Causality in relativistic quantum interactions without mediators

TL;DR

This paper compares quantum field theoretical and direct-coupling models of relativistic quantum interactions, highlighting conditions where retrocausal effects arise and implications for understanding gravity-induced entanglement.

Contribution

It introduces and analyzes a quantum-controlled model as an alternative to QFT for relativistic interactions, exploring retrocausality and implications for quantum gravity.

Findings

The qc-model can approximate QFT in certain regimes.

Retrocausal effects can occur in the qc-model.

Implications for detecting quantum aspects of gravity.

Abstract

We analyse the interaction between two quantum systems in spacetime and we compare two possible models to describe it: 1) a fully quantum field theoretical (QFT) description of the coupling of two quantum systems mediated by a quantum field and 2) a quantum-controlled model (qc-model), which is an effectively relativistic direct-coupling in which the interaction of two quantum systems is not mediated by a field with local quantum degrees of freedom. We show that while there are regimes where the qc-model can approximate QFT arbitrarily well, it can suffer from retrocausal effects. We discuss in what regimes those retrocausal predictions of the qc-model are non-negligible and whether they can be used to argue that gravity induced entanglement experiments can reveal genuinely quantum aspects of the gravitational interaction or not.