It is not only famous and outspoken artists that worry the government of China. The state is reportedly turning its attention to TV programmes that portray time travel. Time travel television, said China's government administrator of radio and television, is "frivolous" in its approach to history – a verbal warning that was seen as tantamount to an official ban.

The time travel crackdown comes just as British television audiences tune their sonic screwdrivers, adjust their bow ties and get ready for a new series of Doctor Who. This reincarnated classic has already shown the Daleks working for Winston Churchill and Queen Victoria battling a werewolf, so it is likely that it would be censored in China. But should the British government also be taking an interest? Is the Tardis a threat to the political and social order?

Science fiction is a subversive genre because it invites us to imagine completely different ways of life, past and future utopias and dystopias. In fact let's begin with Utopia itself, the book written by Thomas More in the early 16th century that is arguably the founding classic of science fiction. Like legions of later science fiction authors, More takes the latest contemporary discoveries as a starting point. The traveller who tells of Utopia is, we are told, a veteran of Vespucci's voyages to the New World. The island he describes is even more amazing than Vespucci's stories of America. It is a place where people live in communistic harmony, with strict laws to keep them harmonious. More was a famous wit, and Utopia is a witty book – to take it literally is a misreading. But what it does is to challenge the assumptions of More's society by the mere fact of being able to imagine an alternative.

Later science fiction has imagined as many dystopias – from 1984 to Philip K Dick's The Man in the High Castle – as utopias. But in each case the effect is to free us from the assumptions of the present.

The Marxist literary critic Frederic Jameson once pointed out that science fiction and historical imagination are actually aspects of the same thinking. He believed that the effect of immersing yourself in another historical age in, say, The Eagle of the Ninth, was very similar to travelling in your mind to the world of Isaac Asimov's Foundation Trilogy. Both genres are set apart from pure fantasy, because they claim some basis in reality. Sci-fi expounds futures that might be possible, or at least that follow basic rules of human nature in how they portray people; historical fiction is, likewise, rooted in actual events.

So China is right to fear time travel. No state that wants to keep governing forever can afford to let people imagine alternatives. Perhaps this is the true reason for Doctor Who's popularity in Britain. When it screens, millions of people defy the rule of reality and escape together into worlds of possibility, worlds of otherness, dreams of utopia.

To the Medusa Cascade, comrades!