CHAPTER THREE

SPLIT ATOMS

THE FAMILY HAD BEEN in New York for just two weeks, looking for an apartment and getting settled, when the liner Drottningholm sailed into New York harbor. On an upper deck, peering down at the welcoming crowd was the Danish Nobel Prize-winning physicist Niels Bohr. Bohr was a mentor to Werner Heisenberg and a luminary in the theoretical physics community. He brought with him incredible news: two German scientists had proven Einstein’s theory correct. Otto Hahn and Fritz Strassmann had split the uranium atom, unlocking a mighty storehouse of atomic energy. And they had done it using the procedure Fermi had pioneered: bombarding uranium with slow-moving neutrons.

In the world of physics there were two kinds of scientists: the theoreticians like Einstein who use their fertile imaginations to explain natural phenomena, and the experimentalists who search for answers using laboratory experiments. As Fermi’s work in Rome demonstrated, he excelled at both; a rare combination.

Back in Rome his students, awed by Fermi’s brilliance, had nicknamed their professor “Il Papa,” the Pope, because they believed that he, just like the head of the Catholic Church, was infallible. But the atomic nucleus was a place of mystery, a new world that scientists had only begun to explore. And this time, “Il Papa” had lost his way.

The news that Hahn and Strassmann had split the atom left Fermi uneasy. His thoughts raced back to his own neutron bombardment experiments at the University of Rome five years earlier. He recalled his difficulty trying to decipher the uranium results. Was element 93 not a new atom as he had thought? Was it, in fact, a composite of split atoms? The more he considered the possibility, the more he realized the answer had to be yes. He too had split the atom, but didn’t know it.

Embarrassed, Fermi acknowledged the apparent error in a footnote to his not-yet-printed Nobel acceptance speech: “The discovery by Hahn and Strassmann,” he wrote, “. . . makes it necessary to re-examine all the problems of transuranic elements, as many of them might be found to be products of a splitting of uranium.” 23

THE RESULTS OF THE NEUTRON bombardment experiment had baffled the two Germans just as they had Fermi. It appeared that a neutron had penetrated the uranium nucleus and broken off a large chunk of the atom. But that was impossible. Scientists using atom-smashing cyclotrons had fired particles packing 9,000,000 volts of energy at atoms and failed to do any damage. 24 The idea that an exhausted neutron could break one apart was beyond absurd.

Unable to explain what had happened, the two German chemists wrote their former collaborator Lise Meitner, a Jewish physicist who had been forced to flee Nazi Germany. It was she who unraveled the mystery.

In the atom’s nucleus, positively charged protons repel each other. They push; they shove; they battle trying to get away from each other, but never can. That’s because something called the “strong force” holds them captive. Fermi’s snail-paced neutrons had disrupted that force. The question was how.

Meitner was on Christmas vacation, skiing with her nephew, when the answer came to her. All of a sudden she understood what her German colleagues had not. A tiny, slow-moving neutron has very little energy. But, like the straw that broke the camel’s back, it was just enough to tip the balance. The additional muscle it brought to the fight enabled the protons in the uranium nucleus to overpower the restraining strong force. In a process later called nuclear fission, the nucleus split, expelled two of its neutrons and along with them a burst of atomic energy. Why slowing the neutrons down made them more effective was still a mystery.

The energy generated by each atom was infinitesimal, only enough “to make a visible grain of sand visibly jump.” 25 But multiply that energy by trillions and trillions of atoms, and the total would become staggering. According to one estimate, “the energy in the mass of one raisin would supply most of New York City’s energy needs for a day.” 26

The news of Hahn and Strassmann’s breakthrough rocked the scientific world. Distinguished scientists in three countries had performed Fermi’s neutron bombardment experiment, and none of them had understood what happened. “Everyone kicked themselves,” says atom bomb historian Richard Rhodes.

Rhodes interviewed U.C. Berkeley physicist Glenn Seaborg who had also missed the historic discovery. “And he told me, ‘I spent the next three days in a blue funk.’” 27

NOW THAT THE ATOMIC GENIE was out of the bottle, there was no turning back. Each new experiment brought new evidence that split atoms emit neutrons. In theory, those neutrons could go on to split more atoms, yielding more neutrons triggering a chain of events that would produce more and more atomic energy.

In late January 1939, Fermi and colleagues at Columbia University confirmed the theory. Their chain reaction generated what The New York Times called “the greatest amount of atomic energy so far liberated by man on earth.” 28 But the Columbia research only hinted at the atom’s true potential. Once dismissed as the stuff of science fiction, an atom bomb was now—theoretically at least—within reach. Scientists were at the frontier of a strange new world that was at once exciting and terrifying. The earth’s atmosphere was 78 percent nitrogen. Would an atom bomb ignite the atmosphere? Would it turn the hydrogen in the oceans ablaze? Scientists didn’t think so, but no one could say with absolute certainty.

AS THE RUMBLINGS OF A WAR in Europe grew louder, the specter of Adolf Hitler armed with an atomic bomb terrified many scientists, especially those Jewish scientists who had fled Nazi anti-Semitism. They took comfort, however, knowing that myriad problems still lay ahead. When the Danish physicist Niels Bohr, now a visiting professor at Princeton University, identified one problem that seemed to push the possibility of a bomb into the far distant future, they all breathed a sigh of relief.

Bohr was at breakfast with colleagues in the Princeton faculty club when the conversation turned to nuclear fission. The scientists were puzzling over why slow-moving neutrons had split the atom’s nucleus and not fast ones, when Bohr, trailed by a young associate, suddenly jumped up and raced to his office across campus.

Breaking several pieces of chalk in his rush to get his thoughts down on a blackboard, he drew a diagram of two atomic nuclei—one, extremely rare; the other, extremely common. The rare atom, which accounted for less than one percent of all uranium atoms in nature, was Uranium 235 (U235). Admiring his work, Bohr explained to his associate that the U235 nucleus was the secret storehouse where nature hid its nuclear energy. The other atom, Uranium 238 (U238), was, in effect, its security guard. For every storehouse there were 139 guards standing sentry, capturing neutrons, absorbing them and preventing them from gaining access to the U235 storehouse. 29

Under normal conditions the defenses were impenetrable, but nature’s grand design had one weakness. Nature had programmed U238 atoms to capture intermediate and fast-moving neutrons, but not slow ones. Low-energy slowpokes could sneak past the guards and break into the nuclear energy storehouses undetected. Fermi didn’t know it in 1934, but he had stumbled upon a back door, a secret passage into those storehouses open only to slowpokes.

That discovery enabled him and Hahn and Strassmann in Germany to split the U235 nucleus. As long as the scarce U235 atoms remained buried among all those U238 atoms, the chances of anyone releasing enough nuclear energy to fuel a bomb were virtually nil.

In 1939 there was no practical way to separate U235 atoms from U238, at least not on the mammoth scale required. And, even if one were invented, conventional wisdom held that it would take years to harvest enough U235 atoms for a single bomb. Some experts estimated that “over 191 years would [roll by] before a single gram of U235 was obtained, and 75,000 years for a single pound.” 30 “It can never be done,” Bohr told his colleagues, “unless you turn the United States into one huge factory.” 31

H.G. Wells’ vision of a world shattered by “crimson conflagrations of atomic bombs” was still just science fiction. Across the Atlantic, British Prime Minister Winston Churchill assured his Secretary of State for Air, “the fear that this new discovery has provided the Nazis with some sinister, new, secret explosive with which to destroy their enemies is clearly without foundation.” 32

But, then again, if any nation had the scientific know-how and the industrial capacity to overcome all the obstacles, it was Nazi Germany. In the 1920s and ’30s Germany was the indisputable world leader in physics, chemistry and mathematics. German scientists had won one-third of the world’s Nobel Prizes in science. 33 Germany’s chemical and armaments industries were second to none. Germany had access to the latest nuclear fission research from British, Soviet, Italian, American and French scientists, all of whom had openly published their work before the war. Nazi Germany controlled the world’s largest uranium mines in occupied Czechoslovakia and the Belgian Congo. And it was the fatherland of a man universally recognized as a leading light in the field of atomic physics, Germany’s youngest full professor at age 26 and a Nobel Prize-winner at 33, Werner Heisenberg.