Science of Discworld III

Auntie Janie at the train station even though she’s expecting me because I told her I would … the universe will take a different path from the one it would have taken if I had done the expected. But we’ve just seen that even saving Abraham Lincoln from the assassin would have the tiniest, most local, of effects. Neighbours such as the gas-bag aliens on Jupiter wouldn’t notice Lincoln’s survival at all, or at least not for a very long time. After all, we haven’t yet noticed them . 2
    In fact, how will they, or we, notice? How will we be able to say, ‘Just a minute, this newspaper shouldn’t be called the Daily Echo … There must have been a time traveller interfering, so that we’re now in the wrong leg of the Trousers of Time’?
    Auntie Janie making her own way from the station won’t topple empires – unless you believe, with Francis Thompson’s The Mistress of Vision , that
    All things by immortal power
    Near or far
    Hiddenly
    To each other linkéd are
    That thou canst not stir a flower
    Without troubling of a star .
    That is, all contingent chaos butterflies are responsible in some sense for all important events like hurricanes and typhoons – andnewspaper titles. When a typhoon, or a newspaper tycoon, topples an empire, that event is caused by everything, all those butterflies, that preceded it. Because change in any one – or perhaps just in one of a very large number – can derail the important event.
    So everything must be caused by everything before it, not just by a thin string of causality.
    We think about causality as a thin string, a linear chain of events, link following link following link … probably because that’s the only way we can hold any kind of causal sequence in our minds. As we’ll see, that’s how we deal with our own memories and intentions, but none of this means that the universe can isolate such a causal string antecedent to any event at all, important or not. And surely ‘important’ or ‘trivial’ is usually human judgement, unless the universe really does ‘smear out’ most small changes (whatever that means), and major events are those whose singular influence can be distinguished at later times.
    Because they are stories, committed to the way our minds work and not to the way the universe works its own causality, most time-travel stories assume that a big (localised) change is needed to have a big effect – kill Napoleon, invade China … or save Lincoln. And time travel stories have another convention, another ‘conceit’, because they are stories, nearer fee-fi-fo-fum than physics. This is the remembered timeline of the traveller. Usually the plot depends on it being unique to him. When he comes back to his present he remembers stepping on the butterfly, or killing his grandfather, or telling Leonardo about submarines … but no one else is conscious of anything other than their ‘altered’ present.
    Let’s move from large events, large or small causes, to how we influence the apparent causality in our own lives. We have invented a very strange oxymoron to describe this: ‘free will’. These words appear prominently on the label of the can of worms called‘determinism’. In Figments of Reality we titled the free will chapter: ‘We wanted to have a chapter on free will, but we decided not to, so here it is’ in order to expose the paradoxical nature of the whole idea. Dennett’s recent book Freedom Evolves is a very powerful treatment of the same topic. He shows that in regard to ‘free will’ it doesn’t matter whether the universe, including humans, is deterministic. Even if we can do only what we must, there are ways to make the inevitable evitable. Even if it is all butterflies, if tiny differences chaotically determine large historical trends, nevertheless creatures as evolved as us can have ‘the only free will worth having’, according to Dennett. He writes of dodging a baseball coming for his face, and this being perhaps a culmination of a causal chain going right back to the Big Bang – yet if it will help his team, he might let it hit his face.
    But then, what decides it is: will it help his team? That’s not a free choice.
    Inevitable, evitable.
    Dennett’s best example is more ancient: Odysseus’s ship approaching the Sirens. Inevitably, if his men hear the Sirens’ song, they will steer the ship on to the rocks. But the steersman must be able to hear the surf, so there seems no way to avoid their lure. Odysseus