  We study the response function which describes the evolution of energy
density induced by an initial disturbance for a Bjorken-expanding quark-gluon
plasma (QGP). We compare the results from solving linearized Boltzmann equation
under the relaxation time approximation with those from viscous hydrodynamics.
While in long time limit the response becomes hydrodynamics, the
non-hydrodynamic response is important at intermediate times when excitations
of wavelength larger than the inverse of the relaxation time $1/\tau_{R}$ are
not fully damped. Therefore observables sensitive to the jet-induced medium
excitations might be employed to explore the properties of QGP at the
"mesoscopic scale".
