Miércoles 10 de Mayo de 2006, Ip nº 152

Black Holes Collide, and Gravity Quivers
Por KENNETH CHANG

In the most precise effort yet to detect gravitational waves — the quiverings of space-time predicted by Einstein's theory of general relativity — the National Science Foundation in the late 1990's carved two large V's, one in the barren landscape of central Washington State, the other among the pines outside Baton Rouge, La.

The tunnels are part of the Laser Interferometer Gravitational-Wave Observatory, known as LIGO. If something astronomically violent, like a collision of two black holes, shakes the fabric of the universe within 300 million light-years of Earth, an expanse that encompasses several thousand galaxies, LIGO should see the resulting gravitational ripples.

The observatory is sensitive enough to detect a change of less than one ten-quadrillionth of an inch, or about a thousandth of the diameter of a proton, in the length of the 2.5-mile-long tunnels.

After several years of testing and fine-tuning — special dampers had to be installed at the Louisiana site to counteract vibrations generated when nearby loggers cut down trees, for instance — the observatory began full operation in November. The centers cost nearly $300 million to build and $30 million a year to operate.

The data so far, reported last week at a meeting of the American Physical Society in Dallas, contain nothing of interest. In fact, scientists would not be surprised if the initial run of the experiment over the next year or so found nothing at all.

"I would still sleep well about general relativity," said Peter R. Saulson, a physics professor at Syracuse and an observatory spokesman.

Jay Marx, LIGO's executive director, estimated that the chance of success was "25 percent, if nature's kind."

General relativity, formulated 90 years ago by Einstein to explain the properties of space and time, fits well with measurements of gravity in and around the solar system. But predictions about what happens around black holes and other places where gravity is extremely strong remain largely untested. One of the predictions is that in such conditions, sizable gravitational waves will be produced.

With new research, scientists have a better idea of what LIGO should look for. Researchers led by Joan M. Centrella, chief of the Gravitational Astrophysics Laboratory at NASA's Goddard Space Flight Center, announced last month that they had succeeded in calculating the shape of the gravitational waves that should result when two black holes, orbiting one another, merge.

"This is not something made up like in a science fiction movie," Dr. Centrella said in a news conference announcing the findings. "Rather, we have confidence that these results are the real deal, that we have the true gravitational wave fingerprint predicted by Einstein for the black hole merger."

The equations of general relativity can be easily written down but are notoriously hard to solve. Astrophysicists were able to simulate the head-on collision of two black holes three decades ago, but computing the paths of orbiting black holes and their violent merger proved much harder.

"This has been a holy grail type of quest for the last 30 years," Dr. Centrella said.

Dr. Centrella's simulations still contain some simplifications that do not reflect attributes of actual black hole pairs: the two black holes have the same mass, and neither is spinning. The calculations predicted, for example, that 4 percent of the mass of the black holes should be converted into gravitational waves.

"That's a very important number," Dr. Saulson said. "That tells us that these gravitational waves are going to be about as strong as we hoped they could be." He added, "And that's got those of us working on the detectors very excited, making it seem more likely we'll bump into something."

Einstein's theory of general relativity changed the idea of gravity from a simple force dragging apples from a tree to a puzzle of geometry. Imagine a rubber sheet pulled taut horizontally and then tossing a bowling ball and a tennis ball onto it. The heavier bowling ball sinks deeper, and the tennis ball will move toward the bowling ball not because of a direct attraction between the two, but because the tennis ball rolls into the depression around the bowling ball.

In this two-dimensional analogy of space-time, one can also imagine a sudden collision of objects creating ripples that skitter across the sheet. Those are the gravitational waves LIGO hopes to detect.

At each site, a laser beam generated at the base of the V is split in two and shot through tunnels buried along each 2.5-mile-long arm. The light bounces back and forth in the two tunnels. When a gravitational wave speeds past, it should stretch and shrink the distance that the laser beams travel, causing the laser light to flicker into a detector at the base of the V.

Because the instruments are susceptible to tiny disturbances, only signals seen by both LIGO detectors, nearly 2,000 miles apart, would likely be convincing to scientists.

The skepticism about whether LIGO will actually spot gravitational waves comes not from questions about general relativity — "People would be incredibly surprised if it wasn't right," Dr. Marx said — but uncertainty about how often events that create gravitational waves occur in the universe.

Pairs of orbiting black holes should be the end result of star systems consisting of two massive stars. Over time, the black holes would spiral inward and eventually collide. Astronomers can see plenty of pairs of massive stars twirling in the sky, but they cannot be sure that they ultimately collapse into pairs of black holes.

Because astrophysicists do not fully understand how stars age, "There are multiple factors of uncertainty," said Vassiliki Kalogera, a professor of physics and astronomy at Northwestern University. "We don't know that binary black holes exist."

At the optimistic end, her calculations suggest that LIGO could detect up to 10 black hole mergers a year. But the calculations are still uncertain by a factor of 100, which means that at the pessimistic end, the rate of detectable black hole mergers may be just one every 50 years or so.

A more common event is the merger of neutron stars, the dense, burned-out cores left over by some exploding stars. The most convincing evidence so far for gravitational waves was the observation in 1974 by two Princeton physicists, Joseph H. Taylor and his student Russell A. Hulse. They saw a pair of pulsating neutron stars spiraling inward toward each other. The amount of energy lost in the decaying orbits turned out to match the amount of energy expected to be emitted in gravitational waves.

However, the gravitational waves produced by orbiting neutron stars are too weak to be detected by LIGO. And even when the neutron stars slam into each other, the cataclysm is not nearly as violent as the merger of black holes, so a neutron star collision would have to occur much closer in order for LIGO to see it. Dr. Kalogera's calculations suggest that the observatory will see a neutron star merger once every seven or eight years, at best.

For LIGO to detect gravitational waves routinely, the instruments will need a proposed $200 million upgrade, which includes more powerful lasers, to increase their sensitivity by a factor of 10, Dr. Marx said.

Astronomers hope that LIGO and its successors, as well as similar detectors in Europe and Japan, will become a new type of telescope. If the detection of gravitational waves becomes common, astronomers should be able to deduce many physical properties of black holes and neutron stars. They may also find that such objects are more common in certain types of galaxies.

The upgraded observatory may also be able to detect gravitational waves produced by exploding stars or even reverberations of the Big Bang 13.6 billion years ago.

Sometime in the next decade, NASA and the European Space Agency hope to launch a space-based gravitational wave detector called the Laser Interferometer Space Antenna, or LISA. Consisting of three satellites flying around the sun in the formation of an equilateral triangle 3.1 million miles apart, LISA would be able to detect gravitational waves with much larger wavelengths, like those produced when mega-black holes at the center of galaxies merge.

For now, the scientists await their first gravitational wave.

"We are all hoping we are lucky," said Gabriela González, a physics professor at Louisiana State and a LIGO scientist. "Even if we are not, we will know more about nature."


  02/05/2006. The New York Times.