What an event is not: unravelling the identity of events in quantum theory and gravity

TL;DR

This paper investigates the concept of events in quantum physics and gravity, proposing an operational, context-dependent definition that considers non-classical phenomena and their implications for causal order and quantum spacetime.

Contribution

It introduces a novel, operational approach to defining events in quantum spacetime, emphasizing their context-dependent properties without disturbing observed phenomena.

Findings

Events are context-dependent and can be accessed without disturbance.

Implications for indefinite causal order and Wigner's friend scenario.

First step towards a quantum spacetime event framework.

Abstract

We explore the notion of events at the intersection between quantum physics and gravity, inspired by recent research on superpositions of semiclassical spacetimes. By going through various experiments and thought experiments -- from a decaying atom, to the double-slit experiment, to the quantum switch -- we analyse which properties can and cannot be used to define events in such non-classical contexts. Our findings suggest an operational, context-dependent definition of events which emphasises that their properties can be accessed without destroying or altering observed phenomena. We discuss the implications of this understanding of events for indefinite causal order as well as the non-absoluteness of events in the Wigner's friend thought experiment. These findings provide a first step for developing a notion of event in quantum spacetime.