With Star Trek: Discovery (DSC) visiting the “Mirror Universe” in their current story arc a quick review of Star Trek’s parallel universes and time travel is in order. Spoilers follow for DSC’s 10th episode “Despite Yourself” (broadcast January 7, 2018 on CBS All Access in the United States), and prior Star Trek series.
“We exist in a universe which coexists with a multitude of others in the same physical space,” said U.S.S. Enterprise (NCC-1701) First Officer Spock in Star Trek: The Original Series (TOS) episode “The Tholian Web” set in 2268. “At certain brief periods of time, an area of their space overlaps an area of ours.” During these periods of “interphase” and other phenomenon it is possible to travel from one universe to another. This plot device has allowed Star Trek to explore a number of alternate realities, most notably the “Mirror Universe”.
The crew of the NCC-1701 a year earlier, in the episode “Mirror, Mirror”, had direct experience with the “Mirror Universe” alternate reality when during a transporter accident Captain James T. Kirk, and three other members of the crew were swapped out with their counterparts from there. In this universe a brutal, totalitarian regime known as the Terran Empire existed in place of the democratic United Federation of Planets at that time.
The origins of the Terran Empire are unclear, with the first precise difference seen in the Star Trek: Enterprise (ENT) episode “In a Mirror, Darkly”. On April 5, 2063 in Bozeman, Montana the human developer of warp drive Zefram Cochrane made “first contact” with the Vulcans as he did in the regular Star Trek universe, but instead of this event being the beginning of a longstanding peaceful relationship between these two species, he shot the first Vulcan to step out of their ship.
A group of humans then stormed the Vulcan ship and stole its technology. A scene from the modified opening credits sequence of this episode also showed the Terran Empire planting their flag on Earth’s moon, but given that the individual is wearing an ENT time period spacesuit it is not clear when this is supposed to be, but suggests that the Empire existed at least as early as the 20th Century. As a result of using Vulcan technology the Terran Empire’s Starfleet of 2155 as seen in ENT appears more advanced than their counterparts in the regular universe, and exists as a galactic power prior to the advent of the Federation.
The Terran Empire gained a further technological advantage during “In a Mirror, Darkly, Part II” when they seized control of the Federation’s U.S.S. Defiant (NCC-1764) originally lost through the dimensional interphase described by Spock in “The Tholian Web”. It is revealed by the Mirror Commander Jonathan Archer of that era’s I.S.S. Enterprise that the Tholians detonated a tri-cobalt warhead in order to create an interphasic rift (a process similar to one seen in Star Trek: Voyager’s “The Voyager Conspiracy” where it is also mentioned that tri-cobalt devices are not typically carried aboard Federation Starfleet ships in the 24th Century), lured an alien ship in with a fake distress call, and pulled the NCC-1764 back in time approximately 113 years, and into the Mirror universe.
Despite these seemingly very significant technological advantages, the Terran Empire seen in ENT (2155), DSC (2256), and TOS (2267) continued along such parallel lines that the exact same ships and crew members are consistently seen in each universe at each of these different points in time. In “Mirror, Mirror”, for example, the NCC-1701’s Captain Kirk, Chief Engineer Montgomery Scott, Dr. Leonard McCoy, and Communications Officer Nyota Uhura are members of the landing party on the Halkan homeworld in both universes.
“Not our universe, not our ship,” said Captain Kirk. “Something parallel, a parallel universe coexisting with ours on another dimensional plane. Everything’s duplicated…almost.” Unlike the Star Trek: The Next Generation episode “Parallels” where far more divergent timelines are seen, the “Mirror” universe continues to track the main universe even after the fall of the Terran Empire through the 24th Century as seen in Star Trek: Deep Space Nine.
“Most places and ships that are found in our universe…exist here,” said Science Specialist Michael Burnham of the of the U.S.S. Discovery (NCC-1031) in the episode “Despite Yourself” after that ship arrives in the Mirror universe because of an apparent malfunction of their prototype displacement-activated spore hub drive. “The same goes for people,” she added. Although this tethered nature of these two universes has never been explained, it is the thread that ties all crossovers (or in the case of ENT appearances as no actual crossovers were shown) between them together.
Later in “Despite Yourself,” searching for a way home, the Discovery crew find records about the captured U.S.S. Defiant, and set out to obtain information about it to recreate the interphase it came through posing as their counterparts to accomplish this mission. If they do locate the information the door is not only open to them returning to their universe, but also possibly to travel through time to the TOS era, or possibly another time.