a fundamental force of nature that underlies some forms of radioactivity, governs the decay of unstable subatomic particles (subatomic particle) such as mesons (meson), and initiates the nuclear fusion reaction that fuels the Sun. The weak force acts upon all known fermions (fermion)—i.e., elementary particles with half-integer values of intrinsic angular momentum, or spin. Particles interact through the weak force by exchanging force-carrier particles known as the W (W particle) and Z (Z particle) particles. These particles are heavy, with masses about 100 times the mass of a proton, and it is their heaviness that defines the extremely short-range nature of the weak force and that makes the weak force appear weak at the low energies associated with radioactivity.

The effectiveness of the weak force is confined to a distance range of 10⁻¹⁷ metre, about 1 percent of the diameter of a typical atomic nucleus. In radioactive decays (radioactivity) the strength of the weak force is about 100,000 times less than the strength of the electromagnetic force (electromagnetism). However, it is now known that the weak force has intrinsically the same strength as the electromagnetic force, and these two apparently distinct forces are believed to be different manifestations of a unified electroweak force (electroweak theory).

Most subatomic particles are unstable and decay by the weak force, even if they cannot decay by the electromagnetic force or the strong force. The lifetimes for particles that decay via the weak force vary from as little as 10⁻¹³ second to 896 seconds, the mean life of the free neutron. Neutrons bound in atomic nuclei can be stable, as they are when they occur in the familiar chemical elements, but they can also give rise through weak decays to the type of radioactivity known as beta decay. In this case the lifetimes of the nuclei can vary from a thousandth of a second to millions of years. Although low-energy weak interactions are feeble, they occur frequently at the heart of the Sun (Cosmos) and other stars where both the temperature and the density of matter are high. In the nuclear-fusion process that is the source of stellar-energy production, two protons interact via the weak force to form a deuterium nucleus, which reacts further to generate helium with the concomitant release of large amounts of energy.

The characteristics of the weak force, including its relative strength and effective range and the nature of the force-carrier particles, are summarized in the standard model of particle physics.