Black Holes Are Invisible. But Now There Are 3 New Ways to ‘See’ Them

Jun 08, 2016

In a world of conspiracy theories and Internet rumors, it's a wonder no one has ever called black holes a hoax. They're mysterious, they're powerful, but—oops!—they're entirely invisible. Trust us though, they're there.

Most people don't doubt the truth of black holes, but the invisibility part does rankle astronomers, who prefer things they can see and measure. Well bad news, science folks, black holes aren't about to pop into view any time soon. But good news: three new developments may make it easier to see what goes on in their immediate vicinity.

The reason for the invisibility of black holes is, of course, their gravity. The hyper-compressed remains of collapsed stars, black holes exert so titanic a gravitational force that anything and everything can fall inside them, but nothing—not even light—can escape. Once something does get swallowed by a black hole, all information about what it was—an apple, a pony, Matthew McConaughey—is forever annihilated without a trace. There is no forensic evidence around a black hole.

But there are ways around that rule. One of them is known as Hawking radiation, discovered in 1974 by—spoiler alert—Stephen Hawking.

According to Hawking, paired particles of matter and antimatter are created around a black hole's event horizon—the threshold at which matter tumbles into the great gravitational maw and disappears forever. For reasons explained only by the mathematical madness that is cosmological physics, while one of those twin particles falls in, the other is ejected, streaming away from the black hole and creating, in effect, a tiny energy leak. Over the course of epochs, that steady loss takes its toll and the black hole runs out of steam and effectively evaporates.

Energy, however, is not the only thing that escapes the black hole. According to a new paper that Hawking and his colleagues will publish this week in the journal Physical Review Letters and that was first reported in The New York Times, information about things that have fallen into a black hole may in fact trickle out too. The new theory suggests that as light waves on the surface of the hole strain to break away from it, they form a bristle of energy that radiates out and away like stalks. As matter falls in, it perturbs the stalks, producing a back-and-forth oscillation called supertranslation. The pattern of that oscillation is unique to the thing that has fallen in, and if you know how to read it, you can determine what that lost object was.

"[Black holes] are not the eternal prisons they were once thought," Hawking said at a June 6 Harvard presentation at which he announced the finding. "If you feel you are trapped in a black hole, don't give up. There is a way out." And if you can't get out, at least evidence of who you once were can.

Other researchers are piercing the black hole veil in other ways. At MIT, a team of investigators led by graduate student Katie Bouman has developed an algorithm that will allow them to visualize the event horizon that surrounds the black hole at the center of our own galaxy. Radio telescopes can cut through the dust that separates us from the heart of the galaxy 25,000 light years away. But that great distance combined with the compactness of a black hole would make the target very hard to image with even the most powerful radio telescopes.

"[It would be] equivalent to taking an image of a grapefruit on the moon," said Bouman in a statement. "To image something this small, we would need a telescope with a diameter of 10,000 kilometers, which is not practical." Indeed it's not, since the diameter of the Earth itself is only 13,000 km (8,000 mi.).

The answer, instead, involves an array of radio antennas around the world, each of which could image a portion of the event horizon. All of the antennae could then pool their data. That would still produce an incomplete picture, but Bouman's algorithm, designed to understand the properties and likely appearance of an event horizon, would fill it all in.

The algorithm is solid and the plan makes sense, but the system is not ready to go online. So far, six observatories have signed on to the project, but it will take more—perhaps twice as many—to gather the minimum data needed for a complete image. That recruitment effort will take at least a year, and perhaps much longer. "[T]here is development and procurement involved," MIT conceded in a press release, which is research institute-speak for "soon—we'll get back to you."

New findings already in hand have gotten a different kind of look at the behavior of a different black hole—this one at the center of a galaxy one billion light years from Earth. Known as Abell 2597 BCG, that one galaxy is part cluster of other galaxies, but judging by size and brightness, it is clearly the alpha dog of the pack.

In a new paper in Nature, a multinational team of 25 scientists from Yale University, the European Southern Observatory and elsewhere report that they have detected three clouds of cold gas falling toward the black hole's event horizon at speeds approaching 800,000 miles per hour (1.3 million k/h). That's exceedingly fast and is not how black holes are supposed to eat.

According to current theories, black holes favor hot gas, which orbits the event horizon in an orderly way and falls gradually in. The new findings suggest a much sloppier, more chaotic way for the black hole to get fed.

"This...process is not smooth, simple and clean, but actually quite chaotic and clumpy," says physicist Michael McDonald of MIT, one of the co-authors of the study. Studying that messiness matters, because the collapse of cold gas is how new stars get formed in galaxies, so the more researchers understand about how the gas behaves, the more they can learn about how new stars are born. That, in turn, provides clues to the origin of the universe itself.

None of this makes black holes easier to see, and unless someone suspends the laws of basic physics, no one ever will see one. But all of it makes black holes—always shrouded in mystery—at least a little easier to understand.

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