Anne-Catherine de la Hamette, "Quantum reference frames and the localisation of events in superpositions of spacetimes"

Wednesday Dec 11, 2024, 4:00 PM → 5:00 PM Europe/Rome

0M04

The study of quantum reference frames (QRFs) is motivated by the idea of taking into account the quantum properties of the reference frames that we use, explicitly or implicitly, in our description of physical systems. Like a classical reference frame, a QRF can be used to define physical quantities such as time, position, momentum, and spin relationally. Unlike its classical analogue, it relativises the notions of superposition and entanglement of quantum systems.

In this talk, I will introduce the key principles of QRFs and illustrate their relevance at the interface of quantum theory and gravity. A crucial step in this context is to consider quantum superpositions of semiclassical spacetimes, as might be sourced by a gravitating object in superposition. But what does it mean for a gravitational source to be in a superposition of different locations? Due to the diffeomorphism invariance of general relativity, there is no natural notion of “same” or “different” points across branches. We address this by constructing a comparison map using coincidences of four scalar fields, allowing us to determine whether events occur at the same or different points across the spacetimes. Different choices of scalar fields can be understood as different instantiations of QRFs.  As an explicit application, we explore how the localisation of events is relative to the choice of QRF and formulate a quantum generalisation of Einstein’s famous hole argument, calling into question the metaphysical significance of spacetime points and their identification.