| abstract
| - A predestination paradox (also called causal loop, causality loop, and (less frequently) 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 traveler is caught in a loop of events that "predestines" or "predates" them to travel back in time. Because of the possibility of influencing the past while time traveling, one way of explaining why history does not change is by saying that whatever has happened must happen. A time traveler attempting to alter the past in this model, intentionally or not, would only be fulfilling their role in creating history as we know it, not changing it, or that the time-traveler's personal knowledge of history already includes their future travels to their own experience of the past. In layman's terms, it means this: the time traveler is in the past, which means they were in the past before. Therefore, their presence is vital to the future, and they do something that causes the future to occur in the same way that their knowledge of the future has already happened. It is very closely related to the ontological paradox and usually occurs at the same time. If Marty had returned to 1985 without noticing any changes in Hill Valley, then the conclusion could be drawn that he had been predestined to be part of its history 30 years earlier. However, at the end of Part I, Marty returns to a Hill Valley different than the one he left, with his family happy and successful, and Biff Tannen answering to George McFly. The predestination paradox is controversial because it seemingly negates the "common sense" notion that we are responsible for our own destinies and, with adequate knowledge and preparation, we can alter their courses. The predestination paradox says the exact opposite, leaving doubt in believers' minds if they really have free will. 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. On the other hand, it's clear that before Marty altered history, Goldie Wilson had been inspired by another event to run for Mayor; that Chuck Berry had already written "Johnny B. Goode" after 1955, and that Lorraine had heard the name "Marty" in some other context and thought it was nice. Put another way, those events were destined to happen whether Marty went back into the past or not. Marty's contribution, if any, would have been to reinforce Goldie's ambition, or Lorraine's liking of a particular name. The closest thing to a predestination paradox might be near the end of Part I, as Marty, having returned to 1985 from 1955, runs across town to the Lone Pine Mall, arriving too late to prevent the Lybians from shooting Doc, and then watching his other self vanish while driving the DeLorean. Since the other Marty hasn't yet departed for 1955, the other Marty hasn't yet altered history by crashing into one of the two twin pines at Old Man Peabody's ranch, and the ripple effect hasn't yet happened, the sign should still say "Twin Pines Mall" as Marty rushes up to it-- unless Marty was predestined to run over the pine tree. However, it should be noted that the ripple effect isn't instantaneous. It's called the ripple effect because the changes in the time stream are affected like throwing a pebble into a pond. The earliest changes are affected first, and the time traveler (due to being in a temporary time bubble) is affected last. Since Marty hit the pine tree when he first traveled to 1955, the effects of that event have already rippled through time. If the other Marty lives in a timeline where history had already been altered by the first Marty's presence, then he would encounter the first Marty if he, too, will be arriving at the Twin Pines Ranch on the early morning of November 5, 1955. Since Doc of Timeline 2 (who met the first Marty in 1955) would have had nearly 30 years to contemplate this paradox, then it's possible that in that timeline, Doc would send the other Marty to a date other than during the eight days of November 5 through 12th, 1955. The film leaves the question open, since only the departure from Lone Pine Mall is seen, and the setting of the time machine at the Lone Pine parking lot was taking place while the first Marty was running there from Hill Valley. However, the Marty that left the Twin Pines Mall is now the same Marty that left the Lone Pine Mall, as he had taken his place. Once the time ripple catches up with a time traveler, the other time traveler is erased from existence. In the new timeline, the Marty that left the Twin Pines Mall overwrote the Marty that left the Lone Pine Mall (possibly when he began to fade from existence at the Enchantment Under the Sea dance in 1955). Doc likely just used his knowledge of the events to make sure that everything played out nearly identically so that Marty would still be sent back to 1955. If he chose another date, Marty would not be able to know when lightning would strike in order to use it to generate the 1.21 gigawatts of power needed to return to the future. The hint at a predestination paradox is not obvious in the two sequels to the film. The details about 2015, 1985A, and Timeline 8 in 1985 are limited to what can be seen for a few minutes. Arguably, Part III seems to disprove the theory of a predestination paradox somewhat, in that Marty avoids a collision with a Rolls-Royce Silver Cloud III that would have led him to a miserable life in 2015, however Doc tells Marty: "Your future hasn't been written yet. Nobody's has!", possibly indicating that events that have already transpired cannot be largely altered while events that have yet to occur can still be changed from the present. Also of note is that in Part III, Doc ends up saving Clara Clayton from falling to her death in 1885 (seemingly undoing a predestined event), however after he builds a time machine in the past he takes her with him to the future, effectively maintaining her fate of no longer living in the past as he instead moved her with him to the future (although with a 10 year gap from her original time of death). Though not canon with the Back to the Future trilogy, a video from The Simpsons Ride contains a predestination paradox which involved Professor Frink going into the past to discover how and why The Institute of Future Technology was replaced by Krustyland. He arrived into the past and accidentally ran over Doc's banker Mr. Friedman with the DeLorean. Frink shortly discovered that the accident he caused is responsible for the very thing he came to investigate as Doc found himself having to sell The Institute to Krusty the Clown.
- 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. It exists when a time traveler 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 traveling, one way of explaining why history does not change is by saying that whatever has happened was meant to happen. A time traveler 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.
- In 2268, after having traveled back in time from 2373, Doctor Julian Bashir speculated that the events in which he was participating constituted a predestination paradox when he met USS Enterprise crewmember Lieutenant Watley. Realizing that his great-grandmother's name was Watley, and that nobody ever met his great-grandfather, Bashir began to suspect that he was destined to fall in love with Watley and become his own ancestor. Bashir's suspicions were later proven incorrect, however, in that he did not pursue a relationship with her, a decision which had no effect on the timeline. Agents Dulmur and Lucsly of the Department of Temporal Investigations, who were sent to investigate the incident, professed to hate predestination paradoxes and were relieved to find out that the incident was not one of them. (DS9: "Trials and Tribble-ations") In 1968, the crew of the Enterprise, who had traveled back in time from 2268 to perform historical research, encountered a Human named Gary Seven, who was attempting to sabotage a nuclear warhead platform. Fearing that Seven was an alien from the future attempting to manipulate the timeline, the Enterprise crew attempted to stop him, but Seven was eventually able to convince them that he was indigenous to the 20th century, and that he was in the employ of a benevolent race of aliens attempting to protect Earth. Later, upon checking their historical records, Commander Spock discovered that the crew of the Enterprise was, in fact, always a part of this series of events, and that, by interfering, they were simply fulfilling their role in history. (TOS: "Assignment: Earth" ) In 1986, needing money after travelling back in time from the year 2286, James Kirk sold a pair of glasses which had been given to him in 2285 by Leonard McCoy as a birthday gift. When Spock expressed his surprise at Kirk's actions, Kirk asserted his belief that they would wind up in his possession again. (Star Trek IV: The Voyage Home) On several occasions Captain Jean-Luc Picard attempted to prevent a predestination paradox from forming by ordering the USS Enterprise-D to stay on its current course, fearing that changing course to avoid a time loop may be the very thing that got the ship stuck in the loop in the first place. (TNG: "Time Squared" , "Cause and Effect" ) In another predestination paradox, two time-traveling Vorgon thieves from the 27th century arrived in 2366 to obtain the Tox Uthat from Captain Picard, who was known to have discovered and destroyed the device on his holiday on Risa. Their attempt to obtain the device was, in the end, the reason why Picard destroyed the Uthat, so that it would not fall into the Vorgons' hands, thus completing the paradox. (TNG: "Captain's Holiday" ) A predestination paradox occurred on Earth in the year 1893, when Enterprise-D crewmember Lieutenant Commander Data, who had traveled back in time from 2368, became trapped in a temporal vortex and had his body separated from his head, which remained in a cavern under the Presidio in San Francisco until 2368. In that year, Data's head was discovered, leading the crew of the Enterprise to mount an investigation. This investigation led to the very time travel event which transported Data back in time in the first place, thereby completing the time loop. (TNG: "Time's Arrow" , "Time's Arrow, Part II" ) In 2371, the devastion of a planet in the Delta Quadrant due to a polaric ion explosion occurred due to a predestination paradox. Kathryn Janeway and Tom Paris were transported back through time by a subspace fracture to the day the explosion occurred. There, a subspace fracture opened and intersected with a polaric conduit and resulted in an explosion. The shock wave from the explosion dissipated into the past, which shattered subspace and created the subspace fractures in the first place. The following day, Voyager arrived at the planet, where the fractures were floating across its surface like icebergs. After Janeway and Paris disappeared through one of the fractures, the crew of Voyager constructed a device that could open the subspace fractures as a means of rescuing their crew. Ultimately, the crew opened a subspace fracture at the location of the explosion, which lead to the explosion that created the fractures and lead to the deaths of Janeway and the planet's natives. This time loop was broken when Kes experienced a vision of what happened. This allowed Janeway to close the fracture before it intersected with the conduit, resulting in the creation of an alternate timeline where the explosion never occurred and Voyager never visited the planet. (VOY: "Time and Again") Captain Braxton, a Starfleet officer from the 29th century, described this type of paradox as "A leads to B leads to C leads to A," when attempting to explain the series of events which led to the destruction of Earth's solar system in his time to the crew of the USS Voyager. Finding debris from Voyager in the aftermath of the temporal explosion that destroyed the solar system, Braxton came to believe that Voyager was somehow responsible for the incident. In an attempt to change history and prevent the explosion, Braxton traveled back in time to 2373 to destroy Voyager. This action caused both ships to be transported back to 20th century Earth, where Braxton's timeship was stolen by an unscrupulous industrialist named Henry Starling. It was at this point that Braxton realized that it would be Starling who would cause the explosion, by taking the timeship to the 29th century without properly recalibrating its temporal matrix. This time loop was eventually broken when the crew of Voyager was able to destroy the timeship moments before it traveled to the future. (VOY: "Future's End", "Future's End, Part II")
- Predestination Paradox is a term for a paradox of time travel. The term also can be used to explain that Prime Evil never really knows what will be even though he is in the future.
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