the fusion of hydrogen into helium, which powers the Sun's thermonuclear process. Most fermions decay by a weak interaction over time. Such decay makes radiocarbon dating possible, as carbon-14 decays through the weak interaction to nitrogen-14. It can also create radioluminescence, commonly used in tritium luminescence, and in the related field of betavoltaics (but not similar radium luminescence). The electroweak force separated into the electromagnetic and weak forces during the quark epoch of the early universe. History In 1933, Enrico Fermi proposed the first theory of the weak interaction, known as Fermi's interaction. He suggested that beta decay could be explained by a four-fermion interaction, involving a contact force with no range. However, it is better described as a non-contact force field having a finite range, albeit very short. In the 1960s, Sheldon Glashow, Abdus Salam and Steven Weinberg unified the electromagnetic force and the weak interaction by showing them to be two aspects of a single force, now termed the electroweak force. The existence of the and bosons was not directly confirmed until 1983. Properties The electrically charged weak interaction is unique in a number of respects: It is the only interaction that can change the flavour of quarks and leptons (i.e., of changing one type of quark into another). It is the only interaction that violates P, or parity symmetry. It is also the only one that violates charge–parity CP symmetry. Both the electrically charged and the electrically neutral interactions are mediated (propagated) by force carrier particles that have significant masses, an unusual feature which is explained in the Standard Model by the Higgs mechanism. Due to their large mass (approximately 90 GeV/c2) these carrier particles, called the and bosons, are short-lived with a lifetime of under seconds. The weak interaction has a coupling constant (an indicator of how frequently interactions occur) between and , compared to the electromagnetic coupling constant of about and the strong interaction coupling constant of ; consequently the weak interaction is "weak" in terms of intensity. The weak interaction has a very short effective range (around to m (0.01 to 0.1 fm)). At distances around meters (0.001 fm), the weak interaction has an intensity of a similar magnitude to the electromagnetic force, but this starts to decrease exponentially with increasing distance. Scaled up by just one and a half orders of magnitude, at distances of around 3 m, the weak interaction becomes 10,000