How a Soviet physicist’s 1960 idea led to the birth of neutrino astronomy and this year’s Nobel Prize in physics

The 2026 Nobel Prize in Physics was awarded to Belgian physicist Francis Halzen for his “decisive contribution to the IceCube neutrino observatory and the discovery of high-energy neutrinos of astrophysical origin.” Neutrinos were long considered almost impossible to detect, especially those arriving from deep space. Tracing them to their cosmic sources is even harder here on Earth because the vast majority come from the Sun or are produced in the planet’s magnetic field. Yet scientists managed to detect them with a project of astonishing scale, first proposed by Soviet physicist Moisey Markov in the 1960s and built through Halzen’s efforts in recent decades: IceCube, an observatory buried deep in the Antarctic ice. What should we know about the project’s history? How does a cosmic neutrino detector work? And what can studying these particles tell us?

The Soviet roots of the 2026 Nobel: How Moisey Markov’s idea won over the physics world

Russian-language popular science writing about physics, like the field itself, is in much better shape than its counterparts in the other Nobel science disciplines, biology and chemistry. Experts have already written many in-depth articles about neutrinos and the IceCube neutrino observatory, the brainchild of this year’s laureate, Francis Halzen. It’s hard to add anything new to these accounts, so they’re a sensible place to start.

Readers are especially fortunate that Mark Bowen’s book “The Telescope in the Ice: Inventing a New Astronomy at the South Pole” was translated into Russian a few years ago. It tells the story of this remarkable project, conceived and built through Halzen’s efforts. Written in 2017, the book is now quite dated, but only in its account of neutrino “discoveries” (modern physics moves fast). Its central story — that of Halzen and his creation — remains accurate and fascinating.

In 2021, physicists Dmitry Naumov and Igor Ivanov of the Joint Institute for Nuclear Research’s Laboratory of Nuclear Problems gave our readers a detailed explanation of what neutrinos are and how scientists search for cosmic neutrinos in the depths of Russia’s Lake Baikal. That article focused on IceCube’s counterpart, the Baikal-GVD neutrino observatory. This is one of the rare fields in which Russian science contributes research that complements work elsewhere in the world, rather than trying to catch up.

Baikal-GVD is in the opposite hemisphere from IceCube and operates in water rather than ice. This is a major advantage, rather than a compromise dictated by circumstances (though working at Lake Baikal is certainly easier than working in Antarctica). Data from the two facilities complement each other: Their positions on opposite sides of the Earth help pinpoint the sources of incoming particles, while detectors in water offer greater precision than those in ice, where light scatters more readily.

The Russian — or, more accurately, Soviet — connection to this year’s prize is clear. The USSR began building a similar neutrino telescope back in the early 1980s. More significantly, Soviet physicist and Academy of Sciences member Moisey Markov proposed searching for traces of neutrinos in large natural bodies of water or ice, such as lakes or seas, as opposed to building purely laboratory-based detectors, which also exist.

Markov proposed the idea at a conference in Rochester in 1960. This wasn’t a case of an idea that occurred to a lone Soviet scientist, appeared in an obscure journal, and was rediscovered in the West many years later. Markov presented it in Rochester himself, and many Western physicists immediately recognized its potential and embraced it. Francis Halzen was among those who embraced it. The laureate himself recalled that moment:

These two paragraphs essentially explain everything you need to know about how IceCube works and the idea that earned the Nobel Prize. To unpack this concise description, we’ll start with neutrinos.

What is a neutrino, and why look for it?

“The neutrino joined the ranks of elementary particles tentatively and timidly,” Moisey Markov wrote in his 1964 monograph. “For several years, it was unclear whether the neutrino was a real particle or a theoretical concept that successfully described, in quantitative terms, the disappearance of energy and angular momentum in various reactions.”

In plainer, modern language, this means that physicists initially just made neutrinos up. They invented them to save the law of conservation of energy in certain radioactive decay processes that produced electrons. Under the conservation laws, these electrons should have had specific, fixed energies — but for some reason, they didn’t: Their energies formed a continuous spectrum rather than a set of discrete values. That was when, as the story is now understood, Austrian-German physicist Wolfgang Pauli proposed that these decays also produced a hypothetical particle that carried away some of the energy and angular momentum.

This saved the law of conservation of energy, but at a considerable cost: The “invented” particle had to have no charge, no mass, and almost no interactions with anything at all. Proving that it even existed was virtually impossible at the time, which naturally troubled physicists, who dislike such intangible things.

The neutrino’s defining property — both its strength and its weakness as a tool for studying the world — is that it participates only in what is called the weak interaction. Gravity also affects it, but so slightly that this effect can often be ignored. The weak interaction is one of the four fundamental interactions, alongside the strong interaction, electromagnetism, and gravity. It acts over an extremely short distance, smaller even than an atomic nucleus.

Neutrinos are often described as light, almost ethereal particles that pass through matter like incorporeal angels. But that description isn’t quite right. Nearly 65 billion neutrinos do pass through every square centimeter of our bodies every second without us even noticing. But that isn’t because these particles are so light.

Although the neutrino’s exact rest mass is still unknown (another remarkable property of these particles), its tremendous speed can give it as much energy as a tennis ball delivers on impact. If a neutrino like that somehow decides to interact with you, you’ll certainly notice. The probability is vanishingly small, however: It has to hit a suitable atomic nucleus with precision down to fractions of a femtometer. Only then does the neutrino have a chance to interact and release its energy at the end of its journey. The fact that this never happens to people tells us less about how light and insubstantial neutrinos are than about how people, like all visible matter, consist mostly of empty space.

So how did scientists prove that neutrinos exist, and even build detectors for them? As you might guess, even very rare events can be detected if they have enough opportunities to happen. For neutrinos, this means entering a medium where they can interact to produce other, more tangible particles that scientists can detect. Even in a detector, the neutrino itself remains invisible. What scientists can detect is the electron, or another, heavier charged particle called a muon, produced by the interaction.

From here, the physics becomes somewhat more familiar: Charged electrons and muons moving at tremendous speeds through a medium such as water begin to emit what is called Cherenkov radiation. It resembles the shock wave produced by a supersonic aircraft in air and occurs when a particle moves faster than light travels in that particular medium — water, for example. (Don’t worry: The speed of light in a vacuum is still unattainable!)

Cherenkov radiation is what makes the water tanks in nuclear reactors glow pale blue. Essentially, all that neutrino detectors such as IceCube and Baikal-GVD do is record flashes of this radiation.

Of course, this makes everything sound too simple. In practice, neutrino physicists’ main challenge is contending with constant noise from an endless stream of other particles. These particles also produce Cherenkov radiation that the detectors pick up, but they aren’t neutrinos. Or they are neutrinos but come from other sources: those same nuclear reactors, the center of the Earth, where radioactive decay occurs, the center of the Sun (the main source of noise), or space.

But cosmic neutrinos come in different kinds. Some — the overwhelming majority by number — are produced near Earth through the decay of protons that have entered the region of the planet’s magnetic field. These are neutrinos too, of course, but they aren’t the ones most physicists and astronomers are interested in.

Much more interesting are particles produced outside our solar system, or, better still, outside our galaxy altogether. They are far, far less numerous than neutrinos from other sources, but only these can have extremely high energies, in the hundreds of teraelectronvolts. No processes on Earth can produce particles with such energies, but supernova remnants and the centers of quasars can. Detections of these particles are fairly rare, and some remain unique, such as the recent detection of a neutrino with an energy of 120 petaelectronvolts by KM3NeT, a deep-sea telescope in the Mediterranean.

Exactly which processes produce these extraordinarily energetic particles remains a mystery. IceCube, Baikal-GVD, KM3NeT, and other similar detectors are being built to investigate that question. This means creating a new kind of astronomy in which neutrinos take the place of light or radio waves. Gravitational waves recently opened a second “window” onto the sky; detectors such as IceCube have opened a third, a neutrino window. The Nobel Committee recognized the opening of that window this year.