Einstein Relation for a Tagged Particle in Simple Exclusion Processes

 It is known that the rescaled position of a tagged particle in symmetric simple exclusion processes converges to a diffusion. If now the tracer particle is driven by a small force, then it picks up a velocity. The Einstein relation states that in the limit, this velocity is proportional to the small force, and the constant of proportionality can be computed from the diffusion matrix of the tracer particle with no driving force. Such a relation is believed to be generally valid. In this article we establish its validity for all symmetric simple exclusion processes in dimension and we prove a density property for certain invariant states of the driven system. Received: 2 September 2001 / Accepted: 28 March 2002 Published online: 31 July 2002