From traders on the stock market to colonies of co-operating ants, "complex systems" are found everywhere in nature. Theorists who study such systems, which are usually composed of elementary units that interact only at a local level, have traditionally tried to explain why intriguing global properties or patterns emerge from them. Why, in other words, does the whole turn out to be much more than the sum of its parts? This approach has been hugely successful, with one of its most prominent accomplishments being the development in the 1970s of the theory of "critical phenomena".
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