Einstein's general relativity confirmed near the black hole – ScienceDaily

The Large Telescope (VLT) has now allowed astronomers to follow one of these stars, called S2, passing very close to the black hole in May 2018 at a speed exceeding 25 million kilometers per hour – three percent of the speed of light – and at a distance of less than 20 billion kilometers.

These extremely delicate measurements were made by an international team led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, in collaboration with collaborators around the world. The observations are the culmination of a series of 26 years of increasingly precise observations of the Milky Way's center using ESO instruments. This is the second time we observe the narrow passage of S2 around the black hole of our galactic center. But this time, thanks to improved instrumentation, we were able to observe the star with unprecedented resolution, "says Genzel. "We have been preparing intensely for this event for several years because we wanted to take advantage of this unique opportunity to observe the general relativistic effects."

The new measurements clearly reveal an effect called gravitational redshift. The star light is stretched at longer wavelengths by the very strong gravitational field of the black hole. And the stretch in the wavelength of the S2 light precisely matches that predicted by Einstein's theory of general relativity. This is the first time that this deviation from the simpler Newtonian gravity predictions has been observed in the movement of a star around a supermassive black hole. The team used SINFONI to measure the motion of S2 to and from the Earth and the GRAVITY interferometric instrument to perform extraordinarily accurate measurements of the position of S2 in order to define the shape of its orbit. GRAVITY creates images so crisp that it can reveal the movement from the evening star to the night passing near the black hole – 26,000 light-years away from Earth.

Our first observations of S2, about two years ago already showed that we would have the ideal black hole laboratory, "adds Frank Eisenhauer (MPE), co-principal investigator of the instrument GRAVITY. "During the close passage, we managed not only to accurately track the star in its orbit, we could even detect the faint glow around the black hole on most of the images. By combining the position and speed measurements of SINFONI and GRAVITY, as well as previous observations using other instruments, the team could compare them to the predictions of Newtonian gravity, general relativity and general relativity. other theories of gravity. As expected, the new results are inconsistent with the Newtonian predictions and in excellent agreement with the predictions of general relativity. More than a hundred years after publishing his article exposing the equations of general relativity, Einstein has been proven once again.

The material contribution of the Institute of Physics I of the University of Cologne was the development and construction of the two GRAVITY spectrometers. Spectrometers analyze the observed stellar light wavelength and convert the received photons into electronic signals. & # 39; GRAVITY is a technological challenge. However, after more than two decades of astrophysical research on high-velocity stars of the Galactic Center and on the development of astronomical instrumentation, the effort has been rewarded with an excellent result in experimental physics, "says Andreas Eckhart from the University of Cologne.

Continued observations should reveal another relativistic effect later in the year – a small rotation of the star's orbit, known as the Schwarzschild precession – while S2's Away from the black hole.

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