A large microwave antenna transmits communications for the Kennedy Space Center in Florida. (Corbis-Bettmann)

Conservation of energy has two very different meanings. In the popular sense, "conserve" means to "save" or "preserve." Electric energy for lights likely had its origin in burning coal, so turning off lights tends to preserve coal, a valuable natural resource. In the scientific sense, conservation alludes to constancy. Succinctly stated, the energy of the universe is constant. Energy can be converted from one form to another, but ultimately there are as many joules of energy after the conversion as before. Every second, an operating 100-watt lightbulb converts 100 joules of energy. If 10 joules are in the form of light, then 90 joules are in some other form, notably heat to the room. Even though the total amount of energy following the conversion is unchanged, the energy may not be available for...