Neutrinos from deep space

The Universe sends particles to Earth with far more energy than any laboratory on the ground can produce. Where they come from is one of the oldest open questions in astrophysics.

Cosmic rays cannot answer it. Because they carry an electric charge, magnetic fields in space bend their paths. By the time they arrive, the direction they come from no longer points back to where they started.

Neutrinos can. They have no charge, so they fly in straight lines from wherever they were made, passing through almost anything along the way. That is also the difficulty: they interact so rarely that catching even a few requires an enormous detector.

Catching the uncatchable

Francis Halzen realised the ice at the South Pole could be that detector. Completed in 2011, IceCube watches a cubic kilometre of Antarctic ice with 5,160 light sensors across 86 strings, frozen at depths between 1,450 and 2,450 metres. On the rare occasions a neutrino hits the ice, it produces charged particles that emit a faint flash of blue light. The sensors record the flash, allowing the team to calculate the neutrino's direction and energy. (In the 1990s, a smaller precursor array called AMANDA had proven that deep ice could work.)

What IceCube found

Then IceCube found what it was built to find:

• 2013: The two most energetic neutrinos recorded at the time, nicknamed "Bert" and "Ernie", were followed by the first evidence of high-energy neutrinos arriving from deep space.
• 2017 to 2018: An alert sent within minutes of a neutrino's arrival prompted about 20 observatories to look. They identified a flaring galaxy, the blazar TXS 0506+056, as the first convincing source of high-energy neutrinos.
• 2022: Evidence revealed neutrinos originating from NGC 1068, a nearby active galaxy.
• 2023: The first portrait of our own galaxy drawn in neutrinos was produced.

The impact reaches far beyond individual discoveries. IceCube turned neutrino astronomy from a theoretical prediction into an active way of observing the Universe. Its alerts changed how observatories collaborate: a single neutrino can now prompt telescopes around the world to aim at the exact same patch of sky. Its data is shared openly, including a public catalogue of 275 alerts from 2011 to 2020, so researchers outside the collaboration can verify and build upon it. Today, about 450 scientists from 58 institutions across 14 countries work on IceCube.

The Nobel Prize in Physics 2026 was awarded to Francis Halzen of the University of Wisconsin–Madison "for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin".