Testing General Relativity with Infrared Interferometry of the Massive Black Hole in the Galactic Center

  Reinhard Genzel  
Ludwig Maximilian University, Munich, Germany

Ever since their discovery in the 1960s, Quasars and other active galactic nuclei are thought to be accreting massive black holes. To prove this hypothesis it is necessary to determine the gravitational potential to scales of the event horizon. While this cannot be done in the distant quasars, dynamical measurements in nearby galaxies, and in particular in the Center of the Milky Way promise to be the most robust and direct path of proving the massive black hole paradigm.

Over the past 25 years we have carried out increasingly precise measurements of the stellar dynamics in the Galactic Center, which prove the existence of a central mass concentration centered on the compact radio source SgrA*, which is likely a black hole of 4 million solar masses, beyond any reasonable doubt.

The next big step is to use SgrA* as a laboratory to test General Relativity in this yet unexplored limit. After 10 years of development, an international team of us in Europe led by Frank Eisenhauer at MPE last year put into operation a novel near-infrared interferometric imager/spectrometer, GRAVITY. This instrument combines the light of all four 8m telescopes of the Very Large Telescope of the European Southern Observatory in Chile, for a 'super-telescope' of 130m equivalent diameter.

I will report the status of the experiment after the first season of observing with GRAVITY. We have successfully carried out imaging of SgrA* will milli-arcsecond resolution. With a sensitivity more than a hundred times better than previous infrared interferometers, we are now on our way to test General Relativity for the first time around a million solar mass, black hole, as one of the orbiting stars is getting read to reach its peri-bothron of 17 light hours in summer of 2018.