This week the Royal Swedish Academy of Sciences awarded the 2026 Nobel Prize in Physics to Francis Halzen “for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin.”

IceCube is a facility at the South Pole that uses light sensors arrayed across a cubic kilometer of ice deep beneath the Antarctic surface to register the energy and direction of elusive neutrino particles arriving from space. Work on the facility first began in 2001, and it has been operating at full capacity since 2011, providing a wealth of data on the torrents of neutrinos created in violent astrophysical environments such as supernovas.

Notably, this is the first time since 1992 that the physics prize has gone to a lone recipient—it is usually divided among two or three. Yet, while Halzen has been IceCube’s leader for its entire history and played a central role in preceding efforts stretching back to the 1980s, he is not best thought of as a singular driving force or mastermind. His highly respected contributions have complemented those of other key figures, and, in a 2020 oral history interview with AIP, he wryly cast himself as a reluctant leader, describing his position within the project as an almost accidental result of circumstances.

Halzen’s road to IceCube

Halzen is a Belgian theoretical physicist who originally focused on accelerator-based particle physics. He joined the physics department at the University of Wisconsin–Madison in 1971, and he has remained there ever since. He turned to the idea of embedding a neutrino detector in ice in 1987, after hearing about a Soviet proposal to detect neutrino captures in Antarctic ice using radio receivers. The following year he presented the concept at a conference in Poland with John Learned, who was leading DUMAND, an embattled effort to build an underwater neutrino detector array near Hawaii.

In his interview Halzen recalled

“I was giving a talk on [quantum chromodynamics] and how QCD was relevant to cosmic ray physics. But there was a small parallel session about detector techniques, and I gave a talk on the AMANDA concept … just to see if anyone would laugh me off the stage. And then, of course, nobody laughed, so I continued to worry about it and think. And we started talking about this more and more seriously, but we really didn’t form a collaboration until two or three years later.”

The Antarctic Muon and Neutrino Detection Array, or AMANDA, was IceCube’s predecessor project. It began to cohere circa 1990 as the idea attracted collaborators and then exploratory funding from the National Science Foundation. Reflecting on his involvement, Halzen said

“You have to understand, I really never had the intention to get involved in AMANDA and IceCube. I got dragged into it by accident. The first Berkeley people who started the project were two students of Buford Price [Doug Lowder and Andrew Westphal], and one of my colleagues, Bob Morse, and me. So, we were four people. And I was the only one who had ever written a proposal, even though Bob was older than me, but he had been a scientist in the Madison high energy group. And so I wrote the proposal, and so I got dragged into this and tried to get out of it many times.”

The development of AMANDA has been extensively recounted in Mark Bowen’s 2017 book The Telescope in the Ice. Although Halzen wrote the initial proposal, Morse was the project’s principal investigator, and much of the work revolved around the formidable practical challenges of embedding detectors in boreholes drilled into an ice sheet. As Bowen stresses, figures working in the field such as Bruce Koci, a world-leading expert in ice drilling, played a crucial role making the project work.

Halzen was also deeply involved in AMANDA, but, as a theorist, he never actually visited Antarctica, either during that period or after. Nevertheless, he became the principal investigator for the follow-on IceCube project in 2001. He recalled

“In the beginning, [the leadership] was shared. I mean, the NSF would talk to Bob [Morse] and me, and formally, actually, he was the PI. But it was pretty much a two-man show. Of course, once IceCube came along, it had to be a big managed project. I was told that the project wouldn’t go if I were not a PI at the time that they were about to release the money. And I can tell you, I would not have been the PI if they hadn’t told me.”

He elaborated that his continued involvement was tied to the money that both NSF and the University of Wisconsin had already put into the effort.

“I didn’t get into this saying, ‘Ahh, I’m going to build a kilometer-cubed detector.’ … But you also get dragged into IceCube because you spend money. By the time that we were struggling to detect atmospheric neutrinos with AMANDA, we had spent some millions of dollars. And you cannot walk away from something after you’ve spent that much money. Now, from the perspective of today, three or four million, that’s a small amount, right? But the university itself, at several stages when NSF wasn’t generous enough, gave or lent us money. At one point UW put one million in AMANDA, and at some point when we were preparing IceCube, it put into the preparations for construction and the design close to five million dollars. They were paid back by NSF, but I didn’t know that then. And you had to succeed first, so I think you’re in a situation where you cannot say, ‘I’m going back to Belgium.’”

Halzen remains IceCube’s PI today.

Project leadership and project management

The modesty and humor that Halzen brought to the interview meant he was reserved about why he became a successful, long-serving leader of IceCube. At one point he remarked:

“I always thought it was a bad idea—I still think so—to have a theorist trying to lead an experiment. But actually, in practice, it worked very well. At different stages, I tried to get rid of this job and it never worked.”

One might presume the theorist simply transformed over time into a skilled administrator, but Halzen resisted that idea, pointing out that he handed off those responsibilities.

“I am not [Barry] Barish. I do not have his skills and his discipline, and I know that. I think the biggest contribution I made to IceCube was to realize that I didn’t have the skills needed to run the project, so I hired a very talented project director who built the thing. His name is Jim Yeck. And I didn’t make the mistake that most physicists would make in thinking they are smarter than everyone else in the room.”

Yeck is a highly sought-after project director, who, in addition to his work on IceCube, has played similar leadership roles for the European Spallation Source, the US component of the Large Hadron Collider project, and Brookhaven National Lab’s Relativistic Heavy Ion Collider, and he is currently project director for RHIC’s successor facility, the Electron-Ion Collider.

As detailed in Bowen’s book, Yeck stepped into the IceCube project in 2003 as it was ramping up and facing severe management difficulties, and he guided it through to its successful completion. Halzen’s reference to Barry Barish is apt and striking, as in 1994 Barish joined the Laser Interferometer Gravitational-Wave Observatory, or LIGO—another large NSF-funded project—just as it was facing potentially fatal problems. Ultimately, Barish won a share of the 2017 Nobel Prize for his role in moving the project forward, alongside two of the project’s early leaders, Kip Thorne and Rainer Weiss.

The opaque logic of Nobel Prize partitioning

Given the Nobel Prize’s extraordinary grip on the attention of the scientific community and the public, it is often regarded as an imprimatur that the winning achievement is of historic significance. Yet the prizes are also an awkward way of marking scientific history, awarding discrete achievements rather than integrated bodies of work, and doing so by recognizing no more than three people who must also still be alive.

One might contemplate in vain as to whether there is some consistent rationale underlying why the Nobel Prize for LIGO was split three ways while the prize for IceCube was awarded to one person. If Halzen won for originating the concept, John Learned might have taken a share, though his involvement ended quickly. Like Halzen, Bob Morse played a long-term leading role in the project, nearly from its beginnings, while Jim Yeck played a crucial role in seeing it through. And these are just a few important figures.

Perhaps one could argue Halzen’s various contributions were of particular importance, even if they were somewhat reluctantly strung together into a career-defining endeavor. Perhaps the prize committee deemed there was no elegant way to expand the number of winners to two or three. We could wait decades for the relevant Nobel archives to open to try to answer such questions, but, meanwhile, better, more answerable questions about IceCube and its place in the history of science surely call for our attention.

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William Thomas
American Institute of Physics
wthomas@aip.org

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