Black holes are essentially invisible, but astronomers are developing technology to image the immediate surroundings of these enigmas like never before. Within a few years, experts say, scientists may have the first-ever picture of the environment around a black hole, and could even spot the theorized "shadow" of a black hole itself.
Black holes are hard to see in detail because the large ones are all far away. The closest supermassive black hole is the one thought to inhabit the center of the Milky Way, called Sagittarius A* (pronounced "Sagittarius A-star"), which lies about 26,000 light-years away. This is the first target for an ambitious international project to image a black hole in greater detail than ever before, called the Event Horizon Telescope (EHT).
The EHT will combine observations from telescopes all over the world, including facilities in the United States, Mexico, Chile, France, Greenland and the South Pole, into one virtual image with a resolution equal to what would be achieved by a single telescope the size of the distance between the separated facilities. [The Strangest Black Holes in the Universe]
"This is really an unprecedented, unique experiment," said EHT team member Jason Dexter, an astrophysical theorist at the University of California, Berkeley. "It's going to give us more direct information than we've ever had to understand what happens extremely close to black holes. It's very exciting, and this project is really going to come of age and start delivering amazing results in the next few years."
From Earth, Sagittarius A* looks about as big as a grapefruit would on the moon. When the Event Horizon Telescope is fully realized, it should be able to resolve details about the size of a golf ball on the moon. That's close enough to see the light emitted by gas as it spirals in toward its doom inside the black hole.
Very Long Baseline Interferometry
To accomplish such fine resolution, the project takes advantage of a technique called very long baseline interferometry (VLBI). In VLBI, a supercomputer acts as a giant telescope lens, in effect.
"If you have telescopes around the world you can make a virtual Earth-sized telescope," said Shep Doeleman, an astronomer at MIT's Haystack Observatory in Massachusetts who's leading the Event Horizon Telescope project. "In a typical telescope, light bounces off a precisely curved surface and all the light gets focused into a focal plane. The way VLBI works is, we have to freeze the light, capture it, record it perfectly faithfully on the recording system, then shift the data back to a central supercomputer, which compares the light from California and Hawaii and the other locations, and synthesizes it. The lens becomes a supercomputer here at MIT."
A major improvement to the Event Horizon Telescope's imaging ability will come when the 64 radio dishes of the ALMA (Atacama Large Millimeter/submillimeter Array) observatory in Chile join the project in the next few years.
"It's going to increase the sensitivity of the Event Horizon Telescope by a factor of 10," Doeleman said. "Whenever you change something by an order of magnitude, wonderful things happen."
Very long baseline interferometry has been used for about 50 years, but never before at such a high frequency, or short wavelength, of light. This short-wavelength light is what's needed to achieve the angular resolution required to measure and image black holes.
Grand Technical Challenge
Pulling off the Event Horizon Telescope has been a grand technical challenge on many fronts.
To coordinate the observations of so many telescopes spread out around the world, scientists have needed to harness specialized computing algorithms, not to mention powerful supercomputers. Plus, to accommodate the time difference between the various stations, extremely accurate clocks are needed.
"We had to prove you could keep time well enough at all the stations, and that the detectors at all the telescopes were good enough, that when you multiply the two signals from two telescopes you wouldn’t get just noise," said Dan Marrone, an astronomer at the University of Arizona's Steward Observatory who's building a receiver to enable the South Pole Telescope to join the project.
The researchers have been using atomic clocks made of what's called hydrogen masers to keep time to an accuracy of about a trillionth of a second per second.
"We use this property of the structure of the hydrogen atom to create a fundamental time reference for us that transitions between two states of the electron in a hydrogen atom," Marrone said. "It creates a low-frequency signal that through careful design you can make a very precise oscillator. It creates very perfect oscillations for a short time period. That means we can average our data over those time periods because they will all have kept time very perfectly."