Einstein faces super-massive black hole challenge

• Astronomers have discovered the closest star yet orbiting a jumbo black hole
• Observations of the star and a second one will test Einstein's theories
• Gravity bends light and twists star orbits, according to Einstein

Undefeated for nearly a century, Albert Einstein now faces his biggest challenger — the super-massive black hole hiding at the center of our Milky Way galaxy.

The challenge comes not from the black hole itself, a collapsed star known as Sagittarius A* that is roughly 4 million times more massive than the sun, but from a pair of stars orbiting this monster. One discovered just this week is the closest star yet spotted orbiting the black hole. And that star, dubbed S0-102, promises to re-open the test that first made Einstein famous, making his theory of gravity face its heaviest level of scrutiny yet.

"Why do we need two stars? Well, because the center of the galaxy is definitely a strange place," says UCLA astrophysicist Andrea Ghez, an author on the S0-102 study in the journal Science. About 26,000 light years away (where one light year is about 5.9 trillion miles), the center of the galaxy hosts not just a super-massive black hole but likely a collection of smaller black holes.

That extra stuff complicates calculations that test Einstein's theory of gravity, ones that first made the famed physicist's name. In 1916, Einstein calculated that a slight slip in the orbit of Mercury around the sun, where the planet's orbit seemed to backslide ever so slightly every century as it circled our star, could be explained precisely by his then-new theory.

The theory concluded that anything with mass essentially warps space and time, creating the attraction between objects that we call gravity. It also predicts that gravity should bend the path that light takes over long distances. In 1919, a series of expeditions that measured the light from distant stars during a solar eclipse showed the star's apparent position seemed to bend away from its known location in just the way that Einstein's theory predicted this light would bend under the influence of the sun's gravity.

"We want to do essentially the same thing as Einstein's tests with the light from stars orbiting this super-massive black hole," Ghez says. Why? Because the gravitational oomph that S0-102 and its sibling S0-2 feel from the pull of a black hole is about 100 times as strong as the pull that Mercury feels from the sun. The two stars take 11.5 and 16 years to circle Sagittarius A*, respectively, putting them outside the monster's grasp, Ghez says, "but space is much more strongly warped there making its effects much stronger."

So, in 2018, when S-02 comes as close as it ever does to the super-massive black hole, astronomers will measure how its gravitational pull bends the wavelength of light departing from the star ("climbing out of the gravitational well," as Ghez puts it). To astronomers the light looks redder than it would be otherwise, a "red-shift" that provides a precise check on Einstein's equations.

And observations of S0-102 will allow them to subtract away the interfering effects of other junk in the super-massive black hole's orbit to measure the red-shift effect. Three years later, S0-102 comes as close on its orbit as it ever does, letting astronomers repeat the experiment.

A test of how the gravity of Sagittarius A* causes the orbits of the two stars to backslide, an effect called "precession," awaits the next generation of telescopes, Ghez says, notably the international Thirty-Meter Telescope, planned to start operations in Hawaii in the next decade.

Does Ghez think her observations will finally prove Einstein wrong? "I'm not betting against him," she says. "I think we will find all sorts of unexpected things we didn't even anticipate. That's the pattern with science."

Indeed, predictions were that young stars like S-02 and S0-102 wouldn't be found near the super-massive black hole at the center of the galaxy. Astronomers thought it would have eaten all the gas clouds needed to form young stars before they took seed. But there they are.

"It says something really profound about how our understanding of the universe has expanded in the last 100 years that we are now contemplating these tests around a super-massive black hole now," Ghez says. "We are definitely looking ahead. It's 2018 or bust."