  We present a stochastic theory for the nonequilibrium dynamics of charges
moving in a quantum scalar field based on the worldline influence functional
and the close-time-path (CTP or in-in) coarse-grained effective action method.
We summarize (1) the steps leading to a derivation of a modified
Abraham-Lorentz-Dirac equation whose solutions describe a causal semiclassical
theory free of runaway solutions and without pre-acceleration patholigies, and
(2) the transformation to a stochastic effective action which generates
Abraham-Lorentz-Dirac-Langevin equations depicting the fluctuations of a
particle's worldline around its semiclassical trajectory. We point out the
misconceptions in trying to directly relate radiation reaction to vacuum
fluctuations, and discuss how, in the framework that we have developed, an
array of phenomena, from classical radiation and radiation reaction to the
Unruh effect, are interrelated to each other as manifestations at the
classical, stochastic and quantum levels. Using this method we give a
derivation of the Unruh effect for the spacetime worldline coordinates of an
accelerating charge. Our stochastic particle-field model, which was inspired by
earlier work in cosmological backreaction, can be used as an analog to the
black hole backreaction problem describing the stochastic dynamics of a black
hole event horizon.
