Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature

by Steven Weinberg

The electroweak theory is the part of the standard model that deals with weak and electromagnetic forces. It is based on an exact symmetry principle, which says that the laws of nature take the same form if everywhere in the equations of the theory we replace the fields of the electron and neutrino with mixed fields—for instance, one field that is 30% electron and 70% neutrino and another field that is 70% electron and 30% neutrino—and at the same time similarly mix up the fields of other families of particles, such as the up quark and the down quark. This symmetry principle is called local, meaning that the laws of nature are supposed to be unchanged even if these mixtures vary from moment to moment or from one location to another. There is one other family of fields, whose existence is dictated by this symmetry principle in something like the way that the existence of the gravitational field is dictated by the symmetry among different coordinate systems. This family consists of the fields of the photon and the W and Z particles, and these fields too must be mixed up with each other when we mix up the electron and neutrino fields and the quark fields. The exchange of photons is responsible for the electromagnetic force, while the exchange of W and Z particles produces the weak nuclear force, so this symmetry between electrons and neutrinos is also a symmetry between the electromagnetic force and the weak nuclear force.