Unitary time evolution in quantum mechanics is a stronger physical postulate than linear time evolution

TL;DR

This paper examines whether unitarity in quantum mechanics is a necessary consequence of linearity and other axioms, concluding that unitarity is an independent, testable axiom rather than a logical consequence.

Contribution

It demonstrates that unitarity of time evolution can be independent of linearity within different formulations of quantum mechanics, revealing it as a potentially falsifiable axiom.

Findings

Unitarity follows from axioms in one formulation but not in another.

Allowing arbitrary linearity leads to a distinct physical theory with new phenomena.

Unitarity should be considered an independent axiom, not a tautology.

Abstract

Discussions of quantum mechanics often loosely claim that time evolution logically must be unitary, in order for the probabilistic interpretation of the amplitudes of the state vector to make sense at all times. We discuss from first principles whether this claim is true: if we assume only that the time-evolution operator is *linear*, then does the stronger requirement that it be *unitary* follow from the other axioms of quantum mechanics? The answer is subtle. We discuss two mathematically distinct but physically equivalent formulations of the axioms of quantum mechanics, and consider generalizing each to postulate only that time evolution is linear. Within one formulation, the unitarity of time evolution follows logically from the other axioms -- but within the other formulation, it does not. Allowing the time-evolution operator be (a priori) arbitrarily linear does not change the physical observables in one formulation of quantum mechanics, but changes the other formulation to a *distinct* (internally consistent) physical theory that allows new phenomonology like (e.g.) faster-than-light communication. Therefore, the unitarity of time evolution is arguably better thought of as a logically independent and experimentally falsifiable axiom of quantum mechanics, not as a tautological consequence of the other axioms.