This artist's impression provided by the European Southern Observatory in July 2018 shows the path of the star S2 as it passes close to the supermassive black hole at the center of the Milky Way galaxy. Image Credit: AP

Paris: A consortium of astronomers said on Thursday they had for the first time confirmed a prediction of Albert Einstein's theory of general relativity by observing the gravitational effects of a supermassive black hole on a star zipping by it.

The German-born theoretical physicist had posited that large gravitational forces could stretch light, much like the compression and stretching of sound waves we perceive with the change of pitch of a passing train.

Researchers from the GRAVITY consortium led by the Max Planck Institute for Extraterrestrial Physics realised that they had a "perfect laboratory" to test Einstein's theory with the Sagittarius A* black hole in the centre of the Milky Way.

Black holes are so dense that their gravitational pull can trap even light, and the supermassive Sagittarius A* has mass four million times that of our sun, making it the biggest in our galaxy.

Astronomers followed the S2 star as it passed close to the black hole on May 19 at a speed in excess of 25 million kilometres per hour.

They then calculated its velocity and position using a number of instruments and compared it with predictions made by Einstein that the light would be stretched by the gravity in an effect called gravitational redshift. Newtonian physics doesn't allow for a redshift.

"The results are perfectly in line with the theory of general relativity" and are "a major breakthrough towards better understanding the effects of intense gravitational fields," said the research team, whose findings are published in Friday's issue of Astronomy & Astrophysics.

This is the first time observers have been able to measure such an effect.
The European Southern Observatory, whose Very Large Telescope in Chile was used to make the observations, had watched S2 pass by Sagittarius A* in 2016 but the instruments it was using then were not sensitive enough to detect the gravitational redshift.

"More than 100 years after he published his paper setting out the equations of general relativity, Einstein has been proved right once more - in a much more extreme laboratory than he could have possibly imagined," said the ESO in a statement.

Astronomers already use another effect predicted by Einstein's theory of general relativity - that a black hole can bend passing light. Called gravitational lensing, researchers have used it to peer behind black holes.

Astronomers hope they can make practical use of the latest confirmation of Einstein's theory to track shifts in S2's trajectory due to gravity, which could yield information on mass distribution around the black hole.

"I am always blown away by Einstein's predictions, by the power of his reasoning which yielded this theory and which has never been faulted," French

astrophysicist Guy Perrin, a member of the GRAVITY consortium, told AFP.
Partnering with the Max Planck Institute in the consortium were French research institute CNRS, the Paris Observatory and several French universities along with Portugal's CENTRA astrophysics centre.