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Why are second order partial differential equations able to model such a wide variety of physical phenomena? Why do we never need to go to third or fourth order ?It seems just about everything can be described with such equations, as if physical reality is just second order in nature. Is there a reason for this?

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marked as duplicate by AccidentalFourierTransform, John Rennie, Kyle Kanos, ACuriousMind♦ Jan 15 '17 at 13:24

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