Nobel Prize in Physics 2026 Explained: Francis Halzen, IceCube and the Neutrino Ghost Particle

Neutrinos are electrically neutral particles that usually pass through ordinary matter without interacting. Different detectors study different neutrino energies, from the Sun's fusion reactions to powerful sources far beyond the Solar System.

The YouTube player loads only when you press Play. Playing connects to YouTube, which may use cookies and storage. See Privacy.

What a ghost particle means

“Ghost particle” is a nickname for a neutrino. It does not describe anything supernatural. Neutrinos have very small masses and no electric charge.

The opening estimate gives about 100 trillion neutrinos crossing a person's body each second. This is an approximate scale based mainly on solar flux and body area. It is not an exact personal count.

IceCube studies a much higher energy range than ordinary solar fusion neutrinos. The opening moves between these populations, so the distinction must remain clear throughout the explanation.

The missing energy problem

In beta decay, an unstable nucleus can emit an electron. Early measurements show a range of electron energies, rather than one value carrying all the available energy.

The apparent deficit threatens an incomplete account of energy conservation. It does not establish that energy actually vanishes. The missing participant could carry energy that the experiment does not detect.

Pauli's proposed particle

In 1930, Wolfgang Pauli proposes an electrically neutral particle to account for the missing energy and momentum. Enrico Fermi later develops the decay theory and popularises the name neutrino.

The name means little neutral one. A particle with no charge can escape instruments that depend on strong electrical interactions. The proposal explains an accounting problem before direct detection becomes possible.

Why neutrinos rarely interact

Electric charge makes many ordinary detection methods effective. Light interacts with charged matter, and electromagnetic forces strongly affect how atoms respond to one another.

A neutrino has no electric charge. It participates in weak interactions and gravity. The weak interaction has a very short characteristic range, on a scale much smaller than a proton.

The video's comparison places that range near a thousandth of a proton's width. This is a rough scale comparison, rather than a sharp edge around the force. Interaction probability also depends strongly on neutrino energy and the target.

Saying atoms are mostly empty space is not the complete explanation. Other particles interact readily despite that geometry. The small neutrino interaction probability is the central reason for its penetrating ability.

The wall of lead comparison

A light-year is the distance light travels in a year. The video imagines a lead wall with that thickness to show how penetrating low-energy neutrinos can be.

The fraction that survives depends on energy and interaction assumptions. A Fermilab teaching example gives about half surviving such a thickness. The narration's “most” must not become a universal precise claim.

This example concerns low-energy neutrinos. It does not imply that every high-energy neutrino crosses any amount of matter unhindered.

The 1956 reactor detection

Clyde Cowan and Frederick Reines use the intense antineutrino supply from a nuclear reactor. In 1956, they report convincing detection, about 26 years after Pauli's proposal.

Cowan and Reines send Pauli a telegram in June 1956 to report the detection.

An electron antineutrino can interact with a proton and produce a positron and a neutron. A positron is the electron's positively charged counterpart. Its annihilation produces a prompt light signal.

The neutron later produces another signal after capture. The relationship between the prompt and delayed signals helps distinguish the interaction from background. The experiment therefore uses a pattern, rather than merely one unexplained flash.

The detected particles are reactor antineutrinos. That detail matters when connecting the historical experiment with the broader neutrino family.

Neutrinos from the Sun

Nuclear fusion in the Sun's core converts hydrogen into helium through reaction chains. Some steps release neutrinos. At Earth, the total solar flux is about 65 billion per square centimetre each second.

A square centimetre gives the scale of a small fingernail. Multiplying flux by a body's projected area gives the large opening estimate. Orientation and the chosen area affect the result.

Energy carried by radiation takes a long, indirect route through the Sun's interior. Repeated interactions delay its escape over many thousands of years. The light finally leaving the surface needs about eight minutes to reach Earth.

Most solar neutrinos escape nearly directly. Crossing a solar-radius distance at nearly light speed takes about two seconds. Their arrival therefore provides much more immediate information about the core's nuclear reactions.

Solar neutrinos at night

When the Sun lies below the local horizon, its neutrinos approach through Earth. Most low-energy solar neutrinos still pass through. The signal does not stop when visible daylight ends.

Super-Kamiokande in Japan studies solar neutrinos and reconstructs their directional distribution. Its neutrino image of the Sun combines many events. It is not a conventional photograph taken with visible light.

Day and night measurements can differ because passage through Earth affects neutrino flavour. Flavour describes the electron, muon, or tau interaction type. The fact that neutrinos arrive at night does not require identical detection rates at all times.

Why neutrinos are useful messengers

High-energy particles from space reach Earth continually. Research on cosmic rays develops from Victor Hess's 1912 observations. Many cosmic rays carry electric charge.

Magnetic fields bend charged-particle paths. Their arrival directions therefore need not point directly toward their original sources. Light also encounters absorption and scattering in matter along its route.

Neutrinos avoid electromagnetic deflection and can escape some regions that block light. Their reconstructed directions can help identify source regions. The method still has angular uncertainty and needs enough statistical evidence.

Why use Antarctic ice?

High-energy astrophysical neutrinos are rare at a detector. A large target increases the opportunity for an interaction. IceCube uses roughly a cubic kilometre of ice as an instrumented target region.

Francis Halzen and collaborators help develop the approach. Antarctica supplies a large, dark ice volume without requiring a manufactured tank of comparable size.

Early AMANDA work in the 1990s encounters light scattering from air bubbles in shallower ice. Deeper ice offers better optical conditions as pressure changes the trapped air structure. Depth therefore improves the ability to reconstruct travelling light.

Building IceCube

The original deep array uses 86 strings, each carrying 60 optical modules. That gives 5,160 sensors. The modules detect small amounts of light in the surrounding ice.

Hot-water equipment melts holes to depths near 2.5 kilometres. The team lowers a sensor string before the water freezes again. The deployed sensors occupy depths of about 1.45–2.45 kilometres.

The drilling system can prepare about one hole every two days during successful operation. Construction spans seven short Antarctic field seasons. This does not mean uninterrupted drilling throughout seven calendar years.

The final string enters the ice in December 2010. Full-array operation begins in 2011. The count describes the original array, rather than every later upgrade or surface instrument.

Cherenkov light

A neutrino occasionally interacts in or near the detector. The interaction can produce charged particles that move through the ice.

Some move faster than light propagates through ice. They remain slower than light's vacuum limit. Their motion produces a cone of Cherenkov radiation.

The sonic-boom comparison explains the cone formed when a disturbance outruns waves in a medium. It does not make neutrinos faster than light in vacuum. IceCube sees secondary light, rather than a neutrino glowing along its entire route.

The sensors record light arrival times with nanosecond-scale precision. A nanosecond is one billionth of a second. Researchers use the pattern to estimate event direction, energy, and type.

Looking through Earth

Cosmic rays interacting in the atmosphere produce many downward-moving background particles. Selecting upward-going events uses Earth to remove much of that background.

Neutrinos can cross Earth more readily than ordinary charged background particles. However, Earth significantly absorbs sufficiently high-energy neutrinos. The amount depends on energy and travel path.

The video therefore needs a qualification to “nothing stops them”. Detector analysis must include absorption instead of treating the whole planet as transparent at every energy.

The 2013 high-energy result

In 2013, IceCube announces evidence for high-energy neutrinos from astrophysical sources. Two prominent events receive the existing nicknames Bert and Ernie.

Their energies lie near the petaelectronvolt scale. Compared with a typical megaelectronvolt-scale solar neutrino, that is about a billion times more energy. Solar neutrinos have a spectrum, so the ratio is an order-of-magnitude comparison.

The result concerns evidence above expected backgrounds. It is not a claim that those events alone identify their individual source objects.

The 2017 blazar association

On September 22, 2017, IceCube detects a high-energy event and sends a rapid alert. The alert reaches other observatories within about a minute. Telescopes investigate the corresponding region of sky.

A flaring blazar, TXS 0506+056, lies in that direction. A blazar is an active galaxy whose relativistic jet points about toward Earth. The central black hole's environment powers the jet; matter does not escape from inside the event horizon.

The source lies at a cosmological distance commonly described as roughly four billion light-years. The observations support a likely association, rather than an absolutely unique proof from one particle.

NGC 1068 and the Milky Way

In 2022, IceCube reports evidence for neutrino emission from NGC 1068. The galaxy lies about 47 million light-years away. Its active central region contains material that obscures much of its high-energy light.

Neutrinos supply information that electromagnetic observations can miss. Their signal complements other observations rather than replacing them.

In 2023, IceCube reports a neutrino view of the Milky Way. The result uses reconstructed events and statistical analysis. It maps a high-energy signal associated with our galaxy, not the ordinary solar-neutrino flow through your body.

The 2026 physics prize

Francis Halzen receives the 2026 physics prize for the IceCube work and its discoveries. The video describes him as 82 at the announcement.

IceCube remains the work of a large collaboration, including scientists, engineers, drill teams, and support staff. A prize for one researcher does not make the observatory a single-person construction.

Recap

Neutrinos have no electric charge and interact weakly with matter. Their penetrating ability brings information from some otherwise hidden regions. Large detectors convert rare interactions into measurable signals.

What this means

The same particle family supports different kinds of astronomy at different energies. Solar-neutrino detectors and IceCube need separate explanations of their targets, backgrounds, and limits.

FAQ

Does IceCube catch ordinary solar fusion neutrinos?

Those neutrinos lie below its usual energy range. Other detectors study them directly.

Can Earth stop a neutrino?

Yes. Absorption becomes important at sufficiently high energies.

Does IceCube directly photograph a neutrino?

No. It detects light from particles produced by interactions and reconstructs the event.

Does one arrival direction prove one source?

No. Directional uncertainty, background, and supporting observations matter.