done… But the more we worked on our report, the more obvious it became that the roadblocks which I had erected for Fermi's idea were not so high after all. We hurdled them one by one, and concluded that heavy hydrogen actually could be ignited by an atomic bomb to produce an explosion of tremendous magnitude. By the time we were on our way to California… we even thought we knew precisely how to do it.
That was not news Edward Teller was likely to hide under a bushel, whatever Oppenheimer's official agenda. Bethe was ushered into the glare as the streamliner clicked west: “We had a compartment on the train to California, so we could talk freely… Teller told me that the fission bomb was all well and good and, essentially, was now a sure thing. In reality, the work had hardly begun. Teller likes to jump to conclusions. He said that what we really should think about was the possibility of igniting deuterium by a fission weapon — the hydrogen bomb.”
At Berkeley the luminaries began meeting in Oppenheimer's office, “in the northwest corner of the fourth floor of old LeConte [Hall],” an older colleague remembers. “Like all those rooms, it had French doors opening out onto a balcony, to which there was easy access from the roof. Accordingly a very strong wire netting was fastened securely over his balcony.” Only Oppenheimer had a key. “If a fire had ever started… in Oppenheimer's absence, it would have been tragic.” But the fires that summer were still only theoretical.
The theoreticians let Teller's bomb distract them. It was new, important and spectacular and they were men with a compulsion to know. “The theory of the fission bomb was well taken care of by Serber and two of his young people,” Bethe explains. They “seemed to have it well under control so we felt we didn't need to do much.” The essentials of fast-neutron fission were firm — it needed experiment more than theory. The senior men turned their collective brilliance to fusion. They had not yet bothered to name generic bombs of uranium and plutonium. But from the pre-anthropic darkness where ideas abide in nonexistence until minds imagine them into the light, the new bomb emerged already chased with the technocratic euphemism of art deco slang: the Super, they named it.
Rose Bethe, who was then twenty-four, understood instantly. “My wife knew vaguely what we were talking about,” says Bethe, “and on a walk in the mountains in Yosemite National Park she asked me to consider carefully whether I really wanted to continue to work on this. Finally, I decided to do it.” The Super “was a terrible thing.” But the fission bomb had to come first in any case and “the Germans were presumably doing it.”
Teller had examined two thermonuclear reactions that fuse deuterium nuclei to heavier forms and simultaneously release binding energy. Both required that the deuterium nuclei be hot enough when they collided — energetic enough, violently enough in motion — to overcome the nuclear electrical barrier that usually repels them. The minimum necessary energy was thought at the time to come to about 35,000 electron volts, which corresponds to a temperature of about 400 million degrees. Given that temperature — and on earth only an atomic bomb might give it — both thermonuclear reactions should occur with equal probability. In the first, two deuterium nuclei collide and fuse to helium 3 with the ejection of a neutron and the release of 3.2 million electron volts of energy. In the second the same sort of collision produces tritium — hydrogen 3, an isotope of hydrogen with a nucleus of one proton and two neutrons that does not occur naturally on earth — with the ejection of a proton and the release of 4.0 MeV of energy.
The D + D reactions' release of 3.6 MeV was slightly less by mass than fission's net of 170 MeV. But fusion was essentially a thermal reaction, not inherently different in its kindling from an ordinary fire; it required no critical mass and was therefore potentially unlimited. Once ignited, its extent depended primarily on the volume of fuel — deuterium — its designers supplied. And deuterium, Harold Urey's discovery, the essential component of heavy water, was much easier and less expensive to separate from hydrogen than U235 was from U238 and much simpler to acquire than plutonium. Each kilogram of heavy hydrogen equaled about 85,000 tons TNT equivalent. Theoretically, 12 kilograms of liquid heavy hydrogen — 26 pounds — ignited by one atomic bomb would explode with a force equivalent to 1 million tons of TNT. So far as Oppenheimer and his group knew at the beginning of the summer, an equivalent fission explosion would require some 500 atomic bombs.
That reckoning alone would have been enough to justify devoting the summer to imagining the Super a little way out of the darkness. Teller found something else as well, or thought he did, and with his usual pellmell facility he scattered it before them. There are many other thermonuclear reactions besides the D + D reactions. Bethe had examined a number of them methodically when looking for those that energized massive stars. Now Teller offered several which a fission bomb or a Super might inadvertently trigger. He proposed to the assembled luminaries the possibility that their bombs might ignite the earth's oceans or its atmosphere and burn up the world, the very result Hitler occasionally joked about with Albert Speer.
“I didn't believe it from the first minute,” Bethe scoffs. “Oppie took it sufficiently seriously that he went to see Compton. I don't think I would have done it if I had been Oppie, but then Oppie was a more enthusiastic character than I was. I would have waited until we knew more.” Oppenheimer had other urgent business with Compton in any case: the Super itself. Not to risk their loss, the bomb-project leaders were no longer allowed to fly. Oppenheimer tracked Compton by telephone at the beginning of a July weekend to a country store in northern Michigan where he had stopped to pick up the keys to his lakeside summer cottage, got directions and caught the next train east. In the meantime Bethe applied himself to Teller's calculations.
The Cornell physicist's instant skepticism gives perspective to Comp-ton's melodramatic recollection of his meeting with Oppenheimer:
I'll never forget that morning. I drove Oppenheimer from the railroad station down to the beach looking out over the peaceful lake. There I listened to his story…
Was there really any chance that an atomic bomb would trigger the explosion of the nitrogen in the atmosphere or the hydrogen in the ocean? This would be the ultimate catastrophe. Better to accept the slavery of the Nazis than to run a chance of drawing the final curtain on mankind!
We agreed there could be only one answer. Oppenheimer's team must go ahead with their calculations.
Bethe already had. “I very soon found some unjustified assumptions in Teller's calculations which made such a result extremely unlikely, to say the least. Teller was very soon persuaded by my arguments.” The arguments — Bethe's and others' — against a runaway explosion appear most authoritatively in a technical history of the bomb design program prepared under Oppenheimer's supervision immediately after the war:
It was assumed that only the most energetic of several possible [thermonuclear] reactions would occur, and that the reaction cross sections were at the maximum values theoretically possible. Calculation led to the result that no matter how high the temperature, energy loss would exceed energy production by a reasonable factor. At an assumed temperature of three million electron volts [compare the 35,000 eV known for D + D] the reaction failed to be self-propagating by a factor of 60. This temperature exceeded the calculated initial temperature of the deuterium reaction by a factor of 100, and that of the fission bomb by a larger factor… The impossibility of igniting the atmosphere was thus assured by science and common sense.
Oppenheimer returned to that good news and they proceeded with the Super. Teller recaptures the mood: “My theories were strongly criticized by others in the group, but together with new difficulties, new solutions emerged. The discussions became fascinating and intense. Facts were questioned and the questions were answered by still more facts… A spirit of spontaneity, adventure, and surprise prevailed during those weeks in Berkeley, and each member of the group helped move the discussion toward a positive conclusion.”
There was serious trouble with Teller's D + D Super. The reactions would proceed too slowly to reach ignition before the fission trigger blew the assembly apart. Konopinski came to the rescue: “Konopinski suggested that, in addition to deuterium, we should investigate the reactions of the heaviest form of hydrogen, tritium.” This, Teller explains, was at that time “only… a conversational guess.” One tritium reaction of obvious interest was the fusion of a deuterium nucleus with a tritium nucleus, D + T, which results in the formation of a helium nucleus with the ejection of a neutron and the release of 17.6 MeV of energy. The D + T reaction kindled at a mere 5,000 eV, which corresponds to a temperature of 40 million degrees. But since tritium does not exist on earth it would have to be created. Neutrons bombarding an isotope of lithium, Li6, would transmute some of that light metal to tritium much as neutrons made plutonium from U235, but the only obvious source of such necessarily copious quantities of neutrons was Fermi's unproven pile. The luminaries did, however, consider the possibility of making tritium within the Super itself by packing the bomb with a dry form of lithium, lithium deuteride. But lithium in its natural form, like
