Conservation rules, symmetries, and the Standard Model of particle physics

Abstract

Abstract One of the most important concepts in physics is that of the symmetry or invariance of a system under a particular operation. For example, a snowflake is invariant under a 60 rotation in the plane of the flake, and this tells us something about the physics of the molecular bonding in water. In fact, conservation rules and the associated symmetries have been called the backbone of high-energy particle physics. Not all conservation rules are absolute. While conservation of electric charge or of energy and momentum are, as far as we know, sacrosanct in all situations, some quantities—for example, conservation of parity—that is, invariance under spatial reflection—hold for certain types of fundamental interaction but not for others. Moreover, on the enormous distance and time scales of the universe at large, it turns out that some quantum numbers, such as baryon number, which seem to be exactly conserved under laboratory conditions, are violated in cosmology. This violation presumably occurred at a very early, very hot stage of the universe, involving some new type of interaction at energies far above what can be obtained by accelerators on Earth.