In 'Cold War' when Clara said that the world didn't end in 1983 or she would be there, the Doctor replied, "History's in flux. It can be changed... re-written."

In 'Hide', when the psychic said that he [the Doctor] can change history, he said, "No, no I can't actually. There are fixed points in time, you see."

So... can history be changed/re-written or not? Or is it that history CAN be changed/re-written, but not by the Doctor?

This is all explained in The Fires On Pompeii.

There are certain moments in history that must always happen - events in The Waters of Mars, The Fires Of Pompeii, The Wedding Of River Song are examples where key moments MUST occur.

Everything else is in flux and since the Timelords died, the rest of time is more vunerable.

That scene really reminded me of when Sarah Jane said virtually the same thing to the fourth Doctor in the Pyramids of Mars, and he takes her to see an alternate version of 1980 if Sutekh wasn't destroyed.

But who, or what, decides what are key moments, and therefore must occur, and what aren't?

The Doctor, being a Time Lord just sort of knows, I think, but if you think about it too much then it really doesn't make sense and whether something is a fixed point or not just depends on what suits the plot for that episode

The Doct- uh, the writer.

But seriously, the best example I've heard comes from The Empty Child. It was alright to save the child and allow everyone to live as none of those people would have had a major impact on the future. But The Doctor had to ensure the explosion goes off, because it was written about and documented in the future... a fixed point that determined the future. The object of the explosion was also what led the Doctor there in the first place, thus securing the proper future.

There are of course holes in the concept, but it's the one thing that you have to permit any time-travel TV show if you want it to remain interesting.

It's a bit like being an experienced rock climber, or an experienced jungle guide. You have an intuitive sense for what rocks aren't safe to stand on, which vines are actually snakes, where the quicksand is. Of course, his biology as a Time Lord probably helps with that too.

Or, to think of it another way, it's like fish that have a sense for electrical fields, or bats that can navigate in the dark using echoes. The Doctor has an extra 'sense' for time.

Of course, what always gets me is that, if, say, in Cold War, the Doctor hadn't arrived, the sub crew would have still freed and angered Skaldak, and the world would have blown up. So either it's already pre-determined that the Doctor has arrived and fixed the problem, or the Doctor is actually fighting to not screw up what's already in the timeline.

Doesn't it slightly contradict the theory of parallel universes as well though? Where it's perfectly fine to have a version of time where the Doctor doesn't turn up and the world does end in 1983 but that just exists as a parallel universe. Or have I got completely confused? I knew this would happen if I started thinking about it too much!

Absolutely, but then Who's been pretty inconsistent about that too. I think Who's always avoided the many-worlds interpretation, because it makes it infinitely complicated to keep track of which reality we're actually in, or what changes have happened in what timeline. But then again, we've had parallel realities in Inferno, or Rise of the Cybermen, although they've been treated as completely separate and not just one of infinitely many branches. Turn Left is a bit different in that it was one reality created for a specific purpose.

Hmm, and I guess the desolate 1980 that the Fourth Doctor showed Sarah Jane in the Pyramids of Mars was a parallel universe as well?

Depends on the story.
There's the idea of "fixed events" and "events in flux", but this is really just a plot device. Doctor Who is a television show which is designed to entertain. It isn't real and it doesn't make any sense in reality.

Sadly, I've not seen that one yet. Shocking, I know. I shall make amends.

Edit: having consulting the Oracle of Wikipedia, it sounds like that the Doctor moved forward in the main timeline to 1980, to a changed future that they must avert, rather than an alternate timeline.

It is indeed quite shocking! I can't recommend Pyramids of Mars enough. Tom Baker plays the role of the Fourth Doctor quite dark, under orders from the director and much to the protests of Tom

I'll have a bash.
You could maybe think of it like this:
It's a bit like a copy - paste job.
Using the Pyramids of Mars as an example, The whole series of events occurred with the Doctor and Sarah Jane there.
But they weren't there until they were there. If you get me. They were pasted in from outside that series of events.
If you think now from the point of view of people in 1981 and beyond, the world didn't end (obviously). What happened was- Some god was re-awoken and tried to destroy everything, but the Doctor arrived and sorted it all out.
Until the Doctor arrived and sorted it all out, he wasn't there to sort it all out, therefore until he DID arrive, the course of the events followed to the destroyed earth of 1980.
While still in the process of sorting it all out ( the 'copy' had been pressed but the 'paste' hadn't been done yet) that was still what happened. (the earth was destroyed). He had to complete the 'paste' before the destroyed earth of 1980 had been 'cut', or 'pasted over' from the timeline.

I think in Cold War the Doctor must have known that events couldn't really lead to the nukes being set off, causing a 'hot war'. (Although there would be a very real danger of all that happened was the nukes malfunctioned and exploded, killing some marine life up at the arctic circle. The next day in the news the Russians might deny one of their subs was missing. Meanwhile in other news an unimportant earthquake was registered somewhere near the north pole.)
So there was still plenty of danger, although I'm betting he doesn't think the earth is going to be nuked from all angles. I bet he would STILL tell Clara and everyone else on board that the big danger was the whole earth being nuked as a result of this. The Ice warrior especially, as he already thought he had nothing to lose and might just kill everyone with his bare hands anyway. Him thinking he could work something out to take the world with him buys them all time while he sorts that out. The crew -so they would fight harder to stop the ice warrior and not give up hope that they were dead anyway. Clara -because the last time he let slip something of this nature to a companion, Rose nearly got both of them killed by time eating paradox things, when she saved her dad's life. (there's probably loads more examples, but this is the first that springs to mind). He wants Clara to be VERY careful with time and what she is doing in it. He probably also doesn't quite trust her yet ( he does take her to the haunted house next, with the alterior motive of having the psychic scan her).

So anyway- I think I confused myself somewhere there lol. Maybe we just aren't meant to know some of the answers to the riddles of the universe

I like trying to make sense out of things that make no sense

It makes a change from discussing when series 8 will be broadcast or how rubbish the latest episode is because RTD didn't make it

It sure does

A lot of things like this are still debated by physicists. Stuff that was not thought possible 150 years ago are now believed to be true, like wormholes and obtaining energy from atoms. Some we have actually achieved, some still just theories.
Who knows- maybe Some things in Doctor Who may turn out to be right

Quite, similarly Rory both "never existed" and appeared on a photo in 2010 in "The Pandorica Opens." Big Paradox. Not to worry!

This is how the dr knows....

"It's like when you're a kid. The first time they tell you that the world's turning and you just can't quite believe it 'cause everything looks like it's standin' still. I can feel it. The turn of the Earth. The ground beneath our feet is spinnin' at 1,000 miles an hour and the entire planet is hurtling around the sun at 67,000 miles an hour, and I can feel it. We're fallin' through space, you and me, clinging to the skin of this tiny little world, and if we let go... That's who I am."

Also.

Sorry, I have to make one small correction here:

'Explained'.

:P

Or...

'explained away'

Not a big paradox, a tiny paradox. An insignificant detail that isn't big enough to bother the Universe. A microscopic speck of sand in the flow of time, a shadow of a memory of an event that never happened. One of those inexplicable things around us every day that we never stop to think about.

Like De Ja Vu

Or ( this kind of thing happens to me ALL the time) you are walking sown a corridor you have many times. You spot a door and think 'That's never been there before'. You ask if they've recently put a new door in and everyone says it's always been there.

Whatever you do, DON'T go in there. And if you do, don't look at anything in the corner of your eye.

We've all got pictures that happen to feature faces that we weren't aware of at the time and have had no impact on our lives. They might as well be non-existent.

All this talk of Pyramids of Mars inspired me to watch it again...

After the Doctor takes Sarah Jane to a desolate wasteland in 1980, she says "but it can't be. I'm from 1980" the Doctor's reply is "every point in time has it's alternative Sarah, you've just looked into alternative time" So from that it certainly sounds like the Doctor travelled to a parallel universe. The only time that the TARDIS has easily done that I think. Of course we only have the Doctors word for it that it really was Earth in 1980. I forgot about rule number 1

Well maybe a teensy bit bigger a paradox seeing it's Amy's and Rory's daughter who's looking at the photo (which in my head is one Mels took!)