The Science of Discworld IV

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.
    Most of the scientific effort to resolve these questions starts from the premise that fine-tuning is genuine, so our universe really is virtually unique when it comes to its ability to harbour life. From here it is easy to become convinced that we are the purpose of the whole thing, or even that without us there would be no observers to collapse the quantum wavefunction of the universe and maintain its existence. Less human-centred explanations have also been offered, including a virtually endless cycle of creation and destruction of different universes, which can be noticed by their intelligent inhabitants only when they are the kind of universe that can
have
intelligent inhabitants, or a vast multiverse of parallel or independent universes in which every physical possibility is realised. Either removes the need to explain any
particular
universe. The sheer scope of the imaginative proposals that emerge from a few numbers is breathtaking.
    There is, however, another way. Instead of accepting the premise of fine-tuning and trying to explain it – or explain it away – we can challenge the premise itself. For a start, it’s strange that physicists can think of no other way to make a universe than to keep ours but change a few constants. It’s even stranger that believers don’t hesitate to impose the same restriction on their omnipotent deity’s creative abilities. But even accepting this limitation, it has been clear for at least a decade that the usual description of fine-tuning is needlessly mystical, and verges on the mythical.
    The issues are deep, and it is important not to avoid them by offering glib ‘explanations’ that miss the point. For example, the Weak Anthropic Principle – that we can observe a universe only if it is suited to our own existence – really does explain why our universe must satisfy some pretty stringent constraints. Since we exist, it has to. But that is just another way of saying ‘the universe is how it is’. It’s no different from reasoning from the existence of, say, sulphur, and concluding that atomic theory has to be much as we think it is. The Weak Anthropic Principle only seems different from the equallyvalid Weak Sulphuric Principle fn2 because it’s about us rather than a lump of yellow rock. But the Copernican principle cautions us not to imagine that there’s anything special about us, and in this case, there isn’t. We are just one piece of evidence. An equally convincing case can be made that the universe is