Why neutrinos matter as Francis Halzen wins 2026 Nobel Prize in Physics?

STOCKHOLM, Oct. 6 (Xinhua) -- Buried deep beneath the Antarctic ice, thousands of light sensors wait for flashes left by neutrinos, elusive particles that can travel across the universe almost undisturbed. By turning a cubic kilometre of South Pole ice into a giant detector, physicist Francis Halzen helped open a new window on the cosmos. The Royal Swedish Academy of Sciences awarded Halzen the 2026 Nobel Prize in Physics on Tuesday "for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin." WHAT ARE NEUTRINOS, AND WHY DO THEY MATTER? Neutrinos are among the smallest known particles. They carry no electric charge, have extremely little mass, and interact so weakly with matter that most pass straight through the Earth without leaving a trace. Olle Eriksson, a member of the Nobel Committee for Physics and professor at Uppsala University, told Xinhua that detecting neutrinos is extremely challenging: "Through my thumbnail, there are 65 billion neutrinos that go through every second, and I don't feel it." But once detected, they can reveal information about events occurring far out in the universe. Scientists have long known that the cosmos contains natural particle accelerators capable of producing particles with energies far beyond those achievable in laboratories on Earth. The origins of the highest-energy cosmic particles, however, have remained difficult to trace. Cosmic rays carry electric charge and are deflected by magnetic fields as they travel through space. Neutrinos, unlike other particles, are electrically neutral and travel essentially in a straight line, allowing scientists to trace their direction back toward the violent environments where they were produced. HOW DID HALZEN TURN SOUTH POLE ICE INTO A TELESCOPE? In 1988, Halzen, the Belgian-born scientist, presented the idea of installing light sensors deep in Antarctic ice. When one of the rare neutrinos collides with an atomic nucleus, it can produce a charged particle that generates a faint flash of blue light as it moves through the transparent ice. Sensors can capture that light and help scientists reconstruct where the neutrino came from. The idea eventually became IceCube, a detector using an entire cubic kilometre of Antarctic ice. Completed in 2011, it contains 5,160 light sensors, mostly between about 1,450 and 2,450 metres below the surface. The depth matters. "There, the ice is very clear, no air bubbles, no dirt, no dust," said Eriksson, adding that this condition allows faint flashes to travel hundreds of metres before being absorbed. The effort paid off. In 2013, IceCube researchers reported the first strong evidence for high-energy neutrinos arriving from distant space. With more data, they were able to establish that some of the particles originated far outside the solar system. Eriksson said the achievement helps humanity better understand its surroundings and its place in the universe by providing information about extreme cosmic events that cannot easily be obtained in other ways. The immediate significance lies mainly in expanding fundamental knowledge, he said, adding that major scientific discoveries can also lead to practical applications that were impossible to foresee when the research began. WHY HALZEN, AND WHAT COMES NEXT? IceCube is the work of a large international collaboration involving hundreds of scientists and engineers. "Even though it is a large collaboration, we could still identify one person who was the pivotal, most important contributor to that experimental buildup and the experimental discovery," said Eriksson. "It was a long journey, but ... it finally worked," Halzen recalled the early phases at Tuesday's Nobel press conference, describing how the success was not guaranteed nearly 40 years ago but made possible by critical contributions from many people. The journey is continuing. Scientists are planning IceCube-Gen2, which is expected to expand the instrumented volume at the South Pole to about eight cubic kilometres, while other neutrino observatories are being developed around the world. Halzen said opening a new window on the universe often leads to unexpected discoveries. For neutrino astronomy, that window may only have begun to open.