Retrocausality
Encyclopedia
Retrocausality is any of several hypothetical phenomena or processes that reverse causality
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

, allowing an effect
Result
A result is the final consequence of a sequence of actions or events expressed qualitatively or quantitatively. Possible results include advantage, disadvantage, gain, injury, loss, value and victory. There may be a range of possible outcomes associated with an event depending on the point of...

 to occur before its cause.

Retrocausality is primarily a thought experiment
Thought experiment
A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...

 in philosophy of science
Philosophy of science
The philosophy of science is concerned with the assumptions, foundations, methods and implications of science. It is also concerned with the use and merit of science and sometimes overlaps metaphysics and epistemology by exploring whether scientific results are actually a study of truth...

 based on elements of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, addressing the question: Can the future affect the present, and can the present affect the past? Philosophical considerations of time travel
Time travel
Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the...

 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.

As philosophy

Although philosophical efforts to understand causality extend back at least to Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

's discussions of the four causes, the idea that the arrow of time
Arrow of time
The arrow of time, or time’s arrow, is a term coined in 1927 by the British astronomer Arthur Eddington to describe the "one-way direction" or "asymmetry" of time...

 could be reversed is substantially more recent. In fact, retrocausality was long considered an inherent self-contradiction
Contradiction
In classical logic, a contradiction consists of a logical incompatibility between two or more propositions. It occurs when the propositions, taken together, yield two conclusions which form the logical, usually opposite inversions of each other...

 because, as 18th century philosopher David Hume
David Hume
David Hume was a Scottish philosopher, historian, economist, and essayist, known especially for his philosophical empiricism and skepticism. He was one of the most important figures in the history of Western philosophy and the Scottish Enlightenment...

 discussed, when examining two related events, the cause, by definition, is the one that precedes the effect.

Furthermore, the ability to affect the past suggests that causes could be negated by their own effects, creating a physical paradox
Physical paradox
A physical paradox is an apparent contradiction in physical descriptions of the universe. While many physical paradoxes have accepted resolutions, others defy resolution and may indicate flaws in theory...

, such as the well-known grandfather paradox
Grandfather paradox
The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent . The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's...

.

In the 1950s, Michael Dummett
Michael Dummett
Sir Michael Anthony Eardley Dummett FBA D.Litt is a British philosopher. He was, until 1992, Wykeham Professor of Logic at the University of Oxford...

 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
Antony Flew
Antony Garrard Newton Flew was a British philosopher. Belonging to the analytic and evidentialist schools of thought, he was notable for his works on the philosophy of religion....

 and, later, by Max Black
Max Black
Max Black was a British-American philosopher, who was a leading influential figure in analytic philosophy in the first half of the twentieth century. He made contributions to the philosophy of language, the philosophy of mathematics and science, and the philosophy of art, also publishing studies...

. 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
Free will
"To make my own decisions whether I am successful or not due to uncontrollable forces" -Troy MorrisonA pragmatic definition of free willFree will is the ability of agents to make choices free from certain kinds of constraints. The existence of free will and its exact nature and definition have long...

 relates to the issues Black raised is summarized by Newcomb's paradox
Newcomb's paradox
Newcomb's paradox, also referred to as Newcomb's problem, is a thought experiment involving a game between two players, one of whom purports to be able to predict the future. Whether the problem is actually a paradox is disputed....

. Essentialist philosophers have proposed other theories, such as proposing the existence of "genuine causal powers in nature" or by raising concerns about the role of induction
Inductive reasoning
Inductive reasoning, also known as induction or inductive logic, is a kind of reasoning that constructs or evaluates propositions that are abstractions of observations. It is commonly construed as a form of reasoning that makes generalizations based on individual instances...

 in theories of causality.

More recent philosophical inquiries into the topic have often adapted aspects of modern physics, including the hypothetical tachyon
Tachyon
A tachyon is a hypothetical subatomic particle that always moves faster than light. In the language of special relativity, a tachyon would be a particle with space-like four-momentum and imaginary proper time. A tachyon would be constrained to the space-like portion of the energy-momentum graph...

 particle
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

 and certain time-independent
T-symmetry
T Symmetry is the symmetry of physical laws under a time reversal transformation: T: t \mapsto -t.Although in restricted contexts one may find this symmetry, the observable universe itself does not show symmetry under time reversal, primarily due to the second law of thermodynamics.Time asymmetries...

 aspects of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

. Jan Faye of the University of Copenhagen
University of Copenhagen
The University of Copenhagen is the oldest and largest university and research institution in Denmark. Founded in 1479, it has more than 37,000 students, the majority of whom are female , and more than 7,000 employees. The university has several campuses located in and around Copenhagen, with the...

 has argued that logical objections to macroscopic time travel may not necessarily prevent retrocausality at other scales of interaction. 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.

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 prominent scientists or have received meaningful evaluation by the scientific community.

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
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

 and antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...

.

The Wheeler–Feynman absorber theory, proposed by John Archibald Wheeler
John Archibald Wheeler
John Archibald Wheeler was an American theoretical physicist who was largely responsible for reviving interest in general relativity in the United States after World War II. Wheeler also worked with Niels Bohr in explaining the basic principles behind nuclear fission...

 and Richard Feynman
Richard Feynman
Richard Phillips Feynman was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics...

, uses retrocausality and a temporal form of destructive interference to explain the absence of a type of converging concentric wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

 suggested by certain solutions to Maxwell's equations
Maxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.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
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

 as an electron moving backward in time, reinterpreting the negative-energy solutions of the Dirac equation
Dirac equation
The Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928. It provided a description of elementary spin-½ particles, such as electrons, consistent with both the principles of quantum mechanics and the theory of special relativity, and...

. Electrons moving backward in time would have a positive electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

. 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
Yoichiro Nambu
is a Japanese-born American physicist, currently a professor at the University of Chicago. Known for his contributions to the field of theoretical physics, he was awarded a one-half share of the Nobel Prize in Physics in 2008 for the discovery of the mechanism of spontaneous broken symmetry in...

 later applied it to all production and annihilation
Annihilation
Annihilation is defined as "total destruction" or "complete obliteration" of an object; having its root in the Latin nihil . A literal translation is "to make into nothing"....

 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 curve
Closed timelike curve
In mathematical physics, a closed timelike curve is a worldline in a Lorentzian manifold, of a material particle in spacetime that is "closed," returning to its starting point...

s, in which the world line
World line
In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" by the time dimension, and typically encompasses a large area of spacetime wherein...

 of an object returns to its origin, arise from some exact solutions
Exact solutions in general relativity
In general relativity, an exact solution is a Lorentzian manifold equipped with certain tensor fields which are taken to model states of ordinary matter, such as a fluid, or classical nongravitational fields such as the electromagnetic field....

 to the Einstein field equation. Although closed timelike curves do not appear to exist under normal conditions, extreme environments of spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

, 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
Exotic matter
In physics, exotic matter is a term which refers to matter which would somehow deviate from the norm and have "exotic" properties. There are several uses of the term....

 or topological defect
Topological defect
In mathematics and physics, a topological soliton or a topological defect is a solution of a system of partial differential equations or of a quantum field theory homotopically distinct from the vacuum solution; it can be proven to exist because the boundary conditions entail the existence of...

s required for the creation of those environments have not been observed. Furthermore, Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

 has suggested a mechanism he describes as the chronology protection conjecture
Chronology protection conjecture
The chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of closed timelike curves...

, 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
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

 called "spooky action at a distance" occurring as a result of quantum entanglement
Quantum entanglement
Quantum entanglement occurs when electrons, molecules even as large as "buckyballs", photons, etc., interact physically and then become separated; the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description , which is...

. 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
American Association for the Advancement of Science
The American Association for the Advancement of Science is an international non-profit organization with the stated goals of promoting cooperation among scientists, defending scientific freedom, encouraging scientific responsibility, and supporting scientific education and science outreach for the...

 symposium, University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...

 physicist John Cramer
John G. Cramer
John G. Cramer is a professor of physics at the University of Washington in Seattle, the United States. When not teaching, he works with the STAR detector at the new Relativistic Heavy Ion Collider at Brookhaven National Laboratory, and the particle accelerator at CERN in Geneva, Switzerland...

 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
Delayed choice quantum eraser
A delayed choice quantum eraser, first performed by Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih, and Marlan O. Scully, is an elaboration on a quantum eraser experiment involving the concepts considered in Wheeler's delayed choice experiment...

.

The hypothetical superluminal
Faster-than-light
Faster-than-light communications and travel refer to the propagation of information or matter faster than the speed of light....

 particle called the tachyon, proposed in the context of the Bosonic string theory
Bosonic string theory
Bosonic string theory is the original version of string theory, developed in the late 1960s.In the early 1970s, supersymmetry was discovered in the context of string theory, and a new version of string theory called superstring theory became the real focus...

 and certain other fields of high-energy physics, moves backward in time. Despite frequent depiction in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 as a method to send messages back in time, theories predicting tachyons do not permit them to interact with normal "time-like
Minkowski space
In physics and mathematics, Minkowski space or Minkowski spacetime is the mathematical setting in which Einstein's theory of special relativity is most conveniently formulated...

" 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.

As para-science

Outside the mainstream scientific community, retrocausality has also been proposed as a mechanism to explain purported anomalies, paranormal
Paranormal
Paranormal is a general term that designates experiences that lie outside "the range of normal experience or scientific explanation" or that indicates phenomena understood to be outside of science's current ability to explain or measure...

 events or personal events, but mainstream scientists generally regarded these explanations as pseudoscientific
Pseudoscience
Pseudoscience is a claim, belief, or practice which is presented as scientific, but which does not adhere to a valid scientific method, lacks supporting evidence or plausibility, cannot be reliably tested, or otherwise lacks scientific status...

. Most notably, parapsychologist
Parapsychology
The term parapsychology was coined in or around 1889 by philosopher Max Dessoir, and originates from para meaning "alongside", and psychology. The term was adopted by J.B. Rhine in the 1930s as a replacement for the term psychical research...

 Helmut Schmidt
Helmut Schmidt (parapsychologist)
Helmut Schmidt is a German-born parapsychologist. In the early 1970s he pioneered research into the effects of human consciousness on machines called random number generators or random event generators at the Rhine Research Center Institute for Parapsychology...

 presented quantum mechanical justifications for retrocausality, eventually claiming that experiments had demonstrated the ability to manipulate radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 through retrocausal psychokinesis
Psychokinesis
The term psychokinesis , also referred to as telekinesis with respect to strictly describing movement of matter, sometimes abbreviated PK and TK respectively, is a term...

. These results and their underlying theory have been rejected by the mainstream scientific community, although they continue to have some support from fringe science
Fringe science
Fringe science is scientific inquiry in an established field of study that departs significantly from mainstream or orthodox theories, and is classified in the "fringes" of a credible mainstream academic discipline....

 sources.

Efforts to associate retrocausality with prayer healing
Faith healing
Faith healing is healing through spiritual means. The healing of a person is brought about by religious faith through prayer and/or rituals that, according to adherents, stimulate a divine presence and power toward correcting disease and disability. Belief in divine intervention in illness or...

have been similarly discounted by legitimate scientific method.
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