Definitions
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 n. The quality or state of being unitary.
Etymologies
from Wiktionary, Creative Commons Attribution/ShareAlike License
Examples

Since unitarity is fundamental to field theory, this result had a certain importance.

Until now, unitarity triangle measurements have been maddeningly consistent with the amount of CP violation predicted by the standard model alone.
A NonTechnical Explanation of Flavo(u)r Physics « Imaginary Potential

So, why should a quantum theory of gravity retain unitarity?

Any such theory that retains unitarity would require, for example, that each of the patches into which a de Sitter region evolves be different in some way from the original patch, such that they individually have fewer degrees of freedom.

Since there are more such regions, it would appear that the total number of degrees of freedom has increased, in violation of unitarity.

When the system S+O is considered from the point of view of O², the measurement can be seen as an interaction whose dynamics is fully unitary, whereas by the point of view of O the measurement breaks the unitarity of the evolution of S.

And then the problem is that either the entropy is decreasing during that collapse, for no good reason and in contradiction with everything we think we know about gravitational dynamics, or it is increasing during the collapse, yet supposedly gives rise to an extraordinarily lowentropy condition on the other side, for no good reason and in contradiction with everything we think we know about unitarity and thermodynamics.

What will be tested are general properties of unitarity, analyticity, and lorentz invarianceproperties that are not unique to string theory.

There are many known examples: Lagrangian theory, thermal properties, foundations of quantum formalism, unitarity, all the crackpotism about Landscape and CC, epiroktic scenarios, etc.

The unitarity theorem would break down, thus signaling Quantum Mechanics needs to be overhauled!
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