The Event Horizon Telescope: Observing Black Holes with Schwarzschild
Radius Resolution

It is now almost certain that at the center of our Milky Way Galaxy
lies a super massive black hole - 4 million times more massive than
our Sun.  Because of its proximity to Earth, this object, known as
Sagittarius A*, presents astronomers with the best opportunity in the
Universe to spatially resolve and image a black hole Event Horizon. To
do this requires using Very Long Baseline Interferometry (VLBI), the
technique whereby radio telescopes around the world are linked
together in a Global phased array. Very short wavelength VLBI
observations have now confirmed structure on ~4 Schwarzschild radius
scales within SgrA*, and have revealed time variability in this source
on the same spatial scales.  For the much more massive (6 billion
solar mass) black hole powering the relativistic jet in M87, similarly
compact structures have been detected.  I will describe the
instrumentation efforts that enable these observations, discuss what
current and future VLBI observations can tell us about these
super-massive black holes, and describe plans for assembling a Global
submm-VLBI Event Horizon Telescope.