Time travel and the paradoxes it can lead to have always fascinated the science fiction author and reader.

Although Einstein's theories demonstrate that travel at or near the speed of light could theoretically allow a human to travel into the future, travel into the past is deemed to be impossible. Yet much memorable science fiction, beginning with H.G. Wells' The Time Machine, enables a human to travel at will through time as well as space.

Time travel leads to all sorts of problems, most notably the possibility that a man could travel back in time to kill his own youthful grandfather and thus himself cease to exist.

But then if he ceases to exist, how could he have traveled back in time?

Such paradoxes lead to some common conventions related to time travel:

• One might travel back to the past but one is unable to change it. Thus Christ always dies on the cross, Abraham Lincoln dies in Ford's Theater, and John F. Kennedy dies in a Dallas motorcade.
• One might travel back to the past and be under strict orders not to interact with anyone for fear that history could be disrupted by someone with knowledge of the future.
• One might travel back to the past but be unable to meet his past self.
• One might travel back to the past, but be unable to bring any person or artifact back to the future with him.

Of course, in some science fiction any one of these conventions is an immutable rule, but in other works these are laws that are broken with unpredictable consequences.

Other works related to time travel are works that deal with time dilation; the effects on characters of varying rates of the passage of time. Such time dilation usually occurs:

• due to faster than light travel; or
• due to existence at the event horizon of a black hole where for all practical purposes time stops.

Time Travel

• "All You Zombies--" by Robert A. Heinlein
• "By His Bootstraps" by Robert A. Heinlein
• Doomsday Book by Connie Willis
• The End of Eternity by Isaac Asimov
• "Fire Watch" by Connie Willis
• "The Little Black Bag" by C.M. Kornbluth
• The Time Machine by H. G. Wells
• The Time Traveler's Wife by Audrey Niffenegger
• To Say Nothing of the Dog by Connie Willis

Time Dilation

• The Forever War by Joe Haldeman
• Gateway by Frederik Pohl
• "Kyrie" by Frederik Pohl
• Time for the Stars by Robert A. Heinlein

Time Loops

• Groundhog Day (film)
• Replay by Ken Grimwood

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