Spontaneous symmetry breaking

Spontaneous symmetry breaking is a concept of theoretical physics, the Standard Model of elementary particle physics in particular plays an important role. This is called spontaneous symmetry breaking when the ground state ( the lowest energy state ) of a physical system less symmetries than the equations of motion underlying.

Symmetry is important physical properties of a system since they may cause the performance of maintenance records or the existence of elementary particles, for example.

The concept also plays a role in solid state physics, where it has its origin. Cools such as a ferromagnet below the Curie temperature, as developed - even for arbitrarily weak internal magnetic field - one oriented in the direction of " spontaneous magnetization " what the pre-existing rotational symmetry breaking. ( By the weak magnetic field that is the direction of the spontaneous magnetization can be specified, while the amount is independent of it. )

Example

The picture illustrates a rotationally symmetric potential. As a mechanical analogue, one can imagine this potential as a surface on which a ball rolls, such as the dished bottom of a bottle or a sombrero. In this case, the potential of two spatial coordinates, X and Y depends on. In particle physics, instead of the spatial coordinates a complex field (or two real fields and ) are considered. A certain value corresponds to a field configuration which sets out to make predictions for observable variables in the experiment.

This potential has stable local minimum values ​​in a full circle around the axis of symmetry, at the "bottom " of the potential. In the mechanical analog, these are the places in which a ball can rest. In particle physics, the minimum values ​​of configurations that can take the field in the vacuum correspond; therefore it is called the " vacuum circuit ".

In addition, there is an unstable ground state exactly in the center of the potential energy surface, at the highest point of the arch. Assume now. that the ball, or the field is initially in this unstable ground state. At the slightest disturbance, the ball will roll away from that point, and finally at a point - remain on the minimum circle - braked by friction. Analogously, from the unstable state transition ( the so-called "false vacuum " ) spontaneously into a stable ground state on the vacuum circuit the field. The configuration, however, has thus lost the rotational symmetry ( " symmetry breaking " ), since the system has decided on a particular position on the circle, which is against all other documents received on the circle.

Consequences

The spontaneous breaking of a global continuous symmetries, the Goldstonetheorem result. This means that for each broken generator of the symmetry group of a massless scalar so-called Goldstone boson exists.

For local custody symmetric theories the Goldstonetheorem not true because they do not meet the conditions for its validity. Instead there makes the Higgs mechanism, the Goldstone bosons to an additional longitudinal polarization degree of freedom of the gauge fields, which would be massless particles without spontaneous symmetry breaking and as such possess only two transverse polarizations.

Nobel Prize 2008

Yoichiro Nambu was awarded for his idea of ​​spontaneous symmetry breaking of the Nobel Prize for Physics in 2008. He shares the prize with Makoto Kobayashi and Toshihide Masukawa, in whose work also symmetries or their injury played a prominent role. Specifically, it is about the violation of the universe originally existing particle-antiparticle symmetry.

Importance of the area for other branches of physics

The concept of spontaneous symmetry breaking plays in other areas of physics an important role, especially in statistical physics (eg the so-called critical behavior at phase transitions ) in solid state physics (eg in the theory of superconductivity) and in the particle (for example, in the above- mentioned Higgs mechanism ).