Science of Discworld III

imagine that the histories converge. No, each universe goes its own way, spitting out new and divergent universes as it goes. The Trousers of Time are a tree: their legs can branch but never merge.
    The Worlds of If stories were divided on this issue. Some had each tiny change in the past getting amplified, resulting in vast changes now: we’ve mentioned Bradbury’s story where you trod on a butterfly in the far past, on a dinosaur hunt, and came back to find afascist regime. Or the changes you made were all wiped out, because there was a gigantic all-powerful inertia-of-events Kismet that you couldn’t change. However you tried to avoid your fate, that only made it more certain to happen. And some stories took a middle way; some things converged and others didn’t.
    This, we think, is the rational way to think about time travel and altering the past.
    After all, we don’t change the rules by which the past works. Gravity still operates, sodium chloride crystals are still cubical, people fall in and out of love, misers hoard and spendthrifts squander. What we change is what physicists call the ‘initial conditions’. We change the positions of a few of the pieces on the Great Chessboard of Life, The Universe and Everything, but we still keep to the rules of chess. That’s how the wizards operated in The Science of Discworld II . They went back in time to remove the Elves from the game board; then they went back again to stop themselves making that mistake.
    We are now ready to think about our question above: would the names of newspapers have changed if Abraham Lincoln had lived to a ripe old age?
    Perhaps some of them would, because some cultures would have become rather different. Perhaps Quebec wouldn’t have been French; perhaps New York would have been Dutch. But names like Daily Mail, Daily News and New York Times are so obvious, so appropriate, that even if the Roman Empire were still running things, the Latin equivalents would seem fitting. Someone would have invented flush toilets, and there would have been a steam engine time, when several people invented steam power. Some things in Western culture seem so likely , from toilet paper on up to (as soon as paper is invented) daily newspapers to plastics to artificial wood … Technology seems to have a set of rules for its advancement, so that it seems rational to expect gramophones of some kind if people make music with musical instruments, then tape players when people get used to electricity and its possibilities for amplification.Then from analogue to digital, to computers … some things seem inevitable.
    Perhaps this feeling is misleading, but it’s silly to insist that absolutely everything in a slightly divergent future has to end up different.
    Organic evolution has lessons for us here, and these lessons can instruct us about how likely various advances in animal organisation were. Innovations like insect wings, vertebrate jaws, photosynthesis, life coming out from the seas on to the land … if we ran evolution on Earth again, would the same things happen? If we went back to the beginning of life on this planet, and killed it, would another system evolve and give us a whole different range of creatures, or would Earth remain lifeless? Or would we be unable to decide whether we’d done anything, because everything would be just the same the second time around?
    If history ‘healed up’, we wouldn’t be able to tell if it was the second, or the hundredth, or the millionth time around – each time sooner or later producing a version of us, whose time machine goes back to The Origin . There would be a consistent time loop, as happened with the Elves in The Science of Discworld II . If life is ‘easy’ to originate (and the evidence does look that way) then this isn’t an exercise in going back and killing your grandfather, or if it is, your grandfather is a vampire and doesn’t remain killed. If life is easy to invent, then preventing it happening once, or a million times, will make no difference in the long run. The same process that generated it will happen again.
    Looking at the panorama of life on this planet, in time as well as space, we can see that there are two kinds of evolutionary innovation. Photosynthesis, flight, fur, sex, and jointed limbs have all arisen independently in several different lineages. Surely, like toilet paper, we would expect to see them again each time we ran life on Earth.And, presumably, we’ll see