The Science of Discworld IV

chapter 16 we examined how Roundworld’s rules affect not just the answer to the question of its shape, but what we mean by the question itself. Now we turn to its origins.
    When the Archchancellor told Marjorie about the origins of Roundworld, he naturally did so from the wizards’ perspective, in which the entire human universe is somehow located inside a small sphere, about the size of a football. And was brought into being by H EX saving Discworld from annihilation, and the Dean poking an exploratory finger into the resulting magical containment field.
    What about the perspective of its inhabitants? Since ancient times, they have wondered how (or whether) their world began, but until recently their answers were resolutely human-centred: mainly stories about creator gods. In contrast, today’s scientific theories of the origins of the universe are (surprise!) universe-centred. They are based not on stories about gods, but on the rules that the universe seems to obey.
    Since the rules are not written down in a book of magical spells, the wizards have to infer the rules from what Roundworld does. Roundworld scientists are in the same position, but their difficulty is compounded, being stuck inside the thing and confined to the present. Even so, they have outperformed the wizards by working out many of the rules that govern their world and their universe.
    When it comes to the origin of the universe, what we really need is a time machine. Despite some hints at the frontiers of today’s physics, discussed in
The Science of Discworld II
and
III
, no practical time machine exists, or is likely to. But that doesn’t stop us wanting to know exactly how the universe began, or taking a serious stab at working it out from the evidence it left behind.
    The origin of the universe is a weighty philosophical question, leading to profound scientific and mathematical ideas. Mathematics, after all, is humanity’s best-developed and most powerful system of logical inference, and if you can’t go back to the distant past to take a look, you have to remain in the present and infer.
    We’ve already seen that questions of shape and origin tend to go hand in hand. That is especially true of the universe, because it is dynamic: what it looks like now depends on what happened to it in the past. So cosmology and cosmogony are intimately intertwined, just as they were in ancient mythology. The current theory of the origins of the universe is of course the Big Bang, which emerged unexpectedly from astronomical observations that were intended to sort out its size and shape. So before we examine the universe’s origins, we’ll take a quick look at these observations and what they led to.
    In ancient times, the world and the universe were pretty much identified. The Sun, Moon, planets and stars were little more than added decoration in the sky; the world on which we lived dominated everything else. Now we realise that our planet is a very tiny lump of rock in a universe so huge that we find it hard to comprehend.
    Humanity first caught a glimpse of the sheer vastness of the universe in 1838, when the astronomer Friedrich Bessel measured the distance to the star 61 Cygni. Until then, people who did not believe that the Earth goes round the Sun could offer a fairly plausible argument to show that the Earth had to be fixed. If it moved round the Sun, then the nearer stars would appear to move slightly against the background of more distant stars, an effect called parallax. But they didn’t. Bessel found out why: even the nearest stars are a tremendous distance away, making this apparent motion too small to detect. He used a sensitive new telescope to observe the star 61 Cygni. This had been nicknamed the ‘flying star’ by Giuseppe Piazzi in 1804 because its apparent motion across the sky, though very small, was unusually large compared to that of most other stars. This suggested that the star might be unusually close to the Earth. Bessel discovered that it is 11.4 light years away, roughly 10 14 (one hundred trillion) kilometres. The modern figure is 11.403, so Bessel was spot-on.
    The diminution of humanity had barely begun. As well as twinkling points of light, the night sky boasted a glowing river of light, the Milky Way. In fact it is a disc of stars, most of them too far away to be resolved as individuals, and we are inside it. We now call such a disc a galaxy. Hints that other galaxies might exist appeared when astronomers