An Etymological Dictionary of Astronomy and Astrophysics
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A physical phenomenon whereby a symmetric system becomes permanently asymmetric. A simple example is a ball lying on top of a hill in equilibrium. The hill-ball system is symmetric about the vertical axis through the top of the hill. Moreover, there is no preferred horizontal direction to the system. However, its state is unstable, since the slightest perturbing force will cause the ball to roll down the hill in some particular direction. The system becomes permanently asymmetric because the ball will not roll uphill by itself. Symmetry breaking is found in several fields of physics, for example in → magnetism (→ ferromagnetism), → thermodynamics (→ crystallization), and → particle physics, where it constitutes the basis of → electroweak interactions. In cosmology, according to the → Big Bang model, the fundamental forces of the Universe split off from one another in a form of spontaneous symmetry braking. If a single, unified force existed with a certain symmetry just after the Big Bang, if that symmetry were somehow broken so that the unified force were fractured, then the result might be several fundamental forces. See also → grand unified theory, → theory of everything, → phase transition.

→ spontaneous; → symmetry; → break.