Time Travel Mechanics

How time travel would work has been a stable of fiction for a long time and GURPS Infinite Worlds consolidates it down it down to something manageable. 
==How to Diverge History==
How history can be changed depends on what factors are important
* Great Man Theory:GURPS Infinite Worlds 97 holds that a man or woman of sufficient willpower, ability, and vision can alter history and change the course of nations. Effectively history as biography.
* Great Moment:GURPS Infinite Worlds 98 The times make the person. For example, take out Hitler and somebody else likely would have taken his place. Stephen Fry's Making History has a far more charming, patient, and effective Rudolf Gloder come to power instead. This mixes the Great Man and Great Moment theories together. James Burke's Connections series looks at history in a manner similar to this as does the Knowledge Web.
* Great Motherland:GURPS Infinite Worlds 98 History as geography.
Realistically all of these should be in play. Sure odds are good that someone else would have sailed west around 1500 and "discovered" the Americas but who that was and who their patron nation was could ripple throughout history.
===Other Considerations===
* Generational Cycles - GURPS give the four parts of the cycle as Idealists, Reactives, Civics, and Adaptives
* PlaguesWilliam H. McNeill's Plagues and Peoples is an excellent look at how plagues can alter culture.
* Politics
* Economics
* War
* Ecology
==Mechanics of Time Travel==
===Fixed Time===
The past cannot be changed.GURPS Infinite Worlds 154 This comes in two versions.
* Time loop: you being in the past is part of history and despite you thinking you are changing history your efforts will fail somehow or will cause what you are trying to prevent.
:* Examples: Robert Forward's novel Timemaster, the Twilight Zone episodes "No Time Like the Past" and "Cradle of Darkness", 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 La Jetée/12 Monkeys.
* Time phantom: traveling to the past turns you into a noncorporeal phantom unable to physically interact with it such as in the Pre-Crisis Superman story "Superman Returns to Krypton" Superman Vol 1 #61 (November, 1949) and Michael Garrett's "Brief Encounter"Twilight Zone Magazine May 1981.
===Plastic Time===
The classic you can change the past and it will affect the present.GURPS Infinite Worlds 154 This has several variants:
* Changes to history are easy and can impact the traveler, the world, or both
: Examples include Doctor Who, the Terminator films, and the Back to the Future trilogy. In some cases, any resulting paradoxes can be devastating, threatening the very existence of the universe. In other cases the traveler simply cannot return home. The extreme version of this (Chaotic Time) is that history is very sensitive to changes with even small changes having large impacts such as in Ray Bradbury's "A Sound of Thunder".
* Plastic Time with resistance in direct relationship to the importance of the event i.e., small trivial events can be readily changed but large ones take great effort.GURPS Infinite Worlds 154
: Examples:
:* In the Twilight Zone episode "Back There" a traveler tries to prevent the assassination of President Lincoln and fails, but his actions have made subtle changes to the status quo in his own time (e.g. a man who had been the butler of his gentleman's club is now a rich tycoon).
:* In the 2002 film adaptation of The Time Machine, it is explained via a vision why Hartdegen could not save his sweetheart Emma—doing so would have resulted in his never developing the time machine he used to try and save her.
:* In The Saga of Darren Shan, major events in the past cannot be changed, but their details can change while providing the same outcome. Using this model, if a time traveler were to go back in time and kill Adolf Hitler, another Nazi would simply take his place and commit his same actions, leaving the broader course of history unchanged.
===Alternate timelines/parallels universes===
Time travel to your own past is impossible. Either your very arrival creates a new timeline (nearly all of Marvel comics time travel) or you arrive in a reality similar or identical to your past up to that point (such as the Echo Earths in GURPS Infinite Worlds and James P. Hogan's The Proteus Operation)
==Types of Time Travel==
===Symmetrical Time===
Time passes at the same rate on both sides. So two years spent in the past cause two years spent in the present. Examples: "Why Papa Left Home"Weird Science #11 and The Time Tunnel series (mostly, there were exceptions).
===Asymmetrical Time===
Time on the "down end" passes at a much faster rate then the up end. In James P. Hogan's The Proteus Operation) the speed at which time in the past is proportional to the difference in years between the past and the present. In the TV pilot movie Time Travelers (1976) where time is implied not to pass on the up end at all.
==Additional Information==
* Trope Talk: Time Travel
*Time Travel (Isaac Arthur)
*Time Travel in Fiction Rundown
==Notes==
==References==
Category:Concepts