From Cherenkov's Glow to IceCube: The Long Road to Francis Halzen's Nobel Prize in Physics

On October 6, the Royal Swedish Academy of Sciences announced that Francis Halzen would receive the 2026 Nobel Prize in Physics, citing his decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin. He will collect the prize at the ceremony in Stockholm on December 10. IceCube is the world's largest neutrino detector: 5,160 light sensors buried in Antarctic ice, watching a cubic kilometre of it.

The Federal's account insists the prize rests on a long chain of earlier work, and it begins with Pavel Alekseyevich Cherenkov. Born in 1904 into a poor peasant family in the village of Novaya Chigla, he was unloading railway wagons for a living at 13. After the 1917 revolution the Soviet government opened education to peasant and working-class children, and a secondary school opened in his village in 1920. Tuition was abolished and entry requirements relaxed. Cherenkov entered university without all the usual prerequisites, graduated four years later, and taught physics and mathematics at an evening school for workers in Kozlov.

In 1934, while studying how liquids respond to radiation, he saw a faint blue glow when gamma rays passed through them. He did not dismiss it. The glow appeared even in pure water and sulphuric acid, which ruled out ordinary fluorescence, and further experiments showed that the gamma rays set fast electrons moving, and the electrons made the light. A charged particle can outrun light inside a material such as water, where light travels more slowly, while still staying below the speed of light in a vacuum. The result is an optical shock wave, loosely like a sonic boom. Ilya Frank and Igor Tamm later explained the effect theoretically, and all three shared the 1958 Nobel Prize in Physics.

The second thread is the neutrino itself. In beta decay, electrons emerge with a continuous range of energies rather than one fixed value, so some energy seemed to vanish. In 1930 Wolfgang Pauli proposed that an uncharged, nearly massless particle was carrying it away. Pauli first called it a neutron; after James Chadwick found the heavier particle that now bears that name, Enrico Fermi coined "neutrino." Detecting something that passes through matter almost unhindered looked close to impossible.

Bruno Pontecorvo refused to accept that. Born in Pisa in 1913 into a Jewish family, he studied under Fermi in Rome and moved to Paris in 1936 to work with Irène and Frédéric Joliot-Curie as fascism and antisemitism spread; Italy's racial laws followed in 1938. He worked in Canada and Britain, then left Britain abruptly in 1950 for the Soviet Union, a move the West regarded as defection. In 1946 he had proposed looking not for the neutrino but for its mark: an electron neutrino striking chlorine-37 could produce radioactive argon-37, which could be extracted and identified by its decay.

The scale shows the difficulty. Ray Davis's later detector at the Homestake mine in the United States held 100,000 US gallons, about 380,000 litres, of chlorine-rich dry-cleaning fluid, roughly 2 x 10^30 chlorine-37 nuclei. Even so, only about 500 to 600 argon-37 atoms were expected per year, and with a half-life near 35 days the gas had to be extracted periodically.

The idea that led toward IceCube came from Moisey Markov. Halzen recalls a photograph of Markov standing beside Pontecorvo, and observes that Pontecorvo thought of nearly every major idea in neutrino physics except this one. Markov's student Igor Zheleznykh had estimated, in his 1958 diploma work, the neutrino flux from cosmic rays striking the atmosphere. Laboratory detectors were far too small, so Markov proposed watching for Cherenkov flashes instead. A muon neutrino interacting in water or nearby rock could produce a muon, which would emit light; sensitive detectors could use the pattern and timing of that light to reconstruct its path. The method needs a huge volume of material, a transparent medium and darkness, all of which a deep lake or ocean could offer.

In 1980 Soviet scientists began building such a telescope in Lake Baikal, Siberia. Its water was remarkably transparent, deep enough to shield against background, and close to shore for connection to a laboratory. In winter, thick ice gave a stable platform for lowering and repairing detector strings. A first stationary string went in during 1984, and the full NT-200 detector, with 192 optical modules, began operating on April 6, 1998. The source text breaks off there, but the line from Baikal to Antarctica is the one the Stockholm ceremony will formally honour.