ABSTRACT
This chapter discusses the linear response, and in particular the density and current response of an electron gas. It demonstrates how correlation functions can be measured in scattering experiments, as illustrated by considering neutron scattering from matter. The kinetic-equation approach to transport is a general method, and allows in principle nonlinear effects to be considered. However, in many practical situations one is interested only in the linear response of the system to the external force. The linear response limit is a tremendous simplification in comparison with general nonequilibrium conditions, since the linear response is uniquely determined by the equilibrium properties of the system. The most important hallmark of linear response is the relation between equilibrium fluctuations and dissipation. The chapter shows that the thermal equilibrium state is stable that is manipulating the system by coupling its physical properties to a weak classical field which vanishes in the past and future can only increase the energy of the system.
