The Knowledge Machine: How Irrationality Created Modern Science

by Michael Strevens

The earth is actually over 4.5 billion years old, and it has harbored life for at least 3.5 billion of those years. How did Kelvin get it so badly wrong? Like Todd Willingham’s jurors, who were presented with an inadequate theory of the way in which house fires develop and burn, he was relying on assumptions that were mistaken in several respects. First, though he had no way of knowing it, the heat of the rock making up the continents is considerably increased by the decay of radioactive elements. Second, heat is transported from the earth’s core not by conduction through solid rock, as Kelvin had supposed, but by convection, in which rock in the earth’s mantle flows from the core to the crust carrying heat with it, warming the crust far more efficiently than conduction could. As a consequence, an old earth can have, as ours does, a surprisingly warm crust. In effect, Kelvin sipped the top layer of a cup of coffee and, finding it to be piping hot, concluded that it had been freshly poured. In fact it had been there for ages, but sitting on a warmer and continually stirred. Huxley was right. Kelvin had poured chaff into his finely calibrated mathematical mill and produced indigestible grit. Were Kelvin, Tait, and other advocates of a “youngish earth” being unscientific? Not at all; there were good reasons and great uncertainty on both sides. The physicists saw their assumptions about the geological structure of the earth as natural and reasonable compared with the largely s...