Retrocausality or Backwards causation is a concept of cause and effect where the effect precedes its cause in time, so a later event in time can affect an earlier event in time. In quantum physics, the distinction between cause and effect is not made at the most fundamental level, so time-symmetric systems can be viewed as causal or retro-causal.[page needed] Philosophical considerations about the nature of time and causality often involve time travel and address the same issues as retrocausality, as do treatments of the subject in fiction, but the two terms are not universally synonymous.
Philosophical efforts to understand causality extend back at least to Aristotle's discussions of the four causes. It was long considered that an effect preceding its cause is 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.[page needed]
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"[page needed] or by raising concerns about the role of induction in theories of causality.[page needed]
The ability to affect the past is sometimes taken to suggest that causes could be negated by their own effects, creating a logical contradiction such as the grandfather paradox. This contradiction is not necessarily inherent to retrocausality or time travel; by limiting the initial conditions of time travel with consistency constraints, such paradoxes and others are avoided.
Aspects of modern physics, such as the hypothetical tachyon particle and certain time-independent aspects of quantum mechanics, may allow particles or information to travel backward in time. 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.[page needed] 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.[page needed]
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 simply 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.[page needed]
Feynman, and earlier Ernst 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 world line. 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.
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[page needed] or the region near certain cosmic strings,[page needed] 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.
Retrocausality is sometimes associated with the nonlocal correlations that generically arise from quantum entanglement, including the notable special case of the delayed choice quantum eraser. However, verifying nonlocal correlations requires ordinary subluminal communication: the no communication theorem prevents the superluminal transfer of information, and 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 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 system is considered. Ongoing experiments by physicist John G. Cramer explore various proposed methods for nonlocal or retrocausal quantum communication, finding them all flawed and unable to transmit nonlocal signals.
Retrocausality is also associated with the two-state vector formalism (TSVF) in quantum mechanics, where the present is characterised by quantum states of the past and the future taken in combination.
Hypothetical superluminal particles called tachyons have a spacelike trajectory, and thus can appear to move backward in time, according to an observer in a conventional reference frames. Despite frequent depiction in science fiction as a method to send messages back in time, tachyons do not interact with normal tardyonic matter in a way that would violate standard causality. Specifically, the Feinberg reinterpretation principle means that ordinary matter cannot be used to make of a tachyon detector capable of receiving information.
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,[page needed][page needed] although they continue to have some support from fringe science sources.
In an experiment by psychologist Daryl J. Bem, a subject was shown two sets of curtains, and was instructed to guess which one had a picture behind it, but the picture was not displayed behind the curtain until after the subject made their guess. Most of the results were statistically indistinguishable from chance (50%), but some results showed a higher margin of success (p. 17) for a subset of erotic images — 53.1%. Subjects which identified as "stimulus-seeking" in the pre-screening questionnaire scored even higher — 57.6%. However its methodology has been strongly criticised.
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