The Science of Discworld IV

socially brainwashed into wanting to. Religions exploit this tendency by making faith paramount: strength of belief trumps contrary or absent evidence. Science deliberately triesto 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 universe – as opposed to innumerable conceivable alternatives – is uniquely fine-tuned for life to arise. Many scientists and most cosmologists now seem to accept that.
    A common image dramatises the point. Take a shiny metal rod and a sharp knife. Rest the rod on the knife’s edge, and try to balance it. You can’t. Unless the rod’s centre of mass is
exactly
above the edge of the blade, the rod will slip, then slide and fall to the ground.
    Life is balanced on a cosmic knife edge.
    Less metaphorically: the laws of nature are exquisitely finely tuned. Change any of the fundamental constants of nature by the smallest amount, and life’s delicate cycles will fail. Poise humanity one micron away from cosmic perfection, and it will topple.
    Alongside this human-centred view of the universe goes a human-centred view of humans. Forget all those weird and wonderful aliens that infest science fiction, living in the hydrogen-helium atmospheres of gas giants, or the frigid cold of worlds so far from their suns that the temperature is barely above absolute zero. It’s much simpler than that. The only viable aliens will be just like us. They will live on a rocky world with oceans and plenty of oxygen in its atmosphere; it will need to be just the right distance from its sun. The world will need a strong magnetic field to keep radiation at bay, a large companion like our Moon to keep its axis stable, and a gas giant like Jupiter to protect it from comets.
    The aliens’ sun will also have to be special. Remarkably like ours, in fact. Not just in its spectral type, its general shape, size, and the kind of nuclear reaction it uses, but in its location. The sun needs to be reasonably far away from any of its galaxy’s spiral arms, because the process of star formation creates a lot of radiation, and most stars form in the spiral arms. On the other hand, it can’t be
too
far away, as our own Sun demonstrates. Moreover, the aliens’ sun must be near enough to the galactic centre for there to be enough heavy elements to provide the planet with a rocky core, but far enough from the centre to avoid being subject to intense radiation, which would destroy life.
    Well, carbon-based life, like ours … but that’s the only kind that can exist. The element carbon is unique: it forms the complex molecules required to make living creatures. Carbon is a key element inthe claim that life anywhere on the universe has to be much like life on Earth.