Francis Halzen won the 2026 Nobel Prize in Physics for IceCube, a cubic kilometre of South Pole ice that detects neutrinos from deep space. Here is how it works.
At the South Pole, the ice is more than two kilometres thick, older than human civilisation and, at depth, astonishingly clear. Buried in it, in a block one kilometre on each side, are more than 5,000 glass spheres strung like beads on 86 cables. They sit in total darkness, waiting for flashes of blue light that last a few billionths of a second. This is the IceCube Neutrino Observatory, and it won Francis Halzen this year’s Nobel Prize in Physics, “for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin”.
The ghost particle
Neutrinos were proposed in 1930 by Wolfgang Pauli, almost apologetically, to balance the books in a kind of radioactive decay. They were not detected until 1956. The delay says everything about them. Neutrinos have no electric charge and almost no mass, and they feel only the weak nuclear force and gravity. Trillions of them, most from the Sun, pass through your body every second without touching a single atom. A neutrino could cross a light-year of lead with a good chance of coming out the other side.
That elusiveness is exactly what makes them precious to astronomers. Light from distant objects can be absorbed by dust and gas, or blocked by the dense regions where the most violent processes happen. Cosmic rays, the charged particles that rain onto Earth with enormous energies, are bent by magnetic fields on their way here, so their direction no longer points back to where they came from. Neutrinos are neither absorbed nor bent. They travel in straight lines from their source, carrying news from places nothing else can leave.
And those places matter. Cosmic rays were discovered in 1912, yet more than a century later, where the most energetic of them are accelerated remained an open question. Wherever such particles are produced, they should also produce high-energy neutrinos. Find the neutrinos, and you find the accelerators.
Why it had to be enormous
The catch is that almost nothing stops a neutrino, so almost nothing detects one. Very occasionally, a neutrino collides head-on with an atomic nucleus and produces a charged particle, often a muon, which carries on in nearly the same direction. That particle moves faster than light travels in ice. The result is the optical equivalent of a sonic boom: a faint cone of blue Cherenkov light. Record the light with enough sensors, and you can reconstruct the particle’s direction and energy.
Cosmic high-energy neutrinos are so rare that theorists had calculated a detector would need a volume of about a cubic kilometre to see them in useful numbers. Building a tank that size was out of the question. Halzen’s insight, put forward in 1988 together with a colleague, was to use a natural one. The deep Antarctic ice is clear, dark, stable and free.
From AMANDA to IceCube
Halzen was an unlikely leader for such a project. Born in Tienen, Belgium, in 1944, he earned his doctorate at the University of Louvain in 1969, worked at CERN, and moved to the University of Wisconsin–Madison in 1972 as a theoretical particle physicist. He later recalled being lucky to be in Wisconsin when the idea came up.
The first test was a smaller array called AMANDA, which proved that light sensors lowered into the ice could detect neutrinos. It also taught hard lessons about the ice itself, whose clarity and structure had to be mapped in detail before any measurement could be trusted. In 2005 AMANDA became part of its far larger successor.
Building IceCube was a feat of engineering as much as physics. Crews used jets of hot water to melt holes about two and a half kilometres deep, then lowered strings of sensors into them before the water refroze, locking the instruments in place for good. Drilling was possible only in the short Antarctic summer. Construction began in 2004 and ended in December 2010, with 86 strings and more than 5,000 light sensors. A denser inner section called DeepCore lowered the energy threshold to around 10 GeV, opening the detector to lower-energy studies as well.
IceCube also uses the whole Earth as a filter. Most of the signals it records come from ordinary particles made when cosmic rays strike our own atmosphere. By looking for neutrinos that come up through the planet from below, physicists can screen out much of this background, because only neutrinos can cross the Earth.
A prize for a leader, and a team
IceCube is the work of a large international collaboration, and Halzen has been its principal investigator from the start. Mark Pearce, chair of the Nobel Committee for Physics, said Halzen had led an international team of researchers and engineers that gave science a fantastic instrument. He is the first person to win the physics prize alone since Pierre-Gilles de Gennes in 1991, a rare choice that reflects how central his leadership was.
Halzen said he hoped the prize would help win approval for IceCube’s planned successor, a much larger detector. After almost four decades in the ice, he is already looking at the next block.
– Sudhakar Garlanka
