Seventy years in the past, the physicists Clyde Cowan and Frederick Reines took a custom-built 10-ton detector, surrounded it with thick lead partitions and moist sandbags, and positioned it close to a robust nuclear reactor on the Savannah River Plant in South Carolina. They referred to as the experiment “Challenge Poltergeist,” designed because it was to catch a ghost.
Greater than 1 / 4 of a century earlier than, physicists had been puzzling over why vitality seemed to be misplaced throughout a radioactive course of referred to as beta decay. One thing was lacking, and there was no identified physics to elucidate it. Then in 1930, the Austrian physicist Wolfgang Pauli proposed a radical resolution: A nearly undetectable particle was silently carrying the lacking vitality away. “I’ve achieved a horrible factor,” Pauli instructed a good friend. “I’ve postulated a particle that can’t be detected.” It might come to be often known as the neutrino. Having virtually no mass and no cost, these particles can move via Earth and the whole lot on it, together with our our bodies, nearly unimpeded.
The large system that Cowan and Reines deployed in early 1956 was meant to search out what Pauli thought was unattainable. That June, the pair of physicists from the Los Alamos Nationwide Laboratory despatched Pauli a telegram: “We’re completely satisfied to tell you that we have now undoubtedly detected neutrinos.”
Consideration then shifted to a broader query. If nuclear reactions produce neutrinos, may we use them to see on the nuclear fireworks inside stars, together with the solar? This introduced an enormous problem: How will you probably catch particles taking pictures from distant stars if these particles can move via virtually something undetected? The suspicion was that detecting a particle that not often collides with matter requires an unlimited quantity of matter for it to collide with. Furthermore, the matter must be shielded from the noise of different types of radiation. So the reply scientists got here up with was to construct among the greatest, deepest, and most unique experimental traps in scientific historical past … after which wait.
Within the Sixties, Raymond Davis Jr. and colleagues at Brookhaven Nationwide Laboratory positioned a tank 1.5 kilometers underground within the Homestake mine in South Dakota and crammed it with almost 400,000 liters of a chlorine-based cleansing fluid referred to as perchloroethylene. On the uncommon event {that a} passing neutrino struck a chlorine nucleus, it will be remodeled right into a radioactive type of argon that may very well be detected and counted. The experiment, which might run for 25 years, discovered simply one-third the variety of neutrinos coming from the solar that had been predicted in theoretical fashions. This grew to become often known as the photo voltaic neutrino drawback.
A long time handed earlier than it was solved — by but extra huge experiments. Deep within the Kamioka mine in Japan, Masatoshi Koshiba constructed a distinct type of detector referred to as Kamiokande, which used 3 million liters of ultrapure water. On this setup, neutrinos sometimes work together with atomic nuclei within the water. The interplay creates an electron that strikes so quick, it generates a flash of what’s referred to as Cherenkov gentle. This gentle will get picked up by detectors.
Kamiokande and Koshiba confirmed Davis’ shortfall, and a second, even bigger detector, Tremendous-Kamiokande, in addition to Canada’s Sudbury Neutrino Observatory, defined the discrepancy. Neutrinos are available three completely different “flavors” (electron, muon, and tau) and may oscillate, or change, between them. To take action, neutrinos will need to have mass, which the legal guidelines of physics failed (and nonetheless fail) to foretell.
Newer neutrino detectors proceed the custom of grand ambitions and stunning outcomes. The IceCube Neutrino Observatory beneath the Amundsen-Scott South Pole Station makes use of Antarctic ice as an alternative of water. It has developed a map of the Milky Manner made up solely of neutrinos and traced these high-energy cosmic particles again to lively galaxies powered by supermassive black holes. On the ground of the Mediterranean Sea, the Cubic Kilometer Neutrino Telescope (KM3NET) has detected the highest-energy cosmic neutrino on report. Its supply stays unknown.
Neutrino oscillations, and the myriad mysteries they offer rise to, have pushed the event of the latest wave of detectors. China’s Jiangmen Underground Neutrino Observatory (JUNO) launched in 2025; preliminary knowledge printed in June 2026 supplied essentially the most exact measurements of neutrino oscillation reported to this point. Japan’s Hyper-Kamiokande (Hyper-Ok) and the Deep Underground Neutrino Experiment (DUNE) within the American Midwest are each anticipated to start operation later this decade.
Due to these and different audacious experiments, the particle that Pauli was positive may by no means be caught has slowly been revealing its secrets and techniques. The recipe for discovery hasn’t modified in seven a long time: Suppose huge, go deep, and summon endurance.
