Time travel

Time travel is the concept of moving forward and backward to different points in time, much as we do through space. It also includes traveling sideways in time between parallel realities or universes.

Time travel in fiction

Types of time travel

Time travel themes in science fiction and the media can generally be grouped into two types (based on effect—methods are extremely varied and numerous), each of which is further subdivided. These type classifications do not address the issue of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of the time line.

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:1. The time line is consistent and can never be changed.

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::1.1 One does not have full control of the time travel. One example of this is The Morphail Effect.

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::1.2 The Novikov self-consistency principle applies (named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University).

Related Topics:
Novikov self-consistency principle - Igor Dmitrievich Novikov - Copenhagen University

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::1.3 Any event that appears to have changed a time line has instead created a new one.

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:::1.3.1 Such an event can be the life line existence of a human (or other intelligence) such that manipulation of history ends up with there being more than one of the same individual, sometimes called time clones.

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:::1.3.2 The new time line may be a copy of the old one with changes caused by the time traveler. For example there is the Accumulative Audience Paradox where multitudes of time traveler tourists wish to attend some event in the life of Jesus or some other historical figure, where history tells us there were no such multitudes. Each tourist arrives in a reality that is a copy of the original with the added people, and no way for the tourist to travel back to the original time line.

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:2. The time line is flexible and is subject to change.

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::2.1 The time line is extremely change resistant and requires great effort to change it.

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::2.2 The time line is easily changed.

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There are also numerous science fiction stories allegedly about time travel that are not internally consistent, where the traveler makes all kinds of changes to some historical time, but we do not get to see any consequences of this in our present day. For example in SPI's time travel wargame a player's traveler machine gunned a roman legion has no effect on same traveler's subsequent feudal era adventures.

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Immutable timelines

Time travel in a type 1 universe does not allow any paradoxes, although in 1.3, events can appear to be paradoxical.

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In 1.1, time travel is constrained to prevent paradox. If one attempts to make a paradox, one undergoes involuntary or uncontrolled time travel. Michael Moorcock uses a form of this principle and calls it The Morphail Effect. In the time-travel stories of Connie Willis, time travelers encounter "slippage" which prevents them from either reaching the intended time or translates them a sufficient distance from their destination at the intended time, as to prevent any paradox from occurring.

Related Topics:
Michael Moorcock - Connie Willis

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In 1.2, the Novikov self-consistency principle asserts that the existence of a method of time travel constrains events to remain self-consistent (i.e. no paradoxes). This will cause any attempt to violate such consistency to fail, even if extremely improbable events are required.

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:Example #1: You have a device that can send a single bit of information back to itself at a precise moment in time. You receive a bit at 10:00:00 PM, then no bits for thirty seconds after that. If you send a bit back to 10:00:00 PM, everything works fine. However, if you try to send a bit to 10:00:15 PM (a time at which no bit was received), your transmitter will mysteriously fail. Or your dog will distract you for fifteen seconds. Or your transmitter will appear to work, but as it turns out your receiver failed at exactly 10:00:15 PM. Etc, etc. Two excellent examples of this kind of universe is found in Timemaster, a novel by Dr. Robert Forward, and the 1980 Jeannot Szwarc film Somewhere In Time (based on Richard Matheson's novel Bid Time Return).

Related Topics:
Timemaster - Jeannot Szwarc - Somewhere In Time - Richard Matheson - Bid Time Return

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:Example #2: In the case of Somewhere In Time, the film deals with events that have already or about to happen which the lead character Richard Collier (played by Christopher Reeve) could not control. Here, Collier is given a watch by a lady he has not yet known (but who already knew him in the past). Sometime later, Collier is fascinated by a picture taken in 1912 of a young actress. Eventually he learns that the woman in the picture is the old lady who gave him the watch, and that he was actually there in 1912 to meet her. Collier chooses to willfully go back in time 68 years in the past to fulfill what was written in the history books. He meets her and falls in love with her, but one day finds a penny in his pocket that he had brought back in time accidentally; the minting date on it is 68 years in the future. Holding tangible proof that he does not "belong" in the past hurls him back to the present day, and so everything that will be/was written in history has happened and Collier could not do anything to change that history. Had he remained in 1912, history would have been altered, and everything that happened at the beginning of the film would not have come true.

Related Topics:
Christopher Reeve - 1912

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An example which could conceivably fall into either 1.1 or 1.2 can be seen in book and film versions of Harry Potter and the Prisoner of Azkaban. Harry and Hermione go back in time to change history. As they do so it becomes apparent that they are simply performing actions that were previously seen in the story, although neither the characters nor the reader were aware of the causes of those actions at the time. This is another example of the predestination paradox. It is arguable, however, that the mechanics of time travel actually prevented any paradoxes, firstly, by preventing them from realising a priori that time travel was occurring and secondly, by enabling them to recall the precise action to take at the precise time and keep history consistent.

Related Topics:
Harry Potter and the Prisoner of Azkaban - Harry - Hermione - Predestination paradox

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In a universe that allows retrograde time travel but no paradoxes, any present moment is the past for a future observer, thus all history/events are fixed. History can be thought of as a filmstrip where everything is already fixed. See block time for a detailed examination of this way of considering the nature of time.

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In 1.3, any event that appears to have caused a paradox has instead created a new time line. The old time line remains unchanged, with the time traveller or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated for the origin timeline and the destination timeline. A possible solution to this is to have the mechanics of time travel require that mass-energy be exchanged in precise balance between past and future at the moment of travel, or to simply expand the scope of the conservation law to encompass all timelines. Some examples of this kind of time travel can be found in David Gerrold's book The Man Who Folded Himself, the Robert Zemeckis film Back to the Future Part II (1989), and the (1994) film '.

Related Topics:
David Gerrold - The Man Who Folded Himself - Robert Zemeckis - Back to the Future Part II - 1989 - 1994

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:Example: In Back to the Future Part II, Marty McFly and Doc Brown decide (after Doc returns from the 21st century to 1985) to travel to 2015 to save McFly's future son. While there, McFly buys an almanac of sporting events from 1951 forward, and decides to use it for financial gain via time travel. Doc Brown forbids him to take the book with him, and inadvertently leaves it lying around for the aged Biff Tannen to take with him. That night, without McFly and Doc Brown knowing it, Tannen takes the time-traveling DeLorean with the book and goes back in time to change history (using the sports almanac for his own financial success). By the movie audience's point of view, Tannen shortly after returns to 2015 and leaves the DeLorean, and McFly and Doc Brown again use the car in an attempt to go back to 1985. But soon the two discover what Tannen had done: Tannen went back to a certain point in 1955, met up with his younger self, and gave the younger Tannen the almanac for him to use for personal and financial gain, so the 1985 that McFly and Brown returned to was the future of a tangent that started in the now alternate 1955, with Hill Valley now corrupt and its citizens' lives changed because of Tannen. McFly and Brown could not just go back to 2015-A (A for alternate) to nab Tannen because whatever they would have done there would have been the future of that particular tangent. In simple words, once you go back in time to change history in this particular instance, whatever happens next will be the future of that particular tangent you just altered (so, for example, if you went back in time to prevent the assassination of President Kennedy in 1963, which in the Twilight Zone episode Time Storm saved Kennedy but simulatneously led to Kruschev assassinated instead, and nuclear war in Europe, or, in the case of Star Trek: Generations, change the fate of a planet and thus saving the crew of the Starship Enterprise, the future after that will be the future based on whatever you altered).

Related Topics:
Back to the Future Part II - Almanac - President Kennedy - Enterprise

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Mutable timelines

Time travel in a Type 2 universe is much more difficult to explain. The biggest problem is how to explain changes in the past. One method of explanation is that once the past changes, so too do the memories of all observers. This would mean that no observer would ever observe the changing of the past (because they will not remember changing the past). This would make it hard to tell whether you are in a Type 1 universe or a Type 2 universe. You could, however, infer such information by knowing if a) communication with the past were possible or b) it appeared that the time line had never been changed as a result of an action someone remembers taking, although evidence exists that other people are changing their time lines fairly often. An example of this kind of universe is presented in Thrice Upon a Time, a novel by James P. Hogan.

Related Topics:
Thrice Upon a Time - James P. Hogan

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Larry Niven suggests that in a type 2.1 universe, the most efficient way for the universe to "correct" a change is for time travel to never be discovered, and that in a type 2.2 universe, the very large (or infinite) number of time travellers from the endless future will cause the timeline to change wildly until it reaches a history in which time travel is never discovered. However, many other "stable" situations may also exist in which time travel occurs but no paradoxes are created; if the changeable-timeline universe finds itself in such a state no further changes will occur, and to the inhabitants of the universe it will appear identical to the type 1.2 scenario.

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Gradual and instantaneous

In literature, there are two (commonly used) methods of time travel:

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1. The most commonly used method of time travel in science fiction is the instantaneous movement from one point in time to another, like the hand of a boy lifting a toy train from the rails with the wheels still turning, and putting it back at a different place. There is not even the beginning of a scientific explanation for this kind of time travel; its popularity is probably due to the fact that it is more spectacular and makes time travel easier.

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2. In The Time Machine H.G. Wells explains that we are moving through time with a constant speed. Time travel then is, in Wells' words: stopping or accelerating one's drift along the time-dimension, or even turning about and travelling the other way. This method of gradual time travel fits best in quantum physics, but is not popular in modern science fiction. Perhaps the oldest example of this method of time travel is in Lewis Carroll's Through the Looking-Glass (1871): the White Queen is living backwards, hence her memory is working both ways. Her kind of time travel is uncontrolled: she moves through time with a constant speed of –1 and she cannot change it. This would make Lewis Carroll the inventor of time travel. T.H. White, in the first part of his Arthurian novel The Once and Future King, The Sword in the Stone (1938) used the same idea: the wizard Merlyn lives back in time, because he was born "at the wrong end of time" and has to live backwards from in front. "Some people call it having second sight".

Related Topics:
The Time Machine - H.G. Wells - Lewis Carroll - Through the Looking-Glass - T.H. White - The Once and Future King - The Sword in the Stone

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