Parallel universe

A parallel universe is a hypothetical universe which exists separately from our own. Some theories of physics postulate the existence of many parallel universes, possibly even an infinite number. Depending on the details of the theory, these universes may or may not interact with each other. The theories involving parallel universes are all controversial, and many physicists believe that there is no need to postulate the existence of any universes other than our own.

The word "multiverse" has been proposed to refer to the collection of all universes.

Everett's "many-worlds" theory

Of the theories of parallel universes, that taken most seriously by modern physicists is the many worlds interpretation of quantum mechanics, proposed by Hugh Everett III in 1956, or some minor modification of it. A champion of this theory is the Oxford-based physicist David Deutsch.

Time as a set of parallel universes

According to one theory, each time frame is related to a particular universe. For example, there is a parallel universe where the First World War is being fought and another where the Second World War is being fought. Similarly, there is a universe where the 4th millennium has dawned. Time travel, according to this theory, is merely travelling from one parallel universe to the other. This may be achievable by passing through a wormhole.