Quantum Theory Needs (And Probably Has) Real Reduction

TL;DR

This paper argues that standard quantum theory may be inconsistent without genuine physical non-unitarity, supporting the need for objective collapse models to ensure well-defined single outcomes in a realist interpretation.

Contribution

It demonstrates that standard quantum theory can produce empirically consequential inconsistencies, advocating for the consideration of objective collapse models with real non-unitarity.

Findings

Standard quantum theory can lead to empirically consequential inconsistencies.

Genuine physical non-unitarity is necessary for a realist interpretation.

Objective collapse models provide a plausible solution to quantum theory's issues.

Abstract

The traditional, standard approach to quantum theory is to assume that the theory ``really'' contains only unitary physical dynamics--i.e., that the only physically quantifiable evolution is that given by the time-dependent Schrodinger equation. This leads to two distinct classes of interpretations for the standard theory in its orthodox form: (i) an Everettian-type approach assuming that all mutually exclusive outcomes occur in different ``branches'' of the universe; or (ii) single-outcome approaches that assume a ``projection postulate'' (PP) with no accompanying physical account within quantum theory. A contrasting, unorthodox approach is to suggest forms of quantum theory that involve physical non-unitarity; these are called ``objective collapse models.'' Among these are Penrose's theory of gravitation-induced collapse and the Transactional Interpretation. The primary focus of this paper is an example demonstrating that standard quantum theory (with or without the projection postulate) can in-principle yield empirically consequential inconsistencies. Thus, it appears that for quantum theory to be viable in a realist sense (as opposed to being an instrumentalist protocol in which inconsistencies are evaded by changing the protocol), it must possess genuine, physical non-unitarity yielding well-defined single outcomes. This leads to the conclusion that objective collapse models should be more seriously considered.