What is Nonlinearity?

Abstract

As time goes by all things in nature change: Animals are born, they grow, live and die, so do plants. Planets, stars, etc. move around all the time changing their positions continuously. Oceans, rivers, clouds etc. again change their state continuously. Crystals grow and chemicals interact. Even inanimate objects like furniture, buildings, sculptures, etc. change their physical state, perhaps more slowly and over a longer period of time. Change is inevitable in nature (though often on a finite time scale a system may be considered stationary). This change of state of physical systems as a function of time is their evolution, whose study constitutes the subject of dynamics. How do such changes arise? Obviously changes take place due to the interplay of forces, simple and complex, which act on the systems. Think of any natural system, it is always being acted upon by one or more forces.