Quantum theory cannot consistently describe the use of itself

TL;DR

This paper presents a thought experiment demonstrating that quantum theory cannot consistently describe itself when applied to complex systems involving agents, challenging the idea of its universal applicability.

Contribution

It introduces a Gedankenexperiment showing the inherent inconsistency in applying quantum theory universally to agents using the theory themselves.

Findings

Agents' conclusions become inconsistent when modeling each other within quantum theory

Quantum theory cannot straightforwardly describe complex systems involving observers

The experiment reveals fundamental limitations in the universality of quantum mechanics

Abstract

Quantum theory provides an extremely accurate description of fundamental processes in physics. It thus seems likely that the theory is applicable beyond the, mostly microscopic, domain in which it has been tested experimentally. Here we propose a Gedankenexperiment to investigate the question whether quantum theory can, in principle, have universal validity. The idea is that, if the answer was yes, it must be possible to employ quantum theory to model complex systems that include agents who are themselves using quantum theory. Analysing the experiment under this presumption, we find that one agent, upon observing a particular measurement outcome, must conclude that another agent has predicted the opposite outcome with certainty. The agents' conclusions, although all derived within quantum theory, are thus inconsistent. This indicates that quantum theory cannot be extrapolated to complex systems, at least not in a straightforward manner.