From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture

TL;DR

This paper argues that various physical effects, including quantum and gravitational phenomena, collectively prevent the formation of time machines, supporting Hawking's Chronology Protection Conjecture.

Contribution

It demonstrates through simple models that multiple physical mechanisms act together to inhibit time travel, reinforcing the conjecture's validity.

Findings

Gravitational back reaction becomes significant before reaching the quantum gravity cutoff.

Casimir and wormhole disruption effects contribute to preventing time machine formation.

Results support Hawking's hypothesis that the universe defends against time travel.

Abstract

The recent interest in ``time machines'' has been largely fueled by the apparent ease with which such systems may be formed in general relativity, given relatively benign initial conditions such as the existence of traversable wormholes or of infinite cosmic strings. This rather disturbing state of affairs has led Hawking to formulate his Chronology Protection Conjecture, whereby the formation of ``time machines'' is forbidden. This paper will use several simple examples to argue that the universe appears to exhibit a ``defense in depth'' strategy in this regard. For appropriate parameter regimes Casimir effects, wormhole disruption effects, and gravitational back reaction effects all contribute to the fight against time travel. Particular attention is paid to the role of the quantum gravity cutoff. For the class of model problems considered it is shown that the gravitational back reaction becomes large before the Planck scale quantum gravity cutoff is reached, thus supporting Hawking's conjecture.