To be or not to be, but where?

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, canonical linearized quantum gravity disrupts this framework by preventing the formation of gauge-invariant local algebras, thereby undermining the statistical independence required in measurements. This presents a major obstacle for modeling early universe cosmology, gravity-induced-entanglement experiments, and poses a significant roadblock toward a comprehensive theory of quantum gravity. A pivotal shift is proposed: the identification of classical and quantum systems should be dynamically evolving rather than static, 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.