Time travel

"See that's the thing about time travel Barry, the more you do it, the less the rules apply to you."

- Savitar explaining time travel to his past self.

Time travel is a concept in the multiverse that involves physical and temporal teleportation within an Earth's history without directly affecting other universes. Time travel is not without consequences, resulting in multiple possible, or at least altered, timelines. The mechanics of time travel differ greatly between that of meta-human speedsters and technological timeships, as used by Time Masters and time pirates. A specific timeline is considered the "main" or "true" timeline, and is protected by the Legends, and formerly the Time Masters, from possible alterations, the results of which are described as aberrations.

General concepts
"You've all seen how the timeline is malleable... in flux until it is set."

- Rip Hunter

Methods of travel
Time can be ventured through with the use of futuristic transport, such as a Time Sphere or a timeship, like the Waverider. Rip Hunter designed the first Time Sphere, and its designs were obtained by the Reverse-Flash, who would eventually need to use them to have Cisco Ramon build the 21st century version. The reason for that was the Reverse-Flash's loss of Speed Force control required for his speedster ability to time travel after killing Nora Allen. According to Vandal Savage, he also figured out another, unexplained form of travel of his own; and in a similar way Abra Kadabra was able to venture into the past without a timeship, but not back to the future. At some point Rip Hunter's Time Bureau created the Time Courier, a portable device allowing to create portals to different places in time and space.

Real and "mirage" timelines
For the reasons not entirely explained there exists a distinction between which timelines are considered "true", and thus protected by the Time Masters, and which are disregarded as "possible", "fluctuating" or "mirage" timelines. The so-called "Flashpoint" timeline, created as the result of The Flash of 2016's actions in the year 2000, followed by traveling back to the future of 2016, was described by Eobard Thawne as "a mirage", which, however, started to cement in reality, removing Barry's memories and powers every time he used the Speed Force. A variant of 2046's Star City encountered by Rip Hunter's team during their timeship's emergency landing after being shot down by Chronos was described by Rip Hunter as a future that is "not set", a result of time being malleable and in flux. As said by that timeline's version of Oliver Queen, however, the destroyed Star City as seen in 2046 was the result of Sara Lance and Ray Palmer never returning from their time travel quest. It is possible that this future would not come to pass should the two return and assist Oliver in stopping the new Deathstroke's army. Another possibility is Oculus shaping the time this way. It is unknown how the "truth" of a timeline is being figured out after the destruction of Oculus by Rip Hunter's team. For most members of Time Masters, the device was described as a way of seeing the true timeline that must be protected. However, in reality, the senior Time Masters, such as Druce, used it to manipulate the timeline, changing the effects of time travel when needed, with only the Vanishing Point, where the Time Masters HQ were located, being the sole place outside of the device's influence. It is possible that it was through the use of Oculus that the Time Masters prevented Rip Hunter's every time travel attempt to save his family from Vandal Savage and his shocktroopers, considering it of great importance to the timeline.

Array of timelines
When Barry Allen and Eobard Thawne changed the timeline in 2016, a notable change was the erasure of Sara Diggle from existence, and the addition of John Diggle, Jr. to the timeline. Very likely the future "Connor Hawke"/Green Arrow whom the Legends encountered in a "potential future", his older self's existence can be explained by an array of timelines - with each timeline within the array being the past and future of the succeeding and preceding ones, respectively. Thus, if someone were to travel to a future set in a timeline that seemingly contradicts the events and occurrences of the relative past they know of, it could be simply explained by changes from time travel, which created the future timeline, of the past timeline. This goes for all timelines created through time ruptues - each an "item", iterated through. As a result, the Legends not only went to a potential future, but a post-Flashpoint one. This itself is an example of temporal mutability - timelines simultaneously existing with other, or branching off the previous ones, whether they're coexisting with or "overwriting" them.

Multiverse & timelines
Different universes have different timelines, even if the "timeline" of one an into "contact" with that of another - for example, in the case of inhabitants of Earths One and Two crossing dimensions - the changes to the timeline of one Earth won't be reflected in that of another. When Barry Allen changed the timeline in 2016, creating a "Flashpoint", and then when Eobard Thawne subsequently created another timeline, closer to the pre-Flashpoint one - Harrison Wells and Jesse Quick weren't affected by the changes upon returning back to Earth One, being their pre-Flashpoint selves. This can be explained by Barry travelling through time after they returned to Earth Two (and its timeline).

It is possible that said characters' post-Flashpoint selves must have ceased to exist and been replaced by their pre-Flashpoint selves when the event of their return to Earth Two's timeline as if it occurred in the post-Flashpoint reality. The existence of multiple timelines coexisting together, across the multiverse, is yet another example of temporal mutability.

In the case of speedsters, it's possible to break the time and dimensional barriers altogether, shown when Barry Allen traveled to an alternate Earth and stayed there for almost a day, but when he returned back to his, only a few seconds had passed by. Alternatively, it's possible that time flows differently in different universes (and by extension, Earths) as a whole - and the differences may depend on how many dimensions away the Earths are, as with Earths One and Two, time flows roughly, if not exactly, the same, despite multiple other differences, like the changes in vibrations. It's also possible that when Barry accidentally traveled to Earth Thirty-Eight and then returned to Earth One, (seemingly seconds later) that he may have travelled through space and time simultaneously, and therefore it would appear that he was only away from Earth One for a brief moment.

Predestination and loops
Since, as said by Rip Hunter, "time wants to happen", a number of time travel attempts into the past to change effects of a future end up causing said future to happen in the first place. For example, finding his wife and son slain by Vandal Savage prompted Rip Hunter to travel back in time and attempt to kill Savage at the early period of his life, as Hath-Set. He failed, and was captured, making Hath-Set swear to hunt him down, and centuries later, after finding Rip's locket, to kill his family. Eobard Thawne's time remnants, would also be a part of ensuring a time loop exists, even if it is contradictory to the speedster's original life story, or to him being erased from existence. The predestined time travel effects included Martin Stein's healing of Herbert G. Wells in the 19th century, or Mick Rory being hunted down by Chronos, his future self. A possible reason for such loops and predestined events is the Time Masters' control of Oculus, as time loops ensured that the current timeline would remain stable and free of major changes, even if altered, thus the free will of time travelers was ultimately seen and manipulated by the Time Masters. Without such a device, however, the time was now uncertain, forcing Time Masters to stop their support of Vandal Savage. It may have also resulted in the increase of aberrations.

Erasure from time
A dangerous effect related to both Time Master and speedster time travel is the possibility of people being erased from timeline. After losing his Speed Force connection and becoming stranded in the year 2000, Eobard Thawne killed Tess Morgan and Harrison Wells, the original creators of S.T.A.R. Labs in the timeline Eobard hailed from, bringing major changes to the "main" timeline. Thawne himself, or at least a version of him that stole Wells's identity, was erased when his ancestor Eddie Thawne killed himself to stop Eobard from killing Team Flash.

As a side effect of Eobard Thawne's and Barry Allen's efforts to prevent the Flashpoint timeline, a number of lives were also altered while the two traveled, shielded in the Speed Force, such as Dante Ramon dying, and Sara Diggle getting erased from existence, with John Diggle, Jr. ending up Lyla Michaels's child instead. The Pilgrim, an agent of the Time Masters, preferred to kill her targets in childhood, erasing them from further existence in the timeline. However, the time wouldn't cement instantaneously, which allowed the Legends to kidnap their younger selves and protect them from harm aboard the Waverider's temporal zone until Pilgrim was destroyed. Eobard Thawne, who would abuse time travel to achieve the best outcome in a conflict, after living through a timeline in which Rex Tyler figured out his plan and time-traveled himself to warn the Legends, went back to before his first meeting with Tyler and killed the vigilante, erasing him from that point in time. However, instead of preventing Tyler's meeting with the Legends altogether, it altered their conversation, before ending it in an abrupt way, with Rex disappearing before the Legends' eyes.

Time Vaults
Time Vaults are specific structures that allow their users to monitor the array of timelines and see how changes in the present can lead to the result, either different or same, as that of the other timeline. For example, the Time Vault used by Eobard Thawne, disguised as the director of S.T.A.R. Labs Harrison Wells, was used to compare his current timeline's future to that of the timeline he claimed to hail from: the changing structure of a newspaper indicated that both timelines would see the Flash disappearing during a crisis with red skies above in 2024.

However, if the timeline changed to the effect of Barry losing those powers, that part of the timeline would disappear. Interestingly, Harry Wells of Earth Two also used a Time Vault of his own, that he called by the same name. It's unknown how exactly did Harry figure out how to create and use it, considering that he's not a known time traveler.

Time fragmentations
Time fragmentations are specific points in space and time that are invisible to the Time Masters, even with the power of the Oculus. One such fragmentation is the town of Salvation in 1871, which the Legends used while on the run from the Time Masters. This allows people to hide out within these fragmentations which forces the Time Masters or their hired mercenaries to individually check each one. It is unknown whether these fragmentations have any effect on speedster-based time travel.

Timeship time travel
"A Time Master's sacred charge is to do no harm to the timeline. Can you imagine what a timeship would look like in, say, Victorian England?"

- Rip Hunter

Features of timeships
Time Masters and time pirates would use timeships for time travel. These vessels appeared to operate like maneuverable spaceships with weaponry and capacity to use a Time Drive to enter the time stream, a space between time periods, from which a timeship could travel to one or the other point in time. The timeships would rely on the "Temporal Zone", protecting their users from aberrations, and having an emergency option of "scattering" its crew throughout timeline. Timeships could pursue each other in the time stream, allowing Chronos to attack the Waverider.

"Rules of time travel"
According to Rip Hunter, the Time Masters have created a number of rules to keep the timeline stable. The prime of these rules is that time travelers can't go back and change events in which they participated. According to the Time Masters' rules, in this case time would fold in on itself, creating a temporal vortex. To punish those who would ignore the rules, the Time Masters employed bounty hunters and assassins. However, only by ignoring the rules did the Legends manage to stop Vandal Savage's final plot to revert the timeline back to Ancient Egypt, and then remain in one piece.

Co-existing with the other self
Meta-human speedsters use the Speed Force to travel through time in two different ways. One way is similar to that of timeship time travel, allowing a past or future version of the speedster to exist in the same time and even interact with their other self. This way, time travel would not result in a new timeline, but bring alterations to the existing one. In Barry Allen's case, one of his time travel attempts led to Hartley Rathaway's reformation and the creation of weaponry that could be used against Time Wraiths. Barry used a similar form of time travel when traveling to the year 2000, as multiple other versions were seen at his house.

Superseding the other self
The other version of speedster time travel involves superseding their selves from another time period, "becoming" them, instead of coexisting. Barry Allen used this variation of time travel when saving Central City from a tidal wave sent by the Weather Wizard, and later after Vandal Savage sent a destructive wave of energy using the Staff of Horus. In both cases, Barry became himself from a day ago, allowing him to change the course of events. After stopping Eobard Thawne from killing his mother, Barry traveled the same way to a changed version of 2016, dubbed "Flashpoint", and was later bought back to the "main" timeline by Eobard in a similar manner.

Dangers of speedster time travel
The speedsters' time travel is especially dangerous, due to its tendency to fundamentally change and substitute the main timeline, as what happened when a version of Eobard Thawne lost his Speed Force connection after killing Nora Allen and traumatizing Barry. The time traveler was now stranded in 2000 and shaped history, including the creation of the Flash, according to his plans. The resulting timeline, while further altered multiple times, was now considered the main one, with the others being merely "possible" or "mirage" timelines. As a result of Eobard's actions, Tess Morgan and Harrison Wells were dead, Wells's reputation tarnished.

However, while Thawne's timeline changes were part of a plan, some changes could arise as unforeseen consequences of time travel, all the while not affecting the speedsters, protected by the Speed Force as a bunker, keeping them safe from the effects of time paradoxes, as well as memories they were supposed to have in this new timeline. After Eobard Thawne, at Barry's request, prevented Flashpoint from happening, he traveled together with Barry to 2016 using a superseding form of time travel, resulting in multiple changes. For the starters, protected by the Speed Force, Eobard was not erased from existence like a version of him from this timeline, disguised as Dr. Wells, had due to Eddie Thawne's suicide. Furthermore, while Eobard retained memories of his own original timeline (before the changes he brought as "Wells"), and Barry had the memories of his life as shaped by "Wells", the superseding resulted in Barry not knowing what memories he is supposed to have in the new, post-Flashpoint timeline, resulting in a conflict with Cisco Ramon, whose brother the Flash refused to save from death, and a feud in the West family, which Barry had to take time to unravel; as well as seeing people whom he's never seen before, yet has been supposed to, like Julian Albert. When Barry tried to go back in time again to correct these issues, he was stopped by Jay Garrick, who told the younger speedster that the altered timeline is akin to a broken cup: you can try to glue it together, but it will never be the same.

According to Savitar, the more a speedster uses time travel, the less the rules will apply which will lead to different causes and effects. Eddie Thawne killed himself so that his descendant Eobard Thawne would never be born, yet he still lived as a Time remnant before Hunter Zolomon erased him from existence.

Time remnants
"In the year 2020, you and your wife, Tess Morgan, successfully launched a Particle Accelerator that changed the course of history. I need it to happen a bit sooner if I'm going to get back. Much sooner."

- Eobard Thawne describes his original timeline to Harrison Wells

A time remnant is a side-effect of a speedster's time travel that appears out of the Speed Force to prevent further time paradoxes. Although Eobard Thawne originally fought the Flash created in 2020 and thus couldn't have a first encounter with him in 2016, after changing the main timeline by killing Tess Morgan and creating the Flash in 2013, before being erased from existence, Thawne inadvertently altered his own backstory as the Reverse-Flash, heavily depended on interactions with the Flash. This forced a time remnant to appear in 2016 and recreate Eobard's first encounter with the Flash.

As time remnants come from erased and aborted timelines, some speedsters, like Hunter Zolomon and Barry Allen, found out how to abuse time travel in order to force the creation of a time remnant, via not superseding their past selves, aborting their timeline. Zolomon's time travel abuse, in particular, had him hunted down and caught by Time Wraiths, who trapped him inside the Speed Force.

Another idea is that time remnants are parts of a stable loop, with memories of timelines that they themselves doomed to non-existence, but having to continue their existence in the new timeline (created by their future actions - including those that occur in the relative past - being triggered to take effect at once) up until the point where they cease to exist, due to past instances of themselves having travelled through (and thus, been protected by) the Speed Force, in order to cause those changes in the first place.

Possible other methods
After being captured and interrogated by Rip Hunter, Vandal Savage claimed that after witnessing a time travel attempt by Rip in 1975, he studied for 200 years to devise his own time travel ability, eventually figuring it out on his own, allowing him to move in and out of the timestream, letting him proceed undetected. However, Savage may have been lying, as he was seen provided with a timeship by the Time Masters, although the two methods may not be exclusive. Additionally, The Pilgrim had a device or an ability to control time around her, stopping her enemies from attacking her. Abra Kadabra also used an unknown way of time travel to get to the 21st century, but could not return to the 64th without a timeship.

History
At an unknown point in time, the Time Masters organization was created to control and oversee the "correct" timeline from the Vanishing Point. Using the Oculus device to manipulate certain events in time, as well as a Time Fleet and bounty hunters to combat the occasional time pirate, the Time Masters held their hold over the main timeline before their device was destroyed by Rip Hunter's team, along with the Oculus Wellspring. It can be noted that the Time Masters' efforts to maintain a "true" timeline were opposed beforehand, with the timeline greatly changing in the year 2000.

The rogue time traveler that brought these changes was Eobard Thawne, the Reverse Flash. After failing to kill a young Barry Allen and killing Nora Allen instead to traumatize the boy and stop him from becoming the Flash, Eobard unexpectedly lost his connection to the Speed Force, presumably since if his plan worked and the Flash never existed, so wouldn't the Reverse-Flash, who based his appearance, name and source of powers on the Flash. To restore his powers and return back to his time as soon as possible, Eobard killed Tess Morgan and Harrison Wells, the original creators of the Flash from the timeline that Eobard hailed from, stealing Dr. Wells's appearance and identity, and deciding on a goal to create the S.T.A.R. Labs particle accelerator, and so, the Flash, as soon as possible, succeeding by the year 2013. He also supervised Barry Allen's movements, making sure that he'd be hit by lightning along with the Dark Matter energy from the particle accelerator and receive a connection to the Speed Force. After that, Eobard, in the guise of Wells, overlooked Barry's recovery in S.T.A.R. Labs.

It's unknown whether these activities of Eobard were intended or accepted by the Time Masters, or whether his changing of history have or have not affected the organization, making them never know whether the timeline was altered or not. It's also possible that the organization itself is a byproduct of the timeline created by Eobard's alteration or the Time Masters knew that Eobard was not going to be born in that timeline that he created (because Eddie's suicide in 2015) and because of this, they did nothing because Eobard would be erased from existence. One way or another, this timeline, with a number of alterations, continued to exist as the "main one", with a major alteration point being the fate of Vandal Savage's world conquest plan. Time Master Druce intended for Savage to conquer the world and then protect it from a Thanagarian invasion, but after the Oculus's destruction, this plan was aborted by the other Time Masters, who ceased their support of Savage's actions.

This "main" timeline was altered by Barry Allen multiple times. Barry changed it twice in 2015, to stop the destruction of Central City. Then again, by traveling from 2016 to 2015 and back to ask Eobard Thawne for help with the Speed Force formula. Then by traveling from 2016 to 2000, stopping the murder of his mother, then returning to 2016 to find it changed in the form of the "Flashpoint" timeline. As Barry's memories started to fade in this "mirage" timeline, he asked Eobard Thawne to try and make things as they were before, to which Thawne complied, with the additional part of returning back to 2016 with Barry, both of them alive, and the timeline altered again. Trying to prevent the new, unwanted changes, Barry tried going back in time again, before ending up dragged into the year 1998 by Jay Garrick, who convinced Barry to let it go, before returning him back to 2016.

Currently, the earliest known point in time visited by time travelers is South Dakota 70 million years ago, when a time-scattered Ray Palmer was saved by Mick Rory and Nate Heywood from an angry dinosaur.

The Flash
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Behind the scenes

 * In the DC comics, time travel is a long-running concept, with many speedsters and other time travelers changing and creating timelines throughout the comics' runs.