As far as time-travel is concerned there are two classic PARADOXES that the gentleman traveller might have to contend with: the creative time paradox and the destructive time paradox, rather more commonly referred to as the “you’re your own father paradox” and the “grandfather paradox”.
The Grandfather Paradox – as pretty much EVERYONE knows by now – is where you go back in time and SHOOT your own grandfather before your father is born. This means that you yourself will never be born and so you cannot travel back in time in order to shoot your grandfather, so he doesn’t get shot and so you DO get born and you DO shoot him and so you DON’T and so you DO and so on forever.
Since scientists would rather like the universe to make SENSE all the time, this sort of thing is BANNED!
Fortunately our old friend QUANTUM MECHANICS gives us a get out clause: the MANY WORLDS hypothesis. Quantum Mechanics is about teeny-tiny events whose outcome is determined by PROBABILITIES. Some people suggest that rather than one single universe, the real world is made up of oodles and oodles of almost identical PARALLEL universes, and the PROBABILITIES connected with quantum events represent the proportion of those universes where the decision goes one way or another.
So, they say, what this means is that if you travel in time, you actually don’t travel to YOUR OWN past, but to a nearly identical past but in a PARALLEL universe. If you SHOOT the person in that universe who matches your grandfather, you are not, ACTUALLY, shooting YOUR OWN grandfather but an innocent STRANGER. If you return to your own time, then your personal history will be intact and your grandfather will have survived without HOMICIDAL attacks by time-warped descendents! The parallel universe, meanwhile, will continue quite happily without you ever having existed.
It is probably worth saying that in Doctor Who, the universe does NOT appear to work this way: there would never be any question of threat from villains CHANGING time if time travel was actually to a different universe and you could just pop home to your own safe timeline at the end. And the Doctor is quite often returning to meet people he has met before, and we are expected to believe that every time they are the same person that he met last time. After all, it would rather undermine the STORY if it is a DIFFERENT Jackie Tyler who Rose meets every time the Doctor takes her home. (Compare with what is established in RISE OF THE CYBERMEN.)
On the other fluffy foot, there is a alternative version of quantum mechanics called the SUPERPOSITION hypothesis. This is the idea that there is only one universe and all the quantum probabilities in a rather mysterious way actually all happen at once and the thing with probabilities just decides which one we see if we are looking.
Obviously, this is even WEIRDER than the parallel universes thing!
Anyway, what this means is that you could give something (or someONE) a measure of how “real” they are by measuring how many superposition states they interact with.
So, in that sense, going back in time and shooting your grandfather makes you a bit less real. So when you shoot him you only shoot as much of him as you are real: or in techno-language you shoot those superposition states that you can interact with, and in that way make grandpa less real. If you get Daddy Richard to do the maths it work something like this: you are four-fifths real, go back and shoot poor old gramps and so he becomes four-fifths DEAD or one-fifth real. Gran stays 100% okay, so if Dad’s reality is then the average of gran and granddad’s, Dad is the average of one-fifth and five-fifths which is THREE FIFTHS real. Then mum is five fifths real as well, so you are the average of three-fifths and five-fifths which IS four-fifths which is where you were at the start!
Actually, this superposition idea might have something to do with what is going on when Dr Who REGENERATES. Rather than just being killed and then his cells magically restarting, perhaps what is happening is that his powers as a TIME LORD mean that at the critical moment he can “shuffle through the deck of probabilities” and instead of being dead find an alternative possible Dr Who who would have survived – like, for example, Dr Twerpee is vulnerable to radiation so regenerates into Dr Tom who would survive, but now Dr Tom is vulnerable to falling from high places when Dr Twerpee was able to survive a similar fall in (pardon my mentioning it) THE PARADISE OF DEATH.
This would help to explain the strange going on in the THREE DOCTORS and the FIVE DOCTORS. When we see Dr Billy and Dr Pat in the THREE DOCTORS they are not being summoned out of the past, but instead the Time Lords are separating out the Doctor into his alternative possible selves.
Like when you get a sweetie wrapper and very, very carefully peel away the top layer of paper so that you get two from one! The Time Lords are able to separate out the probabilities that were Dr Billy and Dr Pat because they know that those were working copies.
Similarly, in the FIVE DOCTORS the time scoop is able to slice off sideways possibilities from Dr Peter – which is why he is so pained by the process. Then the five different Dr Whos that we see are all different versions of the same Dr number five. That is why Dr Pat can remember what happened at his trial – because he is not from the past but is an alternative Dr Who if he had not had to regenerate before getting to where Dr Peter is. Oh and that explains the remark about Rassilon knowing “temporal fusion” at the end because it is Rassilon sticking all the bits of Dr Who back together again like Humpty Dumpty only successfully!
Anyway, all that is the DESTRUCTIVE paradox and if that does not have all the stuffing spilling out of your flappy ears, then never mid because this mystery is about the other one!
The CREATIVE time paradox is about events that cause themselves to happen. Say you hear a great piece of music and then go back in time and WRITE that piece of music so that you can later/earlier HEAR it.
On the face of it, that doesn’t cause the kind of head banging it happens so it doesn’t so it does so it doesn’t contradiction that the grandfather paradox causes.
But WHERE does the music come from? No one ACTUALLY writes it so it just comes from NOWHERE. And the Universe tends to look askance at that sort of thing.
Now, think about what happens in the Doctor’s adventure with the silver Cybermen: “Earthshock”.
The whole of human history depends on the poor old dinosaurs getting wiped out by a huge explosion that happens when a mysterious SOMETHING collides with the Earth. It turns out that the SOMETHING is the Doctor’s companion Adric very not passing his space pilots licence in a spaceship powered by antimatter. The spaceship arrives from the future as a result of the Cybermen trying to smash it into the Earth in the twenty-sixth century. (Look, they have a time machine and really bad aim, alright.)
Except, the REASON for the Cybermen’s plan is that they want to wipe out the human race. The human race that only exists BECAUSE the dinosaurs get wiped out. The human race that only exists BECAUSE the Cybermen try to wipe out the human race.
So where EXACTLY did the human race come from?