Parallel world

Parallel worlds started as a plot device in science fiction. The idea is that every possible decision in the stream of history actually went every possible way, and that all of those possibilities still exist as part of a multiverse. This is one of the classic versions of alternate history.

Of course, this is a fertile source of plots. One classic science fiction series is the Time Police series by H. Beam Piper, including such stories as Lord Kalvan of Otherwhen. In the DC Comics superhero universe during the Silver Age of comic books, it was established the setting is a multiverse with an infinite number of dimensions of that contain various versions of superheroes. This concept was abandoned in the mini-series, Crisis on Infinite Earths, but a variant of the concept was recently revived.

In fantasy fiction, the premise of characters arriving in a parallel world is a popular premise. It allows for the kind of adventures in exotic lands like stories like The Prisoner of Zenda and the John Carter of Mars series portrayed before increasing knowledge in geography and astronomy respectively rendered them unbelievable.

In science, the Many Worlds Interpretation of Quantum Mechanics employs a similar idea. This is often seen as providing a method of reconciling the paradoxical aspects of time travel as predicted by general relativity using the probabilistic nature of quantum mechanics. (However, it should be noted that time travel in general relativity requires such things as matter with negative mass, which has never been observed, so it may be that time travel is impossible and thus there are no paradoxes to be resolved).