Mirror universe
The mirror universe was a parallel universe widely recorded as first being visited by James T. Kirk and several officers from the  in 2267, though in reality already encountered by the  around a decade earlier. This parallel universe coexisted with the prime universe in the same space, but on another dimensional plane. 

The mirror universe was so named because most places, ships, and people that existed in the prime universe also existed there, although usually bearing the antithesis of many of their personal characteristics, in particular swapping their morally good characteristics (for example: altruism, kindness) for morally bad characteristics (for example: selfishness, cruelty) or vice versa, thus "mirror"-like. (; ; )

By 2257, the Terran Empire and the United Federation of Planets were aware of the existence of their respective opposite universe, and both chose to classify that information to prevent deliberate crossovers; the Federation was fighting a costly war at the time they learned of the mirror universe and Starfleet admiralty was afraid of desperate attempts to reunite with loved ones, and the fascistic Terrans feared Federation ideals would inspire rebellion. ()

By the year 3189 the mirror universe had diverged from the prime universe enough that no reported crossings had occurred in the preceding five hundred years. ()

Physical basis 
The mirror universe seemed to be more than just another of infinite quantum realities, and was instead specially linked in some way to the prime universe, as its moral reflection or moral antithesis. (This does not preclude the possibility that other quantum realities may also have their own mirror universes.) Considerations which marked the mirror universe out from other myriad arbitrary quantum realities in relation to the prime universe include:

 The preservation of many historical relationships despite centuries or millennia of historical divergence (for example, groups of individuals such as starship crews coming together in the same times and places, and the birth of visually indistinguishable counterparts in both universes);
 The consistent reversal of those same characters' moral qualities, in most cases;
 The propensity of individuals to "cross over" (even by accident) between these two specific universes, to the exclusion of others.

While the precise physical basis for the moral symmetry between the Prime universe and the mirror universe remained unknown, there were some indications of divergent physical properties. In particular:

 The physical properties of light may be different. Visiting the mirror universe in 2257, Michael Burnham stated: "My eyes open and it's like waking from the worst nightmare I could imagine. Even the light is different. The cosmos has lost its brilliance." ()
 Possibly as a consequence of this "different" light, mirror universe Humans ("Terrans") were comparatively more sensitive to light, manifesting as photophobia when they crossed over to the prime universe. () In 2257,  commented that this sensitivity was "the singular biological difference between our two races" (i.e. between Terrans of the mirror universe and Humans of the prime universe).
 In 3189, a prime universe Starfleet hologram stated "All Terrans are duplicitous by their biology. [....] You may not be aware, but in the past hundred years, we've discovered a chimeric strain on the subatomic level in the Terran stem cell." However, in this case Philippa Georgiou immediately dismissed the possibility of this biological difference, replying: "Silly holo. You cannot rattle me by introducing a completely fabricated biological component to my nastiness and inherently bad behavior." ()

History 

For the main article, see mirror universe history.

Terran Empire
In the mirror universe, much of known history was dominated by the Terran Empire. It is not clear when the Empire began. Captain  once stated that the Empire had existed for "centuries" as of 2155. Archer did not mention how many centuries, but by his statement, the Empire can be traced back to at least 1955, suggesting that it was a Terran political unit before it became an interstellar empire. Emperor Philippa Georgiou also claimed that s had abolished compassionate ideologies "millennia ago" as of the 2250s. (; )

First Contact between Vulcans and the Terran Empire took place in 2063, as it did in the prime universe. A long history of nations warring with each other, the stronger overcoming the weaker, led the people of Terra to believe that conquest was the only means of surviving in the universe. Peace was only a ruse used to determine an enemy's weaknesses and to enable the one offering peace a chance to conquer from within. So, when the Vulcans landed and made their peaceful introduction,  shot the first Vulcan to step onto Terran soil instead of welcoming them with open arms (as in the prime universe), and the Terrans, interpreting the landing as prelude to an invasion, raided the Vulcan ship. The shotgun used by Zefram Cochrane later came into the possession of , who wondered what would have happened had Cochrane not "turned the tables" on the Vulcans' "invasion force." With advanced Vulcan technology at their disposal, the Terran Empire expanded and conquered other races, including the Vulcans, Andorians, Tellarites, Orions, and Denobulans.

In 2155, the , under the command of Captain Maximilian Forrest, was the flagship of the Terran Empire's . In January of that year, Commander Jonathan Archer mutinied against Forrest in order to take the ship into Tholian space to capture the , which had traveled through a spatial interphase from the year 2268 in the prime universe. While the mutiny was ultimately unsuccessful, Captain Forrest had no choice but to continue the mission, since the ship's helm had been locked on auto-pilot. The Enterprise was later destroyed by Tholian ships, but not before Archer was able to take control of the Defiant. () Archer tried to use the Defiant in a grab for power, intending to replace the emperor, but was betrayed by , who declared herself empress. ()

Sometime between 2155 and 2267, the symbol of the Empire appears to have been altered. The earlier symbol closely resembles that of the United Earth government, depicting all of Terra's continents, though replacing a laurel of peace with an aggressive sword. However, by 2267, the symbol, while remaining essentially the same, depicted only the continents of Terra's western hemisphere.

By the mid-23rd century, the Terran Empire was the dominant power in the Alpha Quadrant. The Empire, ruled by Emperor , had at some point in the past century conquered ; some Klingons joined Vulcans, Andorians, and Tellarites in rebellion against the Terrans' rule. By the time of her rule, Kaminar seems to have also been conquered, with Kelpiens reduced to nameless slaves and livestock. In the 2250s, Captain  attempted a coup against Emperor Georgiou. She sent her adoptive daughter, Captain , to apprehend Lorca, but instead Burnham joined Lorca's insurrection. Burnham was subsequently believed to have been killed when her shuttlecraft was destroyed; Lorca was publicly blamed for her death. Eventually, Lorca was betrayed by scientist . When Lorca was attempting to recruit allies on Priors World, the Emperor caught up with him. He attempted to transport to his ship, the , but the transporter beam was caught in an ion storm and Lorca was transported to the prime universe. ()

Lorca successfully impersonated his prime universe counterpart and in 2256 became captain of the . He also recruited the Michael Burnham of the prime universe, though he hid his true origins and motivations from her. Late that year or early in 2257, Lorca used Discoverys displacement-activated spore hub drive to return to the mirror universe. He convinced Burnham to impersonate her mirror universe counterpart and bring him as her prisoner aboard Burnham's ship, the . As he had planned, when Emperor Georgiou heard that Burnham was alive, she summoned her to bring Lorca to her flagship, the . Lorca escaped from an agonizer booth on the Charon and rescued several of his followers who were also being imprisoned and tortured there. With their help, he took control of the Charon, but he and his surviving followers were defeated by Emperor Georgiou and Michael Burnham. Lorca was apparently killed, and Burnham and the Emperor were beamed off of the Charon before it was destroyed by Discovery. () Fearing that people would purposely attempt to cross over in order to bring back a lost loved one, the prime universe's Starfleet classified the very existence of the mirror universe in 2257. ()

In 2267, four Starfleet officers (Captain James T. Kirk, Lieutenant Uhura, Lieutenant Commander Montgomery Scott, and Doctor Leonard McCoy) from the USS Enterprise were exchanged with those same officers from the  in the mirror universe because of transporter interference from an ion storm. They discovered a brutal regime, almost dictatorial in its command structure. Advancement through assassination was commonplace.

During the encounter, Captain Kirk convinced the counterpart of his first officer, , that the Empire could not sustain itself. Indeed, Spock predicted that in its current form, the Empire had 240 years before total collapse. ()

Shortly thereafter, Spock rose to become leader of the Terran Empire, proposing a series of reforms designed to make the Empire more secure and less dictatorial in nature. These included a significant disarmament program. Unfortunately, once these reforms were complete, the Empire was unable to defend itself against the equally aggressive and powerful forces surrounding it. The Klingon-Cardassian Alliance overran the Empire, enslaving Terrans and Vulcans, and freeing several worlds that still remained under Terran occupation, including . ()

Klingon-Cardassian Alliance

With the fall of the Terran Empire, the Klingon-Cardassian Alliance became the dominant power in the Alpha Quadrant. Bajor, which before had been enslaved by the Terrans, soon joined the Alliance. The command post and ore processing facility  was constructed in orbit. 

Also during this time, the forces of the mirror universe began implementing safeguards to prevent another crossover event. Transporter design was altered to prevent inter-dimensional travel, requiring the creation of devices specifically for that purpose, including the multidimensional transporter. In the event of another crossover, those involved would be killed to prevent further interference. ()

By 2370, Terok Nor was commanded by Intendant , with  as her second-in-command. It was at this point in time that contact with the prime universe again took place. A runabout from station Deep Space 9 entered the mirror universe following an incident in the Bajoran wormhole. Kira Nerys and Julian Bashir were captured by forces from Terok Nor, and interrogated by the intendant. Bashir was sent to work in the ore processing plant, where he befriended . After instigating a series of incidents aboard the station, including the death of  during a slave uprising, Kira and Bashir convinced privateer  to rebel against the Alliance and help them to escape back to the prime universe. ()

A year later, Smiley crossed over to the Federation's universe and impersonated his counterpart long enough to capture the prime Sisko and bring him back to the mirror universe, where he convinced Sisko to impersonate the leader of the Terran Rebellion. The mirror Sisko had been killed in a skirmish with Alliance ships, and the rebels needed the other Sisko to win over , Sisko's wife, who in the mirror universe was a scientist working for the Alliance. Jennifer had been developing a transpectral sensor array, which would have allowed the Alliance to locate rebel hideouts in the Badlands. Sisko convinced his wife's mirror counterpart to defect to the side of the rebels. ()

While in the Federation's universe, Smiley downloaded information from Deep Space 9's computers, including the plans for the . In 2372, the rebels constructed their own version of the Defiant, but had trouble getting it to function properly. Jennifer lured Sisko back into the mirror universe to help, though Kira Nerys killed her a short time thereafter. By this time, the rebellion had grown in strength, culminating in the capture of Terok Nor, which became a rebel base of operations. ()

In 2374, Intendant Kira sent a thief, , to the prime universe to steal one of the Bajoran orbs, believing it would permit Kira to unite Bajor under her rule. The attempt failed, and Bareil returned to the mirror universe without the orb. ()

In 2375, Grand Nagus Zek, the leader of the Ferengi Alliance in the prime universe, used the multidimensional transporter to travel to the mirror universe along with his Hupyrian manservant, Maihar'du, hoping to open up business opportunities, but they were captured and held hostage by the Alliance. Kira made arrangements with , the regent of the Alliance, to obtain the prime universe's version of the Klingon cloaking device in exchange for Zek's return. 

She sent , a Trill mercenary and Kira's lover, to the prime side to give Quark the ransom demand. Quark and his brother Rom stole the cloaking device from Klingon General Martok's ship and delivered it to Ezri, but decided at the last minute that they couldn't trust her to keep her side of the deal. Quark and Rom accompanied her to the mirror universe, where all three were captured by the Terran rebels, who planned to keep the cloaking device, until Ezri's companion  freed them, delivering them to Regent Worf. Aboard Worf's flagship, Quark and Rom were imprisoned along with Zek upon discovering Kira's plan, only to be later rescued by Ezri as revenge against Kira for killing Brunt. Ezri ended up joining the rebel cause. Quark, Rom, Zek, and Maihar'du were allowed to return to the primary universe for having aided the rebels' defeat of Worf, a major victory for the rebellion. During the escape from Worf's ship, Garak was killed. When last seen, the rebels' march towards victory showed no apparent signs of slowing. ()

Cultures 
Andorians
Bajorans
Breen
Cardassians
Changelings
Denobulans
Ferengi
Gorn
Halkans
Humans (s)
Kelpiens
Klingons
Orions
Romulans
Suliban
Talosian
Tellarites
Tholians
Trill
Vulcans
Xindi

Technology 
 Agonizer
 Agony booth
 Multidimensional transporter device
 Tantalus field
 Tri-cobalt warhead

Starships 
See also: Mirror universe starships
 
 Bashir's raider
 
 
 
 
  (originated in prime universe)
 
 
 
 Regent's flagship

Appendices

Related topics 
 Mirror universe history
 Mirror universe people
 Mirror universe casualties
 Mirror universe starships
 Terran Empire
 Terran Rebellion
 Klingon-Cardassian Alliance
 
 Story arcs

Appearances 
 
  (Season 4)
 
 
 (Season 1)
 
 
 
 
  (Season 3)
 
 
  (Season 2)
  (Season 3)
 
  (Season 2)
  (Season 3)
  (Season 4)
  (Season 6)
  (Season 7)

Background information

Terminology 
The term "mirror universe" was first mentioned in full in , after having long been used by behind-the-scenes production sources, such as the . Typically, only general terminology has been used of both universes in relation to each other, such as "parallel universe" and "the other universe" in  and , "alternate reality" in , "the other side" in  and , "the alternate universe" in  and , and "the Terran universe" in . Although the term "mirror universe" itself is not used in , the crew of the  specifically referred to their vessel's counterpart as being the "mirror Discovery."

Origins 
Jerome Bixby was inspired to conceptually originate the mirror universe, as a parallel universe, after writing a short story called "One Way Street", which also features a parallel universe. "Having written 'One Way Street', I thought 'OK, parallel universe!" he exclaimed. Allowing the show's regular cast to play alternate versions of the main characters was one motive Bixby had for inventing the mirror universe. "As I was searching for ideas, I knew that they loved to use their cast in unusual ways on Star Trek. Instead of just throwing the cast up against a group of bad guys, or space monsters eating planets, they got a kick out of putting the cast into unusual versions of themselves, in an evil persona." (, issue #164, p. 48) Indeed, Bixby also said, "The universe I created was a very savage counterpart, virtually a pirate ship, into which I could transpose a landing party." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 74) However, Bixby's original version of  didn't actually depict the parallel universe as "a very savage counterpart", with only subtle differences from the prime universe, such as the fact the Enterprise therein was manned by a benevolent crew. Harlan Ellison had proposed including a savage alternate universe in his original take on  and the parallel universe setting in the story that became "Mirror, Mirror" initially worried Robert Justman that it might be too similar to the antimatter universe from . (These Are the Voyages: TOS Season Two)

While the story gradually began to develop into "Mirror, Mirror", the imagined universe evolved too. In a revised story outline (submitted on ), the universe contained a version of Rigel IV and a war which the duplicate Federation, including the alternate Enterprise, was losing against a race called the Tharn, to the point where surrender of the Federation was proclaimed as imminently necessary. Under orders from Admiral McNulty, Kirk took the Enterprise to rendezvous with elements of the Tharn fleet and submitted the ship to them. As Kirk soon realized, however, the Tharn vessels didn't have shields to withstand a phaser blast and the Federation had no knowledge of phaser technology. Next, ten Tharn ships followed the Enterprise, still acting under orders from McNulty, to the nearest starbase, which was Starbase One. During transit, the existence of the other universe, and the fact Kirk had originated there, was discovered by Mirror Spock and Mirror Scott. The crew of the Mirror Enterprise finally managed to create a phaser weapon, with Kirk's help, and used it to defeat the Tharn, before Kirk was returned to his own reality. (These Are the Voyages: TOS Season Two)

Some aspects of the mirror universe were subsequently influenced by comments from Script Consultant D.C. Fontana. For example, she suggested distinguishing the parallel universe from the prime one by doing "something optically or photographically." Thus, the first on-screen view from the mirror universe, depicting the ISS Enterprise and the Halkan homeworld, is a flipped rendition of identical footage from the prime universe. (These Are the Voyages: TOS Season Two)

The mirror universe was drastically changed when Gene Roddenberry submitted a rewrite of the story treatment on . He had found a way to make a parallel universe comprehensible, also changing the name Tharn to that of an individual and renaming the race as the Halkans. Roddenberry had started to think of the mirror universe as analogous to a country under the political control of a fascist military junta. Expressing how he thought of the alternate reality, Roddenberry mused to Gene L. Coon, "Life is valueless, full of fear and terror, and never exploits the full potential of most of the citizens." (These Are the Voyages: TOS Season Two)

A few wording alterations still needed to be made. After Jerome Bixby proceeded from Gene Roddenberry's blueprint by turning in a first draft script for "Mirror, Mirror" on , some clarifications regarding the mirror universe were required. In a memo, Roddenberry told Gene Coon, "Desperately needed are scenes where we, in effect, sit down and say, 'This is where we are; this is what we've seen; what does it add up to?' [....] For example, on Page 11, Kirk says, 'We've got two theories: massive change in our normal setting, or we're someplace else.' I had to read this several times to understand what he meant." Following a second draft of the teleplay (submitted by Bixby in ), NBC Broadcast Standards Department objected to the character of  referring to the mirror universe as "the damned universe," advising that either the word "damned" be deleted or that the phrase be changed to "the universe of the damned." (These Are the Voyages: TOS Season Two) In retrospect, referring to the murderous activities rife in the alternate universe, Marlena Moreau actress Barbara Luna half-jokingly commented, "We take that sort of thing lightly in the Mirror Universe." (, issue #235, p. 80)

First reappearance 
Since "Mirror, Mirror" became a favorite episode with fans of , it seemed only natural for the mirror universe to reappear in a later incarnation of Star Trek. () As early as , David Gerrold jokingly reported that the mirror universe played a part in the only  sequel to a TOS episode that had been suggested so far, in which the Enterprise returned to the parallel universe. (, issue #119, p. 21) The notion of revisiting the mirror universe on TNG was actually pitched to the series many times, ever since it began. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 74) Not intimately familiar with the TNG writers' room, Jonathan Frakes and the rest of the TNG cast were not made aware that there was any consideration of depicting the mirror universe on TNG. () However, one undeveloped mirror universe episode was even written for and submitted to TNG by Jerome Bixby. (, issue #164, p. 45; , issue #167, p. 5) Michael Piller, though, was uninterested in the idea of revisiting the mirror universe. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 74) Explained Brannon Braga, "We were a little frightened at doing it, and doing it badly, and maybe never really figured out what the Next Generation take would have been on it." 

The writing staff of  talked, for a while, about returning to the mirror universe, prior to setting any such plans in motion. () "We were talking about the idea of doing a sort of a mirror universe," commented Robert Hewitt Wolfe, "and so, we started to imagine what that mirror universe would be like, if we put our people into it." ("Section 31: Hidden File 03", DS9 Season 2 DVD special features) The parallel universe reappeared in the DS9 episode  because Michael Piller was intrigued by the possibility of finding out how the reality had been altered since its introduction. "I couldn't get away from the fact that it would be interesting to know what happened after 'Mirror, Mirror' finished," he said. Influenced by watching Schindler's List, Piller was fascinated by the what-if aspect of the grim alternate realm. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 74) Wolfe remarked, "So, that was kinda the idea, was what would the mirror universe be like, a hundred years later? Well, it might not be a very nice place." Changing the mirror universe in this way, pointing out that Kirk had been naive to try to simply improve the parallel universe without any unexpected consequences, was "fun" for Wolfe. ("Section 31: Hidden File 03", DS9 Season 2 DVD special features)

Portraying the mirror universe in "Crossover" involved production requirements that were extremely demanding. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 74) The production staff had the responsibility of convincing the DS9 audience that they were once again viewing events in the mirror universe. () Director David Livingston relished the challenge, since "Crossover" was the first installment to revisit the location. "That was fun to sort of be on the ground floor of creating this other world," he commented. ("Section 31: Hidden File 08", DS9 Season 2 DVD special features) Outlining some of the extra needs, Livingston said, "We had more prosthesis makeup because the [space] station [] is inhabited by Klingons, Cardassians, and Bajorans, and the humans are basically slaves so they're wearing different costumes." Other requirements involved altering the space station itself. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 75) Production Designer Herman Zimmerman noted, "It was more lighting and costuming than anything physical that we needed to do." The alternate universe was deliberately depicted as "weird", such as by using skewed camera angles. "It looked unusual and immediately told the viewer he was in another world," stated Livingston, who devised the idea. () Thus, the notion of making the installment's imagery very strong and dramatic set a visual tone for the mirror universe, helping differentiate it from the prime reality. "Stylistically, I tried to carry that same theme through the episode, so that it had a really strong contrast to what happens on our side of the world, or our side of the universe or reality." ("Section 31: Hidden File 08", DS9 Season 2 DVD special features)

As for clothing the main characters in both "Mirror, Mirror" and "Crossover", many of the costumes were variations on the ones normally worn by the lead actors in their Star Trek roles. () The performances of the characters obviously also changed. "You suddenly get to flip your character. It was part of the  of Star Trek," related Kira actress Nana Visitor, who was delighted not only to appear in the mirror universe of "Crossover" but also to introduce the character of the Intendant. () Although Bashir actor Alexander Siddig was one of three members of DS9's main cast who didn't appear as their usual character's mirror duplicate in "Crossover" (Terry Farrell and Cirroc Lofton being the others), he nonetheless thoroughly enjoyed watching the altered performances of all his cast mates. (; )

When the mirror universe started being revisited on Star Trek: Deep Space Nine, Jerome Bixby was very upset that he received no acknowledgement, in "Crossover", for the reuse of the concept. "They did refer to the visit by Captain Kirk and his landing party, and I perked up when they mentioned that," he admitted. Bixby was so disappointed about not receiving on-screen credit for the recycling of what he felt was an idea he had originated that he consulted the Writer's Guild of America, whose policy was that – if a writer created an original character that was later reused – the writer was entitled to a royalty of some money. "It's arguable," Bixby explained, "that the mirror universe itself might be termed a 'character' [....] I'm not sure that the 'Mirror, Mirror' spin-offs will [result in financial compensation] unless the mirror universe itself can be deemed a character. That was mentioned in passing by the guy at the guild." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 74)

Later appearances 
DS9's second visit to "the other side" was proposed by Robert Hewitt Wolfe to Ira Steven Behr and they proceeded to write the episode, , together. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 94) The writers were faced with a choice of how difficult (or easy) to make accessing the mirror universe. "Every time we wanted to do a mirror universe show," said Behr, "we had to come up with tech [to explain how they got there]. It came down to, did you want to spend half an act finding ways, or did you want to get in there and have fun? So, we made it easier to get in. But at least our [prime universe] people didn't come up with the way to do it. The mirror universe people did." Nonetheless, Wolfe added, "I would say that it was probably extremely difficult to do, but [off screen]." ()

Although Ronald D. Moore hardly had anything to do with "Through the Looking Glass", he highly valued the revisit to the mirror universe, later remarking, "I want to know more of what happened." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 94) However, some viewers seemed concerned that accessing the mirror universe in "Through the Looking Glass" appeared to be too easy. ()

The use of the "alternate dimension" in "Through the Looking Glass" allowed for a cameo appearance by Tim Russ as . "You're dealing with a basic physics concept, theoretical physics – the concept of an alternate universe – and I thought it was wonderful," Russ commented. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 94)

Nana Visitor also enjoyed how the mirror universe was depicted in "Through the Looking Glass". "It was fun to watch everyone else work and see them as this other side of themselves," she said. ()

The mirror universe setting of "Through the Looking Glass" meant the episode took an unusually long time to produce. "These shows have to be shot in a certain style because it's an alternate universe," observed David Livingston. "Even though Deep Space Nine is already moody and dark, this universe has a certain sinewy look to it, and [Director of Photography] Jonathan [West] said, 'If you want it then I have to take the time to do it.' We all kind of bit the bullet, and it went over tremendously. It [the extra time needed to portray the mirror universe] was not a surprise to me. It might have been a surprise to some people, but I had been there." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 95)

Almost everyone working on Star Trek: Deep Space Nine enjoyed the "Crossover" episodes, which is how they generally referred to installments set in the mirror universe. Writers, actors, and directors alike relished the opportunities to take the characters in a direction which could be completely different from their norm. () To the DS9 writers, in fact, a visit to the mirror universe was like a vacation. () Michael Piller noted, "The 'Mirror, Mirror' arc episodes [of DS9] were all great fun." () Hans Beimler enthused, "The alternate universe… [is] a great, fun playground. We get to paint with big brushes. The bad guys are so villainous and the good guys are so good." () He elaborated, "I think the fun thing about the alternate universe, [is] you get to use the Star Trek characters, but you can make up rules as you go along." Laughing, Beimler continued, "It's an alternate universe, but so much of it is blank, that you can really do what you've been wanting to do in our universe and can't do." Ira Behr commented, "Let's face it, the alternate universe is such a wacky idea, and I don't care what anyone says [....] Though we tried to play it with a straight face, at first, in Deep Space Nine, as time went on it just became tougher for us not to see it as what it was, which was kind of a rather flakey alternative to what we had been doing [....] To really enjoy the alternate universe episodes, you just have to let go, you have to relax your sphincter a little bit and not be so anal about it, and just try to have some fun." Behr also described the mirror universe as "something that we were able to exploit pretty successfully for a number of years." (Cinefantastique, Vol. 32, Nos. 4/5, pp. 72 & 73) Ron Moore considered the mirror universe as "just [a] wildly fun" setting, commenting, "It's a cool universe, and it's kind of fun to go over there and see the world turned inside out [....] If anything, you want to see more and more of that world [....] It loosens up the characters a little bit [....] The logic of this parallel existence, where all our people look exactly the same but play other characters, is tenuous at best. Rick [Berman] has always kind of stumbled on it. He always stumbles on it, then he always likes the episode. I think it's how we all feel. It seems like a place to go and relax and have a good time." René Echevarria pointed out, "When you see those characters, you see how much fun the actors have getting to play these shadow versions of themselves." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, pp. 118 & 94) David Livingston remarked, "[One] thing that I like about doing those kinds of episodes […] [is] the cast members get to play other characters and they get a big kick out of it. You know, it's a hoot for them to break out of something that they have to normally play the same every week [....] And they chew the scenery and relish in it. It's fun to watch them do that." ("Section 31: Hidden File 08", DS9 Season 2 DVD special features) Director James L. Conway agreed, "It's great to have these actors be able to free themselves up and play completely different people who are semibasically the same." () One exception to those who praised the mirror universe was Garak actor Andrew Robinson, since he didn't like the mirror Garak. "I never liked those alternate universe shows," Robinson complained. () On the other hand, he admitted, "I know these stories are popular, though." (, issue #237, p. 74)

There was obviously a temptation to go as far afield as possible with differentiating the mirror universe characters from their prime universe counterparts, though there were seemingly limits to this. For example, mirror universe characters that apparently remained much the same as their prime universe counterparts include Spock, Tuvok, and Jadzia Dax. Alternatively, some characters (such as ) could be vastly different in the mirror universe to how they were otherwise portrayed, according to Winrich Kolbe, who directed "Through the Looking Glass". Because the mirror universe episodes of DS9 had a high body count of familiar characters who were eliminated, the writers occasionally worried about running out of characters to populate the alternate realm. "It's a brutal universe," acknowledged Robert Hewitt Wolfe, laughing. ()

Although the mirror universe inspired the DS9 writing staff to have fun, there were some in-house rules the writers had to follow when depicting the alternate universe. "We can't just go over there and interfere with their culture just because we can," said Hans Beimler. "That's not a good enough reason. Ira's very specific about this. He wants the crossover shows […] to have what he calls 'bond,' which means there must be a reason to do them. The crossover universe […] [is] a swashbuckling kind of place. But we can't go there just because we're in a swashbuckling mood. We must go there for a [specific] reason." ()

While directing DS9 Season 4 installment , James Conway bore in mind the prevalent eroticism of the mirror universe. "That's what the mirror universe is about," he remarked. "It's dark and it's sexy, so you can't be afraid of that in those shows because you'd miss opportunities. There's no subtlety over there." Hans Beimler concurred, "The alternate universe is a sexually charged place, and who does what to whom is a matter of great interest to us." ()

At about the end of DS9's fourth season, the show's writing staff thought it likely that the mirror universe would appear again. Ira Steven Behr said, "It's something we'll probably do [....] But you have to find a worthwhile story. There's also a big push to have them come to us.... We'll see." René Echevarria believed it was reasonable to expect there might be another "crossover" episode, commenting, "This is a piece of the franchise we want to do every year, but we're always looking for an in that is more than just playing great games [....] People always say they want those guys to come to our universe, but the idea of them impersonating one of our people, unfortunately, is an angle we sort of cover with the changelings. So what does that really give you? That's why we keep going there." Ron Moore concluded, "I can see still going over there [as a possibility]." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 118)

After freelance writer Michael Taylor tried to tackle an ultimately abandoned DS9 installment during the show's sixth season, he and the series' writing staff began speaking about the possibility of doing another mirror universe episode. () "I wasn't crazy about going back to the alternate universe," Ira Behr conceded. (Cinefantastique, Vol. 30, No. 9/10, p. 44) The DS9 producers consequently decided to stick with the plan of setting their fourth episode about the mirror universe, which was entitled , in the prime universe throughout, meanwhile bringing mirror universe characters there, such as Intendant Kira and the mirror-Bareil. ()

As Star Trek: The Next Generation had completely bypassed the mirror universe, LeVar Burton – who starred as Geordi La Forge on TNG and directed numerous DS9 episodes – wasn't exposed to the mirror universe until he was called upon to direct "Resurrection". The producers' decision to set the entire installment in the prime universe meant Burton had little difficulty with directing the episode. The only element of the mirror universe he may have experienced trouble with – namely, filming scenes of the two Kiras interacting – was actually no problem for him, as Burton had already shot two Rikers in the TNG offering . ()

Shortly before the start of DS9's seventh and final season, Ira Behr began to plan another mirror universe episode. Timing was one of the main factors that motivated the return of the alternate universe in seventh season entry . () Hans Beimler remembered, "We knew we had to do an alternate universe story, that it would be the last time we did an alternate universe story, so we wanted to make it a really great send-up." Behr added, "We all felt the need, with a small n, to revisit the alternate universe one last time." (Cinefantastique, Vol. 32, Nos. 4/5, p. 72) Additionally, Behr proclaimed, "The mirror universe was due for a Ferengi show." () Regarding how the mirror universe was somewhat comedically depicted in "The Emperor's New Cloak", Behr stated, "We finally got to question the whole lunatic idea of the mirror universe." () This was done via the Ferengi character of Rom, who comes to the realization that the alternate universe "just doesn't make any sense."

Slightly more than a year after having directed "Resurrection", LeVar Burton again returned to the subject of the mirror universe by directing "The Emperor's New Cloak". However, he didn't see the entire scope of Deep Space Nines mirror characterizations until he started working on the episode. These performances pleased Ezri Tigan actress Nicole de Boer, who noted, "I liked seeing everybody else's take on the mirror characters." () She elaborated, "That was so much fun [....] I had never seen the mirror universe episodes before, so I didn't really know what to expect. I thought everybody was so great [....] We just all have so much fun with it, being bigger, and campy." (Cinefantastique, Vol. 32, Nos. 4/5, p. 46)

Armin Shimerman referred to "The Emperor's New Cloak" as "the one last acknowledgement of the mirror universe people." (Cinefantastique, Vol. 32, Nos. 4/5, p. 36) Ira Behr commented, "I thought all in all it was a doff of the hat, and a little bow, and a wink to […] the alternate universe." Hans Beimler concurred, "I think it was a nice way to end the last alternate universe [episode], at least the last one we'll do." (Cinefantastique, Vol. 32, Nos. 4/5, p. 73)

Following his work on "The Emperor's New Cloak", Hans Beimler hypothesized that the mirror universe might appear on . "I'm sure Voyager will steal our thunder, and go in there," Beimler contemplated. (Cinefantastique, Vol. 32, Nos. 4/5, p. 73) His prediction turned out to be an incorrect one, as the alternate universe never appeared on VOY. However, Robert Duncan McNeill did once refer to the event horizon of a specific type-four quantum singularity, which appears in , as a "mirror universe." (, issue #213, p. 38)

Prequel developments 

According to the writers and producers of  in a panel discussion at the  Grand Slam XIII convention, the concept of featuring the mirror universe in ENT Season 4 had a long gestation period. "We had talked about doing a mirror universe episode ever since we got into Season Four," recalled Manny Coto.  In fact, Brannon Braga, who worked as co-creator and Executive Producer on ENT, even initially considered that an alternate timeline which had been introduced in the season finale of the previous season, , might be "the beginnings of the Mirror Universe." ()

The mirror universe was intended to be featured in an ultimately undeveloped two-parter which was concocted in the hopes of enabling William Shatner to appear. It would have been revealed that a method of exiting the mirror universe was via the Tantalus field, as it didn't actually disintegrate but in fact relocated its victims – which had included "Tiberius", the mirror universe  from "Mirror, Mirror" (as played by Shatner) – to a penal colony in the prime universe.  "And so by that interesting, fun notion, you would have a world where all these Mirror Universe characters had been living," stated Manny Coto. Returning to the mirror universe was of paramount importance to Tiberius and thus, when  arrived at the penal colony, he was excited by the prospect of it allowing him to journey back to that reality. Much to his chagrin, however, the mirror universe didn't exist yet. In this narrative, it would have been created by Tiberius himself, as an alternate timeline split off from the prime timeline following the events of the show. ("Before Her Time: Decommissioning Enterprise, Part Two: Memorable Voyages", ENT Season 4 Blu-ray special features)

Although negotiations between William Shatner and Paramount failed to reach an agreement, the concept of using the mirror universe as the major setting of a story still appealed to the ENT team of writer-producers. Another hurdle was a continuity issue caused by the fact that the visit to the mirror universe in "Mirror, Mirror" had clearly been its first contact with the prime universe. Manny Coto said, "We started bandying about ideas, trying to figure out a way to get our characters into the mirror universe."  Coto further explained, "When we were playing with the idea we kept going back to the old 'okay, they're beaming over and they get transported there' thing [....] I hit upon the idea that why don't we just do the story in the Mirror Universe? Not a crossover, but the whole episode in there, and then it hit me that we could treat it like the whole episode sort of beamed over from the Mirror Universe, and that turned into a two-parter." (Cinefantastique, Vol. 37, No. 2, p. 41) Speaking from the viewpoint of the mirror universe, he concluded, "There's very little interaction with our regular universe."  Essentially, the creative team wanted to deliver the message that we're "not in Kansas anymore." (Information provided by Mike Sussman)

The concept that the mirror universe existed at least as far back as when First Contact took place between Humans and Vulcans was thought up by Manny Coto. ("Before Her Time: Decommissioning Enterprise, Part Two: Memorable Voyages", ENT Season 4 Blu-ray special features)

Prior to writing the script for "In a Mirror, Darkly, Part I", Mike Sussman did some further research on the mirror universe by reading the non-canon novel Dark Mirror. (Information provided by Mike Sussman)

The use of the mirror universe as the only setting of the "In a Mirror, Darkly" two-parter meant that aliens (such as the Gorn and Tholian species) which were otherwise prohibited from appearing on Star Trek: Enterprise, without violating canon, could be depicted in the two-parter. Hence, Garfield Reeves-Stevens related, "One of the great attractions that Manny had for why he wanted to do a Mirror episode was that there were very few continuity issues [within it]." (Cinefantastique, Vol. 37, No. 2, p. 69) Coto himself rhetorically asked, "The mirror universe, you know, gave us so much and an original Star Trek vehicle, so how can you go wrong?" ("Before Her Time: Decommissioning Enterprise, Part Two: Memorable Voyages", ENT Season 4 Blu-ray special features)

Returning to direct events in the mirror universe by helming "In a Mirror, Darkly, Part I" was enjoyable for James L. Conway, having directed DS9: "Shattered Mirror". Of the ENT mirror universe show, he reminisced, "Taking the characters and flipping their personalities completely was a delight." ("Before Her Time: Decommissioning Enterprise, Part Two: Memorable Voyages", ENT Season 4 Blu-ray special features)

In a cut scene from "In a Mirror, Darkly, Part II", Archer gives a motivational speech to the assembled crews of the former ISS Enterprise and the Avenger.

Brannon Braga revealed – via his Twitter account – that, before the cancellation of Star Trek: Enterprise, there was even some talk about setting the entire fifth season in the mirror universe.  When interviewed, he explained, "We were going to continue on with the Mirror Universe, in a major way [....] Manny Coto […] had some big plans for the Mirror Universe."  Coto himself stated, "The mirror universe was a way, kind of, to expand old mythology, or touch on mythology without breaking canon, and that was gonna be our way to explore fun avenues that we weren't able to explore in regular universe." ("Before Her Time: Decommissioning Enterprise, Part Two: Memorable Voyages", ENT Season 4 Blu-ray special features) Coto's plans for season five included featuring the mirror universe in four or five interconnected episodes. ("Before Her Time: Decommissioning Enterprise, Part Three: Final Approach", ENT Season 4 Blu-ray special features)

In , Simon Pegg jokingly theorized that the origins of the mirror universe crew from "Mirror, Mirror" might actually turn out to be depicted with the evolution of the alternate reality crew of the  from that universe, revealing that they are actually one and the same crew. 

The mirror universe was further developed and explored in the first season of . Production Designer Tamara Deverell recalled, "TJ Scott came in and said, 'We're going to the mirror universe,' and he said, 'Just give me some mirrored stuff everywhere.' We started playing around with adding black, shiny floors. The DPs were really in tune with bringing the whole thing down." ("Designing Discovery", DIS Season 1 DVD and Blu-ray special features)

Dating divergence 
The credits sequence for the mirror universe Star Trek: Enterprise television series used footage of battles going back at least to the "Age of Sail". The mirror  noted that the "great works" of literature in both universes were roughly the same, except that their characters were "soft and weak" (except for Shakespeare), pushing back the latest possible date for a divergence to the 16th century. In mirror-Archer's deleted speech from "In a Mirror, Darkly, Part II", he invokes the favor of the "gods." This, together with 's statement about "being the woman of a Caesar", in "Mirror, Mirror", suggests that the Terran Imperial tradition extends at least back to ancient Rome. However, given the nature of the mirror universe, these statements should be taken with a grain of salt.

Crossover characters 
This is a list of individuals known to exist both in the prime reality and in the mirror universe.

Apocrypha 
The existence of a "point of divergence" from the traditional Star Trek universe has not been confirmed, though according to the novel Fearful Symmetry, the mirror universe is in fact a parallel quantum universe, as quantum signature scans used to match Worf with his  in  were also able to differentiate natives of the mirror universe from those of the prime reality. This suggests that even though the two universes were always separate, they shared a similar past up to some point in their history.

According to the FASA role-playing games and The Best of Trek, the mirror universe diverges from the prime timeline around the Eugenics Wars, while DC Comics' The Mirror Universe Saga comics speculate the Earth-Romulan War was the point of divergence, with Earth having lost that war, and then embarking on a policy of conquest after overthrowing the Romulans (it is not known what kind of contact 's Empire had with the Romulans). Still other works, the novels of William Shatner's Star Trek: The Mirror Universe Trilogy (co-written with Judith and Garfield Reeves-Stevens) and the ization of  seem to indicate that time travel of the Borg to Zefram Cochrane's era might be responsible. This explanation would tie in with  when the Vulcans first arrived and were killed by Cochrane.

Dark Mirror, a Pocket TNG novel by Diane Duane, at first appears to place the mirror universe as parallel since the Eugenics Wars but Picard discovers that the ending of Shakespeare's Merchant of Venice is drastically different: Shylock is awarded, and accepts, the owed pound of flesh.  On further investigation Picard finds that in the mirror universe  kills old King Priam after the death of Hector when asked to return Hector's body for funeral rites, instead of showing one moment of Humanity. Picard thought of that moment in the original prime universe version as the one time in the poem when "that terrible man showed mercy… but not here."  Picard also discovered that Surak of Vulcan hadn't existed in that version of the mirror universe.  This universe is inconsistent with the others in that it was written before the DS9 mirror episodes and references the Terran Empire as still being active in 2367 (although in a conversation with the mirror Worf Picard confirms Kirk and mirror-Spock's belief that the Empire will soon collapse, encouraging Worf to prepare other slaves to be ready to fight back).

Alternate mirror universes might however explain some of the costume and effects inconsistencies in the DS9 mirror episodes. For example in , Alliance ships were seen de-cloaking, and in  the Alliance didn't have cloaking technology.

The mirror universe is mentioned a few times in the Deep Space Nine book trilogy Millennium. During the second novel, it was revealed that Dukat, possessed by the Pah-wraiths, has taken over the mirror Terok Nor, where he is awaiting a final confrontation with Kai Weyoun 5. Also, General Martok crafted an invasion strategy that would have involved moving the entire Klingon fleet to the mirror universe.

In the novel The Soul Key by Olivia Woods, it is revealed that the mirror  staged his death. In addition, the Bajoran wormhole is discovered by the mirror version of Iliana Ghemor.

The mirror universe made an appearance in Star Trek: Voyager - Elite Force, a video game released for PCs. Voyager, having been pulled into a starship scrapyard, encounters hostile Humans working with various aliens, including Malons, Klingons, and Hirogen. These Human "scavengers" are from the mirror universe and operate from a station made up of the remains of a mirror universe  starship, among other things.

The mirror universe also features prominently in Star Trek: Shattered Universe, a video game released on PS2 and Xbox. It depicts an  of the 2290s, and a crossover to the adventures of Captain Sulu on the ISS Excelsior.

According to the novella "Age of the Empress", World War II occurred in the mirror universe, with Japan as one as its participants. One of the results appears to have been a lessening of the power of Japan's emperor, as the Kyoto Imperial Palace was turned into a tourist attraction.

The  novel The Good That Men Do established that Section 31 was already aware of the existence of the mirror universe by 2155.

A mirror universe of the alternate reality is introduced in issue fifteen of IDW Publishing's Star Trek: Ongoing comic series. In this version of events, 's incursion results in a timeline where the Terran Empire conquers the Klingons in 2258, and  recovers the  from . Having destroyed the  and proclaimed himself captain of the "new" Terran flagship, Kirk subsequently tracks down another anomaly similar to that which brought the Narada into their universe and discovers another version of "Spock Prime", who recognizes this world as the mirror universe. Kirk subsequently attempts to use the red matter from the Jellyfish to destroy Vulcan, but this plan is prevented by the treachery of , who beamed  to safety at the last minute. The two Spocks decide to remain on Vulcan to help their people, while Uhura departs as the new captain of the Narada. 

The mirror universe plays a role in Star Trek Online. It appears in in the mission "Jabberwocky", in which the player must return the mirror version of the Orb of Possibilities to that universe, while braving attacks from both the Cardassian True Way and the reformed Terran Empire.  Personnel from the mirror universe also appear in the mission "Lost and Found", as well as the later missions "Temporal Reckoning" and "Ragnarok".

Apocryphal appearances

Comics 
 DC TOS volume 1: The Mirror Universe Saga:
 #9: "Promises to Keep"
 #10: "Double Image"
 #11: "Deadly Reflection!"
 #12: "The Tantalus Trap!"
 #13: "Masquerade!"
 #14: "Behind Enemy Lines!"
 #15: "The Beginning of the End..."
 #16: "Homecoming..."
 Marvel Comics: "Fragile Glass"
 Malibu DS9:
 "Enemies & Allies, Part One"
 "Enemies & Allies, Part Two"
 IDW Publishing: 
 Star Trek: Mirror Images series
 Star Trek: Ongoing: "Mirrored, Part 1", "Part 2"
 Star Trek: Ongoing: "Live Evil, Part 1, "Part 2, "Part 3"
 Star Trek: The Next Generation - Mirror Broken
 Star Trek: The Next Generation - Through the Mirror
 Star Trek: The Next Generation - Terra Incognita
 Star Trek: Voyager - Mirrors and Smoke
 Star Trek: Discovery - Succession

Magazines 
 The Best of Trek #14

Games 
 Star Trek: Voyager - Elite Force
 Star Trek: Shattered Universe
  (Decipher)
 Star Trek Online
 Star Trek Timelines

Novels 
 Dark Mirror
 Star Trek: Dark Passions
 Star Trek: Stargazer: Three
 Star Trek: The Mirror Universe Trilogy
 Spectre
 Dark Victory
 Preserver
 Pocket DS9
 Millennium: The War of the Prophets
 Warpath
 Fearful Symmetry
 The Soul Key
 Star Trek: Mirror Universe
 Glass Empires
 Obsidian Alliances
 Shards and Shadows
 The Sorrows of Empire
 Rise Like Lions

See also
 Star Trek: Myriad Universes

External links 
 
 

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it:Universo dello specchio
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mu:Other universe
nl:Spiegel universum
pt:Universo espelho
ro:Univers oglindă
ru:Зеркальная вселенная
sr:Огледало свемир
 
 
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