  The observable behavior of a complex system reflects the mechanisms governing
the internal interactions between the system's components and the effect of
external perturbations. Here we show that by capturing the simultaneous
activity of several of the system's components we can separate the internal
dynamics from the external fluctuations. The method allows us to systematically
determine the origin of fluctuations in various real systems, finding that
while the Internet and the computer chip have robust internal dynamics, highway
and Web traffic are driven by external demand. As multichannel measurements are
becoming the norm in most fields, the method could help uncover the collective
dynamics of a wide array of complex systems.
