  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.
