The Science of Discworld IV

pleaded for her life: ‘Oh, sir, please don’t kill me, sir, please, sir, I’m on my knees!’
    That ringing plea was then echoed by a black figure that had suddenly been punched in the groin. One small blow for a librarian; one giant step for Roundworld, Marjorie thought, gratified to hear a crunch. And mere seconds after this first challenge, she was pelting down the aisle after the retreating bandit carrying her home.
Her library
and all of the planet surrounding it was accelerating away to only God – or more likely Richard Dawkins – knew where.
    Being the fastest track and field runner in Roedean School helped. The fleeing bandit hadn’t had her training, and certainly didn’t have her stamina, and was flagging as he zigzagged through streets that were quite alien to Marjorie. She had to keep him in sight; she would be completely lost if he got away, so she girded her loins, metaphysically speaking, gulped for breath and sped on. Now it was beginning to look as if the wretched miscreant was weakening – she was sure of it – and this reassurance gave her wings.
    She could hear the sounds of hue and cry dwindling behind her. And then the figure stopped dead, turned round, screamed something incoherent and flung the globe directly at her head.
    fn1 The Quite Reverend Mightily-Praiseworthy-Are-Ye-Who-Exalteth-Om Oats, a mainstream Omnian priest.

TWENTY-TWO
----
FAREWELL, FINE-TUNING

    They didn’t make it; it made them.
    Pastor Oats, a truly wise man, has put his finger on a deep, often unappreciated, truth, which illuminates the misty borderland where science and religion meet. Here lie some of the most perplexing riddles of modern cosmology, where the austere workings of fundamental physics collide with the richness of human experience.
    At the heart of this collision is an astonishing coincidence: universes that can sustain living creatures are extraordinarily unlikely. This coincidence violates, in the most dramatic manner possible, the Copernican principle that humans aren’t special.
    Before Nicholas Copernicus published
On the Revolutions of the Celestial Spheres
in 1543, with a few honourable exceptions, almost everyone viewed humanity as the centre of the universe. This was so obviously true that it seemed ridiculous to deny it. Look around you. Everything else trails off into the distance, and you are smack bang in the middle. Your own senses prove that the stars and other celestial bodies revolve around the Earth. The natural shape for their orbits must surely be a circle, a perfect geometric figure; its perfection provides yet more evidence that everything was created for us, and that we are located at the heart of creation.
    However, ancient astronomers were excellent observers, and when they looked at what the universe was actually doing, they realisedthat circles don’t fit. But they could save the ‘perfect form’ theory, because
combinations
of circles agree very closely with observations. In the second century AD , Claudius Ptolemaeus (Ptolemy) wrote the
Almagest
(‘the greatest’), which represented the movements of the Sun and planets around a stationary Earth. To match the complex trajectories observed, he employed several geometric constructions, involving spheres rotating on axes that are supported by other spheres. In a simplified form, the most important features of the Ptolemaic system were epicycles: circular orbits whose centres themselves revolved in circular orbits. If necessary, those centres might also revolve in circular orbits, and so on. In total, Ptolemy needed more than eighty spheres, but the resulting system was very accurate. Especially at a time when Earth was not recognised as a planet. That term referred to wandering stars, and the Earth was neither a star, nor a wanderer. It was
fixed
.
    We are special.
    Copernicus was clearly a contrarian, and he realised that everything makes a lot more sense if we’re not special, and the Earth is not at the centre. This is an instance of the mediocrity principle: as a working heuristic, it is best to avoid assuming that any given phenomenon has unusual, special features, or violates the laws of nature. One feature of Ptolemy’s system that may have led Copernicus to this view was a suspicious coincidence. The numbers associated with most of the epicycles – size, speed of rotation – were rather haphazard, with no clear patterns. But Copernicus noticed that identical copies of one particular set of epicycle data