What we don't know about time

TL;DR

This paper discusses the profound gaps in our understanding of time in physics, contrasting it with advancements in space and highlighting open questions and future research directions.

Contribution

It provides a critical review of the current state of knowledge about time and outlines key open questions for future exploration in theoretical physics.

Findings

Identifies unresolved questions about the nature of time.

Highlights the limited progress in understanding time compared to space.

Suggests directions for future research on the concept of time.

Abstract

String theory has transformed our understanding of geometry, topology and spacetime. Thus, for this special issue of Foundations of Physics commemorating "Forty Years of String Theory", it seems appropriate to step back and ask what we do not understand. As I will discuss, time remains the least understood concept in physical theory. While we have made significant progress in understanding space, our understanding of time has not progressed much beyond the level of a century ago when Einstein introduced the idea of space-time as a combined entity. Thus, I will raise a series of open questions about time, and will review some of the progress that has been made as a roadmap for the future.