by David Kaiser
Perhaps inspired by their latest exchange, Schrödinger began writing a long essay of his own, on “the present situation in quantum mechanics.” A week and a half after receiving Einstein’s letter about the exploding gunpowder, Schrödinger replied with a novel twist. In place of gunpowder, there was now a cat.
“Confined in a steel chamber is a Geiger counter prepared with a tiny amount of uranium,” Schrödinger wrote to his friend, “so small that in the next hour it is just as probable to expect one atomic decay as none. An amplified relay provides that the first atomic decay shatters a small bottle of prussic acid. This and—cruelly—a cat is also trapped in the steel chamber.” Just as in Einstein’s example, Schrödinger imagined the appointed time elapsing. Then, according to quantum mechanics, “the living and dead cat are smeared out in equal measure.” Einstein was delighted. “Your cat shows that we are in complete agreement,” he wrote in early September. “A ψ-function that contains the living as well as the dead cat just cannot be taken as a description of the real state of affairs.”11
Figure 2.2. Nazi book-burning rally on the Opernplatz in Berlin, 1933. (Source: Imagno, courtesy of Getty Images.)
A few months after Einstein’s September letter, Schrödinger’s now-famous cat example appeared, with nearly identical wording, in the magazine Die Naturwissenschaften.12 But it almost didn’t make it into print. Days after he submitted his draft to the magazine, the founding editor—a Jewish physicist named Arnold Berliner—was fired. Schrödinger thought about retracting the essay in protest and relented only after Berliner himself interceded.13
Schrödinger’s thoughts that summer were preoccupied with more than just concerns about Berliner’s mistreatment. Schrödinger had made no secret of his distaste for the Nazi regime and had become downright fatalistic when forced to flee Berlin, musing in his diary, “might it not be the case that I have already learnt enough of this world. And that I am prepared . . .” Months after arriving in Oxford, a visiting friend noted how unhappy he was, the pressures of displacement compounding the effects of the dismal, daily news. In May 1935—just as the Einstein, Podolsky, and Rosen paper appeared in print—Schrödinger delivered a twenty-minute lecture on BBC radio entitled “Equality and Relativity of Freedom,” recalling the many times throughout history in which “gallows and stake, sword and cannons have served to free respectable people” from political repression.14 Against the drumbeat of advancing fascism, little wonder that talk of balls in boxes morphed so quickly into explosions, poisons, and morbid calculations of life and death.
While his essay was in press, Schrödinger wrote to Bohr, trying again to discern how Bohr and the others could make peace with the bizarre features of quantum mechanics. As with Einstein, Schrödinger longed to discuss such matters with Bohr in person, “but the times are now little suited for pleasure trips.” Larger questions loomed. Schrödinger wrote of his “wish once again to be somewhere permanently, that is, to know with considerable probability what one is to do for the next 5 or 10 years.”15 Living only with probabilities had taken its toll.
Yet Europe sank deeper into darkness. Just a few years after Schrödinger introduced his fable about the quantum cat and prussic acid, Nazi engineers began using the self-same poison—under the trademarked name “Zyklon B”—in their brutally efficient gas chambers. In March 1942, just before his scheduled deportation to a concentration camp, Schrödinger’s former editor from Die Naturwissenschaften, Arnold Berliner, killed himself—choosing, in the end, a terrible certainty.16
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In time, the challenge that Schrödinger thought would undercut quantum mechanics became, instead, one of the most familiar tropes for teaching students about the theory. A central tenet of quantum mechanics is that particles can exist in “superposition” states, partaking of two opposite properties simultaneously. Whereas we often face “either-or” decisions in our everyday lives, nature—at least as described by quantum theory—can adopt “both-and.”
Over the decades, physicists have managed to create all manner of Schrödinger-cat states in the laboratory, coaxing microscopic bits of matter into “both-and” superpositions and probing their properties. Despite Schrödinger’s reservations, every single test has been consistent with the predictions from quantum mechanics. As I take up in the next chapter, for example, my colleagues and I at MIT recently demonstrated that neutrinos—subatomic particles that interact very weakly with ordinary matter—can travel hundreds of miles in such catlike states.17
There is a double irony, then, to Schrödinger’s tale of his twice-fated cat. First, although Schrödinger’s cat remains well known within (and beyond) physics classrooms, few recall that Schrödinger introduced his fable to criticize quantum mechanics rather than elucidate it. Second, and even more telling: Schrödinger’s cat served, in its day, as synecdoche for a broader world that had become too strange—and, at times, too threatening—to understand.
3
Operation: Neutrino
Every fraction of a second, invisible particles called neutrinos whiz past the vans and Winnebagos on Highway 169 headed toward McKinley Park in northeastern Minnesota, just shy of the Canadian border. Having begun their journey at Fermilab, an immense physics laboratory outside Chicago, some of those speeding neutrinos smack into five-ton slabs of steel within an underground mine in the town of Soudan, Minnesota (population: 446), sending sparks of charged particles arcing toward sensitive detectors. Quite unlike the camper vans and RVs, the neutrinos complete their journey—450 miles across the Upper Midwest—in less than three-thousandths of a second.
Neutrinos are fundamental to the construction of the universe. They are tremendously abundant, outnumbering atoms by about a billion to one. They modulate the reactions that cause massive stars to explode as supernovas. Their properties provide clues about the laws governing particle physics. And yet neutrinos are among the most enigmatic particles, largely owing to their reticent nature: they have no electric charge and practically no mass, so they interact extremely weakly with ordinary matter. Some sixty-five billion of them stream through every square centimeter of your body—an area the size of a thumbnail—every second, without your ever noticing them.
Through elaborate sleuthing, physicists have identified three distinct types of neutrinos, which differ in their subtle interactions with other particles. Stranger still, the neutrinos can “oscillate” between types, shedding one identity and adopting another as they travel through space. That discovery led to a significant expansion of the standard theory of how particles behave. Still more recently, my colleagues and I studied neutrinos’ subtle oscillations to probe an even deeper mystery of matter.
We used data on the detection of neutrinos in the Soudan mine to complete one of the longest-distance tests of quantum mechanics ever conducted. In particular, we demonstrated that the tiny neutrinos make the journey in a “superposition” state—miniature versions of Schrödinger’s fabled cat. Along the way, the neutrinos are in no definite state, but rather a quintessentially quantum-mechanical hybrid of all three of the neutrino types that physicists have identified. Only when the neutrinos are measured at the Soudan mine, all those miles away from Fermilab, do they snap into one definite state or another, just as Schrödinger’s cat assumes a definite condition—dead or alive, but not both—once an observer opens the box to look.
In scarcely more than half a century, then, neutrinos have gone from wispy, exotic particles at the edge of detectability to tools for investigating matter at its most essential—from prizeworthy quarry to something more like a forensics kit. In retracing that transformation, we catch glimpses of a larger story, of physicists groping toward an abstruse, beguiling account of nature, set against (and at times engulfed by) larger dramas of the nuclear age.
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The discovery of the neutrino dates back to the 1930s, when the Italian physicist Enrico Fermi helped hammer out the first workable theory of nuclear phenomena like radioactive decay. To make h
is calculations work—in essence, as a bookkeeping device to ensure that all the energy entering a nuclear reaction would balance all the energy at the end—Fermi’s colleague Wolfgang Pauli had postulated the existence of a new, undetected particle that carried some of the energy away. Fermi developed the idea more fully and dubbed the mystery particle a neutrino, or “little neutral one,” since it was theorized to carry no electric charge.1
Neither Fermi nor anyone else at the time thought that such tiny wisps of matter could ever be detected directly. Before long, the spread of fascism in Europe quickly overshadowed such questions. As nations mobilized for war, physicists on all sides of the conflict were absorbed into top-secret projects. Meanwhile, the introduction of Nazi-inspired racial laws in Italy put Fermi’s family in danger (Fermi’s wife, Laura, was Jewish). Late in 1938, he managed a Sound of Music–like escape, exploiting a trip to Stockholm to accept the Nobel Prize in order to slip out of Europe and head for the United States, where he became one of the early scientific leaders of the Manhattan Project. In December 1942, Fermi’s group in Chicago coaxed the first nuclear reactor to go critical, inducing controlled nuclear fission. Their reactor design was scaled up during the war to produce plutonium for atomic bombs.2
By the time peacetime research resumed, physics had undergone an enormous transformation. The bloodiest armed conflict in history had thundered to an end, punctuated by the use of nuclear weapons against the cities of Hiroshima and Nagasaki. Throughout much of the war, the hastily built laboratory at Los Alamos, New Mexico, had served as the main coordinating site for the Manhattan Project.3 After the war the laboratory continued to focus on improving and expanding the nation’s nuclear arsenal, giving physicists a newfound level of prestige—and funding. In this new environment, the first serious effort to detect neutrinos began at Los Alamos, in the early 1950s.
Frederick Reines was a young physicist at the Los Alamos Laboratory, part of the team that tested new weapons at the Eniwetok Atoll in the middle of the Pacific Ocean. Returning from a series of bomb tests in late spring 1951, he fell into a discussion about neutrinos with Fermi, who was then visiting the lab. Reines realized that the aboveground nuclear blasts that he and his team were studying at places like Eniwetok should produce enormous floods of neutrinos—so many that a wayward few just might be detectable.4
Reines and another Los Alamos colleague, Clyde Cowan, convinced the laboratory director to let them conduct an experiment at an upcoming bomb test. They would first excavate a narrow, deep hole near where the bomb would be detonated. Inside the shaft they would suspend a one-ton detector, so big that they nicknamed it “El Monstro.” When the bomb went off, carefully sequenced electronics would release the detector, letting it fall freely while the enormous shock wave from the bomb rumbled through the surrounding earth. (If they had bolted their detector in place so close to the blast, the shock wave would have ripped it apart.) Moments later, after the shock wave had passed, the detector would land on a pile of feathers and foam rubber.
Buried at the bottom of the shaft, the device would be awash in neutrinos from the nuclear fireball. Sensitive electronics on the detector—a huge vat filled with a solution of toluene, an organic molecule usually found in paint thinners—would monitor for telltale flashes of light. A flash would indicate that one out of hundreds of trillions of neutrinos had struck some matter in the liquid and shaken loose a positron, the antimatter twin of the electron. Meanwhile, the physicists would have to wait several days, until the hazardous radioactivity on the surface had died down sufficiently, before they could return to ground zero, dig back down the 150-foot shaft, and retrieve their instrument.5
While preparing for their bomb-based test, Reines and Cowan realized that they could also hunt for neutrinos in a less dramatic way. With one more tweak to their protocol to better rule out spurious readings, they could install their liquid-filled detector next to a nuclear reactor rather than beneath an exploding bomb. The two researchers ran a pilot test near one of the huge reactors in Hanford, Washington—supersized versions of Fermi’s original reactor. Satisfied with the results, they installed an improved device at the newer, more powerful reactor at Savannah River, in South Carolina, during the fall of 1955. (The Savannah River plant was built to produce tritium for hydrogen bombs, which are thousands of times more destructive than the original nuclear bombs.) Within months, Reines and Cowan had clocked in enough tiny flashes of light to convince their colleagues—and, in time, the Nobel Prize committee—that they had finally caught sight of the elusive neutrino.6
Figure 3.1. Frederick Reines (left) and Clyde Cowan conduct an early test to try to detect neutrinos, using reactors in Hanford, Washington, in 1953. (Source: Courtesy of the University of California–Irvine.)
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Fermi’s former assistant Bruno Pontecorvo followed these developments with particular interest. Pontecorvo had begun his career in the 1930s as the youngest member of Fermi’s group in Rome. (Older members nicknamed him the “Puppy.”) He immersed himself in the esoterica of nuclear physics, including Fermi’s new theory of radioactivity and the still-emerging, and still-hypothetical, ideas about neutrinos. Coming from a large Jewish family, Pontecorvo eventually found his situation in Italy untenable. His flight from fascism was even more dramatic than Fermi’s, closer to Casablanca than The Sound of Music: first a fellowship to study in Paris, then a harrowing, nighttime escape through the countryside in June 1940, as Nazi tanks rolled into the city. From the south of France, Pontecorvo boarded a train to Madrid and then on to Lisbon before catching a steamship to New York City.7
Once in North America, Pontecorvo, too, began working on the Manhattan Project. He was assigned to one of the British contingents working in Montreal, aiming to perfect a different type of nuclear reactor than Fermi’s in Chicago. After the war, he took a position with the new British nuclear research facility in Harwell, near Oxford, to continue his research on reactors. Around that time, he proposed a scheme to try to detect neutrinos streaming out of a nuclear reactor, years before Reines and Cowan independently pursued the idea.
Two fascinating books—Simone Turchetti’s The Pontecorvo Affair (2012) and Frank Close’s Half-Life (2015)—document the next bizarre twists in Pontecorvo’s life story. He had been named as one of the inventors, together with Fermi and other members of the Rome group, on a patented technique to slow down certain nuclear particles and thereby increase the rate of particular nuclear reactions. The technique proved to be pivotal to wartime studies of nuclear fission, both for reactors and for bombs. After the war, the patents, which had been granted in Italy in 1935 and in the United States in 1940, assumed a radically different patina.8
In 1949, other members of the Rome group sought compensation for their patented technique, which had since been built directly into the sprawling infrastructure of the US nuclear complex. The patent-wrangling triggered an FBI investigation, which dredged up long-forgotten material on several of Pontecorvo’s relatives who had been outspoken Communists in Italy. A few weeks later, one of Pontecorvo’s colleagues at Harwell, Klaus Fuchs, confessed to having passed atomic secrets to the Soviets during the war. Like Pontecorvo, Fuchs was an émigré from the Continent who had served as part of the British delegation on the Manhattan Project. The associations suddenly looked suspicious.9
What came next reads like a Le Carré novel. While vacationing in Italy in early September 1950, Pontecorvo and his family abruptly zigzagged from Rome to Munich to Stockholm, then on to Helsinki, where they met up with Soviet agents. Pontecorvo’s wife and young children got into one car while Pontecorvo climbed into the trunk of another, and the secret caravan drove through the forest into Soviet territory. Hours later the group arrived in Leningrad; within days they had been delivered to Moscow. Weeks went by before the British or American authorities caught on. Finally, the US Joint Congressional Committee on Atomic Energy released a thick report, Soviet Atomic Espionage, proclaiming Pontecorvo’s defection to be only sl
ightly less damaging than Fuchs’s disloyalty, and even worse than the alleged espionage for which Ethel and Julius Rosenberg would later be executed.10
While lurid headlines swirled in Britain and the United States, Pontecorvo settled into a new position at the vast nuclear research facility at Dubna, outside Moscow. As Close reveals, on the basis of his scrutiny of Pontecorvo’s personal notebooks from the time, Pontecorvo consulted on aspects of the secret Soviet nuclear weapons project, at least for a while. But soon he was given space to pursue more basic research as well. Upon learning of Reines’s and Cowan’s discovery, his thoughts returned to his long-standing love, the neutrino.11
Figure 3.2. Bruno Pontecorvo walks through the streets of Moscow in March 1955, after defecting to the Soviet Union with his family. (Source: Photograph by Hulton Archive, courtesy of Getty Images.)
In 1957, Pontecorvo published an article in the leading Soviet physics journal suggesting that neutrinos could oscillate between different varieties, or “flavors.” (Translations of the journal into English had recently begun, secretly underwritten in part by the Central Intelligence Agency.)12 He refined this idea in a series of papers, reasoning from quantum theory that the neutrinos should occur in superposition states, neither in one flavor state nor the other but both. (He considered only two types of neutrinos at the time.) Whenever physicists made a measurement, they should find a given neutrino in one flavor state only. But in between observations the neutrinos would not have a fixed identity. They would live in a state of statistical indeterminacy, partly one flavor and partly the other.13