Chapter 9: Fundamental Physics

Section 11: Uniqueness and Branching in Time

Parallel universes

The idea of parallel universes which somehow interact with each other has been much explored in science fiction. And one might think that if the history of each universe corresponds to one path in a multiway system then the convergence of paths might represent interactions between universes. But in fact, much as in the case of time travel, such connections do not represent additional observable effects; they simply imply consistency conditions, in this case between universes whose paths converge.

From Stephen Wolfram: A New Kind of Science [citation]