Anderson localization has become an industry in solid state physics. The experimental observation of electron localization is not without controversies. The omnipresence of electron-electron interactions makes it very difficult to observe the effect in Rheinkultur . Weak localization will be discussed in the next section. After having heard one of these talks we realized immediately that this phenomenon was observable for light as well.
Weak localization is a precursor effect of localization.
Standard diffusion theory is obtained when neglecting all interference effects. In the localization regime this neglect is absolutely wrong, but even in the weak multiple scattering regime it is possible to pinpoint a serious flaw in making the assumption that interference terms vanish. The probability for a wave returning to its point of origin can never be well described by neglect of all interferences. Every returning path can also be traversed in opposite direction and these paths have the same phase and interfere constructively. An enhanced returning probability translates in light scattering into enhanced backscattering .