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Supermassive Black Hole stretches Starlight and proves Einstein right again



Artist's impression of the starlight's wavelength stretched by a black hole
Illustration: ESO / M. Kornmesser

Albert Einstein's theory of general relativity is magnificent. For a hundred years, it has consistently predicted all sorts of crazy phenomena that scientists later observed in outer space. An international team now announces that a 26-year observational campaign has re-confirmed the theory.

Astronomers using the Very Large Telescope (VLT) of the European Southern Observatory observed a star orbit believed to be a supermassive black hole at the center of our Milky Way galaxy. They measured the Doppler effect from the gravity of the black hole by attracting the light of the star – the effect was exactly what Einstein's theory predicted.

Astronomers have been using the Very Large Telescope since 1992 to observe a star named S2 hoping to use it as a probe for the gravitational field at the center of the galaxy. The star orbits Sagittarius A * (pronounced A-star), a source of radio waves believed to be a black hole four million times the size of the Sun, three times as long as it is between Earth and Pluto. But with Sag A *'s immense gravity, S2 completes its orbit in just 16 years, driving at 17,650 kilometers per second at 17 million miles per hour.

In an environment like this, the star should behave as predicted by Einstein's theories of special and general relativity. Isaac Newton's older, simpler laws of gravity lose their accuracy when applied to these extreme systems.

In early 2016, researchers began observing S2 with a powerful four-telescope instrument called GRAVITY as the star approached its closest access to Sag A *. The black hole has deformed the room so that the star's light waves were stretched, as predicted by Einstein, according to the paper published in Astronomy & Astrophysics.

That's an impressive thing. "The study has been carried out carefully, and it's amazing to see how well data accuracy has improved over the last two decades as instrumentation has improved," said Ingrid Stairs, a physicist at the University of British Columbia Not participating in the study, Gizmodo said. She also pointed out that astronomers are excited about a side effect of the authors with their work: a new precision measurement of the distance of the sun from the galactic center: 26,490 light-years, give or take some.

Why do scientists keep testing general relativity? It's a rock-solid theory that has not left us in the lurch for over a hundred years. But there are still many unexplained secrets throughout the universe. Some hope that the answers to these mysteries can live in tiny divergences between the predictions of general relativity and what scientists actually observe.

This story is not over yet. Physicists would still like to measure another general relativity prediction that the star should precess, meaning that the orbit itself is rotating more than predicted by Newton. Scientists may have enough data to confirm the existence of this precession by 2020, the paper says. So the measurement will continue. Stairs said, "It will be interesting to see what they find if they continue to measure the shape of the orbit over the next few months." [A&A via ESO]


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