Doctor Who Christmas Special
But anyways, something's been bugging me ever since I saw it... why could Kazran Sardick come in contact with his younger self?
The Doctor Who topic offers the explanation that, since he was changing and becoming nicer, he technically wasn't the same person anymore. The question is, if that applies, how far does the "different person" part extend?
For example, in the Father's Day episode, couldn't baby Rose count as a different person because, unlike older Rose, her father actually made it to the wedding alive? Can the existence of one person make a difference? Or is it only inner changes?
(On another note, if Kazran could remember his past as it changed, why couldn't Rose? Why could she tell her father about him not being there when she and everyone else were, at that time, probably going to be eaten, and thus she'd have no memories?)
Returning to the original topic...
In the fifth Doctor episode Mawdryn Undead, two versions of the Brigadier (the leader of UNIT), from two different times, are unknowingly on the same ship at the same time. The Doctor mentions that if they touch each other, to paraphrase, time would pretty much blow up, or at least suffer serious repercussions. In a marvelous coincidence, when they inevitably touched, it was at the same moment the Doctor was being forced to transfer his remaining regenerations to another person, so all the energy went into that and both time and the Doctor went on their merry way.
But the Doctor wasn't performing anything like that, so why didn't time blow up? D=