COLUMN ONE : Shedding Light on Black Holes : Evidence is mounting that the objects, with virtually limitless gravity, do exist. Recent discoveries could answer one of the biggest riddles of physics and help explain how galaxies are formed.
In the realm of exotic concepts, black holes have always been strange and fearsome things.
They would be the result of incredibly powerful pressures--if Earth, for example, were compressed to the size of a marble. A black hole would have virtually limitless gravity, a pull so strong that nothing, not even light, could travel fast enough to escape. Inside the black hole, time and space would behave in utterly unfamiliar ways.
Of course, there is no known way to dense-pack the Earth so much. But colliding giant stars could conceivably reach the black hole state, sucking in anything--a planet, another star, perhaps an entire galaxy--that ventured too close.
Once gobbled up, matter would be sealed off from the rest of the universe, never to reappear. Or it could pour out into another universe. Some think it might even create one.
So go the theories. But could circumstances in nature ever be right to form such an outlandish creation?
Evidence is mounting that the answer is yes--monstrous black holes do exist. In fact, they may be downright commonplace, so ordinary that they lurk at the center of most galaxies. Recent discoveries of possible black holes could answer one of the biggest riddles of physics and could also help decipher how galaxies are formed.
“In 1975, if you’d said, ‘Do massive black holes exist in just about every galactic center?’ (scientists) would mostly say no,” said John Tonry, an associate professor of physics at the Massachusetts Institute of Technology. “In 1985, they would have split 50-50.
“In 1992, I think most people would say yes.”
The launch of the orbiting Hubble Space Telescope and improvements in ground-based instruments led to a spate of announcements this year of tangible evidence of black holes, each at the core of a galaxy. Since January, at least five galactic centers have been tagged in the pages of prestigious journals and onstage at astronomers’ conventions as apparent black hole sites.
Another galaxy, targeted on one June day by seven ground-based and five orbiting observatories, will probably be added to the list, according to an Ohio State University astronomer who is a principal investigator of the study.
And a NASA airplane, equipped with delicate instruments, carried three astronomers to 41,000 feet above Hawaii last month to examine indications of a black hole at the heart of our own Milky Way.
Astronomers and physicists say it appears that a black hole eating its meal of stars and gas is the central engine of quasars and other exceptionally luminous galaxies.
The extreme brightness in these galaxies detected only at the edges of the universe are thought to be caused by streams of captive stars falling toward the black hole, just as bath water swirls into a drain.
In less bright galaxies, the center may contain a dormant black hole--an unseen, still-insatiable object that has simply exhausted the fuel within its reach. Scientists now speculate that these galaxies started out as quasars. Perhaps the black hole never grew massive enough to generate that much energy. Perhaps it takes thousands of years to pull in the next object and astronomers aren’t watching at the right time.
In any case, the gravity exerted by inactive black holes actually helps keep the surrounding galaxy from flying apart, the newest thinking goes.
“I think it looks pretty good--there is no alternative model to explain the observations,” said Stephen W. Hawking, the University of Cambridge mathematician whose theories about very tiny black holes--which no one is claiming to have observed--are the subject of a current movie, “A Brief History of Time.”
Belief in the larger black holes, scientists caution, is not the same as proof. “People have been more ready to accept the idea that these structures are around, but the real challenge to astronomers is to firm up the case,” said Roger Blandford, a theoretical physicist at Caltech.
Added Blandford: “. . . The circumstantial evidence is just stronger and stronger. . . . That’s one of the things that makes this such an exciting time.”
The theorists have a saying: “Black holes have no hair.” Translation: There should be no outward identifying characteristics at all. Black holes are invisible by definition--they are black because no light or other radiation escapes to be captured by scientific instruments.
But astronomers are learning how to spot prospective black holes in distant space by recognizing the presence of a brutal, yet compact, gravitational field.
Using the Hubble Space Telescope, astronomers Tod R. Lauer of the National Optical Astronomy Observatories in Arizona and Sandra M. Faber of UC Santa Cruz have taken images of the centers of distant galaxies, obtaining finer detail than previously available. They compare the images to computer models of what the galactic cores should look like if a black hole lay at the heart.
Stars typically cluster at the center of any galaxy. “But then they plateau,” Lauer said. The presence of a black hole “makes a particular interesting shape that rises steeply in the center. It’s a subtle distinction.”
Twice now, Lauer and Faber have made a match. In January, they announced at the American Astronomical Society meeting in Atlanta that the central stars in M87, a giant elliptical galaxy in the constellation Virgo, were at least 300 times as dense as expected. But nothing visible is generating so much gravity. The conclusion: a black hole weighing 2.6 billion times as much as the sun is gradually pulling the stars inward.
The black hole--if that’s what it is--appears to be active, Lauer said, citing a jet of ionized gas spewing forth from M87’s nucleus. The stream extends for more than 5,000 light years. What it probably means, Lauer said, is that black holes are messy eaters. The jet, he believes, is driven by the tremendous energy of the gases falling into the hole.
In April, Lauer and Faber declared they had found what they thought was a second black hole. The core of M32, a galaxy in the constellation Andromeda, showed a star pattern consistent with a black hole 3 million times as massive as the sun. The M32 object seems to be dormant, they said.
The astronomers expect that further analysis of Hubble images will reveal additional examples of what they call black hole candidates. “It gives us evidence that would satisfy Columbo,” Lauer said, “but it wouldn’t convince Perry Mason.”
To nail down proof of black holes, astronomers want to calculate the speed at which individual stars near the galaxy’s center orbit the gravitational source. The faster the speeds, the more gravity is being exerted. Such information could be used to more precisely determine how much invisible mass is at the core.
The Hubble’s flawed mirror, ground to slightly wrong specifications, means the images are too blurred for such exacting analysis. A repair job is planned for several years from now.
Meanwhile, John Kormendy of the University of Hawaii and Douglas O. Richstone of the University of Michigan have taken that extra step, thanks to some extraordinary clear conditions on the slope of Mauna Kea volcano on the island of Hawaii. It took them two years to accumulate three hours of useful observations, but eventually they obtained detailed enough information to “weigh” the center of NGC 3115, a galaxy 30 million light-years away.
The scientists used changes in the wavelength of the starlight to determine the speed at which stars swirled toward the center of the galaxy. They calculated how much nearby gravity was necessary to bring the stars to those speeds--and found the amount was hundreds of times higher than the stars themselves could account for.
Consequently, they announced in the July 10 issue of the Astrophysical Journal that they’d discovered indicators of a black hole with the mass of a billion suns. Their observations and analysis, Blandford said at the time, made “a good strong case, probably about the best we’ve got” for a black hole with the potential to have at one time powered a quasar.
Not everyone is convinced that the measurements in the various galaxies could only be caused by a black hole. Critics say these methods don’t answer whether the unseen mass is one mega-black hole or many small objects clumped together--either tiny black holes or collapsed stars known as neutron stars. Neutron stars do not have quite enough mass to reach the black hole state, but they are extremely dense. Unlike black holes, the existence of neutron stars has been confirmed.
Still, an Ohio State astronomer, Bradley M. Peterson, says he can show that at least one galaxy, NGC 3783, has a single huge gravitational source at its core that could only be a black hole.
Peterson led a team that coordinated 12 observatories--in Chile, South Africa, Mexico, the United States and in orbit around the Earth--which all focused attention on NGC 3783 on June 29. The instruments measured, among other things, rapidly fluctuating X-ray levels in the gases swirling around the unseen mass at the core. The levels, responding to the invisible gravitational source, changed as rapidly as every few minutes.
“If there were many objects, they would have had to be connected” to vary so quickly--an unlikely prospect, Peterson said.
“The weight of the evidence,” he said, “now does point to supermassive. One thing rather than lots of little things.”
In the end, said University of Chicago astrophysicist David Schramm, the big-or-little argument may not matter. A cluster of tiny black holes or a swarm of neutron stars should eventually collapse into a huge black hole anyway.
Except . . . there are those who still can’t quite accept a black hole even as an idea, despite its roots in both Isaac Newton’s laws of physics and Albert Einstein’s general theory of relativity.
They feel like mavericks these days. One eminent astrophysicist recently warned, “You can’t tell anybody this,” then dropped his voice to a whisper: “I don’t believe in black holes.”
Another doubter is more willing to go public. The concept of a black hole is “such an extraordinary thing,” said Daniel W. Weedman, a Pennsylvania State University astronomer. “To me, it’s almost a fundamental contradiction in philosophy to state that things exist in the universe which science can never understand. I’m more comfortable with a swarm of neutron stars because I understand neutron stars.”
To persuade Weedman that a gravity source is a black hole, astronomers will have to show him an image of the stellar gases swirling in. The gases should form an inflated doughnut shape--the hole would be where they disappear. No radiation of any kind should come from inside the doughnut hole.
“This kind of an observation is simply not possible today,” Weedman said. “Clearly, that’s one of the Holy Grails of astronomy.”
Whatever is in the center of these galaxies, “we’re indeed talking about something that is unquestionably exceptional,” Weedman said.
But not to worry. The closest prospect for a supermassive black hole is at the center of the Milky Way, more than 25,000 light-years away.
Charles Townes, a Nobel laureate in physics and UC Berkeley professor emeritus, says he believes the incredible gravity there will eventually pull our whole galaxy in. “But we won’t be around,” he said, “and in any case, we’ll fall first into the sun.”
How a Black Hole Might Be Formed
Long, long ago, in a galaxy far away--a hypothetical galaxy we’ll call Galaxy X--a black hole may have formed in the center and continued to grow in a manner something like this:
1. About 15 billion years ago, at the time of the galaxy’s birth, young stars in the very dense center kept colliding. The collisions released massive amounts of energy in the form of heat and light.
2. During the collisions, so much gas was so compacted that the mass of 100 million suns was packed into an area the size of the Earth’s orbit. The inward gravitational pull of all mass overwhelmed the outward push of the gases. The core collapsed. This did not happen gently.
3. The gravity of this new object was so powerful that even something traveling at the speed of light could escape into space. The result was a black hole.
4. It is thought that the core of the hole must have kept shrinking. But it also added mass by sucking in stars that came too close. The black hole might have eaten about one sun’s worth of material each year--and its meal would disappear from the universe forever. This process could continue as long as there was matter within reach.
Today, the dormant black hole helps stabilize the orbits of the billions of stars in Galaxy X.
