Francis Halzen was awarded the Nobel Prize in physics Tuesday for pioneering a telescope that paved the way for a new kind of astronomy. Scientists used this telescope, embedded deep in the ice at the South Pole, to detect ghostly particles known as neutrinos whizzing in at high energies from the far reaches of the universe.

“It’s a way of bringing us information about distant cosmic sources, which we are unable to acquire in other ways,” Mark Pearce, chair of the Nobel Committee for Physics, said at the prize announcement. In a statement following the announcement, he added that Halzen had “provided us with a fantastic instrument.”

Halzen conceptualized the IceCube Neutrino Observatory in the late 1980s as a way to observe the natural particle accelerators of the cosmos, such as supermassive black holes. These objects generate neutrinos at far higher energies than what can be produced on Earth.

The IceCube team, which today consists of more than 400 scientists around the world, completed construction of the telescope in 2011 and announced the discovery of high-energy neutrinos originating from far beyond our solar system two years later. With the conception of IceCube, Halzen birthed the field of neutrino astronomy, which provides a different lens from which to explore the universe. IceCube also laid the groundwork for a suite of other neutrino telescopes around the world, including an underwater observatory that in 2023 detected the most energetic neutrino to date.

“The future, I think, is guaranteed,” Halzen said of neutrino astronomy in an interview with the New York Times. Predicting what astronomers will discover in that future, he added, “I will leave to someone else.”

Physicist at UW-Madison

Halzen, 82, is a theoretical physicist at the University of Wisconsin-Madison. Born in Tienen, Belgium, he is the first Nobel laureate to win the physics prize individually since 1992.

The Nobel Prize earns Halzen 12 million Swedish kronor (about $1.2 million).

In an interview, Halzen said he learned about the news while in a hotel room in Italy. He was working on a proposal for a new physics project when the phone rang.

“You always think first that it’s a joke, right?” he said.

He added that the IceCube Neutrino Observatory was a highly collaborative project and that the most important contribution was not any of the discoveries his team had made over the past 15 years, but simply proving that neutrino astronomy was possible.

Echoes of the origin

Neutrinos are subatomic particles that carry no electrical charge, so they pass through nearly all matter without colliding or otherwise interacting. And they are everywhere: More than a billion neutrinos, emitted by the sun, pass through the palm of your hand every second, according to Pearce.

But the most energetic of these particles come from farther away in the universe and are rarer, making them even more difficult to catch. Unlike other types of emission — namely charged particles and light — neutrinos traverse the universe mostly undisturbed. Their paths point straight back to the distant, and mostly unknown, cosmic sources that generate them.

In 1988, Halzen presented the idea for a neutrino telescope embedded deep in the Antarctic ice and fitted with thousands of light sensors. On rare occasions, a neutrino traveling through Earth would interact with the ice and produce a flash of light, which could be detected by the telescope’s sensors.

More than two decades later, construction of the IceCube Neutrino Observatory was complete, spanning 1 cubic kilometer, or about a quarter of a cubic mile, of ice at the South Pole. It has been capturing neutrinos ever since, at energies 1,000 times higher than the particles produced by the Large Hadron Collider at CERN. The highest energy neutrinos detected by the telescope are so rare that only about 10 are observed each year, said Erin O’Sullivan, the current IceCube spokesperson.

In its statement, the Nobel Committee praised Halzen’s “vision and scientific leadership” for the project, which gave astronomers a fresh look at the universe.

Mysteries remain

Though a new window to explore the universe has been opened, the question of where these high-energy neutrinos are coming from, and how exactly they are generated, remains. One neutrino, detected by IceCube in 2017, was traced back to a supermassive black hole — but other sources remain a mystery.

With IceCube, scientists can discern the direction of incoming neutrinos as precise as about half a degree, or roughly half the width of your thumbnail when your arm is stretched out, Ignacio Taboada, a physicist at the Georgia Institute of Technology and a former IceCube spokesperson said.

“But there’s a lot of universe behind that thumbnail,” he said.

The IceCube Neutrino Observatory has inspired a worldwide network of next-generation neutrino telescopes — rooted in the ice, submerged in the sea and tucked into mountains — to solve this mystery. A second-generation IceCube observatory has also been proposed that would be able to detect 10 times as many neutrinos.