If there’s one thing that science-fiction and fantasy fans do well, it’s recursively subcategorizing the bejeezus out of their respective genres. You can’t just call a book “science-fiction” or “fantasy” anymore. In somebody’s basement somewhere, there is a rule that says that science fiction must be further pigeonholed as hard sf, steampunk, post-apocalyptic, alternate history, military sf, space opera, new space opera and so on. Similarly with fantasy—calling a book a fantasy book is not descriptive enough. Is it epic fantasy? Urban fantasy? Dark fantasy? Is it paranormal fantasy or any other of various sub-categorizations that end in “fantasy”?

These labels have their uses, of course—they steer readers toward books they are specifically looking for. It’s how we find stories we think we might like. Without genre labels, bookstores and libraries are haystacks hiding a needle of interest. (More accurately, multiple needles of interest. And stop being picky!) Think of your last trip to the bookstore. The effect of categorizing is the difference between the overwhelming feeling you get hunting for a interesting science-fiction book in a bargain bin of random selections, versus the giddy feeling you get ogling the rows of shelves specifically reserved for yummy science-fiction and fantasy books.

Which is all a roundabout way of saying that, with so many sf/f sub-genres, readers are bound to have favorites. For me, one of favorite subgenres is Time Travel.

Why time travel? First, you have to understand that there are different types of time travel stories. As I noted in my review of The Accidental Time Machine by Joe Haldeman, time-travel stories can be subdivided (by overanalytical sf fans like myself who recursively subcategorize the bejeezus out of the genre) into several loosely defined and almost certainly overlapping categories:

THE TIME TRAVEL OBSERVER

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Time Travel Observer stories are those in which the protagonist travels through time for no other reason than to see what the past was like or what the future will bring. The definitive example of this is H.G. Wells’ classic The Time Machine, where the unnamed traveler hurtles toward the future, spending a significant amount of time millions of years from his present era. Wells used this as a vehicle for social commentary, of course. The Accidental Time Machine also seems to fit as a Time Travel Observer story, as does Project Pendulum by Robert Silverberg. Through Time Travel Observer stories, the reader himself gets to travel to new worlds, thus fulfilling the hunger for science-fictional “sensawunda.”

THE TIME TRAVEL PARADOX

Some of the best science-fiction stories are the ones that address the time travel paradox head-on. They neither tiptoe around the issue nor ignore it. Alternatively, these types of stories use the concept of time loops to bend the mind of the reader. Examples of the former are Isaac Asimov’s The End of Eternity and David Gerrold’s The Man Who Folded Himself. Robert A. Heinlein’s short story “All You Zombies” and Ray Bradbury’s “A Sound of Thunder” are examples of the latter. Film examples might include 12 Monkeys, Primer and Timecrimes. Time Travel Paradox stories are brain food when done right, despite sometime-predictable plot twists. (“Oh noes! It’s a younger version of himself!”)

THE TIME TRAVEL ADVENTURE

This sub-subcategory is my kitchen sink—it’s the place that holds all other types of time travel stories. Sometimes they involve using time travel to facilitate fish-out-of-water stories, sometimes time travel is used as a tool to right some wrong, and sometimes it’s used for evil or personal gain. Examples include Timescape by Gregory Benford, Millennium by John Varley, Doomsday Book and To Say Nothing of the Dog by Connie Willis, The Anubis Gates by Tim Powers, The Plot to Save Socrates by Paul Levinson, Time Travelers Never Die by Jack McDevitt, and Kage Baker's Company series (time-traveling cyborgs!).

Film examples include Back to the Future and Bill and Ted's Excellent Adventure (but not the sequel because that never happened). This category might be further subcategorized (as if!) into “Time Police” stories—stories like Poul Anderson’s Time Patrol series, Fritz Leiber’s The Big Time, and in the film world, Timecop. The key to well-written time-travel adventure stories is that the time travel is integral to the plot. These scenarios stoke the "what if?” scenario of science-fiction.

So, with so many sf-nal buttons being pushed by the Time Travel subgenre, is it any wonder why it’s one of my favorites?

How about you? What's your favorite subgenre?

John DeNardo is the editor of SF Signal, a group science-fiction and fantasy blog featuring news, reviews and interviews.  He also like bagels.