From Cherenkov’s glow to the IceCube: The road to Halzen’s Nobel

On October 6, the Royal Swedish Academy of Sciences announced that Francis Halzen would receive the 2026 Nobel Prize in Physics for his decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin. He will receive the prize at the Nobel ceremony in Stockholm on December 10.

Buried deep beneath Antarctic ice, IceCube is the world's largest neutrino detector, with 5,160 light sensors monitoring a cubic kilometre of ice. This achievement has opened a new window on the universe's most energetic events.

Yet this feat was not the work of one person alone. Isaac Newton once wrote, "If I have seen further, it is by standing on the shoulders of giants."

The journey to IceCube was much like climbing a high mountain. Generations of scientists had cleared the undergrowth, found footholds and painstakingly paved a path up the slopes. Every finding brought the summit a little closer. Halzen could climb higher because others before him had made the ascent possible. His achievement was to carry their work further, turning decades of ideas and experiments into a remarkable new way of observing the universe.

An unlikely journey to scientific discovery

"When you want something, all the universe conspires in helping you to achieve it," wrote Paulo Coelho in The Alchemist. For Pavel Alekseyevich Cherenkov, it wasn't the universe; it was Vladimir Lenin plotting to overthrow the Tsar and establish a socialist state.

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Born in 1904 into a poor peasant family in the Russian village of Novaya Chigla, Cherenkov's future seemed bleak. He had barely finished primary school when, at 13, he began earning a living unloading railway wagons, like other boys around him.

Then history took a dramatic turn. The Russian Revolution of 1917, led by Vladimir Lenin, brought the world's first workers' state to power. The new Soviet government sought to open education to children from working-class and peasant families. A secondary school opened in Cherenkov's village in 1920, allowing him to resume his studies.

The communist ambition went beyond removing formal barriers based on class, gender, ethnicity and religion. The Soviet state also sought to compensate for disadvantages inherited from the past. An expanding industrial economy needed trained engineers and scientists. At the same time, the new political vision held that scientific knowledge should serve society rather than remain the privilege of a narrow educated class.

Tuition was abolished, and stipends, preferential admission and relaxed entry requirements helped students from poorer backgrounds. In this changing educational landscape, Cherenkov entered university without completing all the usual prerequisites. Four years later, he graduated and became a physics and mathematics teacher at an evening school for workers in Kozlov.

Cherenkov's scientific breakthrough took place in 1934, while he was investigating how liquids responded to radiation. He observed a faint blue glow when gamma rays from a radioactive source passed through certain liquids. It would have been easy to dismiss this weak light as an experimental nuisance. Instead, he investigated it. The glow appeared even in pure liquids such as water and sulphuric acid, where ordinary fluorescence could not explain its unusual properties. Further experiments showed that gamma rays did not produce the light directly. Rather, they set fast-moving charged particles, particularly electrons, in motion; these particles produced the glow.

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But how could a particle travel faster than light? This does not violate the cosmic speed limit: nothing can travel faster than light in a vacuum. However, light moves more slowly through water or glass, and a sufficiently fast charged particle can outrun light travelling through the same material while still travelling below the speed of light in a vacuum. The particle then produces a kind of optical shock wave called Cherenkov radiation. The effect is somewhat like the sonic boom of an aircraft travelling faster than sound, although the underlying physics is different. The emitted light is often blue, giving the water around operating nuclear reactors its characteristic glow.

Cherenkov discovered the phenomenon; Ilya Frank and Igor Tamm later explained it theoretically. In 1958, all three received the Nobel Prize in Physics.

Following the footprints of the invisible

"Seek, and ye shall find." The biblical saying sounds reassuring, but what if the object of your search is a particle that can pass through Earth with almost no trace?

The puzzle began with beta decay, a form of radioactivity in which an unstable atomic nucleus emits an electron and transforms into another nucleus. Physicists knew that the difference in mass between the original one and final nuclei fixed the total energy released in the decay.

If the only products were the new nucleus and the electron, conservation of energy and momentum required the electron to emerge with a definite energy, apart from a tiny correction due to the recoil of the nucleus. But experiments revealed something unexpected: electrons emerged with a continuous range of energies, from very little to a maximum value. When an electron carried away less energy, the rest seemed to have vanished. Yet the energy released by the decay should be the same. Where had the missing energy gone? Was a fundamental law of physics being violated inside the atomic nucleus?

In 1930, the Austrian physicist Wolfgang Pauli proposed a startling solution. Perhaps another particle was escaping alongside the electron, carrying away the missing energy and momentum. This particle would have no electric charge, very little mass and an extraordinarily weak interaction with matter. Pauli initially called it a neutron; after James Chadwick discovered the much heavier particle now known by that name, Enrico Fermi coined the term neutrino. Yet a particle that passed through matter almost unhindered seemed all but impossible to detect.

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Bruno Pontecorvo, an Italian-born physicist who later made his home in the Soviet Union, refused to accept that this elusive particle must remain forever beyond reach. Born in Pisa in 1913 into a Jewish family, he studied under Enrico Fermi in Rome. He became one of the brilliant young physicists associated with Fermi's research group. He moved to Paris in 1936 to work with Irène and Frédéric Joliot-Curie as Fascism and antisemitism spread across Europe; Italy's racial laws of 1938 imposed severe restrictions on Jewish people.

Already abroad, Pontecorvo continued his scientific career amid the upheaval. He also became increasingly committed to communism. After working in Canada during the war and later in Britain, he left Britain abruptly in 1950 and settled in the Soviet Union. His departure was regarded in the West as a defection.

In 1946, Pontecorvo proposed an ingenious way to detect neutrinos: do not try to see the particle itself; look for the mark it leaves behind. He suggested using chlorine-37. If an electron neutrino struck one of these atomic nuclei, it could transmutate it into radioactive argon-37, releasing an electron. The rare argon atoms could then be extracted from the liquid and identified through their radioactive decay. The neutrino would have disappeared without a trace—but the transformed atom would betray its passage.

The scale of it all

The scale of the challenge becomes clear from an experiment built decades later using Pontecorvo's method. Ray Davis's detector at the Homestake mine in the United States held 100,000 US gallons (about 380,000 litres) of a chlorine-rich dry-cleaning fluid. It contained roughly 2 x 1030 chlorine-37 nuclei: a two followed by thirty zeroes. Even with this astronomical number of targets, scientists expected about 500-600 argon-37 atoms would be produced in a year. Argon-37 itself has a half-life of about 35 days, so the scientists had to extract the gas periodically and look for the telltale signs of its decay.

Imagine a detective coming to a house following a break-in. Although the thief has vanished, a muddy bootprint close to a window shows how he got in. The detective need not have to apprehend the burglar in the act of breaking in; the evidence left behind tells the whole story.

Pontecorvo's suggestion applied the same principle to the world of the subatomic. The idea would take many years before it could be turned into a working experiment. Yet its fundamental insight was revolutionary: in order to discover the invisible, one had to learn to recognise its footprints.

Turning water into a telescope

"Nothing is hidden that will not be revealed"—particularly when persistent human resourcefulness takes up the challenge.

Francis Halzen, whose work would eventually lead to the IceCube Neutrino Observatory, recalls a photograph of the Soviet physicist Moisey Markov standing alongside Pontecorvo. Reflecting on the history of neutrino physics, Halzen observed that Pontecorvo had thought of almost every major idea in the field—"except this one. This one was Markov's idea." What idea had even a mind as fertile as Pontecorvo's missed?

Markov had been thinking about a problem that his student, Igor Zheleznykh, was also investigating. In his 1958 diploma work, Zheleznykh estimated the expected flux of neutrinos produced when cosmic rays struck the Earth's atmosphere. But how could physicists catch these elusive particles? Ordinary laboratory detectors were far too small. Neutrinos interact so rarely with matter that scientists needed an enormous number of potential targets to have a reasonable chance of detecting even a few interactions.

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Markov's solution was audacious. Instead of looking for a transformed atom, as in Pontecorvo's proposal, why not watch for the faint flash of Cherenkov radiation produced when a neutrino interacted with matter? A muon neutrino interacting with an atomic nucleus in water or nearby rock could produce a muon. If the muon travelled faster than light travels through water, it would emit Cherenkov radiation. Sensitive light detectors could record the flash and, from the pattern and timing of the light reaching them, help reconstruct the particle's path. The neutrino itself would remain invisible; its passage would be inferred from the trail left by the particle it produced.

The idea demanded three things: an immense volume of material in which neutrinos might interact, a medium transparent enough for the light to travel, and darkness in which the faint flashes could be distinguished from background light. A lake or the deep ocean offered all three in principle. Markov proposed placing detectors deep underwater, where the surrounding water would serve both as the target and as the medium through which Cherenkov light travelled.

Ice, water, and the way forward

The idea slowly became reality. In 1980, Soviet scientists began developing a neutrino telescope in Lake Baikal, Siberia. The lake was a good choice for several reasons. Its deep water was remarkably transparent, the lake was sufficiently deep to shield the detector from much of the unwanted background, and its proximity to the shore made it easier to connect the instruments to a laboratory.

There was another unusual advantage: in winter, a thick cover of ice provided a stable platform from which scientists could lower detector strings into the water and retrieve them for repairs. A first stationary string was deployed in 1984, and the telescope grew through successive stages. The full NT-200 detector, with 192 optical modules, began operation on April 6, 1998—four years after Markov died in 1994.

But turning deep water into a reliable scientific instrument was a continuing struggle. Underwater currents could sway the detector strings and strain their cables; delicate sensors and electrical connections had to survive immense pressure. Bioluminescent organisms produced flashes of their own, while natural radioactivity in the water added further background signals. Scientists had to distinguish these distractions from the faint, fleeting Cherenkov light they sought.

The ocean presented even greater engineering challenges. The ambitious DUMAND project (Deep Underwater Muon And Neutrino Detector project), planned for the Pacific near Hawaii, encountered serious technical difficulties, including a short circuit in a deployed test string. In 1995, the project was cancelled before the full detector could be built. Markov's vision was sound, but realising it demanded more than an ingenious idea: it required technologies suited to working reliably in one of the most unforgiving environments on Earth.

From troubled waters to Antarctic ice

A smooth sea never made a skilled sailor. The proverb reminds us that difficulties can force us to think differently. For Halzen, the solution to the difficulties of building a neutrino telescope in water lay in an unexpected place: the ice underneath the South Pole.

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Halzen, a theoretical physicist at the University of Wisconsin–Madison, US, had been working on ways to detect high-energy neutrinos since the 1980s. Deep underwater, currents could move detector strings, damage cables, and complicate measurements. Why not use the immense Antarctic ice sheet instead? Deep below the surface, the ice is remarkably transparent, stable and dark. The challenge was to drill deep enough to reach the clearest layers and embed thousands of light sensors in them.

The result was IceCube, a neutrino observatory unlike any other. Using powerful hot-water drills, scientists melted holes more than 2 km deep. They lowered strings of optical sensors into them. As the water froze again, the sensors became embedded in the ice.

Completed in 2010, the detector uses 5,160 light sensors distributed across 86 strings, monitoring about a cubic kilometre of ice. When a neutrino interacts with matter in or near the detector and produces a charged particle, the resulting Cherenkov light is recorded by the sensors. From the pattern and timing of the flashes, scientists can reconstruct the particle’s path and infer the neutrino’s direction and energy.

IceCube has transformed neutrino astronomy. In 2013, it offered compelling evidence of high-energy neutrinos arriving from outside our Solar System. Later, researchers found evidence linking neutrinos to the distant active Galaxy TXS 0506+056 and to the Galaxy NGC 1068, both powered by supermassive black holes. In 2023, IceCube produced the first image of our own Milky Way in high-energy neutrinos, opening a new way to study the cosmic-ray interactions taking place within our Galaxy.

Could this extraordinary instrument also help solve the mystery of dark matter? Scientists suspect that dark matter may accumulate in places such as the Sun. If its particles annihilate, they might produce certain kinds of neutrinos that escape and reach Earth. IceCube searches for such signals. It has not yet detected convincing evidence of dark matter, but its observations help narrow the possibilities.

The next step is IceCube-Gen2, a proposed expansion designed to monitor about eight cubic kilometres of ice and greatly increase the number of high-energy neutrinos detected. Meanwhile, an upgrade to the existing observatory was completed in 2026.