Predestination paradox

A predestination paradox, also called a causal loop or causality 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 to travel back in time. This 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 born, thereby precluding his own travel to the past by cancelling his own existence.

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, not changing it.

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.

Examples

A typical example of a predestination paradox 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.

Another example:

A man travels back in time and impregnates his great-great-grandmother. She would, as a result, give birth to one of the man's great-grandparents, who would then give birth to his grandmother or grandfather, who would then be able to give birth to one of the man's parents, to whom will be born the man himself. This man would have to travel back in time in order to ensure his own existence.

A variation on the predestination paradox which involves information, rather than objects, travelling 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 first example, the person would not have travelled back in time but for the fire that he or she caused by travelling back in time. Similarly, in the third example, the man would not have overexerted himself but for the future information he receives. In the second example, the man's very existence would be pre-determined by his time traveling adventure. This also raises the paradox of which came first — the time travel or his existence (see the closely-related Ontological paradox).

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.

Examples from fiction

Template:Spoiler 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.

Prior to the use of time travel as a plot device, the self-fulfilling prophecy variant was more common, with one of the earliest and most famous examples being the ancient Greek legend of Oedipus. There, it is prophesied that the baby Oedipus will one day kill his father and marry his mother. His father, Laius, attempts to circumvent the prophecy by abandoning the baby in the wilderness. Years later, Oedipus — unaware that he was adopted — learns of the prophecy and leaves home to avoid it. He kills a man and marries the widow, but does not learn until later that they are, in fact, his biological parents. The attempts to avoid fate result in the fulfilment of the prophecy.

Literature

Numerous pieces of science fiction and fantasy literature involving time travel make use of the predestination paradox. Notable examples include Isaac Asimov's The End of Eternity.

• In Isaac Asimov's Robots in Time series, a scientist travels two centuries into the future and is shown an utopian civilization free from illness, war and aging. When he returns and reports this, one of the persons who hears his account is a prototype human-looking robot who realises that the future "humans" are actually robots and that mankind will succumb to its own decadence. The robot then buries a note for the robots of the future to discover so that they can convince the time traveller that humanity will triumph.

• In Stephen Baxter's The Time Ships, a sequel to H. G. Wells's The Time Machine, the Time Traveller explains that his researches into time travel began when a mysterious yet vaguely familiar stranger passed him a mineral, the Plattnerite, which he used to construct the machine. Over the course of his subsequent travels which involve the alteration of history, he discovers that the stranger was in fact his future self. Eventually, with the help of humanity's descendants, he restores the timeline and travels into the past to pass the Plattnerite to his younger self.

• In Michael Moorcock's Behold the Man, time traveller Karl Glogauer travels back to 28 AD in search of Jesus. The Jesus he finds is a mentally retarded hunchback, but Glogauer himself becomes known by that name, attracts a mass following, and is captured and crucified.

• In Harry Potter and the Prisoner of Azkaban, Harry and Hermione travel back in time three hours to save Harry's godfather, Sirius Black, and Hagrid's hippogriff, Buckbeak, from being executed. Harry saw someone he believed to be his father casting a powerful Patronus spell saving him from the Dementors. Although hoping to see his father during this time journey, he finds no one there. He is therefore forced to cast the Patronus and save his past self, realizing in the process that it had been himself all along. When later asked how he was able to conjure such a powerful Patronus, Harry explains that he knew he could do it because he had already seen himself doing it. The film version of the book adds more examples, revealing that some shells that were mysteriously thrown into Hagrid's hut earlier were actually hurled by Hermione's future-self to warn them that Cornelius Fudge and company were arriving.

• In Michael Crichton's Timeline, several graduate students who are excavating several medieval castles and towns from 14th century France are given the opportunity to travel back in time to the very place and time period they are studying. On a mission to rescue their Professor, who had left his time machine and gotten lost, the students end up causing some of the historical events they had studied.

• In Christopher Pike's The Starlight Crystal, the main female character, Paige, is actually most of the characters who appear in the novel due to a causal loop, including living through the end and recreation of the universe, creating a race of aliens from her own genetic material who later invade Earth and stumbling across a corpse who turns out to be her older self.

• Robert A. Heinlein's short story "—All You Zombies—" tells of a young man who is taken back in time and tricked into impregnating his younger, female self (before he underwent a sex change), and who turns out to be the offspring of that very union, with the paradoxical result that he is both his own mother and father. In fact, as it turns out, all the major characters in the story are the same person, at different stages of her/his life. This also creates an ontological paradox.

• In the Doctor Who Tenth Doctor Adventures novel The Stone Rose, an ancient Roman statue that looks exactly like Rose Tyler leads to the Doctor and Rose travelling back to Ancient Rome. When their adventure concludes without the statue that inspired it being made, the Doctor sculpts it himself. The book also includes a vial of curative liquid which the Doctor acquires in mysterious circumstances. After it is used up, it is recreated based on the remains, and subsequently taken to the place and time the Doctor first found it.

• In Margaret Weis and Tracy Hickman's Legends Trilogy of the Dragonlance novels, the wizard Raistlin Majere traveled back in time to gain the knowledge needed to cross the threshold between god and man. He encountered the evil wizard Fistandantilus (with whose spirit Raistlin had made a pact in his original timeline) and killed him, but Raistlin merely took Fistandantilus's place in the timeline. A prominent example of the predestination paradox in the story came when, in the past timeline, Raistlin entered the Tower of High Sorcery which he inhabited in the future timeline. When entering his private study, he noted that, though the room should have been untouched during the centuries between the timelines, it was more orderly in the past timeline than when he first entered it in the future timeline. When he could not locate what he sought within the room, he grew extremely angry, and in his fury, he disturbed the items in the room to the degree that he found them in the future timeline.

• In Terry Pratchett's Discworld novel The Last Continent, the Archchancellor Mustrum Ridcully explains the paradox neatly to Ponder Stibbons, who fears that any small change (such as stepping on an ant) in the past could destroy the future. Riduclly claims that clearly any changes he makes in the past were ones he is meant to make, demonstrating that 'the inherent paradoxes in time travel can be resolved by a sufficiently large ego'.

• In Harry Harrison's novel Technicolor Time Machine, Barney Hendrickson travels back in time to present his earlier self with a note explaining how to resolve a seemingly insurmountable difficulty. The younger Barney carefully folds the note and puts it in his wallet, expressing his intention to leave it there until he reaches the point in his life where he travels back in time to hand it to his younger self. This prompts some discussion of how the note actually got written, and by whom, which the older Hendrickson dismisses by saying that the note was written by "time" because it needed to exist to allow the predestination paradox to play out. At the close of the novel, Hendrickson also discovers that by travelling back in time to film the Viking settling of America, he actually caused it to occur.

Film

• Movies in the Terminator series deal with predestination paradoxes. In the first movie, Reese, the soldier sent back in time to protect Sarah Connor, the future mother of his commander John Connor, ends up fathering John Connor with her. Paralleling this, the Terminator cyborg sent back to kill Sarah is destroyed, but its components are salvaged to form the basis of the artificially intelligent computer network Skynet that will, in the future, send it back in time on its murderous mission.

• In the film Donnie Darko, the title character is lured out of bed during the night by a six-foot tall bunny rabbit named Frank right before a jet engine falls through the ceiling of Donnie's room. Frank then informs Donnie that in 28 days the world will end. Over the next four weeks, Donnie is instructed to perform acts which benefit himself but ultimately cause the deaths of Donnie's mother and sister (who die on the plane from which the jet engine is ripped), Donnie's girlfriend, and Frank himself. Through a book entitled Philosophy of Time Travel Donnie realizes that the jet engine is an artifact from the future ripped off by a wormhole and sent back through time to kill him and complete a causal loop that will prevent the world from collapsing into a black hole. On the night of the 28th day, history repeats itself, but instead of leaving his house, Donnie remains in bed and accepts his fate, closing the loop.

• In the film Kate and Leopold, Kate McKay (Meg Ryan) lives in the present day (2001) and falls in love with a time traveller from 1876, Leopold (Hugh Jackman). After Leopold returns to his time, Kate also travels to 1876 to marry Leopold and consequently becomes the great-great-great-grandmother of her ex-boyfriend, Stuart (Liev Schreiber).

• Back to the Future uses several different forms of time travel, and primarily deals with the concept of history being altered, or alternate realities being created. However, several minor details deal with the predestination paradox. For example, in 1955, Marty McFly (Michael J. Fox) discovers that he is the one who inspired Goldie Wilson, his town's African American mayor in 1985, to run for office by accidentally informing Wilson of his future in 1955. Also, by playing "Johnny B. Goode" at the 1955 high school dance, Marty becomes responsible for Chuck Berry's rock and roll composing the very song that Marty would learn to play. His friendship with his future parents led his mother-to-be into thinking that Marty is a nice name, implying that Marty inspired his own naming. The two sequels to the movie deal with similar variation, as well as other paradoxes.

• In Bill and Ted's Excellent Adventure, the scene in which Bill and Ted break the "dudes from the past" out of jail relies on a series of actions that they have to remember to perform in the future. Notably, Bill and Ted are able to open the jail cells thanks to their future selves stealing the keys from the past and planting them outside the police station before they get there. Ted's father's missing keys are mentioned at the beginning of the movie, before any of the time travel events occur. "So it was me that stole my dad's keys!"

• In The Butterfly Effect, when his teacher asks him to draw what he wants to be when he grows up, seven-year-old Evan Treborn (Logan Lerman) draws a murderer standing over two corpses with a bloody knife. He quickly "blacks out" the memory of having drawn the picture and never sees the drawing afterwards. In response to the picture, Evan's mother takes him to a doctor, who suggests he write about the incident in a journal. As an adult, Evan (Ashton Kutcher) uses the journal to return to the past and draw the picture. Also, seven-year-old Evan visits his father Jason in a psychiatric hospital and "blacks out" the few minutes that lead Jason to attack Evan until a guard accidentally deals a lethal blow to Jason's head. Later, in order to speak to his dead father, Evan uses his journals to relive the visit, during which he provokes the attack.

• In Timerider, Lyle Swann (Fred Ward) accidentally gets sent back in time to the old west without him knowing it. There, he meets a beautiful woman in a small town and has sex with her. By the end of the movie, it is evident that the beautiful woman is Lyle's great-great-grandmother and he is his own great-great-grandfather, thus setting up the circumstances for his own birth and, therefore, setting up the circumstances for him to travel through time.

• In Happy Accidents, Sam Deed (Vincent D'Onofrio) in 2470 falls in love with a woman, Ruby, in a photograph he finds. He travels back to 1999 to prevent her death. They fall in love and he eventually convinces her that he comes from the future, but she does not believe his warnings about her death. On the appointed day, Ruby is distracted as she looks at a photograph of herself and Sam at the beach; she is nearly run down by a car but Sam manages to save her. It transpires that the out-of-focus picture is the one Sam finds in the future that inspired him to return to the past. However, in saving Ruby and breaking the loop he creates an ontological paradox, as his knowledge of Ruby's accident in the original timeline no longer has an origin.

• in Spongebob Squarpants episode SB-129 Squidward runs from spongebob and patrick because he does not wan't to jellyfish with them. HE is frozen and then later unfrozen in 2000 years. Eventually he finds a time machine, but goes to far into the past finding himself in the past. But, unfortunatley for squidward, to far in the past. He finds himself with cavemen versions of spoengebob and patrick that annoy him with yelping. He sees them touching a jellyfis reapetedly (and getting a painful sting each time) and he makes them a crud jellyfish net. He eventually works the machine and gets back home. He, of course, is then bothered by spongebob and patrick to jellyfish. He gets annoyed and asks what dunderhead invented this stupid sport anyway, to which they reply, "You did, squidward!"

Television

• In "Roswell That Ends Well", an episode of Futurama, Philip J. Fry travels back in time. He is instructed by Professor Farnsworth not to do anything to change history, "like killing your own grandfather". Fry is so determined to protect a reckless Enos (who he believes is his grandfather) from harm that he hides him in an abandoned building. The building turns out to be part of a nuclear weapon test which kills Enos. Fry then has sex with Enos's fiancee Mildred, believing that she cannot be his grandmother since he still exists. However, Farnsworth points out that in doing so, Fry has become his own grandfather.

• In the Star Trek: Voyager episode "Relativity", Captain Braxton of the future timeship Relativity recruits Seven of Nine to prevent the USS Voyager from being blown up by a temporal intruder. Her first two attempts are unsuccessful, and she ends up recruiting Captain Kathryn Janeway to find the intruder who planted the bomb. The intruder turns out to be a future version of Braxton, seeking revenge against Janeway, whom he blames for interfering with the timeline on numerous occasions and causing him to endure a 30-year exile on 20th century Earth (as seen in the episode "Future's End"). The First Officer on the Relativity arrests the present-day Braxton for "crimes he will commit," and promises Janeway that he will clean up the timeline. How this is to be done, however, or whether the events of the episode will continue to exist if he does so, is never explained. Here, the paradox was called the Pogo paradox (after the phrase "We have met the enemy and he is us" from the Pogo comic strip).

• In the Star Trek: The Next Generation episode "Captain's Holiday", Jean-Luc Picard is contacted by two Vorgons from 300 years in the future. They claim that he is destined to find a powerful weapon that was stolen and hidden in the past, the Tox Uthat. Compelled by this prophecy, Picard finds it, but on discovering that the Vorgons were the ones who stole it in the first place, chooses to destroy it instead. The Vorgons then admit that this was what history had actually recorded and their attempts to change it for their own gain failed.

• In the Star Trek: The Next Generation episode "Time's Arrow", the USS Enterprise is recalled to Earth because Data's severed head has been discovered in an abandoned mine shaft underneath San Francisco from 500 years in the past. While investigating possible causes, Data is sent back to 19th century San Francisco, and after a battle with aliens who are sucking brain energy from humans, his head is severed and the mine shaft collapses.

• In the Star Trek: Deep Space Nine episode "Trials and Tribble-ations", the USS Defiant is sent back through time to the 23rd Century, where they become embroiled in a plot to kill Captain James T. Kirk. In so doing, they end up effectively participating in the Star Trek (the Original Series) episode "The Trouble with Tribbles". A humorous reference is made to the predestination paradox when Julian Bashir suggests that he is meant to have sex with a Lieutenant Watley (who he thinks is one of his ancestors) so that he can become his own great grandfather, lest he cease to exist.

• In the movie Star Trek: First Contact, a Borg ship travels to 2063 from the 24th century to prevent humans from making contact with aliens. The USS Enterprise follows the Borg into the past and destroys the ship. However, debris and some Borg survivors land in the Arctic where they go into suspended animation. Accidentally revived several decades later in the Star Trek: Enterprise episode "Regeneration", these Borg escape Earth and send a message to the Borg Collective of that period. It is implied that this signal may be why the Borg decided to invade the Alpha Quadrant in the 24th century. This disregards the fact that the Borg originally gained Earth's coordinates from the Enterprise's computer in the episode Q Who?.

• In the Stargate SG-1 episode "1969", a wormhole transports the SG-1 team to 1969, where they are arrested as communist spies. One of their guards, Lieutenant George Hammond, who will be their commanding officer in the future, finds a note in Samantha Carter's equipment. The note, in Hammond's own handwriting, states, "George, help them." Because of this, the younger Hammond helps SG-1 escape. In his relative future, General Hammond will remember the incident and write the note, giving it to Carter just prior to SG-1 leaving through the wormhole, thus closing the loop.

• In the 1973 Doctor Who serial Day of the Daleks, guerillas from the 22nd century travel back in time to prevent their own future from coming to pass. However, they discover that it is their actions that actually cause that future to happen. In that case, the loop is broken, not by them, but by the Doctor. As his existence is not dependent on the loop, he is not caught in the paradox and can act freely, his actions presumably causing that future to cease to exist.

• In the 2005 Doctor Who episode The Parting of the Ways, the Doctor's companion, Rose Tyler, absorbs the energy of the spacetime vortex to save the Earth from the Daleks. She also uses the power to scatter the phrase "Bad Wolf" throughout history as clues, to lead her past self to the position where she will absorb the energy of the vortex to save the world.

• In The Flipside of Dominick Hide, Dominick Hide is a time traveller assigned to learn about 20th century transport systems. Learning that his great-great-grandfather lived in the time and place he is studying (London in 1980), he breaks the rules of his job in order to track down his relative. Though he does not find him, he does have an affair with a woman which leads to her giving birth to a son, Dominick's great-grandfather. Thus the great-great-grandfather Dominick was searching for was in fact himself. It is eventually revealed that Hide's superior knew about the loop and deliberately allowed Hide to break the rules in order to fulfill it.

• In the Red Dwarf episode "Future Echoes", there are several predestination paradoxes caused by images of the future appearing in the present. The first echo occurs when Lister is shaving, and his reflection shows him cutting himself. This distracts him, causing him to cut himself. Later, Lister has a nonsensical conversation with Rimmer, unaware he is talking to a future echo. When the real Rimmer enters the room, Lister's attempts to work out what's happening lead to Rimmer making the same responses heard in the echo. Lister later sees a future echo of the Cat with a broken tooth, and decides that if he can prevent this happening, he can prevent other events seen in the echoes. Realising the Cat is trying to eat a robot goldfish, Lister tackles him to the floor before he can bite, breaking the Cat's tooth in the process.

• In the Red Dwarf episode "Ouroboros" Lister encounters an alternate reality where Kochanski survived instead of him, after adding his contribution to an in-vitro tube he finds a supplies case labeled ouroboros (which was also written on the cardboard box he was abandoned in). He then realises that he is his own father, and when the child is 9 months old he goes back in time and places the child where he was left.

• The eponymous clairvoyant computer in the Red Dwarf episode Cassandra lies about what she has seen in an attempt to get revenge on Lister, since he will kill her. Her subterfuge leads directly to Lister inadvertently killing her, after declaring his intention not to. Cassandra's genuine predictions include the concise predestination paradox "You die in about four seconds time of a heart attack, after hearing the news that you're going to die of a heart attack".

• In the Twilight Zone episode "No Time Like the Past", the main character uses a time machine to go back in time to alter past events. After failing to warn a Hiroshima police captain about the atomic bomb, assassinate Adolf Hitler, and change the course of the Lusitania to avoid being torpedoed, he accepts that the past cannot be changed. He then uses the time machine to return to the town of Homeville in the year 1881. After reading in a history book that Homeville's schoolhouse will burn down because of a kerosene lantern thrown from a runaway wagon, he spots the wagon and attempts to prevent the fire, but instead causes the fire he intended to prevent.

• In the Twilight Zone episode "A Most Unusual Camera" three thieves acquire a camera whose pictures foretell the future. After a French waiter translates an inscription on the camera, there is a struggle between them during which a picture is accidentally taken of one of the thieves in which she is screaming. This causes the other two thieves to engage in another struggle, this time resulting in their falling out of the window into the courtyard below. The last remaining thief then takes a picture of the bodies in the courtyard and when the picture develops sees four bodies and not two. Upon seeing this she rushes to the window, trips, and falls out of it. The waiter also manages to fall out, but is not visible at the time.

• In Babylon 5, it is established that the titular station's predecessor Babylon 4 vanished several years before. When it mysteriously reappears in the series' first season (taking place in 2258), it appears to have been dragged through time. The crew of Babylon 5 manage to evacuate those on Babylon 4 before it vanishes again ("Babylon Squared"). Two years later, led by a letter left in the distant past by a Minbari prophet named Valen, the crew of Babylon 5 realised that it was they who "stole" Babylon 4 in the first place to serve as a base of operations for a war a thousand years in the past ("War Without End"). They then travel back in time to accomplish this before Babylon 4 can be destroyed by their enemies, trying to avoid their earlier selves when the station gets shunted to 2258. The former Babylon 5 commander, Jeffrey Sinclair, stays behind and takes Babylon 4 back to 1260, where it fulfils its place in Minbari history. Sinclair is also transformed during the journey into a Minbari, and introduces himself to those he meets in the past as Valen, eventually leaving the prophecies that will guide his future friends and self full circle.

• Also in Babylon 5, Londo Mollari has a recurrent prophetic dream in which he kills, and is killed by G'Kar. To avoid this future, he sparks a genocidal war by his own Centauri people against G'Kar's Narn, but in the long run, this leads to his own and his race's enslavement by the Drakh. After years, he captures John Sheridan and Delenn, and explains the situation to them. He sets them free, in return for their promise to liberate his people. But the only way this plan can succeed is to keep it from the Drakh, who have implanted him with a Keeper that can read his mind. He realizes that the only way to save his people is for G'Kar to strangle him; the violence awakens the Keeper, forcing him to fight back, and so the prophecy is fulfilled.

• In a Bionic Six episode, when Professor Sharp creates a time portal to send a team to pre-history to learn which power killed the dinosaurs, his brother, Scarab, sends henchmen to steal this power. In the end, nobody learns what killed the dinosaurs, but its hinted that they were killed by the radiation that came out from a gun the villains left in the past.

• In the Hong Kong TV series A Step into the Past and the book it was based on, the main character Hong Siu Lung travels back in time to the Warring States Period of China and accidentally changes a few events. He thinks he has altered history, but in the end he creates history by helping the tyrannical Qin Shihuang rise to power.

• In an episode of The Twilight Zone entitled Cradle of Darkness (2002) a woman named Andrea Collins travels back in time in order to kill Adolf Hitler as a baby. In order to do that she commits suicide, jumping from a bridge holding the baby. However, a maid from the Hitler household sees the whole thing and replaces the child with a baby she bought from a gypsy. That baby grows up to be the evil Adolf Hitler we all know. Therefore, Andrea sacrificed her life for the good of mankind, but she also created the very monster she sought to destroy.

Video games

• The console RPG Chrono Trigger is famous for exploring several varieties of time travel ethics, including predestination, mostly when it comes to the creation of the Masamune. After acquiring the two broken halves of the sword from Frog and taking them to Melchior in 1000 AD, he informs you that it needs Dreamstone to be repaired, which no longer exists. Later, before going into the Undersea Palace in 12,000 BC, the same Melchior's past self gives you a ruby knife with which to destroy the Mammon Machine, both being made from the same material. When the knife stabs into the machine, Lavos' power transforms the knife into the great sword which would become known as the Masamune in the centuries to come.

• In the first-person shooter TimeSplitters: Future Perfect, the player character Sgt. Cortez often meets a near-future version of himself who helps him progress with the game. Later on the player must perform that exact role to help his past self. One such example is at one point on the level 'Scotland the Brave', there is a locked door which Cortez cannot get through. This is sorted however when he looks up through a grate to see a future version of himself. After a brief conversation, the future version hands him the key to the door. Later in the level, he travels through a time portal and looks down a grate to see the past version of himself by the locked door. This is when he hands the past version the key to the door.

• In the Legacy of Kain series, the character Raziel is born a human and lives the life of a religious zealot that, along with his Sarafan brethren, eventually meets his end at the hands of a demonic abomination. Raziel is resurrected as a vampire by Kain, who later sentences him to burn in the Abyss. Rising again, and following a quest for vengeance against Kain that covers the five games and time travel, Raziel discovers that it was he, in his wraith form, who killed himself and the other Sarafan, and that Kain had sentenced him to the Abyss to fuel the chase through time that would bring Raziel full circle for a larger purpose, making Raziel escape the clutches of the otherwise all-dominating wheel of fate that dictates the lives of all living creatures in Nosgoth. Raziel eventually succumbs to his fate of becoming the soul-harvesting spirit within the Soul Reaver blade that was previously merged metaphysically with him.

• In Clive Barker's Undying, the character Patrick Galloway discovers a journal that mentions a powerful mystical weapon, the Scythe of the Celt, which was stolen from a now-ruined monastery hundreds of years in the past, and describes a means of travelling in time via a magical portal. The journal writer intended to use the portal to take the Scythe before it was stolen but was mortally injured before he could do so. Using clues in the journal, Galloway travels back to a time when the monastery was intact and populated and steals the Scythe, becoming the very thief mentioned in the journal. The theft of the Scythe also releases energy that causes the destruction of the monastery whose ruins Galloway finds in the present day.

• In Shadow of Memories, the predestination paradox and many other time travel concepts are explored as the protagonist, Eike Kusch, is murdered by a mystery assailant, and he is empowered to prevent his own demise via time travel. In fact, one of the endings results in an alchemist from the past making a wish to be, forever more, just like someone of Eike's character, having met him previously. The wish comes true, with the alchemist being metamorphosed into an exact duplicate of Eike; speculation is that the Eike the player controls, is the future self of the alchemist, having lived immortally for centuries as Eike, hence why his survival is so important to Homunculus.

• In Prince of Persia (Ubisoft), The Sands of Time and Warrior Within, the Prince is able to manipulate time, and the effects or causes of time travel are explored (with a light air in Sands of Time, and with a more far-reaching philosophical logic in Warrior Within) and the very nature of causality is questioned. However, all manipulations seamlessly integrate themselves into the timeline, such that when one is sure that one has just cheated fate (the main theme in Warrior Within), one finds oneself merely fulfilling one's own destiny. Also seen is a self-fulfilling prophecy by the "Empress of Time" that predicts that the Empress will die at the Prince's hand, prompting the Empress to decide to engage the Prince in battle to try and prevent her own death by killing the Prince first. She underestimates the Prince and is killed, and the same event releases the "Sands of Time" that start the Prince's ordeal in the first place.

• In The Legend of Zelda: Ocarina of Time, the main character Link, who is the destined "Hero of Time," goes back and forth between a seven-year time period to save Hyrule from the evil Ganondorf. At one point in the game, as an adult, Link enters a windmill in Kakariko Village where a man teaches him the Song of Storms, which he says "messed up the windmill seven years ago when some kid played it". Then in the past, Link plays the song inside the windmill, causing it to speed up, thus "teaching" the man the same song Link had learned from him in the future.

• In Final Fantasy VIII, it is because of the direct actions of Ultimecia, knowing of her destiny to fall at the hand of Squall, that the events of the game take place. The main character, Squall, ends up briefly in the past in front of the orphanage he grew up in. By telling the matron of his origins, he ensures the creation of Garden and his appointment as commander of the SeeD forces -- the very events that Ultimecia wanted to prevent.

• In the PC game Escape From Monkey Island, the player character Guybrush Threepwood travels through a special time-travel-inducing swamp. There, he meets his future self, who hands him three items and speaks certain phrases to him. The object of this part of the game is to ensure the proper conversation and item-exchanging occur in the same order both when the player is controlling past-Guybrush, and later when the player controls future-Guybrush. Otherwise, a time storm erupts. The three items, of course, are trapped in an endless loop of changing-hands. An interesting side-note is that often one of the items will be a gun. If it is, when the player controls past-Guybrush, he may choose to shoot the future-Guybrush with the gun. However, later when the player controls future-Guybrush, the past-Guybrush inevitably shoots him.

• In Breath of Fire, Ryu and his party encounter an exact duplicate of fellow party member Nina, but dressed in blue rather than pink. The duplicate reacts with confusion to any attempt to speak with her. Later in the game, Nina is pulled into a time stream and separated from the group, and the duplicate is then revealed to be the future version of Nina after she had been pulled through time and become significantly more powerful.

• In Ecco the Dolphin, the eponymous protagonist Ecco meets an ancient creature named the Asterite, who seems to recognize him. The Asterite is willing to help Ecco on his quest, but is unable to, as one of the globes that comprises its body is missing, so it sends Ecco back into a prehistoric era to find it. In the past, Ecco finds a younger version of the Asterite, who attacks him. Ecco fights against the younger Asterite, and in the process takes one of its globes, at which point he is sent back to the present, where he gives the Asterite back the globe that he had stolen millions of years before.

• In the Futurama video game, the crew is sent back in time, after being chased through the universe by Mom and her sons. They are fleeing Mom because the Professor sold Planet Express to Mom, giving her 50% majority ownership of Earth. Thus, under Earthican law, she owns the world and its population. Back in time, they attempt to stop Mom from completing her plan, resulting in their deaths. Having no crew or ship, Prof. Farnsworth reluctantly sells, at which point gameplay restarts at the first scene. Also, at the beginning of the game, when the crew tries to escape, they discover that their ship has been badly damaged. They fix it, but later in the game, they fly their ship into the sun, which results in it getting damaged. After they travel back in time, Fry remarks that they now know who damaged their ship. They then take the other ship that has not yet been damaged, leaving the destroyed ship to their past selves.

• In Shadow Hearts: Covenant, Karin is given a photograph that depcits her as Yuri's mother. Because of this photo, instead of returning home like the other characters, Karin stays in the past and eventually gives birth to Yuri. Karin also assumes the alias of "Anne" as Yuri told her that was his mother's name. Yuri also added that he was named after his mother's first love — Yuri himself. Yuri also gives Karin his mother's cross which is taken back in time and eventually given back to Yuri so that he can close the loop, creating an ontological paradox in relation to the cross.

• In Valkyrie Profile, the valkyrie Lenneth visits Dipan, a city that had thrived in centuries past but in the present is in ruins. Upon arriving, she is attacked by the ghost of the deceased king, who accuses Lenneth of killing him and destroying his kingdom. Lenneth has no recollection of this, as she had never been to Dipan before. After fighting the king's ghost, she explores the ruined palace and is sent back in time to the day of the king's execution. While there, Lenneth discovers that her sister Hrist (who looks very similar to Lenneth save for the color of her hair and armor) had come to the city under Odin's orders to execute the king for attempting to discover immortality. Lenneth then locates the queen, who gives her the king's crown and explains how the king's three mages had tricked him. Before Lenneth can return the crown to the king, he is executed by Hrist, but it is at that point that Lenneth returns to the present. She returns the crown to the king's ghost, putting him to rest, and then hunts down the three (now immortal) mages that had tricked the king.

Machinima

• In the machinima comedy series Red vs Blue: The Blood Gulch Chronicles, the character Church is sent into the past by a massive explosion. After travelling forward again, but to his recent past, he attempts to prevent the events of the previous seasons, primarily the deaths of himself and his on-again-off-again girlfriend Tex. Most of his efforts backfire, and he not only is unable to prevent Tex's death (and becomes responsible for his own), but also becomes responsible for nearly every main event that occurred during the series (including his travel through time as well). Finally, after a series of failed attempts to fix the past, Church gets into the explosion and is sent back to the distant future. On arrival, his attempts to remake the past are undone, since he never went to the past, and the timeline is restored.

Miscellaneous

• Black Sabbath's song Iron Man tells the story of a man who travels to the future and witnesses the end of the world. He then travels back to his own time to attempt to warn the people, but in his travels he enters a magnetic field, the likes of which turn his body to iron. His appearance causes those he warns to fear him rather than heed is warning, and so he ends up causing the very apocalypse which he tried to prevent by going on a frustrated rampage and causing mass destruction.

• In the DC Comics crossover storyline Our Worlds At War, an energy being called Imperiex believes there is an imperfection in the universe, and plans to break it down and create a new one. The storyline concludes when the forces allied against him manage to send both Imperiex and Brainiac-13 to moments after the Big Bang, where they merge with the energies already there. Imperiex then realises that this was the imperfection he detected.

See also

• Newcomb's paradox
• Ontological paradox
• Time travel in fiction
• Grandfather paradox
• Self-fulfilling prophecy
• Pogo Paradox at Memory Alpha