A Cubic Kilometre of Ice to Catch a Ghost: The Numbers Behind This Year’s Physics Nobel

This week the Royal Swedish Academy of Sciences awarded the 2026 Nobel Prize in Physics to Francis Halzen, a Belgium-born physicist at the University of Wisconsin–Madison, for his decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos from beyond our solar system. It was one of three science prizes announced in three days: medicine went to the scientists behind optogenetics, which switches brain cells on and off with light, and chemistry to two chemists who solved a long-standing puzzle about molecules that are mirror images of each other.

IceCube is the one with the best numbers, and they are the kind of numbers that look impossible until you take them apart. That is worth doing, because taking a big number apart is exactly what congak trains you to do.

Start with what it is trying to catch. A neutrino is one of the fundamental particles of nature, and it barely interacts with anything. Neutrinos pour out of the Sun in such quantities that about 65 billion of them pass through every square centimetre of your body every second — roughly the area of a thumbnail. The palm of your hand is about 100 square centimetres, so the count through your palm is somewhere around six and a half trillion a second. Almost none of them touch a single atom on the way through. They pass through the whole Earth the same way.

That is what makes them so valuable to astronomers. Light from a distant galaxy can be absorbed or bent on its way here; a neutrino travels in a straight line through almost anything, so it points straight back to where it came from. The problem is the same property in reverse. Something that passes through a planet without stopping is extraordinarily hard to catch.

Halzen’s answer was to use a block of matter so large that, every so often, one neutrino in an enormous number would happen to hit an atom inside it. The block he chose is the ice under the South Pole.

IceCube is a cubic kilometre of that ice, wired with light sensors. A cubic kilometre sounds like a figure of speech, so put it in congak terms. One kilometre is a thousand metres, so a cubic kilometre is a thousand times a thousand times a thousand cubic metres — a billion of them. Ice weighs a little under a tonne per cubic metre, so the detector is close to a billion tonnes of ice, which is why the University of Wisconsin describes it as surveying a billion tons.

The sensors hang on 86 cables lowered into holes in the ice, 60 sensors to a cable, spaced 125 metres apart on a hexagonal grid. Eighty-six times sixty is a sum worth doing from the left, the technique in our Malay post at https://cerdikcerdik.com/ms/blog/kira-dari-kiri: eighty times sixty is 4,800, six times sixty is 360, and together that is 5,160 — the number of sensors IceCube actually has.

They sit deep. The top sensors are 1,450 metres below the surface and the bottom ones 2,450 metres down, so the instrumented part of the ice is exactly one kilometre tall. For a sense of that depth, the Petronas Twin Towers are about 452 metres high. The shallowest sensor in IceCube is buried deeper than three Petronas Towers stacked on top of each other, and the deepest is more than five towers down.

Getting them there is its own story in numbers. Each hole was melted with a high-pressure jet of hot water that cut through the ice at about two metres a minute. At that rate, reaching 2,450 metres takes about 1,225 minutes — divide by sixty and that is roughly twenty hours of drilling for a single hole, before the cable of sensors is lowered in and the water refreezes around it. The team managed about one hole every two days, and 86 holes of nearly two and a half kilometres each adds up to more than 200 kilometres of hole — about the distance from Kuala Lumpur to Ipoh by road. Construction finished on 18 December 2010.

So how does a block of ice see a particle? When a neutrino does hit an atom in the ice, the collision produces charged particles that race through the ice faster than light itself can travel in ice. That produces a faint blue flash, a kind of optical sonic boom, and the deep Antarctic ice is so dark and so clear that the flash can travel a long way to the nearest sensors. The pattern of which sensors light up, and when, gives the direction the neutrino came from.

There is one more clever trick in the design. Cosmic rays hitting the atmosphere above the South Pole produce a constant rain of other particles that also make flashes in the ice. But almost nothing except a neutrino can pass through the entire planet. So when IceCube sees a track heading upwards — coming from below, having crossed the Earth — it can be confident it is looking at a neutrino. The whole Earth is used as a filter.

The results have arrived over more than a decade. In 2013 IceCube reported the first high-energy neutrinos from beyond the solar system. In September 2017 it caught one that was traced to a supermassive black hole in a distant galaxy. In 2022 it found evidence of neutrinos from an active galaxy about 47 million light-years away, and in 2023 it saw neutrinos coming from our own Milky Way.

Forty-seven million light-years is the last number worth taking apart. A light-year is about 9.5 trillion kilometres, so that galaxy is around 450 billion billion kilometres away. No one can picture that, and there is no need to. Two things are enough to know: it is a number with twenty zeros, and the particle that crossed it went straight through the Earth before a sensor in the Antarctic ice happened to notice it.

That is the real skill congak builds. Not fast arithmetic for its own sake, but the habit of breaking a big number into pieces small enough to reason about — thousands times thousands, hours from minutes, towers from metres. It is the same habit we wrote about for reading the national budget at cerdikcerdik.com/blog/cara-baca-bajet-2027, and it is what the daily challenge at cerdikcerdik.com/math practises in a few minutes a day. If you want to check any of the conversions above, the unit converter at cerdikcerdik.com/tools/converter will turn metres into feet and kilometres into miles.