TPPC weekly seminar (in-person): Steffen Gielen - "Unitarity and clock dependence in quantum cosmology"

Wednesday 24 May 2023, 12:30 → 14:00 Europe/London

Strand Building, S7.06

Speaker: Steffen Gielen (Uni of Sheffield)

Title: Unitarity and clock dependence in quantum cosmology

Abstract: General relativity gives us the freedom to choose different time coordinates to describe evolution, but also says that only relational notions of evolution (such as "what is the value of A when B = b_0"?) are meaningful, and possibly related to observation. In contrast, coordinate-dependent statements are not physical. When we quantise, this leads to various basic technical and conceptual questions known under the heading of the "problem of time": depending on the viewpoint we seem to have either no dynamics at all or too many, potentially inequivalent, ways of defining time evolution in quantum theory. The standard quantum-mechanical demand of unitary time evolution then becomes ambiguous in the general-relativistic context, as it may refer to different notions of time. I will discuss some of these issues in a simple cosmological model, where three inequivalent quantum theories can be defined. Demanding unitarity in these theories leads to quite radically different "predictions" for the resulting cosmology, illustrating the problem of time.

 

Kings2023-Gielen.pdf