  Theoretical approaches to nonequilibrium many-body dynamics generally rest
upon an adiabatic assumption, whereby the true dynamics is represented as a
sequence of equilibrium states. Going beyond this simple approximation is a
notoriously difficult problem. For the case of classical Brownian many-body
dynamics we present a simulation method that allows to isolate and precisely
evaluate superadiabatic correlations and the resulting forces. Application of
the method to a system of one-dimensional hard particles reveals the importance
for the dynamics, as well as the complexity, of these nontrivial
out-of-equilibrium contributions. Our findings help clarify the status of
dynamical density functional theory and provide a rational basis for the
development of improved theories.
