Nobel Prize in Physics 2026

Neutrino Astronomy: A Telescope Made of Ice

On 6 October 2026 the Royal Swedish Academy of Sciences awarded the prize to Francis Halzen (University of Wisconsin–Madison), “for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin”.

An unusual prize: a single laureate, for an instrument. This course works out why that instrument had to be the size it is, how it sees a particle that almost never interacts, and what it has found. Level: advanced undergraduate physics.

Every feature of IceCube is forced by one fact

Neutrinos hardly interact. Follow that through and the whole observatory falls out of it.

  1. 1A neutrino barely interacts with anything.So build the detector a thousand metres across, and let the Earth itself filter the background.
  2. 2You cannot see a neutrino.See the charged particle it makes, by the cone of blue light it leaves behind.
  3. 3You cannot buy a cubic kilometre of detector.Use ice that is already there, two kilometres down, and drill.
  4. 4You need to know where it came from.A high-energy muon runs for kilometres in a straight line. Time the light and the track points back.

Where to start

📰 A Telescope Made of Ice

The story, for any reader: a particle almost nobody could detect, a proposal almost nobody believed, and a detector drilled into the South Pole.

🧭 Neutrinos in One Page

What a neutrino is, the three flavours, and the one number that governs everything: a cross-section some twenty orders of magnitude below a nuclear one.

The three physical steps

1Seeing a neutrino: Cherenkov lightWhy a particle that outruns light in a medium leaves a cone behind it, at exactly 40 degrees in ice, and how many photons that cone contains per metre.2Why a cubic kilometreThe cross-section rises with energy and the flux falls faster. Multiply them and you get the detector volume — and about four events a year.3Reconstructing the skyA 100 TeV muon runs 14 km through ice. Why that makes a track point to half a degree, and why a contained cascade does the opposite.

Then check it yourself

🔭 What it is used for

Multi-messenger astronomy, cosmic-ray origins, particle physics at energies no accelerator reaches — and glaciology, by accident.

🖥️ Simulations

Four simulations that run in your browser and reproduce every number in the course.

✏️ Exercises

Six problems with full worked solutions.

📚 Glossary

Terms, key papers and the prize sources.

What you should be able to do afterwards

  1. Derive the Cherenkov angle from a wavefront construction, and say why there is a speed threshold.
  2. Use the Frank–Tamm result to estimate how many photons a track produces per metre.
  3. Combine a cross-section, a target density and a flux into an event rate, and so justify a detector volume.
  4. Explain why a muon track gives direction and a cascade gives energy, and why no single event gives both well.

Prerequisites: special relativity, electromagnetism to the level of refractive index, and enough particle physics to meet a neutrino without alarm — the primer supplies the rest.

Numerical values are approximate teaching estimates, labelled where they appear. Text and figures CC BY-SA 4.0; simulation code MIT.

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