The Making of the Atomic Bomb

of the river, then across the frozen river and into the open woods beyond.

“But it's impossible,” Frisch remembers them saying in their collective effort to understand. “You couldn't chip a hundred particles off a nucleus in one blow. You couldn't even cut it across. If you tried to estimate the nuclear forces, all the bonds you'd have to cut all at once — it's fantastic. It's quite impossible that a nucleus could do that.” Thirty years afterward Frisch summarized their thinking in more formal terms:

But how could barium be formed from uranium? No larger fragments than protons or helium nuclei (alpha particles) had ever been chipped away from nuclei, and the thought that a large number of them should be chipped off at once could be dismissed; not enough energy was available to do that. Nor was it possible that the uranium nucleus could have been cleaved right across. Indeed a nucleus was not like a brittle solid that could be cleaved or broken; Bohr had stressed that a nucleus was much more like a liquid drop.

The liquid-drop model made a division of the nucleus seem possible. They sat down on a log. Meitner found a scrap of paper and a pencil in her purse. She drew circles. “Couldn't it be this sort of thing?”

Frisch: “Now, she always rather suffered from an inability to visualize things in three dimensions, whereas I had that ability quite well. I had, in fact, apparently come around to the same idea, and I drew a shape like a circle squashed in at two opposite points.”

“Well, yes,” Meitner said, “that is what I mean.” She had meant to draw what Frisch had drawn, a liquid drop elongated like a dumbbell, but had drawn it end-on, indicating with a smaller dashed circle inside a larger solid circle the dumbbell's waist.

Frisch: “I remember that I immediately at that instant thought of the fact that electric charge diminishes surface tension.” The liquid drop is held together by surface tension, the nucleus by the analogous strong force. But the electrical repulsion of the protons in the nucleus works against the strong force, and the heavier the element, the more intense the repulsion. Frisch continues:

And so I promptly started to work out by how much the surface tension of a nucleus would be reduced. I don't know where we got all our numbers from, but I think I must have had a certain feeling for the binding energies and could make an estimate of the surface tension. Of course we knew the charge and the size reasonably well. And so, as an order of magnitude, the result was that at a charge [i.e., an atomic number] of approximately 100 the surface tension of the nucleus disappears; and therefore uranium at 92 must be pretty close to that instability.

They had discovered the reason no elements beyond uranium exist naturally in the world: the two forces working against each other in the nucleus eventually cancel each other out.

They pictured the uranium nucleus as a liquid drop gone wobbly with the looseness of its confinement and imagined it hit by even a barely energetic slow neutron. The neutron would add its energy to the whole. The nucleus would oscillate. In one of its many random modes of oscillation it might elongate. Since the strong force operates only over extremely short distances, the electric force repelling the two bulbs of an elongated drop would gain advantage. The two bulbs would push farther apart. A waist would form between them. The strong force would begin to regain the advantage within each of the two bulbs. It would work like surface tension to pull them into spheres. The electric repulsion would work at the same time to push the two separating spheres even farther apart.

Eventually the waist would give way. Two smaller nuclei would appear where one large nucleus had been before — barium and krypton, for example:

“Then,” Frisch recalls, “Lise Meitner was saying that if you really do form two such fragments they would be pushed apart with great energy.” They would be pushed apart by the mutual repulsion of their gathered protons at one-thirtieth the speed of light. Meitner or Frisch calculated that energy to be about 200 MeV: 200 million electron volts. An electron volt is the energy necessary to accelerate an electron through a potential difference of one volt. Two hundred million electron volts is not a large amount of energy, but it is an extremely large amount of energy from one atom. The most energetic chemical reactions release about 5 eV per atom. Ernest Lawrence was that year building a cyclotron with a nearly 200-ton magnet with which he hoped to accelerate particles by as much as 25 MeV. Frisch would calculate later that the energy from each bursting uranium nucleus would be sufficient to make a visible grain of sand visibly jump. In each mere gram of uranium there are about 2.5 x 10²¹ atoms, an absurdly large number, 25 followed by twenty zeros: 2,500,000,000,000,000,000,000.

They asked themselves what the source of all that energy could be. That was the crux of the problem and the reason no one had credited the possibility before. Neutron captures that had been observed before had involved much smaller energy releases.

When she was thirty-one, in 1909, Meitner had met Albert Einstein for the first time at a scientific conference in Salzburg. He “gave a lecture on the development of our views regarding the nature of radiation. At that time I certainly did not yet realize the full implications of his theory of relativity.” She listened eagerly. In the course of the lecture Einstein used the theory of relativity to derive his equation E = mc², with which Meitner was then unfamiliar. Einstein showed thereby how to calculate the conversion of mass into energy. “These two facts,” she reminisced in 1964, “were so overwhelmingly new and surprising that, to this day, I remember the lecture very well.”

She remembered it in 1938, on the day before Christmas. She also “had the packing fractions in her head,” says Frisch — she had memorized Francis Aston's numbers for the mass defects of nuclei. If the large uranium nucleus split into two smaller nuclei, the smaller nuclei would weigh less in total than their common parent. How much less? That was a calculation she could easily work: about one-fifth the mass of a proton less. Process one- fifth of the mass of a proton through E = mc². “One fifth of a proton mass,” Frisch exclaims, “was just equivalent to 200 MeV. So here was the source for that energy; it all fitted!”

They converted not quite so suddenly as that. They may have been excited, but Meitner at least was profoundly wary. This new work called her previous four years' work with Hahn and Strassmann into doubt; if she was right about the one she was wrong about the other, just when she had escaped from Germany into the indifferent world of exile and needed most to confirm her reputation. “Lise Meitner sort of kept saying, ‘We couldn't have seen it. This was so totally unexpected. Hahn is a good chemist and I trusted his chemistry to correspond to the elements he said they corresponded to. Who could have thought that it would be something so much lighter?’”

Christmas dinner at the Bergiuses' came and went. Frisch skied and Meitner walked. Nineteen thirty-eight was ticking to its end. With a week to pass in a small village they would certainly have visited the fortress and looked down from its ramparts onto the snow-covered valley, onto centuries of violent graves. Though they understood its energetics now, the discovery was still only physics to them; they did not yet imagine a chain reaction.

Harm's letter of December 21, confirming lanthanum, was still not forwarded from Stockholm, nor was the carbon copy of the Naturwissens-chaften paper. Hahn was eager to win Meitner's support and wrote Kungalv directly on the Wednesday after Christmas to woo her. Careful not to seem to usurp her place, he called the discovery his “barium fantasy” and questioned everything except the presence of barium and the absence of actinium, taking the humble chemist's part. “Naturally, I would be very interested to hear your frank opinion. Perhaps you could compute and publish something.” He had continued to hold off telling other physicists, though he itched for physical confirmation of his chemistry. It was as though a maker of hand axes had discovered fire by striking flints while the sorcerers pondered how to harness lightning. He might hardly believe his luck and urgently seek their authentication even though he knew what burned his hand was real.

The letter reached Kungalv on Thursday; by return mail that day Meitner responded that the radium-barium finding was “very exciting. Otto R[obert] and I have already puzzled over it.” But she let slip no answer to the puzzle and she asked about the lanthanum result.

Friday she sent Hahn a postcard: “Today the manuscript arrived.” An important page was missing but it was all “very amazing.” Nothing more; Hahn must have bitten his Up.

In Dahlem Rosbaud passed along the galley proofs. Hahn was more certain now of his findings. The