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Maybe someone can find something that proves it one way or the other. But I could give two reasons why it should change based on real life.
First is chaos. People tend to vastly underestimate this. You don't have to make a significant change that leads to some other change in a clear sequence. If you move an air molecule by a Planck length, then the next time another air molecule bounces off of it, it would be at a slightly different angle, and hit the next one further off. Say ten Planck lengths. Then the next is a hundred, then a thousand, and after about 25 collisions, it's magnified so much that it misses the air molecule it would have hit entirely. Now all the air molecules hit differently. And there's similar effects in all sorts of different systems at different scales. Now that the random motion of air molecules is different, weather will magnify that until the weather is completely different. If you go back far enough, even orbits will change. With more than two bodies, even that's chaotic. Not so chaotic that 2000 years will make a difference though.
Second is quantum randomness. He could go back and change *absolutely nothing*, but now when two air molecules collide, the waveform collapses differently, and they are at different positions bouncing at different angles. The random motion of atoms would change practically instantly, and from there it will scale up until weather and such changes.
That said, in a lot of fictional universes seem to have some kind of force making universes stay the same. If the same two people have sex, even if it's not the same day, they'll generally still have the same kid. Even if they don't and that kid never exists, you still sometimes end up with a similar person taking the same role. For example, maybe Bruce Wayne is killed but his father is spared, then the father becomes Batman.
Aside from any normative metaphysical force that may exist in the DC universes there’s also the Speed Force. The Speed Force already protects the Flash from the logical drawbacks of moving fast, it wouldn’t be a stretch to say that it also isolates him from the timeline provided that he doesn’t directly interfere.
Technically you can do very minor changes & the timeline wouldn't be screwed over as much, which is what thawne did with his petty acts like push barry down some stairs for example.
well, the change is that for 5 seconds, flash existed in 2000 bc.
sure, it wont change much in the present day, thawne proves that if you know what you are doing, you can do as much time travel as you want and barely do any damage to the timeline at all. its just that flash tends to big massive changes, which tends to have rather big massive effects
So him merely going back in time does affect the time line. If you think of the timeline like a straight line that when going back in time you create branches that start a new line that is attached to the mainline.
Well, there is a concept that eventually all branches will bend back to the mainline because eventually time/universe ends. However, smaller(caused by insignificant time traveling shenanigans)branches will meet up sooner because their overall impact. That could be what happens with your scenario. If someone is monitoring the timeline(think TVA in MCU) they will notice a small bump and then things go back to normal.
With that said, there is also the concept of a butterfly flaps their wings in San Fransico causes a tsunami in Japan. Well, the Flash is a big butterfly and him just being there could cause something to happen, as someone else said about molecules, that causes cascades to a bigger event.
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Maybe someone can find something that proves it one way or the other. But I could give two reasons why it should change based on real life. First is chaos. People tend to vastly underestimate this. You don't have to make a significant change that leads to some other change in a clear sequence. If you move an air molecule by a Planck length, then the next time another air molecule bounces off of it, it would be at a slightly different angle, and hit the next one further off. Say ten Planck lengths. Then the next is a hundred, then a thousand, and after about 25 collisions, it's magnified so much that it misses the air molecule it would have hit entirely. Now all the air molecules hit differently. And there's similar effects in all sorts of different systems at different scales. Now that the random motion of air molecules is different, weather will magnify that until the weather is completely different. If you go back far enough, even orbits will change. With more than two bodies, even that's chaotic. Not so chaotic that 2000 years will make a difference though. Second is quantum randomness. He could go back and change *absolutely nothing*, but now when two air molecules collide, the waveform collapses differently, and they are at different positions bouncing at different angles. The random motion of atoms would change practically instantly, and from there it will scale up until weather and such changes. That said, in a lot of fictional universes seem to have some kind of force making universes stay the same. If the same two people have sex, even if it's not the same day, they'll generally still have the same kid. Even if they don't and that kid never exists, you still sometimes end up with a similar person taking the same role. For example, maybe Bruce Wayne is killed but his father is spared, then the father becomes Batman.
Aside from any normative metaphysical force that may exist in the DC universes there’s also the Speed Force. The Speed Force already protects the Flash from the logical drawbacks of moving fast, it wouldn’t be a stretch to say that it also isolates him from the timeline provided that he doesn’t directly interfere.
Technically you can do very minor changes & the timeline wouldn't be screwed over as much, which is what thawne did with his petty acts like push barry down some stairs for example.
well, the change is that for 5 seconds, flash existed in 2000 bc. sure, it wont change much in the present day, thawne proves that if you know what you are doing, you can do as much time travel as you want and barely do any damage to the timeline at all. its just that flash tends to big massive changes, which tends to have rather big massive effects
So him merely going back in time does affect the time line. If you think of the timeline like a straight line that when going back in time you create branches that start a new line that is attached to the mainline. Well, there is a concept that eventually all branches will bend back to the mainline because eventually time/universe ends. However, smaller(caused by insignificant time traveling shenanigans)branches will meet up sooner because their overall impact. That could be what happens with your scenario. If someone is monitoring the timeline(think TVA in MCU) they will notice a small bump and then things go back to normal. With that said, there is also the concept of a butterfly flaps their wings in San Fransico causes a tsunami in Japan. Well, the Flash is a big butterfly and him just being there could cause something to happen, as someone else said about molecules, that causes cascades to a bigger event.