The Science of Discworld II

Lords and Ladies we find the following passage:
    There are indeed such things as parallel universes, although parallel is hardly the right word – universes swoop and spiral around one another like some mad weaving machine or a squadron of Yossarians 4 with middle-ear trouble.
    And they branch. But, and this is important, not all the time. The universe doesn’t much care if you tread on a butterfly. There are plenty more butterflies. Gods might note the fall of a sparrow but they don’t make any effort to catch them.
    Shoot the dictator and prevent the war? But the dictator is merely the tip of the whole festering boil of social pus from which dictatorsemerge; shoot one and there’ll be another one along in a minute. Shoot him too? Why not shoot everyone and invade Poland? In fifty years’, thirty years’, ten years’ time the world will be very nearly back on its old course. History always has a great weight of inertia.
    Almost always …
    At circle time, when the walls between this and that are thinner, when there are all sorts of strange leakages … Ah, then choices are made, then the universe can be sent careening down a different leg of the well-known Trousers of Time.
    This kind of question can be asked of any dynamical system, emergent or not; but it takes on a special aspect when the dynamic ‘makes itself up as it goes along’. In a rerun, would it make up the same thing? Would it tell the same story? If so, that story is robust: it has a degree of inevitability, not just in some particular run of history, but in all of them.
    Science fiction writers explore historical phase space in ‘alternate 5 universe’ stories, where one historical event is changed and the author develops possible consequences. Philip K. Dick’s The Man in the High Castle explores a history in which Germany won World War II. Harry Harrison’s West of Eden trilogy explores a world in which the K/T meteorite missed and the dinosaurs survived. Science writers also ask about historical phase space, especially in the context of evolution. The most celebrated example is Stephen Jay Gould’s Wonderful Life , which asks whether humans would arise again on Earth if evolution were to be run again. His answer, ‘no’, rests on a very literal interpretation of ‘human’. Harrison’s answer in West of Eden is that intelligent mosasaurs – contemporaries of the dinosaurs that had returned to the sea – would evolve, and play the same role on the evolutionary stage that humans have played in this world. (For plot reasons he also has genuine humans in his alternate universe, but the Yilané, the smart mosasaur descendants, were there first.)
    Where Gould sees divergence and massive changes brought aboutby chance events, Harrison sees convergence: same play, different actors. To Gould, a change of actor is significant; to Harrison, what matters is the play. Both have good arguments to present, but the main point is that they are tackling different questions.
    A second way in which science fiction writers explore alternative historical tracks is through the time travel story, and this brings us back to the wizards of Unseen University and their battle against the elves. There are two kinds of time travel story. In the first kind, the protagonists mainly use their ability to travel in time as a way of observing the past or future; a good example is the first significant time travel novel, H.G. Wells’s The Time Machine of 1895. The time machine is a vehicle for Wells to discuss the future of humanity, but his Time Traveller makes no real effort to change history. In contrast, the narrative theme of Robert Silverberg’s 1969 novel Up the Line is the paradoxes that arise if it is possible to travel into the past and change it. In this story, the Time Service does not set out to change the past; on the contrary, its prime objective is to preserve the past and avoid paradoxes, despite the activities of observers from the future, who are cataloguing the past by visiting it and seeing what actually happened.
    The classic time travel paradox is ‘what if I went back and killed my grandfather?’ The logic of the situation, of course, is that with granddad dead, you wouldn’t have been born, so you wouldn’t be able to go back and kill him, so he’d have lived, so you would have been born … All attempts to resolve this self-contradictory