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.
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