What are neutrinos?

Neutrinos are subatomic particles with a vanishingly small mass. They carry no electric charge, which means they almost never interact with ordinary matter. The sun pours them out constantly, and trillions zip through every human body each second without effect.

That near-invisibility is what makes neutrinos scientifically valuable: they travel through the universe essentially unimpeded, carrying information from regions of space that other signals cannot easily escape. The trade-off is that they are extraordinarily hard to catch. Scientists cannot glimpse neutrinos directly; they instead measure the secondary signals left behind by rare collisions.

Halzen focused his career on a rare population of neutrinos with extremely high energies, ones scientists treat as messengers from the cosmos. Tracking these particles back to their points of origin offers clues about how the universe evolved.

Why the committee honored Halzen

"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, in a news release.

Halzen's push to capture cosmic neutrinos dates back to the 1980s. Turning the IceCube concept into a working observatory required years of construction in one of the most remote places on the planet.

Who is Francis Halzen?

Born in Belgium, Halzen built his research career in the United States at the University of Wisconsin–Madison, where he has long led the IceCube collaboration. The project assembled researchers from multiple countries and operates instruments in one of the harshest environments on Earth. The Associated Press reported that efforts to reach him directly on Tuesday were not immediately successful.

What else is happening this Nobel week?

The 2026 Nobel Prize in medicine, announced Monday, went to three scientists whose work led to a tool that uses light to help researchers study how the brain works. The chemistry prize will be announced Wednesday, followed by literature on Thursday and the peace prize on Friday. The Nobel Memorial Prize in Economic Sciences closes the week on Monday.

Last year's physics prize went to three scientists for studies of quantum tunneling, a quantum-mechanical effect in which subatomic particles pass through energy barriers that classical physics would block. That work enabled the ultra-sensitive measurements at the heart of MRI machines and laid groundwork for faster cellphones and computers.