Retrocausality (also called retro-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 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? [cite journal |last=Barry |first=Patrick |title=What's done is done... or is it? |journal=New Scientist |date=2006-08-28 |url= |accessdate=2006-12-19] 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. [cite journal |author=Faye, Jan |title=Backward Causation |journal=Stanford Encyclopedia of Philosophy |url= |date=2001-08-27, rev. 2005-08-29 |accessdate=2006-12-24]

While a few legitimate scientific theories have also proposed limited forms of retrocausality, no experimental observations have been reliably reported that verify such hypotheses.cite book |author=Sheehan, D. P., ed. |title=Frontiers of Time: Retrocausation - Experiment and Theory, San Diego, California, 20-22 June 2006. Record of a symposium held by the Pacific Division of the American Association for the Advancement of Science |publisher=American Institute of Physics |date=2006-11-14 |id=ISBN 0-7354-0361-9]

As philosophy

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 which precedes the effect. [cite book |title=Hume and the Problem of Causation |author=Beauchamp, Tom L. and Alexander Rosenberg |publisher=Oxford University Press |year=1981 |id=ISBN 978-0195202366] Furthermore, the ability to affect the past suggests that events could be negated by their own effects, creating a physical paradox, [cite journal |title=Causality violation and paradoxes |author=Krasnikov, S. V. |journal=Physics Review D |volume=55 |issue=6 |pages=3427–3430 |date=Mar 1997 |doi=10.1103/PhysRevD.55.3427] such as the well-known 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. [cite journal |last=Dummett |first=Michael |title=Can an Effect Precede its Cause |journal=Proceedings of the Aristotelian Society |issue=Supp. 28 |year=1954] This argument was rebutted by fellow philosopher Antony Flew [cite journal |last=Flew |first=Anthony |title=Can an Effect Precede its Cause |journal=Proceedings of the Aristotelian Society |issue=Supp. 28 |year=1954] 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. [cite journal |last=Black |first=Max |title=Why Cannot an Effect Precede Its Cause |journal=Analysis |issue=16 |year=1956 |doi=10.2307/3326929 |volume=16 |pages=49] 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" [cite book |title=The Philosophy of Nature: A Guide to the New Essentialism |author=Ellis, Brian |publisher=McGill-Queen's University Press |date=2002 |id=ISBN 978-0773524746] or by raising concerns about the role of induction in theories of causality. [cite book |title=Hume on Causation |author=Beebee, Helen |publisher=Routledge |date=2006-10-25 |id=ISBN 978-0415243391]

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. [cite book |author=Faye, Jan, Uwe Scheffler and Max Urchs, eds. |title=Logic and Causal Reasoning |publisher=Wiley-VCH |date=1994-10-13 |id=ISBN 3050025999] Even if such effects are possible, however, they may not be capable of producing different effects than would have resulted from normal causal relationships. [cite book |author=Elitzur, A., S. Doley and N. Kolenda, eds. |title=Quo Vadis Quantum Mechanics? |publisher=Springer |date=2005-05-25 |id=ISBN 3540221883]

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. [cite journal |last=Peijnenburg |first=Jeanne |title=Shaping Your Own Life |journal=Metaphilosophy |issue=37] According to Cornelis van Putten, however, there is no need to literally modify the past to achieve the results Peijnenburg discusses. [cite journal |last=van Putten |first=Cornelis |title=Changing the Past: Retrocausality and Narrative Construction |journal=Metaphilosophy |issue=37]

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.

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. [cite journal |author=Wheeler, John and Richard Feynman |title=Interaction with the Absorber as the Mechanism of Radiation |journal=Review of Modern Physics |issue=17 |year=1945] 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.cite book |last=Price |first=Huw |title=Time's Arrow and Archimedes' Point |publiser=Oxford University Press |date=1997-12-04 |id=ISBN 0195117980]

Feynman also employed retrocausality to propose a model of the positron [cite journal |last=Feynman |first=Richard |title=The Theory of Positrons |journal=Physical Review |issue=76 |year=1949 |doi=10.1103/PhysRev.76.749 |volume=76 |pages=749] 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. [cite speech |title=The Development of the Space-Time View of Quantum Electrodynamics |last=Feynman |first=Richard |date=1965-12-11 |location=Nobel Lecture |url= |accessdate=2007-01-02] 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." [cite journal |last=Nambu |first=Yoichiro |title=The Use of the Proper Time in Quantum Electrodynamics I |journal=Progress in Theoretical Physics |issue=5 |year=1950] Although further understanding of antimatter has rendered this model largely obsolete, [cite journal |last=Earman |first=John |title=On Going Backward in Time |journal=Philosophy of Science |issue=34 |year=1967 |doi=10.1086/288153 |volume=34 |pages=211] it is still employed for conceptual purposes, such as in Feynman diagrams.

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, 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 [cite book |last=Thorne |first=Kip |title=Black Holes and Time Warps: Einstein's Outrageous Legacy |publisher=W W Norton |year=1994 |id=ISBN 0-393-31276-3] or the region near certain cosmic strings, [cite book |last=Gott |first=John Richard |title=Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time |publisher=Houghton Mifflin |year=2002 |id=ISBN 0-618-25735-7] 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 which he described as the chronology protection conjecture that would destroy any such closed timelike curve before it could be used. [cite journal |last=Hawking |first=Stephen |title=The Chronology Protection Conjecture |journal=Physical Review D |issue=46 |year=1992 |doi=10.1103/PhysRevD.46.603 |volume=46 |pages=603] These objections to the existence of closed timelike curves are not universally accepted, however. [cite journal |last=Li |first=Li-Xin |title=Must Time Machine Be Unstable against Vacuum Fluctuations? |journal=Classical and Quantum Gravity |issue=13 |year=1996]

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. [cite journal |last=de Beauregard |first=Costa |title=Time Symmetry and the Einstein Paradox |journal=Il Nuovo Cimento |issue=42B |year=1977] At an American Association for the Advancement of Science symposium, University of Washington physicist John Cramer presented the design for an experiment to test for backward causation in quantum entanglement, subsequently receiving some attention from the popular media. [cite journal |last=Paulson |first=Tom |title=Going for a blast in the real past |journal=Seattle Post-Intelligencer |date=2006-11-15 |url= |accessdate=2006-12-19] [cite web |last=Boyle |first=Alan |title=Time-travel physics seems stranger than fiction |publisher=MSNBC |date=2006-11-21 |url= |accessdate=2006-12-19] Although Cramer's experiment has never been performed as of 2008, retrocausality has also been proposed as an explanation [cite paper |title=Backward Causation and the EPR Paradox |author=Wharton, William R. |date=1998-10-28 |url= |accessdate=2007-06-21] for the delayed choice quantum eraser. [cite journal |title=Complementarity and the Quantum Eraser |author=Herzog, T. J. et al. |journal=Physics Review Letters |volume=75 |issue=17 |pages=3034–3037 |date=1995 |doi=10.1103/PhysRevLett.75.3034]

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 send 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. [cite journal |last=Feinberg |first=Gerald |title=Possibility of Faster-Than-Light Particles |journal=Physical Review |issue=159 |year=1967 |doi=10.1103/PhysRev.159.1089 |volume=159 |pages=1089]

As pseudoscience

Outside the mainstream scientific community, retrocausality has also been proposed as a mechanism to explain purported anomalies or paranormal events, but mainstream scientists generally regarded these explanations as pseudoscientific.

Most notably, parapsychologist Helmut Schmidt presented quantum mechanical justifications for retrocausality, [cite journal |last=Schmidt |first=Helmut |title=Can an effect precede its cause? A model of a noncausal world |journal=Foundations of Physics |volume=8 |date=Jun 1978] eventually claiming that experiments had demonstrated the ability to manipulate radioactive decay through retrocausal psychokinesis. [cite journal |last=Schmidt |first=Helmut |title=Collapse of the state vector and psychokinetic effect |journal=Foundations of Physics |volume=12 |date=Jun 1982] These results and their underlying theory have been rejected by the mainstream scientific community, [cite book |author=Druckman, Daniel and John A. Swets, eds. |title=Enhancing Human Performance: Issues, Theories, and Techniques |publisher=National Academy Press |date=Jan 1988|id=ISBN 978-0309037921] [cite book |last=Stenger |first=Victor J. |title=Physics and Psychics: The Search for a World Beyond the Senses |publisher=Prometheus Books |date=May 1990 |id=ISBN 978-0879755751] although they continue to have some support from fringe science sources. [cite journal |last=Shoup |first=Richard |title=Anomalies and constraints: can clairvoyance, precognition, and psychokinesis be accommodated with known physics? |journal=Journal of Scientific Exploration |volume=16 |year=2002]

Efforts to associate retrocausality with prayer healing [cite journal |last=Leibovici |first=L. |title=Effects of remote, retroactive intercessory prayer on outcomes in patients with bloodstream infections: a controlled trial |journal=British Medical Journal |volume=323 |year=2001 |doi=10.1136/bmj.323.7327.1450 |pages=1450] have been similarly discounted by mainstream science. [cite journal |author=Bishop, Jeffrey P. and Victor J. Stenger |title=Retroactive prayer: lots of history, not much mystery, and no science |journal=British Medical Journal |date=2004-12-18 |volume=329]


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