The concept of parallel realities has been explored throughout Doctor Who's history and in related media. In fact, parallel realities played an essential role in returning showrunner, Russell T Davies', first era, most notably in Doctor Who Seasons 2 and 4, both of which starred David Tennant as the 10th Doctor. The first parallel reality was introduced in the Season 2 episodes, "Rise of the Cybermen" and "The Age of Steel," and was revisited in "Army of Ghosts," "Doomsday," "The Stolen Earth" and "Journey's End." The Season 4 episode, "Turn Left," explored a different type of parallel reality created by a Time Beetle. Now, with some teases and the release of set photos for the 60th Anniversary Special, it appears that more parallel realities may be explored.
At the time of Davies' first era, the rules regarding the existence of parallel realities weren't fully explained or clearly defined. At best, the 10th Doctor stated in Season 2 that before the Time War, the Time Lords were able to travel between parallel realities without a problem, implying that they had some degree of control over them. This idea was further elaborated upon in Doctor Who: Flux, which established that Division -- a Black Ops organization from Gallifrey -- could travel between parallel realities with its leader, Tecteun, revealing the Doctor herself may have originated from a parallel reality since she was found near a wormhole. Season 11 even established the existence of sentient pocket universes like the Solitract in "It Takes You Away," which can create almost exact duplicates of a known universe.
With all the different ways parallel realities have been established throughout the course of the modern Doctor Who era, it's no surprise that the multiverse could be explored further in the upcoming Anniversary Special. The strongest evidence of this possibility is the 10th Doctor, his companion, Donna Noble, and her father Wilf appearing older than when we last saw them in Season 4. The 10th Doctor is also seen sporting his classic outfit and a new sonic screwdriver. Additionally, Yasmin Finney's Rose appears to be the daughter of Donna and her husband Shaun and could end up being her world's counterpart to Rose Tyler.
The bigger question, of course, is how a parallel reality of this nature could possibly exist. Could another Time Beetle or Solitract be creating an alternate reality in which Donna never had her memories erased, or could the existence of a multiverse be more of a consequence of the Doctor's meddling around the universe? One thing that's been consistently established throughout Doctor Who is that almost every major universe-ending crisis has converged around the infamous Time Lord and required a major temporal or universal reset to undo the astronomical damages that ensued. This alone could be facilitating the creation of multiple parallel realities, starting with the Time War itself.
The exact details of the Time War between the Daleks and Time Lords have not been explored, but the way it ended did facilitate a major paradox within the Doctor's own timeline. As shown in the Doctor Who 50th Anniversary Special, the moment in which the War Doctor made the decision to end the Time War, wormholes opened up in his own timeline to allow him to meet his future 10th and 11th incarnations. The three Doctors worked together to change their original decision to kill all Daleks and Time Lords with one cosmic bomb. This resulted in all of their previous and future incarnations returning to Gallifrey to time-lock the Time War to a single moment in a pocket universe.
While the Doctor did not end up remembering how he ended the Time War (believing he went with his original decision), the repercussions of his paradoxical solution seemingly rippled across his own timeline in catastrophic ways. One of those major repercussions was the emergence of the Dalek Emperor from the Time War, who infiltrated the future Earth's systems to recreate the Dalek race using human bodies. This resulted in the ninth Doctor's companion, Rose Tyler, absorbing the energy of the Time Vortex and scattering herself across time and space as the Bad Wolf to destroy the Dalek Emperor and his followers.
Other major consequences of the Doctor's paradoxical solution to the Time War were the emergence of the Master as the British Prime Minister, Harold Saxon, the return of the Time Lord President, Rassilon, and the resurrection of Dalek creator, Davros, all with villainous intentions. As Harold Saxon, the Master had humanity's future descendants mass murder their ancestors in 2007, resulting in a major time crisis that could only be resolved by the 10th Doctor and companion Martha Jones erasing a whole year from existence. The resurrected Davros created a bomb to destroy reality itself, which weakened the walls of the multiverse and was resolved only by committing mass genocide against the Daleks.
When Rassilon returned, he attempted to get Gallifrey out of its pocket dimension and save the Time Lords from extinction by having them ascend to a higher plane of existence. He intended to accomplish this by ending time itself, a plan which was stopped by the 10th Doctor and Master, who broke the link between the pocket universe and the main universe. All of this eventually culminated in cracks emerging throughout time and space which threatened to destroy reality, leading to the 11th Doctor's imprisonment inside the Pandorica by his most dangerous enemies. The 11th Doctor was only able to save the universe by rebooting it with a second Big Bang, effectively creating a brand-new universe similar to the old one but also changing the future of his companions Amy Pond and Rory Williams.
Interestingly, despite the birth of a brand-new universe, the Doctor's presence only resulted in more efforts to destroy him, which led to the formation of The Silence and the weaponization of River Song, the daughter of Amy and Rory. This resulted in another time crisis that was resolved with the Doctor marrying River. Unsurprisingly, efforts to destroy the Doctor continued with the 13th incarnation. In this case, the Doctor's adoptive mother, Tecteun, used Division's resources to unleash a Flux event on the universe while Division itself moved on to a parallel reality implied to be the Doctor's home universe. The Flux event wiped out huge chunks of the universe that will most likely be repaired in the upcoming Centenary Special.
With all the universe-ending crises that have happened since the start of the modern Doctor Who era, there have been at least eight opportunities for new parallel realities to come into existence. As the 60th Anniversary Special looks to celebrate the history of the show, it would be far from surprising if Russell T Davies took the audience on a journey across the Doctor Who multiverse and cemented the concept as a platform for future stories. The upcoming 10th Doctor and Donna story is the perfect jumping-off point for such an endeavor.
