For the first time, observations made using ESO’s Very Large Telescope have uncovered the effects predicted by Einstein’s general relativity about the motion of a star that passes through the extreme gravitational field close to the supermassive black hole at the center of the Milky Way. This long-awaited outcome symbolizes the culmination of a 26-year-long observation campaign with the help of ESO’s telescopes in Chile.
The supermassive black hole closest to the Earth is hidden under thick clouds of absorbing dust and is located 26,000 light-years away at the center of the Milky Way. A small group of stars orbits around this gravitational giant, which has a mass equal to four million times that of the Sun, at high speed. This harsh environment—the strongest gravitational field in the Milky Way galaxy - makes it the ideal place to investigate gravitational physics, and specifically to test Einstein’s general theory of relativity.
At present, new infrared observations using the magnificently sensitive GRAVITY , SINFONI, and NACO instruments on ESO’s Very Large Telescope (VLT) have enabled astronomers to follow one of these stars, named S2, when it passed nearer to the black hole during May 2018. At the closest point, this star was located at a distance of less than 20 billion kilometers from the black hole and moving at a speed more than 25 million kilometers per hour - nearly 3% of the speed of light .
The researchers compared the velocity and position measurements from SINFONI and GRAVITY, respectively, together with earlier observations of S2 with the help of other instruments, applying the predictions of general relativity, Newtonian gravity, and other theories of gravity. The new results are incompatible with Newtonian predictions and in exceptional agreement with the predictions of general relativity.
These highly precise measurements were carried out by an international group of scientists headed by Reinhard Genzel from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, in collaboration with colleagues across the globe, at the Paris Observatory–PSL, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA–Centro de Astroﬁsica e Gravitação, and ESO. The observations are the climax of a 26-year sequence of ever-more-accurate observations of the center of the Milky Way with the help of ESO instruments .
This is the second time that we have observed the close passage of S2 around the black hole in our galactic centre. But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution. We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects.
The new measurements evidently uncover an effect known as gravitational redshift. The star’s light is extended to longer wavelengths by the extremely strong gravitational field of the black hole. Moreover, the variation in the wavelength of light from S2 is in exact agreement with that predicted by Einstein’s theory of general relativity. This is the first-ever time that this divergence from the predictions of the less complex Newtonian theory of gravity has been observed in a star’s motion around a supermassive black hole.
SINFONI was used by the researchers to measure the velocity of S2 toward and away from Earth, and the GRAVITY instrument in the VLT Interferometer (VLTI) was used to make extremely precise measurements of the varying position of S2 to outline the shape of its orbit. GRAVITY produces exceptionally sharp images such that it uncovers the motion of the star from night to night as it passes nearer to the black hole - 26,000 light-years from Earth.
Our first observations of S2 with GRAVITY, about two years ago, already showed that we would have the ideal black hole laboratory. During the close passage, we could even detect the faint glow around the black hole on most of the images, which allowed us to precisely follow the star on its orbit, ultimately leading to the detection of the gravitational redshift in the spectrum of S2.
Frank Eisenhauer (MPE), Principal Investigator of GRAVITY and the SINFONI spectrograph
Einstein has been proved right again - in a far more extreme laboratory than he could have probably imagined - over 100 years after he published his paper detailing the equations of general relativity!
Françoise Delplancke, head of the System Engineering Department at ESO, explained the importance of the observations: “Here in the Solar System we can only test the laws of physics now and under certain circumstances. So it’s very important in astronomy to also check that those laws are still valid where the gravitational fields are very much stronger.”
It is anticipated that continuous observations will disclose another relativistic effect any time soon—a small rotation of the orbit of the star, called Schwarzschild precession—when S2 moves away from the black hole.
Xavier Barcons, ESO’s Director General, concluded saying, “ESO has worked with Reinhard Genzel and his team and collaborators in the ESO Member States for over a quarter of a century. It was a huge challenge to develop the uniquely powerful instruments needed to make these very delicate measurements and to deploy them at the VLT in Paranal. The discovery announced today is the very exciting result of a remarkable partnership.”
 GRAVITY was developed by a collaboration consisting of the Max Planck Institute for Extraterrestrial Physics (Germany), LESIA of Paris Observatory–PSL/CNRS/Sorbonne Université/Univ. Paris Diderot and IPAG of Université Grenoble Alpes/CNRS (France), the Max Planck Institute for Astronomy (Germany), the University of Cologne (Germany), the CENTRA–Centro de Astrofisica e Gravitação (Portugal) and ESO.
 S2 orbits the black hole every 16 years in a highly eccentric orbit that brings it within 20 billion kilometers—120 times the distance from Earth to the Sun, or about four times the distance from the Sun to Neptune—at its closest approach to the black hole. This distance corresponds to about 1500 times the Schwarzschild radius of the black hole itself.
 Observations of the center of the Milky Way must be made at longer wavelengths (in this case infrared) as the clouds of dust between the Earth and the central region strongly absorb visible light.