To be or not to be, but where?

TL;DR

This paper discusses the conceptual challenges in defining quantum subsystems in quantum gravity, proposing a dynamic identification between classical and quantum systems to better understand the emergence of spacetime and address foundational issues.

Contribution

It introduces a novel perspective that the classical-quantum identification should be dynamically evolving, facilitating a unified approach to quantum gravity and the measurement problem.

Findings

Traditional local algebras are disrupted in covariant linearized quantum gravity.

A dynamic identification approach may resolve issues with gauge invariance and statistical independence.

This perspective could unify quantum mechanics and gravity in a single-world framework.

Abstract

The identification of physical subsystems in quantum mechanics as compared to classical mechanics poses significant conceptual challenges, especially in the context of quantum gravity. Traditional approaches associate quantum systems with classical ones localized in spacetime, using either Hilbert space factors for finite-dimensional systems or local operator algebras in algebraic quantum field theory. These methods ensure statistical independence for state preparations and measurements. However, covariant linearized quantum gravity disrupts this framework by preventing the formation of gauge-invariant local algebras, thereby undermining statistical independence. This presents a major obstacle for modeling early universe cosmology and gravity-induced entanglement experiments, and poses a significant roadblock toward a comprehensive theory of quantum gravity. A pivotal shift is proposed: the identification between classical and quantum systems should be dynamically evolving rather than fixed, opening the possibility of a single-world unitary quantum mechanics. This perspective aligns with the broader aim of understanding how classical spatiotemporal existence emerges from quantum mechanics and connects the measurement problem with quantum gravity.