Introduction

The actual initiation of jets is a subject that remains extremely difficult to discuss in a detailed and convincing manner despite the obvious importance of this fundamental topic. There are two main reasons for this. The first is a lack of unequivocal observations. Although VLBI measurements have provided structural information on scales corresponding to ≲ 0.1 pc in extragalactic sources, the phenomena that govern the beginnings of jets almost certainly occur on scales at least two to three orders of magnitude smaller. Other observations, especially of X-ray and optical variability, are indubitably important and provide useful constraints on models; however, they do not yield information that is clearly interpretable in a model-independent fashion.

The second generic difficulty has to do with the certainty that the physical processes involved in producing jets are extraordinarily complex. The core of the picture, that accretion onto a super-massive black hole (SMBH) of somewhere between 106 and 1010M≲ is at the heart of beam generation as well as the other properties of active galactic nuclei (AGN), has been commonly accepted for about a decade. However, in attempting to add details to this picture, astrophysicists find that general relativity, hydrodynamics, plasma physics and radiation transport all form thick blobs on their palettes, and the portraits which emerge from combining them in different proportions are, not surprisingly, rather different.