Causality and gravity

TL;DR

This paper discusses how quantum gravity introduces fundamental uncertainties in causal structure, such as ill-defined lightcones, which grow with energy and impact the understanding of causality in high-energy regimes.

Contribution

It demonstrates the inevitability of causal uncertainty in quantum gravity and analyzes this effect within effective field theory and quadratic gravity frameworks.

Findings

Causal structure uncertainty is unavoidable in quantum gravity.

Lightcones become ill-defined at high energies.

Uncertainty grows with energy and affects UV completions.

Abstract

We show how uncertainty in the causal structure of field theory is essentially inevitable when one includes quantum gravity. This includes the fact that lightcones are ill-defined in such a theory. This effect is small in the effective field theory regime, where it it independent of the UV completion of the theory, but grows with energy and represents an unknown uncertainty for a generic UV completion. We include details of the causality uncertainty which arises in a particular UV completion, i.e. quadratic gravity. We describe how the mechanisms uncovered in the effective field theory treatment, and some of those in quadratic gravity, could be common features of quantum gravity.