Francis Halzen and the team that turned Antarctic ice into a telescope

Why try the ice?

To detect something that usually passes through matter, it helps to have a very large target. But building and filling a huge tank is expensive. Antarctic ice offered another possibility: put light sensors into a natural volume of ice and watch for the rare interactions that make it glow.

Francis Halzen, born in Belgium and based at the University of Wisconsin–Madison, helped turn that possibility into a sustained research program. His contribution to IceCube was recognized with the 2026 Nobel Prize in Physics.

The proposal left plenty to find out. Was the deep ice clear enough? How far could light travel? Could its path be understood well enough to locate the particle that produced it? The earlier AMANDA experiment let researchers test and develop this approach before IceCube.

Sources: [1] · [2] · [3]

A clock error can look like a different track

Imagine that light reaches one sensor slightly before another. That difference helps tell us which way a particle was moving. Now imagine that the first sensor’s clock is slightly ahead. A reconstruction could mistake an instrumental error for a change in the particle’s direction.

That is why calibration matters. Each optical module contains a light sensor and electronics that turn a brief flash into a time-stamped signal. The modules must agree on the time, and researchers must know how each electrical signal relates to the amount of light received.

These measurements are part of making the telescope work. Without them, combining thousands of readings would produce a more complicated picture, but not necessarily a better one.

Sources: [3]

The ice has to be measured too

Light does not always travel straight from its source to a sensor. Some is absorbed. Some is scattered, changing direction and arriving later. The real ice also has layers with different optical properties, so equal distances do not always mean equal journeys.

To interpret a late flash, researchers must decide how much of the delay the ice can explain. Otherwise a longer light path might be mistaken for a different particle path. Measuring the ice is as necessary as measuring the sensors.

Many people share this work. Drilling and deployment place the modules; electronics and software preserve the signals; calibration, simulation and analysis connect those signals to physics. According to the collaboration, it includes hundreds of scientists at institutions in many countries, backed by long-term support from institutions and funding agencies.

Sources: [2] · [3]

From a working detector to a discovery

A working observatory can catch an unusual event without yet knowing what it means. Could the sensor response explain it? Could the ice model be wrong? Could a particle made in the atmosphere have produced it? Answering these questions calls on different parts of the collaboration.

The 2013 high-energy neutrino result brought this work together. The researchers selected a sample of events, estimated the atmospheric backgrounds and compared those expectations with what the detector had seen. The case for an astrophysical contribution came from that comparison.

Halzen’s story and the collaboration’s story meet here. Leadership helped keep the observatory going over a long period. The published result also depended on the people who built, operated, understood and analyzed it. The long author list on an instrument paper reflects how the science was done.

Sources: [4] · [3]

Try changing the ice yourself

In the reconstruction Lab, generate an event and fit a direction from its sensor times. Reveal the generated direction to see how close the fit came. Then choose stronger scattering, generate another event and repeat the comparison.

Scattering can delay the first light that reaches a sensor. The fit tries to explain those times with a simple model of direct light, so the delays can pull its answer away from the original direction. The timing residuals show how far the measured times lie from the fitted prediction.

The Lab uses a small teaching array and assumes that one point on the track is already known, leaving out much of the real reconstruction. Even this limited exercise shows why calibration matters: learning where a particle went depends on understanding what happened to its light.

Sources: [3] · [5]

Try it in the Lab

Primary sources & revision

1. Royal Swedish Academy of Sciences · Physics 2026 (6 October)
2. IceCube · Francis Halzen, 2026 Physics Nobel Prize
3. IceCube Collaboration · Instrumentation and Online Systems (2017; arXiv v3)
4. IceCube Collaboration · Evidence for High-Energy Extraterrestrial Neutrinos (2013)
5. IceCube · Public data releases

First published 2026-10-09; last revised 2026-10-11. Original explanatory text and diagrams by Sandbox Physics. Illustrations are schematic; no experimental event records are reproduced here. This is an independent educational publication, not an official Nobel or experiment collaboration publication.