Now that the Standard Model has been completed with the Higgs discovery, I have had several requests to explain the Standard Model of physics in laymen's terms, for those who are interested in the current state of particle physics. I have written about this in the past, but since some people are still asking I will re-post it here.

The Standard Model of physics describes all of the known particles and forces, but we also know that it is incomplete as 95% of the energy in the Universe is still unexplained. It was developed around forty years ago, although it took a decade or two to refine it into the modern form (and in truth it still isn't complete).

The first division of the Standard Model is into gauge bosons and fermions. The gauge bosons are the particles that create the fundamental forces of nature (although at present only three of the four forces can be explained in this way). The details of the gauge mechanism are too complicated for this entry, and may be added as a separate article later. The general idea is that if particles of matter are required to satisfy certain symmetry laws, then the theory will automatically include a force.  

By requiring individual particles to obey a gauge symmetry (which quantum mechanics predicts) then the model will automatically include electric and magnetic effects. Furthermore, if pairs of particles can be swapped without changing the laws of physics then the same gauge mechanism predicts the existence of the weak nuclear force which is responsible for the decay of some nuclei. And finally, if there are triplets of particles that can be interchanged then the gauge mechanism predicts the existence of strong nuclear forces that bind nuclei together. (As far as I know, no one has found a physical force that corresponds to quartets or quintets of particles, but perhaps that will happen at higher energies).

And so with just those three symmetry rules, the Standard Model predicts the existence of three of the four fundamental forces of nature.

Next time, on to the fermions!