Retrocausality

Retrocausality or Backwards causation is a concept of cause and effect where the effect precedes its cause in time.[1]

Retrocausality is primarily a thought experiment in philosophy of science based on elements of physics, addressing whether the future can affect the present and whether the present can affect the past.[2] 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.[1]

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.[3]

Philosophy

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.[4]

In the 1950s, Michael Dummett wrote in opposition to such definitions, stating that there was no philosophical objection to effects preceding their causes.[5] This argument was rebutted by fellow philosopher Antony Flew[5] 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.[6] 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"[7] or by raising concerns about the role of induction in theories of causality.[8]

Physics

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.[9] 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.[10]

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.[11] 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.[12]

Time runs left to right in this Feynman diagram of electron–positron annihilation. When interpreted to include retrocausality, the electron (marked e−) was not destroyed, instead becoming the positron (e+) and moving backward in time.

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.[13] 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.[14]

Feynman, and earlier Ernst Stueckelberg, proposed an interpretation of the positron as an electron moving backward in time,[15] 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.[16] 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."[17] The backwards-in-time point of view is nowadays accepted as completely equivalent to other pictures,[18] but it doesn't have anything to do with the macroscopic terms "cause" and "effect", which do not appear in a microscopic physical description.

Relativity

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[19] or the region near certain cosmic strings,[20] 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.[21] These objections to the existence of closed timelike curves are not universally accepted.[22]

Quantum physics

Retrocausality is sometimes associated with the nonlocal correlations that generically arise from quantum entanglement,[23] including the notable special case of the delayed choice quantum eraser.[24] 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.[25][26] 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.[27] 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.[28]

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.[29]

Tachyons

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.[30] 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.

As fringe science

Parapsychologist Helmut Schmidt presented quantum mechanical justifications for retrocausality,[31] eventually claiming that experiments had demonstrated the ability to manipulate radioactive decay through retrocausal psychokinesis.[32] These results and their underlying theory have been rejected by the mainstream scientific community,[33][34] although they continue to have some support from fringe science sources.[35]

Efforts to associate retrocausality with prayer healing[36] have been similarly discounted by legitimate scientific method.[37]

One of the methodology–deficient experiments[38] of psychologist Daryl J. Bem is discussed in a paper by Richard Shoup.[39] In Bem's experiment, a subject is shown two sets of curtains, and is instructed to guess which one has a picture behind it, but the picture is not displayed behind the curtain until the subject makes 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%.

See also

References

  1. 1 2 Faye, Jan (2001-08-27). "Backward Causation". Stanford Encyclopedia of Philosophy. Retrieved 2006-12-24.
  2. ↑ 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.
  3. ↑ 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.
  4. ↑ Beauchamp, Tom L.; Rosenberg, Alexander (1981). Hume and the Problem of Causation. New York: Oxford University Press. ISBN 9780195202366.
  5. 1 2 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.
  6. ↑ Black, Max (January 1956). "Why Cannot an Effect Precede Its Cause?". Analysis. 16 (3): 49. doi:10.2307/3326929. JSTOR 3326929.
  7. ↑ Ellis, Brian (2002). The Philosophy of Nature: A Guide to the New Essentialism. Montréal: McGill-Queen's University Press. ISBN 9780773524743.
  8. ↑ Beebee, Helen (2006). Hume on Causation. London: Routledge. ISBN 9780415243391.
  9. ↑ 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.
  10. ↑ 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.
  11. ↑ Faye, Jan (1994). Logic and Causal Reasoning. Berlin: Akad.-Verl. ISBN 978-3050025995.
  12. ↑ Elitzur, A.; Dolev, S.; Kolenda, N. (2005). Quo Vadis Quantum Mechanics?. Berlin: Springer. ISBN 978-3540221883.
  13. ↑ 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.
  14. ↑ 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.
  15. ↑ 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.
  16. ↑ Feynman, Richard (1965-12-11). The Development of the Space-Time View of Quantum Electrodynamics (Speech). Nobel Lecture. Retrieved 2007-01-02.
  17. ↑ 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.
  18. ↑ 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.
  19. ↑ Thorne, Kip S. (1995). Black Holes and Time Warps: Einstein's Outrageous Legacy. New York: W.W. Norton. ISBN 978-0393312768.
  20. ↑ 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.
  21. ↑ 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.
  22. ↑ 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.
  23. ↑ 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.
  24. ↑ Wharton, William R. (1998-10-28). "Backward Causation and the EPR Paradox". Retrieved 2007-06-21.
  25. ↑ Costa de Beauregard, Olivier (1977). "Time Symmetry and the Einstein Paradox" (PDF). Il Nuovo Cimento (42B).
  26. ↑ 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.
  27. ↑ 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.
  28. ↑ 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
  29. ↑ Aharonov, Yakir & Lev Vaidman. "The Two-State Vector Formalism: An Updated Review" (PDF). Retrieved 2014-07-07.
  30. ↑ 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.
  31. ↑ 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.
  32. ↑ 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.
  33. ↑ Druckman, Daniel; Swets, John A. (1988). Enhancing Human Performance: Issues, Theories, and Techniques. Washington, D.C.: National Academy Press. ISBN 9780309037921.
  34. ↑ Stenger, Victor J. (1990). Physics and Psychics: The Search for a World Beyond the Senses. Buffalo, New York: Prometheus Books. ISBN 9780879755751.
  35. ↑ Shoup, Richard (2002). "Anomalies and constraints: can clairvoyance, precognition, and psychokinesis be accommodated with known physics?". Journal of Scientific Exploration. 16.
  36. ↑ 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.
  37. ↑ 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.
  38. ↑ 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.
  39. ↑ Shoup, Richard (2012-05-19). "Understanding Retrocausality -- Can a Message Be Sent to the Past?" (PDF). Boundary Institute. Archived from the original (PDF) on 2015-09-19. Retrieved 2016-12-11.
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