WEAK NUCLEAR FORCE

a fundamental force of nature that underlies some forms of radioactivity and certain interactions between subatomic particles. The weak force acts upon all known fermions-i.e., elementary particles with an intrinsic angular momentum, or spin, of half a unit. Particles interact through the weak force by exchanging bosons (particles with integer spin) that are known as W and Z particles (qq.v.). These bosons are heavy, with masses about 100 times the mass of a proton, and it is their heaviness that makes the weak force appear weak at the low energies associated with radioactivity. In radioactive decays, the strength of the weak force is about 100,000 times less than the strength of the electromagnetic force. 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 single "electroweak" force. 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 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 and other stars where both the temperature and the density of matter are high. They are responsible for the initial reactions between protons that lead to the conversion of hydrogen to helium and the resultant release of energy. Christine Sutton