“For almost a hundred years science has been haunted by a dark secret, that there might be mysterious, hidden worlds beyond our human senses. Mystics have long claimed there were such places. They were, they said, full of ghosts and spirits … but ever since the 19th century physicists have been trying to make sense of an uncomfortable discovery. When they tried to pin point the exact location of atomic particles like electrons, they found it was utterly impossible. They have no single location … the only explanation that anyone could come up with is that the particles don’t just exist in our universe.
They slip into existence in other universes, too, and there are an infinite number of these parallel universes, all of them slightly different. In effect, there’s a parallel universe in which Napoleon won the Battle of Waterloo; in another the British Empire held on to its American colonies; in one you were never born … they are even stranger than Elvis being alive.” (Opening narrative - Horizon, BBC 2002 )