An interpretation of quantum mechanics in which it is supposed that any ‘measurement’ leads to all the possible outcomes of the measurement coexisting in different versions of the universe. One common way of thinking about the many-worlds view is to assume that every quantum event results in a split. For example, the spin of a particle may have two states, say ‘up’ and ‘down’. In the standard Copenhagen interpretation of quantum mechanics the state is indeterminate until a measurement is made, at which point the particle becomes either ‘up’ or ‘down’. In the many-worlds interpretation the measurement creates two universes—one in which the spin is ‘up’ and the other in which the spin is ‘down’. The idea is that the universe is constantly multiplying into an enormous number of parallel worlds. The theory was first put forward in 1957 by the US physicist Hugh Everett III (1930–82). See also multiverse.