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The future isn't what it used to be. One hundred years after the publication of HG Wells's The Time Machine, Science Fiction seems lost in a literary black hole, a poor relation of the Fantasy genre that has outsold it for the past 20 years at a ratio of four books to one.

It was the Fifties and Sixties that saw the "golden age" of SF (the term "sci-fi", by the way, is a no-no among true aficionados, used only of SF that isn't very good). Writers like Robert Heinlein, Frank Herbert, Michael Moorcock, JG Ballard and especially Kurt Vonnegut reached out to a far wider readership than the sci-fi nerds who had previously provided the genre's core audience, while Stanley Kubrick's 1968 movie of Arthur C Clarke's 2001: A Space Odyssey also boasted broad appeal.

Indeed, science fiction films have always figured high among the box- office blockbusters, especially in recent years when the technological advances in special effects (SFX rather than SF) have lent themselves to futuristic movie-making. Steven Spielberg's Jurassic Park and ET are the two most profitable films ever made. Close Encounters, Total Recall, Star Wars, Mad Max, Alien, The Terminator and all their many and various sequels have all been major money-spinners.

Following the surprise success of last year's Stargate, this year's audiences can thrill to films featuring British SF comic book heroes Judge Dredd and Tank Girl, while Kevin Costner's $200 million Waterworld, a spectacular futuristic ride, proved to be much, much better at its first British preview screening this month than all the pre-release sneering had suggested.

But where movies boldly go, book sales sadly do not follow. "SF novels have never sold proportionately as well as successful science fiction films," concedes Jayne Johnson, senior editor at HarperCollins,"although Michael Crichton's Jurassic Park obviously sold very well on the back of the film. But science fiction's readership has broadened in recent years. More women, for example, read it - and write it - now." (And, she might well have added, edit it too.)

HarperCollins recently published a sequel to The Time Machine, "hard science" writer Stephen Baxter's The Time Ships. Hard science writers are those who base their SF on a deep knowledge of real science. But Baxter, much acclaimed as a "new voice for the new millennium", recognises a need to soften his science to reach a mainstream audience.

"These days readers don't want to sit through pages of technical stuff," says Baxter, when we meet at a reception in Imperial College, London, during the course of a three-day international conference held to celebrate the centenary of The Time Machine. "SF writers have to write real novels with genuine characters - lack of characterisation has long been recognised as a problem in SF. The challenge is to translate SF themes in a way that is accessible to mainstream audiences. Nowhere on the cover of my Time Ships does it say it's SF - that's important from a marketing point of view."

So how exactly does one define SF? According to the conference organiser, Professor Patrick Parrinder of Reading University, "Science fiction is not necessarily or entirely fiction set in the future, but it is closely cognate with it. All observers agree that the presence of an innovation - 'the strange property or the strange world', as Wells put it - distinguishes the SF story."

That makes SF a broad church with occasionally unwilling members in the congregation. SF folk still reserve particular scorn for PD James, who specifically denied that her Children of Men, a futuristic novel set in the year 2020, was science fiction because it portrayed real characters.

Doris Lessing, who has written several SF novels in recent years, is at the reception too. "I didn't know that I was writing science fiction," she says disarmingly. "I think it has been largely absorbed into the mainstream, hasn't it? I've always thought Salman Rushdie's work, for example, is science fiction."

Indeed, SF readers are quick to point out that a number of successful "mainstream" writers have used SF tropes in their novels: Margaret Atwood in The Handmaid's Tale, say, or Martin Amis in London Fields and Time's Arrow (which utilises an idea - time running backwards - explored some years before in Brian Aldiss's Cryptozoic). Yet when Iain Banks, a novelist with undoubted literary credentials, took up writing SF, it caused consternation: SF is still largely seen as a ghetto it's best not to enter.

Terry Pratchett, a best-selling fantasy writer but also a keen SF reader, says: "A lot of SF has been absorbed into the mainstream. The techno-thriller, for example: before, that would have been sci-fi. And how do you define Philip Kerr's The Gridiron? Besides, SF is not about the future, it's about the present - creating another world to look at the world we live in now."

"Most writers I know delight in blurring the boundaries," adds William Gibson, one of the creators of a new form of SF - cyberpunk - in the 1980s. "If you're trying to write a contemporary mimetic novel in a naturalistic sense, you're going to be dealing with some areas that have the stamp of science fiction."

One problem for contemporary SF writers is that the future is already here. Wells and the other scientific romantics wrote of rockets going to the moon. Forty years into the Space Age we've been there, done that. Ron Howard's blockbuster movie Apollo 13, about four men in a spaceship, is billed not as SF but as "real-life drama".

"There isn't as much future as there used to be," Pratchett agrees. "The future came and bit everyone on the bum. But nothing defines an age like its future. You can spot the Victorian, the Fifties, the Sixties future."

Cyberpunk - a form of SF, heavily influenced by Ridley Scott's cult movie Blade Runner, in which hi-tech meets low-life - seeks to renew the future's sell-by date by hypothesising from current developments in computer technology. Yet, no matter how fast SF writers try to keep up, the future always seems to outstrip them. Gibson's Neuromancer, for example, in which he introduced the concept of Virtual Reality and attracted a new, hip readership to SF, now seems old hat only 10 years on.

Gibson's screenplay, based on his own early short story, for the forthcoming Keanu Reeves film Johnny Mnemonic evinces what he calls a great "nostalgia for the future".

"The world of Johnny Mnemonic is a world where the capital F future isn't going to arise, which I think is pretty much our situation in 1995. The old futures have a way of hanging around and I find that very poignant. I think that everyone sort of knows that the real future is going to be cluttered with all the same junk we have today, except it will be old and beat-up and there will be more of it."

Despite all that, publishers are convinced that SF, after 20 years in the doldrums, is about to boom again. The Orion publishing group's Anthony Cheetham, who has been publishing SF for 30 years, set up a special SF imprint, Millennium, a couple of years ago on that very assumption. "I think it goes in cycles, and these cycles are connected to what is going on in real science. SF is in the doldrums because exciting things are happening in real science. But science fiction will catch up again."

According to Cheetham, the last golden age of SF was blighted by the rise of Fantasy - "the Tolkien clones took over". One happy consequence was that SF actually broadened its readership. "Before the fantasy boom it was mainly blokes who read SF. But women read fantasy and moved on to SF. For a publisher, it makes no difference. We don't distinguish between the two."

In the 1970s the Star Wars films also had a revitalising effect on SF publishing. Now pre-production of the trilogy's long-awaited four "prequels" is well advanced in Hollywood. Steven Spielberg is also looking back up at the stars: he's bought the rights to Arthur C Clarke's latest book, The Hammer of God. Ridley Scott is rumoured to be working on plans for a sequel to Blade Runner.

That will bear no relation to the SF books event of the year, Millennium's publication this autumn of KW Jeter's novel Blade Runner 2: The Edge of Human, a sequel to Scott's film. Jeter's work has the seal of approval of the estate of Philip K Dick, on whose work Blade Runner the movie was based.

So maybe the publishers are right. As we approach the second millennium, perhaps SF's future is not quite all behind it.