Was Einstein Right? Single Star Will Let Astronomers Test Theory of Relativity

Later this year, a special star in our galaxy called S0-2 will take the ride of its—and our—lifetime. And astronomers are particularly excited about the speedy journey because they have just confirmed the star is all alone, which means it will let them test whether Albert Einstein's theory of general relativity is correct.

The plan is the result of a recent paper published in The Astrophysical Journal that shows S0-2 is a single star, without a companion orbiting it closely. To test Einstein's theory, astronomers need to be able to take extremely precise measurements of where exactly the star is. That's not something they could have done if S0-2 had a binary, with two stars circling each other closely but blending into one smudge of light.

03_01_sagittarius_milky_way Giant black hole Sagittarius A* is invisible but tucked deep in the heart of the Milky Way. NASA

“We have been waiting 16 years for this,” first author Devin Chu, a graduate student at the University of California, Los Angeles, said in a press release. “We are anxious to see how the star will behave under the black hole’s violent pull. Will S0-2 follow Einstein’s theory? Or will the star defy our current laws of physics? We will soon find out!”

The test itself will take place as S0-2 zooms past Sagittarius A*, the giant black hole that lurks at the heart of the Milky Way. It will come close to the black hole—about three times the distance from the sun to Pluto, which is extremely close in astronomical terms. Sagittarius A* is so massive that its gravity will pull S-02 along at speeds up to 18.5 million miles per hour. That's still a fraction of the speed of light but pretty darn fast for a giant ball of gas to be moving.

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But Sagittarius A* won't just speed up S0-2. If Einstein was right about how the universe works, the star's light will slow down. That's because the black hole should be warping space-time, creating what scientists call a gravity well. Just as you'd use up energy climbing out of an actual well, the star's light loses energy climbing out of the gravity well.

Knowing that S0-2 is a single star, astronomers can measure how its light changes as it deals with the gravity well around Sagittarius A*. And unlike the star's last swing past the black hole 16 years ago, astronomers now have tools precise enough to do so.

If they see the effects they're expecting, they'll know Einstein's theory has passed another test. If not—well, they'll have to come up with a new theory that fixes the holes in his.

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