Science fiction has always been interested in other worlds. Sometimes, even, it has been interested in other Earths.

The idea that there might exist other versions of the Earth, on which history traveled slightly or wildly different paths, has fascinated SF writers for decades. It recently has found even mainstream expression in the form of a Gwyneth Paltrow film, "Sliding Doors."

In "Sliding Doors," Paltrow plays a woman whose future hinges upon her catching a train home. The film alternates between two parallel story lines: one in which she catches the train and the other in which she doesn't. The film's point is to show the difference even seemingly inconsequential events can make in our lives.

The parallel-Earth idea is a fanciful extrapolation on real-world quantum physics, which posits that for every possible outcome in our world, there are an infinite number of alternate outcomes. Many-worlds theory holds that all those alternate outcomes have, in fact, actually happened, but on other-dimensional Earths parallel to our own.

Many-worlds theory is the guiding force behind the Sci-Fi Channel's hit series "Sliders."

Many alternate-Earth stories follow the same change-one-thing formula used in "Sliding Doors." Harry Turtledove's novel, "How Few Remain," for example, begins with one small change in history. But that one change results in the Confederate States of America's winning the Civil War and, a generation later, a second War Between the States.

Time-travel stories are one popular take on alternate Earths.

In "Back to the Future Part II," a bleak, dystopian Earth results when Biff Tannen (Thomas F. Wilson) travels back in time and gives his younger self information about the future. It's then up to Marty McFly (Michael J. Fox) and Dr. Brown (Christopher Lloyd) to travel back as well and set things right.

In his later years, literary SF's grand master, Robert A. Heinlein, delved heavily into alternate-Earth themes, with his novels "To Sail Beyond the Sunset," "The Cat Who Walks Through Walls" and "The Mark of the Beast."

Alternate universes have also played an entertaining part of the most famous of sci-fi series, "Star Trek."

The original-series episode, "Mirror, Mirror," which features a universe inhabited by evil doppelgangers of Capt. Kirk and Mr. Spock (remember the Spock with the goatee?), consistently ranks among Trekkers as one of the most popular Trek adventures. "Mirror, Mirror" has even spawned sequels in the form of several "Star Trek: Deep Space Nine" episodes.

But perhaps my favorite use of the alternate-Earths concept is the one used by DC Comics.

How do you explain, for example, how Superman could be a member of the Justice Society of America during World War II and still youthful today?

The answer is simple if you have more than one Earth.

For years, DC Comics maintained that the Golden Age, WWII-era Superman lived on "Earth 2," while the Superman of the '60s, '70s and '80s lived on "Earth 1." And there were young (Earth 1) and old (Earth 2) versions of Wonder Woman and Batman as well.

In the mid-'80s, however, the Powers That Be at DC Comics decided that readers were having too much trouble keeping up with DC's multiple-Earths continuity.

As the years had progressed, the multiple Earths had proliferated wildly.

Earths 1 and 2 were joined by Earths S, X, 3, C, Prime and countless others, although few were ever used apart from 1, 2 and S. (Earth S was the home of Captain Marvel and the rest of the Marvel Family.) So, DC's editors decided house cleaning was in order.

The result was "Crisis on Infinite Earths," the one truly great epic of mainstream superhero fiction.

Written by Marv Wolfman and pencilled by George Perez, "Crisis" destroyed all but five of DC's alternate-Earth universes, and then combined the remaining worlds into one.

"Crisis" gave DC a more streamlined continuity.

Unfortunately, the new continuity also had lots of gaps, and many fans of the Golden Age versions of DC's heroes were distressed to learn that the adventures they so loved "never happened," at least as far as DC Comics' new continuity was concerned.

So, earlier this month, DC returned to its old, alternate-Earth continuity -- sort of.

No one is quite certain, exactly, what DC's new concept, "Hypertime," is, including, if the published interviews are any indication, the comic-book writer most involved with inventing it, Mark Waid ("The Flash," "Captain America"). But it seems that Hypertime, whatever it is, at least means that all those old Golden Age stories "really happened," in a fictional sense.

And, really, isn't it a bit silly to claim that some fiction is more fictional than other fiction, anyway?

And aren't all fictional worlds, equally "alternate," after all?