Normally, causal influence is assumed to go only one way—from cause to effect—and never back from the effect to the cause—the ringing of a bell does not cause the pressing of the button that triggered it. Now, researchers from the University of Oxford and the Université libre de Bruxelles have developed a theory of causality in quantum theory, according to which cause-effect relations can sometimes form cycles. This theory offers a novel understanding of exotic processes in which events do not have a definite causal order. The study has been published in Nature Communications.