Retrocausality (also called retro-causation, retro-chronal 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 thought experiment in philosophy of science based on elements of physics, addressing the question: Can the future affect the present, and can the present affect the past? 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.

A few legitimate physical theories have sometimes been interpreted as leading to retrocausality. This is not considered part of science, since the distinction between cause and effect in physics is not made at the most fundamental level.

Retrocausality and Antimatter
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.

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. These advanced waves don’t have anything to do with cause and effect, they are just 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.

Feynman, and earlier 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 worldline. 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.

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 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. 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, however.

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.

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. Work on Cramer’s non-local communication test started in January 2007. Cramer included a status report on the “UW Test of Nonlocal Quantum Communications with Momentum-Entangled Photon Pairs” in his “Five Decades of Physics” talk at a symposium in his honor at the University of Washington, Seattle, Washington, September 11, 2009. Work on the experiment will continue during 2010.

Retrocausality has also been proposed as an explanation for the delayed choice quantum eraser.

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.

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

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