Imaging an Event Horizon: submm-VLBI of a Super Massive Black Hole
  A long standing goal in astrophysics is to directly observe the immediate
environment of a black hole with angular resolution comparable to the event
horizon. Realizing this goal would open a new window on the study of General
Relativity in the strong field regime, accretion and outflow processes at the
edge of a black hole, the existence of an event horizon, and fundamental black
hole physics (e.g., spin). Steady long-term progress on improving the
capability of Very Long Baseline Interferometry (VLBI) at short wavelengths has
now made it extremely likely that this goal will be achieved within the next
decade. The most compelling evidence for this is the recent observation by
1.3mm VLBI of Schwarzschild radius scale structure in SgrA*, the compact source
of radio, submm, NIR and xrays at the center of the Milky Way. SgrA* is thought
to mark the position of a ~4 million solar mass black hole, and because of its
proximity and estimated mass presents the largest apparent event horizon size
of any black hole candidate in the Universe. Over the next decade, existing and
planned mm/submm facilities will be combined into a high sensitivity, high
angular resolution "Event Horizon Telescope" that will bring us as close to the
edge of black hole as we will come for decades. This white paper describes the
science case for mm/submm VLBI observations of both SgrA* and M87 (a radio loud
AGN of a much more luminous class that SgrA*). We emphasize that while there is
development and procurement involved, the technical path forward is clear, and
the recent successful observations have removed much of the risk that would
normally be associated with such an ambitious project.
