One of the powerful ways of probing the behavior of a complex system is observing how it responds to a force applied to it, especially the "indirect" effects that take place at different places or at other times than the force. This is a way of probing the direct and indirect relationships of cause and effect. In this context we are using the term force quite loosely to refer to any interaction with the system. For a simple system the effects of a force are immediate, both in space and in time. Indirect in space means that the system transfers the effect from the place where the force is applied to other places. Indirect in time means that the system shows different effects later on than when the force is applied. This method can be used experimentally and it can also be studied theoretically. Comparing the experimental and theoretical response of a system helps us determine whether the theory correctly describes the behavior of the system. Ultimately, if we want to influence the system to change its behavior, knowing the dynamic response of the system is essential.
Among the general principles that apply to our understanding of dynamic response are:
Fluctuation dissipation theorem
Restoring / maintaining homeostasis
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