The author of this year's most exciting SF debut, The Quantum Thief, talks about how 24 pages of an unfinished first novel won him a three-book deal, his split writing personality and why science fiction is more honest than the mainstream

Sitting in his office on a sunny day in autumn 2008, Hannu Rajaniemi picked up the phone and shifted into a parallel world. Science fiction imprint Gollancz wanted to offer him a three-book deal on the strength of just 24 pages from the opening of an unwritten first novel. Looking back now, with The Quantum Thief just published, the 32-year-old Finnish mathematician remembers being stunned.

"It was hard to accept or understand what had happened," he says. "So I paced around the room in circles for a while before my colleague Sam dragged me out to the pub to celebrate."

Sitting upright at the table in his neatly-pressed shirt, hands folded in his lap as his answers emerge with quickfire precision, it's easy to believe that Rajaniemi's first impulse was one of reflection. Then there was the daunting question of completing the novel. Pacing around the office on that bright October afternoon, Rajaniemi says that he wondered what he'd got himself into.

It was only a few years earlier, in 2002, that he had begun writing in English. Born in Ylivieska in 1978, as a child Rajaniemi dreamed of studying theoretical physics, a passion which led him to a degree in maths at Cambridge. But while he was working on a PhD in string theory and quantum gravity at Edinburgh, he went to a performance by the spoken-word group Writers' Bloc and got chatting with some of the members over a beer after the show. He took a story along to one of their monthly workshop sessions, where he found writers such as Charlie Stross and Andrew Wilson, and was soon hooked.

"It's a really professional group, with brutally honest feedback on your work, which was really excellent. There was some pain," he says with a wry grin. But writing in English made the criticism a little less wounding because it gave him a bit of distance. "It made it easier to be an outsider and look at your own text in a problem-solving way." He hesitates. "Maybe in a mathematical way."

It seemed natural to start writing in English, Rajaniemi continues, because it was the language he was speaking in his daily life. There was also no question of getting feedback from the others in the writing group if he was writing in Finnish. But he soon discovered that he had a different personality when he was writing in English – a personality he liked.

"It's probably a bit of a cliche, but I'm a bit more outgoing in English, whereas in Finnish I tend to be quieter, more reserved," he says. "It's maybe because, for me, Finnish is very much a personal language – it's the language I speak with my very, very close friends and my parents – whereas pretty much my whole professional career, my scientific career and my writing career has been in English, so it's outward facing."

Rajaniemi describes Finnish as a language of poetry and song, with great facility for shaping words and making compounds. His English writing style is simpler, a little more pared-down. Reading some of the first few chapters from the Finnish translation, which he says he'd love to have done himself but hasn't got the time, was a "strange experience ... It felt like they had been written by some Finnish evil twin. It was very good Finnish, but not the Finnish I would have written had I done it myself."

The story cooked up by the outgoing, outward-facing English side of his personality is a classic heist, set in a far future where radical advances in technology have transformed humanity. Protagonist Jean le Flambeur is sprung from a "dilemma prison" – a crystalline structure where his jailers have been confronting him with representations of himself to teach him the value of cooperation – and conscripted on a mission to a Mars by one of the expanded intelligences which control the inner solar system. Pursued by a young detective, he plans a daring theft in a city where the inhabitants tap into an all-seeing network of information as they perform a complicated dance through layers of privacy and forgetting. Le Flambeur winds up following a memory trail he created himself before he was imprisoned.

The plot was inspired by one of Rajaniemi's favourite characters as a teenager, Maurice Leblanc's gentleman thief Arsène Lupin. A charismatic figure who operates on both sides of the law in a series of stories that began appearing in 1905, Lupin is a kind of anti-Sherlock Holmes – a master of disguise who burgles more for the love of a challenge than from any hope of personal gain. But what intrigued Rajaniemi were the cycles of redemption and relapse Lupin goes through as he tries to go straight, always falling gloriously short. At various stages in his career Lupin gets married, joins the French Foreign Legion and even spends five years as a chief of police, investigating himself – but always finds himself drawn to life on the wrong side of the law.

"It's somehow clear that he's never going to succeed, that the pull of the illicit is too strong, that somehow fundamentally, he is Arsène Lupin – though of course Arsène Lupin is not even his real name. Arsène Lupin is the identity he has created for himself, but he can no longer escape it." This underlying thread offers an element of tragedy, Rajaniemi says. "He can break the rules, but in the end he's also imprisoned by some higher order of rules, rules of identity." What would become of Lupin in a future where people really can switch identities or bodies? Could he change at heart? Could he actually redeem himself? "That became the central theme not only of the first book, but probably the sequels that will be forthcoming."

After examining questions of identity through memory in The Quantum Thief, the second instalment will revolve around how we construct stories of ourselves – a theory of consciousness which offers intriguing possibilities for a novelist. According to Rajaniemi, who despaired of the increasing abstraction of string theory and set up a company to apply advanced mathematics to "real-life problems", it's coming along well in the gaps around the day job, with a third in prospect that will focus on how game-playing allows us to adopt radically different selves.

He dismisses the suggestion that a physics PhD is becoming part and parcel of the science-fiction writer's job description, arguing instead that what makes science fiction is "some sort of understanding of the scientific method – that if you make a hypothesis, you need to figure out what consequences that hypothesis has".

"The science part in science fiction really comes from taking this scientific process seriously – the subject matter and expertise is really very secondary," he says. Writing a novel then becomes like "making a thought experiment", with the story emerging from the choices the writer has made about the world in which it is taking place. "That's the fiction part of science fiction – to choose those constraints in such a way that they allow you to tell an interesting story, and emphasise the elements of the story that you want to tell."

This game of consequences – of taking something unreal or exaggerating a trend and playing with the implications – has always appealed to Rajaniemi, who suggests that the fantastical inventions of science fiction are more straightforward than the hidden distortions offered by conventional literature.

"Mainstream fiction is not about real people, it's not about the real world," he says. "It's about a world which is very much like our world, but which is essentially the author's perception of all the elements in our reality. Somehow I feel it's more honest to accept that, in fact, you can introduce any fantastical element you like."