How neutrinos, which barely exist, just ran off with a Nobel Prize — again!

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How neutrinos, which barely exist, just ran off with a Nobel Prize - again!

Ohio State professor breaks down science behind the 2026 Nobel Prize in Physics

On Oct. 6, the 2026 Nobel Prize in Physics was awarded to Francis Halzen (University of Wisconsin) "for decisive contributions to the IceCube Neutrino Observatoryand the discovery of high-energy neutrinos of astrophysical origin."

What's all this about?

I have known Francis - as an inspiration, mentor and friend - for over 35 years. This is almost as much time as he and hundreds of his collaborators have been working on the science and engineering behind this year's announcement. I'm delighted to see this recognition for all of them, including some of my Ohio State colleagues. My own research is in theoretical neutrino astrophysics. I am not part of the IceCube Collaboration.

What are neutrinos?

Neutrinos are almost massless, almost non-interacting particles. They barely exist as the partners of electrons, but are stripped of the electron's charge and mass, making them nearly ghosts. First theorized in 1932, it wasn't until 1956 that neutrinos were discovered in the laboratory through their extraordinarily rare interactions. For example, about 100 billion neutrinos from the sun streamed through your eyeballs - no matter which way you were facing - in the time it took to read that sentence. Almost all did so harmlessly, but maybe once in your lifetime one will interact, which would be much less than what cosmic rays do all the time.

How can you observe astrophysical neutrinos?

Except for being almost impossible to detect, neutrinos are ideal astrophysical messengers. They are created in huge numbers in stars and other objects where material is hot and dense. Unlike other particles, including photons of light, they can easily escape from behind vast amounts of material. And unlike charged cosmic rays, which can have their directions scrambled by magnetic fields, nothing happens to neutrinos as they cross the universe. If we could somehow figure out how to easily detect these neutrinos when they arrive at Earth, this would all be great.

The trick to making this work is simple to say but hard to do - we need huge detectors where almost nothing ever happens. Although the probability of a neutrino interacting in a detector is tiny, it gets bigger in proportion to the size of the detector and how long it operates. And although it is relatively easy to build a huge detector that is sensitive to small signal events, the hard part is reducing the rates of background events, or noise, that mimics neutrino signal events.

Prior to IceCube, physics experimentalists had observed astrophysical neutrinos produced in the sun, from one exploding star (supernova 1987A) near our galaxy, and by cosmic rays in Earth's atmosphere. But these were not yet enough sources to develop the potential of neutrino astronomy.

The long road to IceCube

What is IceCube? Located in one cubic kilometer of deep ice at the South Pole, it is an array of several thousand light detectors called photomultiplier tubes (PMTs). Ordinarily, deep under the ice is a dark and boring place. But when a neutrino interacts, it can produce charged particles that emit a flash of light that triggers the PMTs, allowing IceCube to reconstruct the position, direction and energy of the neutrino.

This required the development of extensive new technology and large, well-functioning experimental collaborations. Think how complicated it can be for a group of several people to cooperate on planning just a simple dinner out. Now imagine what is needed for many hundreds of people from around the world to work together for decades to build and operate a complex detector. Francis led this effort.

What did IceCube see and why does it matter?

In 2013, IceCube announced the detection of two astrophysical neutrinos, each at energies very close to 1 PeV, a specialized physics unit that means a single neutrino has 1,000,000,000,000,000 times more energy per particle than a visible photon of light.

In some sense, this was an incredible shock. Why should such high-energy particles even exist? The entire field of astronomy would be largely unchanged if they didn't. The highest-energy particles produced by humans are protons in the Large Hadron Collider in Geneva, Switzerland, and those are almost as energetic (only 100 times less) as those detected by IceCube. But the key distinction is in the power of the sources. You can walk around Geneva without being fried alive because you remain far enough from the beam. But astrophysical sources must be so bright that they can be seen from halfway across the universe, which is obviously very, very far away. It took many thousands of people and advanced technology to make the Large Hadron Collider work. In comparison, it's astonishing how easily the universe can put together something incredibly more powerful.

Since those first two astrophysical neutrino signal events, IceCube has detected hundreds more. There are a few surprises. First, these events are dominantly coming from very distant sources, with some coming from the Milky Way, and none yet from the sun. This is the opposite order of the history of optical astronomy, where astronomers first discovered the sun, then the Milky Way, and then the vast universe of other galaxies. Second, we don't know what the sources are! We know they are distant galaxies, but we don't know which types.

This is both frustrating and fun, because although there are conflicting hints in favor of different kinds of sources dominating, we just aren't sure. It's fun because we have so much new to learn.

What's next?

To progress, we're going to need a bigger boat. That could be IceCube-Gen2, a laboratory that would house an optical-based detector about 10 times bigger than IceCube, intended to discover the individual sources behind the cosmic haze detected by its predecessor. IceCube-Gen2 would also include a new radio-based detector that is needed to allow the frequent detection of neutrinos at even higher energies. Together, these will help solve the mystery of how the universe makes such powerful accelerators of cosmic rays. They will also make tests of neutrino properties and interactions that could never be duplicated in laboratory experiments.

IceCube-Gen2 has been endorsed in the U.S. National Academies' report on the Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020), for which Laura Cadonati (Georgia Tech) and I served as co-chairs for the Panel on Particle Astrophysics and Gravitation. It has also been endorsed by the 2023 Particle Physics Project Prioritization Panel, convened by the U.S. Department of Energy and the National Science Foundation. As with IceCube, substantial international participation would be needed, and there is evident enthusiasm for this.

Potential competitors to IceCube-Gen2 include two proposals led by China - TRIDENT (TRopIcal DEep-sea Neutrino Telescope) and High-energy Underwater Neutrino Telescope (HUNT) - that would go directly to detectors that would significantly exceed even the size of IceCube-Gen2, but without U.S. participation.

This is thus a pivotal moment for U.S. leadership in growing the field of neutrino astronomy. From decades of work, we have learned an important lesson - catching neutrinos takes patience, but they reward it richly.

John Beacom is an Arts and Sciences Distinguished Professor of physics and astronomy at The Ohio State University. This article is abridged and edited from one originallyself-published in Medium.

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