# Definitions for **unitarity**

### This page provides all possible meanings and translations of the word **unitarity**

### Wiktionary

unitarity(Noun)

The quality or state of being unitary.

### Freebase

Unitarity

In quantum physics, unitarity is a restriction on the allowed evolution of quantum systems that ensures the sum of probabilities of all possible outcomes of any event is always 1. More precisely, the operator which describes the progress of a physical system in time must be a unitary operator. When the Hamiltonian is time-independent the unitary operator is . Similarly, the S-matrix that describes how the physical system changes in a scattering process must be a unitary operator as well; this implies the optical theorem. In quantum field theory one usually uses a mathematical description which includes unphysical fundamental particles, such as longitudinal photons. These particles must not appear as the end-states of a scattering process. Unitarity of the S-matrix and the optical theorem in particular implies that such unphysical particles must not appear as virtual particles in intermediate states. The mathematical machinery which is used to ensure this includes gauge symmetry and sometimes also Faddeev–Popov ghosts. Since unitarity of a theory is necessary for its consistency, the term is sometimes also used as a synonym for consistency, and is sometimes used for other necessary conditions for consistency, in particular the condition that the Hamiltonian is bounded from below. This means that there is a state of minimal energy. This is needed for the second law of thermodynamics to hold.

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"unitarity." *Definitions.net.* STANDS4 LLC, 2014. Web. 23 Apr. 2014. <http://www.definitions.net/definition/unitarity>.