Fantasy might have made him his fortune, but for his next project, Terry Pratchett is set to venture into the world of science fiction, returning to a concept he first dreamed up almost 25 years ago in collaboration with the award-winning British science fiction writer Stephen Baxter.

Pratchett began work on a novel about a chain of parallel worlds, The Long Earth, in 1986, just after completing Equal Rites, the third book set in his Discworld universe in which the world is held up by four elephants, standing on a giant turtle. "I thought to myself [Discworld] is fantasy, and I want to get back to my first love, which is science fiction," Pratchett told the Guardian. The author had previously written two science fiction novels in the late 70s and early 80s, The Dark Side of the Sun and Strata.

But the Discworld books took off, and Pratchett decided to pursue the fantasy path. "Equal Rites was very successful, and without me making an actual decision about it, I found I enjoyed writing the Discworld books, and The Long Earth remained on the back burner until the back burner receded over the horizon," he said.

Digging through his archive recently in search of material for a compilation, his agent, Colin Smythe, stumbled across the unfinished novel and two short stories, and Pratchett began to realise "there was such a lot that could be done with it".

"It was simply based on the quantum theory idea, that the earth is one of an immeasurable number of earths, each one differing by one electron. Like a lot of science fiction, or at least like the stuff which interests me, this is a scenario where we can start to have lots of fun," he said.

"A way is found almost by accident for people to go from one world to another. An overarching theory here is how much our wars are caused by the scarcity of land. Supposing that the thickness of a thought away is another earth, almost exactly like this one, and as far as we can tell not inhabited by anything human. It'd be the land rush to end all land rushes. So off we go, but hang on a minute, there are no mobile phones … so there are certain problems: metal doesn't go well through whatever ether you step through to get to the next world. You're really pretty well having to start again, so it's a strange kind of situation – people are incredibly rare, they're the biggest resource you have."

Pratchett has collaborated with other authors before, working with Neil Gaiman on Good Omens, and was keen to team up with a science fiction writer to develop The Long Earth series. Baxter, author of Flood, Ark and the Time's Tapestry and Destiny's Children series, and winner of the British Science Fiction Association prize and the Philip K Dick award, felt like the right choice.

"I think Stephen Baxter is one of our best science fiction writers, and the best hard science fiction writer," he said. "I really like his stuff. There's a man who's at home with trillions … The nice thing about working with him is that pretty much one or other of us can solve any sort of problem. It'll be fun – an antidote to all the other boys and girls being allowed out to play and you typing away by yourself. I suffer from permanent cabin fever. It's having a mirror to bounce things off, and at the same time you're his mirror as well. Also he is used to writing hard science fiction."

At the moment, the pair are both working on other books (Pratchett's next Discworld novel is out next autumn) but they're bouncing ideas off each other and pinning down the outline for the series, the first book of which will be published by Doubleday in spring 2012. "There are excited phone calls – and there will probably be fights over who does what scene," said Pratchett, who's particularly looking forward to covering Native Americans visiting a pristine version of the Black Hills of Dakota, "and saying this is great but I really hope we get cell phones going, this blanket and smoke thing really sucks".

This being a Terry Pratchett series, there will, of course, be humour involved, but the author is adamant that "you can't just put humour in a book". "There has to be something to play it up against – tragic relief," he stressed. "Humour should come from the situation, not because you think it's a good time to put a joke in."

Both authors are determined to put the parameters of their world(s) down in concrete before getting started. "Once you've got the science and the background you have to be true to it. One of the problems of Doctor Who-type fiction is that you can make it up as you go along. If you do it right, you use the modelling clay you've got," said Pratchett. "You only get one chance to put down the parameters of what's possible. I've been phoning doctors and people like that and saying 'Can this work?' You have to find something which seems right and not too blatantly bad from a scientific point of view."

Generally speaking, "the rule in science fiction is you are allowed one impossible thing," he said. "Ours is, supposing we can step to the world next door – to all the words next door. And let's see what people do, faced with this … Of course, being writers we can play with what we know about mankind – offered an absolute paradise, he will find some way of cocking things up."