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 Earth, 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 typify our Milky Way galaxy. "It's very undistinguished," says Cambridge University astronomer Andrew Fabian, who has been studying it for 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.
A team led by Jörn Wilms of the University of Tübingen in Germany
has recently published the best spectrum yet for MCG-6-30-15. It
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—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," says Wilms. That graph,
he and his colleagues claim, says it all if you read it right: a giant
black hole spinning at nearly the speed of light, the space-time around
it twisted up 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—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 proposed 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. In 1919, during
a solar eclipse, the astrophysicist Arthur Eddington measured how the
sun bent light from a star behind it. That shift, about one
two-thousandth of a degree, agreed with Einstein's calculations and was
verified in later tests to prove that Einstein, not Newton, had it
right.
Anatomy of a Black Hole
One of the weirdest implications of Einstein's
general relativity theory is that as a black
hole spins, it
pulls space-time (represented
above by green grid lines) along for the
twisty
ride. Recent observations of the galaxy
MCG-6-30-15 suggest that
the spinning of its
central black hole inside a huge magnetic
field produces power just like an electric
generator. This energy contributes
to the
bright glow of iron atoms and other ultrahot
matter swirling in
a region called the corona.
Graphic by Matt Zang
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 slower, and at the event horizon
it would stop altogether. If your poor friend carrying the clock 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 companion 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 to tell 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 paths out of the hole exist, 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.
