Are Causality Violations Undesirable?

TL;DR

This paper challenges the common view that causality violations are inherently undesirable, presenting reasons why such violations can be consistent with physical theories and may have meaningful interpretations.

Contribution

It offers a nuanced perspective on causality violations, arguing they can be compatible with physical models and may serve as useful features rather than flaws.

Findings

Causality violations can be singularity free and physically consistent.

Nontrivial topology in causality-violating spacetimes can model particles.

Causal censorship through event horizons can preserve causal well-behaved regions.

Abstract

Causality violations are typically seen as unrealistic and undesirable features of a physical model. The following points out three reasons why causality violations, which Bonnor and Steadman identified even in solutions to the Einstein equation referring to ordinary laboratory situations, are not necessarily undesirable. First, a space-time in which every causal curve can be extended into a closed causal curve is singularity free--a necessary property of a globally applicable physical theory. Second, a causality-violating space-time exhibits a nontrivial topology--no closed timelike curve (CTC) can be homotopic among CTCs to a point, or that point would not be causally well behaved--and nontrivial topology has been explored as a model of particles. Finally, if every causal curve in a given space-time passes through an event horizon, a property which can be called "causal censorship", then that space-time with event horizons excised would still be causally well behaved.