Weak interaction (often called the weak force or sometimes the weak nuclear force) is one of the four fundamental interactions of nature, along with strong interaction, electromagnetic force, and gravitation. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons. Its most familiar effect is beta decay (or the emission of electrons by neutrons or positrons by protons in atomic nuclei) and the associated radioactivity. It is a non-contact force, called weak because the typical field strength is 10−11 times the strength of the electromagnetic force and some 10−13 times that of the strong force, when forces are compared between particles interacting in more than one way.
The weak force was originally (in the 1930s) described by Fermi's theory of a contact four-fermion interaction, that is to say, a "force" without range.
http://en.wikipedia.org/wiki/Weak_nuclear_force
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