ABSTRACT
This chapter presents nonlinear problems from the experimental point of view. Population dynamics, epidemiology and mathematical biology in general count among the most important contributors to the ‘comeback’ experienced by nonlinear science in recent decades. Reynolds states that ‘The internal motion of water assumes one or another of two broadly distinguishable forms — either the elements of the fluid follow one another along the lines of motion which lead in the most direct manner to their destination, or they eddy about in sinuous paths the most indirect possible’. These are laminar and turbulent forms of motion. The plasma experiment illustrates nonlinear phenomenon associated with systems presenting symmetries, namely spontaneous symmetry breaking. Spontaneous symmetry breaking appears in number of physical phenomena. For Bénard instability, experiments preceded the theory at the beginning and lately they have developed simultaneously. Finally, for the laser system many of observed phenomena have been, if not predicted specifically for the case in study, pre-existent as abstract theory.
