The quantum physics of chronology protection

TL;DR

This paper reviews the current understanding of Stephen Hawking's conjecture that fundamental physics prevents the formation of time machines, discussing examples of spacetimes with closed causal curves and the community's reactions.

Contribution

It provides a survey of the theoretical and conceptual status of chronology protection, highlighting the mechanisms and debates surrounding the prevention of closed timelike curves.

Findings

Examples of spacetimes with closed causal curves are examined.

Theoretical arguments for and against chronology protection are discussed.

The community's reactions to potential time machine scenarios are summarized.

Abstract

This is a brief survey of the current status of Stephen Hawking's ``chronology protection conjecture''. That is: ``Why does nature abhor a time machine?'' I'll discuss a few examples of spacetimes containing ``time machines'' (closed causal curves), the sorts of peculiarities that arise, and the reactions of the physics community. While pointing out other possibilities, this article concentrates on the possibility of ``chronology protection''. As Stephen puts it: ``It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.''