On August 13, 1943, the first drop tests of a prototype atomic bomb were made at the Dahlgren Naval Proving Ground [by a Navy TBF aircraft] to determine stability in flight. These tests were on a 14/23 scale model of a bomb shape which was then thought probably suitable for a gun assembly. Essentially, the model consisted of a long length of 14-inch pipe welded into the middle of a split standard 500-pound bomb. It was officially known at Dahlgren as the “Sewer Pipe Bomb.”… The first test… was an ominous and spectacular failure. The bomb fell in a flat spin such as had rarely been seen before. However, an increase in fin area and a forward movement of the center of gravity provided stability in subsequent tests.
In the meantime Seth Neddermeyer, whose implosion experimentation group Parsons inherited, had visited a U.S. Bureau of Mines laboratory at Bruceton, Pennsylvania, to experiment with high explosives. Edwin McMillan, who was interested in implosion, went with the Caltech physicist:
At that point it was just Seth and myself with a few helpers. The first cylindrical implosions were done at Bruceton. You take a piece of iron pipe, wrap the explosives around it, and ignite it at several points so that you get a converging wave and squash the cylinder in. That was the birth of the experimental work on implosion, long before experimental work on the gun method.
Back at Los Alamos Neddermeyer set up a small research station on South Mesa, the next mesa south of the Hill across Los Alamos canyon. He fired his first tests in an arroyo on Independence Day, 1943, using iron pipe set in cans packed with TNT. Experimenting with cylinders rather than spheres simplified calculation. Because he wanted to recover the results he packed only limited amounts of explosive. “Those tests of course could not be very sophisticated,” says McMillan. “… They did show that you could take metal pipes and close them right in so that they became like solid bars, indicating that this was a practical method.” They also showed that the squeeze was far from uniform: the pipes emerged from the arroyo dust twisted and deformed.
When Parsons, a thoroughly pragmatic engineer, had time to look over Neddermeyer's work he was openly contemptuous. He doubted if implosion could ever be made reliable enough for field use. Neddermeyer presented his initial results at one of the weekly colloquia Oppenheimer had instituted at Hans Bethe's suggestion to keep everyone with a white badge — everyone cleared for secrets — informed of Tech Area progress. Richard P. Feynman, a brilliant, outspoken New York-born graduate-student theoretician from Princeton, summarized the opinion of the assembly in a phrase: “It stinks.” In the name of lightheartedness Parsons was cruder. “With everyone grinding away in such dead earnest here,” he told the group, “we need a touch of relief. I question Dr. Neddermeyer's seriousness. To my mind he is gradually working up to what I shall refer to as the Beer-Can Experiment. As soon as he gets his explosives properly organized, we will see this done. The point to watch for is whether he can blow in a beer can without splattering the beer.” Implosion was even harder to do than that.
John von Neumann, the Hungarian mathematician who had come to the United States in 1930 and joined the Institute for Advanced Study, had been examining for the NDRC the complex hydrodynamics of shock waves formed by shaped charges, technology which was being applied to the American tank-killing infantry weapon known as the bazooka. Like Rabi, von Neumann had agreed to serve as an occasional Oppenheimer consultant. He visited Los Alamos at the end of the summer and looked into implosion theory, another warren of hydrodynamic complexity. Neddermeyer had devised “a simple theory that worked up to a certain level of violence in the Shockwave.” Von Neumann, he says, “is generally credited with originating the science of large compressions. But I knew it before and had done it in a naive way. Von Neumann's was more sophisticated.”
“Johnny was quite interested in high explosives,” Edward Teller remembers. Teller and von Neumann renewed their youthful acquaintance during the mathematician's visit to the Hill. “In my discussions with him some crude calculations were made,” Teller continues. “The calculation is indeed simple as long as you assume that the material to be accelerated is incompressible, which is the usual assumption about solid matter… In materials driven by high explosives, pressures of more than 100,000 atmospheres occur.” Von Neumann knew that, Teller says, as he did not. On the other hand:
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.
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… 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.” 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 Chad-wick. “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.” 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.”
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. 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
