The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,123
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 in the claim that life anywhere on the universe has to be much like life on Earth. But in the cosmic scheme of things, carbon is highly unlikely. It exists only by virtue of a remarkably precise alignment of the energy levels of nuclear reactions inside stars. So stars are special, and the reason is life.
Not just stars. The whole universe is special, finely tuned for life to exist. The basic physics of our universe, on which everything else rests, depends on about thirty fundamental constants: numbers such as the strength of gravity, the speed of light and the strength of atomic forces. Those numbers appear in the deep laws of nature, relativity and quantum theory, and there seems to be no clear mathematical reason why they might not be different. They are ‘adjustable parameters’ – knobs that a creator god could twiddle to any value He/She/It desired. But, tellingly, if you do the sums, it turns out that if any of these constants were even slightly different from its actual value, then not only would life be impossible: there would be no planets for life to inhabit, no stars to provide energy, and no atoms to assemble into matter.
Our universe, like life, is also improbably balanced on the finest of knife edges, and the slightest deviation would have spelled disaster.
This scenario of cosmological fine-tuning is widely viewed as one of the biggest puzzles in cosmology, a series of wildly unlikely coincidences that demands rational explanation but seems to lead only to imaginative speculations involving physics that has not yet been supported by evidence. Religious fundamentalists have seized upon it as a proof of the existence of God. It is difficult, even for atheists, not to have some sympathy with them, because the usual presentation of the science involved points unerringly towards some kind of design principle for our universe.
Fine-tuning, be it terrestrial or cosmological, makes perfect sense from a human-centred viewpoint. In contrast, it seems to pose some very difficult questions for universe-centred thinking.