The last of the four known forces is the weak force. It is responsible for the spontaneous disintegration of certain radioactive nuclei – like uranium – where a neutron in the nucleus decays into a proton, an electron and an anti neutrino. The weak force is thus the force behind fission energy. Becquerel and the Curie couple discovered such decays already in 1896, but it took until about 1970 before a satisfactory theory, compatible with the principles of quantum mechanics, was completed.

The weak force acts upon quarks as well as electrons, neutrinos and similar particles. Most remarkable is the fact that the consistent theory also includes the electromagnetic interactions. A forced but nevertheless exuberant marriage between both interactions was achieved. The electro-weak theory is described in terms of four particles (three W and a single B particle) that mediate the forces, three of which have to do with the weak force (usually denoted as W+, W- and Z particles), while another combination of them is to be identified with electromagnetism (the photon, denoted as γ).

The defining expression looks horribly complicated, and yet it is not so hard to recognize the same generic structure as in other examples discussed before. The total Lagrangian Lt-w is composed of four parts. The first part Lg describes the particles carrying the forces and the interaction between them (completely analogous to the F squared term for the gluons in the equation for the strong force). The second part Lf contains all the Dirac-type particles and their interactions with the force-carrying particles. The label i indicates the various particle species – electrons, quark flavors, neutrinos, etc.

There is an important subtlety, which has to do with the fact that the weak interactions work differently on the left-handed (L) and on the right-handed (R) spin component of the Dirac-type particles. In other words: nature breaks mirror symmetry on a very fundamental level, preferring the lefthanded variety of matter. This means that if a certain process may take place, the mirror image of that process may turn out to be impossible.

In the table of the Standard Model in the side bar, we see quarks (in three colors) and leptons, six types of each. The six quark types up, down, charm, strange, top and bottom are the ‘flavors’ referred to before.