Except if the time loop completes, Equestria doesn't collapse because Twilight averts the disaster by clipping Pinkie's hair.
What I'm saying is that I'm not sure if there's a way for both timelines to diverge from the same basic timeline. Let's say we have the initial timeline, timeline A. This timeline diverges into:
A1: In which Future Twilight never delivers the message and Equestria collapses due to metaphysical weirdness
A2: In which Future Twilight
does deliver the message, and Equestria is safe.
Wryte's theory only allows for Timeline A2 to occur because as soon as Future Twilight appears the disaster is averted because Future Twilight's appearance results in Pinkie's hair being clipped. It's a stable time loop. This is all well and good except for the fact that the A1 timeline was never actually in danger of occurring.
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Okay, I agree with that, the problem is that Pinkie wouldn't be
in the library looking for the spell unless Future Twilight had delivered the message. Pinkie wouldn't be there in the first place unless Twilight had found the spell in the previous timeline. By that point the time loop is already in place, and it won't deviate from the set path (Twilight receives the message and disaster proofs Equestria, including trimming Pinkie's hair) unless an outside force manipulates it.
Basically, the fact that Twilight received the message in the first place means that the scene of "Pinkie in the library with hair in her eyes" doesn't happen unless something outside the timeline changes it. Either Twilight doesn't get the message in any timeline (A1), or she does, and averts the disaster (A2).
edited 6th Jan '13 5:44:28 PM by JapaneseTeeth