The physicist Enrico Fermi is famous not only for solving fundamental questions of science in the restricted space on the back of an envelope but for formalizing that process. This new form of calculation—the scientific equivalent of a haiku—is called an order of magnitude calculation. This way of looking at the world prizes above all not exact answers but answers that are easily understandable and say something fundamental about the world using only the information available on a bus. They must be accurate by “an order of magnitude,” which is to say that they should be correct within a factor
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