Belgian-born physicist honoured for turning Antarctic ice into a giant neutrino detector and opening a new way of studying the universe
SRINAGAR: Francis Halzen, a physicist at the University of Wisconsin–Madison, has been awarded the 2026 Nobel Prize in Physics for his decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin, the Nobel Prize organisation said.
According to the Nobel Prize organisation, Halzen’s scientific vision transformed the ice beneath the South Pole into a giant detector capable of capturing extremely rare particles arriving from the distant universe. The resulting IceCube Neutrino Observatory occupies about one cubic kilometre of ice and is equipped with thousands of light sensors.
The achievement has opened what the Nobel Prize organisation describes as a new kind of astronomy, allowing scientists to study some of the universe’s most violent and mysterious processes through particles that can travel enormous distances without being deflected or losing energy.
Elusive Particles
Neutrinos are among the most difficult particles in nature to detect. The Nobel Prize organisation describes them as having no electric charge and almost no mass. They interact so weakly with matter that they normally pass straight through the Earth and through human bodies without anyone noticing.
The scale is extraordinary: according to the Nobel Prize organisation, about 65 billion neutrinos from the Sun pass through an area the size of a fingernail every second. A neutrino rarely collides with an atomic nucleus, producing a signal scientists can detect.
Yet these almost invisible particles can carry valuable information about where they came from.
Neutrinos are produced in some of the most energetic environments in the universe. Scientists have long known that cosmic particle accelerators can generate energies up to a million times greater than those achievable in laboratories on Earth. But the identity and workings of many of these natural accelerators have remained a major mystery.
Antarctic idea
The breakthrough began with an idea Halzen developed in the 1980s.
While working on cosmic particles, Halzen learned of plans to detect neutrinos in Antarctica using radio receivers. He realised that there might be another way: if a neutrino collided with an atomic nucleus inside Antarctic ice, the collision could produce a flash of light. Sensitive instruments buried deep in the ice could detect that flash.
Halzen and physicist John G Learned developed the concept and presented their proposal for a neutrino observatory in the South Pole ice in 1988, according to the Nobel Prize organisation.
The location offered several advantages. A research station was already operating at the South Pole, providing logistical support. Deep ice was dark, stable and relatively free of interference from living organisms and radioactive material. The region is also geologically stable and does not experience earthquakes.
The disadvantages were equally obvious. The extreme cold makes travel to and from the South Pole impossible for much of the year, meaning that most practical work has to be carried out during the short Antarctic summer, between November and February.
Elusive Particles
Turning the idea into an observatory required solving formidable engineering problems.
According to the Nobel Prize organisation, researchers learned from glaciologists that kilometre-deep holes could be drilled by using hot water to melt the ice. Long cables carrying strings of light sensors could then be lowered into the holes.
Halzen has likened the sensors to lightbulbs in reverse: rather than producing light, they capture it and convert it into an electrical signal.
The researchers first tested their approach in Greenland before beginning construction of the first South Pole neutrino observatory, known as AMANDA.
The early experiments revealed a problem. Bubbles in the upper layers of the Antarctic ice scattered light and blurred information about the direction in which particles were travelling. But below about 1,400 metres, the ice was exceptionally pure and transparent. Light generated by neutrino interactions could travel roughly 300 metres before being absorbed.
AMANDA was completed in January 2000. Although it worked, scientists concluded that a much larger detector would be needed to capture the extremely rare, high-energy neutrinos produced by cosmic particle accelerators.
The IceCube
A cubic kilometre of ice becomes a detector
IceCube was built across one cubic kilometre of Antarctic ice. When it reached its full size in 2011, it contained 5,160 light sensors arranged along 86 cables.
The enormous size was necessary because high-energy cosmic neutrinos are extremely rare. Scientists needed to monitor a huge volume of ice to have a realistic chance of observing enough collisions.
But IceCube was also detecting many other particles.
Cosmic rays constantly strike the Earth’s atmosphere, producing showers of particles. According to the Nobel Prize organisation, more than 100 million such atmospheric particles are registered by IceCube every day. Atmospheric neutrinos also reach the detector, including some that travel through the Earth from the northern hemisphere.
Scientists therefore had to distinguish the rare cosmic neutrinos from this enormous background of ordinary atmospheric events.
The Discovery
The breakthrough came in stages.
In 2013, the IceCube team reported the first evidence that it had detected high-energy neutrinos originating beyond our solar system. Within a few years, enough data had been collected for researchers to establish the discovery with confidence, according to the Nobel Prize organisation.
The significance lies in what these particles can tell scientists about their origins.
Unlike electrically charged particles such as protons, neutrinos are not deflected by magnetic fields as they travel through space. Their direction therefore preserves information about where they came from.
This makes neutrinos potentially valuable messengers from distant cosmic particle accelerators.
Neutrino Advantage
Cosmic rays frequently consist of protons and other atomic nuclei. Some reach Earth with energies far beyond what human-built particle accelerators can produce.
But because protons carry an electric charge, magnetic fields bend their paths through space. By the time they reach Earth, scientists cannot simply trace their trajectory backwards to identify their source.
Neutrinos provide a possible solution. The processes that accelerate these cosmic rays can also produce high-energy neutrinos. Since neutrinos are electrically neutral, they travel without being bent by magnetic fields. Scientists can therefore use their direction to investigate the places where the cosmic rays were accelerated.
The Nobel Prize organisation also points out that gamma radiation, another messenger from violent cosmic events, can diminish as it interacts with matter and light during its journey through space. Neutrinos are much less affected. This allows them to carry information from environments that may be hidden from other forms of observation.
Astronomy Advances
Neutrino astronomy itself is not new.
Scientists began detecting neutrinos from the Sun in the 1960s. In 1987, the Kamiokande observatory in Japan detected a burst of neutrinos associated with a supernova in the Large Magellanic Cloud, a neighbouring galaxy.
These discoveries demonstrated that neutrinos could be used to investigate astronomical events. Raymond Davis Jr. and Masatoshi Koshiba received the 2002 Nobel Prize in Physics for their pioneering neutrino research.
Another breakthrough came with the discovery that neutrinos can change from one type into another as they travel. Takaaki Kajita and Arthur B. McDonald received the 2015 Nobel Prize in Physics for this discovery.
Halzen’s work has taken neutrino astronomy to a new scale by using a massive volume of Antarctic ice to detect the highest-energy neutrinos arriving from the cosmos.
The Nobel Prize organisation says the aim is to obtain information about phenomena that cannot be studied in any other way, including objects hidden behind dust clouds or so distant that other forms of radiation have disappeared during their journey to Earth.
Source Hunt
IceCube has already identified possible sources of cosmic neutrinos, but scientists have not yet solved the mystery of all their origins.
One promising candidate is NGC 1068, also known as M77, an active galaxy. IceCube has recorded 79 neutrinos apparently coming from its direction. The Nobel Prize organisation cautions, however, that the evidence is not yet strong enough to establish conclusively that NGC 1068 is a neutrino source. More observations are needed.
Researchers have also detected high-energy neutrinos coming from within the Milky Way. They believe these may be produced when cosmic radiation collides with atoms in the thin gas between stars.
Every additional neutrino detected could help scientists narrow down the locations and understand the physical processes of the universe’s most powerful natural particle accelerators.
IceCube Future
The Nobel Prize organisation says neutrino astronomy is likely to expand substantially in the years ahead.
Other neutrino telescopes are being developed in the Northern Hemisphere, using water rather than ice. Projects in places including Lake Baikal, the Mediterranean and the South China Sea are part of a growing international effort to detect and study these elusive particles.
Meanwhile, IceCube itself is set to grow.
Scientists are planning IceCube-Gen2, an expanded observatory that would monitor an astonishing eight cubic kilometres of ice, compared with IceCube’s present one cubic kilometre.
For Halzen, the journey from an idea presented in 1988 to a functioning observatory at the bottom of the world has taken more than three decades.
For astronomy, the implications could last much longer.
By turning Antarctic ice into a giant detector, Halzen and the international IceCube team have given scientists a new way to investigate the universe, not simply by observing the light that reaches Earth, but by detecting the almost invisible particles that have travelled across space carrying clues about some of the cosmos’s most violent events.
