  We present a method to calculate the real time effective propagator of a
generic open quantum system, immersed in a medium using a wave function based
framework. The medium is characterised by a set of harmonic oscillators having
a continuous span of frequencies. This technique has been applied to the
Caldeira-Leggett model showing that high frequency modes of the medium do not
contribute towards decay of the population of states of the open system. In
fact, they cause a Rabi type oscillation. Moreover, our wave function based
approach provides an excellent alternative to conventional formalisms involving
the density matrix.
