Ask a Physicist: How Many Calories are in that Sunbeam?

Last week, Joe from Massachusetts wrote in to ask: Life is possible through the transfer of the sun's energy, through photosynthesis, and animals eating and us eating them. Is it possible to measure how much energy a person receives from the sun in order to live an average life, say 85 years being the average? Tall order, yes? Joe,Great question--it's awesome that you recognize the flow of energy involved in biology and physics: it means that ultimately, almost all life on Earth is fusion-powere...