"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 had no single location.
The only explanation is that the particles don't just exist in our universe.
They slip in and out of existence in other universes,
an infinite number of parallel universes,
all of them slightly different."
-opening narrative from 2002 BBC science series Horizon