Symmetry breaking in physics describes a phenomenon where (infinitesimally) small fluctuations acting on a system which is crossing a critical point decide the system's fate, by determining which branch of a bifurcation is taken. To an outside observer unaware of the fluctuations (or "noise"), the choice will appear arbitrary. This process is called symmetry "breaking", because such transitions usually bring the system from a disorderly state into one of two definite states. Symmetry breaking is supposed to play a major role in pattern formation.
Symmetry breaking is distinguished into two types as:
- Explicit symmetry breaking where the laws describing a system are themselves not invariant under the symmetry in question.
- Spontaneous symmetry breaking where the laws are invariant but the system is not because the background of the system, its vacuum, is non-invariant. Such a symmetry breaking is parametrized by an order parameter. A special case of this type of symmetry breaking is dynamical symmetry breaking.
Symmetry breaking can cover any of the following scenarios:
- The breaking of an exact symmetry of the underlying laws of physics by the random formation of some structure;
- A situation in physics in which a minimal energy state has less symmetry than the system itself;
- Situations where the actual state of the system does not reflect the underlying symmetries of the dynamics because the manifestly symmetric state is unstable (stability is gained at the cost of local asymmetry);
- Situations where the equations of a theory may have certain symmetries, though their solutions may not (the symmetries are "hidden").
One of the first cases of broken symmetry discussed in the physics literature is related to the form taken by a uniformly rotating body of incompressible fluid in gravitational and hydrostatic equilibrium. Jacobi and soon later Liouville, in 1834, discussed the fact that a tri-axial ellipsoid was an equilibrium solution for this problem when the kinetic energy compared to the gravitational energy of the rotating body exceeded a certain critical value. The axial symmetry presented by the McLaurin spheroids is broken at this bifurcation point. Furthermore, above this bifurcation point, and for constant angular momentum, the solutions that minimize the kinetic energy are the non-axially symmetric Jacobi ellipsoids instead of the Maclaurin spheroids.
See also 
- Higgs mechanism
- QCD vacuum
- Goldstone boson
- 1964 PRL symmetry breaking papers
- J. J. Sakurai Prize for Theoretical Particle Physics
- Anderson, P.W. (1972). "More is Different". Science 177 (4047): 393–396. Bibcode:1972Sci...177..393A. doi:10.1126/science.177.4047.393. PMID 17796623.
- Jacobi, C.G.J. (1834). "Über die figur des gleichgewichts". Poggendorf Ann. Phys. Chim (33): 229–238.
- Liouville, J. (1834). "Sur la figure d'une masse fluide homogène, en équilibre et douée d'un mouvement de rotation". Journal de l'École Polytechnique (14): 289–296.