I recently read through The Year's Best Science Fiction: Thirtieth Annual Collection edited by Gardner Dozois, in which one of the stories (Vandana Singh's enjoyable and contemplative "Ruminations in an Alien Tongue") includes an alien artifact that serves as a portal to alternate worlds. Portals are not new to science fiction, and Singh's story, which reinvigorates the idea by having it originate from a machine that distorts probabilities, nevertheless had me thinking about portals in science fiction and fantasy books.

The most obvious definition of portals—doorways to other places—is too simplistic to convey the true nature of them. Sometimes those places are real, but far away. Sometimes they are fantasy worlds that shouldn't exist but do. Sometimes they are physical, sometimes metaphorical, mere plot devices to advance the story. They could take many forms, from holes in the ground, to mirrors, to large constructs big enough to fly a starship through. Sometimes they aren't about traveling distance at all, but instead are about traveling through time.

In fantasy novels, portals tend to be ways for characters to pass from their world (usually our own) to a fantastical secondary world. One of the most famous portals in all of literature is the rabbit hole in Lewis Carroll's Alice's Adventures in Wonderland. Through this portal, which she enters accidentally, Alice begins her fantastic adventures in a secondary world filled with colorful characters.

A portal serves a similar purpose in the high fantasy novel The Lion, the Witch and the Wardrobe by C. S. Lewis, the first book in The Chronicles of Narnia. Here, four siblings who are evacuated from London during World War II to escape the Blitz discover a wardrobe that leads to the magical land of Narnia where they become heroes.

In Neil Gaiman's Coraline, the portal isn't a doorway to a better world, but a worse one. Here, a young girl discovers a secondary world that has a home eerily similar to her own from which she must rescue herself and the souls trapped by her “other mother.” The portal between our world and the other one is literally a doorway—a locked door in a downstairs room—bricked up but still the object of young Coraline's curiosity, despite a warning to not go through it.

Turning to science fiction, the purpose of the portal isn't so much the destination as it is a means of cutting down travel time. That is, in science fiction, portals tend to be ways for characters to move great distances. The idea of a portal in a scientifically plausible setting inevitably leads to the idea of a “star gate” through which ships can travel vast distances in an eye blink. About 36 years before the film Stargate existed, science-fiction author Andre Norton wrote Star Gate, the title referring to the mechanism through which an advanced race of Terrans leave an alien world when it is determined that their existence is hindering the natural growth of the planet's natives. 

In Dan Simmons' Hyperion Cantos (comprised of the novels Hyperion, The Fall of Hyperion, Endymion and The Rise of Endymion) there exists a network of so-called "farcasters," which are essentially portals connecting most of the planets inhabited by humans. Travelers use these to quickly planet-hop from one world to another. That's also the preferred method of moving long distances by the characters in Peter F. Hamilton's Commonwealth universe. In his duology Pandora's Star and Judas Unchained, set in the 24th century by which time humanity is occupying more than 600 planets, wormhole technology is used to connect the vast distances. This allows the central story—the mystery of the sudden appearance of barriers around a pair of stars that could signal the a species-ending threat—to play out on a much larger canvas of multiple planets spread out across the universe.

The plot of Robert A. Heinlein's Tunnel in the Sky, one of Robert A. Heinlein's so-called juvenile novels, students undergo tests of survival on other planets through the use of a portal that takes them there. Heinlein's characters aren't the only young people to use portals: Steven Gould's YA novel Wildside sees its teenage protagonist discover a portal to a pristine world never populated by humans. But is it a land of untold riches and possibilities, or one of impending danger by the people who wish to control it?

If there is a literary opposite of Alice's rabbit hole, size-wise, it would have to be the titular artifact from Stephen Baxter's hard-sci-fi novel Ring, one of the books in his mind-expanding Xeelee sequence. In this case, the ring—a toroid of cosmic string more than 10 million light-years across—acts as a portal into another universe, in this case used to escape our doomed one.

C.J. Cherryh's Morgaine Cycle, which rides the sometimes-blurry line between fantasy and science fiction, features a portal as well. The series (comprised of Gate of Ivrel, Well of Shiuan, Fires of Azeroth and Exile's Gate—all of which have recently been optioned for film) is about a time-traveling heroine named Morgaine and her mission to preserve the integrity of the universe. She does this by using portals to travel to various worlds...and then destroying those portals so that their world may be protected from an alien race bent on manipulating time.

An example of a science-fiction book in which the portal is not meant to move people over great distances is The Shining Girls by Lauren Buekes (also being adapted, but to television). Here, a serial killer discovers the portal that allows him to move through time to perpetrate his dastardly deeds. This mechanism adds tension to an already-harrowing tale; how do you stop a killer who has the perfect getaway?

What are your favorite portals in Science Fiction and Fantasy?

John DeNardo is the editor of SF Signal, a Hugo Award-winning group science-fiction and fantasy blog featuring news, reviews and interviews. You can follow him on Twitter as @sfsignal.