The trouble with GDP

ONE of Albert Einstein’s greatest insights was that no matter how, where, when or by whom it is measured, the speed of light in a vacuum is constant. Measurements of light’s price, though, are a different matter: they can tell completely different stories depending on when and how they are made.

In the mid 1990s William Nordhaus, an economist at Yale University, looked at two ways of measuring the price of light over the past two centuries. You could do it the way someone calculating GDP would do: by adding up the change over time in the prices of the things people bought to make light. On this basis, he reckoned, the price of light rose by a factor of between three and five between 1800 and 1992. But each innovation in lighting, from candles to tungsten light bulbs, was far more efficient than the last. If you measured the price of light in the way a cost-conscious physicist might, in cents per lumen-hour, it plummeted more than a hundredfold.