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A temporal paradox is a paradoxical situation in which a time traveler causes, through actions in the past, the exclusion of the possibility of the time travel that allowed those actions to be taken.

The typical example is that of the grandfather paradox, wherein a time traveler goes back in time and kills his grandfather before his father is conceived. It is a paradox because if this occurs, he will never be born, and therefore never be able to travel back in time to kill his grandfather, thus allowing himself to be born.


Currently, temporal paradoxes are the domain of science fiction and philosophy. Various schools of thought exist as to what would happen in the grandfather paradox were a time travel device ever invented.

Timeline protection hypothesis

The timeline protection hypothesis posits that a time traveller would be incapable of causing a paradox, as the natural continuity of the timeline would cause any such interaction to fail. For example, he would appear in the wrong place (or fail to go anywhere at all), various twists of fate would conspire to impede him, or his actions would instead be responsible for the history he remembers.

A similar theory states that time travel may in fact be a factor in making the universe the way it is today; actions of time travelers are responsible for the present situation. Alternatively, any time traveller would take the utmost care in avoiding such paradoxes, inspired by the fear of the damage they could cause.

The animated television series Futurama shows a more lighthearted side of the paradox. In the episode "Roswell That Ends Well", the main character, Philip J. Fry, travels back in time with his friends to 1947 in Roswell, New Mexicomarker. Remembering that his grandfather works at the base, and told that killing the man would nullify his own existence, Fry becomes obsessed with protecting him. Fry's efforts prove counterproductive: he locks the man in a shack to protect him, failing to realize that an atomic bomb is being tested on the grounds. When he doesn't disappear, he assumes that the man could not have been his grandfather and thus proceeds to sleep with and impregnate his grandmother, thereby becoming his own grandfather.

1972 "Doctor Who" adventure Day of the Daleks. Sir Reginald Styles is targeted by 22nd Century guerrillas, who believe he's behind the deaths of VIP delegates. Because of those deaths, the Daleks were able to take over Earth in their time.In truth, a fellow guerrilla who was left behind was to blame, which was the true cause of their timeline ensuing.

In the machinima series Red vs. Blue, Church, one of the characters is sent back in time via the combined energy of a bomb and a weather machine. He attempts to fix past events, as well as preventing the explosion, but his attempts are ultimately responsible for most of the events that took place beforehand in the series, including his own death.

In the Japanese manga Doraemon, the lead character's grandson came back in time to meet his grandfather in his primary school days intentionally to change the life he is in. The lead character questioned the existence of his grandson that if he did married the girl he likes (instead of the one he dislikes but conceived the father of his future grandson), what would happen to his grandson. His grandson along with Doraemon replied that there are multiple paths leading to the same future, and they will still exist even if the lead married another girl. The plot never explicitly told of when the history was altered, but later events in the plot did showed a future where the lead married the girl he likes and lived a better, wealthier life and yet, future characters showed no signs of remembering the original history. However, when being ask of the reason of Doraemon's reason of being in the past, he will reply his role is to make sure the lead marries the girl he likes, which mentally still reserves logic.

Timeline corruption hypothesis

Another idea is that any change in the timeline, even without personal interaction, while allowable, would cause a "butterfly effect" in the timeline. All history after the time the traveler visited would be affected by minute changes the traveler had made in the past, and the history, depending on how severe the time traveller's actions were, would sooner or later be completely changed. This has been coined the "timeline corruption hypothesis." The 2004 film The Butterfly Effect and the Multiverser RPG system prefer this view. There's also the Ray Bradbury science fiction short story "A Sound of Thunder", in which the butterfly effect is caused by a real butterfly.

The most well-known example of this theory is the 1985 film Back to the Future, in which the protagonist Marty McFly goes back in time and interferes with his parents' first romantic encounter, thus erasing his own existence (as well as that of his family). However, the effect only happens gradually, exemplified by a family photo in his possession: each of his siblings begins to disappear limb by limb, starting with the oldest and working down to him (Marty is the youngest of the three). This allows Marty to correct the error and restore the timeline, albeit with a few minor changes that are due to his interference.

This idea also appears in a Family Guy episode, in which Peter goes back in time with the help of "Death" so that he can relive his teen life. When he arrives in the past, rather than spending the time with his present-day wife, Lois, Peter ignores her. His actions cause a corruption in the timeline, and when Peter and Brian (who went with him to the past) return to the present day, all of reality is radically different. But Brian observes that this altered reality is an improvement, as he comments on the better health care system, the lack of violence on the streets, and the fact that Al Gore is President and both Karl Rove and Dick Cheney are dead.

Philip K. Dick also explored timeline corruption paradoxes. In the story "Orpheus with Clay Feet" Slade, a character from the future, goes on a time travel vacation to the past where he can visit famous science fiction writer Jack Dowland and become his muse. Slade, however, fails to inspire Dowland as he had hoped, and Dowland never becomes the master he should have been.

Multiple universes hypothesis

Another hypothesis is that there are an infinite number of universes, one for each possibility. Here, should the time traveller kill his grandfather, one universe would have a live grandfather, and another universe would have a dead one. Failing that, the universe would annihilate itself, for such a paradox would defy its laws.

Another theory concerning the classic grandfather paradox is that such an event would create a new universe, one in which the aforementioned deed was committed. This would not affect the committer's universe, nor the committer himself.

An example of this occurs in the Japanese anime series Dragonball Z in which Trunks, the son of Vegeta and Bulma comes to the present from the future. In the present, Trunks wasn't even born. He warns of the arrival of androids which are more powerful than even Goku and Vegeta, and forewarns the death of Goku in his past. However, by giving the medicine which can cure Goku and undertaking extensive training, Trunks and the Z Fighters manages to defeat the androids as well as Cell. When Trunks goes back to his present age though, his universe is still the same with Goku and other Z-fighters still dead, the only difference being that at that point of time, Trunks, because of his training with the Z-fighters, had become strong enough to defeat the androids of his own time.

Temporal merging hypothesis

This is the opposite of the multiple universes hypothesis, in that each action committed in time travel actually overlaps one reality with another. For instance, if a time traveler were to meet his double from another time, the double would merge with the time traveler, making the traveler a part of the time he is visiting. The same would hold true for events. Two events would merge into the nearest event which does not produce a paradox (a dead grandfather in one universe but not in another would either create a dead grandfather in both universes, but alter the person's heritage so as to allow this, merge both timelines so that the person would fade from all timelines upon return, or produce a mean between life and death such as a coma).

An example of this is seen in the film The One, in which a character travels across time or dimensions, destroying copies of himself to cause them to merge — thus increasing power for the original character.

Choice timeline hypothesis

In the choice timeline hypothesis, history changes the instant the time traveler decides to travel back in time, thereby rendering his actions in that regard pre-destined.

Bill & Ted's Excellent Adventure, as well as its sequel and animated adaptation, feature numerous uses of this hypothesis. Bill and Ted, constantly realizing that their plans are foiled by the lack of a certain item, decide to later travel back in time and deliver themselves the necessary item, often indicating a specific place in which the item will appear. Upon searching the location, the item is invariably there. This is used to somewhat ridiculous proportions in the second film, Bill & Ted's Bogus Journey, where the characters quite literally make objects appear out of thin air with this method.

Can-Not Because Has-Not theory

This theory states that the present is not the forefront of time, and so we are our future selves' past. Thus, if sometime in the future a time travel device were created, someone from the future would have already brought it back to us, thus establishing itself as "already" existing in our time as a result - and likely copied and recopied. Since our present selves are still wondering about time travel, this theory states that we will never be able to build a time machine, because if we are still wondering, then no one from the future has built a time machine and brought it back with them to us, and if no one in the future has built a time machine, then we in the future will not build a time machine, and no one can ever build a time machine because no one in the future has built one. This theory, however, creates aCausality Loop. Also if it is that any time machine made will allow only one way travel, i.e. into the future, or if it is that we can go into the future, but while returning, we can only return to the present (i.e. not set the dial to go back more than we used to go into the future, or before the time machine is invented), this theory fails.There are also other possibilities: someone in the future will build a time machine and many people will use it to travel back in the past, but we don't know this because: "no one can travel so far in the past to reach our time" or "no one has revealed that time-travelling is possible" or "no one that revealed that time-travelling is possible, was trusted" or "anyone with the knowledge to sufficiently understand temporal mechanics enough to build such a device, will also be enlightened enough not to interfere with the known written past"

Temporal Disinterest Hypothesis

This hypothesis states that if someone builds a time machine for a specific purpose, and fulfills this purpose by means of time travel into the past, the resulting being (the maker of the time machine) would be satisfied and not have a reason to create a time machine, thus causing yet another paradox. This type of paradox is avoided in the 2002 adaptation of H.G. Wells' The Time Machine where the main character creates a time machine to save his dead fiance, but upon doing so she is killed another way. When traveling to the future he finally finds out that if she did not die he would never create the time machine, thus it is impossible to save her. Even though this does not create a physical paradox, it creates a mental one in which the time traveler who still builds the time machine for the same reason, remembers a different cause for that reason/outcome.

Harmony Theory

This theory suggests that the universe tends toward harmony and will correct itself even if an actor, event or idea “destroys” any one particular timeline. For example, if one were to create a time machine, travel back in time and kill Hitler before his rise to power in an effort to save the lives of six million Jews, the Holocaust would nevertheless have to happen—even without Hitler's presence—because in our universe, the events of World War II, including the Holocaust, must take place in order for the United States to become a superpower and use its influence to help create the United Nations, which in turn helped create the modern state of Israelmarker. Some see an example of Harmony Theory in the fact that Isaac Newton and Gottfried Leibniz—without any overtly obvious connection—both invented infinitesimal calculus independently of one another. Another example of Harmony Theory might be seen in The Hitchhiker's Guide to the Galaxy, in which two pan-dimensional creatures expect Arthur Dent's brain to know the answer to the Ultimate Question to Life, the Universe, and Everything because it should already be in his thought patterns.

See also

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