The Science of Discworld IV

uniquely finely tuned to make sulphur.
    The Weak Anthropic Principle only goes so far. It doesn’t explain
why
this kind of universe exists, rather than something different – especially when almost any alternative would allegedly fall apart or explode the moment it came into being, or would be so boring that only very simple structures could form. However, the Strong Anthropic Principle – that the universe was made
in order for
humans to exist – doesn’t explain any of that either. We could just as readily formulate the Strong Sulphuric Principle: the universe was made
in order for
sulphur to exist.
    Why us?
The Strong Anthropic Principle just assumes it’s obvious that we are the purpose of the whole thing. Sulphur? Don’t be silly.
    Let’s warm up with the carbon story, which is easier to grasp. Then we’ll take a look at those puzzling fundamental constants. We discussed both of these in
The Science of Discworld II: the Globe
, and we have to cover some of the same ground again before going further. We’ll keep it brief.
    Astrophysicists have put together a careful account of how the chemical elements formed. Combinations of elementary particles – protons, neutrons, or their more exotic precursors – came together in vast clouds to form atoms of the lightest element, hydrogen. The early universe was hot enough for hydrogen atoms to fuse together, making the next lightest element, helium. Then the clouds collapsed under their own gravity, triggering nuclear reactions. Stars were born, and within those stars new elements assembled, with atomic weightsup to and including iron. Subtler processes, occurring in red giant stars, put together heavier elements, as far as bismuth. Everything else required high-energy processes occurring only in supernovas – massive stellar explosions.
    In 1954 the astronomer Fred Hoyle realised that there was a problem with carbon. The universe contains a lot more of it than the known nuclear reactions can explain. And carbon is the vital element for life. Carbon can form in red giants through the ‘triple-alpha’ process, in which three helium nuclei (atoms minus their electrons) collide, pretty much simultaneously. A helium nucleus comprises two protons and two neutrons. So three of them combined must yield a nucleus with six protons and six neutrons. This is carbon.
    In the dense environment of a red giant star, nuclei collide relatively often. But it’s not terribly likely that just as two of them come together, a third joins the party. So the process has to happen in two stages. First, two helium nuclei collide and fuse, making beryllium. Then another helium nucleus fuses with that. Unfortunately for this theory, the form of beryllium involved falls apart after one tenth of one quadrillionth of a second. The chance that a helium nucleus can hit such a rapidly vanishing target is much too small.
    Hoyle knew this, and he also knew that there is a loophole. If the combined energies of beryllium and helium just happen to be very close to an energy level of carbon, then the nuclei can fuse much faster and the sums work out fine. Such a near-coincidence of energies is called a resonance. No suitable resonance was then known, but Hoyle insisted that it had to be there. Otherwise
Hoyle
wouldn’t be there, being made from quite a lot of carbon. That led him to predict an unknown energy level of carbon around 7.7 MeV (million electronvolts, a convenient unit of energy for nuclear reactions). By the mid-1960s the experimentalist William Fowler had found such a resonance at 7.65 MeV, within 1% of Hoyle’s prediction. Hoyle presented this discovery as a triumph of ‘anthropic’ reasoning: deducing something about the universe from the existenceof humans. Without that finely tuned resonance, we wouldn’t be here.
    It sounds impressive, and it is when told that way. But already we see a tendency to exaggerate. For a start, the link to humans is unnecessary and irrelevant. What matters is the amount of
carbon
in the universe, not what it can make. We do not need to appeal to our own existence to know how much carbon there is. In
The Fallacy of Fine-Tuning
, Victor Stenger refers to an investigation of the history of Hoyle’s prediction by the philosopher Helge Kragh. Hoyle did not initially link the resonance to the existence of life, let alone human life. The anthropic connection was not made for nearly thirty years. ‘It is misleading to label the prediction of the 7.65