Alternate timeline
Alternate timelines were altered versions of the timeline. Unlike parallel universes, alternate timelines often did not seem to diverge from the universe, but instead rewrote history to the point of wiping out the original timeline. However, alternate timelines did sometimes co-exist as alternate realities. (; ; ; ; ; ; )

Alternate timeline creation 
There were several methods of temporal manipulation that could create an alternate timeline:
 A temporal incursion into the past could cause an alternate chain of events to unfold. However, in the cases where the time travel event was part of a predestination paradox, an alternate timeline would be created if the paradox was not completed. () In at least one case, when accidental time travel from 2371 led to the premature death of Gabriel Bell in 2024, an altered future existed for the same amount of time it took for Benjamin Sisko to replace and complete Bell's vital role in the "Bell Riots" of 2024. ()

 Time travel to the future of a timeline could create an alternate future, where the time traveler had never returned back to the past from the future. Such was the case, for example, when the  traveled from 2344 to 2366 through a temporal rift. (; ; ) This however did not always occur, such as in the case when Samuel Clemens visited the 24th century through a temporal distortion created by the Devidians. ()

 Sometimes objects and people who had traveled through time from the future into the past would vanish when the timeline is altered enough, such as an alternate Captain Jean-Luc Picard who came to the past through an energy vortex in 2365. () When objects were pushed completely out of the space-time continuum, they were also erased from history, creating an altered timeline, where the object never came into being. Such was the case with the effect of the main energy beam weapon of the Krenim weapon ship and destroying the interspatial parasites that had infected Captain Jonathan Archer in the 22nd century. (; ) According to the Borg Queen, a similar fate would have awaited the alternate future Vice Admiral Kathryn Janeway, if the Queen had managed to kill her counterpart Captain Janeway in 2378. () More commonly though, changes to the past of a timeline did not cause the objects and people from an alternate future to vanish. Multiple versions would then coexist. These alternate versions could be reintegrated into a single person using 29th century technology. () 
 Sometimes, the original version of the future would come to be regarded as an alternate timeline after changes to history were made. For example, Kes of 2376 in the original timeline damaged , murdered B'Elanna Torres, and vanished by traveling five years into the past. Her presence in the past altered the future, creating a new timeline where, at the time of Kes' return, Captain Janeway prevented harm from coming to her crew, and Kes returned to  instead of time traveling. ()

As a rule, Starfleet officers were forbidden to cause changes in the timeline or to share their knowledge of future events by the Temporal Prime Directive. () The Department of Temporal Investigations was tasked to ensure that time travel events did not contaminate the timeline. () By the 28th century, changing the timeline had become more universally illegal after the Temporal Accord was established. () By the 29th century, Starfleet had taken it on as a mission to use time travel as a means of upholding the integrity of the timeline by fixing changes in the past. It was also Starfleet policy by then to integrate different versions of people into one, when several coexisting ones appeared due to paradoxes and time travel. () By the 32nd century, time travel had been banned outright because the Temporal Wars made it clear that time travelling societies could not be trusted to shun fanatics such as Vosk who felt entitled to alter the timeline regardless of the consequences. (; )

While the prime timeline was usually restored by operatives from the 29th to the 31st centuries, in some cases, the influence of alternate timelines remained as a part of the chain of events in the prime timeline. The crew of the , for instance, would not have learned that they had to travel to the past to convince Zefram Cochrane to make the test warp flight, had they not accidentally witnessed an alternate timeline, where the Borg prevented Humanity's First Contact with the Vulcans and assimilated Earth. (; ) Warnings, temporal incursions and information of the original future were also an integral part of the prime timeline. Such was the case, when Captain Jean-Luc Picard shared his experiences of the future shown to him by Q, to his crew in 2370, causing them to make different life choices that created a new and entirely different timeline of the future. ()  was rescued and aided by its crew members from the future. In 2375, a transmission from Harry Kim from 2390 averted the crash landing of the Voyager. In 2378, technology and assistance from Admiral Janeway from 2404 saved the ship from a longer disastrous journey through the Delta and Beta Quadrants. ()

Alternate realities 
Different versions of a timeline also sometimes coexist as parallel universes. Most active temporal anomalies allow interaction between two different alternate timelines for a moment, making them parallel realities in relation to one another. (; ; ; ) 

Borg Queens had transtemporal awareness, which granted them knowledge bridging into other alternate timelines and realities, described as hearing "echoes" of alternate versions of themselves that co-existed with their own. ()

 The term "alternate reality" was briefly referenced by  and  when the two debated whether such a phenomenon even exists. () 

 Time travel to the past by the Romulan mining vessel Narada through a black hole created with red matter resulted in the creation of an alternate reality. () 

{{bginfo|The writers of , Alex Kurtzman and Roberto Orci, were asked about the implications of the new alternate reality that was introduced in the film in an interview. They explained the new reality runs parallel to the prime reality as a new quantum reality, as the concept was explained by Data in the episode .  The continued existence of the prime reality was later confirmed with  and .}}

 In 2370, Q allowed Picard to shift his consciousness between three different timeframes, one in the past, one in the present and one in the future from Picard's point of view, in order for him to create an anti-time eruption. The eruption was linking these timelines together, and when it collapsed the timelines ceased to exist. ()

 In 2371, radiation poisoning and the temporal energy emissions of an artificial quantum singularity of a Romulan Warbird allowed Miles O'Brien to jump between the prime timeline and several alternate timelines, in some of which Deep Space 9 was destroyed. At least during some of the jumps two of the timelines coexisted. O'Brien was eventually replaced by one of his future selves. ()

 Harry Kim considered an alternate reality as one possible cause for his sense of déjà vu and familiarity to a region of space in the Delta Quadrant, until the Taresian retrovirus was discovered to be the actual cause. ()

 Pockets, folds, and fragments 

 It was possible to artificially create pockets where a previous timeline continued, while the timeline of the rest of the universe was changed. Such technology as a subspace bubble, a subspace isolation field, temporal shields, and the wake of a temporal vortex kept any enveloped object in a pocket with its own timeline. The Guardian of Forever was also capable of maintaining the time planet in its original timeline. (;; ; ; )

 The race of the alien who took the shape of Cosimo existed in temporal inversion folds of the space-time matrix. The folds were parallel time streams visible as temporal anomalies intersecting the prime reality. Inside the folds, reality remained unaffected by changes in the timeline. It was possible to utilize a fold to exchange one's consciousness with an alternate timeline version of oneself. ()

 Temporal causality loops create independent fragments of time, inside which the time of the universe repeats itself. From outside the loop, it appears as if anything inside it simply vanished from the space-time continuum. For people inside the loop, the memories from previous loops begin to assert themselves as a sense of déjá vu and eventually as clearer memories. ()

 Time continuums 

Within the universe there were also several parallel time continuums occupying the same space but in different times. ()
 The Devidians lived in a different time continuum, only a fraction of a second away, with a positive phase variance of 0.004 percent from the normal timeline. ()

 The quantum singularity lifeforms were native to a parallel time continuum. They utilized temporal apertures to travel between continuums and delivered their embryos to mature in nests inside the gravity wells of quantum singularities in our universe. The adult beings of the species were capable of taking humanoid form and were unaffected by temporal fractures. ()

 Elysia was a small parallel time continuum that periodically touched the prime universe in the Delta Triangle region. Elysia was described by Devna as a pocket in the garment of time. The collisions between the two alternate universes produced time warp vortexes in the time barrier between the universes, that allowed passage from one side to the other at high warp speed. ()

 List of alternate timelines 

 Appendices 
 Appearances 
Alternate timelines are depicted in the following episodes or films:
 
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 Related topics 
 Temporal anomaly
 Temporal mechanics
 Time
 Alternate reality
 Parallel universe

 Background information 
The following is a list of episodes involving alternate timelines.

 Apocrypha 
According to the Star Trek: Myriad Universes story "Places of Exile" by Christopher L. Bennett in the  novel of the series, Infinity's Prism, the concept of quantum realities is synonymous with all types of alternate timelines. According to the 2008 novel Fearful Symmetry by Olivia Woods, the mirror universe is also a parallel quantum universe, further suggesting that even trans-dimensional realms are alternate timelines. 

The Star Trek: Department of Temporal Investigations novel Watching the Clock goes into detail about how and why some forms of time travel create parallel alternate realities and others lead to the overwriting of the same timeline. According to the novel, the only way one timeline could replace another is if they coexisted side by side from the moment of their divergence and were merged together again at a later point. Timelines diverge when they shift sufficiently out of phase to become non-interacting, but it is not impossible for them to interfere again at a later point in time. If they did become entangled as a single system, quantum information theory demands that only one of the two conflicting sets of information survive, as quantum history has to be self-consistent. After the merge, it would be as if one timeline suddenly transformed into another. The previous events would still have occurred, but they would no longer be remembered, as the information would have been destroyed. Two different timelines coming back into phase would violate entropy, so there needs to be some kind of force acting to merge them back together.

The game Star Trek: Armada features the , a starship that came from an alternate future where the Borg controlled most of the Alpha Quadrant. Captain Thaddius Deming of the Premonition hoped to warn the Federation of a coming Borg invasion in time to prevent the Borg victory. With the help of Captain Jean-Luc Picard of the , and after making two further temporal incursions, the Premonitions mission was a success and she returned back to the future.

The Star Trek: Coda novels establish the material seen in the relaunch novels for the series ,  and  as being set within a quantum reality known as the  in order to reconcile the differences resulting from the creation of series such as Star Trek: Picard and Star Trek: Prodigy''.

External link 
 

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 Alternate timeline