First successful test of Einstein’s general relativity near supermassive black hole

A culmination of 26 years of ESO observations of the heart of the Milky Way.

Artist’s impression of S2 passing supermassive black hole at centre of Milky Way
This artist’s impression shows the path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. As it gets close to the black hole the very strong gravitational field causes the colour of the star to shift slightly to the red, an effect of Einstein’s general thery of relativity. In this graphic the colour effect and size of the objects have been exaggerated for clarity.

or the first time, scientists have observed the impacts anticipated by Einstein’s general relativity on the movement of a star going through the extreme gravitational field close to the supermassive black hole in the center of the Milky Way. The study allowed astronomers to follow the star, called S2, as it passed near the black hole amid May 2018.

The star is located at a distance of fewer than 20 billion kilometers from the black hole, moving at a speed in excess of 25 million kilometers per hour. Using the observations made by exquisitely sensitive GRAVITY [1], SINFONI and NACO instruments on ESO’s Very Large Telescope (VLT), scientists can precisely make Newtonian predictions in excellent agreement with the predictions of general relativity.

Artist’s impression of S2 passing supermassive black hole at centre of Milky Way - annotated
This artist’s impression shows the path of the star S2 as it passes very close to the supermassive black hole at the centre of the Milky Way. As it gets close to the black hole the very strong gravitational field causes the colour of the star to shift slightly to the red, an effect of Einstein’s general thery of relativity. In this graphic the colour effect and size of the objects have been exaggerated for clarity.

They compared the position and speed estimations from GRAVITY and SINFONI individually, alongside past observations of S2 utilizing different instruments, with the predictions of Newtonian gravity, general relativity and different hypotheses of gravity. They thus now came up with the culmination of a 26-year arrangement of perpetually exact observations of the center of the Milky Way utilizing ESO instruments.

Orbits of stars around black hole at the heart of the Milky Way
This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018. This is a perfect laboratory to test gravitational physics and specifically Einstein’s general theory of relativity.

Their measurements revealed an effect called gravitational redshift. Light from the star is extended to longer wavelengths by the exceptionally solid gravitational field of the black hole. Furthermore, the adjustment in the wavelength of light from S2 concurs unequivocally with that anticipated by Einstein’s theory of general relativity. This is the first time when that this deviation from the forecasts of the simpler Newtonian theory of gravity has been seen in the movement of a star around a supermassive black hole.

GRAVITY tracks star passing supermassive black hole
The GRAVITY instrument in the VLT Interferometer has tracked the motion of the star S2 as it made a very close approach to the black hole at the centre of the Milky Way. This image shows the star and black hole shortly before their closest approach in May 2018.

Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany said, “This is the second time that we have observed the close passage of S2 around the black hole in our galactic center. 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.”

In order to create precise measurements of the changing position of S2 in order to define the shape of its orbit, scientists used SINFONI and quantified the velocity of S2 towards and away from Earth. They also used GRAVITY instrument in the VLT Interferometer (VLTI) that unveils the motion of the star from night to night as it passes close to the black hole. his long-sought result represents the climax of a 26-year-long observation campaign using ESO’s telescopes in Chile.

Françoise Delplancke, head of the System Engineering Department at ESO, explains the significance 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.”

NACO observation of the stars at the centre of the Milky Way
This image is one of hundreds collected using the NACO instrument on ESO’s Very Large Telescope in Chile over a two-decade period to monitor the motions of stars orbiting the supermassive black hole at the centre of the Milky Way.

Scientists are further planning to continue their exploration in order to unveil another relativistic effect very soon.

Xavier Barcons, ESO’s Director General, concludes: “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.”

Orbit diagram of S2 around black hole at centre of the Milky Way
This diagram shows the motion of the star S2 around the supermassive black hole at the centre of the Milky Way. It was compiled from observations with ESO telescopes and instruments over a period of more than 25 years. The star takes 16 years to complete one orbit and was very close to the black hole in May 2018. Note that the sizes of the black hole and the star are not to scale.

These extremely precise measurements were made by an international team led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, in conjunction with collaborators around the world, 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.