[Paper Review] Semi-quantum Gravity and Testing Gravitational Bell Non-locality

Adrian Kent|arXiv (Cornell University)|Aug 18, 2018

Quantum Mechanics and Applications25 references4 citations

TL;DR

This paper explores semi-quantum gravity models in which gravity remains classical while matter evolves unitarily under quantum mechanics, proposing that psychophysical parallelism—linking consciousness to physical states—may apply to the classical gravitational field rather than quantum matter. It argues such models predict non-Newtonian gravitational anomalies testable via low-energy experiments measuring gravitational Bell non-locality.

ABSTRACT

Semi-classical gravity attempts to define a hybrid theory in which a classical gravitational field is coupled to a unitarily evolving quantum state. Although semi-classical gravity is inconsistent with observation, a viable theory of this type might be appealing, since it potentially might preserve the basic features of our two most successful theories while unifying them. It might also offer a natural solution to the quantum measurement problem. I explore the scope for such "semi-quantum" hybrid theories, and note some interesting, though daunting, constraints. Consistency with observation generally requires pyschophysical parallelism with the classical gravitational field rather than the quantum matter. Solvability suggests the gravitational field at a point should be determined by physics in its past light cone, which requires local hidden variables and predicts anomalously non-Newtonian gravitational fields. These predictions could be tested by low energy, although technologically challenging, experiments in which the Bell non-locality of the gravitational field is verified by direct measurement.

Motivation & Objective

• To investigate the viability of hybrid theories in which gravity is classical but matter evolves unitarily under quantum mechanics.
• To examine whether psychophysical parallelism could apply to the classical gravitational field rather than quantum matter, offering a potential solution to the quantum measurement problem.
• To identify testable predictions—specifically, anomalous gravitational fields due to local hidden variables—that could distinguish such theories from standard semi-classical gravity.
• To propose a feasible experimental pathway to test gravitational Bell non-locality using low-energy, technologically challenging setups.
• To assess the conceptual and empirical consistency of local hidden variable models for gravity in the context of unitary quantum evolution.

Proposed method

• Formalize a hybrid theory where the gravitational field at a spacetime point P is determined by physics in its past light cone Λ(P), requiring local hidden variables.
• Adopt the assumption that the gravitational field is not sourced by the quantum matter's expectation value but by a classical field correlated with observer experience via psychophysical parallelism.
• Use the framework of Everettian quantum mechanics to analyze how a single classical spacetime could emerge from multiple quantum branches.
• Derive predictions of non-Newtonian gravitational fields arising from local hidden variables, even in low-energy regimes.
• Propose that gravitational Bell non-locality can be tested directly by measuring correlations in gravitational fields across entangled quantum systems.
• Evaluate the consistency of such models by analyzing the mutual dependence between spacetime structure and quantum branch structure.

Experimental results

Research questions

• RQ1 Can a hybrid theory with classical gravity and unitary quantum matter be consistent with observation if psychophysical parallelism applies to the gravitational field rather than quantum matter?
• RQ2 What are the testable predictions of semi-quantum gravity models that differ from standard semi-classical gravity?
• RQ3 Can local hidden variables in the gravitational field lead to measurable deviations from Newtonian gravity in low-energy regimes?
• RQ4 Is it possible to experimentally verify gravitational Bell non-locality through direct measurement of gravitational correlations?
• RQ5 How do the conceptual challenges of branch structure and spacetime dependence affect the consistency of hybrid gravity models?

Key findings

• Semi-quantum gravity models that postulate psychophysical parallelism with the classical gravitational field rather than quantum matter can be consistent with observation, offering a potential resolution to the quantum measurement problem.
• Such models predict anomalously non-Newtonian gravitational fields due to local hidden variables, even in regimes where gravity is otherwise Newtonian.
• The gravitational field at a point P is determined by physics in its past light cone, implying a causal, local structure that requires hidden variables and breaks standard semi-classical expectations.
• These models predict measurable deviations from standard gravity that could be tested via low-energy experiments probing gravitational Bell non-locality.
• Although no specific local hidden variable model for gravity has been identified, the framework is empirically testable and could be falsified by future experiments.
• The paper argues that such hybrid theories, while conceptually inelegant, are logically consistent and may offer a viable path to unifying quantum mechanics and gravity without requiring quantized gravity.