Parallel universes really do exist, according to a mathematical discovery by Oxford scientists described by one expert as "one of the most important developments in the history of science". The parallel universe theory, first proposed in 1950 by the US physicist Hugh Everett, helps explain mysteries of quantum mechanics that have baffled scientists for decades, it is claimed. In Everett's "many worlds" universe, every time a new physical possibility is explored, the universe splits. Given a number of possible alternative outcomes, each one is played out - in its own universe. A motorist who has a near miss, for instance,...