Quantum Langevin Equations and Stability
  Different quantum Langevin equations obtained by coupling a particle to a field are examined. Instabilities or violations of causality affect the motion of a point charge linearly coupled to the electromagnetic field. In contrast, coupling a scatterer with a reflection cut-off to radiation pressure leads to stable and causal motions. The radiative reaction force exerted on a scatterer, and hence its quasistatic mass, depend on the field state. Explicit expressions for a particle scattering a thermal field in a two dimensional space-time are given. 
Marc-Thierry Jaekel (Laboratoire de Physique The'orique de l'Ecole   Normale Supe'rieure) and Serge Reynaud (Laboratoire Kas...