at the University of Wisconsin鈥揗adison has won the 2026 Nobel prize in physics for his work on spotting ghostly particles called neutrinos using a detector at the South Pole.
Neutrinos are subatomic particles that rarely interact with material, are electrically neutral and perfuse the whole universe. Because of this, we can detect neutrinos that originated far away in the cosmos, trace their origin and learn more about the nature of the universe. Cosmic neutrinos are produced when a proton collides with another particle.
If you hold your hand out flat, more than a billion neutrinos will pass through it every second. These originate from the sun, but Halzen鈥檚 work concerns the much more rare and high-energy neutrinos coming from deep within the cosmos and what their detection can tell us about the universe.
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Neutrinos act like 鈥済host-like messengers from the cosmos鈥�, said , chair of the Nobel committee for physics, at a press conference. 鈥淚t鈥檚 a way of bringing us information about distant cosmic sources which we鈥檙e unable to acquire in other ways.鈥�
In 1988, Halzen proposed using the Antarctic ice as a neutrino detector, then went on to spearhead an international collaboration of more than 450 people from 58 institutions in 14 countries to make the IceCube Neutrino Observatory a reality.
The detector uses thousands of light sensors buried 2 kilometres under the surface of the ice in deep holes drilled using hot water. As neutrinos shoot through Earth, they interact with the ice, creating what鈥檚 known as Cherenkov radiation and emitting blue light. The network of light sensors allows researchers to trace their trajectory back to their origin point, revealing vital information about their formation.
Halzen calculated that, if you built a neutrino collector 1 cubic kilometre in size, you would detect one neutrino per day. IceCube was completed in 2010 and began full operations the following year. The first high-energy extraterrestrial neutrinos were detected in 2013.
鈥淲hen I reflect on this moment, I have to emphasise how lucky I was,鈥� said Halzen, addressing the press conference by phone. 鈥淏ecause when we started this project, everybody realised this was maybe a good idea, but very few thought it would work 鈥� including myself. So this was kind of an adventure where success was not guaranteed, and we were lucky to overcome the various challenges. It was a long journey, but finally it worked.鈥�
Halzen said he hopes the prize will reflect on the collaborators who worked on the project in the early days.
The IceCube detector has gone on to make many more discoveries. In 2017, it helped identify the first definitive source for a single high-energy cosmic neutrino: a blazar called TXS 0506+056 that blasted a huge jet of energy towards Earth. In 2023, it identified neutrinos from within our own galaxy, the Milky Way, which are typically overwhelmed by stronger signals from the rest of the universe.
Of winning the award, Halzen said: 鈥淚t was a great surprise and I didn鈥檛 expect it. It鈥檚 a great pleasure.鈥�
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