The theory has its origins in the dual resonance
model - first proposed in 1969 by Gabriele Veneziano -
which described the strongly interacting hadrons as strings.
Then will someone please explain comic book author Gardner Fox's Flash story (taken also from Wikipedia):
Fox's script for "Flash of Two
Worlds!", from The Flash #123 (Sept.
1961), introduced the concept that the Golden Age heroes existed on a parallel
Earth named Earth-Two.
Remember, there were two Flash superheroes in the comic books. The Flashes figured out that the parallel worlds were only separated by vibrations.
How did Gardner Fox learn of this? I know the man was a genius and a real reader and gatherer of information, especially science information, but where then did HE get the idea?
Also, unrelated, Star Trek had an episode called "Mirror, Mirror" that brought up this parallel universe idea and that was first broadcast October 6, 1967.
Apparently, Jorge Luis Borges may have done some inventing of this idea back in 1941. From Wikipedia:
"The Garden of Forking Paths" (original Spanish title: "El Jardín de
senderos que se bifurcan") is a 1941 short story by Argentine
writer and poet Jorge Luis Borges. It
was the title story in the collection El jardín de senderos que se bifurcan
(1941), which was republished in its entirety in Ficciones ("Fictions") (1944).
It was also the first of Borges's works to be translated into English when it
appeared in Ellery
Queen's Mystery Magazine in August 1948.