To be or not to be, but where?
Guilherme Franzmann
TL;DR
The paper argues that fixed, spacetime-local identifications of quantum subsystems are incompatible with gravity, since gauge-invariant local observables cannot be defined in linearized quantum gravity. It proposes a dynamical, context-dependent tensor-product structure (TPS) to identify quantum subsystems, linking emergence, ontology, and the measurement problem within a quantum-gravity framework. By combining emergence theory, algebraic QFT insights, and gravity-induced nonlocality, the work outlines a path toward a single-world unitary quantum mechanics and explores implications for early-universe cosmology and gravitationally induced entanglement experiments. The framework aims to reconcile classical spacetime emergence with quantum subsystems, offering a conceptual and mathematical program to address foundational questions in quantum gravity and measurement.
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.