The equivalence principle and inertial-gravitational quantum backreaction

This work is divided into two parts. The first examines recent proposals for “witnessing” quantum gravity via entanglement from the point of view of Bronstein’s original objection to a quantization of gravity. Using techniques from open quantum systems, we sketch how unavoidable inertial and gravitational backreaction between probe and detector could affect the experimental detection of the quantization of gravity. We argue that this is actually an inherent feature of any quantum description that attempts to incorporate the equivalence principle exactly within quantum dynamics. In the second part, we speculate on how an exact realization of the equivalence principle might be implemented in an effective quantum field theory via the general covariance of correlators. While we are far from giving an explicit construction of such a theory, we point out some features and consequences of such a program.