A Quest to See a Black Hole's Shadow

At the core of the Milky Way is a supermassive black hole that sucks in light, rendering it virtually invisible. But astronomers say they will be able to see the black hole's overall shadow within a few years.

"The Holy Grail of black hole astronomy is within our grasp," says Avery Broderick of the Harvard-Smithsonian Center for Astrophysics. "We could see the shadow that the black hole casts on surrounding material, and determine the size and spin of the black hole itself."

Nothing can escape the intense gravitational field of a black hole, not even light. And since they can't emit light, or any other form of matter, there's no visible evidence of their existence. But as matter gets pulled in, it heats up and radiates energy in "hot spots."

Some of this radiation escapes and can be detected.

Astronomers have already detected radiation from hot spots just outside the black hole, and they believe that these will paint a background against which the black hole's profile, or shadow, will stand out.

Since the technology to view the shadow won't be in place for another few years, Broderick and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics have designed a model that anticipates what the shadow will look like.

The hot spot of radiation rotates around the black hole, but researchers don't know if the hole itself is spinning or not, so Broderick and Loeb created two scenarios - one with a motionless black hole, and another with one spinning at the maximum rate. 

In each scenario, the hot spot is depicted as a rainbow-colored blob rotating around a solid blue disk representing the black hole's accretion disk, where matter heats up and collects before finally being sucked into the black hole.

"It will be really remarkable when observers can see all the way to the edge of the Milky Way's central black hole - a hole 10 million miles in diameter that's more than 25,000 light-years away," Broderick said.

To view the shadow, astronomers will need a radio telescope as large as the Earth. One is already in the works, more or less. Instead of building one impossibly giant telescope, astronomers will combine readings from a collection of submillimeter telescopes from across the continent.

This technique, known as interferometry, has already been used to study long wavelength emissions from outer space. Astronomers believe that studying short wavelength emissions could yield a high-resolution view of the outer region of the black hole.

The gravity well at the Milky Way's center is the best target for observation by interferometry because it covers the largest area in the sky of any known black hole. And an even higher resolution image could be achieved by combining observations from infrared instruments.

"Submillimeter and infrared observations are complementary," said Lincoln Greenhill, also of the Harvard-Smithsonian Center for Astrophysics. "We need to use both to tackle the problem of getting high-resolution observations. It's the only way to get a complete picture of the galactic center."

But a clear picture of this black hole won't be the only benefit of spotting its shadow. This data will ultimately help astronomers test Einstein's general theory of relativity within the intensely strong gravitational field of a black hole.

"When astronomers achieve it, that first image of the black hole's shadow and inner accretion disk will enter textbooks, and will test our current notions on gravity in the regime where spacetime is strongly curved," Loeb said.

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