What is hard science fiction?
I've just returned from Denver and the World Science Fiction Convention (worldcon) where I spoke on issues such as privacy, DRM and creating new intelligent beings. However, I also attended a session on "hard" science fiction, and have some thoughts to relate from it.
Defining the sub-genres of SF, or any form of literature, is a constant topic for debate. No matter where you draw the lines, authors will work to bend them as well. Many people just give up and say "Science Fiction is what I point at when I say Science Fiction."
Genres in the end are more about taste than anything else. They exist for readers to find fiction that is likely to match their tastes. Hard SF, broadly, is SF that takes extra care to follow the real rules of physics. It may include unknown science or technology but doesn't include what those rules declare to be impossible. On the border of hard SF one also finds SF that does a few impossible things (most commonly faster-than-light starships) but otherwise sticks to the rules. As stories include more impossible and unlikely things, they travel down the path to fantasy, eventually arriving at a fully fantastic level where the world works in magical ways as the author found convenient.
Even in fantasy however, readers like to demand consistency. Once magical rules are set up, people like them to be followed.
In addition to Hard SF, softer SF and Fantasy, the "alternate history" genre has joined the pantheon, now often dubbed "speculative fiction." All fiction deals with hypotheticals, but in speculative fiction, the "what if?" is asked about the world, not just the lives of some characters. This year, the Hugo award for best (ostensibly SF) novel of the year went to Chabon's The Yiddish Policemen's Union which is a very clear alternate history story. In it, the USA decides to accept Jews that Hitler is expelling from Europe, and gives them a temporary homeland around Sitka, Alaska. During the book, the lease on the homeland is expiring, and there is no Israel. It's a very fine book, but I didn't vote for it because I want to promote actual SF, not alternate history, with the award.
However, in considering why fans like alternate history, I realized something else. In mainstream literature, the cliche is that the purpose of literature is to "explore the human condition." SF tends to expand that, to explore both the human condition and the nature of the technology and societies we create, as well as the universe itself. SF gets faulted by the mainstream literature community for exploring those latter topics at the expense of the more character oriented explorations that are the core of mainstream fiction. This is sometimes, but not always, a fair criticism.
Hard SF fans want their fiction to follow the rules of physics, which is to say, take place in what could be the real world. In a sense, that's similar to the goal of mainstream fiction, even though normally hard SF and mainstream fiction are considered polar opposites in the genre spectrum. After all, mainstream fiction follows the rules of physics as well or better than the hardest SF. It follows them because the author isn't trying to explore questions of science, technology and the universe, but it does follow them. Likewise, almost all alternate history also follows the laws of physics. It just tweaks some past event, not a past rule. As such it explores the "real world" as closely as SF does, and I suspect this is why it is considered a subgenre of fantasy and SF.
I admit to a taste for hard SF. Future hard SF is a form of futurism; an explanation of real possible futures for the world. It explores real issues. The best work in hard SF today comes (far too infrequently) from Vernor Vinge, including his recent hugo winning novel, Rainbows End. His most famous work, A Fire Upon the Deep, which I published in electronic form 15 years ago, is a curious beast. It includes one extremely unlikely element of setting -- a galaxy where the rules of physics which govern the speed of computation vary with distance from the center of the galaxy. Some view that as fantastic, but its real purpose is to allow him to write about the very fascinating and important topic of computerized super-minds, who are so smart that they are as gods to us. Coining the term "applied theology" Vinge uses his setting to allow the superminds to exist in the same story as characters like us that we can relate to. Vinge feels that you can't write an authentic story about superminds, and thus need to have human characters, and so uses this element some would view as fantastic. So I embrace this as hard SF, and for the purists, the novels suggest that the "zones" may be artificial.
The best hard SF thus explores the total human condition. Fantastic fiction can do this as well, but it must do it by allegory. In fantasy, we are not looking at the real world, but we usually are trying to say something about it. However, it is not always good to let the author pick and choose what's real and what's not about the world, since it is too easy to fall into the trap of speaking only about your made-up reality and not about the world.
Not that this is always bad. Exploring the "human condition" or reality is just one thing we ask of our fiction. We also always want a ripping good read. And that can occur in any genre.