Unitarity means that if a future state, F, of a system is unique, the corresponding past point, P, is also unique, provided that there is no information lost on the transition from P to F.
— Me@2019-05-22 11:06:48 PM
In quantum physics, unitarity means that the future point is unique, and the past point is unique. If no information gets lost on the transition from one configuration to another[,] it is unique. If a law exists on how to go forward, one can find a reverse law to it. It is a restriction on the allowed evolution of quantum systems that ensures the sum of probabilities of all possible outcomes of any event always equals 1.
Since unitarity of a theory is necessary for its consistency (it is a very natural assumption, although recently questioned), the term is sometimes also used as a synonym for consistency, and is sometimes used for other necessary conditions for consistency, especially the condition that the Hamiltonian is bounded from below. This means that there is a state of minimal energy (called the ground state or vacuum state). This is needed for the third law of thermodynamics to hold.
— Wikipedia on Unitarity (physics)
2019.05.23 Thursday (c) All rights reserved by ACHK