  We study the interaction of a fast moving particle in the Quark Gluon Plasma
with linearized hydrodynamics. We derive the linearized hydrodynamic equations
on top of an expanding fireball, and detail the solutions for a static medium.
There are two modes far from the jet -- a sound mode and a diffusion mode. The
diffusion mode is localized in a narrow wake behind the jet while the sound
mode propagates at the Mach angle, $\cos(\theta_M) = c_s/c$. A general argument
shows that the strength of the diffusion mode relative to the sound mode is
directly proportional to the entropy produced by the jet-medium interaction.
This argument does not rely on the linearized approximation and the assumption
of local thermal equilibrium close to the jet. With this insight we calculate
the spectrum of secondaries associated with the fast moving particle. If the
energy loss is large and the jet-medium interaction does not produce
significant entropy, the flow at the Mach angle can be observed in the
associated spectrum. However, the shape of associated spectra is quite fragile
and sensitive to many of the inputs of the calculation.
