Acceleration and Collimation Mechanisms in Jets - R. Blandford 5. It is clear that we are still not in possession of a working theory of jet collimation. I have argued the case that magnetic fields, originating on accretion disks are involved but, as I have also indicated, alternative explanations cannot be excluded. Indeed, the diversity of jet forms may be matched by a similar variety of physical explanations.
Further progress requires a combination of observational input and theoretical calculation.
Let us take these in turn. As I have indicated above, there is already observational evidence in the case of both AGN and YSO jets that they do not emerge from their energy sources fully collimated. Instead they seem to have to propagate through 100 scalelengths before they satisfy an observer's definition of a jet. Prospects for probing the jet collimation scale using VLBI are good. In the case of the optical jets associated with YSO, the improved infrared resolution possible with large telescopes like the Keck and VLT telescopes may allow us to address the same issue in an environment where we have a more accurate measurement of the radius of the central object.