If a shell moves in one-third of the way toward the center you obtain under the assumption of an incompressible material a pressure in excess of eight million atmospheres. This is more than the pressure in the center of the earth and it was known to me (but not to Johnny), that at these pressures, iron is not incompressible. In fact I had rough figures for the relevant compressibilities. The result of all this was that in the implosion significant compressions will occur, a point which had not been previously discussed.1851
It had been clear from the beginning that implosion, by squeezing a hollow shell of plutonium to a solid ball, could effectively “assemble” it as a critical mass much faster than the fastest gun could fire. What von Neumann and Teller now realized, and communicated to Oppenheimer in October 1943, was that implosion at more violent compressions than Neddermeyer had yet attempted should squeeze plutonium to such unearthly densities that a solid subcritical mass could serve as a bomb core, avoiding the complex problem of compressing hollow shells. Nor would predetonation threaten from light-element impurities. Develop implosion, in other words, and they could deliver a more reliable bomb more quickly.
It was possible at that point to estimate roughly the size and shape of a bomb that worked by fast implosion. The big gun bomb would be just under 2 feet in diameter and 17 feet long. An implosion bomb—a thick shell of high explosives surrounding a thick shell of tamper surrounding a plutonium core surrounding an initiator—would be just under 5 feet in diameter and a little over 9 feet long: a man-sized egg with tail fins.
Norman Ramsey started planning full-scale drop tests that autumn as the aspens brightened to yellow at Los Alamos. He offered to practice with a Lancaster. The Air Force insisted he practice with a B-29 even though the new polished-aluminum intercontinental bombers were just beginning production and still scarce. “In order that the aircraft modifications could begin,” Ramsey writes in his third-person report on this work, “Parsons and Ramsey selected two external shapes and weights as representative of the current plans at Site Y. . . .1852, 1853 For security reasons, these were called by the Air Force representatives the ‘Thin Man’ and the ‘Fat Man,’ respectively; the Air Force officers tried to make their phone conversations sound as though they were modifying a plane to carry Roosevelt (the Thin Man) and Churchill (the Fat Man). . . . Modification of the first B-29 officially began November 29, 1943.”
* * *
A captain of the Danish Army who was also a member of the Danish underground visited Niels Bohr at the House of Honor in Copenhagen early in 1943. After tea the two men retired to Bohr’s greenhouse where hidden microphones might not overhear their conversation. The British had instructed the underground that they would soon be sending Bohr a set of keys. Blind holes had been drilled in the bows of two of the keys, identical microdots implanted and the holes sealed. A captioned diagram located the holes. “Professor Bohr should gently file the keys at the point indicated until the hole appears,” the document explained. “The message can then be syringed or floated out onto a micro-slide.”1854 The captain offered to extract the microdot and have it enlarged. Bohr was no secret agent; he accepted the offer gratefully.
When the message arrived it proved to be a letter from James Chadwick. “The letter contained an invitation to my father to go to England, where he would find a very warm welcome,” Aage Bohr remembers. “ . . . Chadwick told my father that he would be able to work freely on scientific matters. But it was also mentioned that there were special problems in which his co-operation would be of considerable help.”1855 Bohr understood that Chadwick might be hinting about work on nuclear fission. The Danish physicist was still skeptical of its application. He would not stay in Denmark, he wrote Chadwick in return, “if I felt that I could be of real help . . . but I do not think that this is probable. Above all I have to the best of my judgment convinced myself that, in spite of all future prospects, any immediate use of the latest marvelous discoveries of atomic physics is impracticable.” If an atomic bomb were a serious possibility Bohr would leave. Otherwise he had compelling reasons to stay “to help resist the threat against the freedom of our institutions and to assist in the protection of the exiled scientists who have sought refuge here.”1856
The threat against Danish institutions that Bohr was helping to resist was peculiar to the German occupation of Denmark. Germany relied heavily on Danish agriculture, which supplied meat and butter rations to 3.6 million Germans in 1942 alone.1857 It was a labor-intensive agriculture of small farms and it could only continue with the cooperation of the farmers and, more broadly, of the entire Danish population. Not to arouse resistance the Nazis had allowed Denmark to keep its constitutional monarchy and continue to govern itself. The Danes in turn had extracted an extraordinary price for agreeing to cooperate under foreign occupation: the security of Danish Jews. To the Danes the eight thousand Jews in Denmark, 95 percent of them in Copenhagen, were Danish citizens first of all; their security was therefore a test of German good faith. “Danish statesmen and heads of government,” reports a historian, “one after the other, had made the security of the Jews a conditio sine qua non for the maintenance of a constitutional Danish government.”1858
But resistance, especially strikes and sabotage, gradually increased as the Danish people felt the occupation’s burden and as the tides of war began to turn against the Axis powers. The German surrender at Stalingrad on February 2, 1943, may have appeared to many Danes to be a turning point. Mussolini’s resignation and arrest the following summer on July 25 and the impending surrender of Italy certainly did. On August 28 the Nazi plenipotentiary for Denmark, Dr. Karl Rudolf Werner Best, presented the Danish government with an ultimatum at Hitler’s orders demanding that it declare a state of national emergency, forbid strikes and meetings and introduce a curfew, a ban on arms, press censorship at German hands and the death penalty for harboring arms and for sabotage. With the King’s permission the government refused. On August 29 the Nazis reoccupied Copenhagen, disarmed the Danish Army, blockaded the royal palace and confined the King.
One reason for the takeover was Nazi determination to eliminate the Danish Jews, whose exemption from the Final Solution infuriated Hitler. The Nazis had arrested several Jewish notables on August 29 (they had planned to arrest Bohr but had decided the deed would be less obvious during a general roundup). In early September Bohr learned from the Swedish ambassador in Copenhagen that his emigré colleagues, including his collaborator Stefan Rozental, were slated for arrest. He contacted the underground, which helped the emigrés escape across the Öresund to Sweden. Rozental endured nine stormy hours crowded with other refugees in a rowboat borrowed from a city park before his exhausted party made Swedish landfall.
Bohr’s turn came soon after. The Swedish ambassador took tea at the House of Honor on September 28 and hinted that Bohr would be arrested within a few days. Even professors were leaving Denmark, Margrethe Bohr remembers the diplomat emphasizing.1859 The next morning word came through her brother-in-law that an anti-Nazi German woman working at Gestapo offices in Copenhagen had seen orders authorized in Berlin for the arrest and deportation of Niels and Harald Bohr.
“We had to get away the same day,” Margrethe Bohr said afterward. “And the boys would have to follow later. But many were helping. Friends arranged for a boat, and we were told we could take one small bag.”1860, 1861 In the late afternoon of September 29 the Bohrs walked through Copenhagen to a seaside suburban garden and hid in a gardener’s shed. They waited for night. At a prearranged time they left the shed and crossed to the beach. A motorboat ran them out to a fishing boat. Threading minefields and German patrols they crossed the Öresund by moonlight and landed at Linhamm, near Malmö.
Bohr had learned at the last minute that the Nazis planned to round up all the Danish Jews the next evening and deport them to Germany. Leaving his wife in southern Sweden to await the crossing of their sons he rushed to Stockholm to appeal to the Swedish government for aid. He discovered that the Swe
des had offered to intern the Danish Jews but the Germans had denied that any roundup was planned.
In fact it proceeded on schedule while Bohr worked his way through the Swedish bureaucracy, but fell far short of success. The Danes, warned in advance, had spontaneously hidden their Jewish fellow citizens away. Only some 284 elderly rest-home residents had been seized.1862 The more than seven thousand Jews remaining in Denmark were temporarily safe. But few of them planned at first to leave the country; it was far from certain that Sweden would accept them and there seemed nowhere else to go.
Meeting with the Swedish Undersecretary for Foreign Affairs on September 30 Bohr had urged that Sweden make public its protest note to the German Foreign Office.1863 He saw that publicity would alert the potential victims, signal Swedish sympathy and bring pressure to bear on the Nazis to desist. The Undersecretary told him Sweden planned no further intervention beyond the confidential note. Bohr appealed to the Foreign Minister on October 2, failed to win publication of the note and determined to dispense with intermediaries. Rozental says the Danish laureate “went to see Princess Ingeborg (the sister of the Danish king Christian X) and while there expressed the desire to be received by the King of Sweden.”1864 Bohr also contacted the Danish ambassador and influential Swedish academic colleagues.1865 Rozental describes the crucial meeting with the King:
The audience . . . took place that afternoon. . . . King Gustaf said that the Swedish Government had tried a similar approach to the Germans once before, when the occupying power had started deporting Jews from Norway.1866 The . . . approach, however, had been rejected. . . . Bohr objected that in the meantime the situation had changed decisively by reason of the Allied victories, and he suggested that the offer by the Swedish government to assume responsibility for the Danish Jews should be made public. The King promised to talk to the Foreign Minister at once, but he emphasized the great difficulties of putting the plan into operation.
The difficulties were overcome. Swedish radio broadcast the Swedish protest that evening, October 2, and reported the country ready to offer asylum. The broadcast signaled a route of escape; in the next two months 7,220 Jews crossed to safety in Sweden with the active help of the Swedish coast guard. One refugee’s report of what first alerted him in hiding to the idea of escape is typical: “At the pastor’s house I heard on the Swedish radio that the Bohr brothers had fled to Sweden by boat and that the Danish Jews were being cordially received.”1867 With personal intervention on behalf of the principle of openness, which exposes crime as well as error to public view, Niels Bohr played a decisive part in the rescue of the Danish Jews.
Stockholm was alive with German agents and there was fear that Bohr would be assassinated. “The stay in Stockholm lasted only a short time,” remembers Aage Bohr. “ . . . A telegram was received from Lord Cherwell . . . with an invitation to come to England. My father immediately accepted and requested that I should be permitted to accompany him.” Aage was twenty-one at the time and a promising young physicist. “It was not possible for the rest of the family to follow; my mother and brothers stayed in Sweden.”1868
Bohr went first. The British flew their diplomatic pouch back and forth from Stockholm in an unarmed two-engine Mosquito bomber, a light, fast aircraft that could fly high enough to avoid the German anti-aircraft batteries on the west coast of Norway—flak usually topped out at 20,000 feet. The Mosquito’s bomb bay was fitted for a single passenger. On October 6 Bohr donned a flight suit and strapped on a parachute. The pilot supplied him with a flight helmet with built-in earphones for communication with the cockpit and showed him the location of his oxygen hookup. Bohr also took delivery of a stick of flares. In case of attack the pilot would dump the bomb bay and Bohr would parachute into the cold North Sea; the flares would aid his rescue if he survived.
“The Royal Air Force was not used to such great heads as Bohr’s,” says Robert Oppenheimer wryly.1869 Aage Bohr describes the near-disaster:
The Mosquito flew at a great height and it was necessary to use oxygen masks; the pilot gave word on the inter-com when the supply of oxygen should beturned on, but as the helmet with the earphones did not fit my father’s head, he did not hear the order and soon fainted because of lack of oxygen.1870 The pilot realized that something was wrong when he received no answer to his inquiries, and as soon as they had passed over Norway he came down and flew low over the North Sea. When the plane landed in Scotland, my father was conscious again.
The vigorous fifty-eight-year-old was none the worse for wear. “Once in England and recovered,” Oppenheimer continues the story, “he learned from Chadwick what had been going on.”1871 Aage arrived a week later and father and son toured Britain observing the developing activities there of the Tube Alloys project, which included a section of a pilot-scale gaseous-diffusion plant. But the center of gravity had long since shifted to the United States. The British were preparing to recover a share of the initiative by sending a mission to Los Alamos to help design the bombs; they wanted Bohr on their team to increase its influence and prestige. By then the Danish theoretician had taken what Oppenheimer calls a “good first look.” At how nuclear weapons would change the world, Oppenheimer means. He emphasizes Bohr’s developing understanding then with a potent simile: “It came to him as a revelation, very much as when he learned of Rutherford’s discovery of the nucleus [thirty] years before.”1872
So Niels Bohr prepared in the early winter of 1943 to travel to America once again with an important and original revelation in hand, this one in the realm not of physics but of the political organization of the world.
He was willing to be impressed by a mighty progress of industry. “The work on atomic energy in the USA and in England proved to have advanced much further than my father had expected,” Aage Bohr understates.1873 Robert Oppenheimer pitches his summary closer to the shock of surprise a refugee released from the suspended animation that had been occupied Denmark would have felt: “To Bohr the enterprises in the United States seemed completely fantastic.”1874
They were.
15
Different Animals
The 59,000 acres of Appalachian semiwilderness along the Clinch River in eastern Tennessee that Brigadier General Leslie R. Groves acquired for the Manhattan Engineer District as one of his first official acts, in September 1942, extended from the Cumberland foothills in a series of parallel, southwestern-running ridge valleys. Groves liked the geology, which offered isolation for his several enterprises, but the new reservation was nearly as primitive as Los Alamos would be. The Clinch, a meandering tributary of the Tennessee, defined the reservation’s southeastern and southwestern boundaries. Eastward twenty miles was Knoxville, a city of nearly 112,000, farther east the wall of Great Smoky Mountains National Park. Five unpaved county roads traversed the ninety-two square miles of depleted valleys and scrub-oak ridges, an area seventeen miles long and seven miles wide that supported only about a thousand families in rural poverty. In the ridge-barricaded valleys of this impoverished hill country, far from prying eyes, the United States Army intended to construct the futuristic factories that would separate U235 from U238 in quantity sufficient to make an atomic bomb.
To do so it had first to improve communications and build a town. Into the gummy red eastern-Tennessee clay in the winter of 1942 and the spring of 1943 its contractors cut fifty-five miles of rail roadbed and three hundred miles of paved roads and streets. They improved the important county roads to four-lane highways. Stone & Webster, the hard-pressed Boston engineering corporation, laid out a town plan so unimaginative that the MED rejected it and passed the assignment to the ambitious young architectural firm of Skidmore, Owings and Merrill, which produced a wellsited arrangement of housing using innovative new materials that saved enough money to allow for such amenities in the best residences as fireplaces and porches. The new town, planned initially for thirteen thousand workers, took its name from its location lining a long section of the northwesternmost valley: Oak Ridge. The e
ntire reservation, fenced with barbed wire and controlled through seven guarded gates, was named, after a nearby Tennessee community, the Clinton Engineer Works. Its workers would come to call it Dogpatch in homage to the hillbilly comic strip “Li’l Abner.” The new gates closed off public access on April 1.
Groves planned to build electromagnetic isotope separation plants and a gaseous-diffusion plant at Clinton; plutonium production, he realized during his first months on the project, would proceed at such a scale and generate so vast a quantity of potentially dangerous radioactivity that it would require a separate reservation of its own. Of the three processes, Ernest Lawrence’s electromagnetic method was farthest along.
Electromagnetic isotope separation enlarged and elaborated Francis Aston’s 1918 Cavendish invention, the mass spectrograph. As a 1945 report prepared by Lawrence’s staff explains, the method “depends on the fact that an electrically charged atom traveling through a magnetic field moves in a circle whose radius is determined by its mass”—which was also a basic principle of Lawrence’s cyclotron.1875 The lighter the atom, the tighter the circle it made. Form ions of a vaporous uranium compound and start them moving at one side of a vacuum tank permeated by a strong magnetic field and the moving ions as they curved around would separate into two beams. Lighter U235 atoms would follow a narrower arc than heavier U238 atoms; across a four-foot semicircle the separation might be about three-tenths of an inch. Set a collecting pocket at the point where the U235 ion beam separately arrived and you could catch the ions. “When the ions strike the bottom of the collecting pocket . . . they give up their charge and are deposited as flakes of metal.”1876 Schematically, with slotted electrodes to accelerate the ions, the arrangement would look like the illustration on page 488.