Francis Halzen of the University of Wisconsin–Madison, US, was awarded the 2026 Nobel Prize in Physics on 6 October for his decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin. The Royal Swedish Academy of Sciences recognised Dr Halzen’s scientific leadership in developing IceCube, a neutrino observatory embedded in a cubic kilometre of Antarctic ice at the South Pole. The facility has enabled researchers to detect extremely high-energy neutrinos originating from distant regions of the universe.
Neutrinos are electrically neutral particles with almost no mass that interact only very weakly with matter. Billions pass through the human body every second, but because they rarely collide with atomic nuclei, they are exceptionally difficult to detect.
Their ability to travel through matter largely undisturbed makes high-energy neutrinos valuable messengers from the cosmos. Unlike charged cosmic rays, whose paths are bent by magnetic fields, neutrinos travel in straight lines, allowing researchers to trace them back towards their sources. They can also pass through matter and radiation that would obstruct other forms of astronomical observation.
Halzen first proposed using Antarctic ice to detect neutrinos in 1988. His approach involved placing light sensors deep within the ice to capture flashes generated when neutrinos interacted with atomic nuclei. The South Pole offered several advantages, including exceptionally clear and stable ice, low levels of radioactive material and the absence of biological interference found in ocean-based detectors.
The concept eventually led to AMANDA, an early neutrino observatory, before work began on the much larger IceCube detector. Constructing the facility required sensors to be lowered into kilometre-deep holes drilled into the ice using hot water. IceCube reached its full size in 2011, with 5,160 light sensors distributed across 86 cables.
Detecting astrophysical neutrinos required researchers to distinguish them from the much larger background of particles produced by cosmic rays interacting with Earth’s atmosphere. In 2013, the IceCube team reported the first evidence that it had detected cosmic neutrinos, followed by confirmation of the discovery after collecting additional data.
The findings opened a new field of neutrino astronomy, offering a way to investigate some of the universe’s most energetic environments. Scientists hope the particles will help identify the sources responsible for accelerating cosmic rays to extreme energies, including potentially violent stellar explosions and active galaxies.
IceCube has already identified possible neutrino sources, including the active galaxy NGC 1068, although the evidence is not yet sufficient to establish it definitively as a source. Researchers have also detected high-energy neutrinos originating within the Milky Way.
“Francis Halzen has led an international team of researchers and engineers who have provided us with a fantastic instrument. His tenacity and scientific vision has paved the way for a new kind of astronomy,” said Mark Pearce, Chair of the Nobel Committee for Physics.
IceCube continues to collect neutrino data, while plans are underway for IceCube-Gen2, an expansion that would increase the instrumented volume to eight cubic kilometres of ice. The project could provide further insight into the violent cosmic processes that produce high-energy neutrinos and potentially reveal previously unknown astronomical phenomena.
By Rosie Bannister
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