scientific papers. That was the program the leaders of the Reich mistakenly received. Himmler regretted: he would be away from Berlin that day. Keitel was “too busy at the moment.” Raeder would send a representative. None of the leaders chose to attend.
What Heisenberg had to say might have surprised them. He emphasized atomic energy for power but also discussed military uses. “Pure ura-nium-235 is thus seen to be an explosive of quite unimaginable force,” he told his staff-level auditors. “The Americans seem to be pursuing this line of research with particular urgency.” Inside a uranium reactor “a new element is created [i.e., plutonium]… which is in all probability as explosive as pure uranium-235, with the same colossal force.” At the same time at Harnack House, where Leo Szilard once lodged, bags packed, Army Ordnance was learning that “it would suffice to bring together two lumps of this explosive, weighing a total often to a hundred kilograms, for it to detonate.”
Basic knowledge of one direct route to an atomic bomb — via plutonium — was at hand. What was lacking was money and materials. The February 26 meeting won over at least the Minister of Education. “The first time large funds were available in Germany,” Heisenberg recalled at the end of the war, “was in the spring of 1942, after that meeting with Rust, when we convinced him that we had absolutely definite proof that it could be done.” Heisenberg's “large” is relative to the modest funds that had been available before, however. Not Bernhard Rust but Albert Speer needed to be convinced of the military promise of atomic energy to swell the scale of funding anywhere near the billions of reichsmarks that production of even ten kilograms of U235 or plutonium would require.
Speer did not recall the February 26 invitation after the war. Atomic energy first came to his attention, he writes in his memoirs, at one of his regular private luncheons with General Friedrich Fromm, the commander of the Home Army. “In the course of one of these meetings, at the end of April 1942, [Fromm] remarked that our only chance of winning the war lay in developing a weapon with totally new effects. He said he had contacts with a group of scientists who were on the track of a weapon which could annihilate whole cities… Fromm proposed that we pay a joint visit to these men.” Speer also heard that spring from the president of the Kaiser Wilhelm Society, who complained of lack of support for uranium research. “On May 6, 1942, I discussed this situation with Hitler and proposed that Goring be placed at the head of the Reich Research Council — thus emphasizing its importance.”
That shift to the obese Reichsmarshal who commanded the Luftwaffe and whom Hitler had designated to be his successor carried only symbolic promotion. More crucial was a June 4 conference at Harnack House that Speer, Fromm, automobile and tank designer Ferdinand Porsche and other military and industrial leaders attended. In February Heisenberg had devoted most of his lecture to nuclear power. This time he emphasized military prospects. The secretary of the Kaiser Wilhelm Society was surprised: “The word ‘bomb’ which was used at this conference was news not only to me but for many others present, as I could see from their reaction.” It was not news to Speer. When Heisenberg took questions from the floor, one of Speer's deputies asked how large a bomb capable of destroying a city would have to be. Heisenberg cupped his hands as Fermi had done sighting down Manhattan Island from Pupin Hall. “As large as a pineapple,” he said.
After the briefings Speer questioned Heisenberg directly. How could nuclear physics be applied to the manufacture of atomic bombs? The German laureate seems to have shied from committing himself. “His answer was by no means encouraging,” Speer remembers. “He declared, to be sure, that the scientific solution had already been found… But the technical prerequisites for production would take years to develop, two years at the earliest, even provided that the program was given maximum support.” They were crippled by an absence of cyclotrons, Heisenberg said. Speer offered to build cyclotrons “as large as or larger than those in the United States.” Heisenberg demurred that German physicists lacked experience building large cyclotrons and would have to start small. Speer “urged the scientists to inform me of the measures, the sums of money and the materials they would need to further nuclear research.” A few weeks later they did, but their requests looked picayune to a Reichsminister accustomed to dealing in billions of marks. They requested “an appropriation of several hundred thousand marks and some small amounts of steel, nickel, and other priority metals… Rather put out by these modest requests in a matter of such crucial importance, I suggested that they take one or two million marks and correspondingly larger quantities of materials. But apparently more could not be utilized for the present, and in any case I had been given the impression that the atom bomb could no longer have any bearing on the course of the war.”
Speer saw Hitler regularly and duly reported the findings of the June conferences:
Hitler had sometimes spoken to me about the possibility of an atom bomb, but the idea quite obviously strained his intellectual capacity. He was also unable to grasp the revolutionary nature of nuclear physics. In the twenty-two hundred recorded points of my conferences with Hitler, nuclear fission comes up only once, and then is mentioned with extreme brevity. Hitler did sometimes comment on its prospects, but what I told him of my conferences with the physicists confirmed his view that there was not much profit in the matter. Actually, Professor Heisenberg had not given any final answer to my question whether a successful nuclear fission could be kept under control with absolute certainty or might continue as a chain reaction. Hitler was plainly not delighted with the possibility that the earth under his rule might be transformed into a glowing star. Occasionally, however, he joked that the scientists in their unworldly urge to lay bare all the secrets under heaven might some day set the globe on fire. But undoubtedly a good deal of time would pass before that came about, Hitler said; he would certainly not live to see it.
Following that, according to Speer, “on the suggestion of the nuclear physicists we scuttled the project to develop an atom bomb… after I had again queried them about deadlines and been told that we could not count on anything for three or four years.” Work on what Speer calls “an energy-producing uranium motor for propelling machinery” — the heavy-water pile — would continue. “In the upshot,” Heisenberg wrote in
“We may be engaged in a race toward realization,” Vannevar Bush wrote Franklin Roosevelt on March 9, 1942; “but, if so, I have no indication of the status of the enemy program, and have taken no definite steps toward finding out.” Why Bush was not more curious remains a mystery. Conant, Lawrence and Compton, not to mention the emigres, fretted continually about the possibility of a German bomb. It was their primary reason for urging an American bomb. It was not Bush's or Roosevelt's — to them the bomb offered offensive advantage first of all — but the two leaders were alert to the German danger and surprisingly indifferent to assessing it.
The report that accompanied Bush's letter stated that five to ten pounds of “active material” would be “fairly certain” to explode with a force equivalent to 2,000 tons of TNT, up from 600 tons in the third National Academy of Sciences report of the previous November 6. It recommended building a centrifuge plant at a cost of $20 million that could produce enough U235 for one bomb a month and estimated that such a plant could be completed by December 1943. A gaseous diffusion plant, its cost unspecified, might deliver by the end of 1944. An electromagnetic separation plant — Ernest Lawrence's project — won the most attention in the report: it might “offer a short-cut,” wrote Bush, and deliver “fully practicable quantities of material by the summer of 1943, with a time saving of perhaps six months or even more.” In summary, “present opinion indicates that successful use is possible, and that this would be very important and might be determining in the war effort. It is also true that if the enemy arrived at results first it would be an exceedingly serious matter. The best estimate indicates completion in 1944, if every effort is made to expedite.”
Roosevelt responded two days later: “I think the whole thing should be pushed not only in regard to development, but also with due regard to time. This is very much of the essence.” Time, not money, was becoming the limiting factor in atomic bomb development.
A meeting on May 23 brought all the program leaders together with Conant to decide which of several methods of making a bomb should be moved on to the pilot-plant and industrial engineering stages. The centrifuge, gaseous barrier diffusion, electromagnetic and graphite or heavy-water plutonium-pile approaches all looked equally promising. Given wartime scarcities and budget priorities, which should be advanced? Conant used an arms-race argument to identify the point of decision:
