A Conservative Theory of Semiclassical Gravity

We argue that semiclassical gravity can be made consistent by assuming that quantum systems source or are classically affected by a gravitational field only when they undergo certain non-gravitational interactions that give rise to environment-induced decoherence. When systems are not affected by this decoherence-inducing process, they do not source a gravitational field, and the expectation value of their stress-energy tensor does not enter the semiclassical equations describing the gravitational field in a region. In the absence of these interactions in a region, spacetime may be flat. We argue that this can be tested by investigating the gravitational field sourced by quasi-isolated systems and the absence of gravity-mediated entanglement in the Bose-Marletto-Vedral (BMV) experiment, providing distinct predictions. We propose a possible kind of decoherence-inducing interactions that give rise to gravity, which involve chains of causally ordered non-gravitational localized interactions between quantum systems modeled via decoherence and test functions that we call Stable Determination Chains (SDCs). SDCs obey conditions that aim to address the measurement problem and allow for a conservative theory of gravity. It is conservative because it does not need to modify the fundamental equations of quantum theory, unlike spontaneous and gravity-induced collapse approaches to semiclassical gravity, and without invoking relationalism. Furthermore, it does not appeal to nonlocal, retrocausal, or superdeterministic hidden variables. We argue that these SDCs provide additional benefits, such as a semiclassical estimate of the value of the cosmological constant, the prediction of a time-varying cosmological constant that weakens with time, in agreement with some of the recent evidence, and a proposal about how SDCs give rise to gravity.