In April 1946, four years before I got to know him, von Neumann joined with Klaus Fuchs to offer a design for igniting a Super. In a patent application that they submitted in May of that year,{18} they gave as the title of their invention, “Improvements in methods and means for utilizing nuclear energy.” A cover sheet on their patent submission called it, more succinctly and more revealingly, “One proposed design for ‘Super.’” It is interesting to visualize their “unlikely collaboration,” as Jeremy Bernstein has called it.{19} Klaus Fuchs, lean, exceedingly quiet, always polite, a favorite babysitter among Los Alamos wives during World War II. John von Neumann, well rounded in physique and in interests, an ebullient bon vivant, probably very far down anyone’s list of prospective babysitters. What they had in common was intellectual brilliance—and curiosity.
Their patent application says that their invention was conceived on April 18, 1946 and that it was disclosed at that time to Edward Teller and Robert Serber. As it happens, Thursday, April 18, was the first day of a three-day conference at Los Alamos, chaired by Teller and devoted to prospects for the Super.{20} Earlier, just after the end of World War II, Enrico Fermi had delivered a series of lectures at Los Alamos on the Super as then conceived. (Thanks to Fuchs, notes on Fermi’s lectures promptly reached the USSR, and were eventually published in Russia in the original English as well as in translation.{21}) Now, in April, thirty-one people gathered to pull together what was known about thermonuclear burning and to look further ahead. Among the participants, in addition to Teller, Serber, von Neumann, and Fuchs, was Stan Ulam.{22}
On the patent application, von Neumann’s name is first. Four years later, Fuchs, while being interrogated in England after his arrest for spying, claimed that the idea was his.{23} We will never know. I am free to imagine the following conversation during a coffee break on April 18.
Von Neumann: Klaus, I’d like to learn more about your idea for igniting DT using radiation. Let’s get together for a drink after today’s session.
Fuchs: Ja, bestimmt, Johnny. I’ll see you at the Lodge.
Von Neumann may have known more about the paper work involved in a patent application.
As the imaginary conversation above suggests, their idea was to use radiation from a fission bomb to ignite thermonuclear fuel, but through a different process than the one later envisioned by Teller and Ulam. I save the details of the Fuchsvon Neumann invention for the next chapter.
Chapter 9
The Classical Super
According to Tom Glazer and Dottie Evans in their album “Space Songs,”{1}
It is that, and it is an H bomb. In the Sun, just as in an H bomb on Earth, hydrogen nuclei fuse to make helium nuclei, releasing energy. One big difference between the solar H bomb and the terrestrial H bomb is speed.[48] The process in the Sun is slow, very slow. It will take about ten billion years for the Sun to consume most of its hydrogen fuel (it’s about half gone now). An H bomb on Earth runs through a sizeable part of its fuel (likely to be a compound of lithium and deuterium)—in little more than a millionth of a second. What accounts for that difference in time scale is, in a word, gravity. All that holds the terrestrial H bomb together, if only for a small fraction of a second, is inertia. With stupendous acceleration, the bomb’s case gains speed and blows apart. By contrast, the Sun is held together, almost “forever,” by gravity. The thermonuclear explosion taking place in the center of the Sun, although sufficient to transform more than four million tons of mass into energy every second and sufficient to bathe the Earth in life-sustaining light, is insufficient to overcome the gravity that holds it all together. So the Sun keeps on “exploding,” and we reap the benefit.[49]
When, in the summer of 1942, Robert Oppenheimer convened a group of nine theoretical physicists (seven already recognized for their achievements,[50] and two bright “youngsters”[51]) to spend a few weeks together in Berkeley reflecting on how nuclear physics might be applied to war, their subject matter was relatively new in the firmament. Nuclear fission dated only from December 1938, and was a big surprise when it made its appearance. Nuclear fusion had been discussed earlier but came of age as an explanation of stellar energy and as quantitative science only with the work of Hans Bethe in the late 1930s. So here was this small band of “luminaries,”{2} as Oppenheimer called them, charged with the awesome task of considering how to apply the new knowledge of nuclear physics to the pursuit of war.
Fission had no sooner been discovered than physicists recognized its potential to generate large quantities of energy on Earth—either explosively or under controlled conditions. Such energy generation requires a “chain reaction.” Enrico Fermi and Leo Szilard, working at Columbia University in New York, had demonstrated the likelihood of such a chain reaction soon after Niels Bohr brought the news of fission to America in January 1939. On average, they found, a fission event stimulated by a single neutron generates more than one additional neutron, providing the possibility of an ever-increasing cascade of energy release. Later that year Hitler launched World War II. In December 1941, the United States entered the war. Add to these events the deep concern about German prowess in science, and it is easy to see why Oppenheimer’s study group was convened. And easy to see why nuclear fission was the group’s initial focus—a focus that, before long, led to “Project Y” in Los Alamos and to a fission bomb, or “A bomb.”
But the Berkeley gathering was barely under way when the subject of fusion intruded.
Within just a few days, the participants reached the conclusion that a fission bomb could probably be built—a conclusion based on what they already knew or could calculate about the mechanism of fission, the diffusion of neutrons through matter, the likelihood of random neutrons in the environment, and, not incidentally, how fast it might be possible to fire one piece of uranium or plutonium against another. Serber, Frankel, and Nelson had already been working on these questions in the weeks preceding the conference. Now they went into another room, so to speak, to address in more detail the question how a fission bomb might actually be constructed. Serber thought so carefully about that question that by the time the Los Alamos project got under way in March 1943, he was the leading expert on the prospective A bomb and was tapped to give introductory orientation lectures to newly arrived scientists an engineers on “the hill”—lectures that are still famous and still in print under the title Los Alamos Primer.{3} So oracular was Serber that when notes on his postwar lectures on nuclear physics at Berkeley were later gathered together, they were titled Serber Says; an updated version is still in print.{4}
48
Another difference is that the particular nuclear reactions are different, but both in the Sun and on Earth the net effect is the same: the transformation of hydrogen into helium.
49
The Sun’s energy output is equivalent to about ten billion terrestrial H bombs per second. Yes, we do call that slow.
50
The six notables besides Oppenheimer were Felix Bloch, Hans Bethe, Emil Konopinski (later my colleague at Indiana University), Robert Serber, Edward Teller, and John Van Vleck. Also on tap for some of the discussions were the local Berkeley experimental physicists John Manley, Edwin McMillan, and Emilio Segrè. (The group included no less than five future Nobelists: Bloch, Bethe, McMillan, Segrè, and Van Vleck.)