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The Nobel Prize in Physics 2026

The Telescope Made of Ice

About 65 billion neutrinos from the Sun pass through every square centimetre of your body each second, and almost none of them touch anything. On 6 October Francis Halzen won the Nobel Prize in Physics for the instrument that catches the rarest neutrinos of all, the high-energy ones from deep space: a cubic kilometre of Antarctic ice, wired with 5,160 light sensors, up to 2,450 metres beneath the South Pole.

Solar neutrinos through your thumbnail since you arrived 0 At about 65 billion per square centimetre per second; a thumbnail is roughly one square centimetre.

mariuscomper.uk · 14 October 2026

#What is a neutrino, and why is it so hard to catch?

A neutrino has no electric charge and almost no mass. It ignores electromagnetism and the strong force that holds atomic nuclei together, so it passes through matter as if it were empty space: through you, through buildings, through the whole planet. The Sun makes them in staggering numbers. Each time four hydrogen nuclei fuse into one of helium in its core, two neutrinos fly out. They do not stop at night; they come up through the ground beneath your bed.

That indifference is what makes them precious to astronomers. Light from a distant galaxy can be absorbed by dust and gas on the way. Cosmic rays, the charged particles that also stream in from space, are bent by magnetic fields until they arrive from no particular direction. A neutrino flies straight. Catch one, and you know which way to look.

Catching one is the problem. Very rarely, a neutrino strikes an atomic nucleus. The collision throws out charged particles that move through ice faster than light itself travels in ice, and they shed a faint blue glow, an optical cousin of the sonic boom called Cherenkov light. To see enough of those flashes from the rarest, most energetic neutrinos, the ones that come from beyond the solar system, you need a detector about a kilometre across.

In 1988 Francis Halzen, a Belgian-born theoretical physicist at the University of Wisconsin–Madison, presented a way to get one. He did not propose to build a tank. The ice sheet at the South Pole is about 2,800 metres thick, and deep down it is extraordinarily clear, because the pressure has turned its air bubbles into crystals that scatter almost no light. Hang light sensors in it, and the ice becomes the detector.

#How deep is IceCube?

Sink through the ice at the South Pole, metre by metre, and back through 100,000 years of snowfall.

0 m · the surface

The South Pole sits on a plateau of ice about 2,800 metres thick. The snow at the top fell last winter. Every metre down is a step back in time.

About 700 m

This ice fell as snow roughly 10,000 years ago, as the last ice age was ending.

828 m

The height of the Burj Khalifa, the world's tallest building. Nothing of the observatory is here yet.

The holes

Each of IceCube's 86 holes was melted out by a 5-megawatt hot-water drill firing a jet at about 88°C. In the best season a 2.5 km hole could be drilled in as little as 27 hours, on just over 15,000 litres of fuel. A cable of sensors was lowered in before the water froze around it.

1,450 m · the first sensors

The top of the detector. Each cable, or string, carries 60 glass spheres the size of basketballs, spaced about 17 metres apart down the next kilometre on most strings; eight central strings pack theirs more tightly. Each holds a light sensor and its own electronics. The ice here is about 40,000 years old.

1,950–2,100 m · the dust layer

The ice turns murky. This band of dust was laid down 60,000–70,000 years ago, in one of the dustiest spells of the last ice age, and it scatters light. IceCube sees less clearly through this slice of itself.

2,450 m · the last sensors

The bottom of the detector, in ice laid down roughly 95,000–100,000 years ago. Below about 2,200 metres there is so little dust that the ice is clearer still.

About 2,800 m · bedrock

Above you hangs a cubic kilometre of watched ice, about 900 million tonnes of it, threaded with 86 strings and 5,160 sensors. The last string went in on 18 December 2010.

#How does IceCube detect neutrinos?

Each sensor counts the photons that reach it and stamps each with a precise time. A particle crossing the ice lights up the sensors along its path one after another; the order in which they light up gives its direction, and the amount of light helps estimate its energy: the energy deposited in the detector for a contained burst, and a lower limit for a muon that passes straight through. Pick an event below to see what each kind looks like.

Drag to turn the detector

A neutrino that has crossed the Earth hits a nucleus in or below the ice and makes a muon, which carries on in nearly the same direction and leaves a straight line of light. For a good track, the direction back to the source is known to within about a degree.

Illustration of the three kinds of event, not recorded data. The 86 strings and 60 sensors per string are true to count; the layout of the strings is simplified. Sphere size shows how much light a sensor caught; colour shows when, from amber (first) to blue (last).

#Why does IceCube look down through the Earth?

Most of what crosses the ice is not a neutrino at all. Cosmic rays smashing into the atmosphere make muons, heavier cousins of the electron, which rain down through the ice. The collisions also make neutrinos, and those are hard to tell apart from the cosmic kind. Here is IceCube's own tally for a year:

Muons from the atmosphere, coming down≈ 90,000,000,000
Neutrinos made in the atmosphere≈ 100,000
Neutrinos from spacea few dozen to ≈ 100
Events per year, logarithmic scale: each step along the bar is ten times more. Figures from IceCube; the atmospheric muon and neutrino counts are rounded from its published rates, and the cosmic count is a range because estimates differ.

That is roughly a billion falling muons for every neutrino from space. So the telescope mostly looks the other way. No muon can cross the planet; a neutrino can. Anything travelling upwards through the ice was almost certainly made by a neutrino that crossed the Earth. The planet is the shield, though an imperfect one: it stops the falling muons but not the neutrinos the atmosphere makes on the far side of the world, and at the very highest energies it begins to absorb even neutrinos.

Because the observatory sits on the pole, "below" is always the same half of the sky. A star at the South Pole never rises or sets; it circles at a fixed height. From the South Pole, the northern half of the sky is always on the far side of the Earth, so IceCube watches it through the planet, 24 hours a day.

The southern sky needs a different trick. A burst of light that starts inside the detector, with no track of light leading in from outside, is a good neutrino candidate. A veto rejects incoming muons, though some can slip through. Using those contained events, IceCube can also see the half of the sky above it.

#What did IceCube find in 2017?

One neutrino, and the galaxy it probably came from.

22 September 2017

A muon crosses IceCube heading upwards. The neutrino that made it carried an estimated 290 teraelectronvolts, about 40 times the energy of a proton in the beams of CERN's Large Hadron Collider.

Through the planet

It arrived from 5.7 degrees below the horizon. Traced backwards, its path ran through about 1,300 km of rock and ice before it reached the detector.

43 seconds later

IceCube's software sent an automatic alert with the direction to observatories around the world.

A week later

The Fermi gamma-ray space telescope reported that the patch of sky held a known galaxy in outburst: the blazar TXS 0506+056, brighter in gamma rays than it had been at any time in the previous decade. A blazar is a galaxy whose central black hole fires a jet of matter almost straight at us.

3.7 billion years

The light from TXS 0506+056 that reaches us now set out about 3.7 billion years ago. When it left, the Earth was less than a billion years old.

Evidence, not proof

One neutrino could be a coincidence. If neutrinos and flaring blazars were unrelated, a match this close would turn up by chance only about once in 770 to once in 4,000 tries. But when IceCube searched its archive, it found an earlier excess of neutrinos from the same direction. Two papers in Science in July 2018 made TXS 0506+056 the first identified candidate source of high-energy cosmic neutrinos.

#Where do cosmic neutrinos come from?

The first evidence came in 2013: 28 neutrinos between 2010 and 2012 with energies far above what the atmosphere usually makes, where about 11 would have been expected. Two of them, nicknamed Bert and Ernie after the Muppets, each carried more than a petaelectronvolt, more than 150 times the energy of a proton in the beams of CERN's Large Hadron Collider. Where they came from could not be told. Pinning neutrinos to particular places has taken a decade more.

IceCube's neutrino sky An all-sky map. The northern half of the sky, which IceCube sees through the Earth with muon tracks, is shaded. The band of the Milky Way crosses it. Three neutrino findings are marked: the blazar TXS 0506+056, the galaxy NGC 1068, and the plane of the Milky Way. Below the horizon at the South Pole: seen through the Earth Above the horizon at the South Pole Celestial equator Centre of the Milky Way TXS 0506+056 NGC 1068
Below the horizon at the Pole Milky Way Neutrino findings
Equatorial coordinates, Hammer projection, north at top. The galactic plane is computed from standard coordinates; source positions are the published ones. At the South Pole, everything north of the celestial equator is permanently below the horizon; IceCube sees it through the Earth with muon tracks, and sees the rest with contained bursts of light.

NGC 1068 is a surprise in its own way. It is a galaxy with a feeding black hole hidden behind thick gas and dust, and it shines far more faintly in high-energy gamma rays than in neutrinos. That suggests the neutrinos are made somewhere so dense that the gamma rays made alongside them cannot get out. That is the case for neutrino astronomy in one object. It shows parts of the universe that telescopes using light cannot see.

The firmest result of all is the closest. Our own galaxy glows in neutrinos along its plane, most of it in the southern sky that IceCube sees above it rather than through the Earth. First reported in 2023, the signal reached 5.7 sigma in an analysis of 12 years of data published in July 2026, making the Milky Way the first source of high-energy neutrinos to pass the 5-sigma threshold physicists use for a discovery. It is faint for a galaxy: by one estimate from Halzen and colleagues, far dimmer in neutrinos than the average distant galaxy, which they called a neutrino desert.

#Why did Francis Halzen win the Nobel Prize?

The Royal Swedish Academy of Sciences gave him the prize, alone, "for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin". He was the principal investigator from the start, and led the collaboration of scientists and engineers that turned a block of the Antarctic into an instrument. He is the first sole winner of the physics prize since 1992.

"His tenacity and scientific vision has paved the way for a new kind of astronomy."

Mark Pearce, chair of the Nobel Committee for Physics

Halzen was born in Tienen, Belgium, on 23 March 1944, took his doctorate at the University of Louvain (Leuven) in 1969 and has taught at Wisconsin since the early 1970s. Asked in his Nobel telephone interview whether he was proud, he said his "main reaction to this project over time was not proud, but was surprised." In IceCube's announcement he called the prize "a great relief for me to finally deliver the recognition that this great collaboration deserves."

  1. 1988Halzen presents the idea of catching neutrinos in the ice at the South Pole.
  2. 1999The proposal for a cubic-kilometre detector, with Halzen as principal investigator.
  3. 201018 December: the last of 86 strings is lowered into the ice.
  4. 2013First evidence for neutrinos from beyond the solar system: 28 events, including Bert and Ernie.
  5. 201722 September: the alert that leads to the blazar TXS 0506+056.
  6. 2022Neutrinos from the active galaxy NGC 1068.
  7. 2023The Milky Way seen in neutrinos.
  8. 2026July: the Milky Way passes 5 sigma, the first discovery-grade neutrino source.
  9. 20266 October: the Nobel Prize in Physics.

#What comes next for IceCube?

In the 2025–26 Antarctic summer, five more strings went into the middle of the array, packed with newer sensors and calibration devices, to measure the ice more precisely and to catch lower-energy neutrinos. A proposed successor, IceCube-Gen2, would watch about eight cubic kilometres of ice. It is still a proposal.

Halzen's telescope has no lens and no mirror, and it cannot be pointed. It waits in the dark for something that almost never happens, and it lets the whole Earth stand between it and the noise.

#Sources and method

Detector figures come from the IceCube collaboration and its instrumentation papers; the prize details from the Royal Swedish Academy of Sciences and the Nobel Foundation. Ice ages at depth come from published South Pole depth–age scales and are approximate. Several figures are our own arithmetic from those sources: the solar neutrino rate (the Sun's luminosity divided by the energy released per pair of neutrinos, spread over a sphere one astronomical unit in radius); the 1,300 km path through the Earth (a chord of a 6,371 km sphere at 5.7 degrees); the 900 million tonnes of a cubic kilometre of ice at 917 kg per cubic metre; the 17-metre sensor spacing on standard strings (one kilometre over 59 gaps); the energy comparisons with the LHC's 6.8 TeV protons; the Earth's age when the blazar's light left (4.54 billion years minus 3.7 billion); the odds of a fluke (the inverse of the published chance probabilities); and the ratio of falling muons to cosmic neutrinos.