Unitarity

The unitarity principle is a fundamental concept in quantum mechanics that states that the time evolution of a quantum state according to the Schrödinger equation is mathematically represented by a unitary operator. This means that the probability of all possible outcomes of a quantum interaction must sum to 100%. The principle of unitarity is a basic postulate of quantum mechanics, while generalizations of or departures from unitarity are part of speculations about theories that may go beyond quantum mechanics. The unitarity principle is a general feature of time evolution under the Schrödinger equation, and it is typically taken as an axiom in quantum mechanics.