Event Horizon Telescope: Seeing a Black Hole using VLBI

Theoretical calculations predict that the Milky Way's central black hole, called Sagittarius A*, will look like this when imaged by the Event Horizon Telescope. 

The false-colour image shows light radiated by gas swirling around and into a black hole. 

The dark region in the middle is the "black hole shadow," caused by the black hole bending light around it.

CREDIT: Dexter, J., Agol, E., Fragile, P. C., McKinney, J. C., 2010, The Astrophysical Journal, 717, 1092.

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 that inhabits the centre 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.

"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. 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."

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