  The formation of correlations due to collisions in an interacting nucleonic
system is investigated shortly after a disturbance. Results from one-time
kinetic equations are compared with the Kadanoff and Baym two-time equation
with collisions included in second order Born approximation. A reasonable
agreement is found for a proposed approximation of the memory effects by a
finite duration of collisions. The formation of correlations and the build up
time is calculated analytically for the high temperature and the low
temperature limit. This translates into a time dependent increase of the
effective temperature on time scales which interfere with standard fire ball
scenarios of heavy ion collisions. The consequences of the formation of
correlations on the two- particle interferometry are investigated and it is
found that standard extracted lifetimes should be corrected downwards.
