Observations of the galaxy MCG-6-30-15 suggest that the spinning of its central black hole is producing power just like an electric generator.
That power contributes to the bright glow of iron atoms and other ultrahot matter swirling in a region called
the corona. In this NASA illustration, the event horizon is the central black bulge; the corona is the bright ring around it,
and the magnetic field is blue.
XXM-Newtopn/NASA
In the heart of MCG-6-30-15, a galaxy 130 million light-years away, there is a hole. It’s as big around as the orbit of Mars. Into this hole stars and star stuff are always falling—a lot of stuff, the equivalent of a hundred million suns so far. From this hole nothing escapes, not even light. It is perfectly black, like the mouth of a long tunnel. If you were to get into a spaceship and put it into orbit around this perfect blackness, you would find, once you got close enough, and even before you started your final descent into darkness, that you were no longer in control. You would be swept along by an irresistible current, not of swirling gas or stardust but of space-time itself.
That’s because the black hole in MCG-6-30-15 is spinning. And as it spins, it drags space-time around with it.
No spaceship has been there to check it out, of course. And none of this is directly visible from Earth. From our planet, MCG-6-30-15 doesn’t look like much. It’s a lenticular galaxy, a lens-shaped blob of stars without the photogenic spiral arms that mark our Milky Way galaxy. “It’s very undistinguished,” says Cambridge University astronomer Andrew Fabian, who has been studying it for more than a decade. “If you were to use an optical telescope and just look at images, you wouldn’t jump up and down.” But if you look at the galaxy with a different kind of telescope, it comes alive. As gas falls toward the central black hole, before it disappears from the universe forever, it becomes so hot that it emits X-rays, which astronomers can collect and plot on a spectrum.
In 2002 a team led by Jörn Wilms of the University of Tübingen in Germany published the best spectrum yet for MCG-6-30-15. That doesn’t look like much either, just a gently sloping line of data points, with a small spike at the top. But it was Figure 1 in the researchers’ paper [pdf]—there was no Figure 2—and although they did not actually jump up and down when they first saw it, they did get quite excited. “We just didn’t believe what it was,” Wilms says. That graph, he and his colleagues claim, says it all if you read it right. It represents a giant black hole spinning at nearly the speed of light, the space-time around it twisted like a whirlpool, and the fluorescent iron atoms that trace that fantastic motion cast like leaves on swirling water.
All that—and one more thing. The X-ray glow of those iron atoms is so intense that gravitational heating alone cannot explain it. What that unassuming little graph may represent is the detection of a new source of cosmic energy, one predicted a quarter century ago but never before observed. Some theorists believe a large fraction of all the light in the universe, including its most spectacular displays—the jets of radiant gas that shoot out of certain galaxies at near- light speed—may be generated this way. Its basic principle is familiar; Michael Faraday discovered it in 1831. But the setting is exotic, to say the least. If Wilms and his colleagues are right, there is not just a hole but also an electromagnetic generator at the heart of MCG-6-30-15, one that takes the rotational energy of swirling space-time and converts it into light, much as an alternator spinning atop an auto engine spits out electricity.
There was a time, before Faraday, when generators would have seemed more exotic than black holes; black holes were actually conceived first. The Reverend John Michell of Yorkshire, England, a geologist and astronomer as well as a clergyman, predicted their existence in 1784, using Newtonian physics. To Newton, light was made of particles with mass, and gravity was a force exerted by massive objects on one another. The more massive and compact an object, the greater the velocity required to escape its gravity. Michell calculated that a star 500 times as large as the sun and just as dense would have an escape velocity of the speed of light. Light particles directed upward would fall back to the star’s surface the way arrows or cannonballs do on Earth. Because light could never reach us from such a star, it would appear totally dark.
This is the misconception that most of us still harbor today, that a black hole is simply a star so massive that even light cannot escape it.
The reality is more disturbing, because a black hole obeys Einstein’s rules and not Newton’s. In a way, Einstein’s rules, which were contained in the theory of general relativity he formulated in 1915, are more intuitive. Whereas Newtonian gravity was a mysterious force that somehow emanated from mass and acted instantaneously over long distances, in Einstein’s view a massive object simply curves the space-time fabric around it. It thereby bends the path of anything traveling through space-time, including light. It does that despite the fact that light particles, or photons, have no mass, contrary to what Newton thought.
The gap between Einstein and Newton increases as gravity gets stronger and the curvature of space more extreme—black holes being the most extreme case of all. Einstein himself never believed they could exist. He was convinced that nature had a way, not yet discovered by physicists, to protect us from what he considered an absurd implication of his theory. Today, though, it would be hard to find a physicist or an astronomer who doesn’t believe in black holes. One reason is that when enough mass is concentrated in a small enough space—as, for instance, in a large star that has exhausted its nuclear fuel—no force known can resist the implosive force of gravity.
That is what a black hole is, according to Einstein’s theory of general relativity: a never-ending implosion. It is not just a star that is dark; it is an infinitely deep hole in the fabric of four-dimensional space-time. It forms when a massive object implodes and shrinks below a critical circumference, called the event horizon, and then keeps on imploding until all that mass is concentrated in a singularity, a point far, far smaller than a subatomic particle. At that point, space-time ends and the pull of gravity becomes infinite.
“Think of a black hole not simply as a place where gravity is extremely strong but as a place where the fabric of space-time is being pulled continuously into the hole,” says astrophysicist Mitchell Begelman of the University of Colorado, one of the authors of the Wilms paper. “Space isn’t sitting there stationary outside the hole. It’s always being stretched and pulled into the hole.”
Time is being stretched too. If you were to watch from a distant spaceship as a clock fell into a large black hole, you would see it ticking more and more slowly, and at the event horizon it would stop altogether. If you had a friend carrying the clock and he were to shine a light back toward you, you would see the light waves getting stretched out just like the ticks of the clock. This is called gravitational red shift. A light that started out blue would shift to red, then to infrared, then to radio wavelengths as it approached the event horizon. There the waves would become infinitely long and the light would wink out.
Your doomed friend would be utterly unaware of this. In his frame of reference, his clock and his blue light would be behaving normally (that’s relativity). He would not splatter off the event horizon because it is not a material surface; he would fall through it without noticing a change. Your desperate signals telling him to turn back would follow him into the hole, and he would receive them without difficulty. Perhaps he might respond with some poignant blue flashes of his own. But that last message would never reach you. Inside the event horizon, space is so curved that no path out of the hole exists, even for light. Once your friend penetrated the horizon, the darkness would close over him. You would not see his fate—to be ripped into his constituent particles as he approached the singularity.
So that is a black hole: a place where the future leads only inward, with unpleasant results. Now imagine it spinning very rapidly.
Most black holes must spin at least a little bit. Stars also spin, and when a large one collapses, the resulting black hole must spin even faster, since the same amount of angular momentum is stuffed into a much smaller amount of space. There may be millions of stellar black holes floating around our own galaxy, each 5 or 10 times as massive as our sun and roughly 50 miles around, each spinning more or less furiously—once a millisecond or so would be possible.
Black holes on an altogether different scale are believed to squat in the centers of most galaxies, including our own and MCG-6-30-15; the latest estimate has ours weighing in at a relatively puny 2.6 million suns. No one is quite sure how such monsters form. Perhaps it is through the spiraling collision of stars or star-size black holes in the overcrowded galactic core. In any case, a giant black hole would be born spinning, and as more clouds of star stuff spiraled into it, adding their angular momentum to its own, it would speed up. Ultimately, the theory goes, its event horizon should be moving at nearly light speed—the upper limit. A black hole with a mass 100 million times that of our sun, like the one in MCG-6-30-15, would have a circumference of more than 100 million miles, yet it could be rotating once every hour and three-quarters.
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