Gauge Field Theory Coherent States (GCS) : IV. Infinite Tensor Product and Thermodynamical Limit

TL;DR

This paper extends the Hilbert space framework for Lorentzian Quantum General Relativity to non-compact spacetimes using Infinite Tensor Products, enabling new approaches to quantum topology change and semi-classical gravity.

Contribution

It introduces the application of von Neumann's Infinite Tensor Product theory to quantum gravity, allowing for the treatment of infinite graphs and non-compact spacetimes.

Findings

Extended Hilbert space framework for non-compact spacetimes.

New techniques for studying quantum topology change.

Potential for analyzing quantum fields on fluctuating spacetimes.

Abstract

In the canonical approach to Lorentzian Quantum General Relativity in four spacetime dimensions an important step forward has been made by Ashtekar, Isham and Lewandowski some eight years ago through the introduction of an appropriate Hilbert space structure. This Hilbert space, together with its generalization due to Baez and Sawin, is appropriate for semi-classical quantum general relativity if the spacetime is spatially compact. In the spatially non-compact case, however, an extension of the Hilbert space is needed in order to approximate metrics that are macroscopically nowhere degenerate. For this purpose, in this paper we apply von Neumann's theory of the Infinite Tensor Product (ITP) of Hilbert Spaces to Quantum General Relativity. The cardinality of the number of tensor product factors can take the value of any possible Cantor aleph as is needed for our problem, where a Hilbert space is attached to each edge of an arbitrarily complicated, generally infinite graph. The new framework opens a pandora's box full of techniques, appropriate to pose fascinating physical questions such as quantum topology change, semi-classical quantum gravity, effective low energy physics etc. from the universal point of view of the ITP. In particular, the study of photons and gravitons propagating on fluctuating quantum spacetimes is now in reach, the topic of the next paper in this series.