Does The Ancient One's explanation create a paradox?

In Endgame, it seems that the Ancient One's explanation of

time-travel, and something I believe Hulk and Tony alluded to as well, is that going back in time does not change what has already happened, but merely creates a new, parallel universe, so that "time" is now shaped like a horizontal Y, that shoots off in two directions. This happened in the movie when Thanos travelled to the future and got snapped, since here now exists a timeline where Thanos no longer does his original snap.

However,

this seems to create a paradox, because if we go back in time to the section of the time which is still linear, where does that time-line end up? In the newly created timeline or the original time-line?

See this drawing below (SPOILERS):

This is very similar to the imagery created by the Ancient One when talking to Hulk. The blue line represents time, which splits into two parallel lines. For example, the lower line could represent the original time-line where everything happens as we have seen it in the movies. The upper line could represent the time-line of the universe where Thanos, Nebula, and his entire army no longer exist, since they travelled to the future and got snapped. So, now imagine you are living at the time-point indicated by the red cross. Where does a person living in this time-line end up? The upper or lower time-line?

• Hi, welcome to SF&F! Since you're here, you might want to take the tour. Apr 27, 2019 at 1:35
• BTW, I think the answer is "both." Apr 27, 2019 at 1:35
• @Niffler for future reference, MCU tag should always be accompanied by the Marvel tag Apr 27, 2019 at 7:32
• I appreciate the excellent technical drawings featured in this question. Apr 27, 2019 at 9:20
• Apr 27, 2019 at 9:22

Your mistake is in thinking time stops being linear.

Rather than the branch of a tree, consider a section of railway track.

No matter where you are on this track, the origin and the destination are the same. It's linear.

Now, let's do something drastic, like take the Time stone out of this track's reality.

This is equivalent to flicking a switch, and changing the track's destination. The path is still linear. You can't get to (or start on) the gray section of the track because it will have never existed, which means no matter where you are on this track, the origin and the destination are still the same. (The destination has been altered, but the track itself is still the track).

The Ancient One's concern was that her ability to protect Earth would falter because she had "looked ahead" at the track before the switch was thrown. This is why returning the stone, i.e. "flipping the switch" twice, is ultimately an acceptable compromise to her.

• This is what they were going for, except that their depiction of time travel is contradictory, imo. They remove the stones from the timeline in the past, and return to their present. By your analogy, it seems that they have gone back to their original branch of the train track. Then they go back to the past again and return the stones, which would create yet another branch of the track. The branch they took the stones from would still exist, and still be doomed.
– Kai
Apr 28, 2019 at 14:13
• Rather than "flipping the switch twice" their plan was dependent upon "unflipping the switch" as if it were never flipping initially. May 5, 2019 at 16:01
• Indeed! For binary switches, one might argue there is not a difference between "flipping twice" and "not flipping at all"! May 5, 2019 at 18:27

Based on the fact that the plot explicitly rejects the

Back To The Future based time travel mechanics,

Safe to say that they would end up

in a new reality, where Thanos and co. time travelled to the future, got snapped off, and hence timeline 1 never occurred anymore. This has no tangible effect on the new timeline. What the ancient one says is, as long as the infinity stones are not moved away from their original timelines, there will remain only one timeline. The "Y" will occur only if an infinity stone is completely removed from its own timeline, which is why in the end Cap travels back to the past and places all the stones at their appropriate locations.

Q: Where does a person living at the red-crossed point end up? The upper or lower time-line?

A: Both. When a new timeline comes into existence, it copies everything from the original timeline.

There's no paradox here. Unless a person jumps across time to switch timelines, he/she will continue to exist in both timelines.