Time Travel
From Self Indulgence
It is easy for us to imagine traveling through time in ways that are not actually possible. Exploring such fictional movements through time can quickly lead to "time paradoxes" such as the grandfather paradox: what would happen if you went back in time and killed your grandfather before your parents were born? The authors of time travel stories have invented many different imagined types of time travel that restrict the possible movements in time and resolve the issues of time travel paradox.
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Rules of time travel
Given the many different types of imagined time travel that have been used as plot devices in stories, we can ask: What rules for time travel, when followed, allow the most interesting stories?
Time travel fantasy
There are many stories that involve a single act of time travel with no attempt to explain what makes time travel possible; the act of traveling in time is essentially a magical plot device. A Connecticut Yankee in King Arthur's Court (1889) is an example of such a story: Hank is hit on the head and magically goes back into a fictional past. In Harry Potter and the Prisoner of Azkaban (1999) a magical "time turner" allows for a trip back in time.
Time travel in science fiction
The Time Machine (1895) featured time travel that is made possible by scientific means and construction of a device that makes time travel possible. In The End of Eternity (1955), time travel is made possible by the construction of "Eternity", a kind of space/time bubble that exists outside of normal space/time. The time travelers reside in "Eternity" and are safe from the changes that they cause in time. Timescape (1980) makes use of tachyons, newly discovered particles that make it possible to send information into the past. Both The End of Eternity and Timescape include the idea of multiple alternative timelines or "realities"; by causing a change in the past, a new "parallel" or "alternative" timeline can be created.
An interesting feature of The End of Eternity is that time travelers from the future decide to take action that prevents the further use of time travel.
In The Last Starship from Earth (1968), most of the story depicts an "alternate timeline", a version of Earth's history that is slightly different than the Earth we know. The protagonist ends up being sent back in time on a special mission and ends up creating our timeline.
