The talk of biology’s coming preeminence was probably right, thought West. But the idea that this meant physics would go out of fashion struck him as ridiculous. Like D’Arcy Thompson, he saw mathematical descriptions as the root of any true science, and most areas of biology were still lacking in such theories. To West’s mind this gave physicists an entry into biology. He began thinking about what sort of biology problems he might tackle.

West had recently turned 50, and his mind had turned to thoughts of mortality. He decided to look into what determined life span. On the one hand, longevity is predictable. Insurance firms and actuaries can calculate life expectancy based on diet, occupation, income, and so on. Half-an-hour’s research would have given him a good idea of how much longer he had left. On the other hand, although biologists have some ideas about the mechanisms of aging, no one could explain why a mouse should live for a year or two and a man, built from much the same molecules, with much the same genes and biology, should live for a century. Explanations based in genetics or physiology struck him as superficial. “If biology is to be a real science, you ought to have a theory that can predict why we live 100 years,” says West. “That’s real science, not some qualitative nonsense about gene expression. That’s not an explanation of anything.”

He began to teach himself biology in the evenings and on weekends, picking up bits and pieces of knowledge wherever he could. It was, he says, “like learning about sex on the street.” The library at Los Alamos is devoted almost entirely to physics, so he resorted to reading his children’s high school biology textbooks. He discovered that larger species live longer and that across species life span increases proportional to the 1/4 power of body mass. And he discovered that many researchers had linked life span to metabolic rate, believing that animals that burn energy relatively quickly live shorter lives than the slow burners. But he also discovered that no one could explain why a species’ average life span was as long as it was.

West, too, has a background in scaling. Like living things, the behavior of physical systems depends on their size, and the laws of physics are a matter of scale. Quantum theory is most useful for describing what happens inside the atom. Relativity comes into its own