Exotic processes with cycles of consequences

Causal quantum loops. Credit: @ULB

Causal reasoning is ubiquitous – from physics to medicine, economics and the social sciences, as well as in everyday life. Every time we press the button, the bell rings and we think that pressing the button causes the bell to ring. Normally, causal impact is assumed to go in only one direction – from cause to effect – and never reversed from effect to cause: the ringing of the bell does not cause the push of the button that caused it. Now researchers from Oxford University and the Université libre de Bruxelles have developed a theory of causality in quantum theory, according to which cause-effect relationships can sometimes form cycles. This theory offers a new understanding of exotic processes in which events do not have a definite causal order. The study is published in Nature Communications.

One of the ways in which quantum theory challenges classical intuition is by challenging our ideas of causality. Quantum intertwining can be used to produce correlations between distant experiments that are known to avoid satisfactory causal explanations within the framework of classical causal models. Moreover, a unification of quantum theory and gravity is expected to allow situations in which the causal structure of space-time is subject to quantum uncertainty, suggesting that events should not be ranked causally at all. Recently, a team of researchers from Oxford and Brussels have developed a theory of causality in quantum theory, in which causal concepts are defined in fundamentally quantum terms rather than belonging to a prominent classical level of measurement results. This has provided, in particular, a causal understanding of the correlations produced by confused states. Now, they have generalized the theory to allow causal influence to go in cycles, providing a causal understanding of events with events in indefinite cause order.

“The main idea behind our proposal is that causal relationships in quantum theory correspond to influence through so-called unitary transformations – these are the types of transformations that describe the evolution of isolated quantum systems. fundamental and places causal relationships in functional dependence between variables, ”says Jonathan Barrett of Oxford University.

The main idea of ​​the new study is to apply the same principle to processes in which the sequence of operations can be dynamic or even indeterminate, seeing that a large class of these processes can be understood to derive from unitary transformations, too, not just those unfolding in an ordinary sequence.

“Previously, processes with indefinite causal order were usually considered simply incompatible with any causal account. Our work shows that a major class of them – those that can be understood to come from unitary processes and that are believed to be those that can have a physical realization in nature – can in fact be seen to have a certain causal structure, though one that involves cycles, ”says Robin Lorenz, a lead author of the study.

“The idea of ​​cyclical causal structures may seem counterintuitive, but the framework of the quantum process within which it is formulated guarantees that it is without logical paradoxes, such as the possibility of going back in time and killing your newest self,” explains Ognyan Oreshkov from Free University of Brussels. “Exotic as they appear, some of these scenarios are known to have experimental realizations in which the variables of interest are delocalized in time.”

Does this mean that space-time does not have the acyclic causal structure it is normally supposed to have? Not exactly, because in the mentioned experiments the events that are causally related in a cyclical way are not local in space-time. However, researchers believe that the causal structure of time space itself can become cyclical in this quantum way at the intersection of quantum theory and general relativity, where processes analogous to those feasible in the laboratory are expected, but with events that are local in their relevance. time space reference frames.

Reference: “Causal cyclic causal models” by Jonathan Barrett, Robin Lorenz and Ognyan Oreshkov, February 9, 2021, Nature Communications.
DOI: 10.1038 / s41467-020-20456-x

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