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Like an Olympic athlete, the general theory of relativity has passed many tests in its century-long career. Its string of successes began in 1915, when Albert Einstein's picture of gravity as curved spacetime neatly explained shifts in the orbit of Mercury that had vexed astronomers for more than half a century. In recent decades it has faced more exotic and extreme tests, such as explaining why pairs of superdense neutron stars whirling around each other appear to be gradually spiraling toward collision. Here, too, general relativity triumphed: The stars are losing energy at exactly the rate expected if, as the theory predicts, they emit gravitational waves.
Yet physicists remain unsatisfied. The tests so far have been too easy, they say. The gravitational fields involved have been fairly weak, coming from single stars and bending or slowing light only very slightly. If the theory is going to show cracks, it will be under more extreme, high-field conditions. That matters because�on paper, at least�general relativity isn't the only game in town. Theorists have put forward alternative models for gravity, but in low fields they look identical to Einstein's theory. In strong fields, they begin to change.
Now, searching for a tougher test, researchers are looking toward the center of our galaxy. There, shrouded in dust, lurks a bright, compact source of radio waves known as Sagittarius A* (Sgr A*) for its position in the sky, near the edge of the constellation Sagittarius. Because of the way stars move in its vicinity, astronomers think that Sgr A* marks the dark heart of the Milky Way: a supermassive black hole weighing as much as 4 million suns but crammed into a space smaller than the distance between the sun and Mercury. That black hole produces the most intense gravitational field in our galaxy and so provides a unique laboratory for testing the predictions of general relativity. Over the next few years, using a range of new instruments tuned to infrared light and radio waves�radiation capable of penetrating the clouds of dust and gas around the galaxy's core�astronomers are hoping to see whether Sgr A* is bending relativity beyond the breaking point.
Two teams of astronomers�one led by Andrea Ghez of the University of California, Los Angeles (UCLA), and the other by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany�are staring at the center of the galaxy more intently than anyone before them. They are tracking a handful of stars that swoop close to the center�one of them to a distance equal to that between the sun and the edge of the solar system. Meanwhile, a unique new radio telescope array�still being assembled�is gearing up to carry the scrutiny right up to the edge of the putative black hole itself. In each case, the mission is the same: to spot discrepancies that Einstein's formulae cannot explain.
General relativity has "never before been tested at the high-field limit," says astrophysicist Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts. Elsewhere in the galaxy, astronomers have observed stars apparently caught in the grip of smaller black holes. But the stars close to Sgr A* "are 100 times closer to the event horizon [the boundary of a black hole] and the mass scale is a million times greater," Ghez says. "Does general relativity work down at scales 100 times closer? You're getting into the realm of basic physics: What is gravity? That's why people care."
Testing general relativity in this distant laboratory isn't easy. The black hole at Sgr A* is a small object, by galactic standards, and it emits no light. What radiation we do see is from superheated dust and gas falling in toward the event horizon. Once material passes that boundary, no trace of it remains. All astronomers can observe is the effects of the black hole's gravity on things around it. The UCLA and MPE teams aim to do just that.
Both groups began work in the early 1990s, when the current generation of 8- to 10-meter optical and infrared telescopes was coming online. At first, it was very difficult to pick out the movement of individual stars. The teams first determined that the stars were moving very fast (consistent with orbits around a very large mass) and then that they were accelerating around something. In 2002, the brightest of the near-in stars appeared to make its closest approach to the black hole and swing away again, essentially allowing the researchers to calculate its full orbit. It was following an ellipse so tight and so fast�5000 kilometers per second at closest approach�that it had to enclose an enormous, compact mass. "Then the community began believing in the supermassive black hole," says astronomer Stefan Gillessen of the MPE team.
Observations stepped up a gear during the 2000s, thanks to adaptive optics: systems that rapidly deform a telescope's mirror to compensate for the blurring effect of Earth's atmosphere. The sharper images that resulted enabled the teams to see more stars and to track them more accurately. Now the researchers could start looking for signs that relativity was making the stars' orbits deviate from a classical Newtonian course. So far, the effects of relativity have not emerged.
Both teams expect that to change starting in 2018, when that same bright star from 2002�known as S2 in Europe and S0-2 in North America�has its next close encounter with the black hole and the gravitational field it experiences is at a maximum. By then, both the W. M. Keck telescope in Hawaii, which the UCLA team uses, and Europe's Very Large Telescope in Chile, used by the MPE team, will have been upgraded. "We're trying to line up all the tools and methods ready for 2018," says UCLA's Gunther Witzel.
The teams will be looking for two telltale relativistic effects during and after the close approach, Ghez says. First, they expect to see the star's light shift toward longer, redder wavelengths as the photons strain against the black hole's intense gravity. |
