Weak Interaction

Weak interaction (often called the weak force or sometimes the weak nuclear force) is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars. Weak interactions affect all known fermions; that is, particles whose spin (a property of all particles) is a half-integer.

In the Standard Model of particle physics the weak interaction is theorised as being caused by the exchange (i.e., emission or absorption) of W and Z bosons; and as such, is considered to be a non-contact force, like the other three forces. The best known effect of this emission is beta decay, a form of radioactivity. The Z and W bosons are much heavier than protons or neutrons and it is the heaviness that accounts for the very short range of the weak interaction. It is termed weak because its typical field strength is several orders of magnitude less than that of both electromagnetism and the strong nuclear force. Most particles will decay by a weak interaction over time. It has one unique property – namely quark flavour changing – that does not occur in any other interaction. In addition, it also breaks parity-symmetry and CP-symmetry. Quark flavour changing allows for quarks to swap their 'flavour', one of six, for another.

The weak force was originally described, in the 1930s, by Fermi's theory of a contact four-fermion interaction: which is to say, a force with no range (i.e., entirely dependent on physical contact). However, it is now best described as a field, having range, albeit a very short range. In 1968, the electromagnetic force and the weak interaction were unified, when they were shown to be two aspects of a single force, now termed the electro-weak force. The theory of the weak interaction can be called Quantum Flavordynamics (QFD), in analogy with the terms QCD and QED, but in practice the term is rarely used because the weak force is best understood in terms of electro-weak theory (EWT).

Weak interactions are most noticeable when particles undergo beta decay, and in the production of deuterium and then helium from hydrogen that powers the sun's thermonuclear process. Such decay also makes radiocarbon dating possible, as carbon-14 decays through the weak interaction to nitrogen-14. It can also create radioluminescence, commonly used in tritium illumination, and in the related field of betavoltaics.