"Dark Tower" was a work that famed author Stephen King spent thirty years writing. Although he claimed that this epic novel had been inspired by Tolkien's "The Lord of the Rings," the two were actually vastly different.
"The Lord of the Rings" was a fantasy epic, set in a very straightforward fantasy world. Its story was not particularly complicated, anyone could understand it, and everyone yearned for that fantastical continent.
"Dark Tower," on the other hand, was more complex—one could even say difficult to understand, at least in the era when it was published.
Around 1950, quantum physicists were the first to propose the concept of Parallel Universes. They used an example of a fox eating grapes, explaining the subsequent differences caused by the two possibilities of "eating them" and "not eating them."
At first, this was merely a discussion within scientific circles. Ordinary citizens at the time generally did not care about such things, and there were certainly no convenient learning channels like the Internet. For a long time afterward, it remained only a scientific hypothesis, a topic of conversation among elite talents in a small circle.
By 1961, the concept of Parallel Worlds had appeared in issue 123 of "The Flash V1," which told of Barry, The Flash of Earth-One, meeting Jay, The Flash of Earth-Two, thereby introducing the setting of DC's Parallel Universe.
It could be said that The Flash was a true trailblazer in the comics world. In every sense, he was a pioneer who opened the way.
After that, the concept of Parallel Universes entered many households through comics and gradually became fashionable. Back then, if an American teenager had not read The Flash and did not know about Parallel Universes, they would practically be too ashamed to show their face at school.
Though that