The problem of time for non-deparametrizable models and quantum gravity

TL;DR

This paper distinguishes between two types of reparametrization invariant models, showing that non-deparametrizable models like general relativity face fundamental issues in canonical quantization, challenging quantum gravity development.

Contribution

It introduces a classification of reparametrization invariant models and analyzes how the problem of time affects their quantization, highlighting issues in non-deparametrizable models such as general relativity.

Findings

Deparametrizable models allow solving the problem of time.

Non-deparametrizable models lack a clear notion of time in their phase space.

Canonical quantization may fail for non-deparametrizable models like general relativity.

Abstract

In this article I introduce a distinction between two types of reparametrization invariant models and I argue that while both suffer from a problem of time at the time of applying canonical quantization methods to quantize them, its severity depends greatly on the type of model. Deparametrizable models are models that have time as a configuration or phase space variable and this makes it the case that the problem of time can be solved. In the case of non-deparametrizable models, we cannot find time in the configuration or phase space of the model, and hence the techniques that allow solving the problem in the deparametrizable case do not apply. This seems to signal that the canonical quantization techniques fail to give a satisfactory quantization of non-deparametrizable models. As I argue that general relativity is non-deparametrizable, this implies that the canonical quantization of this theory may fail to provide a successful theory of quantum gravity.