How big is Neutrino? | Scale of the Universe

A neutrino is an elementary particle, one that is not made of anything smaller as far as experiments can tell. It has no electric charge and a mass so small that experiments have only set an upper limit on it. No experiment has measured a size for it either, so the 1 yoctometer shown above is not a measurement. A yoctometer is 10⁻²⁴ meters, a trillionth of a trillionth of a meter.

Size

A neutrino has no surface to measure. Instead, physicists watch how neutrinos scatter off electrons. A neutrino with some size, described by a quantity called its charge radius, would scatter slightly differently from a point.

The TEXONO experiment in Taiwan caught antineutrinos, the antimatter partners of neutrinos, streaming from a nuclear reactor 28 meters away. They scattered off electrons as the standard theory of particles predicts, within the experiment's precision. The Particle Data Group, which collects particle measurements from around the world, uses this 2010 result as its limit. It corresponds to a charge radius of less than about 1.8 × 10⁻¹⁸ meters, or a width of less than about 3.6 × 10⁻¹⁸ meters.

Physicists disagree about whether a neutrino's charge radius is a true size, so the figure is best read as a limit on how neutrinos scatter. The explorer's 1 yoctometer is far below it, so no experiment could confirm or rule it out.

Size comparisons

A proton, about 1.7 femtometers (1.7 × 10⁻¹⁵ meters) across, is about 460 times as wide as the upper limit for a neutrino.
A hydrogen atom, about 0.1 nanometers across, is nearly 30 million times as wide as the upper limit. A nanometer is a billionth of a meter.
If the upper limit were enlarged to the width of a human hair, about 0.1 millimeters, a proton would be about 4.6 centimeters (1.8 inches) across. A human hair enlarged by the same amount would be about 2.8 million kilometers (1.7 million miles) wide, about seven times the distance from Earth to the Moon.
The upper limit is about 200 million billion (2 × 10¹⁷) times the Planck length, the next smaller entry in the explorer.

The ghost particle

Wolfgang Pauli proposed the neutrino in 1930 to explain beta decay, a kind of radioactivity in which an atomic nucleus gives off an electron. Some energy seemed to go missing, and Pauli suggested that an unseen particle with no charge carried it away. Enrico Fermi named it the neutrino, Italian for "little neutral one."

Neutrinos react so rarely with matter that they were thought almost impossible to catch. Frederick Reines and Clyde Cowan of Los Alamos finally detected them at the Savannah River Plant in South Carolina, using the flood of antineutrinos from its nuclear reactor. They published the discovery in July 1956, and Reines shared the 1995 Nobel Prize in Physics for it.

Everywhere, and very light

After photons, the particles of light, neutrinos are the most common particles in the universe. Many are made by nuclear fusion in the Sun. About 66 billion neutrinos from the Sun reach each square centimeter at Earth every second, and hardly anything stops them.

Neutrinos come in three types: electron, muon and tau neutrinos. Takaaki Kajita and Arthur McDonald won the 2015 Nobel Prize in Physics for showing that neutrinos switch between types as they travel, which means they must have mass. In 2025, the KATRIN experiment reported that the neutrino's mass is less than 0.45 electronvolts, a unit of energy that physicists also use for mass. An electron has more than a million times as much mass.