In A Nutshell
A 1961 issue of DC Comics’ The Flash depicted the hero visiting a parallel reality and encountering an alternate Flash—leading to the development of the popular concept of the “multiverse” or infinite alternate realities, which quantum physics has since suggested and may prove to be true.
The Whole Bushel
When DC Comics rebooted their Golden Age characters of the ’40s during the Silver Age of the late ’50s and ’60s, most characters were given new identities and backgrounds. Such was the case with the Flash, and in issue #123 of September 1961—titled “Flash Of Two Worlds”—the Silver Age Flash, Barry Allen, found himself in a parallel reality teaming up with the Golden Age Flash, Jay Garrick (who, in a nice touch, is a comic book hero in Allen’s world). Allen’s ability to vibrate faster than light enabled him to travel to this alternate world, which introduced the concept of the “multiverse”—infinite parallel realities, some with only minute differences, some radically different, and everything in between—to not just the world of comics, but popular culture in general.
It was only a few years earlier that Princeton student and theoretical physicist Hugh Everett III had begun developing a theory of the “Universal Wave Function,” which eventually came to be known colloquially as the “Many-Worlds Interpretation” of quantum physics. According to this theory—which some astoundingly smart people believe to be correct—the multiverse is the nature of reality. Put as simply as possible, every event that could ever possibly happen does happen in physical reality in a different and non-communicating worldline.
Of course, writer Gardner Fox was really just trying to come up with a cute way to have his new creation meet up with his old one when writing “Flash Of Two Worlds,” and the multiverse can’t be proven because of the whole “non-communicating” thing. Right? Well, yes and no. Perhaps we on this worldline—where super-powered humans are just fanciful stories—will never peek into another reality, but that doesn’t mean no others on different worldlines haven’t already.
In fact, if the multiverse does exist, travel between realities is absolutely possible and has been accomplished many times. It was first done by a man who is just as real on his worldline as you are on yours—Barry Allen, the Flash.