What is the Structure of Relativistic Jets in AGN on Scales of Light Days?

The structure of relativistic jets in AGN on scales of light days reveals how energy propagates through jets, a process that is fundamental to galaxy evolution. A deeper understanding of jet physics will clarify the differences between radio-loud and radio-quiet quasars that manifest themselves near the supermassive black hole. We can also use relativistic jets to identify supermassive binary black holes remaining from galaxy mergers, and use orbital motion to derive the masses of the black holes. The search for binary black holes in the nuclei of galaxies will yield important information on their overall lifetime and on the processes occurring in galaxies that affect black holes and quasars. High-precision astrometric measurements, made using the technique of optical interferometry on a space-based platform, such as SIM Lite, are the key to answering these questions.