None of this is to say, of course, that the entirety of deterministic eternity is written, every moment a stepping-stone in an inevitable series of events and arrangements of matter that were set in motion by the acceleration and trajectories of every subatomic particle at the moment of the Big Bang. It’s not as simple as that, as you can probably imagine once time travellers start wandering around, murdering and stepping on things.
It might help to look at it in terms of Uncertainty, or some other state of quantum what-have-you. Conscious observation really does determine a surprising amount.
See, right at this moment, for you the following fifteen minutes could go several ways. You’re an ostensibly sentient and self-willed organism. You can carry on doing what you were going to be doing (hopefully reading this, I feel obligated to say), or you can do something else. You can hold your breath until you pass out. You can go and find a rock and start hitting people with it until other people stop you. For the strictly legal record, I urge you not to do that. But the point is, you can. Those were just two examples. You could also do a lot of other things.
 Although if you do, I hope you also come back here and see that I fucking called it, you jerk.
Now, at the end of those fifteen minutes, what has happened has happened. Can you go back in time – either replacing your fifteen-minutes-ago self or replicating yourself depending on the school of time travel you belong to – and change what happened? Probably. Heck, if there’s two of you, you can add a whole lot of mostly-gross things to the repertoire of things you can spend fifteen minutes doing, and that includes smoking a cigarette for the remaining thirteen minutes.
But, until you get there and go back, there’s no way of knowing which series of events actually happened. And even if you do get there and go back, there’s no way of knowing whether you’re going to avert, expand upon, cause, forget, or completely unravel the future you came from. There’s no way of even knowing if you’re from the future of the past you’ve just arrived in. For all you or your possible replica knows, you’re from another timeline altogether and all you’re going to do is spin off a whole bunch more timelines, and smoke a cigarette.
Incidentally, also for the legal record, I would urge any time-travelling smokers out there to go back in time and stop themselves from smoking. Just … I don’t know, give it a try and see whether you manage it. What’s the worst that can happen? Because I assure you lung cancer is a really close second.
Still, the point is, the future isn’t really determined until it’s the present. When you go back in time to change that present, sure – the present becomes the future again, technically, and maybe a future that’s been nailed in place by your observation and participation … but it’s still now your experiential past that you’re trying to re-write. For you, the future happened previously, so going back to change it is like taking off all your clothes and putting them back on again in an attempt to alleviate your embarrassment at having worn a T-shirt with a necrophilia joke on it to your grandfather’s funeral. Not only are you still wearing the shirt, you just stripped naked at your grandfather’s funeral.
In the very process of existing, you change time from future to present to past, nailing it down behind you as you go. If you go back again, the nailed-down path is there. If some giant douche from fifty billion years in the future turns up, they’ve probably nailed down everything that happened in between.
It’s probably just as well nobody noticed them.
As an amusing counterpoint to this, of course, there’s the interesting case of Adolf Hitler who, depending on which redacted government file you get your hands on, was either killed by a time traveller just before the tide of the war turned dramatically back in his favour following a weird-bunker-sex-fuelled epiphany about something called nuclear transpersion … or was shot in the head by Eva Braun, a time traveller who entered his life earlier on with the intention of directing his advancement and who then killed herself because she was from a one-way time-travel school of thought … or was himself a time traveller, sent to avert World War 2 and winding up causing it in a classic paradox-correction event, finally killing himself when the futility of changing the future became clear, which still more ironically changed the future … but changed it to what it was going to be like anyway. Causality, like Eva Braun, is a cruel mistress.
Most of the files agree that the original time traveller was Adolf Hitler’s grandson. Who didn’t exist because he killed his grandfather before his father could be conceived in a weird bunker-sex incident with a time traveller. It just goes to show how the universe cleans up after itself – the whole thing conspiring to set entities like the Myconet so hopelessly adrift from physical, causal reality that it’s hardly any wonder they speak in riddles all the damn time.
And it’s not even as simple as that, but it’s probably best we leave it there.