Time travel themes in science fiction and the media can generally be grouped into two general categories (based on effect-methods are extremely varied and numerous), each of which can be further subdivided. However, there are no formal names for these two categories, so concepts rather than formal names will be used with notes regarding what categories they are placed under. Note: These classifications do not address the method of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of what happens to history.

1. There is a single fixed history, which is self-consistent and unchangeable. In this version, everything happens on a single timeline which doesn't contradict itself and can't interact with anything potentially existing outside of it.

A man travelling a few seconds into the past in a single self-consistent timeline. This scenario raises questions about free will, since once the traveller has decided to enter the time machine, then as soon as his own double appears, there is absolutely no way for him to change his mind.

2. History is flexible and is subject to change (Plastic Time)

Time travel under the parallel universe hypothesis. This scenario has the potential to preserve free will, but breaks symmetry between universes.

3. Alternate timelines. In this version of time travel, there are multiple coexisting alternate histories, so that when the traveler goes back in time, she ends up in a new timeline where historical events can differ from the timeline she came from, but her original timeline does not cease to exist (this means the grandfather paradox can be avoided since even if the time traveler's grandfather is killed at a young age in the new timeline, he still survived to have children in the original timeline, so there is still a causal explanation for the traveler's existence). Time travel may actually create a new timeline that diverges from the original timeline at the moment the time traveler appears in the past, or the traveler may arrive in an already existing parallel universe (though unless the parallel universe's history was identical to the time traveler's history up until the point where the time traveler appeared, it is questionable whether the latter version qualifies as 'time travel').

A type of story which could be placed in this category is one where the alternate version of the past lies not in some other dimension, but simply at a distant location in space or a future period of time that replicates conditions in the traveler's past. For example, in a Futurama episode called "The Late Philip J. Fry," the professor designed a forward-only time travel device. Trapped in the future, he and two colleagues travel forward all the way to the end of the universe, at which point they witness a new Big Bang which gives rise to a new universe whose history mirrors their own history. Then they continue to go forward until they reach the exact time of their initial departure. Although this journey is not exactly a backward time travel, the final result is the same.

Immutable timelines

Time travel in a Type 1 universe does not allow paradoxes such as the grandfather paradox to occur, where one deduces both a conclusion and its opposite (in the case of the grandfather paradox, one can start with the premise of the time traveler killing his grandfather, and reach the conclusion that the time traveler will not be able to kill his grandfather since he was never born) though it can allow other paradoxes to occur.

In 1.1, the Novikov self-consistency principle asserts that the existence of a method of time travel constrains events to remain self-consistent. This will cause any attempt to violate such consistency to fail, even if seemingly extremely improbable events are required.

Example: 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 p.m., then no bits for thirty seconds after that. If you send a bit back to 10:00:00 p.m., everything works fine. However, if you try to send a bit to 10:00:15 p.m. (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 p.m., etc. Examples of this kind of universe are found in Timemaster, a novel by Dr. Robert Forward, the Twilight Zone episode "No Time Like the Past", and the 1980 Jeannot Szwarc film Somewhere In Time (based on Richard Matheson's novel Bid Time Return).

In 1.2, time travel is constrained to prevent paradox. How this occurs is dependent on whether interaction with the past is possible.

If interaction with the past is possible and one attempts to make a paradox, one undergoes involuntary or uncontrolled time travel. 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.

Example: A man who travels into the past with intentions to kill Hitler finds himself on a Montana farm in late April 1945.

If interaction with the past is not possible then the traveler simply becomes an invisible insubstantial phantom unable to interact with the past as in the case of James Harrigan in Michael Garrett's "Brief Encounter".

While a Type 1 universe will prevent a grandfather paradox it doesn't prevent paradoxes in other aspects of physics such as the predestination paradox and the ontological paradox (GURPS Infinite Worlds calls this "Free Lunch Paradox").

The predestination paradox is where the traveler's actions create some type of causal loop, in which some event A in the future helps cause event B in the past via time travel, and the event B in turn is one of the causes of A. For instance, a time traveler might go back to investigate a specific historical event like the Great Fire of London, and their actions in the past could then inadvertently end up being the original cause of that very event.

Examples of this kind of causal loop are found in Timemaster, a novel by Dr. Robert Forward, the Twilight Zone episode "No Time Like the Past", EC comics stories like "Man who was Killed in Time" (Weird Science #5), "Why Papa Left Home" (Weird Science #11), "Only Time will Tell" (Weird Fantasy #1), "The Connection" (Weird Fantasy #9), "Skeleton Key" (Weird Fantasy #16), and "Counter Clockwise" (Weird Fantasy #18), the 1980 Jeannot Szwarc film Somewhere In Time (based on Richard Matheson's novel Bid Time Return), the Michael Moorcock novel Behold the Man, and Harry Potter and the Prisoner of Azkaban. It is also featured in 1972's Doctor Who, in the three part The Day of the Daleks, where three freedom fighters from the future attempt to kill a British diplomat they believe responsible for World War Three, and the subsequent easy conquest of Earth by the Daleks. In the future they were taught an explosion at the diplomat's (Sir Reginald Styles) mansion with foreign delegates inside caused the nations of the world to attack each other. The Doctor (Jon Pertwee), figures out that they caused the explosion all along by way of a temporal paradox. This is also seen in the 2006 crime thriller Deja Vu.

A version of the ontological paradox. The appearance of the traveler is the result of his disappearance a few seconds later. In this scenario, the traveler is traveling along a closed timelike curve.

The Novikov self-consistency principle can also result in an ontological paradox (also known as the knowledge or information paradox) where the very existence of some object or information is a time loop. GURPS Infinite Worlds gives the example (from The Eyre Affair) of a time traveler going to Shakespeare's time with a book of all his works. Shakespeare pressed for time simply copies the information in the book from the future. The "free lunch" is that no one really writes the plays!

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in an ontological paradox scenario involving a physical object, there can be a violation of the second law of thermodynamics. Ross uses Somewhere in Time as an example where Jane Seymour's character gives Christopher Reeve's character a watch she has owned for many years, and when he travels back in time he gives the same watch to Jane Seymour's character 60 years in the past. As Ross states

"The watch is an impossible object. It violates the Second Law of Thermodynamics, the Law of Entropy. If time travel makes that watch possible, then time travel itself is impossible. The watch, indeed, must be absolutely identical to itself in the 19th and 20th centuries, since Reeve carries it with him from the future instantaneously into the past and bestows it on Seymour. The watch, however, cannot be identical to itself, since all the years in which it is in the possession of Seymour and then Reeve it will wear in the normal manner. It's [sic] entropy will increase. The watch carried back by Reeve will be more worn that [sic] the watch that would have been acquired by Seymour."

On the other hand, the second law of thermodynamics is understood by modern physicists to be a statistical law rather than an absolute one, so spontaneous reversals of entropy or failure to increase in entropy are not impossible, just improbable (see for example the fluctuation theorem). In addition, the second law of thermodynamics only states that entropy should increase in systems which are isolated from interactions with the external world, so Igor Novikov (creator of the Novikov self-consistency principle) has argued that in the case of macroscopic objects like the watch whose worldlines form closed loops, the outside world can expend energy to repair wear/entropy that the object acquires over the course of its history, so that it will be back in its original condition when it closes the loop.

Time travel in a Type 2 universe is much more complex. 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. The Back to the Future trilogy films also seem to feature a single mutable timeline (see the "Back to the Future FAQ" for details on how the writers imagined time travel worked in the movies' world). By contrast, the short story "Brooklyn Project" by William Tenn provides a sketch of life in a Type 2 world where no one even notices as the timeline changes repeatedly.

In type 2.1, attempts are being made at changing the timeline, however, all that is accomplished in the first tries is that the method in which decisive events occur is changed; final conclusions in the bigger scheme cannot be brought to a different outcome.

As an example, the movie Déjà Vu depicts a paper note sent to the past with vital information to prevent a terrorist attack. However, the vital information results in the killing of an ATF agent, but does not prevent the terrorist attack; the very same agent died in the previous version of the timeline as well, albeit under different circumstances. Finally, the timeline is changed by sending a human into the past, arguably a "stronger" measure than simply sending back a paper note, which results in preventing both a murder and the terrorist attack. As in the Back to the Future movie trilogy, there seems to be a ripple effect too as changes from the past "propagate" into the present, and people in the present have altered memory of events that occurred after the changes made to the timeline.

The science fiction writer Larry Niven suggests in his essay "The Theory and Practice of Time Travel" 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 travelers 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 might 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 1.1 scenario. This is sometimes referred to as the "Time Dilution Effect".

Few if any physicists or philosophers have taken seriously the possibility of "changing" the past except in the case of multiple universes, and in fact many have argued that this idea is logically incoherent, so the mutable timeline idea is rarely considered outside of science fiction.

Also, deciding whether a given universe is of Type 2.1 or 2.2 can not be done objectively, as the categorization of timeline-invasive measures as "strong" or "weak" is arbitrary, and up to interpretation: An observer can disagree about a measure being "weak", and might, in the lack of context, argue instead that simply a mishap occurred which then led to no effective change.

An example would be the paper note sent back to the past in the film Deja Vu, as described above. Was it a "too weak" change, or was it just a local-time alteration which had no extended effect on the larger timeline? As the universe in Deja Vu seems not entirely immune to paradoxes (some arguably minute paradoxes do occur), both versions seem to be equally possible.

Alternate histories

In Type 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 traveler 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 and The Time Ships by Stephen Baxter, plus several episodes of the TV show Star Trek: The Next Generation and the Android Saga in the Japanese TV series Dragon Ball Z.

Gradual and instantaneous

In literature, there are two methods of time travel:

Time travel or space-time travel

An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date. However, the theory of relativity rejects the idea of absolute time and space; in relativity there can be no universal truth about the spatial distance between events which occurred at different times (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time. In the theory of special relativity, which deals with situations where gravity is negligible, the laws of physics work the same way in every inertial frame of reference and therefore no frame's perspective is physically better than any other frame's, and different frames disagree about whether two events at different times happened at the "same position" or "different positions". In the theory of general relativity, which incorporates the effects of gravity, all coordinate systems are on equal footing because of a feature known as "diffeomorphism invariance".

Nevertheless, the idea that the Earth moves away from the time traveler when he takes a trip through time has been used in a few science fiction stories, such as the 2000 AD comic Strontium Dog, in which Johnny Alpha uses "Time Bombs" to propel an enemy several seconds into the future, during which time the movement of the Earth causes the unfortunate victim to re-appear in space. Much earlier, Clark Ashton Smith used this form of time travel in several stories such as "The Letter from Mohaun Los" (1932) where the protagonist ends up on a planet millions of years in the future which "happened to occupy the same space through which Earth had passed". Other science fiction stories try to anticipate this objection and offer a rationale for the fact that the traveler remains on Earth, such as the 1957 Robert Heinlein novel The Door into Summer where Heinlein essentially handwaved the issue with a single sentence: "You stay on the world line you were on." In his 1980 novel The Number of the Beast a "continua device" allows the protagonists to dial in the six (not four!) co-ordinates of space and time and it instantly moves them there—without explaining how such a device might work.

In Clifford Simak's 1950s short story "Mastodonia" (later broadcast on the X Minus One radio anthology show, and then significantly re-written into a longer novel of the same name) the protagonists are aware of the possibility of changes in ground level while traveling back in time to the same geographical coordinates and mount their time machine in a helicopter so as to not materialize underground. When the helicopter is damaged beyond repair while in the past, they then build a mound of rocks from which to launch their return to the present.

The television series Seven Days also dealt with this problem; when the chrononaut would be 'rewinding', he would also be propelling himself backwards around the Earth's orbit, with the intention of landing at some chosen spatial location, though seldom hitting the mark precisely. In Piers Anthony's Bearing an Hourglass, the potent Hourglass of the Incarnation of Time naturally moves the Incarnation in space according to the numerous movements of the globe through the solar system, the solar system through the galaxy, etc.; but by carefully negating some of the movements he can also travel in space within the limits of the planet. The television series Doctor Who avoided this issue by establishing early on in the series that the Doctor's TARDIS is able to move about in space in addition to traveling in time.

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