The Hitchhiker's Guide to the Galaxy famously features a supercomputer, Deep Thought, that after millions of years spent calculating "the answer to the ultimate question of life, the Universe and everything", reveals it to be 42. Douglas Adams's cruel parody of reductionism holds a certain sway in physics today. Our 42 is Schroedinger's many-body equation: a set of relations whose complexity balloons so rapidly that we cannot trace its full consequences up to macroscopic scales. All is well with this equation, provided we want to understand the workings of isolated atoms or molecules up to sizes of about a nanometre.
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