Last year researchers "heard" black holes for the first time, when they detected the gravitational waves unleashed as two of them crashed together and merged. Now, they want to see a black hole, or at least its silhouette. Next month, astronomers will harness radio telescopes across the globe to create the equivalent of a single Earth-spanning dish—an instrument powerful enough, they hope, to image black holes backlit by the incandescent gas swirling around them. Their targets are the supermassive black hole at the heart of our Milky Way galaxy, known as Sagittarius A* (Sgr A*), and an even bigger one in the neighboring galaxy M87.
Earlier observations using this Event Horizon Telescope (EHT) without its full roster of dishes yielded tantalizing results, but in images the two black holes remained featureless blobs. This year, for the first time, the EHT will add dishes in Chile and Antarctica, sharpening its resolution and raising expectations. Astronomers now hope to see how the black holes whip the hot gas around them into accretion disks and spawn matter-spewing jets. They also hope to chart the size and shape of the event horizon—the boundary of the black hole—to test whether Albert Einstein's theory of gravity, general relativity, still works under such extreme conditions.
"It's a very bold and gutsy experiment," says theoretical astrophysicist Roger Blandford of Stanford University in Palo Alto, California, who is not involved in the project. Blandford believes the EHT may not only show how black holes work, but also deliver a more fundamental message. "It will validate this remarkable proposition: that black holes are common in the universe. Seeing is believing."
The EHT takes aim just once a year, when good weather is likely, when both black holes are visible in the sky, and when it's possible to get time at all the observatories around the globe. This year, the team will observe for 5 nights during a 10-night window from 5 to 14 April. Then, an intensive data processing effort begins, and it may be a year before they know whether they've succeeded. "It's an exercise in delayed gratification. Delayed gratification squared," says EHT director Shep Doeleman at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
Imaging black holes is a formidable challenge, and not just because their intense gravity prevents even light from escaping. They are also surprisingly small. Sgr A* is calculated to contain the mass of 4 million suns, based on the nervy, high-speed orbits of stars in its gravitational grip. But its event horizon, the point of no return for anything approaching a black hole, is 24 million kilometers across, just 17 times wider than the sun. To see something so small from 26,000 light-years away requires a telescope dish of global dimensions.
At optical wavelengths, Sgr A* is hidden by the shroud of dust and gas obscuring the galaxy's heart. Radio waves can pass through more easily, but ordinary radio dishes are still hampered by ionized gas clouds and low resolution. Best are telescopes sensitive to the shortest radio waves—millimeter waves—but the dishes, detectors, and data processing technology for this part of the spectrum were developed only in the past few decades. "There is only a tiny window where we can see the event horizon," says Heino Falcke, an astrophysicist at Radboud University in Nijmegen, the Netherlands, and chair of the EHT science council. "The Milky Way is like a milky glass."
Early this decade, Doeleman and other EHT researchers began testing the idea with millimeter-sensitive dishes in Hawaii, California, and Arizona. Later, they extended the array to include the Large Millimeter Telescope in Mexico. Along the way, they got a good enough image of the black hole in M87 to see the base of its matter-spewing jets—data that are helping them understand how the jets are created. In 2015, they glimpsed the magnetic field around Sgr A*, which may help explain how black holes heat up the material around them.
But to see the event horizon itself, the EHT had to grow even larger. Over the years, it has evolved from a loose, poorly funded group to a worldwide collaboration involving 30 institutions in 12 countries. Next month it will include farflung additions, including the IRAM dish in Spain, the South Pole Telescope, and the Atacama Large Millimeter/submillimeter Array (ALMA), a large international observatory comprising 66 dishes in northern Chile. With its huge dish area, ALMA is the big catch because it will boost the EHT's sensitivity by an order of magnitude. "That's the key for us," Doeleman says.
Adding new instruments isn't simple. The technique for combining signals from distant dishes is known as very long baseline interferometry, and most millimeter-wave telescopes are not equipped to take part. EHT researchers had to visit each facility to tinker with its hardware and install new digital signal processors and data recorders. In the case of ALMA, that took some persuading. "We had to go into the bowels of ALMA and rewire it," Doeleman says. "It required political buy-in at all levels."
The campaign next month will be a nervous time for the EHT team. All eight observatories need clear skies and no technical glitches to get the best possible observations. "The first time, things can go wrong," Falcke says. Data volumes will be so large that they have to be recorded on hard drives and shipped back to the Haystack Observatory in Westford, Massachusetts, and the Max Planck Institute for Radio Astronomy in Bonn, Germany, for processing. There, devices known as correlators, made from clusters of PCs but with the power of supercomputers, will spend months crunching through the data, combining the signals from separate dishes as if they came from a single dish as wide as Earth. Adding further delay, data from the South Pole Telescope won't arrive until September or October, when planes can retrieve the hard drives after the Antarctic winter.
When the data finally all come together sometime next year, the team hopes to see a bright ring of light from photons orbiting close to the event horizon, with a dark disk in its center. The ring should be brighter on one side, where the rotation of the black hole gives photons a boost, although the images on this first attempt may not be as crisp as the team's simulations. "It'll probably be a crappy image, but scientifically it will be very interesting," Falcke says.
Doeleman hopes to see structure in the matter swirling around the event horizon and watch, movielike, as gas falls into it and vanishes. Such observations might help explain why some black holes gorge on matter and shine brightly, whereas others—like Sgr A*—seem to be on a starvation diet. Falcke has a simpler wish. "The event horizon is the defining thing about a black hole," he says. "I hope to see it; to literally see it."