Parallel universes in fiction

A parallel universe, also known as an alternate universe, world, or dimension, is a plot device in fiction which uses the notion of a hypothetical universe co-existing with another, typically to enable alternative narrative possibilities. The sum of all potential parallel universes that constitute reality is often called the "multiverse". The device serves several narrative purposes. Among them, parallel universes have been used to allow stories with elements that would ordinarily violate the laws of nature, to enable characters to meet and interact with alternative versions of themselves or others from their home universe, thus enabling further character development, and to serve as a starting point for speculative fiction, particularly alternate history.