The relativistic jet paradigm seems to explain a wide range of characteristics observed in compact jets. This is remarkable given the relative simplicity of both the geometry and physics assumed. Still, one has the impression from both numerical simulations of jet fluid dynamics and observations such as those of M 87 (see Biretta's contribution to these proceedings) that actual jets are much more complex than the basic model allows. I hope that the high dynamic range allowed by the VLBA and future simulations of jets with relativistic flow velocities will soon allow us to inject a larger dose of realism into the models. It is almost certainly true that we will find that some of our simplifying assumptions are woefully inadequate to the task of describing compact jets.
It is somewhat disappointing that the smallest observable features of
the radio jets
are much larger than the expected interaction scale between the central
engine and
the deepest regions of the plasma flow that emerges downstream as a
well-collimated
relativistic jet. Fortunately, it now appears that this "inner jet"
region produces a high luminosity in X-rays and
rays, and is
therefore observationally accessible.
Multifrequency monitoring of a few selected objects across all wavebands
are now
planned for late 1992 and 1993. Inspection of the data for correlated
variability and time
delays should reveal the relative location of the nonthermal emission at
each waveband.
This will allow us to infer the geometry and physics of the inner jet,
which in turn
should lead to a better understanding of the processes by which the jet
is focused and
accelerated. When combined with observations of the thermal emission
from active
galaxies, this information should allow us to develop a complete model
of active galaxies,
from the central engine to the jet and extended lobes.
The author's work on compact jets is supported in part by National Science Foundation grant AST-9116525 and NASA grants NAGW-1068 and NAG 5-1566.