Black Holes Spin?

That's only one of several incredible new surprises about these whirlpools of darkness

By Robert Kunzig|Monday, July 01, 2002
RELATED TAGS: STARS

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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 thishole stars and star-stuff are always falling—a lot of stuff, theequivalent of a hundred million suns so far. From this hole nothing

escapes, not even light; it is perfectly black, like the mouth of along tunnel. If you were to get into a spaceship and put it into orbitaround this perfect blackness, you would find, once you got closeenough, and even before you started your final descent into darkness,that you were no longer in control. You would be swept along by anirresistible current, not of swirling gas or stardust but of space-timeitself.

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 ofthis is directly visible from Earth. From Earth, MCG-6-30-15 doesn'tlook like much: It's a lenticular galaxy, a lens-shaped blob of starswithout the photogenic spiral arms that typify our Milky Way galaxy."It's very undistinguished," says Cambridge University astronomerAndrew Fabian, who has been studying it for a decade. "If you were touse an optical telescope and just look at images, you wouldn't jump upand down." But if you look at the galaxy with a different kind oftelescope, it comes alive. As gas falls toward the central black hole,before it disappears from the universe forever, it becomes so hot thatit emits X rays, which astronomers can collect and plot on a spectrum.

A team led by Jörn Wilms of the University of Tübingen in Germanyhas recently published the best spectrum yet for MCG-6-30-15. Itdoesn't look like much either, just a gently sloping line of datapoints, with a small spike at the top. But it was Figure 1 in theresearchers' paper—there was no Figure 2—and although they did notactually jump up and down when they first saw it, they did get quiteexcited. "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 giantblack hole spinning at nearly the speed of light, the space-time aroundit twisted up like a whirlpool, and the fluorescent iron atoms thattrace that fantastic motion cast like leaves on swirling water.

All that—and one more thing. The X-ray glow of those iron atoms is sointense that gravitational heating alone cannot explain it. What thatunassuming little graph may represent is the detection of a new sourceof cosmic energy, one predicted a quarter century ago but never beforeobserved. Some theorists believe a large fraction of all the light inthe universe, including its most spectacular displays—jets of radiantgas that shoot out of certain galaxies at near light speed—may begenerated this way. Its basic principle is familiar; Michael Faradaydiscovered it in 1831. But the setting is exotic, to say the least. IfWilms and his colleagues are right, there is not just a hole but alsoan electromagnetic generator at the heart of MCG-6-30-15, one thattakes the rotational energy of swirling space-time and converts it intolight, much as an alternator spinning atop an auto engine spits outelectricity.

There was a time, before Faraday, when generatorswould have seemed more exotic than black holes; black holes wereactually conceived first. The Reverend John Michell of Yorkshire,England, a geologist and astronomer as well as a clergyman, predictedtheir existence in 1784, using Newtonian physics. To Newton, light wasmade of particles with mass, and gravity was a force exerted by massiveobjects on one another. The more massive and compact an object, thegreater the velocity required to escape its gravity. Michell calculatedthat a star 500 times as large as the sun and just as dense would havean escape velocity of the speed of light; light particles directedupward would fall back to the star's surface the way arrows orcannonballs do on Earth. Because light could never reach us from such astar, it would appear totally dark.

This is the misconceptionthat most of us still harbor today, that a black hole is simply a starso massive that even light cannot escape it.

The reality is moredisturbing because a black hole obeys Einstein's rules and notNewton's. In a way, Einstein's rules, which were contained in thetheory of general relativity he proposed in 1915, are more intuitive.Whereas Newtonian gravity was a mysterious force that somehow emanatedfrom mass and acted instantaneously over long distances, in Einstein'sview a massive object simply curves the space-time fabric around it. Itthereby bends the path of anything traveling through space-time,including light. It does that despite the fact that light particles, orphotons, have no mass, contrary to what Newton thought. In 1919, duringa solar eclipse, the astrophysicist Arthur Eddington measured how thesun bent light from a star behind it. That shift, about onetwo-thousandth of a degree, agreed with Einstein's calculations and wasverified in later tests to prove that Einstein, not Newton, had itright.

/~/media/import/images/2/e/a/blackhole_2Anatomy of a Black Hole
One of the weirdest implications of Einstein's
general relativity theory is that as a black
hole spins, itpulls space-time (represented
above by green grid lines) along for thetwisty
ride. Recent observations of the galaxy
MCG-6-30-15 suggest thatthe spinning of its
central black hole inside a huge magnetic
field produces power just like an electric
generator. This energy contributesto the
bright glow of iron atoms and other ultrahot
matter swirling ina region called the corona.
Graphic by Matt Zang

The gap between Einstein and Newton increases as gravity gets strongerand the curvature of space more extreme—black holes being the mostextreme case of all. Einstein himself never believed they could exist.He was convinced that nature had a way, not yet discovered byphysicists, to protect us from what he considered an absurd implicationof his theory. Today, though, it would be hard to find a physicist oran astronomer who doesn't believe in black holes. One reason is thatwhen enough mass is concentrated in a small enough space—as forinstance in a large star that has exhausted its nuclear fuel—no forceknown can resist the implosive force of gravity.

That is whata 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 aninfinitely deep hole in the fabric of four-dimensional space-time. Itforms when a massive object implodes and shrinks below a criticalcircumference, called the event horizon, and then keeps on implodinguntil all that mass is concentrated in a singularity—a point far, farsmaller than a subatomic particle. At that point, space-time ends, andthe pull of gravity becomes infinite.

"Think of a black holenot simply as a place where gravity is extremely strong but as a placewhere the fabric of space-time is being pulled continuously into thehole," says astrophysicist Mitchell Begelman of the University ofColorado, one of the authors of the Wilms paper. "Space isn't sittingthere stationary outside the hole. It's always being stretched andpulled into the hole."

Time is being stretched, too. If youwere to watch from a distant spaceship as a clock fell into a largeblack hole, you would see it ticking slower, and at the event horizonit would stop altogether. If your poor friend carrying the clock wereto shine a light back toward you, you would see the light waves gettingstretched out just like the ticks of the clock. This is calledgravitational red shift. A light that started out blue would shift tored, then to infrared, then to radio wavelengths as it approached theevent horizon. There the waves would become infinitely long, and thelight would wink out.

Your doomed companion would be utterlyunaware of this; in his frame of reference, his clock and his bluelight would be behaving normally. (That's relativity.) He would notsplatter off the event horizon, because it is not a material surface;he would fall through it without noticing a change. Your desperatesignals to tell him to turn back would follow him into the hole, and hewould receive them without difficulty. Perhaps he might respond withsome poignant blue flashes of his own. But that last message wouldnever reach you. Inside the event horizon, space is so curved that nopaths out of the hole exist, even for light. Once your friendpenetrated the horizon, the darkness would close over him. You wouldnot see his fate—to be ripped into his constituent particles as heapproached the singularity.

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So that is a black hole: a placewhere the future leads only inward, with unpleasant results. Nowimagine it spinning very rapidly.

Most black holes must spinat 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 aspinning figure skater speeds up when she pulls in her arms. There maybe millions of such black holes floating around our own galaxy, eachfive 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 bepossible.

Black holes on an altogether different scale arebelieved to squat in the centers of most galaxies, including our ownand MCG-6-30-15; the latest estimate has ours weighing in at arelatively puny 2.6 million suns. No one is quite sure how suchmonsters form. Perhaps it is through the spiraling collision of starsor star-size black holes in the overcrowded galactic core. In any casea giant black hole would be born spinning, and as more clouds ofstar-stuff spiraled into it, adding their angular momentum to its own,it would speed up. Ultimately, the theory goes, its event horizonshould be moving at nearly light speed—the upper limit. A black holewith a mass 100 million times that of our sun, like the one inMCG-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 ahole and is thus being spun around too? Does it get into such a fiercegnarly twist that it rips, spilling stars, planets, andgeneral-relativity theorists out of the universe and into the coldnonexistence of hyperspace? Probably not. "I think you've reached thelimits of the fabric analogy," says Begelman. Space-time is not reallya fabric, he explains; it's a mathematical description of the possiblemotions of matter and energy. A black hole drags all the possibilitiesinward. A spinning black hole first drags them around with it for awhile. Near the event horizon the drag is so strong that nothing canresist it. The only possible motion is to spin along with the blackhole.

The mind reels, begs for a metaphor, a lifeline to amore familiar experience. "If you were to hover just outside thehorizon but still under the influence of this twisting of space," saysBegelman, "it would be as if you were riding on—let me think of anotheranalogy—I would say it might be like a whirlpool."

Like awhirlpool but also like a flywheel, because the tremendous energystored in that twirling patch of cosmos might actually be extractable.

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Cosmic Points of No Return
Black holes at the center of sixactive
galaxies are caught in the act of reeling in
huge swirls ofinterstellar 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
eventhorizon. Because event horizons are
tiny compared with the size of thegalaxies
they inhabit, no one has ever seen one;
the smallest measuredblack hole presumably
has an event horizon on the order of six
milesacross. For reference, the top left image is
about 6,000 light-yearsacross.
Photographs courtesy of
Ohio State University/Hubble Space Telescope


Images of space-time whirlpools did not immediately pop up on JörnWilms's computer screen when he received his data from MCG-6-30-15. Thedata came from a satellite telescope called XMM-Newton, launched in1999 and operated by the European Space Agency. (X rays from spacedon't penetrate Earth's atmosphere, so they must be collected inspace.) XMM-Newton is on a distended orbit that takes it one-third ofthe way to the moon, which keeps it out of Earth's shadow long enoughto stay pointed at—and collecting photons from—the same faint objectfor more than a day.

On June 11, 2000, X-ray photons that hadleft MCG-6-30-15 during the early Cretaceous Period 130 million yearsago poured through the open hatch at one end of XMM-Newton. Theyglanced off gold mirrors, which focused the photons onto a siliconwafer at the other end, 25 feet away. This electronic detector recordedeach photon individually. What Wilms received, back in his office inTübingen, was a long list of several million individual photons postedon 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 anythingin such data without a theory of what you're looking at and lookingfor. MCG-6-30-15, the whole galaxy, is not much more than a point inthe sky. Yet from previous observations of its spectrum, combined withlots of theoretical calculations, astronomers have sketched a pictureof the intense activity in its nucleus. The central black hole, theybelieve, is girdled by a thin disk of gas that is spiraling inwardtoward doom. Most of this accretion disk is relatively cool, "whichmeans its temperature is in the millions of degrees," says Wilms. Atthat temperature it glows mostly blue and ultraviolet.

The Xrays must come from hotter stuff. The theory says they come from atenuous, roiling foam of electrons and protons, called the corona, thatsplashes up from the disk near its center. As blue and UV photonsstream through this billion-degree foam, they ricochet off itshigh-speed particles and are thereby boosted to X-ray energies—thewhole band of X-ray energies, what astronomers call a continuumspectrum. To be at a billion degrees, the corona must be so close tothe black hole that the in-falling gas has already converted most ofits gravitational energy to heat. And because the corona is small, theX-ray emissions from MCG-6-30-15 can change fast. "We've seen itsbrightness double in a hundred seconds," says Andrew Fabian. "If youlooked at it through an X-ray telescope, you would say, 'Wow!'"

+++

More than a decade ago, Fabian and his colleagues discovered a way ofseeing into this shimmering cloud, almost to the edge of its blackheart; and it was this strange, subtle feature of the X-ray spectrumthat the Wilms group went looking for. Some of the X rays from thecorona, the Cambridge researchers realized, would shine back onto theaccretion disk and excite iron atoms there. And some of those ironatoms would thereupon fluoresce, emitting X rays of their own—not overthe whole band this time but at a single precise line in the energyspectrum: 6.4 kilo-electron volts, which is the energy an electronloses when it falls from one shell in an iron atom to a lower one.

An "emission line" like that in the hands of an astronomer is likea radar gun in the hands of a cop: It reveals how fast theX-ray-emitting iron atoms are traveling. Because the iron atoms inMCG-6-30-15 are moving, astronomers don't see the line right at 6.4kilo-electron volts. Instead the X rays are Doppler-shifted, like aradar beam bouncing off a speeding car (the radar waves hit the gunmore often if the car is moving toward the gun and less often if thecar is moving away from the gun). The X rays are thus shifted towardthe blue side of the spectrum, or "blue-shifted" to higher energies,and also intensified on the side of the accretion disk that is movingtoward Earth; they are "red-shifted" to lower energies on the side thatis moving away. When astronomers record a single spectrum for the wholegalaxy, the iron line is smeared in both directions by this Dopplereffect. At the same time it is also gravitationally red-shifted,because some of the iron atoms are very close to the black hole, wheretime itself and thus all light waves are stretched.

The netresult is that the sharp emission line is smeared into a broad,asymmetrical hump—and the broader the hump, the faster the iron must bemoving and the closer it must be to the black hole. Astronomer AndrewFabian predicted all this in 1989. In 1994, working with Japaneseresearchers and the Japanese X-ray satellite ASCA, he found evidencefor a broad iron line in MCG-6-30-15. Wilms and his colleagues hopedfor more conclusive results with the more sensitive XMM-Newton.

"And what we saw right from the beginning was that the iron line waswrong," says Wilms. "It was much broader than what we thought it shouldbe." The initial excitement was followed by fretting about whether theyunderstood their own telescope. "Almost monthly, we'd have these panicattacks where someone would raise a calibration problem," recalls ChrisReynolds of the University of Maryland in College Park, who worked withFabian on the earlier study and with Wilms on this one. "We'd have todo the whole analysis again."

The analysis consisted inbuilding, brick by theoretical brick, a model of MCG-6-30-15 that wouldexplain the data they got from the real galaxy. The researchers startedwith a model that included only continuum X rays from the corona; theyfound that it produced too many X rays at low energies and not enoughat high ones. They added a cloud of warm haze a few light-years fromthe black hole to absorb some of those low-energy X rays. (That hazereally seems to exist; it's what makes MCG-6-30-15 look dull in visiblelight.) They supplemented the X rays coming directly from the coronawith ones that had reflected first off the disk—and found they werestill coming up short. Finally, they added a fluorescent iron line,amazingly bright and red-shifted so strongly that it had to be comingfrom iron atoms streaking just over the event horizon at near lightspeed. Bingo.

"It was almost like a blazing ring right around the black hole," says Reynolds.

For the iron atoms to get that bright so close to the black hole inMCG-6-30-15, the hole has to be rotating rapidly. By draggingspace-time around with it, a rotating hole allows gas to orbit closerto the event horizon without falling in. And if the iron atoms arefluorescing that brightly, it means something is wrong with thestandard model of black-hole accretion disks. In that view the disk islit up by only gravitational energy, which is converted into heat andlight by means of friction. But it's hard to generate blazing ringsthat way. "The gravitational energy is released gradually, so theglowing region of the accretion disk is fairly extended," saysReynolds.

"There's no way you can produce more energy, say,by throwing the stuff down the black hole faster," says Wilms. "Youreally need some other mechanism."

"If what we're seeing iswhat we think we're seeing," says Mitch Begelman, "then it's verysignificant."
The new mechanism for getting energy out of ablack hole is not really new. Roger Blandford and Roman Znajek ofCambridge University proposed it in 1977. And the reason you can getenergy out of a black hole, that swallower of all things, is that theenergy you detect never really got into the black hole to beginwith—it's associated with the space-time whirlpool created outside theevent horizon by the black hole's rotation.

/~/media/import/images/b/3/6/blackhole_5smSpinning Out Electricity
Like a Black Hole

Magnetic fields, Blandford and Znajek realized, could convert thatrotational energy to electricity. The accretion disk is made of chargedparticles, and when the particles move, they generate a magnetic field.From then on, the field lines and the gas tend to stick together andmove together. When the gas plunges into the black hole, it follows themagnetic field lines. In Blandford and Znajek's theory, these linesprotrude from the event horizon like quills from a porcupine. Passingfirst through the space-time whirlpool, they continue far beyond itinto quieter realms. The whirlpool whips these magnetic field linesaround.

+++

It was Michael Faraday who discovered what happenswhen a magnetic field moves through an electrical conductor, or viceversa—though he certainly didn't have the ionized gas of an accretiondisk in mind. "Faraday said changing magnetic flux generates anelectromotive force—a voltage, if you like," says Blandford, who is nowat Caltech. "That's the basis of simple generators. It's the same thinghere. We've got a black hole that's spinning, so it's moving magneticfields around it, and that creates voltages. This time, though, thevoltages can be prodigiously large." In theory the voltage differencebetween the black hole's poles and its equator can be billions oftrillions of volts.

You can think of the magnetic field linesas wires in a titanic electric circuit, with the black hole as thegenerator; or you can think of them as elastic bands that literallyfling electrically charged particles into distant space as theythemselves are whipped around by the rotating black hole. The blackhole acts like a flywheel: As matter falls into it and increases itsspin, it stores energy; it releases energy again and slows down a bitas the magnetic field lines accelerate charged particles. "Probablywhat may happen is that you twist up the field lines by a certainamount, and then they snap back," Begelman speculates. "Then you twistthem up again, and they snap back. This would happen in an unsteady andsomewhat unpredictable way, and as a result you would extract theenergy in fits and starts."

That sort of pulsing certainlygoes on in cosmic jets, which are what Blandford and Znajek inventedtheir theory to explain. Jets are narrow streams of gas that emergefrom the cores of some galaxies, travel at more than 99 percent thespeed of light, and penetrate as much as several million light-yearsinto intergalactic space before fanning out into broad, luminous lobes.How might a black-hole whirlpool generate such a pair of waterspouts?Swirling bundles of magnetic field lines, flinging particles outwardfrom the poles of the hole, provide a natural explanation. It would benice, though, to have some direct observational evidence for thetheory; Blandford has been waiting a quarter-century for that.

MCG-6-30-15, unfortunately, has no jets. For an active galaxy it isrelatively quiet. But it does seem to have that blazing ring rightaround the black hole—and at the moment, say Wilms and his colleagues,the most plausible source of that light is some type of electromagneticgenerator powered by the rotation of the black hole. The details of themechanism have yet to be hashed out—and a lot of people are nowmotivated to work on it. "The theorists have been talking about thiskind of process for years," says Reynolds. "But until now there's neverreally been an observation you can point to and say, 'We think we havehard facts.'"

How hard are those facts? There is no doubt thatthe observation Wilms and his colleagues made was hard in anothersense—"at the limits of our current technology," as Begelman puts it.The only easily recognizable thing on their Figure 1 is the littlespike at the summit of the spectrum: That's the iron line, right whereit should be, at around 6.4 kilo-electron volts. But it's the unshiftediron line, made by slow-moving iron atoms far from the black hole. Thebroad iron line, the feature they were looking for, is so broadenedthat it is almost horizontal, an extra stratum laid over the continuumX rays from the corona. So it is hardly rude to be skeptical. "It'svery tricky telling what's the feature and what's the continuum," saysJulian Krolik of Johns Hopkins University, one of the theorists nowtrying to figure out how magnetic fields could convert a black hole'sspin energy into light. "We're all a little anxious about this."

More data may soon dispel the anxiety. Fabian's team has recentlyobserved MCG-6-30-15 again with XMM-Newton—watching it for three timesas long as the Wilms team did—during which time it got twice as bright;they too found a broad iron line. And last fall Fabian and fellowastrophysicist Jon Miller of the Massachusetts Institute of Technologyrecorded an uncannily similar spectrum from a stellar-mass black holein our galaxy. "It looks just the same as MCG-6-30-5," says Fabian.

Perhaps the remarkable thing is that there should be anyobservational evidence at all for so outlandish a phenomenon. "We'retesting some of the most exotic predictions of the theory of blackholes," says Begelman. "Even beyond the idea that they themselves canexist—the idea that a black hole can actually grab on to space andtwist it around, forcing everything in the vicinity to spin." Einsteinhimself couldn't accept the first idea, even as a matter of theoreticalprinciple; now scientists are on the verge of actually measuring thesecond one. Which doesn't mean they find it any easier than the rest ofus to imagine a space-time whirlpool.

"I can do the math, and it pops out," says Wilms. "But I always have big problems imagining it."


Two X-ray Telescopes Are Better Than One

/~/media/import/images/c/0/7/blackhole_4Photographs: top to bottom,
courtesy of STSCI/NASA;
courtesy of European Space Agency.

XMM-Newton (right, bottom) is not the only telescope casting a pryingeye on the violent X-ray universe. NASA's own five-ton workhorse is theorbiting Chandra X-ray Observatory (right, top), launched in July 1999.Like its European counterpart, Chandra has an elongated orbit swingingit from about 6,000 miles above Earth's surface to nearly 87,000 milesaway. Chandra and XMM-Newton both detect a wide range of X-ray sources,from the "soft," lower-energy emissions of supernova remnants up to theenergetic beams from black holes and neutron stars. (Another satellite,NASA's Rossi X-ray Timing Explorer, monitors ultrahigh energy signalsfrom those objects.)
So why have two big observatories? "Theyare actually quite complementary," says Steve Snowden, an astronomerwith the XMM-Newton Guest Observer Facility at NASA's Goddard SpaceFlight Center in Greenbelt, Maryland. Chandra's four sets ofX-ray-reflecting mirrors are smoother and more accurately shaped andaligned than are XMM-Newton's 174 nested mirrors. "It has much clearervision. It is able to resolve finer objects in the sky," Snowden says,and pick out details in complex structures like supernova remnants andstar clusters. But the large surface area of XMM-Newton's mirrors takesin five times the number of X rays and views a larger patch of the sky,"so it is very useful for looking at large structures," adds Snowden.
— Kathy A. Svitil


Check out a tutorial on black holes: www.howstuffworks.com/black-hole.htm.
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