  Interaction is so ubiquitous that imaging a world free from it is a difficult
fantasy exercise. At the same time, in understanding any complex physical
system, our ability of accounting for the mutual interaction of its
constituents is often insufficient when not the restraining factor. Many
strategies have been devised to control particle-particle interaction and
explore the diverse regimes, from weak to strong interaction. Beautiful
examples of these achievements are the experiments on Bose condensates, or the
recent experiments on the dynamics of spin chains. Here I introduce another
possibility, namely replacing the particle-particle interaction with an
external stochastic field, and once again reducing the dynamics of a many-body
system to the dynamics of single-particle systems. The theory is exact, in the
sense that no approximations are introduced in decoupling the many-body system
in its non-interacting sub-parts. Moreover, the equations of motion are linear,
and no unknown external potential is inserted.
