How can changes to the timeline affect just one universe?

Question

In season 3, a short dialog between Harry and Barry (Harry notices the changes to the timeline because he just came from another universe, that was not affected) establishes that changes to a timeline only affect one universe, so every universe has its own timeline.

On a first glance that sounds logical. But on a second (fridge logic) it does not. If the timeline of Barry's universe is changed, than all events of interaction with the other universe are changed, too. For example in the Flashpoint timeline, I assume Berry never traveled to the other universe (it was "the other" Flash I assume?) And even if the timeline is changed slightly so that he travels over there a bit earlier or later or with another hair cut, the timeline of the other universe has to change, too.

Only in the case the timeline is changed completely, so that he never travels there, the "time remnant" mechanism could kick in and a time remnant Barry will visit the other universe out of nowhere. But what about visitors of the other universe? How can they (in the new timeline) still visit the old timeline? In the Flashpoint timeline doesn't zoom appear? Doesn't Harry and his daughter visit our universe? If yes, how can they, on a later timeline, visit and remember the old timeline? Or in the slightly altered resulting timeline, how can they not remember the speed laboratory? They must have seen it on their visit in this altered timeline.

Is this a plot hole or is it explained somewhere in or out of universe?

Answer 1

I'm sure there's a Cisco-explains-it-all scene somewhere, but I can't remember what episode that would be in.

However, based on evidence of events in season 3, I would say that time in the Flash is actually very stable and is in fact very hard to change.

For one, when Barry created Flashpoint, he changed so much of the timeline that the entire universe started ripping itself apart. Thawn insults Barry, saying that it is because Barry changed time "like an amateur". This would imply that it is possible to change the past and future, but it has to be subtle.

This is further backed up as season 3 progresses. Barry manages to fix Flashpoint, but in doing so there are some very subtle changes. Team Flash spends the entire season trying to change the outcome of the future with varying success.

In the final episode of the season, they show that most of the events they thought have changed or prevented actually still happened. In the end, the only way they were able to save Iris was for Wells to pretend to be her. The events actually still happen exactly as Barry saw in his first trip to the future.

So to answer the question, changes to a timeline do only effect one timeline because those changes are very small. Time in Flash is actually very stable and mostly linear. Most if not all of the events of Thawn and Zoom still occurred during season 1 & 2. Barry still traveled to other Earths. People traveling from Earth 2 and beyond actually are traveling to the new timeline created after Flashpoint. The changes are very subtle that they most likely don't even notice.

Answer 2

I'm pretty sure the show hasn't explained this, having seen every episode and being a bit of a fan, so the only way to approach this question is by flow diagram.

Lets assume, as you said, that Barry changes the past in his universe, nothing changes in other universes - fine. That's fair and that's what Harrison Wells said when he came over and saw some changes. Back to the Future rules. You have one current reality (for Earth one), and any number of past realities.

Season 2 happens. Earth 2 Harrison (Harry-2) comes to Earth-1 for help. They defeat Zoom, Flash's dad dies, Harry-2 goes home, Flash creates Flash-point.

There are two possibilities. Flash created Flashpoint before Harry-2 came to Earth-1. Everything changes. Maybe Harry-2 comes to Earth one and the Flash still stops Zoom. Maybe Harry-2 comes to Earth-1 and Zoom wins (unlikely cause Flash was happy in his flashpoint world). Maybe Harry-2 goes to Earth-4 where the Flash was trained by Thawne and Earth-4 flash stops Zoom, maybe he goes to Earth-19 and Supergirl kicks Zooms but. That's all filed under possibility one.

Possibility two, (and I think this is the one to go with), Flash going back in time after Harry-2 returned home doesn't change what happened in Harry's past. In Harry's past (and in the Flash's past), they defeated Zoom. Their history is the real history. It's everyone in Earth one who had a different history.

Does this create a conflict? No. All it creates is different memories. Harry-2 remembers pre-flashpoint because that's what he lived through. Nobody else remembers pre-flash point because their timeline as changed.

In this scenario, If Harry-2 had returned during Flashpoint he'd have looked around, found Flash and said "WTF did you do??? you can't mess with time like this". As it was, he returned after Flash had tried to undo Flashpoint and there were only minor changes and he recognized the minor changes and had a similar talk with flash about "the first rule of speedsters is - don't mess with the timeline".

You're thinking that because Flash re-wrote the past, Harry-2's time on Earth one should be re-written, but not necessarily, it all could still have happened to Harry-2's experience, just as it happened with the Flash's experience. In other words, all you need to do to keep your memories when the Flash does a reset, is travel to another Earth.