How Big Is A Black Hole?

How Big Is A Black Hole?: An international coterie of astronomers, physicists and other scientist who work at the edge of human observation are meeting in Tucson, Ariz., this week to figure out the seemingly impossible task of directly imaging the "shadow" cast in normal matter by the supermassive black hole believed to lie at the center of our Milky Way galaxy.

How hard can that be? How hard would it be to image a grapefruit on the Moon?

That's the apparent size of the black hole at its range of 26,000 light years. In reality, it's about the size of the orbit of Mercury, but at that distance "to see something that small and that far away, you need a very big telescope," says Dan Marrone, an astronomer at the University of Arizona, which is playing host to the Event Horizon Telescope meeting.

The idea is to build a telescope the size of the Earth by linking radio telescopes all over the planet into a single virtual instrument, and then snapping the outline of the hole at the center of the galaxy.

"Nobody has ever taken a picture of a black hole," says Dimitrios Psaltis, an astrophysicist at at Arizona's Steward Observatory. "We are going to do just that."

By using radio waves instead of visible light or another wavelength, Pstaltis and his colleagues believe they will be able to penetrate the disk of stars, planets and other matter that lies between Earth and the galaxy core. Even so, the image they produce will not be the the hole itself, but the glow from compressed matter swirling the hole's immense gravity field before being pulled into it like water into a bathtub drain.



Illustration by Scott Noble/RIT

The shadow of the black hole, representing the event horizon beyond which light cannot escape, should be a perfect circle of compressed plasma if Einstein's Theory of General Relativity is correct.

"If we find the black hole's shadow to be oblate instead of circular, it means Einstein's Theory of General Relativity must be flawed," says Psaltis in a University of Arizona press release on the meeting, which ends Friday. "But even if we find no deviation from general relativity, all these processes will help us understand the fundamental aspects of the theory much better."

No one has yet seen what's going on so near a black hole, which is considered one of the most extreme environments in the universe. The Hubble Space Telescope captured this near infrared image of a jet of electrons and other subatomic particles streaming from the black hole at the center of the galaxy M87, pushed almost to the speed of light by the violent interactions near the event horizon there.



NASA and The Hubble Heritage Team (STScI/AURA)

Collecting the direct image will take time. Hard drives containing the relevant data will be collected from participating radio telescopes around the world at a central data center at the Massachusetts Institute of Technology's Haystack Observatory, where it will be processed. As more telescopes contribute data, the image of the black hole's shadow will sharpen, the researchers say.

"So far, we have indirect evidence that there is a black hole at the center of the Milky Way," says Psaltis. "But once we see its shadow, there will be no doubt."