Approximation of the theory by a decohered quantum-sector limit

Ascertain whether the sector of classical–quantum dynamics that effectively describes a fully quantum theory with a decohered spacetime subsystem provides a sufficiently accurate approximation to the classical–quantum gravity theory developed in this work.

Background

The classical–quantum framework can arise as an effective limit of a fully quantum theory in which one subsystem (spacetime) has decohered. If this limit well-approximates the present classical–quantum theory, it would help connect the proposal to fully quantum gravity frameworks.

The authors explicitly flag uncertainty about the quantitative adequacy of this approximation for the theory they study here.

References

"There is a sector of the parameter space of classical-quantum dynamics, in which the theory can be taken as an accurate description of a fully quantum theory in which one subsystem (here spacetime), has decohered. Whether that parameter space is a close enough approximation of the theory considered here, is an open question."

— Renormalisation of postquantum-classical gravity  (2402.17844 - Grudka et al., 2024) in Discussion

It also raises the question of whether de Sitter space exists as the classical limit of any quantum state.

— Quantum State of a Gravitating Spacetime Region  (2609.10684 - Bousso et al., 9 Sep 2026) in Section 1, paragraph “Classical-quantum correspondence”