One of the four fundamental forces, responsible for radioactive decay and particle transformations.The weak interaction is one of the four fundamental forces of nature, alongside gravity, electromagnetism, and the strong nuclear force. It is responsible for processes where particles change identity, such as radioactive beta decay, where a neutron transforms into a proton, an electron, and an antineutrino. Despite its name, it is far stronger than gravity at short ranges, but it acts only over subatomic distances, making it difficult to observe directly in everyday life.
The weak interaction was first identified in the late 19th century through studies of radioactivity, but it was formalized in the 1930s by Enrico Fermi, who proposed a theory of beta decay. The modern electroweak theory, unifying it with electromagnetism, was developed in the 1960s by Sheldon Glashow, Abdus Salam, and Steven Weinberg, and confirmed experimentally in 1983 with the discovery of the W and Z bosons at CERN.
The weak interaction is mediated by exchange particles called W and Z bosons, which are massive—about 80–90 times the mass of a proton. Because these bosons are so heavy, the force has an extremely short range, roughly 10^-18 meters. When a particle emits or absorbs a W boson, it can change its flavor (e.g., a down quark becomes an up quark), leading to transformations like beta decay. The Z boson, on the other hand, mediates neutral-current interactions, where particles exchange energy and momentum without changing identity.
The weak interaction is harnessed in nuclear power and medical imaging, such as positron emission tomography (PET), which relies on beta-plus decay, and in neutrino detectors that study the sun and supernovae.
The weak interaction is the only force that can change the flavor of quarks, which is why it is essential for the nuclear reactions that power stars.
