Neutrinos are the most common elementary particles and at the same time the most mysterious. Every second, billions of these tiny particles rush through our body without us feeling it. Even solid steel, lead or our entire planet are no obstacle for these “ghost particles”. Neutrinos carry no electrical charge, have hardly any mass and only interact via the weak nuclear force – one of the four fundamental forces of physics. However, this only works in and on the atomic nucleus. A neutrino can therefore only be detected when it hits an atomic nucleus.
Messengers from the cosmos
At the same time, neutrinos are important helpers for physics, astronomy and cosmology. Because these elementary particles are not only created during radioactive nuclear decay or in the sun. They are also released in some of the most energetic phenomena in the cosmos: supernovae, gamma-ray bursts or the tearing of a star by an active black hole. The crucial thing, however, is that such cosmic neutrinos can travel billions of light years without being deflected.
If you capture neutrinos and determine their origin, you can find out how and where such cosmic events take place. Because the “ghost particles” are virtually indestructible, neutrinos from the first stellar explosions in our universe can still be found in the cosmos.
Signal from a high-energy cosmic neutrino in the IceCube neutrino detector — © IceCube Collaboration
How do you catch neutrinos?
The problem, however: If neutrinos are so “shy” and hardly interact with matter – how do you capture them? This is exactly the question that the new Nobel Prize winner in physics, Francis Halzen, asked himself – and answered. “Francis Halzen is the driving force behind the IceCube neutrino observatory. As lead scientist, he has significantly advanced the joint work of scientists and engineers from the very beginning. His scientific vision also forms the basis for future research in this area,” explains the Nobel Prize Committee.
Halzen got the impetus for the neutrino detector he designed from an idea from the 1960s: At that time, some physicists suggested using light-sensitive sensors in large water tanks as neutrino detectors. When the “ghost particles” collide with an atomic nucleus, excited, charged secondary particles are created that release their excess energy in the form of light.
Because water is transparent, photosensors can capture this light. The path and energy of this glow can then be used to determine where the original neutrino came from and how energetic it was. However, for this to work, a large volume of water is required – which makes such detectors complex. Natural bodies of water, on the other hand, are only suitable to a limited extent because water turbulence and other disruptive influences make measurements difficult.
South Pole ice instead of water tank
In 1988, Francis Halzen came up with the idea of using frozen water as a neutrino detector: the ice of Antarctica, more precisely under the South Pole. The advantage: The ice there is very thick, clear and dark, and there are no disturbances from living creatures or turbulence in the ice cover, which is several kilometers thick. In addition, the South Pole plateau is geologically stable and there are no earthquakes. Halzen’s idea was to insert photodetectors into this ice, which could then monitor a correspondingly large volume of ice for the subtle neutrino signals.
Lowering a photosensor into the Antarctic ice. — © IceCube Collaboration
But what seemed logical in theory was anything but easy in practice: Antarctica is extremely hostile to life. It is correspondingly difficult to build an observatory there. Halzen and his team therefore decided to first try out the necessary methods and technologies in Greenland. There they tested, among other things, how to use hot water drills to get the photosensors housed in transparent spheres hundreds of meters deep into the ice.
From the precursor to the IceCube detector
At the beginning of the 1990s, the construction of the first neutrino observatory in the Antarctic ice began with the IceCube predecessor AMANDA. However, the first test drillings were discouraging because they revealed that the South Pole ice was less clear than hoped: countless small bubbles clouded the view of the photosensors. The ice was only clear enough below a depth of 1,400 meters. The team therefore developed methods to place their sensors at this depth. In 2000, the AMANDA detector was completed and delivered the first data.
However, it soon became clear that the volume of the AMANDA detector was not sufficient to detect cosmic neutrinos. Because they are much rarer, a larger detector volume is required to capture them. Therefore, Halzen and his team began constructing IceCube – a neutrino detector that covers a good 1 cubic kilometer of ice. The first expansion stage of IceCube was completed in 2011: It includes 5,160 photosensors embedded in 86 vertical rows in the ice beneath the South Pole.
Building the IceCube neutrino detector — © IceCube Collaboration
IceCube’s discoveries and what happens next
Since then, we have been able to thank the IceCube neutrino detector for a number of discoveries: in 2013, physicists used it to detect the first cosmic neutrino, and in 2018 they were able to trace one of these extremely high-energy particles back to its origin for the first time. The neutrino detected by IceCube came from a blazar – an active black hole at the heart of a galaxy – around four billion light-years away. In 2023, physicists created the first neutrino map of our Milky Way using data from the South Pole.
However, not all questions about cosmic neutrinos and their sources have been clarified yet. In 2026, physicists used IceCube data to discover a kink in the energy distribution of cosmic neutrinos, the cause of which is still unclear. And the role of these elementary particles in other areas of physics and cosmology is still far from being fully explored. This is one of the reasons why the IceCube Observatory is being further expanded. At the beginning of 2026, six additional strands with a total of more than 600 photosensors were embedded up to 2,400 meters deep into the Antarctic ice.
“We will certainly hear a lot more about neutrino astronomy in the coming years,” states the Nobel Prize Committee. Francis Halzen paved the way for an entire discipline with his theoretical and practical contributions.
Source: Nobelprize.org