Retrocausality, or backwards causation, is a concept of cause and effect in which an effect precedes its cause in time and so a later event affects an earlier one. In quantum physics, the distinction between cause and effect is not made at the most fundamental level and so time-symmetric systems can be viewed as causal or retrocausal.[page needed] Philosophical considerations of time travel often address the same issues as retrocausality, as do treatments of the subject in fiction, but the two phenomena are distinct.
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 the existence of "genuine causal powers in nature" or by raising concerns about the role of induction in theories of causality.[page needed][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. 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]
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.[page needed][page needed] Furthermore, the chronology protection conjecture of Stephen Hawking suggests that any such closed timelike curve would be destroyed before it could be used. These objections to the existence of closed timelike curves are not universally accepted.
Retrocausality is associated with the Double Inferential state-Vector Formalism (DIVF), later known as 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.
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 have nothing to do with cause and effect: they are simply a different mathematical way to describe normal waves. The reason they were proposed is that a charged particle would not have to act on itself, which, in normal classical electromagnetism, leads to an infinite self-force.[page needed]
Ernst Stueckelberg, and later Richard Feynman, 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 has nothing to do with the macroscopic terms "cause" and "effect", which do not appear in a microscopic physical description.
Retrocausality is sometimes associated with the nonlocal correlations that generically arise from quantum entanglement, including for example the delayed choice quantum eraser. However accounts of quantum entanglement can be given which do not involve retrocausality. They treat the experiments demonstrating these correlations as being described from different reference frames that disagree on which measurement is a "cause" versus an "effect", as necessary to be consistent with special relativity. That is to say, the choice of which event is the cause and which the effect is not absolute but is relative to the observer. The description of such nonlocal quantum entanglements can be described in a way that is free of retrocausality if the states of the system are considered. Physicist John G. Cramer has explored various proposed methods for nonlocal or retrocausal quantum communication and found them all flawed and, consistent with the no communication theorem, unable to transmit nonlocal signals.
Even if retrocausality exists at a quantum level, it cannot be used for communication because verifying any nonlocal correlation requires ordinary subluminal communication between observers at the source and destination: the no communication theorem prevents the superluminal transfer of information. Fundamental descriptions of matter and forces require the full framework of quantum field theory in which spacelike-separated operators commute.[clarification needed]
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 frame. 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 a tachyon detector capable of receiving information.
Retrocausality has been claimed to occur in some psychic phenomena such as precognition, however the existence of such phenomena has never been proven. For example psychologist Daryl J. Bem showed experimental subjects two sets of curtains and instructed them to guess which one had a picture behind it, but did not display the picture behind the curtain until after the subject made their guess. Some results showed a higher margin of success (p. 17) for a subset of erotic images, with subjects who identified as "stimulus-seeking" in the pre-screening questionnaire scoring even higher. However the methodology has been strongly criticised and these results discounted.
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.[page needed][page needed]
- Faye, Jan (2001-08-27). "Backward Causation". Stanford Encyclopedia of Philosophy. Retrieved 2006-12-24.
- Barry, Patrick (September 2006). "What's done is done…". New Scientist. 191 (2571): 36–39. doi:10.1016/s0262-4079(06)60613-1. Retrieved 2006-12-19.
- Sheehan, Daniel P. (2006). Frontiers of Time: Retrocausation - Experiment and Theory; San Diego, California, 20-22 June 2006. Melville, New York: American Institute of Physics. ISBN 978-0735403611.
- Beauchamp, Tom L.; Rosenberg, Alexander (1981). Hume and the Problem of Causation. New York: Oxford University Press. ISBN 9780195202366.
- Dummett, A. E.; Flew, A. (11 July 1954). "Symposium: "Can An Effect Precede Its Cause?"". Aristotelian Society Supplementary Volume. 28 (1): 27–62. doi:10.1093/aristoteliansupp/28.1.27.
- Black, Max (January 1956). "Why Cannot an Effect Precede Its Cause?". Analysis. 16 (3): 49–58. doi:10.2307/3326929. JSTOR 3326929.
- Ellis, Brian (2002). The Philosophy of Nature: A Guide to the New Essentialism. Montréal: McGill-Queen's University Press. ISBN 9780773524743.
- Beebee, Helen (2006). Hume on Causation. London: Routledge. ISBN 9780415243391.
- Krasnikov, S. V. (15 March 1997). "Causality violation and paradoxes". Physical Review D. 55 (6): 3427–3430. Bibcode:1997PhRvD..55.3427K. doi:10.1103/PhysRevD.55.3427.
- Earman, John; Smeenk, Christopher; Wüthrich, Christian (7 May 2008). "Do the laws of physics forbid the operation of time machines?". Synthese. 169 (1): 91–124. doi:10.1007/s11229-008-9338-2. ISSN 0039-7857.
- Faye, Jan (1994). Logic and Causal Reasoning. Berlin: Akad.-Verl. ISBN 978-3050025995.
- Elitzur, A.; Dolev, S.; Kolenda, N. (2005). Quo Vadis Quantum Mechanics?. Berlin: Springer. ISBN 978-3540221883.
- Thorne, Kip S. (1995). Black Holes and Time Warps: Einstein's Outrageous Legacy. New York: W.W. Norton. ISBN 978-0393312768.
- Gott, John Richard (2002). Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time (1st ed.). Boston: Mariner Books. ISBN 978-0618257355.
- Hawking, S. W. (15 July 1992). "Chronology protection conjecture". Physical Review D. 46 (2): 603–611. Bibcode:1992PhRvD..46..603H. doi:10.1103/PhysRevD.46.603. PMID 10014972.
- Li, Li-Xin (1 September 1996). "Must time machines be unstable against vacuum fluctuations?". Classical and Quantum Gravity. 13 (9): 2563–2568. arXiv:gr-qc/9703024. Bibcode:1996CQGra..13.2563L. doi:10.1088/0264-9381/13/9/019.
- Watanabe, Satosi (1955). "Symmetry of physical laws. Part III. Prediction and retrodiction". Reviews of Modern Physics. 27 (2): 179–186. Bibcode:1955RvMP...27..179W. doi:10.1103/RevModPhys.27.179. hdl:10945/47584.
- Aharonov, Yakir & Lev Vaidman. "The Two-State Vector Formalism: An Updated Review" (PDF). Retrieved 2014-07-07.
- Wheeler, John Archibald; Feynman, Richard Phillips (1 April 1945). "Interaction with the Absorber as the Mechanism of Radiation". Reviews of Modern Physics. 17 (2–3): 157–181. Bibcode:1945RvMP...17..157W. doi:10.1103/RevModPhys.17.157.
- Price, Huw (1997). Time's Arrow & Archimedes' Point: New Directions for the Physics of Time (1st ed.). New York: Oxford University Press. ISBN 978-0195117981.
- Feynman, R. P. (15 September 1949). "The Theory of Positrons". Physical Review. 76 (6): 749–759. Bibcode:1949PhRv...76..749F. doi:10.1103/PhysRev.76.749.
- Feynman, Richard (1965-12-11). The Development of the Space-Time View of Quantum Electrodynamics (Speech). Nobel Lecture. Retrieved 2007-01-02.
- Nambu, Y. (1 February 1950). "The Use of the Proper Time in Quantum Electrodynamics I". Progress of Theoretical Physics. 5 (1): 82–94. Bibcode:1950PThPh...5...82N. doi:10.1143/ptp/5.1.82.
- Villata, M. (30 November 2011). "Reply to "Comment to a paper of M. Villata on antigravity"". Astrophysics and Space Science. 337 (1): 15–17. arXiv:1109.1201. Bibcode:2012Ap&SS.337...15V. doi:10.1007/s10509-011-0940-2.
- Rave, M. J. (22 October 2008). "Interpreting Quantum Interference Using a Berry's Phase-like Quantity". Foundations of Physics. 38 (12): 1073–1081. Bibcode:2008FoPh...38.1073R. doi:10.1007/s10701-008-9252-y.
- Wharton, William R. (1998-10-28). "Backward Causation and the EPR Paradox". arXiv:quant-ph/9810060. Bibcode:1998quant.ph.10060W. Retrieved 2007-06-21. Cite journal requires
- Costa de Beauregard, Olivier (1977). "Time Symmetry and the Einstein Paradox" (PDF). Il Nuovo Cimento (42B).
- David Ellerman (2012-12-11). "A Common Fallacy in Quantum Mechanics: Why Delayed Choice Experiments do NOT imply Retrocausality". Archived from the original on 2013-06-15. Retrieved 2017-05-12.
- Rubin, Mark A. (2001). "Locality in the Everett Interpretation of Heisenberg-Picture Quantum Mechanics". Found. Phys. Lett. (). 14 (2001): 301–322. arXiv:quant-ph/0103079. Bibcode:2001quant.ph..3079R. doi:10.1023/A:1012357515678.
- J. G. Cramer (April 2014), "Status of Nonlocal Quantum Communication Test" (PDF), UW CENPA Annual Report 2013-14, Article 7.1, retrieved September 21, 2016
- Feinberg, G. (25 July 1967). "Possibility of Faster-Than-Light Particles". Physical Review. 159 (5): 1089–1105. Bibcode:1967PhRv..159.1089F. doi:10.1103/PhysRev.159.1089.
- LeBel, Etienne P.; Peters, Kurt R. (January 2011). "Fearing the future of empirical psychology: Bem's (2011) evidence of psi as a case study of deficiencies in modal research practice" (PDF). Review of General Psychology. 15 (4): 371–379. doi:10.1037/a0025172. Retrieved 2 November 2017.
- Schmidt, Helmut (June 1978). "Can an effect precede its cause? A model of a noncausal world". Foundations of Physics. 8 (5–6): 463–480. Bibcode:1978FoPh....8..463S. doi:10.1007/BF00708576.
- Schmidt, Helmut (June 1982). "Collapse of the state vector and psychokinetic effect". Foundations of Physics. 12 (6): 565–581. Bibcode:1982FoPh...12..565S. doi:10.1007/bf00731929.
- Druckman, Daniel; Swets, John A. (1988). Enhancing Human Performance: Issues, Theories, and Techniques. Washington, D.C.: National Academy Press. ISBN 9780309037921.
- Stenger, Victor J. (1990). Physics and Psychics: The Search for a World Beyond the Senses. Buffalo, New York: Prometheus Books. ISBN 9780879755751.
- Shoup, Richard (2002). "Anomalies and constraints: can clairvoyance, precognition, and psychokinesis be accommodated with known physics?". Journal of Scientific Exploration. 16.[unreliable source?]
- Leibovici, L. (2001). "Effects of remote, retroactive intercessory prayer on outcomes in patients with bloodstream infection: randomised controlled trial". British Medical Journal. 323 (7327): 1450–1. doi:10.1136/bmj.323.7327.1450. PMC 61047. PMID 11751349.
- Bishop, J. P (18 December 2004). "Retroactive prayer: lots of history, not much mystery, and no science". BMJ. 329 (7480): 1444–1446. doi:10.1136/bmj.329.7480.1444. PMC 535973. PMID 15604179.