**OBSERVERS IN QUANTUM GRAVITY**

**Markus Mueller**

**University of Vienna**

**Title**

Quantum theory and the structure of spacetime in the light of operationalism

**Abstract**

Quantum theory and the structure of spacetime in the light of operationalism
Some approaches to quantum gravity, e.g. the AdS/CFT correspondence or
Sean Carroll’s approach, promote the idea that spacetime can ultimately
be reconstructed from the structure of (some underlying) quantum theory.
In this talk, I sketch a research program that aims at studying aspects
of this idea without committing to a particular model or ansatz. Namely,
simple thought experiments of communicating observers, based only on
what we currently know about physics, can shed light on the structural
relation between quantum theory and spacetime — in both directions.
I will first present a simple thought experiment that shows how
relativity of simultaneity constrains the structure of the quantum bit,
without assuming the validity of quantum theory from the outset [1].
Then I will show that this kind of reasoning can be extended and
reversed: if we assume the validity of quantum theory and the existence
of certain types of “universal measurement procedures” (but make no
assumptions about spacetime), then observers will automatically relate
their descriptions of local laboratory physics by elements of the
Lorentz group SO(3,1) [2]. Thus, taking the question of “how observers
operate in a quantum world” seriously can help illuminate the relation
between two seemingly separate aspects of physics.

[1] A.J.P. Garner, M.P. Mueller, and O. C. O. Dahlsten, The complex and
quaternionic quantum bits from relativity of simultaneity on an
interferometer, Proc. R. Soc. A 473, 20170596 (2017); arXiv:1412.7112

[2] P.A. Hoehn and M.P. Mueller, An operational approach to spacetime
symmetries: Lorentz transformations from quantum communication, New J.
Phys. 18, 063026 (2016); arXiv:1412.8462