The weak force is one of the four fundamental forces of nature, and it acts only over very short distances. Its range, the distance it can reach, is about 10⁻¹⁸ meters, or 1 attometer. That is less than a thousandth of the width of a proton.

Distance

An attometer is 10⁻¹⁸ meters, a billionth of a billionth of a meter. The notation 10⁻¹⁸ means 1 divided by a 1 followed by 18 zeros. The range is not an edge that anyone has measured with an instrument. It is an estimate, worked out from the mass of the particles that carry the force.

The weak force is carried by particles called the W and Z bosons. They are heavy: a W boson has about 86 times the mass of a proton, and a Z boson about 97 times. Quantum physics allows a force-carrying particle to exist only for a brief moment, and the heavier the particle, the briefer the moment.

Even moving at nearly the speed of light, a W boson can cover only about 2 × 10⁻¹⁸ meters in that time, according to the textbook University Physics Volume 3. CERN, the European particle physics laboratory, rounds the range to about 10⁻¹⁸ meters. It adds that the force vanishes altogether beyond the radius of a single proton.

Size comparisons

A proton, about 1.7 femtometers (1.7 × 10⁻¹⁵ meters) across, is about 1,700 times as wide as the range of the weak force.
The strong force, which holds quarks together inside protons and neutrons, reaches about 10⁻¹⁵ meters, about 1,000 times as far as the weak force.
Experiments have found that a quark, if it has any size at all, is less than about 8.6 × 10⁻¹⁹ meters across, a little less than the range of the weak force.
If the range of the weak force were enlarged to the width of a human hair, about 0.1 millimeters, a proton would be about 17 centimeters (6.6 inches) across and a hydrogen atom about 11 kilometers (6.6 miles) across.

What the weak force does

The weak force can change one kind of quark into another. That is how a neutron turns into a proton in beta decay, a kind of radioactive decay. The neutron gives off a W boson and becomes a proton, and the W boson then turns into an electron and an antineutrino.

The weak force can also turn a proton into a neutron. That happens in the first step of the nuclear fusion that powers the Sun. Two protons combine, and one of them becomes a neutron, making deuterium, a heavy form of hydrogen. Neutrinos feel only the weak force and gravity, and most pass through a whole planet without being stopped.

Although it is called weak, this force is much stronger than gravity. CERN explains that it is essentially as strong as the electromagnetic force, and appears weak because the heavy W and Z bosons limit its reach.

Finding the W and Z bosons

Enrico Fermi put forward the first theory of the weak force in 1933. In the 1960s, Sheldon Glashow, Abdus Salam and Steven Weinberg proposed that the weak and electromagnetic forces are two forms of a single electroweak force. Their theory predicted the W and Z bosons and, combined with earlier experiments, their masses.

To make the bosons, physicists at CERN turned the laboratory's largest accelerator into a machine that collided protons with antiprotons. The UA1 and UA2 experiments observed both particles there for the first time in 1983, with the predicted masses. Carlo Rubbia and Simon van der Meer received the 1984 Nobel Prize in Physics for the discovery.

Sources

The Z boson. CERN.
The Standard Model. CERN.
The Standard Model. In University Physics Volume 3. OpenStax, 2016.
Introduction to Particle Physics. In University Physics Volume 3. OpenStax, 2016.
The Four Basic Forces. In College Physics, 2nd edition. OpenStax, 2022.
Mass, Energy, and the Theory of Relativity. In Astronomy, 2nd edition. OpenStax, 2022.
Summary Tables: Gauge and Higgs Bosons. Particle Data Group, 2025.
Limits on the effective quark radius from inclusive ep scattering at HERA. H. Abramowicz et al. (ZEUS Collaboration), Physics Letters B, 2016.
SI prefixes. International Bureau of Weights and Measures.
Proton rms charge radius. NIST CODATA, 2022.
Proton mass energy equivalent in MeV. NIST CODATA, 2022.
Bohr radius. NIST CODATA, 2022.
Just How Small Is "Nano"?. National Nanotechnology Initiative.

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