The aim of this section is to discuss recent evidence bearing on the question "why are strong-flavor jets predominantly one-sided?". Three main classes of explanation have been proposed:
Doppler boosting: Jets are intrinsically symmetrical and have relativistic bulk velocities. The nearer jet appears to be brighter as a result of Doppler boosting. Most attention has been focused on this idea, since it makes several simple predictions. With the exception of M 87 (Biretta, these proceedings), there is no direct evidence for apparent motions in excess of c on kiloparsec scales. The indirect arguments are:
Asymmetric dissipation: One of the jets radiates a significantly higher proportion of its energy as synchrotron radiation. Models for such a process are not yet well-developed (but see Fraix-Burnet 1992 for a possible approach).
Alternating ejection: Only one jet is active at any time. This idea encounters severe difficulties when applied to objects where the synchrotron lifetimes of electrons in components on both sides of the nucleus are short compared with the light-travel times between components and nucleus. This was originally pointed out by Hargrave & Ryle (1974) for Cygnus A and applies to many other powerful radio sources. The recent detection of optical synchrotron radiation from the counterjet side of M 87 (Stiavelli et al. 1992; Sparks et al. 1992) allows a similar argument to be made for an FRI source.
For the moment, we adopt the hypothesis that strong-flavor jets are symmetrical and relativistic and attempt to test its predictions.