Retrocausality

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Retrocausality is any of several hypothetical phenomena or processes that reverse causality, allowing an effect to occur before its cause. Also referred to as retro-causation, backward causation or by other, similar terms, it is primarily a philosophy of science thought experiment based on elements of physics, addressing the question: Can what happens in the future affect the present, and can the present affect the past?^[1] Philosophical considerations of time travel often address the same issues as retrocausality, although the two terms are not universally synonymous.^[2] While a few legitimate scientific theories have also proposed limited forms of retrocausality, no experimental observations have been reliably reported^[3] as of 2006.

Contents 1 As philosophy 2 As physics 2.1 Historical models 2.2 Current topics 3 As pseudoscience 4 In Popular Culture 5 References

[edit] As philosophy

Although philosophical efforts to understand causality extend back at least to Aristotle's discussions of the prime mover, 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 which precedes the effect. Furthermore, the ability to affect the past suggests that events could be negated by their own effects, creating a physical paradox, the best known of which example is the grandfather paradox.

In the 1950s, Michael Dummett wrote in opposition to such definitions, stating that there was no philosophical objection to effects preceding their causes.^[4] This argument was refuted by fellow philosopher Anthony Flew^[5] and later, by Max Black's "bilking argument".^[6] A later, more complex discussion of how free will relates to the issues Black raised is summarized by Newcomb's paradox.

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 has argued that logical objections to macroscopic time travel may not necessary prevent retrocausality at other scales of interaction.^[7] Even if such effects are possible, however, they may not be capable of producing different effects than would have resulted from normal causal relationships.^[8]

Jeanne Peijnenburg invokes retrocausality to describe how "broad imagination" can redefine or even alter past events, resulting in changes to present-day personality and perception.^[9] According to Cornelis van Putten, however, there is no need to literally modify the past to achieve the results Peijnenburg discusses.^[10]

[edit] As physics

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 respected scientists or have received meaningful evaluation by the scientific community. In general, models appearing to permit retrocausality or time travel to occur are often thought to possess mathematical artifacts or to simply be conceptually flawed.

[edit] Historical models

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.

John Archibald Wheeler and Richard Feynman proposed a theory using 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.^[11] However, there has been no experimental observation of these "advanced waves", and it has been suggested that they may in fact simply be a different mathematical means to describe normal waves.^[12]

Feynman also employed retrocausality to provide a proposed model of the positron^[13] by reinterpreting the negative-energy solutions of the Dirac equation. In this model, electrons moving backward in time would appear to possess 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.^[14] 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."^[15] Although further understanding of antimatter has rendered this model largely obsolete,^[16] it is still employed for conceptual purposes, such as in Feynman diagrams.

[edit] Current topics

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 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^[17] or the region near certain cosmic strings,^[18] may allow their formation. The exotic matter or topological defects required for the creation of those environments have not been observed. Furthermore, Stephen Hawking has suggested a mechanism which he described as the chronology protection conjecture that would destroy any such closed timelike curve before it could be used.^[19] These objections to the existence of closed timelike curves are not universally accepted, however.^[20]

Retrocausality has also been proposed as a mechanism to explain what Albert Einstein called "spooky action at a distance" occurring as a result of quantum entanglement. Although the prevailing scientific viewpoint is that the effects generated by quantum entanglement do not require any direct communication between the involved particles, Costa de Beauregard proposed an alternative theory.^[21] University of Washington physicist John Cramer presented the design for an experiment to test this theory at an American Association for the Advancement of Science symposium,^[3] subsequently receiving some attention from the popular media,^[22][23] although the experiment has never been performed as of 2006.

The hypothetical superluminal particle called the tachyon, proposed in the context of the Bosonic string theory and certain other fields of high-energy physics, moves backward in time. Despite frequent depiction in science fiction as a method to sent messages back in time, theories predicting tachyons do not permit them to interact with normal "time-like" matter in a manner that would violate standard causality. Specifically, the Feinberg reinterpretation principle renders impossible construction of a tachyon detector capable of receiving information.^[24]

[edit] As pseudoscience

Outside the mainstream scientific community, retrocausality has also been proposed as a mechanism to explain purported pseudoscientific effects. Most notably, parapsychologist Helmut Schmidt presented quantum mechanical justifications for retrocausality^[25], eventually claiming that experiments had demonstrated the ability to manipulate radioactive decay through retrocausal psychokinesis.^[26] These results and their underlying theory have been rejected by the mainstream scientific community,^[27][28] although they continue to have some support from fringe science sources.^[29]

Efforts to associate retrocausality with prayer healing^[30] have been similarly discounted by mainstream science.^[31]

[edit] In Popular Culture

• The novel "Thrice Upon a Time" by James P. Hogan deals with retrocausality using the fictional "Tau Radiation".
• The movie "Retroactive" deals with retrocausality involving "time loops" repeated over and over again.

[edit] References