When I briefly flicked through responses to Who's mid-season finale over the weekend, it seemed that opinion was pretty divided. A great deal of that seems to come down to whether one thought the ending had the necessary emotional punch, or whether one saw the entire thing as a cynical act of button-pressing designed to wring maximum melodrama out of a series of ridiculous ideas.
Personally, I'm somewhere between the two; I enjoyed the episode, but I could hear the gears grinding in the background as Moffat set up each brick in the wall he needed by the episodes' end.
Anyway, I don't want to talk about the ending, because everyone else is. Nor am I going to talk about how much effort the episode put in to contradicting not only itself, but both previous appearances of the Weeping Angels as well (though as someone elsewhere - alas, I forget who and where - pointed out, if the image of an Angel becomes an Angel, sticking them on postcards is a pretty damn stupid idea).
No, I want to talk about theories of time travel.
I'm not putting this behind a cut because I'm not intending to do much in the way of spoilering, but for those so spoilerphobic you'd bitch if someone told you the title of a given episode (and yes, such people exist), you might want to look away now, since this post revolves around a major plot point from ten minutes or so into the episode. Basically, the Doctor discovers that a book he's reading is actually an account of what's going on in the episode - like if he was reading Sharpe whilst watching the Battle of Talavera, only worse - which leads to him saying something quite extraordinary: he can only change the events the book is based on for those sections he has not yet read.
I'll admit, there's something about that idea that's lunatic enough to appeal to me. Basically, it leaves us with only two options: either the universe is subjective, or the universe is intelligent. And weird.
Back during the classic series, the closest we came to a coherent theory of time travel was that once the Doctor arrived, he couldn't start flitting around the nearby area with the TARDIS, or at least not in any way other than to move in space whilst remaining constant in time. This rule was broken at least once ("Pyramids of Mars" springs immediately to mind, but no doubt there are more), but the basic idea was that the Doctor couldn't, even when he had the level of control over his TARDIS necessary to try - which wasn't often - just nip back in time and save the people who died the first time around. If he watched someone get exterminated, that was it, there was never any way for that person to avoid that fate [1]. Moffat even made a small step towards formalising this unwritten rule during "The Girl in the Fireplace", when Mickey suggests using the TARDIS to travel to the other end of a time corridor and is told it can't be done, because they're all "part of events now".
But what have we learned now? That the TARDIS crew become part of events not through their presence, but through their observation. The mere act of being aware of what will happen cements that fact. One might call it Heisenberg's Certainty Principle, though more fairly of course it should be Schrodinger's Causality Cat.
The immediate question then becomes: who is it who needs to do the observing? If Amy reads the book silently to herself, can the Doctor still not change what happens in the book? What if she reads the book then steps of the TARDIS, with the Doctor still none the wiser? What if some other Time Lord (back when there were such things) reads the book, a Time Lord who never so much as meets the Doctor?
This is what I mean about a subjective universe. It seems ridiculous to suggest that the definition of a "fixed point" is something no time-traveler observed either directly or second hand (though see SLIGHTLY SPOILERISH footnote [2]). It's much easier to believe that it's something internal, that a time-traveler cannot rewrite any event they themselves witnessed, or have been made aware of.
When you start thinking about it like that, everything clicks into place. When the Doctor says "fixed point", he just means "I already know how this plays out". The only things he can fiddle with are the bits that he's got no foreknowledge of. In other words, the Doctor could go back in time and save someone who he failed to rescue the first time around, but only if he was unaware he'd failed in the first place.
Put yet another way, it means that had the Doctor somehow forgotten what he learned through the book that risked forcing a fixed end to the episode, everything would once again have been fine. If Schrodinger had got horribly drunk and lifted his hypothetical lid to check on the hypothetical cat, he would know then whether the cat was still alive. But then, if he replaced the lid, passed out, and remembered nothing the next morning, his experiment is unchanged. The cat is still both alive and dead, even if it had died before he'd opened the box, or, for that matter, if he'd killed the cat himself whilst checking its miserable prison for more booze.
This theory also explains why so many "fixed points" turn out to be well-known historical events. This is no accident, but a feature of the Doctor's continued interest in our planet. Moreover, it means every time he visits this world there's a few more things he knows he can't change, either because he witnesses them, or someone tells him about it.
So what happens when a Time Lord with no knowledge of X comes up against one who's seen X and knows what it involves? Actually, that's arguably a large part of the Third Doctor's exile and struggle against the Master. The Master turns up without the slightest clue regarding Earth's future development, and proceeds to gleefully kick over every sand castle he can find, and the Doctor knows he must be stopped, because the Doctor knows how things should look ten years later, and knows they don't involve giant space squids or rampaging sea-lizards sporting fish-net everything.
The next question regards what happens when a time-traveler inadvertently - or deliberately - changes a fixed point. One option is that it goes horribly wrong for them - the Reapers from "Father's Day" show up and start munching through things. The second, more interesting option, is that it genuinely can't be done. The universe is not just subjective - in the sense that it won't allow one time-traveler to do what another could with impunity - but it is self-aware, and actively resists changes a given time-traveler would recognise as such.
This is one solution to the Grandfather Paradox. What happens when you go back and time and try to shoot your grandfather? You can't. The universe will conspire to ensure it never happens. The gun gets lost. The bullet proves to be a blank. Your grandfather survives but is shaken enough he swears to never speak of it again, even if he lives to see his grandchildren grow up.
Some people are quite happy with this argument. I had an ex-girlfriend who loved time-travel stories, and singled this idea out as her favorite. Personally, I dislike it, because of what it suggests about the following example. I write two letters, address them to myself, place first class stamps on them, and travel a few years back into the past. One letter contains a random series of letters and numbers. The other contains enough details of the future to make me rich, Back to the Future 2 style. I then post one envelope, at random, and burn the other.
So what happens? Does the universe stop the letter no matter what it contains, because it could change the future to a noticeable extent - certainly noticeable by me? Or does the universe know somehow if the "right" letter has been posted, and step in then? Or is the choice of letter itself completely determined, and so whether or not the letter is intercepted is "known" before I even post it? That last option seems like the easiest get out, but of course I could choose the letter before I went back into the past. I could even travel into the future and get someone else to choose it for me. There's no way I can think of to get around the problems of the thought experiment except to say that time travel is possible but the universe is completely deterministic to the point where even when time travelers think they've changed the past, they haven't really, because if they knew the past as well as they think they did, they'd realise everything was the way it always was. To take the Star Trek IV approach "How do we know he didn't invent the thing?".
Perhaps, after an awful lot of words, that's where we have to end up. Not so much that the universe is aware, but that time-travelers are forced only in circumstances in which they are unaware of how their actions will influence later events. To return to an old favorite, The Ring Cycle, the curse of Odin is that his knowledge is so complete; that he can see the downside in any action he might choose to take, and can therefore do nothing. There is something strangely attractive about the idea that meddling with good intentions is a net positive, even without any knowledge of the final result. Something perhaps very human, as well.
Anyway, that's where my mind has been wandering for the last three days. Alternative takes or comments on my rickety logic are, of course, more than welcome.
[1] There was a wonderful in a '90s Doctor Who Magazine short story in which the Doctor says that for time travelers, whether or not someone is considered dead is based entirely on whether they've witnessed that death. I think that can be stretched (as in last season, for example) to being informed of their death, as oppose to it just being assumed, explaining why the Doctor was so devastated to learn of the Brigadier's death (and yes, I choked up just a little), even though had his TARDIS been twenty years further on he would be amazed to learn Lethbridge-Stewart was still tottering about.
[2] Given who proves to have written the book, of course, the "second-hand" argument doesn't track, but that didn't seem to be the implication of the episode.
