The Science of Discworld IV

hieroglyphs is not how diverse that culture’s flora and fauna were, but how narrowly its symbolism was tailored to the organisms that were important to everyday Egyptian life.
    As we came to understand our world more deeply, and asked new questions, comfortable answers in terms that we could intuitively understand began to make less and less sense. Conceivably the Sun might, metaphorically, be pushed around by an invisible giant dung-beetle, but the Sun is a vast ball of very hot gas and no ordinarybeetle could survive the heat. You either fix things up by attributing supernatural powers to your beetle, or you accept that a beetle can’t hack it. You then have to accept that the motion of the Sun occurs for reasons that differ significantly from the purposeful shoving of a beetle storing up food for its larvae, raising the interesting question ‘why or how
does
it move?’. Similarly, although the setting Sun looks as if it is disappearing underground, you can come to understand that it is being obscured by the rotating bulk of the Earth. Instead of telling a story that offers little real insight, you’ve learned something new about the world.
    It took time for humanity to realise all this, because our planet is far larger than a village. If you walked 40 kilometres every day it would take you three years to travel all the way round the world, ignoring ocean crossings and other obstacles. The Moon is nearly ten times as far away; the Sun is 390 times as far away as the Moon. To get to the nearest star, you must multiply that figure by a further 270,000. The diameter of our home galaxy is 25,000 times as great again. The nearest galaxy of comparable size, the Andromeda galaxy, is 25 times as far away. The distance from Earth to the edge of the observable visible universe is more than 18,000 times as great as that. In round figures, 400,000,000,000,000,000,000,000 kilometres.
    Four hundred sextillion. That’s some village.
    We have no intuitive feel for anything that large. In fact, we have little intuitive feel for distances of more than a few thousand miles, and those only because many of us now travel such distances by air – which shrinks the world to a size we can comprehend. From London, New York is just a meal away.
    We know that the universe is that big, and that old, because we have developed a technique that consciously and deliberately sets aside the human-centred view of the world. It does so by searching not just for evidence to confirm our ideas, which human beings have done since the dawn of time, but for evidence that could disprove them, anew and rather disturbing thought. This technique is called science. It replaces blind faith by carefully targeted doubt. It has existed in its current form for no more than a few centuries, although precursors go back a few thousand years. There is a sense in which ‘know’ is too strong a word, for scientists consider all knowledge to be provisional. But what we ‘know’ through science rests on much more secure foundations than anything else that we claim to know, because those foundations have survived being tested to destruction.
    Through science, we know how big and how old the Earth is. We know how big and how old our solar system is. We know how big and how old the observable part of the universe is. We know that the temperature at the centre of the Sun is about 15 million degrees Celsius. We know that the Earth has a roughly spherical core of molten iron. We know that the Earth is roughly, though not exactly, spherical, and that (with suitable caveats about moving frames of reference) our planet goes round the Sun rather than being fixed in space while the Sun goes round
it
. We know that many features of an animal’s form are determined, to a significant degree, by a long, complicated molecule that lives inside the nucleus of its cells. We know that bacteria and viruses cause most of the world’s diseases. We know that everything is made from seventeen fundamental particles.
    ‘Know’ is one of those simple yet difficult words. How can we know, to take a typical example, what the temperature is at the Sun’s centre? Has anyone been there to find out?
    Well, hardly. If scientists are right about the temperature at the centre of the Sun, nobody who was suddenly transported there would survive for a nanosecond. In fact, they’d burn up long before they even reached the Sun. We haven’t sent measuring instruments to the centre of the Sun, for