Guardian of Forever
The Guardian of Forever was a mysterious construct of an unknown, ancient alien race, which functioned as a time portal: a gateway to the time vortex that allowed access to other times, locations and dimensions. 

It was located on an ancient planet where the focus of all timelines throughout at least the Milky Way Galaxy converged. It was positioned among the eventual ruins of a large, forgotten city that stretched beyond the horizon in all directions around it. Based on initial observations, the ruins appeared to be at least one million years old. The Guardian said that it was far older than that, even exceeding the Earth's sun's age of 4.6 billion years. () 

After the factions of the Temporal Wars attempted to use it for their own purposes, the Guardian of Forever went into hiding far removed from its original planet. It was eventually located by the  data on Dannus V where, taking on the form of a man named Carl, it tested  before it helped her to survive her deteriorating condition from being displaced from her own time and the mirror universe. ()

Design 
The Guardian of Forever appeared physically as an inert, crudely asymmetrical, circular gateway--a rough-hewn torus--comprised of a quasi-metallic substance that defied sensor analysis and was able to display the flow of time itself, even though it could not alter the "speed at which it displayed the past." 

It generated immense ripples in time that manifested themselves as spatial disturbances in the region around the planet where it was located. Apparently sentient, it was capable of speech and interacted with anyone it encountered, responding to external stimuli such as questions and actions. Spock was unconvinced that the Guardian could be a machine as Federation technology understood mechanics, and it, itself, when James Kirk asked it directly whether it was a machine or a being, called itself "both...and neither," implying that it was neither quite a machine as Federation technology understood mechanics nor quite a being as Federation biology understood life, and yet both a machine as Federation technology did not understand mechanics and a being as Federation biology did not understand life. The object would flash whenever it spoke and emitted a gaseous fog within its center whenever it generated a portal. ()

By the 32nd century, the Guardian was able to assume human form as a man named Carl to test subjects that wished to use the Guardian's portal. By this time, the Guardian could manifest as both its original portal form and Carl simultaneously. It also had the ability to take on the form of a simple wooden door rather than its more elaborate original form. When  passed through the Guardian's portal into an alternate timeline of the mirror universe, her physical form appeared to pass out for a minute rather than vanishing. However, Georgiou's bio-scanner still showed data points from the three months that she had spent in the alternate timeline before the Guardian pulled her back. This showed that the Guardian of Forever was capable of transporting subjects across both time and universes if it so wished. When the Guardian agreed to help Georgiou, as it she did not wish to return to the mirror universe, it was able to set the portal for a time period when the prime and mirror universes were still aligned which would be a place that Georgiou could survive in. ()

History

Contact with the Federation 

The Guardian was first discovered by the crew of the  in 2267. Encountering powerful waves of space displacement, which Spock described as "ripples in time", the Enterprise tracked the waves back to their point of origin on a previously-uncharted planet. Doctor Leonard McCoy, suffering from paranoid delusions as a result of an accidental overdose of cordrazine, beamed down to the surface in an attempt to escape the ship.

Searching for McCoy, Captain James T. Kirk and Spock encountered and made contact with the Guardian, who offered them the chance to explore the past. As the Guardian was displaying images from Earth history, McCoy emerged from hiding and leapt through the time portal, arriving on Earth in the year 1930.

The landing party soon discovered that they had lost all contact with the Enterprise, and the Guardian informed them that McCoy had effected a change in history, wiping out their civilization. Realizing that they would have to correct the damage to history, Kirk and Spock had the Guardian replay Earth history, and traveled through the portal to a point in time prior to McCoy's arrival. Eventually successful in their effort to restore the timeline, the Guardian returned all three of them to their proper place and time only a moment after they initially departed. ()

In 2269, a team of historians, accompanied by Captain Kirk and Spock, used the Guardian to investigate Federation history. Their investigation included firsthand accounts of the formation of the  civilization and the monitoring of  history of the 2230s and 2240s. While Kirk and Spock were visiting Orion, their support team was monitoring Vulcan's past, which, in doing so, inadvertently removed Spock from the proper timeline.

Spock, however, was protected from the change while he was in Orion's past, and the change to the timeline went unnoticed until he and Kirk returned through the Guardian. Once the cause was determined, Spock, using the name Selek, was able to use the Guardian to return to his own childhood on Vulcan, and prevent his death during a personal ordeal to determine the course his own life would take before he was scheduled to go through the kahs-wan ritual. ()

In 2381, what appeared to be a model of the Guardian of Forever was located behind a bar located at Starbase 25. Both Kirk and Spock had previously visited this bar sometime during the 2260s. ()

Temporal Wars 
During the Temporal Wars, many attempted to use the Guardian as a means of traveling through time for the conflict. Because of this, the Guardian moved itself far away from its original location and went into hiding. By the 32nd century, it was located on the planet Dannus V. ()

"Carl"  

By the 32nd century, the Guardian appeared in the form of Carl, a visual representation in the form of a Human male, who was guarding a trans-dimensional door on Dannus V. 

In 3189, while searching for a cure for 's time/dimensional displacement illness, Michael Burnham used the 's  in conjunction with Federation databases of the 32nd century to locate a potential cure on Dannus V.

When initially encountered, Carl was alone reading a newspaper detailing the future death of Georgiou. He advised both Georgiou and Burnham that the door was specifically for Georgiou to pass through. He also advised that, while her current condition would stabilize, she could still die on the other side of the door. ()

When Georgiou returned from an alternate timeline that she had created in the mirror universe, Carl revealed that it was a test to evaluate (to "weigh") how she would affect another universe/timeline. Although Georgiou had failed to save the , she had tried for peace and saved  who would go on to save a lot of other people, thus passing Carl's test. Burnham demanded to know who Carl was, and the Guardian, deeming Georgiou and Burnham trustworthy, revealed his true nature as the Guardian of Forever and explained how he had ended up Dannus V. The Guardian opened a portal to a time in which the prime universe and the mirror universe were still aligned where Georgiou could survive. Carl encouraged Georgiou to consider herself lucky as she was getting a second chance which was pretty unique. However, Carl warned Georgiou that it wouldn't be easy and will be "bumpy and painful. Lots of rainstorms, heartaches. But that's life. Or so I'm told." Georgiou asked if Burnham could come with her, but Carl told the two women that Burnham was right where she was supposed to be and vanished, leaving behind the Guardian's portal. After Georgiou entered it, the Guardian of Forever vanished. ()

Images seen through the Guardian of Forever

Appendices

See also 
 , the original story outline and screenplay published by Harlan Ellison, who "never forgave [the writing team of TOS] for rewriting" his work.
 The Nexus: a formless, infinite plane of existence shaped by its inhabitants's individual desires and dreams; where the rules of the physical universe, such as those of time and space, do not apply. Should anyone wish to leave, they could literally go to any point in the spacetime continuum of their choosing. ()

Background information 
In the original teleplay for "The City on the Edge of Forever," the Guardians of the Time Vortex were 9-foot (2.7 meter) tall, humanoid statue-like beings.

The original Guardian was designed by Desilu Supervising Art Director Rolland M. Brooks, because his colleague, regular Set Designer Matt Jefferies, was sick with the flu that week. (Star Trek: The Animated Series DVDs special feature " text commentary") When Jefferies returned to his duties and saw the lopsided donut-shaped set piece for the first time, he reportedly exclaimed, "What the hell is this?!" according to Story Editor DC Fontana. (These Are the Voyages: TOS Season One, 1st ed, p. 517) Special Effects Artist Jim Rugg was responsible for the light effects for the Guardian. (Cinefantastique, Vol. 17, No. 2, p. 24)

The character voice of the Guardian was provided by Bartell La Rue in "The City on the Edge of Forever".

The footage seen in "The City on the Edge of Forever" through the time portal is, for the most part, lifted from old Paramount Pictures and  films, such as the 1945 swashbuckling film  in the latter case. It should be noted though, that Desilu had not yet been acquired by Paramount's holding company Gulf+Western at the time, and that, while much of RKO's assets were acquired by Desilu Studios after the former went defunct in 1957, its backlog catalog of film productions was not part of it, it being divided over several other major Hollywood studios. The use by Desilu of these clippings in the episode therefore, regardless of their origins, essentially constituted copyright infringements in current understanding.

The Guardian's last words in "The City on the Edge of Forever" were: "Many such journeys are possible. Let me be your gateway." This suggested that further stories could have been written involving the Guardian of Forever appearing in The Original Series, but the Guardian's only other canonical appearances were in the animated episode , and, decades later, the  episodes  and "Part 2". In "Yesteryear," the Guardian's voice was performed by James Doohan; in the Discovery episodes, a human-appearing personification of the Guardian, "Carl", was played by Paul Guilfoyle, with archive footage of Bartell LaRue's voice used when Carl's true identity was revealed.

An original draft of the episode that eventually became  featured a Vulcan science team researching history through the Guardian of Forever. In that story, the team accidentally caused the death of Surak, the father of modern Vulcan philosophy – as a result, the Time of Awakening never occurred, and the Vulcan race had essentially evolved as the Romulans. The timeline was reset when Sarek, aboard the Enterprise-D to greet the scientists, used the Guardian to go back in time and take Surak's place in history. ()

Using the Guardian on  was discussed at one point but decided against.  However, one of the Wadi chula game pieces from  was modeled after the Guardian. ()

During production on the film , it was briefly rumored that the Guardian would be used by the Romulans to go back in time.

Apocrypha 
In The Worlds Of The Federation, as in Alan Dean Foster's prose conversion of "Yesteryear," the Guardian's planet is designated as "The Time Planet." In The Worlds Of The Federation, moreover, instead of its coordinates being provided, it is listed as "Stellar Location CLASSIFIED," and it is described as being quarantined against visitations under "General Prohibition P-119," presumably in consequence of the events in "Yesteryear."

The Guardian appears in the game Star Trek Online in a mission involving the players character breaking the quarantine of the Guardian's planet. The player must travel through the portal in order to follow a group of Klingons and stop them from destroying the Enterprise from the Original series and rescue Miral Paris, who they kidnapped to create and distribute a cure to the Klingon augment virus. Also, if the player questions the Guardian about why it speaks in riddles, it gives the same response that it did to Spock: that it answers as simply as their limited understanding allows.

In the novel Yesterday's Son, Kirk and Spock use the Guardian to rescue Spock's son Zar from the ancient, doomed world of Sarpeidon. They also discover that Zar is able to communicate telepathically with the Guardian. In the novel's sequel, Time for Yesterday, the Guardian is summoned away from its duties of regulating time in the Milky Way by its capricious Creators. Zar uses his telepathic powers to return the consciousness of the Guardian to the gateway and banish the Creators to a universe where they cannot harm our space-time.

In the novel Engines of Destiny, the Guardian appears in an alternate timeline where the Borg have conquered Earth; having learnt of the change from her 'prime' counterpart, an alternate version of Guinan visits the Guardian's planet to ask for help, the Guardian revealing how the timeline can be restored to normal.

In the novel The Devil's Heart, the titular object, an ancient stone rumored to have vast powers, is revealed to be a "seed" created by the same civilization who created the Guardian and meant to create another Guardian on the world it was sent to, before it went astray.

In the novel First Frontier, members of the Clan Ru, a species whose pre-sentient ancestors – Earth dinosaurs – had been rescued by the Preservers before the , used the Guardian to destroy the asteroid which would hit Earth, allowing their species to remain and evolve there. The timeline would be reset by Kirk, with the assent of the Clan Ru, when they discovered that in this altered history, their species destroyed itself in a series of nuclear wars before it could reach the stars.

In the  series, during a flashback to several thousand years ago, the Guardian is used by a younger Q when attempting to find something new, allowing him to make contact with the being known as 0 (although the Guardian briefly tries to deny 0 access to this universe), who subsequently contacts (*), Gorgan, and The One via the Guardian. It is hinted here that the Guardian was built by the race that would eventually evolve into the Q – when looking at the Guardian, the young Q comments "At least our ancestors made things." This reflects his dejection at the stagnant nature of the Q Continuum.

In the alternate future seen in the  book trilogy Millennium, the Guardian of Forever was key to Admiral Kathryn Janeway's Project Forever. Janeway, along with a combined Federation/Borg armada, hoped to use the Guardian to go back in time and wipe out Bajor (the Federation was at war with the Bajoran Ascendancy in this timeline and were facing the end of the universe). However, at the very moment that Janeway and a team arrived at the Guardian, the Grigari set off a singularity bomb, creating a black hole that destroyed the entire Federation/Borg fleet, the Grigari fleet, Janeway, and the Guardian. The timeline was later reset by Benjamin Sisko and the crew of Deep Space 9.

In the alternate future presented in , set in 2406, Admiral Riker used the Guardian to travel back in time to save Deanna Troi's life from an attempt to kill her and prevent her participation in a conference in 2366. In the timeline where she died, the species involved went on to secretly rebuild and become a major military power, but with her survival, Troi's empathic powers revealed that they were lying to gain time, and the conference was abandoned until the species was in desperate need of assistance and were subsequently permitted to rebuild under strict observation.

Although the future Data also attempts to travel back in time to maintain continuity (Riker decides to travel back after a new autopsy reveals that Troi was killed by a poison that was only developed after her death, suggesting that she was killed by a time-traveler, but Data feels that Riker is clutching at straws as the poison in her system could have been a natural fluke), it is revealed at the conclusion of the novel that history had already been changed, and Riker's actions actually set it back on the right path, with the travelers incredulous that the Guardian never told them that detail simply because they never asked.

In Spock Vs. Q, Spock mentions having used the Guardian to travel back to the 20th century. Q describes it as "that lopsided donut thing" before Spock corrects him and asks him if he knows it. Q responds, "Been there, done that, got the T-shirt."

In the novel Provenance of Shadows, the Enterprise returns to the Guardian for a third time, just before the end of its original five-year mission. They respond to a distress call from the science station erected in orbit of the Guardian's planet. Three Klingon ships attack the station and the Enterprise. They are successful in boarding the Enterprises bridge and manage to take over the ship, forcing the crew to abandon ship, but Captain Kirk manages to escape, obtains several phasers, and transports down to the planet. There, he steps through the Guardian and ends up back on the Enterprises bridge moments before the Klingons boarded. As the crew evacuates the bridge, Kirk sets the phasers on overload and sacrifices himself to kill the Klingons, but his now-alternate timeline counterpart survives. As a last resort, the Klingons crash their ship into the Guardian, apparently destroying it.

The image of the sailing ship firing its cannons seen in the Guardian's portal is also later seen in the intro to the mirror universe episodes of .

A 1978 story in an issue of the Gold Key Star Trek comic, titled "," features the Guardian.

In the short story "Guardians" from the anthology book Strange New Worlds VII, the Horta are assigned by the Federation to serve as the protectors of the Guardian after a failed attack/attempt to alter the timeline by the Romulans. Since their lifespan extends for the next forty thousand years, they are witnesses to vast changes in the Federation, the ascendancy of Humanity to incorporeal beings and the eventual exploration of the known universe and beyond. The Guardian undergoes periodic attacks and attempted usage from the Borg, the Romulans, and the Crystalline Entity, each time defeated by the Horta.

In the short story "Orphans" from the anthology book Strange New Worlds 9, the Dominion attempted to seize the Guardian in 2374 during the Dominion War, but the Jem'Hadar were unable to activate it as it did not recognize their altered DNA. Section 31 sent Erik Pressman and Julian Bashir to Angosia III to recruit super-soldiers such as Roga Danar for a mission to aid them in recapturing the Guardian's planet. After successfully completing the mission, Danar asked the Guardian what it desired. When it said it wanted freedom, Danar granted it permission to leave the planet, which it then did. The story also states that the Andorians had their own name for the Guardian: the T'Sh'Iar, which meant "God's Window."

The Guardian is also featured in the novel The Star to Every Wandering, in which it plays a sizeable role. In this novel, the events of the film  play out differently, with Kirk's emergence from the Nexus resulting in a temporal loop with catastrophic effects. This Kirk, returned to the Nexus by these events, then recruits another version of himself from within the Nexus, who then rescues yet another Kirk from the  by employing the Guardian of Forever to travel through time. This third Kirk then travels to Veridian III and dies helping Picard save the Veridian system while the second Kirk ends up contemplating using the Guardian to travel back in time and save Edith Keeler's life without damaging history.

External links 
 
 
 

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Artificial lifeforms
Time travel technology