Inside the Experimental Traps Scientists Set for Ghostly Neutrinos

科学家为捕捉幽灵般的“中微子”所设下的实验陷阱

Seventy years ago, the physicists Clyde Cowan and Frederick Reines took a custom-built 10-ton detector, surrounded it with thick lead walls and wet sandbags, and placed it near a powerful nuclear reactor at the Savannah River Plant in South Carolina. They called the experiment Project Poltergeist, designed as it was to catch a ghost.

七十年前，物理学家克莱德·考恩（Clyde Cowan）和弗雷德里克·莱因斯（Frederick Reines）制造了一个重达10吨的定制探测器，用厚厚的铅墙和湿沙袋将其包围，并放置在南卡罗来纳州萨凡纳河工厂的一座强力核反应堆旁。他们将这项实验命名为“骚灵计划”（Project Poltergeist），正如其名，旨在捕捉一个“幽灵”。

More than a quarter of a century before, physicists had been puzzling over why energy appeared to be lost during a radioactive process called beta decay. Something was missing, and there was no known physics to explain it. Then in 1930, the Austrian physicist Wolfgang Pauli proposed a radical solution: A virtually undetectable particle was silently carrying the missing energy away. “I have done a terrible thing,” Pauli told a friend. “I have postulated a particle that cannot be detected.” It would come to be known as the neutrino. Having almost no mass and no charge, these particles can pass through Earth and everything on it, including our bodies, virtually unimpeded.

在此前二十多年里，物理学家们一直困惑于为何在一种称为“β衰变”的放射性过程中，能量似乎会凭空消失。有什么东西不见了，而当时已知的物理学无法解释这一现象。1930年，奥地利物理学家沃尔夫冈·泡利（Wolfgang Pauli）提出了一个激进的解决方案：一种几乎无法探测的粒子正在悄悄带走这些缺失的能量。“我做了一件可怕的事，”泡利曾对朋友说，“我假设了一种无法被探测到的粒子。”这种粒子后来被称为“中微子”。由于几乎没有质量且不带电荷，这些粒子可以几乎不受阻碍地穿过地球及其上的一切，包括我们的身体。

The massive device that Cowan and Reines deployed in early 1956 was meant to find what Pauli thought was impossible. That June, the pair of physicists from the Los Alamos National Laboratory sent Pauli a telegram: “We are happy to inform you that we have definitely detected neutrinos.”

考恩和莱因斯在1956年初部署的庞大装置，正是为了寻找泡利认为不可能存在的东西。那年6月，这两位来自洛斯阿拉莫斯国家实验室的物理学家给泡利发了一封电报：“我们很高兴地通知您，我们已经确切地探测到了中微子。”

Attention then shifted to a broader question. If nuclear reactions produce neutrinos, could we use them to peer at the nuclear fireworks inside stars, including the sun? This presented a huge challenge: How can you possibly catch particles shooting from distant stars if these particles can pass through almost anything undetected? The suspicion was that detecting a particle that rarely collides with matter requires a vast amount of matter for it to collide with. Moreover, the matter would have to be shielded from the noise of other forms of radiation. So the answer scientists came up with was to build some of the biggest, deepest, and most exotic experimental traps in scientific history … and then wait.

随后，人们的注意力转向了一个更广泛的问题。如果核反应会产生中微子，我们能否利用它们来窥探恒星（包括太阳）内部的核反应烟火？这带来了一个巨大的挑战：如果这些粒子几乎可以穿透任何物质而不被发现，你又该如何捕捉从遥远恒星射出的粒子呢？人们推测，要探测一种极少与物质发生碰撞的粒子，就需要大量的物质供其碰撞。此外，这些物质还必须屏蔽其他形式辐射的干扰。因此，科学家们给出的答案是：建造科学史上最大、最深、最奇特的实验陷阱……然后等待。

In the 1960s, Raymond Davis Jr. and colleagues at Brookhaven National Laboratory placed a tank 1.5 kilometers underground in the Homestake mine in South Dakota and filled it with nearly 400,000 liters of a chlorine-based cleaning fluid called perchloroethylene. On the rare occasion that a passing neutrino struck a chlorine nucleus, it would be transformed into a radioactive form of argon that could be detected and counted. The experiment, which would run for 25 years, found just one-third the number of neutrinos coming from the sun that had been predicted in theoretical models. This became known as the solar neutrino problem.

20世纪60年代，布鲁克海文国家实验室的雷蒙德·戴维斯（Raymond Davis Jr.）及其同事在南达科他州霍姆斯特克矿地下1.5公里处放置了一个储罐，并注入了近40万升一种名为全氯乙烯的含氯清洁剂。在极少数情况下，穿过的中微子会撞击氯原子核，将其转化为一种可被探测和计数的放射性氩。这项持续了25年的实验发现，来自太阳的中微子数量仅为理论模型预测的三分之一。这后来被称为“太阳中微子问题”。

Decades passed before it was solved—by yet more massive experiments. Deep in the Kamioka mine in Japan, Masatoshi Koshiba built a different kind of detector called Kamiokande, which used 3 million liters of ultrapure water. In this setup, neutrinos occasionally interact with atomic nuclei in the water. The interaction creates an electron that moves so fast, it generates a flash of what’s called Cherenkov light. This light gets picked up by detectors.

几十年后，这个问题才通过规模更大的实验得到解决。在日本神冈矿深处，小柴昌俊（Masatoshi Koshiba）建造了一种名为“神冈探测器”（Kamiokande）的不同探测器，使用了300万升超纯水。在这种装置中，中微子偶尔会与水中的原子核发生相互作用。这种相互作用会产生一个运动极快的电子，从而引发一种被称为“切伦科夫辐射”的闪光。这种光会被探测器捕捉到。

Kamiokande and Koshiba confirmed Davis’ shortfall, and a second, even larger detector, Super-Kamiokande, as well as Canada’s Sudbury Neutrino Observatory, explained the discrepancy. Neutrinos come in three different “flavors” (electron, muon, and tau) and can oscillate, or switch, between them. To do so, neutrinos must have mass, which the laws of physics failed (and still fail) to predict.

神冈探测器和小柴昌俊证实了戴维斯观测到的缺失，而第二个规模更大的探测器“超级神冈探测器”（Super-Kamiokande）以及加拿大的萨德伯里中微子天文台解释了这一差异。中微子有三种不同的“味”（电子、μ子和τ子），并且可以在它们之间发生振荡或转换。要做到这一点，中微子必须具有质量，而物理定律未能（且至今仍未能）预测到这一点。

Newer neutrino detectors continue the tradition of grand ambitions and surprising results. The IceCube Neutrino Observatory below the Amundsen-Scott South Pole Station uses Antarctic ice instead of water. It has developed a map of the Milky Way made up only of neutrinos and traced these high-energy cosmic particles back to active galaxies powered by supermassive black holes. On the floor of the Mediterranean Sea, the Cubic Kilometer Neutrino Telescope (KM3NET) has detected the highest-energy cosmic neutrino on record. Its source remains unknown.

更新型的中微子探测器延续了宏大抱负和惊人成果的传统。位于阿蒙森-斯科特南极站下方的“冰立方中微子天文台”（IceCube）使用南极冰层代替水。它绘制了一张仅由中微子构成的银河系地图，并将这些高能宇宙粒子追溯到由超大质量黑洞驱动的活跃星系。在地中海海底，“立方公里中微子望远镜”（KM3NET）探测到了有记录以来能量最高的宇宙中微子，其来源至今仍是未解之谜。

Neutrino oscillations, and the myriad mysteries they give rise to, have driven the development of the newest wave of detectors. China’s Jiangmen Underground Neutrino Observatory (JUNO) launched in 2025; initial data published in June 2026 provided the most precise measurements of neutrino oscillation reported to date. Japan’s Hyper-Kamiokande (Hyper-K) and the Deep Underground Neutrino Experiment (DUNE) in the American Midwest are both expected to begin operation later this decade.

中微子振荡及其引发的无数谜团，推动了新一波探测器的发展。中国的江门中微子实验（JUNO）于2025年启动；2026年6月发布的初步数据提供了迄今为止关于中微子振荡最精确的测量结果。日本的“顶级神冈探测器”（Hyper-K）和美国中西部的“深地下中微子实验”（DUNE）预计都将在本十年晚些时候投入运行。

Because of these and other audacious experiments, the particle that Pauli was sure could never be caught has slowly been revealing its secrets. The recipe for discovery hasn’t changed in seven decades: Think big, go deep, and summon patience.

正是因为这些大胆的实验，泡利曾确信永远无法捕捉到的粒子，正慢慢揭开它的秘密。七十年来，发现的秘诀从未改变：想得宏大，钻得够深，并保持耐心。