Testing the Massive Black Hole Paradigm and GR with Infrared Interferometry

The GRAVITY near-IR beam combiner allows very sensitive (K~19), phase-referenced milliarcsec K-band imaging and polarimetry, 20-100 micro-arcsecond broad-band astrometry, and micro-arcsecond differential spectro-astrometry with the combined 4 UT telescopes of the Very Large Telescope of the European Southern Observatory. GRAVITY is a game changer in studying the massive black hole in the Galactic Center, and in active galactic nuclei. I will summarize the highlights of the last two years and report new results in using GRAVITY for tests of General Relativity near a massive black hole.