Identification is pointless: Localisation of systems and events in superpositions of spacetimes

Abstract: A first step towards scenarios at the interface of quantum theory and gravity is to consider a quantum superposition of semiclassical spacetimes, such as the one, arguably, sourced by a gravitating object in superposition. But what does it mean that a gravitational source is in a superposition of “different” locations? How can one compare points across the different manifolds? Due to the diffeomorphism invariance of general relativity, there is a priori no notion of the “same” or “different” points across the branches. In fact, there are multiple ways of relating points or events on the different spacetimes. Here, we make this concrete by using coincidences of four scalar fields to construct a comparison map between all spacetimes in superposition, which allows us to determine whether a system or an event is located at the “same” or “different” points across the branches. Different choices of scalar fields can be understood as different instantiations of quantum reference frames (QRFs), connecting the construction to recent research in the field of quantum foundations. As an explicit application of this formalism, 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. The latter calls into question the metaphysical meaning of not just spacetime points but also the identification between spacetime points across manifolds in superposition. Finally, we discuss the implications of QRF changes for indefinite causal order, the locality of interaction, and potential empirical consequences in interference experiments.

general relativity and quantum cosmologyhistory and philosophy of physicsquantum physics

Audience: advanced learners

( paper | video )

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Tricontinental Quantum Fundamentals Seminar

Series comments: The Tricontinental Quantum Fundamentals Seminar (3C-QFS) is an online seminar series connecting researchers from Australia, Europe, and America. Our goal is to facilitate discussion between the three continents and bring together those working on foundational topics in relativistic quantum physics, covering but not limited to:

• Measurement, localisation, and causality in relativistic quantum theory
• Energy conditions and quantum inequalities
• Non-locality and observer-dependence of entanglement
• Quantum reference frames and symmetries in quantum theory
• Holography, AdS/CFT, and analogue gravity

For 3C-QFS, we wish to foster an inclusive community and promote open discussion by encouraging a casual atmosphere during the seminars.

The seminars are held online via Zoom, consisting of 40-60 minute talks followed by additional time for questions. We meet every three weeks on Tuesday, alternating between two times in order to connect research communities across two of the three continents.

For those unable to join the seminars live, talks are recorded and hosted on the 3C-QFS YouTube channel.

The seminar series is primarily for students and academics active in the foundations of physics. To sign up for the mailing list and join the seminars live, please complete the registration form on the 3C-QFS website.

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