Originally Posted by stafs:
“My theory is that in Doctor Who, time is not rigid or consistent but elastic, it's contunually changing. So the Doctor bends time to allow the good things to happen instead of the bad, time then snaps back into the shape it was before and everyone is happy.
Rose, however, didn't bend time but broke it because she interfered with her own past causing the Reapers to appear. They would have eradicated all signs of the paradox and time would have streamed around it, like a river whose path has been blocked by a boulder, but Rose's dad repaired time by sacrificing himself to allow time to flow in a mostly straight line again.”
This is a good explanation. There are two things I'd add.
1/ In a Jon Pertwee episode (it's a Dalek story), time travellers from a future Earth have travelled back to the present day to try and kill a government minister (or something like that), and they fail. Jo Grant asks the Doctor why they can't simply travel back to try again - and he says something like 'Oh, because of the ******* effect' (or something). He’s then quickly interrupted before he can elaborate. Basically, it was clear that you can’t keep trying to change the past. You get one shot and that’s all. Rose had two shots at it and so broke this particular rule.
2/ In this story, the Doctor makes a very important point that Rose’s dilemma would never have happened before, because of the Time Lords. The made sure things like that didn’t happen. Probably, they were the ones who enforced the ‘******* effect'. So the reason none of that has ever happened before seems to be because the Time Lords actually did perform some sort of necessary function and now they’re gone, there’s hell to pay.