The VLTI is the only array of 8m-class telescopes that explicitly included interferometry in its design and implementation. No other array is equipped with a comparable infrastructure. The VLTI, with its four 8m telescopes and a total collecting area of 200 m2, is the only interferometer to allow direct imaging at high sensitivity and high image quality. The VLTI is also the only array of its class offering a large 2” field of view. GRAVITY will for the first time utilize this unique capability, providing simultaneous interferometry of two objects. This allows narrow angle astrometry with a precision of order ten microarcsecond. A second new and unique element of GRAVITY is the use of infrared wavefront sensors to observe highly extincted objects. GRAVITY is also the only instrument providing phase-referenced complex visibilities, which is a major advantage for the model-independence and fiducial quality of interferometric maps. The combination of VLTI and GRAVITY will be the world-leading facility for many years to come.

Fundamental measurements over a wide range of fields in astrophysics

Much like in the case of long-baseline radio interferometry, GRAVITY infrared astrometry with order ten microarcsecond accuracy and phase referenced imaging with four milliarcsecond resolution will bring a number of key advances: GRAVITY will carry out the ultimate empirical test whether or not the Galactic Centre harbors a four million solar mass black hole. GRAVITY will finally decide if the near-infrared flares from Sgr A* originate from individual hot spots close to the last stable orbit, from statistical fluctuations in the inner accretion zone, or from a jet. If the current hot-spot interpretation of the near-infrared flares is correct, GRAVITY has the potential of directly determining the space-time metric around this black hole. GRAVITY may even be able test General Relativity in the presently unexplored strong field limit. GRAVITY will also be able to unambiguously detect intermediate mass black holes, if they exist. It will dynamically measure the mass of supermassive black holes in many active galactic nuclei, and will probe with unprecedented resolution the physics of their mass accretion, outflow, and jets. Furthermore, GRAVITY will explore young stellar objects, their stellar disks and jets, and it will measure the properties of binary stars and exoplanet systems. In short, GRAVITY will enable dynamical measurements in an unexplored regime, and it will increase substantially the range and number of astronomical objects that can be studied with the VLTI.

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