Halzen's discovery laid the foundation for a totally new type of astronomy. Cosmic neutrinos, due to their properties, are elusive and able to travel through galaxies, stars and all types of matter without being absorbed or deflected. For this reason they can be considered messengers: they arrive from the most remote corners of the Universe and bring with them information that, otherwise, would never have reached Earth.
They are the most elusive particles in the Universe: electrically neutral, with an almost non-existent mass and, therefore, capable of crossing planets, stars, human bodies and every type of matter without being perceived in any way and without leaving a trace. They are called cosmic neutrinos and their discovery is due to the Belgian Francis Halzen, awarded the 2026 Nobel Prize for Physics on 6 October. The 82-year-old scientist received the recognition for "his decisive contributions to the IceCube experiment", the largest neutrino observatory in the world which is located in the ice of Antarctica and which in 2018 managed to capture these particles arriving from the cosmos.
What are cosmic neutrinos
Thanks to the discovery of high-energy neutrinos arriving from the cosmos, pursued around the world for decades, Halzen's research has laid the foundations for a completely new type of astronomy. But what is it exactly? A neutrino, as anticipated, is an electrically neutral particle, with a very small mass that interacts very rarely with matter. For this reason it can cross planets, stars and entire regions of the Universe almost without being deflected or absorbed. And it is this characteristic that makes it unique and of fundamental importance for astronomy studies: just think that even light can be absorbed or deflected, while a cosmic neutrino can move undisturbed anywhere and arrive on Earth bringing with it direct information from its source.
Halzen's intuition
Francis Halzen, who worked as a scientific collaborator at Cern between 1969 and 1971, later became a professor at the University of Wisconsin-Madison and Principal Investigator of the Amanda and IceCube projects. This last project brought him the most prestigious award: Halzen had the idea of using an enormous volume of Antarctic ice as a detector and, in 2018, managed to capture the most elusive particles in the cosmos. IceCube uses about a cubic kilometer of ice, with thousands of optical sensors inserted deep inside: in the very rare cases in which a neutrino interacts with a particle in the ice, it can produce secondary particles that emit a faint light, called Cherenkov radiation. At that point, optical sensors record these flashes and, by reconstructing them, allow scientists to determine the energy and direction of the neutrino.
The importance of discovery
As commented by many other scientists after the Nobel awarded to Halzen, the discovery of cosmic neutrinos laid the foundations for a completely new type of astronomy. This is because we must think of cosmic neutrinos as a kind of messenger that arrives from the most remote corners of the Universe and brings with it important information. Neutrinos come out from sources that man, from the Earth, is unable to study and, reaching us without being absorbed, deflected or blocked, they bring us direct information. Many events in the cosmos have been studied through light which, however, does not always manage to reach the Earth, as in the case of black holes. Neutrinos, on the contrary, are also able to pass near a black hole, cross a galaxy and the Earth until arriving at the IceCube observatory. The study of cosmic neutrinos, therefore, can lead to understanding black holes themselves, to solving some still open questions in physics and, especially, to understanding where the most energetic particles in the Universe come from.
