What was Einstein's theory of the black hole

And again Einstein was right : The orbit of a star around the black hole of the Milky Way

In the middle of many galaxies - maybe even in all of them - sits a supermassive black hole. Our home galaxy, the Milky Way, also has such a bizarre center. 26,000 light years away from us there is around 4 million times more matter packed together within a relatively small radius of only around 12 million kilometers than in the sun. The attractive gravitational force in the vicinity of this black hole is correspondingly strong.

And precisely because of its gigantic gravitation, it has also given itself away: the stars observed in its vicinity race around it at such enormous speeds that they can only be held in their narrow orbits by the great gravitational pull of a black hole. The international research collective “Gravity” has been keeping a particularly close eye on one of these racing stars for almost 30 years - and has calculated its orbit.

That's easier said than done. Because dense clouds of gas and dust prevent the view into the center of the Milky Way with optical telescopes, which can only collect visible light. Infrared light can penetrate the dust and gas clouds, but because of its longer wavelengths it provides a more blurred image than visible light.

Up to 20 billion kilometers from the black hole

This is why the scientists have been using the complex technical trick of so-called interferometry since 2016 in order to be able to measure the respective positions of the star S2 on its orbit even more precisely: They not only used one of the four giant telescopes of the "Very Large Telescope Array" ( VLT) of the European Southern Observatory ESO on Mount Paranal in Chile. Instead, they aimed the four telescopes at the same time on the star S2.

Each of these telescopes has a diameter of 8.2 meters. If you combine the 4 individual images of these mirror telescopes, you get such sharp images as if you had taken them with a single telescope that has a diameter of 130 meters.

Stefan Gillessen from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, who led the analysis of the measurement data, explained to Tagesspiegel how the position data of the star were obtained in detail: "For S2 it was 118 points with individual telescope data, 54 with interferometry and 92 measurements of its radial velocities. In addition, there were 75 measurements with individual telescope data of the position of the black hole, which always work when its surroundings happen to be a little brighter ”.

From a total of almost 300 individual position and speed measurements of the star S2 with the VLT, its orbit gradually emerged: It orbits the central black hole in the Milky Way almost exactly on an elliptical orbit, one orbit taking 16 years. At the point of its orbit closest to the black hole, the so-called perihelion, S2 approaches the black hole up to 20 billion kilometers; its speed increases to over 25 million kilometers per hour.

A departure from the ellipse that confirms Einstein's theory

To the delight of the research group headed by Frank Eisenhauer from the MPE in Garching, it turned out that S2, as hoped, deviated a bit from the exact elliptical orbit. Stefan Gillessen underlines the precision of the observation: "The measured deviation in the angle corresponds to a distance of 65 cm on the moon". However small this deviation may be, it confirms the correctness of Albert Einstein's general theory of relativity.

In principle, it was the same test that Einstein himself had used to test his new theory as soon as he had published it in 1915. In Einstein's time, the cosmic test laboratory was of course not the distant center of the Milky Way, but the nearby solar system, more precisely: the orbit on which the planet Mercury orbits the sun. As early as the middle of the 19th century, the French astronomer Urbain Le Verrier had precisely measured the orbit of the innermost planet of the solar system. It turned out that Mercury's elliptical orbit rotates slowly.

To put it more precisely: the point of its orbit closest to the sun, i.e. its perihelion, slowly but steadily moves around the sun every 250,000 years. Because of this perihelion shift, Mercury does not orbit the sun on an elliptical orbit, but on a rosette orbit. Even taking into account all conceivable gravitational forces in the solar system - the planets themselves also attract each other - the astronomers before Einstein did not succeed in fully explaining Mercury's rosette orbit with the help of Newton's classical law of gravitation; there remained a small but significant discrepancy of around 40 kilometers per revolution. Only Einstein found the explanation with the help of his general theory of relativity.

Einstein didn't believe in black holes

His joy at this brilliant confirmation of his controversial theory of gravity can already be heard in the opening words of his publication: “In the present work I find an important confirmation of this most radical theory of relativity; it turns out that it explains the rotation of the orbit of Mercury, discovered by Le Verrier, qualitatively and quantitatively, without any special hypotheses having to be based on. "

A little more than 100 years later, the collaboration team "Gravity", founded by Reinhard Genzel from MPE, has carried out the classic Einstein test again, but now, according to Stefan Gillessen, "for the first time with one of the most extreme objects in the universe - a black hole" .

And like Albert Einstein, he and his colleagues again have every reason to be happy: The star S2 races on a rosette around the supermassive black hole in the middle of the Milky Way, like Mercury around the sun. Even in the high gravity range of a black hole, the celestial bodies apparently obey the laws of general relativity. The little irony of this story: Albert Einstein didn't even believe that there could be black holes.

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