The Flash Introduces a New Source of Alternate Earths

Following the events of The CW's "Crisis on Infinite Earths," the heroes of Earth-Prime (and leads of shows like Supergirl, Black Lightning, and The Flash) have had no way of accessing alternate Earths, and did not know that they even still exist. Tonight's episodes of The Flash, titled "A Girl Named Sue," has given the writers a new opportunity to visit alternate Earths, though -- even if it seems that they will be unpopulated. It's a superhero universe, though, so it's hard to imagine that this won't be used in some fun, unexpected way to give us alternate Earths and, potentially, alternate takes on characters.

How does it all work? Well, we'll break it down for you below.

In the meantime, obviously, spoilers ahead for "A Girl Named Sue," tonight's episode of The Flash.

Eva McCulloch (Efrat Dor) was working in her office on the night of the particle accelerator explosion back during the pilot for The Flash. She ended up somehow transported into, and then trapped inside, the giant mirror in her office, discovering a mirror universe that included a duplicate Central City with one major difference: no other people. After being trapped alone for years, she finally met someone new this week: Iris West-Allen (Candice Patton).

McCulloch explained that superstring theory allows for the existence of fractal worlds, apparently existing in other dimensions rather than occupying different vibrational frequencies. It feels like a slight variation on the Arrowverse's existing multiverse theory, but using Mirror Master's powers rather than The Flash's as its basis.

Obviously it's possible that this will not be used as a way of bringing in alternate realities and/or alternate versions of people and objects from Earth Prime, but it seems pretty likely that once a door like this is open it will be hard to fully close again, especially when McCulloch suggested that there are "worlds," not "a world" during her explanation for Iris.

Until we get a better explanation for who the "Mirror Iris" is, and how exactly travel throught he Mirror works, it's up in the air how or whether this will be used by more people...but don't be surprised if we find out that some characters (maybe Harry and Jesse?) could have used the mirrorverse to wait out the Crisis in the same way The Flash and the other Paragons used the Speed Force and Vanishing Point.

The Flash airs on Tuesday nights at 8 p.m. ET/PT on The CW, before episodes of DC's Legends of Tomorrow.