It first appeared in the 1967 episode Mirror, Mirror, in which a transporter malfunction swaps several members of the Enterprise we know with their more sadistic mirror universe doppelgangers. (The Van Dyke beard on the alternate universe variant of Leonard Nimoy's Mr Spock is the episode's scariest conceit.) After things are finally set right, Spock mentions to Captain Kirk (William Shatner) how refreshingly human those doppelgangers seemed, suggesting that our ‘good’ side can be as much of a mask as our ‘evil’ one. Who’s to say the world we know best is necessarily better than any other?

Sometimes alternate realities come in the form of premonitions; characters get a view of how things might be if the world continues on its current course. Among the few lesser-known works by massively influential film-maker Steven Spielberg is his episode of the '70s anthology TV series The Name of the Game, entitled LA 2017.

In it, a book publisher played by Gene Barry blacks out after crashing his car in the Hollywood hills and wakes up to find himself in a desolate world where humans have been driven underground due to pollution. If the environment above ground is toxic, the situation below is much worse, as the surviving populace is at the mercy of a corporatised fascist government.

Like much dystopic fiction, the episode is more about the evanescent fears of the current moment made tactile. Barry's character eventually escapes the nightmare – it's proven to be a literal dream. Yet the image that lingers most is not any of the future visions of anonymous steel structures and a militarised police force, but that of a dead bird lying by the side of the road in the present, an omen of a blasted wasteland that could still be made from humanity's own war on mother earth.

Be careful what you wish for

The Spotnitz-produced X-Files often dipped its toe into alternate universe fiction, one of the best examples of which finds hilarious form in the Season 7 episode Je Souhaite. FBI agent Fox Mulder (David Duchovny) discovers a wisecracking genie wrapped in a carpet and wishes for "world peace". But his good intentions blind him to the literalness of the request. When he steps outside every human being on Earth except for him has vanished, and all the noise pollution with them. Well…it's certainly more peaceful.