Where did this idea of parallel universes come from? Science fiction is an obvious source: in the 1960s, Captain Kirk met his ‘other self’ in a Star Trek episode called ‘Mirror, Mirror’, while Philip K Dick’s novel The Man in the High Castle (1963) imagined an alternate world in which the US was a Nazi puppet state. Since then, the idea has become mainstream, providing the image of forking paths in the romantic comedy Sliding Doors (1998), and the spine-chilling ‘What if?’ in Philip Roth’s novel The Plot Against America (2004), which envisaged the anti-Semitic aviator Charles Lindbergh defeating Roosevelt in 1940. But there’s also science fact. In 1935, Erwin Schrödinger proposed his famous thought experiment involving a cat in a box whose life or death is connected to a quantum event, and in 1957 the American physicist Hugh Everett developed his ‘many worlds’ theory, which proposed that the act of opening Schrödinger’s box entailed a splitting of universes: one where the cat is alive, and another where it is dead. –Andrew Crumey – Aeon.