The Rules of Time TravelEdit
When preparing to either run or play in a time-travel game, it is essential to learn the rules of time travel. Each game will have its own internal rules, but here are the basics to make a time-travel universe make sense.
The past cannot be changed. Under these rules the past cannot be changed and consists of two versions:
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. In many cases this form of time travel results in the Free Lunch Paradox where a piece knowledge or an item was never created.
Examples: Robert Forward's novel Timemaster, 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 La Jetée/12 Monkeys.
Traveling to the past turns you into a incorporeal phantom unable to physically interact with it such as seen in some Pre-Crisis Superman stories and Michael Garrett's "Brief Encounter" in Twilight Zone Magazine May 1981.
You can change the past and it will affect the present. Comes in two broad versions:
Changes to history are easy and can impact the traveler, the world, or bothEdit
Examples: 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) says 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 resistanceEdit
The past can be changed but the resistance to change is in direct relationship to the importance of the event i.e., small trivial events can be readily changed but large ones take great effort.
In the Twilight Zone episode "Cradle of Darkness" a woman travels to the past and becomes a housemaid of the Hitler household allowing her to kidnap and kill Adolf Hitler as a baby. However, another housemaid seeing this buys a homeless woman's baby who she passes off as the baby that was drowned ensuring that there is still an Adolf Hitler.
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. (A Grandfather's Paradox)
Great Man vs Great MomentEdit
This is somewhat related to Plastic Time with resistance but it means that history is prone to flow in certain directions.
Stephen Fry's 1996 novel Making History explores such a possibility. The protagonist uses a time machine to make Hitler's father sterile so Hitler is never born. However, because the social-political factors that created the Nazi party still exist a more harming, patient, and effective leader named Rudolf Gloder takes over the party resulting in Nazi Germany surviving into the present day.
New Timelines/Parallel WorldsEdit
Functionally these are identical in that the past can be easily changed but it has no effect on the present you come from. The main difference is in the mechanics.
With New Timelines the the moment the traveler arrives in the past the timeline branches and to the present they came from is no longer the future they came from.
With Parallel Worlds time travel is actually travel to another dimensions whose the past is identical to yours until your arrival but diverges from that point forward. Echo Earths in GURPS Infinite Worlds and James P. Hogan's The Proteus Operation are examples of this.
These rules need not exist in isolation. In fact, Pacesetter's (now Goblinoid Games) TimeMaster RPG mixes Parallel Worlds and Plastic Time rules together.
The Time Corps setting in GURPS Infinite Worlds inverts the Plastic Time rule with access to the past being endangered by changes to the past.