Three little paradoxes: making sense of semiclassical gravity

TL;DR

This paper reviews the debate on semiclassical gravity, showing that experimental and paradoxical arguments do not exclude it, but highlight the need for an objective wave function collapse mechanism for consistency.

Contribution

It classifies semiclassical models based on wave function collapse mechanisms and clarifies that current paradoxes do not rule out semiclassical gravity, emphasizing the importance of collapse mechanisms.

Findings

Page and Geilker experiment does not exclude semiclassical gravity

Certain paradoxes do not constrain semiclassical coupling

Faster-than-light signaling is avoided with proper collapse mechanisms

Abstract

I review the arguments most often raised against a fundamental coupling of classical spacetime to quantum matter. I show that an experiment by Page and Geilker does not exclude such a semiclassical theory but mandates an inclusion of an objective mechanism for wave function collapse. In this regard, I present a classification of semiclassical models defined by the way in which the wave function collapse is introduced. Two related types of paradoxes that have been discussed in the context of the necessity to quantize the gravitational field can be shown to not constrain the possibility of a semiclassical coupling. A third paradox, the possibility to signal faster than light via semiclassical gravity, is demonstrably avoided if certain conditions are met by the associated wave function collapse mechanism. In conclusion, all currently discussed models of semiclassical gravity can be made consistent with observation. Their internal theoretical consistency remains an open question.