In the Arrowverse, time travel has New Rules as the Plot Demands pretty much whenever it appears.
Spoilers are unmarked so that this page doesn't look like Swiss cheese.
- The events of the first season were kickstarted by the Reverse Flash, aka Prof. Eobard Thawne aka "Harrison Wells", going back in time and killing Nora Allen. He also spends most of the season trying to get back to the future and it's heavily implied that he wants to go back to the same future he left despite going back more than a century and causing massive changes.
- When he killed Barry's mother, he paradoxed himself out of getting speedster powers, finding himself unable to return. He then realized he had to restore his future to get his powers back and return to it, and find a different means of attack that doesn't prevent his gaining speed. The fact that he couldn't have traveled in time at all if he was never a speedster is conveniently ignored.
- One might well wonder what exactly Thawne was thinking in trying to kill Barry, as he only became the Reverse-Flash due to the influence of the Flash. If Barry were to be killed as a child, Thawne would almost certainly never become the Reverse. However, this is never brought up.
- Then the season finale came and Barry was supposed to perform physical time travel and prevent his mother from dying by running at Mach 2 and getting hit by a hydrogen particle. At the same time, Reverse Flash planned to use Barry's travel to get back to the future in some machine he somehow didn't need the first time (presumably since he's lost most of his speed, though you'd think that would be explained in the show itself) and is definitely not needed by Barry. Long story short, Eobard's ancestor Eddie got killed, which erased Eobard from existence, since he'd never be born, but it somehow didn't affect anything that he did throughout the season, nor does it affect anyone's memories of Thawne. Oh, and for some reason, a black hole appeared over Central City, threatening to destroy the Earth.
- Then a younger Reverse Flash returned in Season 2 despite never having been born. This was explained by him being protected by some "time bubble" during his "earlier" time travels. When he gets locked in the pipeline and stopped from going back to the future (from which he would have time traveled again to cause Season 1 to happen) metahuman Vibe is somehow getting erased from existence supposedly because he got powers from Reverse Flash causing the explosion of the particle accelerator. For some reason, only Vibe is affected despite every metahuman getting their powers this way. (Though it could be a side effect of Thawne killing Vibe that one erased time.) Flash has to get Reverse Flash back to the future to save Vibe and yet this time it's achieved merely by running fast enough without using the hydrogen particle.
- Several episodes later, Flash goes back in time to talk with Thawne before he's prevented from ever being born. This time he encounters a so-called Time Wraith, which hunts those who travel through time "and don't know what they're doing" (Never mind that this is the first time when he does know what he's doing. He has a plan and more or less does exactly what he intended to, even if he had to play a bit of Xanatos Speed Chess in the meantime), which have never been so much as mentioned before.
- Also, this season introduces the concept of "time remnants", temporal duplicates who exist when a speedster travels back on their own timeline and interferes with it. The "past" version of the speedster is treated as disposable due to their future being changed, which doesn't really make sense, especially with later episodes showing that there's no time-related problems with remnants existing at the same time as the "originals". Thawne once summons an army of his own time remnants, only for all of them to fade once the original is killed by the Black Flash.
- Time remnants were apparently intended to resolve the Timey-Wimey Ball and make things logically consistent. They make interference in your own past non paradoxical, explained as the Speed Force shielding your past self self so it could survive to become your present self. Except that it turns out, as mentioned above, you can kill your past self without getting yourself erased. This happens multiple times during season 2.
- This issue arises from two distinct types of beings both being called "time remnants." Eobard in Season 2 was protected from erasure so he could kill Nora, activate the particle accelerator early, and keep the timeline in order. The other time remnants we see in Season 2, Season 3, and Legends of Tomorrow are clones created when a speedster travels back in time and brings back the self they met.
- It turns out that the Time Wraiths have a bit of discretion in how they handle things. Barry doubles back on his past to create a Time remnant, which then sacrifices himself to stop Zoom from destroying the multiverse. The death of the time remnant attracts the attention of the Time Wraiths, but they take one look at the situation, and take Zoom instead, excusing Barry's paradoxical action as needed to save the multiverse. Why they didn't grab Zoom earlier isn't very well explained. Apparently it's because Zoom's speed didn't come from the Speed Force, and doesn't show up on the monitoring, so Barry had to get their attention, after which they were then free to act.
- Then all logic is thrown out of the whole thing at the end of Season 2. Barry goes back in time again to prevent his mother's death and succeeds, in effect just undoing all the changes Reverse Flash caused in the first place (which, again, should really have been the result of history being altered so Thawne was never born!) The third season opens with the Flashpoint timeline, a timeline where of course Barry's parents are still alive, but the particle accelerator explosion happened early somehow anyway even though Thawne is the one that made that happen, and there are metahumans running around as a result. It gets even MORE confusing when Barry subsequently reverses his change, which instead of ending up right back in the timeline as he left it resulted in a timeline where Thawne somehow never became trapped in the past, but the events of the first 2 seasons still happened anyway, but with many changes. By this point, it should be obvious that Thawne's timeline is immune to Barry's meddling.
- Savitar, the Season 3 Big Bad who turns out to be a future duplicate of Barry who only exists because of a Stable Time Loop, eventually hangs a big fat lampshade on this mess, when he states that the more speedsters use time travel, the less the rules apply to them anyway.
- Once The Multiverse is added to the equation, it somehow turns out that temporal alterations in one world have zero effect on the others, even though they may have intersected at one point. For example, post-Flashpoint, when Harry and Jesse return to Earth-1, they immediately note certain differences between the pre-Flashpoint and post-Flashpoint versions of Earth-1.
- In Season 4, Barry warns Jesse against going back in time because he doesn't want to risk another Flashpoint...except that they'd only be going back about five minutes, if that, so the chances of a whole new timeline being created are pretty tiny.
- Season 5 attempts to fix much of the discrepancy around Thawne's time travel by retconning that he taps into the Negative Speed Force, which is disconnected from the normal flow of time. Each time that Thawne was apparently erased from existence, he actually went into the Negative Speed Force, and after recuperating can simply go out into the new timeline, protected from any paradoxical changes. Still doesn't explain how he got the memories of living as Wells for 15 years, though. Or how he was able to survive being erased from existence by the Black Flash, which ought to be, as a Speed Force enforcer, able to beat the Negative Speed Force.
- Season 7 has another incredibly egregious example, where Barry worries that he'll create another Flashpoint...by going to the future. How exactly it would be possible to affect the past by traveling to the future is never explained. Maybe Barry's just playing it safe after all the time-travel crap he's already gone through?
- Considering that Eobard's Start of Darkness was him travelling to the future to discover that he was the villain Reverse Flash, this may very well have been a prudent worry. Changing the unknown does seem to be safer than changing the known, unless you are Eobard Thawne.
- And then in season 8, Eobard creates yet another negative force (Negative Still Force), and uses it to travel back and change the past, creating a Reverse Flashpoint, where he manages to become the Flash, making Barry the Reverse Flash. How this works, especially since he killed Barry as a child, is never explained. And then Barry manages to somehow undo the creation of this timeline, which brings things back to normal, except that now Eobard is being erased, even though he was established as immune to erasure before, and is saved from erasure by being disconnected from the Negative Speed Force, even though it was what was protecting him from being erased before!
- Season 1 of Legends made time travel messier than ever before. Not only does Rip Hunter mention "fixed points in time" which cannot be changed for some reason* , but also it is said that changing the future is harder than changing the past, when logically it should be the other way around. It doesn't help that Rip is from a century-plus beyond the rest of the Legends, so his past is the Legends' future, and "the present" for the Legends at large is an arbitrary concept.
- It also mentions nasty aftereffects for health of time travelers, which neither Flash nor Reverse-Flash ever experienced, presumably because they are protected by the Speed Force when time traveling. It wouldn't make sense if the Speed Force could protect them from something as severe as paradoxes and never being born but couldn't protect them from nausea, blindness or anything so mundane.
- Gets even weirder in the Season 1 finale. Vandal Savage plots to set off explosions at three points in time to reset history back thousands of years and do it over. Somehow, stopping and killing his youngest self won't help; the Legends have to stop all three of him.
- In Season 2, it gets even more convoluted when the Legion of Doom acquires the Spear of Destiny, an item that can alter reality, and use it to create a perfect world (for them) and change the entire timeline. And despite the past being changed too via this, the Legends are still somehow able to travel back in time to a point where time hasn't been altered yet.
- To be fair regarding the above, the Spear was destroyed in 1918 and there is nothing to indicate that history was changed at any point before any of the Legends were born.
Crossovers
- In season 2 of Legends, Thawne (wearing his real face) ends up being once again erased from existence by the Black Flash... only to somehow come back in Crisis on Earth-X, once again wearing the face of Dr. Wells. When asked, his answer is just to say that "It's time travel, Barry. So very confusing. I always seem to be saying that to you, don't I?" When Barry has Thawne cornered, he decides not to kill him, possibly acknowledging that Thawne will just return anyway.
Fan Works
- In To Hell and Back (Arrowverse), Barry's first major solo villain is the Eobard Thawne who killed his mother while trying to fight his future self. Later on, Eddie Thawne's psychiatrist is revealed to be a younger version of Eobard Thawne who hasn't experienced those events yet, putting Barry in the awkward position of having to protect Eobard from an enraged Eddie to prevent a dangerous time paradox.
- "Two for the Price of One
" sees Leonard Snart being brought back to life after his apparent death at the Vanishing Point with the aid of Buffy Summers and Willow Rosenberg (Buffy the Vampire Slayer) as part of their efforts to help Snart's counterpart in their world. Once Buffy and Willow have returned to their world, Barry and his allies mention the time they recruited Snart's help in "Infantino Street", which prompts the Legends to realise that the Snart Barry recruited was a future version of Snart as they don't recall visiting the time period Barry retrieved Snart from yet. They speculate that the Snart Barry met on that occasion implied he was a younger version of himself either to avoid mixing up the timeline or just to screw with the Flash's allies, and Snart admits that both are likely to be true.
