THE MIRRORWORLD DOESN’T yet fully exist, but it is coming. Someday soon, every place and thing in the real world—every street, lamppost, building, and room—will have its full-size digital twin in the mirrorworld. For now, only tiny patches of the mirrorworld are visible through AR headsets. Piece by piece, these virtual fragments are being stitched together to form a shared, persistent place that will parallel the real world. The author Jorge Luis Borges imagined a map exactly the same size as the territory it represented. “In time,” Borges wrote, “the Cartographers Guilds struck a Map of the Empire whose size was that of the Empire, and which coincided point for point with it.” We are now building such a 1:1 map of almost unimaginable scope, and this world will become the next great digital platform.