He turned a cubic kilometre of South Pole ice into a telescope that catches particles from deep space.

The quick version

1. Neutrinos fly straight from the most violent places in the universe, yet almost never hit anything.
2. Halzen proposed burying light sensors deep in Antarctic ice to catch the rare flash when one does.
3. IceCube, finished in 2011, found the first high-energy neutrinos from far outside our solar system.

Picture it

A boat's wake at night. Picture a dark lake covered with motion sensors, and a boat you cannot see. You cannot spot the boat, but its wake spreads out in a cone, and the sensors record when the wake reaches each one. From that timing you can work out which way the boat was heading. In IceCube, the rare neutrino that hits an atom makes a charged particle, and that particle leaves a cone of blue light in the ice that the sensors time in the same way. The analogy breaks in one place: the neutrino itself leaves no wake at all, and almost every neutrino passes through the whole detector without a trace.

For younger students

Some tiny particles from space zoom through the whole Earth without bumping into anything. A scientist named Francis Halzen put thousands of light detectors deep in the ice at the South Pole. When one of these particles bumps into the ice, it makes a little flash of blue light, and the detectors catch it.

A 10-minute lesson

Opening question, 2 minutes. Ask the class: why would anyone build a telescope in ice at the bottom of the world instead of in space? Collect a few guesses. Then explain that neutrinos pass through almost everything, so scientists need a huge, clear, dark detector to catch the rare one that hits something.

Hands on demo, 5 minutes. Fill a baking tray with a little water and float small bits of paper around the edges as sensors. Ask one student to drag a pencil tip quickly across the water while the others watch which paper bits move first. Use the order of movement to guess the direction the pencil traveled. Explain that IceCube does the same with blue light, using 5,160 sensors deep in South Pole ice to time when the light reaches each one.

Point out where the demo differs from the real thing. The neutrino itself leaves no trace. Only a charged particle it makes, when it rarely hits an atom, gives off the cone of light.

Check for understanding, 3 minutes. Ask: what do IceCube's sensors actually detect? A good answer is the blue light from a charged particle that a neutrino made, since the neutrino itself makes no light. Close by noting that in 2013 IceCube reported the first evidence of high-energy neutrinos from outside our solar system.

Words to know

Neutrino

A tiny particle with no charge and almost no mass that passes through nearly everything.

Cosmic rays

Charged particles, mostly protons, that hit Earth from space, sometimes with enormous energy.

Cherenkov light

Faint blue light a charged particle gives off when it moves faster than light can in ice.

Optical module

One IceCube sensor: a light detector and electronics inside a glass ball about 35 cm wide.

PeV (petaelectronvolt)

A unit of particle energy, more than a hundred times the energy the LHC gives a proton.