Physics Nobel 2026: A block of ice caught ghosts. Indian scientists explain how

Beneath the South Pole, in a darkness so complete that no sunrise has ever reached it, 5,160 light sensors hang in the ice like lanterns on very long strings, waiting for a faint blue flicker that almost nothing can make. On October 6, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics 2026 to Francis Halzen, the scientist who first imagined that a frozen continent could be turned into a telescope. The flicker appears when a ghost from deep space finally collides with an atom.

The citation honours his decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin, which simply means particles born far beyond our solar system. Space has been sending us messages all along. Halzen built the ear to hear them.

WHO IS FRANCIS HALZEN, AND WHY HIM?

Halzen was born in 1944 in Tienen, Belgium, earned his PhD at KU Leuven in 1969 and soon moved to the University of Wisconsin-Madison in the United States, where he is still a professor. He began in particle physics, the study of the smallest building blocks of matter. Only later did he grasp what particles arriving from space could teach physicists, and what physics could teach astronomers in return.

He is the only laureate this year, although IceCube is the work of hundreds of scientists from dozens of institutions in 14 countries. The Nobel Committee's scientific background explains its reasoning. Halzen was Principal Investigator from the very beginning, meaning the scientist who leads a project and answers for it, and he is credited with its invention, its design, its construction and its everyday science. Debanjan Bose, an astroparticle physicist who joined the IceCube Collaboration in 2009, calls what Halzen and the collaboration achieved almost unimaginable.

WHAT IS A GHOST PARTICLE, AND WHY CHASE IT?

The neutrino carries no electric charge and almost no mass, and it barely notices ordinary matter. According to the Nobel Prize website, 65 billion neutrinos from the Sun pass through your fingernail every second without leaving a trace. Hence, the nickname.

Neutrinos have been to Stockholm before. In 2002, Raymond Davis Jr and Masatoshi Koshiba won for catching neutrinos from the Sun and from an exploding star. In 2015, Takaaki Kajita and Arthur McDonald won for showing that neutrinos change type in flight, which proves they have mass. This year's prize is about neutrinos with vastly more energy.

Why chase them? Because Earth is constantly showered by cosmic rays, mostly protons, carrying more energy than any machine we have built, and where nature makes such accelerators is among the universe's best-kept secrets. A good cosmic messenger, explains Prof Sovan Chakraborty, an astroparticle physicist at IIT Guwahati, must above all point back to its source, and most travellers cannot. Charged particles are bent by magnetic fields at the source, in their host galaxy and again in ours, like a pinball ricocheting off bumpers, so they arrive having forgotten where they began.

Light is more faithful, but not faultless. Space only looks empty, Chakraborty says. Even between galaxies it is filled with faint backgrounds, among them the cosmic microwave background, the afterglow of the early universe, and the accumulated glow of every star that has shone. These can absorb light or alter its character on its long journey, so a gamma ray reaching us has often lost part of what it knew at birth.

Neutrinos, being nearly massless and only weakly interacting, slip out of their sources and cross the cosmos almost untouched. They keep their direction and, Chakraborty notes, their spectra retain the original properties of the source. The one caveat is that neutrinos change type in flight, which alters their flux a little. That behaviour, he says, is far better understood than the dimming of light, and its discovery earned the 2015 prize.

They are also a natural by-product. When a speeding proton smashes into gas or light near its source, it creates particles called pions. Neutral pions decay into gamma rays, a form of light that can be absorbed along the way. Charged pions decay into neutrinos, which arrive undisturbed. Each one is a postcard carrying the accelerator's return address.

Scientists therefore like to read all three messengers together, cosmic rays, gamma rays and neutrinos, an approach called multimessenger astrophysics. IceCube's neutrinos, Chakraborty says, are the most crucial piece of the jigsaw puzzle, and the messengers are not competing; rather, they complement each other.

Their energy is measured in electronvolts, a minuscule unit. These neutrinos reach a petaelectronvolt, or PeV, a million billion electronvolts. The Large Hadron Collider near Geneva, the most powerful accelerator on Earth, pushes protons to about 7 teraelectronvolts. A PeV is well over a hundred times that.

WHY BUILD A TELESCOPE INSIDE ANTARCTIC ICE?

Because ghosts are hard to catch with a small net. Neutrinos rarely collide with anything, and high-energy ones are far scarcer than low-energy ones, so catching them takes a gigantic volume of material. IceCube is built mainly for the high-energy end of the neutrino sky, Chakraborty notes, and that is why it needs a cubic kilometre. Physicists suggested using water around 1960. The pioneering project, DUMAND, sat 4,800 metres deep in the Pacific off Hawaii. Its single string, deployed in 1993, failed after a short time, and the sea proved noisy, thanks to glowing marine life and radioactive potassium.

In autumn 1987, Halzen gave a talk in Kansas, where a glaciologist told him about a Soviet plan to detect neutrinos in Antarctic ice using radio waves. Halzen and colleagues worked out that it would not be sensitive enough for PeV neutrinos. But ice stayed on his mind. In June 1988, he and John Learned, then with DUMAND, put the idea on paper: let the ice be both the target and the detector. Halzen has since said that others probably had the same thought and gave up, and that being unaware of what was known about ice's optical properties kept him going.

Ice has real advantages. The South Pole already had a research station. At depth, the darkness is constant, there are no living creatures, radioactivity is low, and the ground does not shake. The price is a short working season, between November and February, because the cold makes travel impossible for much of the year. Bose points out that there is no road, no power grid and months of darkness, and that the practical challenges were enormous.

Early tests in Greenland and with an earlier detector, AMANDA, delivered a surprise. Microscopic air bubbles near the surface scattered light so badly that tracks blurred. But below about 1,400 metres, the ice turned out to be astonishingly clear.

HOW DOES ICECUBE CATCH A NEUTRINO?

The builders borrowed a trick from glaciologists: a hot-water drill, which the Academy compares to a sophisticated shower head, melts holes a kilometre deep. Cables of sensors are lowered in and the ice freezes around them. Between 2005 and 2010, 86 cables went down, each with 60 sensors spaced 17 metres apart, between 1,450 and 2,450 metres below the surface. IceCube was completed in 2011, instrumenting about a cubic kilometre of ice, roughly a billion tonnes. Halzen describes each sensor as a light bulb in reverse.

When a neutrino does collide with an atomic nucleus in the ice, it creates charged particles that travel faster than light can in ice. Light slows down in ice, so nothing breaks the universal speed limit. The result is a blue glow called Cherenkov radiation, a sort of sonic boom made of light, as Bose describes it. The sensors convert that flash into an electrical signal. The number of photons reveals the neutrino's energy, and their arrival times reveal its direction.

The shape of the flash tells a story too. A muon neutrino, one of three types, leaves a track, a long streak of light that can be traced to within about 0.3 degrees at high energies, narrower than the Moon appears in the sky. Other types, Chakraborty explains, produce a cascade, a spherical burst of light that measures energy well but gives the direction poorly.

The hard part is noise. Over a hundred million particles from the atmosphere above Antarctica are registered every day. Around 100,000 neutrinos are registered each year, mostly from our own atmosphere, while only about 100 are expected to be cosmic. So scientists favour what comes up through the Earth, which acts as a giant shield. This also lets IceCube watch the whole sky around the clock, even the half hidden beneath our feet, Bose notes.

WHAT HAS ICECUBE FOUND SO FAR?

In 2013, after two years of data, the team reported two neutrinos of 1.04 and 1.14 PeV, found by accident while searching for something else. They led to 28 cosmic neutrinos in the same data. In 2014, the team ruled out a purely atmospheric origin at 5.7 sigma. Sigma is physicists' measure of how unlikely a fluke is, and five sigmas, the discovery standard, means roughly a one in 3.5 million chance of a false alarm. The most energetic neutrino IceCube has recorded, on March 31, 2019, carried about 11 PeV. In 2023, the KM3NeT telescope in the Mediterranean recorded one of roughly 220 PeV, the highest energy ever reported.

Then came suspects. On September 22, 2017, a neutrino of about 290 teraelectronvolts arrived from within about a tenth of a degree of TXS 0506+056, a blazar, which is a galaxy whose jet is aimed at us. The blazar had been brightening in gamma rays for months, and an alert went worldwide in under a minute.

In 2022, IceCube reported 79 neutrinos from NGC 1068, also called Messier 77, 46 million light years away, at 4.2 sigma, just short of discovery. At its core, an active galactic nucleus, a black hole 15 million times the Sun's mass feeds on gas and dust. Chakraborty says these neutrinos are considered the strongest evidence yet of what physicists call hadronic processes, in which accelerated protons collide and spawn neutrinos. The same collisions must also make gamma rays, and here they are missing. His reading is that the action unfolds deep inside, close to the supermassive black hole, in surroundings so opaque that gamma rays are swallowed while neutrinos escape.

This year, IceCube reported neutrinos from our own Milky Way at 5.7 sigma after 12 years of data, which the collaboration describes as the first astrophysical source of high-energy neutrinos to cross the five sigma line. Most cosmic neutrinos, however, seem to come from beyond our galaxy, and no single source dominates.

WHERE DO THEY COME FROM, AND WHY IS IT SO HARD TO TELL?

The neutrino sky is wide, Chakraborty says, and the suspects are among the most violent places in the universe: the remnants of exploded stars, gamma-ray bursts, which are brief and brilliant flashes from cosmic explosions, blazars, active galaxies and starburst galaxies, where stars are born at a furious pace. Dr Raghuveer Garani of IIT Madras adds tidal disruption events, where a black hole tears a star apart, and thinks the answer is probably a combination.

Three things make the hunt hard. The first is distance. Sources are remote, and weakly interacting neutrinos leave only a faint signal. The second is noise. Cosmic neutrinos must compete with muons and atmospheric neutrinos made when cosmic rays strike our own air, and Garani points out that only a few cosmic ones hide among huge numbers of atmospheric ones. The third is geometry. Each track points to a patch of sky holding thousands of galaxies, so many tracks must line up over years of data, Chakraborty explains, and gamma-ray telescopes must lend a hand. A larger detector would overcome many of these limits, which is why IceCube-Gen2 is being pursued.

HOW IS INDIA INVOLVED, AND WHAT CAN IT DO NEXT?

Prof Sanjib Kumar Agarwalla and his group at the Institute of Physics, Bhubaneswar, are members of the IceCube Collaboration. They have analysed data from DeepCore, a denser cluster of sensors tuned to lower-energy neutrinos made in our atmosphere, to test physics beyond the Standard Model, the particle physicists' rulebook. They also use neutrinos to probe the Earth's interior, rather like a medical scan of the planet. Agarwalla says Indian researchers can contribute through data analysis, artificial intelligence, detector development and building the next generation of observatories.

At the Tata Institute of Fundamental Research in Mumbai, Garani says, Mohamed Rameez has worked on IceCube since his doctoral days and now develops methods to sharpen where neutrinos come from. His colleague Prof Amol Dighe notes that Indian neutrino scientists already take part in major experiments, including IceCube, and that contributions to its successor may be planned. Neutrinos at these energies, Dighe adds, could reveal whether the Standard Model holds or hides new physics, though the energy available in a collision is still not far above the Large Hadron Collider's.

India's part is not only in the data. At IIT Guwahati, Chakraborty works on neutrino astrophysics, from exploding stars to the highest-energy neutrinos, the kind of theory that helps explain what detectors such as IceCube see.

There is a gap too. The India-based Neutrino Observatory, which the Union Cabinet sanctioned at Rs 1,500 crore in January 2015, has not begun construction. Its proposed site in Tamil Nadu's Theni district has faced environmental objections, and in February 2022 the state government told the Supreme Court it would not allow the project. Dighe says the project has yet to secure an underground site it can build in, though efforts to create an underground laboratory continue.

Meanwhile, India's Mega Science Vision 2035, a high-energy physics roadmap prepared by the Indian community with TIFR as its nodal institution and released in July 2025, names IceCube-Gen2 and says India could contribute components to it. It also backs GRAPES-3 at Ooty and the MACE telescope at Hanle, which watch cosmic rays and gamma rays.

WHAT HAPPENS NEXT?

The next step is IceCube-Gen2, planned at eight cubic kilometres. Telescopes in water are also coming up, among them KM3NeT in the Mediterranean, Baikal-GVD in Siberia's Lake Baikal, P-ONE off Canada and TRIDENT in the South China Sea. Garani hopes the Nobel will help Gen2 win approval.

Dighe has his own wish list: how a star explodes, and which neutrino is the lightest. Nobody knows yet, and the answers may well arrive from the ice.

It is a fitting end to a very quiet story. A particle with no charge and almost no mass, caught by sensors in the dark, has opened a new window on the violent universe. The prize carries one name, but the lanterns in the ice belong to many.