The story so far: Belgian-born U.S. physicist Francis Halzen has won the physics Nobel Prize for 2026 for helping build the IceCube observatory in Antarctica — a telescope that has no mirrors or lenses and doesn’t even point at the stars. Using 5,000 sensors buried deep in the ice, IceCube has explored what natural processes in the cosmos can accelerate subatomic particles to energies far beyond any human technologies have managed so far.
Since the early 20th century, scientists have known that energetic particles called cosmic rays constantly bombard the earth from outer space. Most cosmic rays are protons or the nuclei of elements like helium and iron. Some of these particles have been found to have more energy than the world’s largest particle accelerator — the Large Hadron Collider (LHC) in Europe — can manage.
Scientists thought powerful events like exploding stars or jets of energy emitted by distant galaxies could have given these particles a Herculean push, but they couldn’t know for sure. Just looking in the direction a cosmic ray came from didn’t help. Since cosmic rays are charged particles, they could have been deflected by magnetic fields in space before reaching the earth. So scientists turned to neutrinos. These subatomic particles have no electric charge, a very small mass, and interact very rarely with matter. Nuclear reactions, like fusion in the sun’s belly, produce neutrinos in copious numbers, and trillions of them pass through your body — and through the earth — every second without leaving any trace. And they travel in a straight line through space. Halzen and others came up with the idea of IceCube to ‘catch’ and study neutrinos with very high energy. When a cosmic-ray proton strikes other atoms, it can produce particles called pions, which rapidly decay through reactions that could produce neutrinos. And these high-energy neutrinos are an indication that some high-energy event produced them. But neutrinos’ reluctance to interact with matter also makes them extremely hard to catch. This idea actually predated Halzen’s insight, going back to the 1960s. Scientists, including John Learned, once tried to build a detector in the Pacific Ocean but maintaining the facility’s instruments underwater proved too difficult. With more data and analysis, the collaboration established that a population of high-energy neutrinos was coming to our planet from elsewhere in the universe — and that IceCube could spot them. It was a landmark event in a new field called neutrino astronomy.
This is why the IceCube observatory is in Antarctica: the enormous volume of frozen water ice is the catcher, and the 5,000 sensors embedded in the ice look for the occasional evidence of a neutrino striking the ice. In 1987, after being alerted to a Soviet Union plan to study neutrinos in Antarctic ice, Halzen, Thomas Stanev, and Enrique Zas concluded the approach would be unsuitable for the energies they were interested in. In this sense, the physics prize this year is reminiscent of the same prize for Barry Barish in 2017, who coordinated the complex effort to set up the LIGO gravitational-wave detectors. In 2013, scientists identified two neutrinos with around 140-times the energy of a proton accelerated by the LHC.
Halzen has been feted for proposing the idea as well as keeping hundreds of scientists and engineers working on it for decades, according to the Nobel committee.
