One of the four fundamental forces, an interaction involving the exchange of W and Z intermediate vector bosons, changing one flavour of quark into another. It causes protons to transmute into neutrons, allowing deuterium to form, deuterium-burning powering the Sun, and allowing heavy nuclei to form. The weak force acts over a range of about 10^-18 m (about 0.1 per cent of the diameter of a proton).