Neutrinos from the Edge of the Universe. The 2026 Nobel Prize in Physics

This discovery has opened an entirely new window onto the Universe. Thanks to neutrinos, scientists can study regions from which even the most energetic light cannot reach us and search for answers to one of the oldest questions in modern astrophysics: where do the most energetic particles reaching the Earth come from?

Particles That Pass Through the Earth

Neutrinos are among the most elusive particles known to physics. They carry no electric charge and interact with matter only extremely weakly. This means they can travel enormous distances almost entirely unimpeded.

One might say that the Earth is nearly transparent to them. A neutrino can pass through the entire planet and, in most cases, nothing happens.

This is both their greatest advantage and their greatest challenge.

If neutrinos hardly ever collide with matter, how can they be detected at all?

Neutrinos are extraordinarily difficult to detect because they interact so weakly with matter. On the other hand, this very property makes them exceptional messengers from the most distant and extreme places in the Universe

– Prof. dr hab. Julian Sitarek, Department of Astrophysics, Faculty of Physics and Applied Informatics, University of Lodz.

A Telescope Hidden Beneath the Ice

Imagine a telescope with neither a lens nor a mirror. It is not located on a mountaintop or in orbit either.

It is hidden deep beneath the Antarctic ice.

This is the IceCube Neutrino Observatory, one of the most remarkable astronomical observatories in the world. Its “telescope” consists of approximately one cubic kilometre of ice located at the South Pole.

Embedded within the ice are 86 vertical strings containing a total of 5,160 optical sensors. They are deployed at depths of up to approximately 2.5 kilometres.

Why such a vast detector?

Because catching a neutrino requires an enormous amount of matter. Scientists wait for the exceptionally rare moments when a neutrino does collide with an atom in the ice. Such an interaction can produce a charged particle which, as it travels through the ice, emits a flash of light detected by IceCube’s sensors.

In practice, this means that scientists observe the Universe using light generated inside Antarctic ice.

Cosmic Messengers

Why are neutrinos such a valuable source of information?

Let us imagine a very distant and extreme cosmic object. Processes may occur there during which particles are accelerated to unimaginable energies. Among the products of these processes are high-energy protons and other cosmic-ray particles.

The problem is that protons carry an electric charge. During their journey through the Universe, they are deflected by magnetic fields. As a result, their paths do not point directly back to the place from which they originated.

Neutrinos do not have this problem.

They travel almost in a straight line from their source to the Earth. They can therefore act as cosmic signposts, indicating locations where the most energetic processes in the Universe take place.

This is immensely important because the origin of cosmic rays has remained a mystery for more than a century.

A New Window onto the Universe

IceCube has shown that some of the high-energy neutrinos it detects originate outside our Galaxy. This means they can be used to study distant cosmic sources and processes occurring under extreme conditions.

Neutrinos are also unique “messengers”. Light can be absorbed even within the source that emits it. Neutrinos can overcome such obstacles far more easily.

IceCube allows us to study regions of the Universe that may be opaque to high-energy light. Thanks to neutrinos, we receive information that cannot be obtained through observations of light

– Prof. dr hab. Julian Sitarek, Department of Astrophysics, Faculty of Physics and Applied Informatics, University of Lodz.

This is why astronomers are increasingly moving beyond observing light alone. They combine information from different “messengers” including photons, neutrinos and gravitational waves. This is how multi-messenger astronomy develops, enabling the same phenomena to be examined from multiple perspectives.

When a Neutrino Meets the MAGIC Telescopes

Fully exploiting the information provided by IceCube requires cooperation with other observatories.

One example is the MAGIC Collaboration, in which the Faculty of Physics and Applied Informatics of the University of Lodz also participates.

The MAGIC telescopes observe gamma radiation, light with energies incomparably greater than those of visible light. When IceCube detects a neutrino and identifies the direction from which it arrived, astronomers can point other telescopes towards the same region of the sky.

This happened in the case of the neutrino IceCube-170922A. Following its detection, the MAGIC telescopes observed a gamma-ray flare originating from the active galaxy TXS 0506+056.

This was the most significant example of the simultaneous observation of a neutrino and gamma radiation originating from the same direction.

However, another case is even more intriguing.

IceCube detected evidence of neutrino emission from the direction of the active galaxy NGC 1068. Despite several years of observations, however, the MAGIC telescopes did not detect any associated high-energy gamma radiation from that source.

This may indicate that light is absorbed within the source, while neutrinos are still able to escape.

Thanks to them, we can therefore look into places that remain hidden from light.

The Nobel Laureate Visited Łódź

Prof. Francis Halzen visited Łódź as early as 1988, when he took part in the V International Symposium on Very High Energy Cosmic Ray Interactions. He was also a co-author of numerous presentations delivered during the 31st International Cosmic Ray Conference, held in Łódź in 2009.

Today, his many years of work on IceCube and the study of cosmic neutrinos have been recognised with the highest distinction in the world of science.

What is more, research related to neutrinos and high-energy cosmic radiation also forms part of the scientific activity conducted at the Faculty of Physics and Applied Informatics of the University of Lodz.

This demonstrates that modern astrophysics is no longer solely about looking at the sky through a telescope. Sometimes it is necessary to look through a kilometre of Antarctic ice in order to see what light cannot reveal.

We extend our congratulations to Prof. Francis Halzen and the entire IceCube Collaboration!

Adapted from material prepared by Prof. dr hab. Julian Sitarek, Department of Astrophysics, Faculty of Physics and Applied Informatics, University of Lodz.