The central black hole of the Milky Way puts Einstein’s theories to the test.


Astronomers at the University of California tested Albert Einstein’s theories of relativity in the crucible of the monstrous black hole in the center of our Milky Way and found them rock solid. For now.

The team, led by UCLA astronomer Andrea Ghez and with key analyses from Jessica Lu, an assistant professor of astronomy at Berkeley University, followed a star so close to the black hole that the light it emitted was influenced by the intense gravity of the black hole. The effect, a gravitational redshift, was exactly what Einstein’s theories of special and general relativity predict.

“Measuring gravitational redshift around a supermassive black hole is really the beginning of a new era of testing general relativity,” said Lu, who worked with Ghez as a PhD student in 2003. “Our galactic center is a special place, a unique place, because we can study the physics and astrophysics of a supermassive black hole in detail. It’s almost impossible to do that in any other galaxy.”

General relativity, which regards gravity as a distortion of space and time, has been validated within our solar system and in the interactions between pairs of dense solar neutron stars or pulsars. But tests around extremely massive objects – the black hole at the center of the galaxy is the mass of 4 million suns – could show where general relativity does not explain the universe and changes are necessary.

“We know that general relativity must eventually collapse because it does not mesh with quantum mechanics, so it’s just a constant hunt for where that fracture is,” Lu said.

“We can absolutely exclude Newton’s law of gravity, (and) our observations agree with Einstein’s theory of general relativity,” Ghez said. “His theory, however, definitely shows vulnerability. It cannot fully explain gravity within a black hole, and at some point we must go beyond Einstein’s theory to a more comprehensive theory of gravity that explains what a black hole is.

Ghez, Lu, lead author Tuan Do of UCLA, and her colleagues have published their findings in Science today.

Red Shift, Blue Shift
Black holes are black because the light emitted at the surface or event horizon cannot escape: It does not have enough energy. The light falls back and orbits the black hole before finally disappearing inside, so that we see only black.

The UC team followed the star SO-2, which is far enough from the event horizon to still be visible. Nevertheless, the general theory of relativity says that the light it emits will lose energy and become redder by the time it reaches Earth, which is about 26,000 light years from the galactic center.

In addition, the special theory of relativity, which explains why people traveling at different speeds see time and space differently, says that the speed of the star will cause the light to be bluer when moving toward us and redder when moving away.

The team, using 24 years of observations, saw both effects. When SO-2 got closest to the black hole – approximately 120 times the distance between Earth and our sun, or 120 astronomical units – the light lost about 0.03 percent of its energy while climbing out of the gravitational well of the black hole.

Also, at closest approach, when it was traveling at 16 million miles per hour – nearly 3 percent of the speed of light – the redshifts and blueshifts perfectly matched the predictions of special relativity. Because of general relativity, SO-2 was traveling 107 miles per hour faster than simple Newtonian gravity would predict, based on Isaac Newton’s 17th century theory.

Similar results were reported a year ago by a competing team led by Reinhard Genzel, an astrophysicist at UC Berkeley and director of the Max Planck Institute for Extraterrestrial Physics in Germany. These results came before three key events in the 16-year orbit of SO-2: its closest approach to the black hole, called SagA* (towards the southern constellation of Sagittarius); its fastest and most blue shifted motion relative to Earth; and its slowest, redshifted motion relative to Earth.

The new results include these three events in the analysis, which allow better verification of the general theory of relativity and make it the most detailed study ever conducted on supermassive black holes and Einstein’s theory of relativity.

“The star SO-2 orbits a very eccentric orbit. Farthest from SagA*, it is 16.5 times further than its next approach,” Lu said. “It really dives in, whips around the black hole, then goes out and hangs away for a while. The period in which it goes through the next approach is very short, but very important to measure.”

The measurements also provided a more accurate mass for the black hole at the center of the galaxy – 3.984 million times the mass of the Sun – and determined its distance at 7,971 parsecs (25,916 light years).

Lu led the team’s astrometry group, which precisely measured the position in the sky of SO-2 relative to SagA* with the two 10-meter telescopes at the Keck observatories in Hawaii. These telescopes are equipped with adaptive optics to remove blurs from the atmosphere. Ghez and Do led the group that measured the red and blue light displacements of SO-2. Taken together, these data provided the three-dimensional position of the stellar orbit required to test the theory of relativity.

The team aims to put relativity to the test again by astrometrically measuring the precession of the orbit of SO-2 – a gradual rotation of the orbital surface predicted by general relativity. An important early test of relativity was the explanation for an anomaly in the precession of Mercury’s orbit, which turned out to be a consequence of the distortion of space-time by the Sun’s gravity.

“The stars around SagA*, the supermassive black hole in our galaxy, should also show this orbital precession, just like Mercury,” she said, “We have never measured it against a supermassive black hole before. It will be another probe of general relativity.”

Co-authors with Lu, Ghez and Do are researchers from Japan, Germany, France, Spain and the USA. The National Science Foundation has supported the UCLA Galactic Center Group for 25 years with additional funding from the W. M. Keck Foundation, Gordon and Betty Moore Foundation and Heising-Simons Foundation.


About Author

Mette Frederiksen is a The Washington Newsday correspondent. With her coverage of general science, NASA and the interface between technology and society, Frederiksen has been in the Science Desk's Technology Beat since joining Washington Newsday in 2018.

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