The 2026 Nobel prize for physics has been awarded to Francis Halzen for his discovery of high energy neutrino particles from space.
The Nobel committee said Halzen has made “decisive contributions” to the discovery of “neutrinos of astrophysical origin” with IceCube, an experiment installed under ice at the South Pole.
Ellen Moons, secretary general of the Royal Swedish Academy of Sciences, referred to these neutrinos as “ghostly messengers”. This term refers to the fact the particles have nearly no mass and pass through solid objects with hardly any interaction. In fact, billions of neutrinos pass through any given person on Earth every second.
Most of these neutrinos are emitted by the Sun. However, high energy neutrinos coming from elsewhere in the cosmos also reach our planet. These neutrinos are about a billion times rarer than those that stream out from the Sun.
They originate from diverse sources, such as black holes located at the hearts of distant galaxies that accelerate particles to extreme energies.
The idea for the IceCube experiment was to use the ice at the South Pole as a transparent natural medium to capture the faint flashes of these high energy particles. The experiment comprises thousands of sensors under the frozen surface of Antarctica, distributed over one cubic kilometre.
Commenting on why Halzen’s work is important, Mark Pearce, chair of the Nobel committee for physics, said: “The neutrino has such peculiar properties: the fact that it interacts so infrequently in material – and in this case that it’s a way of bringing us information about distant cosmic sources that we are unable to acquire in other ways.”
Most of what we know about the Universe comes from light. But some of its most extreme environments can hide their secrets from light altogether. High-energy neutrinos give us another way to see them.
Halzen, who is based at the University Wisconsin-Madison in the US, has helped turn these elusive particles into a new tool for astronomy. Neutrinos interact so weakly with matter that they can travel through stars, galaxies and even entire planets almost undisturbed. That makes them extraordinarily difficult to detect — but also uniquely valuable messengers from the cosmos.
The challenge is that high-energy neutrinos coming from objects beyond our Solar System are exceptionally rare. To catch enough of them, physicists need a detector on an enormous scale – around a cubic kilometre.
In 1988, Halzen proposed a remarkably bold solution: use the Antarctic ice itself.
Natural detector
Deep beneath the South Pole, the ice is exceptionally clear. Scientists drill narrow boreholes deep into it and lower long strings of light sensors more than a kilometre beneath the surface. Once the holes refreeze, the sensors become embedded inside an enormous natural detector.
Most neutrinos pass straight through the Earth and through IceCube without leaving any trace. But occasionally, a neutrino collides with matter in or near the ice and produces a charged particle such as a muon.
That muon can move through the ice faster than light does in ice, producing a faint cone of blue light known as Cherenkov radiation. This does not mean it is travelling faster than the universal speed limit: light moves more slowly through ice than through a vacuum.
By measuring the timing and pattern of this light across many buried sensors, IceCube can reconstruct the muon’s path and infer the direction from which the original neutrino arrived.
In effect, the muon leaves a glowing trail through the ice that allows scientists to work backwards towards the neutrino’s source in the sky.
In 2013, IceCube reported the first strong evidence for a population of extremely high-energy neutrinos coming from beyond our solar system.
This was a major breakthrough because neutrinos behave very differently from other particles reaching us from space. Cosmic rays, for example, are electrically charged. Magnetic fields bend their paths as they travel through the universe, so by the time they arrive at Earth it can be extremely difficult to work out where they came from.
Neutrinos have no electric charge. They travel through space in almost straight lines, so their arrival direction can point us back towards their source. They can also escape from dense environments where light may be absorbed or scattered before getting out.
That opens a new window on some of the most violent places in the universe, including the surroundings of supermassive black holes and other powerful cosmic particle accelerators.
The breakthrough also helped establish what we now call multi-messenger astronomy: building a fuller picture of the universe by combining different cosmic signals, including light, gravitational waves, cosmic rays and neutrinos.
Each carries a different piece of information.
Twist in the tale
Neutrinos are no strangers to the Nobel prize. Over the past few decades, Nobel prizes have recognised their detection, the discovery of different neutrino types, the detection of cosmic neutrinos and the discovery that neutrinos oscillate — and therefore have mass.
Halzen’s work adds another chapter: using them not simply as particles to study, but as messengers with which to do astronomy.
Because neutrinos can cross enormous distances and pass through matter almost undisturbed, scientists have even speculated about whether they might one day be used for communication across interstellar distances.
That remains firmly hypothetical: IceCube is detecting neutrinos produced by natural astrophysical processes, not messages from extraterrestrial civilisations.
There is a lovely irony in the story. Neutrinos are useful precisely because they are so difficult to catch. Their reluctance to interact with matter is what allows them to cross enormous cosmic distances almost unchanged.
And the detector that made this new astronomy possible did not need to contain a cubic kilometre of manufactured material. Halzen’s insight was to realise that nature had already provided one beneath the South Pole.
These so-called “ghost particles” have therefore become a remarkable way of probing some of the most energetic – and otherwise hidden – parts of the universe.