Black hole hunters

THEORY OF PROOF A team of researchers from MIT could potentially produce the first indication that Einstein’s theory of gravity, general relativity, needs fixing for the first time since it was introduced a hundred years ago, finds out Dennis Overbye.

Pico De Orizaba National Park, Mexico - Sheperd Doeleman’s project to take the first-ever picture of a black hole was not going well. For one thing, his telescope kept filling with snow. For two weeks at the end of March, Volcan Sierra Negra, an extinct 15,000-foot volcano also known as Tliltepetl that looms over the landscape in southern Mexico, was the nerve centre for the largest telescope ever conceived, a network of antennas that reaches from Spain to Hawaii to Chile.

Known as the Event Horizon Telescope, named after the point of no return in a black hole, its job was to see what has been until now unseeable: an exquisitely small, dark circle of nothing, a tiny shadow in the glow of radiation at the center of the Milky Way galaxy. It is there that astronomers think lurks a supermassive black hole, a trap door into which the equivalent of four million suns has evidently disappeared.

If Doeleman, a 48-year-old researcher from the Massachusetts Institute of Technology’s Haystack Observatory and the Harvard-Smithsonian Center for Astrophysics, and his colleagues succeed, the images they capture will be in textbooks forever, as definitive evidence of Einstein’s weirdest prediction: that space-time could curl up like a magician’s cloak around massive objects and vanish them from the universe. In short, that black holes - objects so dense that not even light can escape their maws - are real. That space and time as we know them can come to an end right under our noses.

Conversely, they could produce the first indication that Einstein’s theory of gravity, general relativity, the rule of rules for the universe, needs fixing for the first time since it was introduced a hundred years ago. “We’re swinging for the fences,” Doeleman, who has spent eight years putting this effort together, said one afternoon in an office in Serdan, a small town at the volcano’s base.

The center of the Milky Way, 26,000 light-years from here, coincides with a faint source of radio noise called Sagittarius A*. Astronomers tracking the orbits of stars circling the centre have been able to calculate that whatever is at the center has the mass of four million suns. But it emits no visible or infrared light. If this is not a black hole, neither Einstein nor anyone else knows what it could be. “That is the strongest evidence so far for an event horizon,” Doeleman said, using the name for the boundary of a black hole, the edge that is the point of no return.

The Event Horizon Telescope involves 20 universities, observatories, research institutions and government agencies, and more than a hundred scientists. Among other things, to keep the radio telescopes in their network suitably synchronised, they had to equip them with new atomic clocks accurate to within one second every 100 million years, and new short-wavelength receivers.

The March observing run was the first time the group would have enough telescopes - seven radio telescopes, on six mountains - to begin to hope they could glimpse the black hole. They would have five chances over a period of two weeks. On each night, they hoped to have two black holes in their sights: Sagittarius A*, and one in a giant galaxy known as M87, which anchors the enormous Virgo cluster of galaxies about 50 million light years away.

If everything went right - if all the elements of Doeleman’s spiderweb of weather and electronics and superprecise timing held together - they would see that any given wavefront would arrive bearing the marks of interference, a complicated pattern of crests and troughs - “fringes,” in the astronomical vernacular. With enough fringes from baselines going in different directions across the sky from the various observatories, the astronomers could reconstruct a map of what was happening out there, thousands of millions of light-years away. Seeing even one fringe from one baseline would be a triumph - it would mean they were achieving the kind of resolution needed to make a detailed image of Sagittarius A* and see if it looks like a black hole.

The Plumbers’ blues
The first piece of Doeleman’s spider silk to break was the radio telescope in Chile. That failure put more of an onus on the Mexican telescope. Sierra Negra was a natural choice as the fulcrum of the Event Horizon Telescope. Not only is it centrally located, but the new Large Millimeter Telescope sited there, with its giant dish designed for short wavelengths, is also the most sensitive radio telescope in the network.

During a dry run, however, the astronomers discovered that the telescope’s new receiver was afflicted by a mysterious electrical buzz. Several days of troubleshooting failed to make the buzz go away. “We’re just plumbers here,” Doeleman said one morning. When the team made its third try, the atmosphere in the control room was almost giddy as the telescope swung into position, staring at the black hole in the fiery galaxy M87. Doeleman typed into his laptop that the Large Millimeter Telescope was taking data. At last. An hour later, the weather went bad and they had to stow the telescope to keep the snow out. Just before dawn, five long hours later, the weather cleared enough for the telescope to rejoin the network, now focused on the Milky Way center. But two hours later, the sun had risen too high for them to continue. The black-hole party now became a race against time and weather. The next night, the Mexican telescope was shut out by the weather completely.

Getting out of dodge
Doeleman went home, satisfied that his team was in good shape to carry on, while he watched by laptop and Skype. They were now down to their last official chance to spin the silk. They clicked with the Event Horizon Telescope for good, first for Virgo and then for Sagittarius, collecting data until dawn. That night marked the end of the Event Horizon Telescope’s official observing run, but as it happened, there was an encore. California, Arizona and Mexico were available for an extra night.

This year, the hundredth since Einstein presented his Theory of General Relativity, the calendar is chock-full of meetings and celebrations devoted to the theory. Perhaps during this yearlong party, astronomers may finally know if the dark shadow of eternity is smiling at us through the star clouds of Sagittarius.

The computers are already running. At the end of April, an e-mail went out to the Event Horizon collaboration, dense with graphs, the result of correlating the observations from one night between two mountains - Sierra Negra and Mauna Kea, in Hawaii. They showed striking signs of an interference pattern. The fringes were there. The spider silk had held. “I had no idea I could hold my breath that long!” Doeleman said.