The Science of Discworld IV

multiplied by their distances from the knife, must be
equal
. Exactly. The slightest imbalance leads to total failure. So, by analogy, any imbalance in the laws of nature, however insignificant, would destroy the conditions required for life to exist. Change the speed of light or various other constants by a few per cent, and the delicate carbon resonance in stars would fail. No resonance, no carbon, no carbon-based life.
    Maybe, though, we’ve accepted these arguments too readily. How relevant, how sensible, is the analogy of a metal rod and a sharp knife? Straight metal rods are an artificial product of technology. In mathematics and nature, most things are nonlinear – bent. What happens if you place a bent rod on top of a knife edge? Assume the bend is not too great, and roughly in the middle. Provided you place the rod on the knife so that it’s reasonably near the balance point, as soon as you let the rod go it turns so that the free ends hang downwards. It slips sideways, but not very far, and then it stops. For a few seconds it seesaws up and down, but eventually it comes to rest.
    Perfectly balanced.
    Reach out a fingertip and push one end up a little. When you let go, the bent rod swings back to its original position, overshoots, reverses direction, and eventually settles back to where it was to begin with. If you push the other end down, the same thing happens.
    Next, move the rod sideways on its pivot, away from the bend. The shiny metal is slippery, and the rod slides back until it balances again. It’s not necessary to
arrange
for the rod to balance. It does soof its own accord. At the balance point, the forces pulling it to either side cancel out just as precisely as they would have to do to balance a straight rod, but the rod no longer falls off if the balance is wrong. It just moves a little, and finds its own balance point. The mathematical reason is straightforward. The rod seeks a state of minimum energy, where its centre of mass is lowest. Because the centre of mass of a bent rod is below the pivot, it ends up hanging in a stable position.
    It’s not
necessary
to fine-tune the universe.
    It can fine-tune itself.
    The ‘knife edge’ thought-experiment is rigged; the analogy with nature is false. The experiment depends on the rod being
straight
. Pretty much any other shape would be self-correcting. In fact, even a straight rod will balance on your finger. As long as the finger is close to the midpoint, the rod no longer slides off. Agreed, a finger is sweaty and sticky, and that can stop the rod sliding, but that’s not the main reason why the rod balances. If one end tilts upwards, the rod rolls sideways and the point of contact with the finger moves away from the raised end. The weight of rod on the raised side is now greater than that on the other side, so the combined forces conspire to return the rod to the horizontal. If it is tilted the other way, the same thing happens. Even a straight rod will find its own balance point if the pivot is not a knife-sharp edge.
    Not only is the thought-experiment rigged: so is the metaphor. A universe doesn’t have to be perfectly linear, and it doesn’t have to pivot on an infinitely thin line. The anthropic, human-centred mentality has unerringly homed in on exactly the wrong metaphor. It ignores the universe’s tendency to respond to change by altering its own behaviour.
    The triple-alpha reaction in the red giant star is just like that. An exact coincidence of energy levels is not necessary. The nuclear energy of beryllium plus that of helium is within a few per cent of one of the energy levels of carbon – but not spot-on. That’s where the red giant comes in. The energies balance only if the star is at the righttemperature.
And it is.
This may seem to be even further evidence of fine-tuning: the astrophysics of the red giant has to compensate precisely for the disparity in nuclear energy levels. But the star is like the bent rod. It has a nuclear thermostat. If its temperature is too low, the reaction proceeds faster, and the star heats up until the energies become equal. If the temperature is too high, the reaction proceeds more slowly, and the star cools down until the same thing happens. It would be just as sensible to admire the exquisite precision with which a wood-burning fire adjusts its temperature to be exactly that at which wood can burn. Or to be amazed that a puddle fits exactly into the dip in the ground that contains