Alternate versions of reality are a staple of science fiction. As with depictions of the alien, science fiction uses depiction of alternate realities to illuminate some aspect of our own present or past or to foresee the future.

Rooted in theories of quantum physics is the possibility of the existence of parallel universes, occupying the same space and time as our own reality but on a different plane of existence. Some theorists conjecture that any exercise of choice splits reality along a variety of timelines that evolve in divergent ways depending on the choice that had been made.

Alternate and parallel universes differ from fantasy worlds in a couple of key ways.

Alternate Universe stories generally point to a particular moment in time when the world diverged from the world that we know and then they extrapolate from that difference to describe the alternate universe.

While parallel universes, much like fantasy worlds, generally feature some sort of mechanism for travelling back and forth between versions of reality, the science fiction parallel universe provides some sort of scientific explanation for this phenomenon or some sort of scientific device that allows characters to move between realities.

Common to many parallel universes are:

• an opposing set of physical laws or moral codes or social conventions
• a lack of previous knowledge of the existence of the other universe
• an inability to communicate with or pass easily to the other universe

Related to the parallel universe story and sometimes incorporated into the parallel universe story is the alternate history, a work that poses the "What If?" question and then extrapolates the answer. In many of these works, the addition or elimination on one small action or decision sends the world in a very different direction. Sometimes characters move freely between one world and the other and can see how a small change in an event has sent the other universe along a different path.

Parallel Universe

• The Apprentice Adept Series by Piers Anthony
• The Dark Materials Trilogy by Philip Pullman
• The Neanderthal Parallax trilogy by Robert J. Sawyer

Alternate History

• Bring the Jubilee by Ward Moore
• The Lathe of Heaven by Ursula K. Le Guin
• "The Lucky Strike" by Kim Stanley Robinson
• The Man in the High Castle by Philip K. Dick

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