Abstract

We have no experimental knowledge about whether spacetime and gravity exhibit quantum phenomena or not. In this talk, I propose an “extended symmetry principle” that states that dynamical laws are invariant when changing quantum reference frames. This principle alone implies that spacetime can be placed in a quantum superposition. We study two concrete situations: a clock near a gravitational source that is in spatial superposition, and a superposition of conformally related spacetime metrics inhabited by a massive quantised Klein-Gordon field. We construct the explicit quantum frame changes and, invoking the extended symmetry principle, show how to transform to a quantum frame in which the standard theories of general relativity and quantum field theory allow the dynamics to be determined.