For the first time in history, Albert Einstein’s theory of general relativity has been confirmed in a new context: in the most extreme gravitational field of the Milky Way , created by Sagittarius A, the supermassive black hole located at the center of our galaxy.
After 26 years of observations in an incredible environment, a star called S2 orbiting this object has just made its closest pass, and it behaved exactly as the theory of relativity predicts. It’s an impressive scientific result, obtained through amazing technology, careful math, and diligent observation.
“This is the second time we have observed the passage of S2 around the black hole in our galactic center. But this time, thanks to improved instrumentation, we were able to observe the star with unprecedented resolution, ” said astrophysicist Reinhard Genzel of the Max Institute. Planck of Extraterrestrial Physics to the journal Astronomy & Astrophysics that publishes the work.
“We have been intensively preparing for this event for several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects,” continues Genzel.
There are three of these S stars (not to be confused with S-type stars) in orbit close to Sgr A, which has a mass roughly equivalent to 4 million suns . S2, in its elliptical orbit, is one of the two stars that comes closest to the black hole at the perimeter. It is located only 17 light hours from the center of the galaxy, about four times the distance between the Sun and Neptune.
It may seem a bit far away, but when it comes to an object that has the gravitational pull of a supermassive black hole, it is very close.
The effect of the supermassive black hole at the center of our galaxy is so strong that it accelerates the star to around 25 million kilometers per hour, almost 3% of the speed of light.
When S2 is this close, according to relativity, the gravitational effect of the black hole must stretch the star’s light into longer wavelengths, toward the red end of the electromagnetic spectrum. Experts have confirmed that the star speeds up when it approaches the black hole and slows down when it moves away.
However, watching him around Sgr A is no easy feat. For starters, it is 26,000 light years away. In addition, the region is enveloped in a thick cloud of dust that makes it impossible to observe visible light.
The research team used various instruments on the European Southern Observatory’s Very Large Telescope to observe the perimeter of the star. SINFONI, GRAVITY and NACO have infrared and near infrared sensors, which can penetrate dust to pick up infrared sources. Using these instruments, the research team measured the velocity and mapped the orbit of S2 as it orbited Sgr A.
And they got the result they hoped for.
The new measurements clearly reveal the redshift , the first direct detection near a supermassive black hole, demonstrating, once again, Einstein’s theory of general relativity.
And it is that when the star approaches the black hole, it appears redder than it really is, because the wavelengths shift towards the red by the incredible gravitational attraction of the black hole.
It is the latest in a long and impressive line of evidence that has proven the theory of relativity time and time again.
If Einstein was right, why are tests still being done? Because if there are circumstances where the theory breaks, it will mark a profound change in the way we understand the universe, leading to a new form of physics. It is clear that if at any point it does break, it will most likely do so under absolutely extreme conditions.
“Here in the solar system we can only test the laws of physics under certain circumstances. So it is very important in astronomy to verify that those laws remain valid where the gravitational fields are much stronger”, clarifies astrophysicist Françoise Delplancke of ESO .
Reference: Detection of the gravitational redshift in the orbit of the star S2 near the Galactic center massive black hole, Astronomy & Astrophysics (2018). DOI: 10.1051 / 0004-6361 / 201833718
