to radically change the political structure of the world”:1207 To a person finding himself at the beginning of an era, its simple fundamental structures may become visible like a distant landscape in the flash of a single stroke of lightning. But the path toward them in the dark is long and confusing. At that time [i.e., 1939] we were faced with a very simple logic. Wars waged with atom bombs as regularly recurring events, that is to say, nuclear wars as institutions, do not seem reconcilable with the survival of the participating nations. But the atom bomb exists. It exists in the minds of some men. According to the historically known logic of armaments and power systems, it will soon make its physical appearance. If that is so, then the participating nations and ultimately mankind itself can only survive if war as an institution is abolished.

Both sides might work from fear of the other. But some on both sides would be working also paradoxically believing they were preparing a new force that would ultimately bring peace to the world.

“How much money do you need?” Commander Hoover wanted to know.1237 Szilard had not planned to ask for money. “The diversion of Government funds for such purposes as ours appears to be hardly possible,” he explained to Pegram the next day, “and I have therefore myself avoided to make any such recommendation.”1238 But Teller answered Hoover promptly, probably speaking for Fermi: “For the first year of this research we need six thousand dollars, mostly in order to buy the graphite.” (“My friends blamed me because the great enterprise of nuclear energy was to start with such a pittance,” Teller reminisces; “they haven’t forgiven me yet.”1239 Szilard, who would write Briggs on October 26 that the graphite alone for a largescale experiment would cost at least $33,000, must have been appalled.1240)

The committee recommended “adequate support for a thorough investigation.” Initially the government might undertake to supply four tons of pure graphite (this would allow Fermi and Szilard to measure the capture cross section of carbon) and, if justified later, fifty tons of uranium oxide. Briggs heard from Pa Watson on November 17. The President had read the report, Watson wrote, and wanted to keep it on file. On file is where it remained, mute and inactive, well into 1940. Even with Szilard and Fermi stalled, fission studies continued at many other American laboratories. Prodded by a late-October letter from Fermi, for example, Alfred Nier at the University of Minnesota finally began preparing to separate enough U235 from U238, using his mass spectroscope, to determine experimentally which isotope is responsible for slow-neutron fission.1244, 1245 But to American physicists and administrators in and out of government a bomb of uranium seemed a remote possibility at best. However intense their sympathies, the war was still a European war.

In the days before the war, Otto Frisch remembers, in Hamburg with Otto Stern, he used to run experiments by day and think intensely about physics well into the night. “I regularly came home,” Frisch told an interviewer once, “had dinner at seven, had a quarter of an hour’s nap after dinner, and then I sat down happily with a sheet of paper and a reading lamp and worked until about one o’clock at night—until I began to have hallucinations. . . . I began to see queer animals against the background of my room, and then I thought, Oh, well, better go to bed.’ ” The young Austrian’s hypnagogic visions were “unpleasant feelings” but otherwise “it was an ideal life. I’d never had such a pleasant life, ever—this concentrated five hours work every night.”1246

The possibility of a critical mass is anchored in the fact that the surface area of a sphere increases more slowly with increasing radius than does the volume (as nearly r2 to r3). At some particular volume, depending on the density of the material and on its cross sections for scattering, capture and fission, more neutrons should find nuclei to fission than find surface to escape from; that volume is then the critical mass. Estimating the several cross sections of natural uranium, Francis Perrin put its critical mass at forty-four tons. A tamper around the uranium of iron or lead to bounce back neutrons might reduce the requirement, Perrin calculated, to only thirteen tons.

There had always been four possible mechanisms for an explosive chain reaction in uranium: (1) slow-neutron fission of U238; (2) fast-neutron fission of U238; (3) slow-neutron fission of U235; and (4) fast-neutron fission of U235. Bohr’s logical distinction between U238 and thorium on the one hand and U235 on the other ruled out (1): U238 was not fissioned by slow neutrons. (2) was inefficient because of scattering and the parasitic effects of the capture resonance of U238. (3) was possibly applicable to power production but too slow for a practical weapon. But what about (4)? Apparently no one in Britain, France or the United States had asked the question quite that way before.

10-23 cm21265 into Peierls’ formula.1266 “To my amazement” the answer “was very much smaller than I had expected; it was not a matter of tons, but something like a pound or two.”1267 A volume less than a golf ball for a substance so heavy as uranium. But would that pound or two explode or fizzle? Peierls easily produced an estimate. The chain reaction would have to proceed faster than the vaporizing and swelling of the heating metal ball. Peierls calculated the time between neutron generations, between 1×2×4×8×16×32×64 . . . , to be about four millionths of a second, much faster than the several thousandths of a second Frisch had estimated for slow-neutron fission.1268 Then how destructive was the consequent explosion? Some eighty generations of neutrons—as many as could be expected to multiply before the swelling explosion separated the atoms of U235 enough to stop the chain reaction—still millionths of a second in total, gave temperatures as hot as the interior of the sun, pressures greater than the center of the earth where iron flows as a liquid. “I worked out the results of what such a nuclear explosion would be,” says Peierls. “Both Frisch and I were staggered by them.”1269 And finally, practically: could even a few pounds of U235 be separated from U238? Frisch writes: I had worked out the possible efficiency of my separation system with the help of Clusius’s formula, and we came to the conclusion that with something like a hundred thousand similar separation tubes o...

The first of the two parts they titled “On the construction of a ‘superbomb’; based on a nuclear chain reaction in uranium.”1275 It was intended, they wrote, “to point out and discuss a possibility which seems to have been overlooked in . . . earlier discussions.”1276 They proceeded to cover the same ground they had previously covered together in private, noting that “the energy liberated by a 5 kg bomb would be equivalent to that of several thousand tons of dynamite.” They described a simple mechanism for arming the weapon: making the uranium sphere in two parts “which are brought together first when the explosion is wanted. Once assembled, the bomb would explode within a second or less.”1277 Springs, they thought, might pull the two small hemispheres together. Assembly would have to be rapid or the chain reaction would begin prematurely, destroying the bomb but not much else. A byproduct of the explosion—about 20 percent of its energy, they thought—would be radiation, the equivalent of “a hundred tons of radium” that would be “fatal to living beings even a long time after the explosion.” Effective protection from the weapon would be “hardly possible.”

The second report, “Memorandum on the properties of a radioactive ‘super-bomb,’ ” a less technical document, was apparently intended as an alternative presentation for nonscientists.1278 This study explored beyond the technical questions of design and production to the strategic issues of possession and use; it managed at the same time both seemly innocence and extraordinary prescience: 1. As a weapon, the super-bomb would be practically irresistible. There is no material or structure that could be expected to resist the force of the explosion. . . . 2. Owing to the spreading of radioactive substances with the wind, the bomb could probably not be used without killing large numbers of civilians, and this may make it unsuitable as a weapon for use by this country. . . . 3. . . . It is quite conceivable that Germany is, in fact, developing this weapon. . . . 4. If one works on the assumption that Germany is, or will be, in the possession of this weapon, it must be realised that no shelters are available that would be effective and could be used on a large scale. The most effective reply would be a counter-threat with a similar weapon. Thus in the first months of 1940 it was already clear to two intelligent observers that nuclear weapons would be weapons of mass destruction against which the only apparent defense would be the deterrent effect of mutual possession.

The surest method for building a reactor, Heisenberg wrote, “will be to enrich the uranium-235 isotope. The greater the degree of enrichment, the smaller the reactor can be made.” Enrichment—increasing the proportion of U235 to U238—was also “the only method of producing explosives several orders of magnitude more powerful than the strongest explosives yet known.”1281 (The phrase indicates Heisenberg understood the possibility of fast-neutron fission even before Frisch and Peierls did.)

The experiment confirmed what Fermi and Szilard had already demonstrated: that ordinary hydrogen, whether in the form of water or paraffin, would not work with natural uranium to sustain a chain reaction. That understanding left the German project with two possible moderator materials: graphite and heavy water.1359 In January a misleading measurement reduced that number to one. At Heidelberg Walther Bothe, an exceptional experimentalist who would eventually share a Nobel Prize with Max Born, measured the absorption cross section of carbon using a 3.6-foot sphere of high-quality graphite submerged in a tank of water. He found a cross section of 6.4 × 10−27 cm2, more than twice Fermi’s value, and concluded that graphite, like ordinary water, would absorb too many neutrons to sustain a chain reaction in natural uranium.

When [Fermi] finished his [carbon absorption] measurement the question of secrecy again came up. I went to his office and said that now that we had this value perhaps the value ought not to be made public. And this time Fermi really lost his temper; he really thought this was absurd. There was nothing much more I could say, but next time when I dropped in his office he told me that Pegram had come to see him, and Pegram thought that this value should not be published. From that point the secrecy was on.1360 It was on just in time to prevent German researchers from pursuing a cheap, effective moderator. Bothe’s measurement ended German experiments on graphite.

In the midst of experiment Fermi found time to theorize. He and Teller had lunch at the University Club one pleasant day in September. Afterward, walking back to Pupin—“out of the blue,” Teller says—Fermi wondered aloud if an atomic bomb might serve to heat a mass of deuterium sufficiently to begin thermonuclear fusion.1465 Such a mechanism, a bomb fusing hydrogen to helium, should be three orders of magnitude as energetic as a fission bomb and far cheaper in terms of equivalent explosive force. For Fermi the idea was a throwaway. Teller found it a surpassing challenge and took it to heart. Teller liked to break new ground. When he understood something theoretically he usually moved on without waiting for experimental confirmation. He understood the atomic bomb. He moved on to consider the possibility of a hydrogen bomb.

No document Franklin Delano Roosevelt signed authenticates the fateful decision to expedite research toward an atomic bomb that Vannevar Bush reported in his October 9 memorandum to James Bryant Conant: the archives divulge no smoking gun. The closest the records come to a piece of paper that changed the world is a banality. Bush personally delivered the third National Academy of Sciences report to the President on November 27, 1941. Roosevelt returned it to him two months later with a note on White House stationery written in black ink with a broad-nibbed pen, a note that would communicate only a commonplace of the housekeeping of state secrets except for the authority of its first vernacular expression and the initials it bears: