Predestination paradox
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A predestination paradox, also called either a causal loop, or a causality loop and (less frequently) either a closed loop or closed time loop, is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveller is caught in a loop of events that "predestines" him or her to travel back in time. Because of the possibility of influencing the past while time travelling, one way of explaining why history does not change is by saying that whatever has happened was meant to happen. A time traveller attempting to alter the past in this model, intentionally or not, would only be fulfilling his role in creating history as we know it, not changing it. It is very closely related to the ontological paradox and usually occurs at the same time.
The predestination paradox is in some ways the opposite of the grandfather paradox, the famous example of the traveller killing his own grandfather before his parent is conceived, thereby precluding his own travel to the past by canceling his own existence.
In physics, the Novikov self-consistency principle proposes that contradictory causal loops cannot form, but that consistent ones can. In a physical sense, a self-consistent causal loop of this kind is not actually a paradox because it produces a logically consistent result rather than a contradictory one. It is only perceived as a paradox because it goes against conventional expectations and assumptions about causality.
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[edit] Examples
A dual example of a predestination paradox is depicted in the classic Ancient Greek play 'Oedipus'
Laius hears a prophecy that his son will kill him and marry his wife. Fearing the prophecy, Laius pierces Oedipus' feet and leaves him out to die, but a herdsman finds him and takes him away from Thebes. Oedipus, not knowing he was adopted, leaves home in fear of the same prophecy that he would kill his father and marry his mother. Laius, meanwhile, ventures out to find a solution to the Sphinx's riddle. As prophesied, Oedipus crossed paths with Laius and this leads to a fight where Oedipus slays Laius. Oedipus then defeats the Sphinx by solving a mysterious riddle to become king. He marries the widow queen Jocasta not knowing she is his mother.
A typical example of a predestination paradox (used in The Twilight Zone episode "No Time Like the Past") is as follows:
A man travels back in time to discover the cause of a famous fire. While in the building where the fire started, he accidentally knocks over a kerosene lantern and causes a fire, the same fire that would inspire him, years later, to travel back in time.
A variation on the predestination paradoxes which involves information, rather than objects, traveling through time is similar to the self-fulfilling prophecy:
A man receives information about his own future, telling him that he will die from a heart attack. He resolves to get fit so as to avoid that fate, but in doing so overexerts himself, causing him to suffer the heart attack that kills him.
In all three examples, causality is turned on its head, as the flanking events are both causes and effects of each other, and this is where the paradox lies. In the second example, the person would not have traveled back in time but for the fire that he or she caused by traveling back in time. Similarly, in the third example, the man would not have overexerted himself but for the future information he receives.
In most examples of the predestination paradox, the person travels back in time and ends up fulfilling their role in an event that has already occurred. In a self-fulfilling prophecy, the person is fulfilling their role in an event that has yet to occur, and it is usually information that travels in time (for example, in the form of a prophecy) rather than a person. In either situation, the attempts to avert the course of past or future history both fail.
[edit] Examples from fiction
[edit] Time travel
Many fictional works have dealt with various circumstances that can logically arise from time travel, usually dealing with paradoxes. The predestination paradox is a common literary device in such fiction.
In The Twilight Zone 2002-2003 revival, there is an episode in which a character (played by Katherine Heigl) goes back in time to assassinate Adolf Hitler while he is a baby. She kills the baby (whom she presumes to be actual Adolf Hitler), but the nanny (discovering the death) replaces the baby with a street gypsy's baby, and she presents this baby to the father as his own. The father proceeds to introduce this son to his guests as "Adolf", presumably the Adolf Hitler known to history in the first place.
In television, the episode "Roswell That Ends Well" of the animated television series Futurama puts a more humorous spin on the paradox. In the episode, the main characters are thrown back in time to Roswell, New Mexico, in 1947, sparking the Roswell UFO Incident. Meanwhile, Philip J. Fry, told that the death of his grandfather Enos would nullify his own existence, becomes obsessed with protecting the man. To this end, he shuts Enos in a deserted house in the middle of the desert in order to protect him, failing to realize that the house is in fact on a nuclear testing site. The resulting atomic test kills Enos, but Fry does not disappear. Fry later comforts Enos' fiancée, no longer believing her to be his future grandmother. He has sex with her, only to realize afterward (thanks to the Professor) that she was/will be his grandmother after all because Fry has just made her pregnant with Fry's own father, making him his own grandfather.
The video game Prince of Persia: Warrior Within, the second in the Prince of Persia trilogy, centers heavily around predestination paradoxes. In the game, the Prince is being chased by the Dahaka, a being whose purpose is to preserve the timeline by erasing the Prince from it. Unable to fight the monster, the Prince travels to the Island of Time in order to kill the Empress of Time, who created the time-manipulating sands from the first game. In doing so, he hopes to prevent the sands from being created, since it was the existence of the sands that put him in his current predicament. However, the Prince realizes too late that killing the Empress is what creates the sands, and hence he becomes the architect of his own fate. A secondary paradox is the Sand Wraith, who seems to stalk the Prince throughout the first half of the game, even trying to kill him at one point. The wraith is killed by the Dahaka shortly before the Prince kills the Empress. After killing the Empress, the Prince realizes that he can change his fate by using the Mask of the Wraith, which transforms him into the Sand Wraith and sends him back in time a short distance. He learns that the wraith (who he now understands to be his future self) was trying to protect him, rather than attack him. Upon reaching the point at which the Dahaka is supposed to kill him, the Prince uses his knowledge of the encounter to have his younger self die instead, ending the mask's power and creating a grandfather paradox as well.
The film Donnie Darko incorporates a very complex example of fictional predestination paradox.
In Harry Potter and The Prisoner of Azkaban, Harry is saved from the dementors by a stag patronus. After traveling back in time, he realizes he was the one who produced the patronus, after watching himself being attacked. Similarly, in the movie, Harry and his friends are alerted to the presence of the Minister for Magic when a rock hits Harry in the head; but after travelling back in time, Hermione recognizes the same rock and throws it at Harry herself. Note that, had this rock not been thrown by her in the first place, she wouldn't have recognized it later on, and wouldn't have known to throw it.
[edit] Prophecies
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Prior to the use of time travel as a plot device, the self-fulfilling prophecy variant was more common.
Shakespeare's "Macbeth" is a classic example of this. The three Weird Sisters give Macbeth a prophecy that he will eventually become king, but the offspring of his best friend will rule instead of his afterwards. Spurred by the prophecy, he kills Duncan, his king, and his friend Banquo, something he never would have done before. In addition to these prophecies, other prophecies foretelling his downfall are given, such as that he will not be attacked until a forest moves to his castle, and that no man ever born of a woman can kill him. In the end, fate (or predestination) is what drives the House of Macbeth mad, and, ultimately, kills them, as Macbeth is killed by a man who was never "born", as the man was torn from his mother's womb by cesarian section.
In the Philip K. Dick short story "The Minority Report", on which the Steven Spielberg film Minority Report is based, murders are prevented through the efforts of three "precogs", psychic mutants who can see the crimes before they are committed. When police chief John Anderton is implicated in a murder-to-be, he sets out on a crusade to figure out why he would kill a man he has yet to meet. Many of the signposts on his journey to meet fate were predicted exactly as they occur, and his search leads him inexorably to the scene of the crime, where he cannot stop himself from killing the other man. In the end, the prediction itself is what had set the chain of events in motion.
In the Harry Potter series a prophecy made by Sybill Trelawney foretelling of "the one with the power to vanquish the dark lord" is what causes Lord Voldemort to attempt to kill Harry and thus imbue him with the power to be able to vanquish Voldemort.
Similarly in the popular network television show Lost, Desmond Hume's future flashes regarding Charlie's deaths eventually lead to his death. Desmond has a vision in which Charlie pushes a button below a flashing light which allows the other castaways to be rescued just before he drowns. However when the event occurs, events happen slightly differently than in Desmond's vision and it is suggested that Charlie may have been able to save himself without jeopardizing the hopes of rescue, if he had not believed his death was crucial in the rescue of the other castaways.
In Star Wars: Revenge of the Sith, Anakin Skywalker is plagued by dreams of his wife dying during child birth. This, as well as the limitations placed upon him by the Jedi council, sends him towards the dark side of the force in an effort to find a way to prevent her death. In his desire to keep Chancellor Palpatine alive, as he is the only one with the knowledge Anakin needs to prevent his wife from dying, Anakin attacks Jedi Master Mace Windu, preventing him from killing Palpatine and thus fully turns to the Dark Side. Becoming Darth Vader, Anakin performs evil acts which cause his wife, Padme to withdraw from him, eventually leading to him using his dark side powers to kill her.
