Nonperturbative calculation of the shear viscosity in hot phi(4) theory inreal time

Starting from the Kubo formula we calculate the shear viscosity in hot phi( 4) theory nonperturbatively by resumming ladders with a real-time version o f the Bethe-Salpeter equation at finite temperature. In the weak coupling l imit, the generalized Fluctuation-Dissipation Theorem is shown to decouple the Bethe-Salpeter equations for the different real-time components of the 4-point function. The resulting scalar integral equation is identical with the one obtained by Jeon using diagrammatic "cutting rules" in the Imaginar y Time Formalism, (C) 1999 Elsevier Science B.V. All rights reserved.