“Atom Man Flies Away,” declared a banner headline in London’s Daily Express. “British intelligence service MI5 has been brought into the hunt for the missing atomic scientist,” reported the BBC. On the other side of the globe, a front-page story in the Sydney Morning Herald quoted a fellow physicist: “No doubt, a man like him would be highly useful to the Russians.” Thus, practically overnight, in the fall of 1950, a low-key Italian-born scientist by the name of Bruno Pontecorvo caused a worldwide sensation.
For sure, Cold War tensions and paranoia fueled the media frenzy and the rampant speculation that ensued. Following a successful atomic-bomb test by the Soviet Union the year before, the United States had decided to develop even mightier “hydrogen bombs” that exploit nuclear fusion as well as fission. War had broken out on the Korean peninsula over the summer. Meanwhile in Washington, Senator Joseph McCarthy had begun his notorious campaign to purge suspected communists from public life. Even physicists, previously held in high esteem by the political establishment for their triumphant work on the Manhattan Project during the Second World War, were not beyond reproach. It didn’t help that one of their tribe, Klaus Fuchs, confessed to spying for the Soviets and was convicted in a high-profile trial in Britain.
As it happened, at the time Pontecorvo was employed at the same government-run atomic-research laboratory in Harwell, near Oxford, as Fuchs. No wonder his sudden disappearance, along with his wife and three sons on their way back to Britain from an Italian vacation, raised a ruckus. Almost immediately, the media—and the security services—suspected that Pontecorvo had defected to the Soviet Union. Sure enough, he turned up at a Moscow press conference five years later, although the central mystery of whether he was a spy remains unsolved to this day.
Pontecorvo’s standing as a physicist, however, is not in doubt. His insights, particularly when it comes to the ghostly neutrinos, have shaped the field. He investigated neutrinos both before and after his defection. In more ways than one, the breakthrough honored by this year’s physics Nobel Prize stems from—and validates—his theoretical work decades ago.
Back in the 1940s, Pontecorvo was the first to suggest that the nuclear-powered Sun should be a copious neutrino source. He also came up with a clever scheme involving a giant tank of dry-cleaning fluid for trapping these pathologically shy particles, which hardly interact with anything and therefore are hard to pin down. Pontecorvo didn’t get to hunt for neutrinos himself, but American scientist Ray Davis detected solar neutrinos two decades later, just as the farsighted Italian had proposed so long ago. (Davis received a share of the 2002 physics Nobel Prize for his discovery.)
Davis’s experiment deployed in a South Dakota mine, and others set up later, registered far fewer neutrinos than solar models predicted, though. Scientists puzzled over the embarrassing mismatch between observation and theory, but Pontecorvo came to the rescue, now from behind the Iron Curtain.
Decades earlier, he had proposed that multiple varieties or “flavors” of neutrinos must exist—and three Columbia University experimentalists proved him right (winning a physics Nobel Prize for it much later, in 1988). Building on that concept and work by other researchers, Pontecorvo and a Russian colleague named Vladimir Gribov suggested that neutrinos could be fickle, changing form like cosmic chameleons. Already in 1968, soon after Davis announced his early findings, they posited that neutrinos “oscillating” from one flavor to another on their way from the Sun could account for the apparent shortfall. As strange as that sounds, the proposal made sense in the wacky world of quantum mechanics.
Three decades passed before two complementary and sophisticated experiments—Super-Kamiokande in Japan and Sudbury Neutrino Observatory in Canada—verified their intriguing suggestion. This year’s physics Nobel Prize went to Arthur McDonald and Takaaki Kajita, two leaders of these large research collaborations, for experimentally confirming the shape-shifting nature of neutrinos. Their breakthrough solved the long-standing mystery of the missing solar neutrinos (because the numbers work out once flavor changes are accounted for), established that neutrinos have non-zero mass (because they couldn’t oscillate among flavors otherwise), and opened up the prospect of new physics. It is a Nobel-worthy discovery for sure—one that confirms Pontecorvo’s foresight yet again, for the third time in Nobel Prize history in case anyone is keeping count.