On today’s show: How we got from dim little candles made out of cow fat, to as much light as we want at the flick of a switch. The history of light explains why the world today is what it is. It explains why we aren’t all subsistence farmers, and why we can afford to have artists and massage therapists and plumbers. (And, yes, people who do radio stories about the history of light.) The history of light is the history of economic growth — of things getting faster, cheaper, and more efficient.