The Online Encyclopedia and Dictionary






Weak nuclear force

The weak nuclear force or weak interaction is one of the four fundamental forces of nature. It is most commonly seen in beta decay and the associated radioactivity. The predicate weak derives from the fact that the field strength is some 109 times less than that of the strong nuclear force. The term nuclear indicates that it is a short-range force, limited to distances smaller than an atomic nucleus.


The weak nuclear force affects all leptons and quarks. It is the only force affecting neutrinos (except for gravitation, which is negligible on laboratory scales). The weak interaction enables all lepton and quark particles and antiparticles to interchange energy, mass, electric charge and flavor—effectively to change into each other.

Although the weak nuclear force used to be described by Fermi's theory of a contact four-fermion interaction, today we know that it is mediated by the W and Z bosons. Because of their large mass of about 90 GeV/c2, their mean life is limited to about 3 \times 10^{-25}seconds by the uncertainty principle:

\Delta E\cdot\Delta t\geq\hbar/2

Even at the speed of light this effectively limits the range of the weak nuclear force to 10 - 18meters, or about 1000 times smaller than the diameter of an atomic nucleus.

The laws of nature were long thought to remain the same under mirror reflection. The results of an experiment viewed via a mirror were expected to be identical to the results of a mirror-reflected copy of the experimental apparatus. This is called the law of parity conservation in physicists' jargon. However, in the mid-1950's Chen Ning Yang and Tsung-Dao Lee suggested that the weak interaction might violate this law. Chien Shiung Wu and collaborators confirmed this prediction in 1957, earning Yang and Lee the 1957 Nobel Prize in Physics.

The Standard Model of particle physics describes the electromagnetic force and the weak nuclear force as two different aspects of a single electroweak force, the theory of which was developed around 1968 by Sheldon Glashow, Abdus Salam and Steven Weinberg (more at W and Z bosons). They were awarded the 1979 Nobel Prize in Physics for their work.

See also

External links

Last updated: 05-13-2005 07:56:04