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THERE’S AN OLD physics joke that goes like this: A dairy farmer asks a physicist how to increase milk production on their farm. The physicist says, “Okay, first we assume a spherical cow...” If you’ve ever spent time around a physicist, or a scientist of any kind, that joke is a crackup because it’s so true. Scientists approximate all sorts of things in order to come up with ballpark figures for whatever problem they’re working on. We call them “back of the envelope calculations,” because they’re often scribbled on whatever piece of paper is close at hand, and that often