Francis Halzen, a physicist at the University of Wisconsin, Madison, received the 2026 Nobel Prize in Physics for his contributions to a neutrino detection observatory — named IceCube — and “the discovery of high-energy neutrinos of astrophysical origin,” according to the Royal Swedish Academy of Sciences. 

Neutrinos are particles known for their evasive behavior. Intuitively, their connection to ice is not immediately evident. Halzen’s prize, therefore, is the result of decades of theoretical and experimental innovation in particle physics and optics, with the potential to explain the origin of incredible astrophysical processes.

Why are neutrinos so hard to detect? 

As a result of decades of scientific research, innovation, and technological advancement, physicists have come to understand the building blocks of matter through a theoretical framework called the Standard Model of particle physics, which accurately predicted the experimental confirmation of particles like quarks, gluons, and — as relates to Halzen’s research — neutrinos. 

While some particles in the Standard Model were experimentally confirmed in large particle collider experiments, the neutrino is more elusive than that. The particle has very little mass, does not have charge, and rarely interacts with any other matter. The neutrino’s neutrality, therefore makes it very difficult to research, as traditional detector and collider facilities cannot fully study the particle’s properties. 

Still, scientists do not fully understand the neutrinos and their fundamental properties. While neutrinos have been split into three different types, called flavors, researchers have never directly measured the neutrino’s mass. Additionally, electromagnetic interactions between neutrinos and other particles have never been experimentally recorded. 

It is important to note, however, that neutrinos are not a rare particle. In fact, more than 100 trillion neutrinos pass through any given human body every second. These neutrinos are produced in a variety of highly energetic cosmological events, including galactic mergers, supernovae, stellar fusion, and even the Big Bang.

Studying these particles, therefore, can allow for physicists like Halzen to probe the earliest moments of the universe, as well as violent, cosmological processes that otherwise evade traditional probing mechanisms. 

What is IceCube?

The IceCube Neutrino Observatory is located in the ice sheet of Antarctica. It consists of a cubic kilometer ice block outfitted with 5,160 digital optical modules (DOMs) that track and record neutrinos as they pass through the ice. 

There is no specific quality that makes the neutrinos interact with the ice in the observatory more often than they do other matter; the IceCube instead utilizes the sheer scale of the Antarctic ice and its lack of dust and dirt to increase the probability of interaction. 

IceCube detects around 275 neutrinos daily, utilizing the interaction between neutrinos and the ice molecules to track movement and behavior of the particles. Upon interaction, the neutrino releases a secondary particle which emits a form of light called Cherenkov light. This light is released as a result of the secondary particle travelling at the speed of light, faster than the speed of light within the ice itself due to the material’s larger index of refraction. 

The DOMs inside of the ice record this Cherenkov light, endowing scientists with the ability to measure the neutrino’s trajectory and energy. Since the neutrino rarely interacts with matter, examining its trajectory can give scientists a good idea of its cosmological origin, by literally tracing the path of the neutrino into the atmosphere and beyond. 

IceCube’s advancements to neutrino astrophysics

Before IceCube, the highest energy neutrinos recorded could be traced back to the Sun, the particles on energy scales of a megaelectronvolt (~106 electronvolts). 

In 2013, just two years after the inception of IceCube’s data collection, the observatory detected neutrinos on the petaelectronvolt (~1015 electronvolts) scale. This energy scale is on the order of one quadrillion electronvolts. To visualize, the age of the universe itself is around 430 quadrillion seconds. 

The origin of these highly energetic neutrinos has been attributed to something astrophysicists call cosmic rays. Cosmic rays, first discovered in 1912, correspond to subatomic particles travelling through space at nearly the speed of light. However, since these particles are not electrically neutral, they often interact with gas, dust and other astronomical objects. 

These interactions between cosmic rays and other particulate matter, can radiate the highly energetic neutrinos observed at IceCube. 

By tracing the origin of these high-energy neutrinos (and their corresponding cosmic rays) scientists can find these “cosmic ray accelerators.” Yet, a definitive answer on what cosmological object accelerates cosmic rays to these energies is still an open question in astrophysics.

The study of neutrinos is a relatively new field of physics, with physicist Wolgang Pauli’s theorization of the particle only coming in the mid-20th century. The expansion of relevant experimental detection devices like IceCube, however, has been paramount in increasing humanity’s understanding of the cosmos. 

Still, the field will continue to evolve with ever-growing technological advancement and new theoretical frameworks concerning the neutrino. 

“Someone asked me what neutrino astronomy will be like 50 years from now. There’s no way to answer that question,” said Halzen, in an interview with the Royal Swedish Academy of Sciences. “We [IceCube Collaboration] have been consistently making new discoveries over the 15 years that we have been taking data. That’s my real wish, that this may continue, and it will.”