Within a few years, scientists should be able to "see" the shadow of an elusive black hole believed to reside at the center of the Milky Way. If it exists, it is currently invisible because matter -- and even light -- cannot escape.
All that's needed, an international group of researchers said Wednesday, is a slight refinement of the ability to measure short-wavelength radiation generated from the area where the incredibly massive object is expected to be in our home galaxy.
The researchers have developed a computer model visualizing what the black hole's shadow may look like. The model, created by Eric Agol at Johns Hopkins University, shows a handful of similar possible outcomes, all based on different inputs.
"Regardless of the structure of the region around the black hole that we tried in our computer models, we saw a shadow in the simulated images," said Agol, one of the authors of a paper appearing in the January 1, 2000 issue of Astrophysical Journal Letters. "This paper is our way of trying to interest astronomers in working together to perform the actual observations, which could produce very exciting results."
Such direct observations, which would display radiation from the mysterious fringes of the black hole -- an area known as the event horizon -- would bring black holes out of the dark recesses of theory and into the realm of the known. The event horizon is thought to be the point of no return for black holes, a somewhat mystical virtual wall behind which the laws of physics are not understood and may be different from what is known.
"Proving that there is an event horizon means there is a part of the world that is separated from our physics," said lead researcher Heino Falcke of the Max-Planck Institute in Germany. "We can't see behind this thing. We can't go behind this thing. And we don't know what's behind. It could be anything."
Falcke told space.com that the motivation to model the shadow stemmed from a 1973 theoretical paper by physicist James Bardeen, which Falcke recalled while pondering other aspects of black holes. When he and his colleagues began looking into the resolution needed to image the event horizon, they realized that rapidly improving observational techniques were coming close to what they needed.
The method relies on a cosmic trick called gravitational lensing. As if going through a giant galactic magnifying glass, the light and radio waves passing near a black hole are bent and re-focused, giving researchers what appears to be a larger target to search for. Depending on how much refinement of current methods is needed, Falcke said it could take anywhere from one to 15 years to image the shadow. His estimate is five years or less.
By tracing radiation, in the form of radio waves, emitted from all light around the black hole, Falcke and his colleagues expect the black hole itself will appear as a shadow against the surrounding light.
"The shadow has to be there," he said. "There's no way around it, if (the theory of) general relativity is correct. I'm pretty much convinced that this method is possible."
Simply, Falcke expressed the importance of the possible, eventual image of a black hole's shadow: "That would be the final proof that black holes exist. That defines a black hole. They are black."
What are we looking for?
Black holes are theoretical massive objects that pull nearby matter into their relatively small areas. Even light cannot escape the intense gravity, so researchers have never seen a black hole. They detect them indirectly by noting the gravitational effects on nearby stars, or by detecting powerful radiation given off by gas and other matter that swirls into the object.
A black hole is believed to exist at the center of the Milky Way, some 25,000 light-years away from Earth. But the density of intervening stars and other matter makes it difficult to see anything near our galactic center.
Near the Milky Way's center, around a star called Sagittarius A*, astronomers have found a compact source of very strong radio emission, perhaps created by highly ionized gas surrounding a black hole, Falcke explains. To study the region, researchers have used Very Long Baseline Interferometry (VLBI), which effectively creates a very wide telescope by combining the results of individual telescopes at various locations on Earth. The technique allows for measuring very short wavelengths of radiation.
"I think we didn't realize before how close the technique is to detecting this shadow," Falcke says. "With the currently available resolution, we could 'see,' from Berlin Germany, a radio source in Los Angeles the size of a mustard seed. Now we have to improve things to the point where we can image a dent on the seed."
For the paper, the authors took what astronomers currently know about the mass of Sagittarius A* and other potential features of the black hole, such as its rotation, and factored this into a "relativistic ray-tracing" program Agol had developed. The program traces the path of electromagnetic radiation through space warped by the gravity of a black hole.
Given the resolution achievable at short radio wavelengths, the calculations showed a distinctive pattern in radiation from Sagittarius A*: a circular shadow.
"With the major observatories working together, and a further improvement of millimeter-VLBI, we should soon be able to actually image the shadow of a black hole. This would be the final test of whether black holes and event horizons exist," Falcke said.
The research was sponsored by the National Science Foundation, NASA, and others.
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