 Another fundamental problem is that classical chaos is a function of nonlinearities whereas Schrödinger’s equation describing quantum systems is linear. Empirical investigation of quantum chaology, hence, usually focuses on externally driven quantum systems (see next section) and scattering processes (e.g., quantum billiards). The focus in these studies is on the unpredictability of the time evolution of such systems and processes. Although unpredictability is a feature of classical chaotic systems, there are many reasons why the time evolution of quantum systems may be as unpredictable (e.g., if commuting observables undergo complicated dynamics). It is not clear that unpredictability in externally driven quantum systems and scattering processes is due to any form of chaos. 