By Maria Bregman
By Maria Bregman
Ice at the South Pole as a telescope: Francis Halzen and the neutrino hunt
On 6 October 2026, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics 2026 to Francis Halzen of the University of Wisconsin–Madison, USA, “for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin”. He is the sole laureate, with a prize share of 1/1, and the prize amount is 12 million Swedish kronor.
A single name on a physics prize invites an obvious question: what did one person do that a collaboration of hundreds could not? The Academy’s answer is in the citation. Halzen supplied the vision and the scientific leadership, and the IceCube Neutrino Observatory at the South Pole supplied the evidence. Mark Pearce, chair of the Nobel Committee for Physics, said: “His tenacity and scientific vision has paved the way for a new kind of astronomy.”
A shy particle
The Academy’s popular background calls the neutrino “the shyest particle in the universe”: it has no electric charge and almost no mass, and it generally passes unnoticed through matter. Every second, it says, 65 billion neutrinos from the Sun flow through your little fingernail. Only seldom does a neutrino make its presence felt by colliding with an atomic nucleus.
That rarity is what makes cosmic neutrinos useful. According to the Academy, neutrinos from distant regions carry clues to the extreme environments where they originated, and the processes that accelerate protons to high energies also give rise to high-energy neutrinos. Cosmic rays, the particles that continuously traverse the cosmos, can reach us with far more energy than any accelerator on Earth can give them, but their origin has been, in the Academy’s words, “one of the universe’s best-kept secrets”. The University of Wisconsin–Madison adds that cosmic rays are charged and their paths cannot be traced directly back to their sources, while neutrinos are neutral and travel nearly undisturbed, which gives scientists an almost direct pointer to the source.
An idea about ice
In the 1980s Halzen heard that Soviet researchers planned to capture neutrino signals in Antarctica with radio receivers. He realised that another method could be to install light sensors in glacier ice and look for flashes of light from the collisions. Around 1960, the Academy notes, physicists had already suggested that water could capture neutrinos: when a neutrino strikes an atomic nucleus, a charged particle is produced that travels in the same direction and emits blue light, which can be detected if the water is transparent. Halzen’s idea was to use naturally occurring ice instead. He contacted John G. Learned, who worked on DUMAND, a similar project in the deep ocean off Hawaii, and together they first presented a concept for a neutrino observatory in the ice of the South Pole in 1988, at a conference in Poland.
The Academy lists the advantages of the South Pole: a research station with regular transports was already there; at depth the darkness is constant; unlike the ocean, there is no interference from living creatures; radioactivity is low; and the area is geologically stable, with no earthquakes. It also names the drawback. The cold makes travel to and from the Pole impossible for much of the year, so the practical work must be done in the brief Antarctic summer, between November and February.
In 1992, a group of tenacious researchers and engineers made the first attempts to lower light sensors into the glacier. IceCube covers a cubic kilometre of ice, with light sensors on long cables and a surface building where the signals are collected, and it was finished in 2011. The breakthrough, the Academy says, came around twenty years after those first attempts: IceCube registered neutrinos with properties that convinced researchers they came from distant regions of space.
What IceCube has found
According to the University of Wisconsin–Madison, IceCube announced the first detection of high-energy neutrinos from outside the solar system in 2013, which opened a new era of “multi-messenger astrophysics”, in which not only light but particles such as neutrinos and gravitational waves carry information about distant events. The discovery won the 2013 Physics World Breakthrough of the Year Award. In September 2017 the observatory detected a high-energy neutrino from a supermassive black hole at the centre of a distant galaxy in the direction of the constellation Orion. IceCube later provided the first evidence of high-energy neutrino emission from an active galaxy 47 million light-years away, in 2022, and from the Milky Way, in 2023. The collaboration’s own account names the galaxies TXS 0506+056 and NGC 1068 among the sources.
Each result bears on a question older than the observatory: where cosmic rays come from, a puzzle that has resisted solution since Victor Hess discovered them, more than a century ago. The Academy adds that the observatory’s continuing measurements could even reveal previously unknown cosmic phenomena.
The person and the team
Halzen was born in 1944 in Tienen, Belgium, and took his PhD in 1969 at KU Leuven. The Academy’s background says he was drawn to particle physics by the progress of the 1960s and moved to the University of Wisconsin–Madison soon after his doctorate; the university says he joined its physics faculty in 1972 and oversaw the design and development of IceCube and its predecessor experiment, AMANDA.
After the announcement, Halzen said: “It’s a great relief for me to finally deliver the recognition that this great collaboration deserves.” He called the prize “a celebration of a very unusual project” and credited the diligence and hard work of the many collaborators he has worked with. The IceCube collaboration includes some 450 scientists from 58 institutions in 14 countries, and its spokesperson, Erin O’Sullivan, said that Halzen “continues to inspire us with the potential of the future of the field”.
A prize to one person for a collective instrument always simplifies. The citation handles this by rewarding contributions and leadership in the observatory and the discovery of astrophysical neutrinos, not a single detection paper.
What comes next
In 2025–2026 the IceCube Upgrade was installed, intended to lower the energy threshold and better calibrate the ice. The collaboration expects the first science data from it later this year, and a larger successor, IceCube-Gen2, is proposed. These plans lie outside the cited award and are context, not part of the prize.
Francis Halzen. Photo: infn.it / Wikimedia Commons, CC0.
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