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Retrocausality (also called retro-causation, retro-chronal causation, backward causation, and similar terms) is any of several hypothetical phenomena or processes that reverse causality, allowing an effect to occur before its cause.
Retrocausality is primarily a thought experiment in philosophy of science based on elements of physics, addressing the question: Can the future affect the present, and can the present affect the past? Philosophical considerations of time travel often address the same issues as retrocausality, as do treatments of the subject in fiction, although the two terms are not universally synonymous.
While some discussion of retrocausality is confined to fringe science or pseudoscience, a few physical theories with mainstream legitimacy have sometimes been interpreted as leading to retrocausality. This has been problematic in physics because the distinction between cause and effect is not made at the most fundamental level within the field of physics.
Although philosophical efforts to understand causality extend back at least to Aristotle's discussions of the four causes, the idea that the arrow of time could be reversed is substantially more recent. In fact, retrocausality was long considered an inherent self-contradiction because, as 18th century philosopher David Hume discussed, when examining two related events, the cause, by definition, is the one that precedes the effect.
In the 1950s, Michael Dummett wrote in opposition to such definitions, stating that there was no philosophical objection to effects preceding their causes. This argument was rebutted by fellow philosopher Antony Flew and, later, by Max Black. Black's "bilking argument" held that retrocausality is impossible because the observer of an effect could act to prevent its future cause from ever occurring. A more complex discussion of how free will relates to the issues Black raised is summarized by Newcomb's paradox. Essentialist philosophers have proposed other theories, such as proposing the existence of "genuine causal powers in nature" or by raising concerns about the role of induction in theories of causality.
More recent philosophical inquiries into the topic have often adapted aspects of modern physics, including the hypothetical tachyon particle and certain time-independent aspects of quantum mechanics. Jan Faye of the University of Copenhagen has argued that logical objections to macroscopic time travel may not necessarily prevent retrocausality at other scales of interaction. Even if such effects are possible, however, they may not be capable of producing effects different from those that would have resulted from normal causal relationships.
Established physics does not generally employ retrocausality. Nevertheless, a number of theories allowing particles or information to travel backward in time have been proposed by prominent scientists or have received meaningful evaluation by the scientific community.
As the modern understanding of particle physics began to develop, retrocausality was at times employed as a tool to model then-unfamiliar or unusual conditions, including electromagnetism and antimatter.
The Wheeler–Feynman absorber theory, proposed by John Archibald Wheeler and Richard Feynman, uses retrocausality and a temporal form of destructive interference to explain the absence of a type of converging concentric wave suggested by certain solutions to Maxwell's equations. These advanced waves don't have anything to do with cause and effect, they are just a different mathematical way to describe normal waves. The reason they were proposed is so that a charged particle would not have to act on itself, which, in normal classical electromagnetism leads to an infinite self-force.
Feynman, and earlier Stueckelberg, proposed an interpretation of the positron as an electron moving backward in time, reinterpreting the negative-energy solutions of the Dirac equation. Electrons moving backward in time would have a positive electric charge. Wheeler invoked this concept to explain the identical properties shared by all electrons, suggesting that "they are all the same electron" with a complex, self-intersecting worldline. Yoichiro Nambu later applied it to all production and annihilation of particle-antiparticle pairs, stating that "the eventual creation and annihilation of pairs that may occur now and then is no creation or annihilation, but only a change of direction of moving particles, from past to future, or from future to past." The backwards in time point of view is nowadays accepted as completely equivalent to other pictures, but it doesn't have anything to do with the macroscopic terms "cause" and "effect", which do not appear in a microscopic physical description.
Open topics in physics, especially involving the reconciliation of gravity with quantum physics, suggest that retrocausality may be possible under certain circumstances.
Closed timelike curves, in which the world line of an object returns to its origin, arise from some exact solutions to the Einstein field equation. Although closed timelike curves do not appear to exist under normal conditions, extreme environments of spacetime, such as a traversable wormhole or the region near certain cosmic strings, may allow their formation, implying a theoretical possibility of retrocausality. The exotic matter or topological defects required for the creation of those environments have not been observed. Furthermore, Stephen Hawking has suggested a mechanism he describes as the chronology protection conjecture, which would destroy any such closed timelike curve before it could be used. These objections to the existence of closed timelike curves are not universally accepted, however.
Retrocausality is sometimes associated with the nonlocal correlations that generically arise from quantum entanglement, which Albert Einstein famously called "spooky action at a distance", including the notable special case of the delayed choice quantum eraser. This is not generally agreed upon within the physics community because verifying nonlocal correlations requires ordinary subluminal communication, since the no communication theorem prevents the superluminal transfer of information, and because fundamental descriptions of matter and forces require the full framework of quantum field theory in which spacelike-separated operators commute. Accounts of quantum entanglement that do not involve retrocausality generally emphasize how the experiments demonstrating these correlations can equally well be described from different reference frames, that disagree on which measurement is a "cause" versus an "effect", as necessary to be consistent with special relativity. The description of such nonlocal quantum entanglements can be described in a way that is manifestly free of retrocausality if the states of the observers are included in the quantum treatment, which is often but not exclusively associated with the many worlds interpretation.
Hypothetical superluminal particles called tachyons would have a spacelike trajectory, and thus move backward in time according to observers in some reference frames. Despite frequent depiction in science fiction as a method to send messages back in time, theories predicting tachyons do not permit them to interact with normal "tardyonic" matter in a way that would violate standard causality. Specifically, the Feinberg reinterpretation principle renders impossible construction of a tachyon detector capable of receiving information. Within modern quantum field theory, tachyons (or particles with imaginary mass) are interpreted to signify that the theory has been expanded about a configuration that is a local maximum of potential energy, instead of a local minimum.
Outside the mainstream scientific community, retrocausality has also been proposed as a mechanism to explain purported anomalies, paranormal events or personal events, but mainstream scientists generally regarded these explanations as pseudoscientific. Most notably, parapsychologist Helmut Schmidt presented quantum mechanical justifications for retrocausality, eventually claiming that experiments had demonstrated the ability to manipulate radioactive decay through retrocausal psychokinesis. These results and their underlying theory have been rejected by the mainstream scientific community, although they continue to have some support from fringe science sources.