Does gravity have to be quantized? Lessons from non-relativistic toy models

Abstract

It is often argued that gravity has to be a quantum theory simply because a fundamentally semiclassical approach would necessarily be inconsistent. Here I review recent Newtonian toy models of (stochastic) semiclassical gravity. They provide one option to implement a force semiclassically without getting into the known problems associated with mean-field. These models are not complete theories and should not be considered too seriously, but their consistency shows that semiclassical gravity is hard to dismiss on purely theoretical grounds.

arXiv comment16 pages -- written for the proceedings of the DICE 2018 workshop in Castiglioncello -- provides a more detailed account of the technical arguments in arXiv:1802.03291

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• Hybrid Geometrodynamics: A Hamiltonian description of classical gravity coupled to quantum matter
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• Tabletop Experiments for Quantum Gravity Are Also Tests of the Interpretation of Quantum Mechanics
• Correlations and signaling in the Schrödinger-Newton model
• Mixture equivalence principles and post-quantum theories of gravity
• Measurement-induced entanglement entropy of gravitational wave detections
• The string theory swampland in the Euclid, SKA and Vera Rubin observatory era
• Newton's laws of motion generating gravity-mediated entanglement
• Hybrid quantum-classical systems: statistics, entropy, microcanonical ensemble and its connection to the canonical ensemble
• Probing the nonclassical dynamics of a quantum particle in a gravitational field
• Can gravity mediate the transmission of quantum information?
• Existing experiments suffice to indirectly verify the quantum essence of gravity