by George Dyson
“It is likely that a super-bomb can be constructed and will work,” wrote Teller in a summary he personally inserted into the final report.36 The conference also produced a patent disclosure, filed jointly by von Neumann and British physicist (and Soviet agent) Klaus Fuchs, for the invention, on or about April 18, 1946, at Los Alamos, New Mexico, of a “proposed design for ‘Super’ ” described as “a device for initiating a thermo-nuclear reaction which employs a quantity of fissile material adaptable to sustain a neutron divergent chain reaction [and] a massive quantity of material in which a thermo-nuclear reaction can be maintained.”37 When Klaus Fuchs was revealed as a Soviet spy, von Neumann knew better than anyone else how much useful information had—and had not—been passed to the Soviet side.
It would take Robert Richtmyer two years to model what had happened in the first microseconds of the Trinity test. Until there were better computers available, further progress was limited, even though, according to Carson Mark, who succeeded Richtmyer as director of T-Division, half of his group’s effort was devoted to the Super between 1946 and 1949. Von Neumann, impatient to get started, began writing code for the machine that did not yet exist. “In that T-Division coffee room, I had watched Johnny, when he was building his Princeton machine, cover a blackboard with the first stirrings of flow-diagram coding,” remembers Françoise Ulam, “while casting unconscious sidelong glances at every feminine pair of legs that went by.”38
After the Soviet explosion of a nuclear weapon on August 29, 1949 (named “First Lightning” by the Soviets and “Joe-1” by the United States), the General Advisory Committee of the Atomic Energy Commission was asked for their opinion on whether the United States should undertake the development of the hydrogen bomb. The answer was no. “It is not a weapon which can be used exclusively for the destruction of material installations of military or semimilitary purposes,” Oppenheimer explained in the introduction to the committee’s report. “Its use therefore carries much further than the atomic bomb itself the policy of exterminating civilian populations. We all hope that by one means or another, the development of these weapons can be avoided.”39
“Its use would involve a decision to slaughter a vast number of civilians,” the majority, including James Conant as well as Oppenheimer, concurred. “We believe that the psychological effect of the weapon in our hands would be adverse to our interest.… In determining not to proceed to develop the Super bomb, we see a unique opportunity of providing by example some limitations on the totality of war and thus of limiting the fear and arousing the hopes of mankind.” An even stronger minority addendum, signed by Enrico Fermi and Isidor Rabi, added that “It is necessarily an evil thing considered in any light.” Von Neumann, not yet a member of the commission, strongly disagreed. “I think that there should never have been any hesitation,” he wrote in 1950, after Truman had made the decision to move full speed ahead.40
Ulam believed that much of this soul-searching was unnecessary, because the ENIAC calculations were flawed and Teller’s Super would turn out to be a dud. With the assistance of Cornelius Everett, a former colleague from Madison, he undertook a first-approximation check on the earlier results, using the hand (and punched card) computing techniques that had been developed for implosion calculations during the war. “Stan, who had a conceptual hunch that the Super Teller envisaged was not practical, undertook the simplified calculations, first with Everett, then with us, the data analysts,” says Françoise, who was working in the Los Alamos hand computing division at the time. “In a couple of months these calculations confirmed his feelings. In other words, Stan was the first to blow the whistle: it was not going to work. Everyone else—von Neumann, Admiral Strauss, the head of the AEC, and the military were all for pursuing and experimenting with Teller’s scheme.”41
“The degree of hope, if you will, or fear, that such a thing is possible gradually changed and, as a matter of fact, it was not even continually in one direction,” Ulam later testified.42 The doubts raised by Ulam and Everett put Teller on the defensive, and left von Neumann impatient to determine whose numbers were correct. He commandeered the ENIAC, and was first in line to use any available new machine. “When the hydrogen bomb was developed,” he testified at the Oppenheimer hearings in 1954, “heavy use of computers was made [but] they were not yet generally available.… It was necessary to scrounge around and find a computer here and find a computer there which was running half the time and try to use it.”43 Ralph Slutz, who had left the IAS to supervise construction of the SEAC, for the Bureau of Standards in Washington, D.C., remembers “a couple people from Los Alamos” (Metropolis and Richtmyer) showing up as soon as the computer began operating, around Easter of 1950, “with a program which they were terribly eager to run on the machine … starting at midnight, if we would let them have the time.”44
“When the larger and more precise electronic calculations of von Neumann and others slowly and gradually brought confirmation of Stan’s point of view, it was a real setback for the whole enterprise,” says Françoise. “In spite of an initial, hopeful-looking ‘flare-up,’ the whole assembly started to cool down,” adds Stan. “Every few days Johnny would call in some results. ‘Icicles are forming,’ he would say.” Much as he believed Teller’s faith in the Super to be misplaced, the germs of an alternative were incubating in the back of Ulam’s mind. “Cycle 10 has been going for the last 24 hours,” he reported to von Neumann on January 27, 1950 (the day Klaus Fuchs signed his confession), while the punched card calculations at Los Alamos were under way. “By the way: warning about conduction: we had to divide the time interval into 5! (sic!) on cycle 9. Hydrodynamics, so far at least, far from being a danger is the only hope that the thing will go!”45
The classical Super depended on heating the deuterium (or deuterium-tritium) fuel to the 100 million degrees or more required to ignite. If this was going to happen at all, it had to happen quickly, before the expansion of hot material blew things apart and escaping radiation allowed things to cool off. “For the ‘Super,’ the hydrodynamical disassembly proceeded faster than the buildup and maintenance of the reaction,” Ulam later explained.46 The bomb would fizzle out.
Ulam had noted, in monitoring the progress of the Super calculations, that hydrodynamic forces, rather than diminishing the prospects for thermonuclear ignition by disassembling things before the fuel could become hot enough to ignite, might instead be persuaded to work the other way. Increased pressure brings increased density. And increased density brings not only higher temperatures but also higher opacity. As you squeeze a region of hot plasma, it not only gets hotter, it gets blacker. And there were ways to take advantage of that.
“What you tell me about the events of cycle 10 is very interesting,” von Neumann replied on February 7. “I need not tell you how I feel about the ‘victory.’ There are, however, plenty of problems left.”47 The “victory” was the public announcement, by President Truman on January 31, that, in response to the Soviet bomb test of August 29 and against the advice of Oppenheimer and the General Advisory Committee, he had “directed the Atomic Energy Commission to continue its work on all forms of atomic weapons, including the so called hydrogen or superbomb.” Lewis Strauss, armed with Klaus Fuchs’s confession, had claimed that the Soviets, without any Oppenheimers to restrain them, might already be ahead. Teller finally had access to unlimited resources, and an actual test, “Greenhouse George,” was scheduled that would show whether a small sample of deuterium-tritium would ignite.
Then Ulam came up with a surprise. According to Bethe, he was not even thinking about the Super problem, but about how it might be possible to construct very-high-yield, two-stage fission bombs. “Unbeknownst to me, Stan had continued to think about the problems in a round-about sort of way, more for their scientific challenge than political or military importance,” Françoise explains. “And suddenly, he came upon a totally new and intriguing approach.”
“I found him at home at noon staring intensely out of a
window with a very strange expression on his face,” she says.
I can never forget his faraway look as peering unseeing in the garden, he said in a thin voice—I can still hear it—“I found a way to make it work.”
“What work?” I asked.
“The Super,” he replied. “It is a totally different scheme, and it will change the course of history.”48
Ulam spoke to Carson Mark and Norris Bradbury immediately, and Edward Teller the following day. Teller, who had been working on the problem for almost a decade, immediately improved upon Ulam’s suggestion, and brought in a young Viennese physicist, Frederic de Hoffmann, who performed the initial calculations establishing the probable feasibility of the new approach. “I wanted to do something about the hydrogen bomb and nobody else wanted to,” Teller says, “and the one man who wanted to do it more than I was Freddy de Hoffmann.”49 It was de Hoffmann, twenty years old at the time, who had calculated the ballistic trajectories for the two bombs that were dropped on Japan.
Teller titled his 1955 review of the H-bomb’s development “The Work of Many People”—a genuine attempt to share the credit, in the face of widespread criticism, with those who had helped. Hans Bethe wrote his own account in 1954, which he opened by giving Teller full credit, during the atomic bomb development, for being “the first to suggest that the implosion would compress the fissile material to higher than normal density inside the bomb.” But he refused to assign Teller chief credit for the breakthrough on the hydrogen bomb. “It is difficult to describe to a nonscientist the novelty of the new concept,” he wrote. “It was an entirely unexpected departure from the previous development. It was also not anticipated by Teller, as witness his despair immediately preceding the new concept.”50
Ulam suggested that overenthusiasm for the classical Super may have delayed Teller’s own arrival at a successful design, and emphasized that the real credit should go to “the enormous number of calculations, all the studies of the general physics of the processes, the engineering planning, all combined with the necessity of predicting and avoiding ‘side effects,’ any one of which could ruin the success of the device.” And if individuals were to be singled out, he noted to Bethe, “it would be hard to exaggerate the importance of the contributions made by Fermi in the decisive switch from the original, hopeless approach.”51
The breakthrough, now known as the Teller-Ulam invention, appeared in February 1951, and was published (in an edition of twenty secret copies) under joint authorship on March 9, 1951. “The arrangement might be called heterocatalytic, involving as it does a setting off of a reaction in one system by a reaction started in another,” Teller and Ulam explained.52 “This new idea transformed the concept of the Super into the beautifully workable hydrogen bomb,” says Harris Mayer, who helped resolve the details of a new concept that was “remarkably complex, and devilishly interesting.” Mayer’s specialty was radiation opacity—how some states of matter are more opaque to radiation at certain temperatures, and some less. Understanding the details can help tailor things so radiation flows where you want it to, and when it reaches its destination is either absorbed or transformed. “Nature had provided generous margins,” he says, “in the properties of radiation flow.” Mayer adds, however, that “nobody thought that Stan was the significant person in the new hydrogen bomb development until the Oppie affair. And the Oppie affair got everybody so mad at Edward that then they spoke of the Teller-Ulam concept.”53
“Ulam kept pressing for squeezing the secondary,” says Theodore B. Taylor, the gifted Los Alamos bomb designer who was friends with both Ulam and Teller at the time. “Now whether he did that with the key perception that then the inverse Compton effect wouldn’t drain the energy, that things would be much closer to equilibrium and that at these high densities you get a fast enough reaction rate and a high enough temperature rise so that it would be very efficient, I don’t know who came up with that.” Taylor gives credit to both sides. “My sense of things is that this direct compression is something that they both saw at the same time, that compressing it was the way to go,” he says. “Then the question was: How do you compress it? The subject had been brought up by Ulam, and what I’ve come to believe is that Teller said, ‘Oh, that’s terrific, but let’s use the radiation, not the hydrodynamics.’ And then everything became clear.”54
The classical Super had been going nowhere for eight years, and now there was a design that went from concept to successful test in nineteen months. The first meeting to discuss the implications of the new approach—which included Oppenheimer, Teller, von Neumann, Bethe, Fermi, and John Wheeler—was held in Oppenheimer’s office at the Institute for Advanced Study in June 1951. “All the top men from every laboratory sat around this table and we went at it for 2 days,” testified Gordon Dean, a Lehman Brothers partner who became chairman of the AEC. “At the end of those 2 days we were all convinced, everyone in the room, that at last we had something.… The bickering was gone.… That is when it began to roll and it rolled very fast.”55
The IAS computer had at last become available, and with Oppenheimer’s support, it was 1943 all over again. “When I saw how to do it, it was clear to me that one had to at least make the thing,” Oppenheimer testified at his security hearing. “The program we had in 1949 was a tortured thing that you could well argue did not make a great deal of technical sense. It was therefore possible to argue that you did not want it even if you could have it. The program in 1951 was technically so sweet that you could not argue about that.”56 Everything depended on the H-bomb working on the first try. “Mike, the first H-bomb,” says Marshall Rosenbluth, “in fact was quite over-designed.”57
Strauss became impatient, and increasingly suspicious of Oppenheimer, even though he had appointed Oppenheimer to the directorship of the Institute and knew that he was now helping the H-bomb effort move ahead. Strauss complained, according to the notes of a conversation concerning the Oppenheimer camp’s opposition to the H-bomb, that “first they objected on moral grounds; then they objected on the ground that there were no military targets; then they objected on the ground that the super would be too costly in term of neutrons as compared with the plutonium which could be alternatively produced; and now they want to build [deleted] which would not be a real open-ended weapon at all.”58 The deletion probably refers to the Super Oralloy Bomb (SOB), the largest fission weapon ever produced. Designed by Ted Taylor, the Super Oralloy Bomb yielded 500 kilotons in the Ivy King test at Enewetak on November 15, 1952, and was intended to demonstrate that for any conceivable military purpose, half a megaton should be enough. Half a megaton was not enough for Lewis Strauss.
Relations between the IAS and the AEC were complex. “By early 1952 there was some change in this game of musical chairs,” Klári explains. “Johnny became a member of the General Advisory Committee of which Oppenheimer was still the chairman; however, Lewis Strauss was no longer one of the commissioners, but had become the President of the Board of Trustees at the Institute for Advanced Study, of which Johnny was a member and Robert the director.” Strauss, appointed to the AEC by President Truman in 1947, served until 1950 and was reappointed in 1953, as chairman, by President Eisenhower. “At the beginning of 1954, the year that all hell broke loose,” says Klári, “these were the relative positions: at the Institute, unchanged—Johnny, member—Oppenheimer, director—Strauss, president. At the Atomic Energy Commission: Johnny member of the G.A.C., Lewis chairman of the Commission, Robert completely not.”59
The extent of actual weapons work at the Institute was kept secret, but the comings and goings of people from Los Alamos and the AEC were hard to conceal, and the purpose not hard to guess. “The objective,” says Ted Taylor of the von Neumann computer, “was pretty specifically to be able to do the coupled hydrodynamics and radiation flow necessary for H-bombs.”60 John Wheeler moved his small team from Los Alamos to Princeton University, launching “Project Matterhorn,” under a subcontract with Los Alamos, to prepare thermonuclear c
alculations for the Institute machine until Los Alamos could build a computer for itself.
“The mathematicians certainly knew there was classified work going on,” says Freeman Dyson, who arrived in 1948. “They may not have known that it was hydrogen bombs, but it was pretty obvious. And they were strongly opposed to that.” The groundswell of public opinion against atmospheric testing would come later, but opposition to the hydrogen bomb on humanitarian principles was there from the start. “It made a very bad impression to have this safe that used to be in Fuld Hall, with all Oppenheimer’s secrets in it,” Dyson explains. “And it wasn’t just the safe, there were two armed guards who were there too. It really looked rather formidable.” Virginia Davis, who came to the Institute in 1952 with logician Martin Davis, remembers writing “STOP THE BOMB” in the dust on von Neumann’s car.
The air force, which would be taking custody of any deliverable weapons, had to be reminded that the Institute was not Los Alamos or RAND. According to Oppenheimer, at a briefing given by Edward Teller and the RAND Corporation, Secretary of the Air Force Thomas K. Finletter “got to his feet and said ‘give us this weapon and we will rule the world.’ ” Oppenheimer, who had willingly served the army under General Groves, resisted the air force. “Johnny steadily increased his defense activities,” says Klári, “while Robert gradually moved away from them.”61
Julian Bigelow received his Q clearance, allowing access to atomic secrets, on February 23, 1950. On March 14, the AEC advised the Institute that all working under AEC contracts “are instructed to refrain from publicly stating facts or giving comment on any thermonuclear reactions,” and on March 17 it was clarified, in response to strong objections from Oppenheimer, that these restrictions would “still permit unclassified discussions of what might be called the classical thermonuclear reactions as long as there is no reference to their relation to weapons.”62 Stellar evolution codes could be run in the open, but weapons codes had to be run in the dark.