by Sam Kean
Elementary logic, then, ruled out lanthanum or barium. But if uranium wasn’t turning into those elements, what the heck was it turning into? Hahn was starting to appreciate Irène’s confusion. So as he always did, he reached out to Meitner and wrote her a letter in mid-December 1938 outlining his odd results.
No one would have blamed Meitner if she’d told Hahn to do something anatomically impossible. But she couldn’t resist this scientific puzzle—nor resist sticking it to the Joliot-Curies again. Moreover, she was cold and lonely in Sweden and desperate to reconnect with the scientific world. She was actually leaving for a Christmas holiday outside Stockholm with her nephew when the letter arrived, and she took it along. The nephew, a fellow physicist, found her frowning over it one morning at breakfast. She’d been stuck for days, she admitted, and they decided to take a walk in the snow and discuss the matter further.
No doubt the truth had been tickling Meitner’s subconscious for some time—and she realized it suddenly on the walk. Hahn had said the transmuted elements acted like lanthanum and barium. So maybe they were lanthanum and barium. But for that to happen, the uranium nucleus had to have broken apart—not just spit out a little atomic shrapnel, but cracked in half. It was an unthinkable idea, one that virtually every other scientist alive would have rejected. Even Irène Curie, a Nobel Prize winner, couldn’t make that leap. Meitner could, and she concluded that, however unlikely it sounded, Hahn had split the atom.
Perhaps to punish him, Meitner didn’t immediately tell Hahn her conclusion. When she did, the news staggered him. How could this be true? Yet he trusted Meitner—she didn’t bungle things—and her conclusions reinforced a decision he’d made a week earlier. He’d recently called up Paul Rosbaud, the editor who’d helped Meitner escape Germany, to ask if he could rush a short paper about his work into print. These experiments thrilled Rosbaud, and he agreed to pull an already typeset paper from the next issue of Naturwissenschaften (The Natural Sciences) and swap in Hahn’s. The date was December 22, 1938.
However excited most physicists were at this news, the more politically astute ones felt something else: a chill of foreboding. Uranium is fairly common in the Earth’s crust, so it’s easy to mine in large quantities. Physicists also knew (per E = mc2) that splitting uranium atoms would liberate gobs of energy, more energy than humankind could safely handle. Until late 1938, this had only been a theoretical worry. No more. From that instant forward, uranium was no longer a second-rate radioactive element. From that instant forward, only uranium mattered.
A few scientifically savvy Nazis later accused Hahn and Rosbaud of treason for their actions that day. The duo hurried the paper into print, they charged, to alert the enemies of the Reich and prevent the Nazis from hoarding the secret of uranium. That might sound like typical Nazi paranoia, but for once the Nazi paranoia was correct: Rosbaud did indeed want to warn France, Great Britain, and the United States about this monumental discovery. He despised the Nazis and feared that war might break out at any moment; delaying publication by even one issue might cost the world dearly.
As it was, the world would suffer enough. December twenty-second happened to be the solstice in 1938, and as one historian commented, “The world’s winter had begun.”
Despite the treason, the world at large would remain ignorant of Hahn’s paper until the new issue of Naturwissenschaften appeared several weeks later. But rumors about Hahn’s discovery—soon called uranium fission, after the process by which bacteria divide—went rippling through the world of physics in the meantime.
It all started in early January 1939, when Lise Meitner’s nephew ran into Danish physicist Niels Bohr in Copenhagen and confided what Tante Lise had discovered. Upon hearing this, Bohr slapped his forehead like a cartoon character. “What idiots we’ve been!” he cried. The nephew made Bohr promise to keep the discovery secret, since Hahn’s paper had merely laid out the chemical evidence for fission. Meitner wanted to write her own paper about the physics involved, and Bohr’s blabbing could undermine her. Bohr vowed to remain silent.
Then promptly broke his vow. A few days later he sailed off for a sabbatical in the United States, and practically before the ship pulled anchor, he spilled the beans to a colleague onboard, who was equally stunned. Bohr and the colleague then scrounged up a blackboard from somewhere on the ship, and despite the choppy waters in the Atlantic—Bohr was seasick the whole voyage—they spent the entire nine days working out the implications of fission. In his excitement, Bohr forgot to mention the need to keep mum. Not bound by any promises, the colleague started yammering the moment he landed in New York on January 16, and pretty soon the entire U.S. physics community knew.
Realizing he’d screwed up, Bohr made an official announcement at a formal dinner in Washington, D.C., on the twenty-sixth. He’d been scheduled to talk about low-temperature physics that evening, but to hell with that—this was much hotter. And while he tried to ensure that Meitner got proper credit, everyone stopped listening after he revealed the basic discovery. Even before he’d finished speaking, in fact, several local physicists pushed their way out of the lecture room—some still wearing tuxedoes from dinner—and ran off to fire up their own experiments. One physicist in Northern California, reading an account of Bohr’s talk in the newspaper the next day, actually jumped up in the middle of a haircut and raced to his lab. Within the nuclear physics community, the news of fission would forever divide the world into Before and After.
Enrico Fermi was one of the politically savvy scientists who shivered at the news of fission. After several years of slow neutrons and fast footraces in Rome, Fermi had watched Italy sink into fascism under Benito Mussolini. By 1938 he could no longer imagine a future there for him and his wife, Laura, who was Jewish, and they began making plans to leave. They finally got their chance in the fall of that year, when Niels Bohr pulled Fermi aside at a scientific meeting.
As a Scandinavian, Bohr had close contact with the Nobel Prize committee in Stockholm, and he told Fermi that he might expect a little award in a few weeks. Normally, this would have been a serious breach of etiquette: the Nobel committee works in secret and guards the identity of winners. But Bohr had a good reason for this peccadillo. The Italian lira was fluctuating wildly in value then, and Italian laws would probably prevent Fermi from converting the money to a more stable currency. Bohr wanted to know if Fermi wished to delay his prize a year, when the economics might be more favorable.
Fermi then let Bohr in on a secret of his own. He was fleeing Rome for Columbia University in New York, so getting the prize money immediately would be a huge help. Bohr said not to worry. A few weeks later Fermi did indeed win the Nobel in physics, and he and Laura left Rome for Stockholm with little more than a suitcase apiece, never to return.
Sadly, with Fermi’s departure, his rambunctious little physics team—most of which was Jewish—underwent its own fission. One scientist fled to California, another to Paris. Top lieutenant Edoardo Amaldi (who’d first noticed the difference between samples irradiated on a marble shelf and a wooden tabletop) also wanted to flee, but as a good Catholic he was in no personal danger in Italy. Colleagues therefore begged him to stick things out; there were few jobs available for foreigners abroad, and others needed those slots more than he did. So Amaldi pasted a smile on his face and accompanied Fermi to the train station to see his mentor off. He returned home miserable, and was soon drafted into the Italian army, destined for the front in North Africa.
Coincidentally, Bohr spilled the beans about fission not long after Fermi had arrived in New York, and with his agile, leaping mind, Fermi saw exactly where the discovery would lead. A colleague remembers him standing in his office one gloomy January day, chatting about the brave new world of fission. Fermi then turned toward the Ozymandias of the Manhattan skyline and cupped his hands into a ball. “A little bomb like that,” he murmured, “and it would all disappear.”
The mood in Paris was equally grim, albeit for different reasons. Upon seei
ng that bastard Hahn’s article on fission in Naturwissenschaften, Frédéric Joliot had locked himself away for two days to study it. He finally emerged dark-eyed and haggard, and broke the news to everyone at his and Irène’s labs: they’d been scooped again. They’d seen the same evidence as Hahn and Meitner, the same fission by-products, and simply hadn’t understood.
Echoing Bohr, Irène wailed, “What assholes we’ve been!” She then turned on her husband in fury. If you hadn’t run away to your own lab and abandoned me, she yelled, we would have made this discovery ourselves. Several people who worked in the lab silently agreed. There had been nothing wrong with Irène’s chemistry, but she lacked the physics knowledge that Joliot had, knowledge crucial for interpreting her results. He’d simply been too distracted with his cyclotron, his midlife scientific crisis, to pay his wife’s work any attention. Joliot was mortified.
Nothing if not ambitious, however, he decided that his Paris team would take the next crucial step in fission research. Despite Fermi’s prophecy, Meitner and Hahn hadn’t actually proved that you could make a bomb with uranium. All they’d proved was that bombarding a uranium atom with a neutron would split the atom and release energy. But what then? The big question was whether, aside from energy, the fissioning atom released anything more. In particular, did it release more neutrons? If so, then other uranium atoms in the vicinity might absorb these neutrons and become unstable, too. Those atoms would then undergo fission themselves and—here’s the key point—release still more neutrons. These secondary neutrons would destabilize more uranium atoms, which would release tertiary neutrons, and so on. As Joliot had predicted in his Nobel speech, it would be a nuclear chain reaction.
It all depended on one thing: the number of neutrons released. If a uranium fission liberated just one neutron, there was no reason to get excited. Each step would cause just one additional atom to fission, and the chain reaction would proceed slowly or even fizzle. But if uranium atoms released two or more neutrons every round, watch out. One fission would lead to two, two to four, four to eight, eight to sixteen, and so on—an uncontrollable cascade of energy. Joliot’s course was clear, then: bombard samples of uranium, and measure the neutron multiplication.
Having already been scooped on several discoveries, Joliot put a gag order on his assistants: no one could discuss their new experiments with outsiders. Ironically, though, these precautions ended up exposing Joliot’s secret. A physicist at Columbia University named George Placzek had recently visited the lab in Paris, and to make extra-double-sure that Placzek didn’t blab about anything, Joliot’s team sent him a cable in early 1939 reading JOLIOT’S EXPERIMENT SECRET. Unfortunately for them, in one of those minor blunders that rewrite history, someone got one Columbia physicist with a funny Eastern European name confused with another Columbia physicist with a funny Eastern European name, and the message ended up down the hall on the desk of Leo Szilard—who read it and froze in horror.
Szilard had actually dreamed up the concept of nuclear chain reactions way back in 1933. He therefore knew better than anyone that chain reactions could lead to bombs. Moreover, as a Jewish refugee from Hungary, the idea held a special horror for him. Above all else in life, Szilard feared the Third Reich, and although Joliot’s telegram never mentioned uranium or fission, Szilard guessed instantly what the “secret experiment” must be.
Never one to hold his tongue, Szilard eventually wrote a letter to Paris outlining his fears: Germany had the top scientists in the world, along with the best industrial plants. If anyone could weaponize nuclear physics, it was the Nazis. He therefore asked Joliot to be prudent. Do all the research you want, he said; you can even submit papers to journals to establish priority for discoveries. But please, please, don’t publish anything related to chain reactions. Don’t tip off Hitler.
Joliot wasn’t the only one Szilard was keeping an eye on. Szilard’s fellow refugee at Columbia, Enrico Fermi, had also plunged forward with fission research, and Szilard made the same plea for secrecy to him. At first the Italian blew him off. (Fermi’s exact response was one word: “Nuts!” Apparently his fondness for American slang had outpaced his command of it; he probably meant something like “Nuts to that.”) But Szilard kept pressing Fermi, and Fermi eventually relented. He even withdrew an already accepted paper from a research journal to keep the information secret.
Joliot proved less accommodating. If we censor our scientific work out of fear, he told Szilard, then Hitler will have destroyed yet another basic freedom. It was a powerful argument, but it’s hard not to suspect Joliot of having selfish motives here: given his reputation for blowing discoveries, he desperately hoped to redeem himself.
To Szilard’s dismay, Joliot published results in April 1939 showing that uranium atoms release an average of 3.5 neutrons every fission, well above the minimum for a chain reaction. This was actually a mistake; the currently accepted figure is 2.5. Joliot never achieved a self-sustaining chain reaction, either; his efforts petered out quickly. But the overall point stood: nuclear chain reactions, and therefore nuclear bombs, were possible. And far from keeping all this a secret, Joliot began discussing wild plans to build and detonate a nuclear weapon in the Sahara desert—the world’s first, if short-lived, atomic bomb project.
Things moved at a gallop after that. No one had heard of uranium fission before January 1939; by December, more than a hundred papers on the topic had appeared worldwide. This explosion of research caused nothing but grief for the chemist at the center of things, Otto Hahn. The Nazis despised him even more now for leaking the secret of fission. He also despaired over the Pandora’s box he’d opened: when he first saw Joliot’s work on neutron multiplication—and realized that his own discovery might lead to the most destructive bombs in history—he decided to kill himself.
Ultimately Hahn changed his mind, but he couldn’t shake off his anguish. He enlisted a few colleagues in a scheme to seize all the stocks of uranium in Germany and dump them into the sea, a plan he abandoned only when someone pointed out its futility: in seizing part of Czechoslovakia that spring, Hitler had inadvertently acquired the richest uranium mines in Europe. The realization that he could not stop nuclear fission put Hahn back on suicide watch. He’d split more than the atom—he’d divided the world.
CHAPTER 6
Spinning out of Control
Even as scientists like Fermi were fleeing Europe in the late 1930s, other physicists were wrestling with the morality, and wisdom, of returning there. Two old friends in particular, Samuel Goudsmit and Werner Heisenberg, faced tough choices on this front, and while both men debated long and hard over what to do, they knew their decisions would end in pain either way.
Of the two, Goudsmit was the less likely scientist. “In my high school days,” he once said, “I wanted to solve mysteries, and there were three professions where one could solve mysteries: the police, archaeology, or science.” Science eventually won out, but it was a close thing. After growing up in a typical Jewish household in The Hague he decided to attend college in Leiden and, to his parents’ disappointment, abandon the family businesses for scholarly pursuits. (His father sold bathroom fixtures, his mother designed frilly hats.) At Leiden his rich black hair earned him the nickname luizebos, or mop-top, and he proved a sharp if erratic student, regularly failing exams in subjects he didn’t care about. He was also prone to enthusiasms: he suddenly began studying hieroglyphics one year, eventually becoming fluent in them, and took an eight-month course on scientific detective work that included units on fingerprints, forgeries, and bloodwork. It took him some time to settle on physics, and when he did so, he made the great discovery of his life almost too easily.
That discovery was quantum spin. The technical details needn’t concern us, but spin describes the intrinsic angular momentum of particles such as electrons and neutrons; along with mass and charge, it’s one of their fundamental properties. When Goudsmit started off, however, he had no idea he was on the trail of something that importan
t. He’d simply heard about some odd new experimental results and wanted to solve the mystery of what produced them. So he and a fellow student, George Uhlenbeck, finally sat down one summer day in 1925 to hash things out. They groped about with no real plan, trying this and that, and the idea of spin fell out almost accidentally. But by the end of the session, they knew they really had something. Invigorated at solving the mystery, Goudsmit finally looked up from his papers after hours of labor—and noticed an ominous black cloud sweeping across the sky. He and Uhlenbeck later learned that a tornado had touched down in Holland, but they’d been so wrapped up in the equations they hadn’t noticed.
Spin would soon send whirlwinds of its own through quantum physics. The two students dashed off a short paper and showed it to their advisor, who scratched his head and said, Well, it’s either brilliant or nonsense. Let’s publish it and find out which. Sure enough, several eminent physicists hated the idea—it seemed too strange. But others took a shine to spin, including Werner Heisenberg, who sent Goudsmit a letter congratulating him on the “courageous” work. An ecstatic Goudsmit ran to Uhlenbeck to show him the message. Uhlenbeck just blinked. Who’s Heisenberg?
This ignorance stunned Goudsmit. Heisenberg was Heisenberg, the top young physicist in the world. Goudsmit already revered the man, and the letter cemented his affection. The two eventually met and became friends, and when Heisenberg later visited Holland, he stayed with Goudsmit’s parents, eating dinner at their house and accompanying Goudsmit to the seaside to watch fireworks. Thanks in part to Heisenberg’s support, the world physics community embraced spin, and mop-top’s reputation soared.