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, for the same reason a
spinning figure skater speeds up when she pulls in her arms. There may
be millions of such black holes floating around our own galaxy, each
five 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.
What happens to the space-time fabric that is being dragged into such a
hole and is thus being spun around too? Does it get into such a fierce
gnarly twist that it rips, spilling stars, planets, and
general-relativity theorists out of the universe and into the cold
nonexistence of hyperspace? Probably not. "I think you've reached the
limits of the fabric analogy," says Begelman. Space-time is not really
a fabric, he explains; it's a mathematical description of the possible
motions of matter and energy. A black hole drags all the possibilities
inward. A spinning black hole first drags them around with it for a
while. Near the event horizon the drag is so strong that nothing can
resist it. The only possible motion is to spin along with the black
hole.
The mind reels, begs for a metaphor, a lifeline to a
more familiar experience. "If you were to hover just outside the
horizon but still under the influence of this twisting of space," says
Begelman, "it would be as if you were riding on—let me think of another
analogy—I would say it might be like a whirlpool."
Like a whirlpool but also like a flywheel, because the tremendous energy stored in that twirling patch of cosmos might actually be extractable.
Cosmic Points of No Return
Black holes at the center of six
active
galaxies are caught in the act of reeling in
huge swirls of
interstellar dust and gas in
combined visible and near-infrared images
taken by the Hubble Space Telescope. This
matter spirals inward,
heating up violently
and finally plunging into a black hole's
event
horizon. Because event horizons are
tiny compared with the size of the
galaxies
they inhabit, no one has ever seen one;
the smallest measured
black hole presumably
has an event horizon on the order of six
miles
across. For reference, the top left image is
about 6,000 light-years
across.
Photographs courtesy of
Ohio State University/Hubble Space Telescope
Images of space-time whirlpools did not immediately pop up on Jörn Wilms's computer screen when he received his data from MCG-6-30-15. The data came from a satellite telescope called XMM-Newton, launched in 1999 and operated by the European Space Agency. (X rays from space don't penetrate Earth's atmosphere, so they must be collected in space.) XMM-Newton is on a distended orbit that takes it one-third of the way to the moon, which keeps it out of Earth's shadow long enough to stay pointed at—and collecting photons from—the same faint object for more than a day.
On June 11, 2000, X-ray photons that had left MCG-6-30-15 during the early Cretaceous Period 130 million years ago poured through the open hatch at one end of XMM-Newton. They glanced off gold mirrors, which focused the photons onto a silicon wafer at the other end, 25 feet away. This electronic detector recorded each photon individually. What Wilms received, back in his office in Tübingen, was a long list of several million individual photons posted on a Web site. It included the energy and arrival time of each one. Data do not get much rawer than that.
You don't see anything
in such data without a theory of what you're looking at and looking
for. MCG-6-30-15, the whole galaxy, is not much more than a point in
the sky. Yet from previous observations of its spectrum, combined with
lots of theoretical calculations, astronomers have sketched a picture
of the intense activity in its nucleus. The central black hole, they
believe, is girdled by a thin disk of gas that is spiraling inward
toward doom. Most of this accretion disk is relatively cool, "which
means its temperature is in the millions of degrees," says Wilms. At
that temperature it glows mostly blue and ultraviolet.
The X rays must come from hotter stuff. The theory says they come from a tenuous, roiling foam of electrons and protons, called the corona, that splashes up from the disk near its center. As blue and UV photons stream through this billion-degree foam, they ricochet off its high-speed particles and are thereby boosted to X-ray energies—the whole band of X-ray energies, what astronomers call a continuum spectrum. To be at a billion degrees, the corona must be so close to the black hole that the in-falling gas has already converted most of its gravitational energy to heat. And because the corona is small, the X-ray emissions from MCG-6-30-15 can change fast. "We've seen its brightness double in a hundred seconds," says Andrew Fabian. "If you looked at it through an X-ray telescope, you would say, 'Wow!'"
