In the past researchers were convinced of the fact that physical systems show unpredictability only because of its complexity (such as the weather), or due to the diversity of the factors which caused the changes in the system.
The developement of the systems theory caused the awareness that this is not true. Even very simple systems where parts of the scheme influence each other in a nonlinear way, show the phenomenon "deterministic chaos". The computer appeared to be a useful instrument to study this subject. A fine example is the double pendulum. A schematic representation:
The pendulum appears to move totally unpredictable, given this example (with L1 = L2 and m1 = m2):
'Infinitely' small differences of initial speed determine different trajectories.