Belgian physicist Francis Halzen has won the 2026 Nobel Prize in Physics for his work in founding the IceCube Neutrino Observatory and for the discovery of high-energy elusive particles, known as neutrinos, that come from deep in space.

Halzen, who is based at the University of Wisconsin–Madison, will take home all of the 12-million Swedish kronor (US$1.2-million) prize, announced by the Royal Swedish Academy of Sciences in Stockholm on 6 October.

“It was a great surprise and I obviously didn’t expect it,” said Halzen, speaking to the Nobel press conference after the prize was announced. “This reflects on the really courageous people who joined me in this project when really no respectable conservative physicist would have joined me, but many talented people did and that’s why I’m here.”

Neutrinos are the second most common particle in the Universe, after photons. More than 1 billion neutrinos pass through a hand every second, but high-energy neutrinos are extremely rare and hard to detect. IceCube, based in the South Pole, was designed to study the particles when they fly out of some of the most energetic environments in the Universe, such as supernovae and γ-ray bursts. Because neutrinos pass straight through matter, they provide a straight line back to these events, providing researchers with a unique way of probing astrophysical phenomena.

Looking into the cosmos

Despite being all around us, neutrinos are fiendishly difficult to detect because they are unaffected by magnetic fields and rarely ever interact. In the 1980s, Halzen had the idea to use threads of detectors, lowered more than 1.5 kilometres into holes drilled into the clear ice of the Antarctic — which is free from many types of interference — to detect the rare and faint flashes of light when a fast-moving neutrino hits an atom. As long as the volume was big enough, Halzen theorized that the set-up could catch neutrinos and the path would reveal the direction from which they come, allowing researchers to trace their origins in the cosmos (see ‘Neutrino observatory’).

The IceCube Neutrino Observatory — an array of 5,160 sensors — was completed in 2011 and now involves a collaboration of more than 450 researchers. “After one and a half decades of development, and another decade of construction, there was no guarantee we would ever see anything,” Halzen, principal investigator of IceCube, told APS news in 2025. “Many people thought we wouldn't, but we did.”

Within two years, the detector had observed neutrinos from beyond the galaxy and with extreme energies. In 2013, IceCube published its result, showing that there was a population of neutrinos at energies so high that could not have been produced anywhere in the Solar System, and establishing the new field of neutrino astronomy¹.

“The biggest risk we took is that nobody knew if the kilometer cube detector was actually large enough to detect neutrinos beyond our atmosphere from the Universe and that was our biggest one. But it only took 2 years to detect that,” Halzen told the Nobel press conference.

“We found evidence for neutrinos coming from supermassive black holes in other galaxies, and they shine so strongly that when you look at a neutrino sky you don’t see the Milky Way.”

In 2014, IceCube discovered three neutrinos with energies that blew all others out of the water. Nicknamed Bert, Ernie and Big Bird, they had energies on the scale of petaelectronvolts, thousands of times more energetic than those produced in Earth’s most advanced particle colliders. And three years later, the collaboration made a long-awaited discovery of a high-energy neutrino that they could for the first time trace back to a specific source — a distant galaxy, called TXS 0506+056, often referred to as ‘the Texas event’.

Follow-up observations suggested that the source was a ‘blazar’, a violent galaxy harbouring a supermassive black hole at its core, that can flare up in brightness. Studies from a suite of telescopes viewing the blazar using different methods described the source across seven papers in 2018.