Causality in quantum field theory is defined by the vanishing of field commutators for spacelike separations. However, this does not imply a direction for causal effects. Hidden in our conventions for quantization is a connection to the definition of an arrow of causality, i.e., what is the past and what is the future. If we mix quantization conventions within the same theory, we get a violation of microcausality. In such a theory with mixed conventions, the dominant definition of the arrow of causality is determined by the stable states. In some quantum gravity theories, such as quadratic gravity and possibly asymptotic safety, such a mixed causality condition occurs. We discuss some of the implications.

• Received 22 August 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.171601

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Published by the American Physical Society