Einstein’s Theory of General Relativity Stands, but Cracks Are Showing

A star known as S0-2 (the blue and green object in this artist’s rendering) made its closest approach to the supermassive black hole at the center of the Milky Way in 2018. (Nicole Fuller / National Science Foundation)

(CN) – No one knows what happens inside a black hole, but Einstein’s theory of general relativity still explains the phenomena occurring just outside.

Research published in the journal Science Thursday confirms observations of the relativity redshift in May 2018 when star S0-2 reached its closest position to Sagittarius A, the super-massive black hole at the center of the Milky Way. Redshift is what happens when light increases in wavelength, often due to objects moving farther apart or the expansion of space itself.

Moving 16 million miles an hour at its closest point, S0-2 drew within 120 astronomical units (11 billion miles) of the black hole 4,000 times bigger than Earth’s sun. Here, researchers observed the relativist redshift predicted by Albert Einstein’s theories of relativity and special relativity.

Black holes are so dense even light gets trapped in their gravitational pull.

Einstein’s 1915 theory predicted electromagnetic wavelengths, like light, would grow longer near a black hole. Photon wavelength varies depending on how fast an object is moving and how much energy it takes to fight the black hole’s gravitation field.

“Our observations are consistent with Einstein’s theory of general relativity. However, his theory is definitely showing vulnerability. It cannot fully explain gravity inside a black hole, and at some point we will need to move beyond Einstein’s theory to a more comprehensive theory of gravity that explains what a black hole is,” Andrea Ghez, UCLA astronomer and professor said in an interview.

More than two dozen researchers analyzed spectra data collected using a spectrograph built at UCLA from the W.M. Keck Observatory atop the dormant Mauna Kea volcano in Hawaii. Described as a “rainbow of light” by Ghez, spectra offer insights into what a star is made of.

Researchers also drew from spectroscopic and astrometry measurements dating back to 1995, covering S0-2’s last complete 16-year orbit.

More questions remain to be answered, like why do there appear to be so many young stars orbiting so close to Sagittarius A? Most physicists expect to find old stars orbiting near black holes since the extreme environment is not conducive to star creation, and yet there is S0-2 and many other young stars.

The researchers are also tracking other stars, including S0-102 which orbits the black hole every 11.5 years.

“General Relativity has a hard time describing things that are very small, but has a lot of mass, like black holes. By testing in extreme environments, like supermassive black holes, we can look for areas where the theory breaks down. This will then give us clues on how to build a better model of gravity,” said Tuan Do, lead author of the study, a UCLA research scientist and deputy director of the UCLA Galactic Center Group.

During the decades Ghez spent collecting evidence of Sagittarius A’s existence, she often butted up against Einstein’s relativity which doesn’t explain what a black hole is or what happens in one.

Still, Ghez said she wasn’t disappointed with the results.

“I was so thrilled. We’ve gone where nobody else has gone before and we have improved our understanding of gravity, one of the four most fundamental forces, but the least tested,” Ghez said.

While sliding into her car, Ghez pointed out that Google Maps and Waze rely on satellites being able to make precise measurements from Earth’s gravitational field. Einstein’s general relativity allows for this, but Newtonian physics does not. Under Newtonian physics, GPS would be unreliable and nothing would change near a black hole since he postulated that space and time are absolute.

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