alternate universe

The first credible suggestion that alternate universes might exist came in the early 1950s when a young physics graduate student named Hugh Everett was toying with some of the more bizarre implications of quantum mechanics.

There are moments when he seems to be writing something on the order of Jorge Luis Borges's "Tlön, Uqbar, Orbis Tertius"—an amazing fragment about the alternate universe where every book contains its counterbook, where things become duplicated and "also tend to become effaced and lose their details when they are forgotten."