A Supermassive Black Hole Just Proved Einstein Right Once Again

The European Southern Observatory (ESO) has announced the results of a 26-year-long observation campaign to test Einstein’s theory of general relativity around one of the most extreme objects in the universe: the supermassive black hole at the center of the Milky Way.
This incredible object, known as Sagittarius A*, weighs over 4 million times the mass of our Sun and produces the strongest gravitational field in the galaxy. Within this field, a small group of stars orbit at high speed and by tracking these stars researchers were able to put theories of gravity to the test. Einstein’s passes with flying colors.

This artist’s impression shows the path of S2 as it passes close to the supermassive black hole. The color effect and size of the objects have been exaggerated for clarity. ESO/M. Kornmesser

As reported in the journal Astronomy & Astrophysics, the researchers capitalized on several technical improvements to the instruments available to the Very Large Telescope, such as GRAVITY, SINFONI, and NACO. These were able to track the stars in exquisite detail. In particular, they followed the star S2, an object that gets closer than 20 billion kilometers (12.4 billion miles) from Sagittarius A*.

“This is the second time that we have observed the close passage of S2 around the black hole in our galactic centre," team leader Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics, said in a statement. "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 team witnessed something called gravitational redshift. The gravitational field around the black hole is so strong that it stretches the wavelengths of the light leaving the star, making it redder. This effect is not predicted in Newton’s theory of gravity but it agrees precisely with the value expected in Einstein’s theory of relativity.
“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,” explained Françoise Delplancke, head of the System Engineering Department at ESO.

While this is a great success for the international collaboration, it does not mark the end of this project. The team will continue to observe S2 and the other stars around Sagittarius A*. They should confirm another relativistic effect soon. The gravitational field should cause the orbit of S2 to rotate a little, something called the Schwarzschild precession, as S2 moves further away from the black hole.

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