Simple Refutation of the Eppley-Hannah argument

TL;DR

This paper presents a straightforward critique of the Eppley-Hannah argument, showing that coupling quantum matter with classical gravity via local quantum states avoids their claimed contradictions, challenging the necessity of quantizing gravity.

Contribution

It offers a simple refutation of the Eppley-Hannah argument by proposing a hybrid theory where gravity interacts locally with quantum states, avoiding inconsistencies.

Findings

Coupling gravity to local quantum states avoids Eppley-Hannah contradictions

Critique challenges the necessity of gravity quantization based on their thought experiment

Hybrid theories with local interactions are plausible and consistent with relativity

Abstract

In an influential paper, Eppley and Hannah argued that gravity must necessarily be quantized, by proposing a thought experiment involving classical gravitational waves interacting with quantum matter. They argue the interaction must either violate the uncertainty principle or allow superluminal signalling. The feasibility of implementing their experiment in our universe has been challenged by Mattingly, and other limitations of the argument have been noted by Huggett and Callender and by Albers et al.. However, these critiques do not directly refute the claim that coupling quantum theories with a Copenhagen collapse postulate to unentanglable classical gravitational degrees of freedom leads to contradiction. I note here that if the gravitational field interacts with matter via the local quantum state, the Eppley-Hannah argument evidently fails. This seems a plausibly natural feature of a hybrid theory, whereas the alternative considered by Eppley-Hannah is evidently inconsistent with relativity.