The Science of Discworld II

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    If we’d not told ourselves stories about the Moon, there would have been no point in going there at all. An interesting view, maybe … but we ‘knew’ about the view only because we had told ourselves scientific stories about images sent back by space probes. Why did we go? Because we’d been telling ourselves that we would, one day, for several hundred years. Because we’d made it inevitable and introduced it into the ‘future story’ of a great many people. Because it satisfied our curiosity, and because the Moon was waiting. The Moon was a story waiting to be finished (‘First human lands on the Moon!’), and we went there because the story demanded it.
    When Mind evolved on Earth, a kind of narrativium evolved alongside it. Unlike the Discworld variety of narrativium, which on the Disc is just as real as iron or copper or praseodymium, our variety is purely mental. It is an imperative, but the imperative has not been reified into a thing. However, we have the sort of mind, that respond to imperatives, and to many other non-things. And so it feels to us as if our universe runs on narrativium.
    There is a curious resonance here, and ‘resonance’ is definitely the word. Physicists tell a story about how carbon forms in the universe. In certain stars there is a particular nuclear reaction, a ‘resonance’ between nearby energy levels, which gives nature a stepping-stone from lighter elements to carbon. Without that resonance, so the story goes, carbon could not have formed. Now, the laws of physics as wecurrently understand them involve several ‘fundamental constants’, such as the speed of light, Planck’s constant in quantum theory, and the charge on an electron. These numbers determine the quantitative implications of the physical laws, but any choice of constants sets up a potential universe. The way that a universe behaves depends on the actual numbers that are used in its laws. As it happens, carbon is an essential constituent of all known life. All of which leads up to a clever little story called the Anthropic Principle: that it’s silly for us to ask why we live in a universe whose physical constants make that nuclear resonance possible – because if we didn’t, there’d be no carbon, hence no us to ask about it.
    The story of the carbon resonance can be found in many science books, because it creates a powerful impression of hidden order in the universe, and it seems to explain so much. But if we look a little more closely at this story, we find that it is a beautiful illustration of the seductive power of a compelling but false narrative. When a story seems to hang together, even consciously self-critical scientists can fail to ask the question that makes it fall apart.
    Here’s how the story goes. Carbon is created in red giant stars by a rather delicate process of nuclear synthesis, called the triple-alpha process. This involves the fusion of three helium nuclei. 5 A helium nucleus contains two protons and two neutrons. If you fuse three helium nuclei together, you get six protons and six neutrons. That, as it happens, is a carbon nucleus.
    All very well, but the odds on such a triple collision occurring inside a star are very small. Collisions of two helium nuclei are much more common, though still relatively rare. It is extremely rare for a third helium nucleus to crash into two that are just colliding. It’s like paintballs and wizards. Every so often, a paintball will go splat! against a wizard. But you wouldn’t bet a lot of money on a second paintball hitting him at the exact same moment. This means that the synthesis ofcarbon has to take place in a series of steps rather than all at once, and the obvious way is for two helium nuclei to fuse, and then for a third helium nucleus to fuse with the result.
    The first step is easy, and the resulting nucleus has four protons and four neutrons: this is one form of the element beryllium. However, the lifetime of this particular form of beryllium is only 10- 16 seconds, which gives that third helium nucleus a very small target to aim at. The chance of hitting this target is incredibly small, and it turns out that the universe hasn’t existed long enough for even a tiny fraction of its carbon to have been made in this way. So triple collisions are out, and carbon remains a puzzle.
    Unless … there is a loophole in the argument. And indeed there