1. Fixed Points
Doctor Who doesnt always portray time and space as flexible and changeable. It seems that there are some events that are so monumental they become unalterable fixed points that cannot be changed no matter how hard you try. These are often centered around death, or at least the appearance of death. Its interesting that fixed points seem to have a lot to do with perception. They only seem to be fixed if either enough people know about them or if they have been recorded, like in the Melody Malone book in Angels Take Manhattan (series 7). They also seem to be more likely to happen if they involve complicated time events, which everyone who time travels would certainly be. Wedding of River Song (series 6) represents the Doctors death as an unalterable point in time. It has to happen in a certain time at a particular space. Does the idea of unchangeable events make sense? Well sort of. Its a convenient way to justify certain plot points. More importantly it ups the dramatic stakes of the story. Lets face it, the Doctor Who universe has been altered and rebooted so much at this point its pretty much accepted that its always in flux. Which is fine. I think in terms of the story though, we need the idea of the fixed points in order to create consequences for certain actions. Otherwise character decisions wouldnt matter because they could just go back and fix whatever they didnt like. Its a delicate thing trying to tell a story about a man without boundaries - when the Doctor is faced with limitations he becomes a much more interesting character. The fantastic thing about Doctor Who is that the premise still allows for fresh ways to explore the idea of travel through all of space and time, what consequences that might have, and how it affects both the Doctor and his companions. As our own awareness of the universe and how it works grows, it only adds to the fascination.