More (#2) from The Making of the Atomic Bomb:

After Alexander Sachs paraphrased the Einstein-Szilard letter to Roosevelt, Roosevelt demanded action, and Edwin Watson set up a meeting with representatives from the Bureau of Standards, the Army, and the Navy...

Szilard began by emphasizing the possibility of a chain reaction in a uranium-graphite system. Whether such a system would work, he said, depended on the capture cross section of carbon and that was not yet sufficiently known. If the value was large, they would know that a large-scale experiment would fail. If the value was extremely small, a large-scale experiment would look highly promising. An intermediate value would necessitate a large-scale experiment to decide. He estimated the destructive potential of a uranium bomb to be as much as twenty thousand tons of high-explosive equivalent. Such a bomb, he had written in the memorandum Sachs carried to Roosevelt, would depend on fast neutrons and might be “too heavy to be transported by airplane,” which meant he was still thinking of exploding natural uranium, not of separating U235.

Upon asking for some money to conduct the relevant experiments, the Army representative launched into a tirade:

"He told us that it was naive to believe that we could make a significant contribution to defense by creating a new weapon. He said that if a new weapon is created, it usually takes two wars before one can know whether the weapon is any good or not. Then he explained rather laboriously that it is in the end not weapons which win the wars, but the morale of the troops. He went on in this vein for a long time until suddenly Wigner, the most polite of us, interrupted him. [Wigner] said in his high-pitched voice that it was very interesting for him to hear this. He always thought that weapons were very important and that this is what costs money, and this is why the Army needs such a large appropriation. But he was very interested to hear that he was wrong: it’s not weapons but the morale which wins the wars. And if this is correct, perhaps one should take a second look at the budget of the Army, and maybe the budget could be cut."

"All right, all right," Adamson snapped, "you'll get your money."

More (#3) from The Making of the Atomic Bomb:

...the British Chemical Society asked [Otto] Frisch to write a review of advances in experimental nuclear physics for its annual report...

Frisch’s review article mentioned the possibility of a chain reaction only to discount it. He based that conclusion on Bohr’s argument that the U238 in natural uranium would scatter fast neutrons, slowing them to capture-resonance energies; the few that escaped capture would not suffice, he thought, to initiate a slow-neutron chain reaction in the scarce U235. Slow neutrons in any case could never produce more than a modest explosion, Frisch pointed out; they took too long slowing down and finding a nucleus. As he explained later: "That process would take times of the order of a sizeable part of a millisecond... and for the whole chain reaction to develop would take several milliseconds; once the material got hot enough to vaporize, it would begin to expand and the reaction would be stopped before it got much further. So the thing might blow up like a pile of gunpowder, but no worse, and that wasn’t worth the trouble."

Not long from Nazi Germany, Frisch found his argument against a violently explosive chain reaction reassuring. It was backed by the work of no less a theoretician than Niels Bohr. With satisfaction he published it.

...Concerned that Hitler might bluff Neville Chamberlain with threats of a new secret weapon, Churchill had collected a briefing from Frederick Lindemann and written to caution the cabinet not to fear “new explosives of devastating power” for at least “several years.” The best authorities, the distinguished M.P. emphasized with a nod to Niels Bohr, held that “only a minor constituent of uranium is effective in these processes.” That constituent would need to be laboriously extracted for any large-scale effects. “The chain process can take place only if the uranium is concentrated in a large mass,” Churchill continued, slightly muddling the point. “As soon as the energy develops, it will explode with a mild detonation before any really violent effects can be produced. It might be as good as our present-day explosives, but it is unlikely to produce anything very much more dangerous.” He concluded optimistically: “Dark hints will be dropped and terrifying whispers will be assiduously circulated, but it is to be hoped that nobody will be taken in by them.”

...[Several months later] Frisch walked home through ominous blackouts so dark that he sometimes stumbled over roadside benches and could distinguish fellow pedestrians only by the glow of the luminous cards they had taken to wearing in their hatbands. Thus reminded of the continuing threat of German bombing, he found himself questioning his confident Chemical Society review: “Is that really true what I have written?”

Sometime in February 1940 he looked again. 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.

If Frisch now glimpsed an opening into those depths he did so because he had looked carefully at isotope separation and had decided it could be accomplished even with so fugitive an isotope as U235. He was therefore prepared to consider the behavior of the pure substance unalloyed with U238, as Bohr, Fermi and even Szilard had not yet been...

...He shared the problem with [Rudolf] Peierls... [and together they worked out that] 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.”

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 one might produce a pound of reasonably pure uranium-235 in a modest time, measured in weeks. At that point we stared at each other and realized that an atomic bomb might after all be possible."

Frisch and Peierls wrote a two-part report of their findings:

The first of the two parts they titled “On the construction of a ‘superbomb’; based on a nuclear chain reaction in uranium.” It was intended, they wrote, “to point out and discuss a possibility which seems to have been overlooked in... earlier discussions.” 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.” 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. 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. Frisch and Peierls finished their two reports and took them to [Mark] Oliphant. He quizzed the men thoroughly, added a cover letter to their memoranda (“I have considered these suggestions in some detail and have had considerable discussion with the authors, with the result that I am convinced that the whole thing must be taken rather seriously, if only to make sure that the other side are not occupied in the production of such a bomb at the present time”) and sent letter and documents off to Henry Thomas Tizard...

“I have often been asked,” Otto Frisch wrote many years afterward of the moment when he understood that a bomb might be possible after all, before he and Peierls carried the news to Mark Oliphant, “why I didn’t abandon the project there and then, saying nothing to anybody. Why start on a project which, if it was successful, would end with the production of a weapon of unparalleled violence, a weapon of mass destruction such as the world had never seen? The answer was very simple. We were at war, and the idea was reasonably obvious; very probably some German scientists had had the same idea and were working on it.”

Whatever scientists of one warring nation could conceive, the scientists of another warring nation might also conceive — and keep secret. That early in 1939 and early 1940, the nuclear arms race began.

More (#1) from The Making of the Atomic Bomb:

Fermi and Szilard had both written reports on their secondary-neutron experiments and were ready to send them to the Physical Review. With Pegram’s concurrence they decided to go ahead and mail the reports to the Review, to establish priority, but to ask the editor to delay publishing them until the secrecy issue could be resolved...

...If Bohr could be convinced to swing his prestige behind secrecy, the campaign to isolate German nuclear physics research might work.

They met in the evening in Wigner’s office. “Szilard outlined the Columbia data,” Wheeler reports, “and the preliminary indications from it that at least two secondary neutrons emerge from each neutron-induced fission. Did this not mean that a nuclear explosive was certainly possible?” Not necessarily, Bohr countered. “We tried to convince him,” Teller writes, “that we should go ahead with fission research but we should not publish the results. We should keep the results secret, lest the Nazis learn of them and produce nuclear explosions first. Bohr insisted that we would never succeed in producing nuclear energy and he also insisted that secrecy must never be introduced into physics.”

...[Bohr] had worked for decades to shape physics into an international community, a model within its limited franchise of what a peaceful, politically united world might be. Openness was its fragile, essential charter, an operational necessity, as freedom of speech is an operational necessity to a democracy. Complete openness enforced absolute honesty: the scientist reported all his results, favorable and unfavorable, where all could read them, making possible the ongoing correction of error. Secrecy would revoke that charter and subordinate science as a political system—Polanyi’s “republic”—to the anarchic competition of the nation-states.

...March 17 was a Friday; Szilard traveled down to Washington from Princeton with Teller; Fermi stayed the weekend. They got together, reports Szilard, “to discuss whether or not these things”—the Physical Review papers—“should be published. Both Teller and I thought that they should not. Fermi thought that they should. But after a long discussion, Fermi took the position that after all this was a democracy; if the majority was against publication, he would abide by the wish of the majority.” Within a day or two the issue became moot. The group learned of the Joliot/von Halban/Kowarski paper, published in Nature on March 18. “From that moment on,” Szilard notes, “Fermi was adamant that withholding publication made no sense.”1135

[About a month later, German physicist] 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."

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.

On the origins of the Einstein–Szilárd letter:

Szilard told Einstein about the Columbia secondary neutron experiments and his calculations toward a chain reaction in uranium and graphite. Long afterward he would recall his surprise that Einstein had not yet heard of the possibility of a chain reaction. When he mentioned it Einstein interjected... “I never thought of that!” He was nevertheless, says Szilard, “very quick to see the implications and perfectly willing to do anything that needed to be done. He was willing to assume responsibility for sounding the alarm even though it was quite possible that the alarm might prove to be a false alarm. The one thing most scientists are really afraid of is to make fools of themselves. Einstein was free from such a fear and this above all is what made his position unique on this occasion.”

And:

By [August 1935] the Hungarians at least believed they saw major humanitarian benefit inherent in what Eugene Wigner would describe in retrospect as “a horrible military weapon,” explaining: "Although none of us spoke much about it to the authorities [during this early period] — they considered us dreamers enough as it was — we did hope for another effect of the development of atomic weapons in addition to the warding off of imminent disaster. We realized that, should atomic weapons be developed, no two nations would be able to live in peace with each other unless their military forces were controlled by a common higher authority. We expected that these controls, if they were effective enough to abolish atomic warfare, would be effective enough to abolish also all other forms of war. This hope was almost as strong a spur to our endeavors as was our fear of becoming the victims of the enemy’s atomic bombings."

From the horrible weapon which they were about to urge the United States to develop, Szilard, Teller and Wigner — “the Hungarian conspiracy,” Merle Tuve was amused to call them — hoped for more than deterrence against German aggression. They also hoped for world government and world peace, conditions they imagined bombs made of uranium might enforce.

More (#5) from The Making of the Atomic Bomb:

With fifty-three people aboard including the concert violinist the Hydro sailed on time. Forty-five minutes into the crossing, Haukelid’s charge of plastic explosive blew the hull. The captain felt the explosion rather than heard it, and though Tinnsjö is landlocked he thought they might have been torpedoed. The bow swamped first as Haukelid had intended; while the passengers and crew struggled to release the lifeboats, the freight cars with their thirty-nine drums of heavy water — 162 gallons mixed with 800 gallons of dross — broke loose, rolled overboard and sank like stones. Of passengers and crew twenty-six drowned. The concert violinist slipped high and dry into a lifeboat; when his violin case floated by, someone was kind enough to fish it out for him. Kurt Diebner of German Army Ordnance counted the full effect on German fission research of the Vemork bombing and the sinking of the Hydro in a postwar interview:

"When one considers that right up to the end of the war, in 1945, there was virtually no increase in our heavy-water stocks in Germany... it will be seen that it was the elimination of German heavy-water production in Norway that was the main factor in our failure to achieve a self-sustaining atomic reactor before the war ended.

The race to the bomb, such as it was, ended for Germany on a mountain lake in Norway on a cold Sunday morning in February 1944.

More (#4) from The Making of the Atomic Bomb:

Two associates of Soviet physicist Igor Kurchatov reported to the Physical Review in June 1940 that they had observed rare spontaneous fissioning in uranium. “The complete lack of any American response to the publication of the discovery,” writes the American physicist Herbert F. York, “was one of the factors which convinced the Russians that there must be a big secret project under way in the United States.”

And:

[An experiment] left the German project with two possible moderator materials: graphite and heavy water. 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 cm^2, 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. Von Halban and Kowarski, now at Cambridge and in contact with the MAUD Committee, similarly overestimated the carbon cross section — the graphite in both experiments was probably contaminated with neutron-absorbing impurities such as boron — but their work was eventually checked against Fermi’s. Bothe could make no such check. The previous fall Szilard had assaulted Fermi with another secrecy appeal:

"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."

It was on just in time to prevent German researchers from pursuing a cheap, effective moderator. Bothe’s measurement ended German experiments on graphite.

And:

Leo Szilard was known by now throughout the American physics community as the leading apostle of secrecy in fission matters. To his mailbox, late in May 1940, came a puzzled note from a Princeton physicist, Louis A. Turner. Turner had written a Letter to the Editor of the Physical Review, a copy of which he enclosed.1365 It was entitled “Atomic energy from U238” and he wondered if it should be withheld from publication. “It seems as if it was wild enough speculation so that it could do no possible harm,” Turner told Szilard, “but that is for someone else to say.”

Turner had published a masterly twenty-nine-page review article on nuclear fission in the January Reviews of Modern Physics, citing nearly one hundred papers that had appeared since Hahn and Strassmann reported their discovery twelve months earlier; the number of papers indicates the impact of the discovery on physics and the rush of physicists to explore it. Turner had also noted the recent Nier/Columbia report confirming the attribution of slow-neutron fission to U235. (He could hardly have missed it; the New York Times and other newspapers publicized the story widely. He wrote Szilard irritably or ingenuously that he found it “a little difficult to figure out the guiding principle [of keeping fission research secret] in view of the recent ample publicity given to the separation of isotopes.”1368) His reading for the review article and the new Columbia measurements had stimulated him to further thought; the result was his Physical Review letter.

...Szilard... answered Turner’s letter on May 30... [and] told him “it might eventually turn out to be a very important contribution” — and proposed he keep it secret. Szilard saw beyond what Turner had seen. He saw that a fissile element bred in uranium could be chemically separated away: that the relatively easy and relatively inexpensive process of chemical separation could replace the horrendously difficult and expensive process of physical separation of isotopes as a way to a bomb.

And:

“Oppenheimer wanted me to be the associate director,” [I.I. Rabi] told an interviewer many years later. “I thought it over and turned him down. I said, ‘I’m very serious about this war. We could lose it with insufficient radar.’” The Columbia physicist thought radar more immediately important to the defense of his country than the distant prospect of an atomic bomb. Nor did he choose to work full time, he told Oppenheimer, to make “the culmination of three centuries of physics” a weapon of mass destruction. Oppenheimer responded that he would take “a different stand” if he thought the atomic bomb would serve as such a culmination. “To me it is primarily the development in time of war of a military weapon of some consequence.” Either Oppenheimer had not yet thought his way through to a more millenarian view of the new weapon’s implications or he chose to avoid discussing those implications with Rabi. He asked Rabi only to participate in an inaugural physics conference at Los Alamos in April 1943 and to help convince others, particularly Hans Bethe, to sign on. Eventually Rabi would come and go as a visiting consultant, one of the very few exceptions to Groves’ compartmentalization and isolation rules.

And:

Work toward an atomic bomb had begun in the USSR in 1939. A thirtysix-year-old nuclear physicist, Igor Kurchatov, the head of a major laboratory since his late twenties, alerted his government then to the possible military significance of nuclear fission. Kurchatov suspected that fission research might be under way already in Nazi Germany. Soviet physicists realized in 1940 that the United States must also be pursuing a program when the names of prominent physicists, chemists, metallurgists and mathematicians disappeared from international journals: secrecy itself gave the secret away.