Time and Quantum Clocks: a review of recent developments

TL;DR

This review discusses recent developments in understanding time within quantum physics, exploring proposals like quantum and intrinsic measurement of time, and their implications for unifying quantum mechanics with relativity.

Contribution

It synthesizes recent advances and clarifies the conceptual foundations for treating time as a quantum observable, highlighting new approaches to quantum gravity.

Findings

Revived and refined the Page-Wootters proposal for quantum time

Discussed the role of covariant loop quantum gravity in the context of time

Identified conceptual pathways toward unifying quantum physics and relativity

Abstract

In this review we present the problem of time in quantum physics, including a short history of the problem and the known objections about considering time a quantum observable. The need to deal with time as an observable is elaborated through some unresolved problems. The lack of a consistent theory of time is currently hindering the formulation of a full-fledged theory of quantum gravity. It is argued that the proposal set forth by several authors of considering an intrinsic measurement of quantum time, besides having the conventional external time, is compelling. Recently several suggestions have been put forward to revive the proposal of Page and Wootters (1983), elaborating and resolving some of the main ambiguities of the original proposal and opening new scope for understanding its content. The approach followed in these new contributions exposes the need to go beyond the limitations enforced by the conventional approach of quantum physics. The attitude of covariant loop quantum gravity, in which it is called to completely ignore time, is also discussed. This review could be a step forward in an endeavour to reform our outlook of the unification of the theory of relativity and quantum physics by furnishing the conceptual ground needed for this goal. Intentionally, some technical details are avoided since we aim to present the approaches to resolve the problem in a simple way with the clearest possible outlook. These can be looked up in the original references provided.