Causality Constraints on Gravitational Effective Field Theories
  We consider the effective field theory of gravity around black holes, and
show that the coefficients of the dimension-8 operators are tightly constrained
by causality considerations. Those constraints are consistent with -- but
tighter than -- previously derived causality and positivity bounds and imply
that the effects of one of the dimension-8 operators by itself cannot be
observable while remaining consistent with causality. We then establish in
which regime one can expect the generic dimension-8 and lower order operators
to be potentially observable while preserving causality, providing a
theoretical prior for future observations. We highlight the importance of
"infrared causality" and show that the requirement of "asymptotic causality" or
net (sub)luminality would fail to properly diagnose violations of causality.
