The linear polarization of the centimeter-wave length radiation from active galactic nuclei (AGN) will be studied at the smallest angular scales. This will be accomplished with the technique of polarization-sensitive Very Long Baseline Interferometry ("polarization VLBI"), whereby the linear polarization structure of compact radio sources may be determined on angular scales of milliarcseconds. At the distance of AGN, this corresponds to physical scales of light years. The linear polarization tells us about the magnetic field structure, radiation mechanisms, particle content, kinematics, and geometry of AGNs, which will help us understand their structure and evolution. The ultimate goal of this work is to elaborate the mechanism that lies at the heart of AGNs, currently thought to be accretion of material onto massive black holes.
