How big is High-Energy Neutrino? | Scale of the Universe

A high-energy neutrino is a neutrino that carries a very large amount of energy, like those that reach Earth from deep space. Neutrinos are elementary particles with no electric charge and almost no mass. No experiment has measured a size for a neutrino, so the figure shown is not a measurement.

Size

The size line reads 15 zeptometers, or 1.5 × 10⁻²⁰ meters. A zeptometer is 10⁻²¹ meters, a thousandth of a billionth of a billionth of a meter. The notation 10⁻²¹ means 1 divided by a 1 followed by 21 zeros. The explorer does not say what energy its high-energy neutrino has, and no experiment has measured a size for neutrinos of any energy.

For quarks, experiments have at least set an upper limit on size. For neutrinos, physicists have not agreed on how a size should even be defined. The Particle Data Group, which collects the world's particle measurements, lists limits on a quantity called the neutrino charge radius. It also notes that the meaning of that quantity is still disputed.

What is known well is how rarely neutrinos interact with anything. A neutrino feels only two of the four fundamental forces, the weak force and gravity. With no electric charge, it is not pushed or pulled by electric and magnetic forces, and it does not feel the strong force that binds quarks.

Size comparisons

Apart from gravity, a neutrino can affect another particle only through the weak force, which reaches about 10⁻¹⁸ meters. That is less than a thousandth of the width of a proton, which is about 1.7 femtometers (1.7 × 10⁻¹⁵ meters) across.
Most neutrinos pass straight through the whole of Earth, 12,742 kilometers (7,918 miles) across, without being stopped.
The chance of a hit grows with energy. A neutrino with an energy of 40 TeV (40 trillion electronvolts, where an electronvolt is a tiny unit of energy) travels on average about the width of Earth before Earth stops it.
To catch the few neutrinos that do interact, the IceCube detector at the South Pole watches a cubic kilometer of ice, with 5,160 light sensors hung between 1,450 and 2,450 meters below the surface.

Neutrinos from space

Between May 2010 and May 2012, IceCube recorded 28 high-energy events, far more than the roughly 11 expected from particles made in Earth's atmosphere. The most energetic two carried about 1 PeV each, or 1,000 TeV. In 2013 the IceCube team reported them as evidence for high-energy neutrinos from space.

On February 13, 2023, the ARCA detector of the KM3NeT telescope, 3,450 meters deep in the Mediterranean Sea off Sicily, recorded a neutrino with an estimated energy of about 220 PeV. As of 2026, it is the most energetic neutrino ever observed. That single particle carried roughly as much energy as a tennis ball dropped from a height of 6 centimeters (2.4 inches).

Scientists think such neutrinos come from the most violent places in the universe, such as the surroundings of supermassive black holes and exploding stars. There, particles called cosmic rays are sped up to enormous energies and can produce neutrinos.

How neutrinos are caught

Neutrinos are the second most common particle in the universe, after photons, the particles of light. Yet they interact so rarely that, in the words of the OpenStax textbook Astronomy, "Earth is more transparent to a neutrino than the thinnest and cleanest pane of glass is to a photon of light." Catching them takes enormous detectors.

IceCube and KM3NeT watch huge volumes of ice or seawater. When a neutrino does hit something, it makes electrically charged particles that give off a bluish glow called Cherenkov light. Sensors record the glow, and physicists use it to work out the neutrino's direction and energy.