ABSTRACT
In many physical systems, we are interested in small time-dependent changes in the state. The approach used in the last chapter cannot deal with this situation, and we must develop perturbation theory further. This will be done in the present chapter. In fact, we will develop the approach twice, first for the Schrödinger representation and then for the Heisenberg representation in a linear vector space. This double approach is followed for two reasons. First, it is important to grasp the concept of the time-dependent perturbation approach in a direct method, without the encumbrance of the mathematics that accompanies the latter approach—the interaction representation. Second, it is important to learn the mathematics of the interaction representation, but in doing so it is quite helpful to understand just where we are heading. The double approach achieves both objectives.
