Equation of Motion of a Mass Point

In this appendix we outline the derivation of the non-unitary part of the equation of motion (3.71) of a mass point under the influence of scattering processes. As explained in the main text, in most situations the approximation of negligible recoil is entirely sufficient. We therefore limit our presentation to this important case. Treatments which analyze scattering events have been given by several authors, see for example Joos and Zeh (1985), Gallis and Fleming (1990), Tegmark (1993), and Alicki (2001). For the more general case of quantum Brownian motion see Sects. 3.2.2 and 5.1 and references cited there. The following presentation follows largely Gallis and Fleming.