In Quantum Mechanics, an operator is called unitary if its hermitian adjoint is equal to its inverse, ie. U+U = UU+ = 1.
Unitary operators can be used to describe symmetries of the space of quantum states; due to their property that they preserve the hermitian inner product: that is to say, if under some transfromation |A> goes to U|A> and |B> goes to U|B>, then <B| goes to <B|U+, and then
    <B|U+U|A> = <B|A>

U"nit*a*ry (?), a.

1.

Of or pertaining to a unit or units; relating to unity; as, the unitary method in arithmetic.

2.

Of the nature of a unit; not divided; united.

Unitary theory Chem., the modern theory that the molecules of all complete compounds are units, whose parts are bound together in definite structure, with mutual and reciprocal influence on each other, and are not mere aggregations of more or less complex groups; -- distinguished from the dualistic theory.

 

© Webster 1913.

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