The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,122
lots of copies of an epicycle are trumped by just one copy, even if it has to be attached to a different body.
In a frame attached to the Earth, the laws of motion become extraordinarily complicated. The nearest major galaxy, M31 in Andromeda, about 2.6 million light years away, has to whiz all the way round the Earth once every 24 hours. More distant objects – the current record is about 13.2 billion light years – must undergo even more outlandish gyrations. In contrast, if we choose a frame of reference centred on the Sun, making it stationary relative to the average positions of the stars, the mathematics becomes far simpler and the physics and metaphysics far more reasonable. Ignoring the gravitational influences of any other bodies, the Sun and Earth both orbit their mutual centre of gravity in ellipses. But because the Sun is so much more massive than the Earth, that centre lies well inside the Sun. So … the Earth goes round the Sun. We foolishly think the Earth is stationary because it is, relative to us. (Sorry, still too human-centred: make that ‘we are stationary, relative to it’.)
Lesson learned – after several centuries, a few burnings and a lot of fuss and bother. But that was just the warm-up act. When astronomers realised that distant blobs of light were galaxies – swirling masses composed of billions of stars – it eventually dawned on them that the familiar Milky Way’s river of light is no accident: it is our own galaxy seen edge-on, from inside. Naturally our Sun will be at the galactic centre … Well, no, it is actually in a very nondescript region about two thirds of the way towards the rim: 27,000 light years from the galactic core, close to one of the galaxy’s spiral arms, the Orion Arm. The glorious Sun is merely one star (and a pretty feeble one at that) among thousands in the Local Fluff, which itself lies inside the Local Bubble. The Sun is not even in the galactic plane, though it’s fairly close – about sixty light years.
After several centuries in which every successive attempt to portray humanity as special was debunked, the Copernican principle became embedded in fundamental physics as a generalisation of Einstein’s basic principle of relativity: there is no such thing as a privileged observer.
We said earlier that a major motivation behind the scientific method is a conscious awareness that people tend to believe things because they want to, or have been socially brainwashed into wanting to. Religions exploit this tendency by making faith paramount: strength of belief trumps contrary or absent evidence. Science deliberately tries to counteract it by demanding convincing evidence. The Copernican principle is one extra reminder about what not to assume. It doesn’t always apply, but it punctures our sense of self-importance.
Broad quasi-philosophical principles like those of Copernicus and Occam are guidelines, not hard-and-fast rules. And, wouldn’t you just know it: as soon as we started to get used to the idea that in the vast scheme of things we are pretty ordinary, evidence began to turn up that this wasn’t a done deal. Maybe we are special. Maybe the Earth is in a privileged position, or a privileged state. Maybe it has to be.
By the time this line of reasoning had run its course, we seemed to be so special that the entire universe must somehow operate in precisely the manner that can give rise to … us. It is as though the universe were created with humanity in mind.
To those of a religious persuasion, this was hardly news, and they welcomed the partial conversion of the scientific world with open arms. But even atheists were coming round to the idea that if the universe were even slightly different from what it is, we wouldn’t be here. There’s even a general principle, a distinctly non-Copernican one, that can be used to justify these claims. It’s called the Anthropic Principle.
There are two flavours. The Weak Anthropic Principle states that the universe has to be of a kind that can give rise to creatures like us, because if it weren’t, we wouldn’t be here to ask awkward questions. The Strong Anthropic Principle states that the universe was in some sense designed for us. We are not just an accidental by-product; we are what it’s all for. In 1986 John Barrow and Frank Tipler compiled an impressive, highly technical, analysis: The Anthropic Cosmological Principle. It discussed the view that in several respects our