We show that causality constrains the sign of quartic Riemann corrections to the Einstein-Hilbert action. Our constraint constitutes a restriction on candidate theories of quantum gravity.
@article{arxiv.hep-th/0612015,
title = {Causality Constrains Higher Curvature Corrections to Gravity},
author = {A. Gruzinov and M. Kleban},
journal= {arXiv preprint arXiv:hep-th/0612015},
year = {2010}
}