Time is wibbly-wobbly timey wimey. What that means to me is that as humans we see it as linear but timelords see it differently. But since we're humans I'll try and explain it in a framework we can understand. It's an old framework that others have used but seems to suffice.
Let us for the moment make the assumption that time is in at least one way linear. Like old celluloid film it moves from one instant to the next ever advancing.
Now, let's suppose that there are no reels to collect it and that it tends to ball up and bend and twist all around itself. So much so that it becomes incredibly tangled to the point of some moments touching other moments. If you had the ability to see it you could see moments that were completely unrelated to where you are and what was happening at your particular moment.
Let’s assume timelords have that ability. It’s been more than hinted at and I’m willing to leave that as a given.
So what happens when a timelord regenerates?
Well we know that there’s an incredible release of energy. We also know that the timelord becomes someone different. I think that during this process two things happen.
One of the classical views of time is that moment to moment the timestream splits from itself to create new streams as events happen in the universe.
Now as timelords are tied to the universe through the time vortex what I believe happens is that when a timelord regenerates he splits off into a new timeline within the larger whole of the time vortex.
If we go back to our film analogy this means that the previous incarnation of the timelord in question goes off on his own personal timeline creating another ribbon of film that continues to make it’s own tangled ball.
The fresh regeneration starts a fresh timeline but two things happen. First the energy spent to regenerate pushes them rapidly along the new timeline. This is one reason why timelords don’t cross their own timelines immediately after a regeneration - because the universe is pushing them apart.
Second the energy spent during their regeneration kind of obscures their past view. A sort of burnt spot on the film so to speak. Kind of like being able to remember past events from years ago but not remembering what you had for lunch.
(This could also be the reason for only 13 regenerations. Maybe massive dementia sets in after that or something. Pure speculation on my part though.)
Then as the timelords move down their own personal timelines the film bundle/ball/whatnot gets larger and more tangled. Once all that happens there certainly will be random contacts between old regeneration timelines and new.
This also explains why past regenerations often look older than when they originally split off. They have not only lost a certain part of their energy creating the new regeneration but they have aged along their own personal timeline as well.
As for previous regenerations not recalling information when meeting newer ones I don’t have a full explanation. Perhaps though when they contact their own timelines there is a push akin to the same magnetic polarity keeping itself away from each other. The universe never seems to want two things occupying the exact same place in time and space. That’s been clearly said in the show in both older episodes and newer ones.
Perhaps as a self defense mechanism the universe prevents something (or in this case someone) from becoming more like another object that it’s already very close to. The fields repel and the information between them during their brief contact is wiped out but not evenly.
Remember that a past regeneration has spent some of his energy and is thus weaker than the newer one. That certainly would make me believe that more of the stronger one remains after a meeting than the weaker one.
Thus newer regenerations remember new data after crossing their own timestreams while older ones do not.
Timey Wimey for me to get some Sleepy Weepy. ;-)
Cross-posted from Dreamwidth ( http://nimitzbrood.dreamwidth.org/274197.html ) but feel free to comment here as well.