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.

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.

Related to the time travel 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.

In Bring the Jubilee Ward Moore writes about an America in which the South won the Civil War.

In The Man in the High Castle by Philip K. Dick, the Axis powers have won World War II, the Japanese have taken over the Pacific States, and Germany controls Europe.

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.

Common to many of these 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

As you will see from the Recommendations, many works that use this motif--beginning with Lewis Carroll's Alice in Wonderland--veer into fantasy.

What makes one work fantasy and another science fiction is the attempt or lack of attempt to explain the existence of and access to the parallel universe in scientific terms.