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Thursday, Nov. 13, 2003


A black hole on our doorstep

The Atacama Desert in northern Chile is one of the most inhospitable places on Earth. It's 2,600 meters above sea level and receives almost no rainfall. Visitors, when they are not tending to dry skin and nosebleeds caused by the altitude, often compare the terrain to the barren red rocks that cover the surface of Mars.

The comparison is more than just subjective. Last week the journal Science published a paper comparing the soil in Atacama with that examined by the Viking landers sent to Mars in the 1970s. Using technology unavailable to the Viking landers, scientists have isolated organic compounds from the Atacama soil. The hope is that the soils will help researchers design better experiments for detecting life in Martian soil.

Almost lifeless though the desert is, however, a different group of scientists has another good reason to go there: the four giant white cylinders of the Paranal Observatory. These form the European Southern Observatory's imaginatively named Very Large Telescope.

Each of the four telescopes has a mirror 8.2 meters in diameter, making each, individually, among the largest telescopes in the world. Yet they can also combine their light to achieve the sensitivity of a single 16-meter telescope and the resolution of a single 200-meter telescope. Working together, the four telescopes of the VLT can see the universe in unprecedented detail.

"Using the VLT, we have seen the highest red shift yet observed," said Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, in a telephone interview. Red shift occurs when the object being observed is moving away from the observer. Since the universe is expanding, all the objects we see in the sky are moving away from us. The further away a stellar body is, the higher the red shift of the light we see from it.

"We have now detected massive black holes formed only 800 million years after the Big Bang," enthused Genzel. "We are now 14 billion years after the Big Bang, so we are looking at objects only 5 or 7 percent of the age of the universe. It's really amazing."

But Genzel was in the news last month for his work with an international team of astronomers on an object much closer to home. At the center of our own galaxy, the Milky Way, sits a ravenous monster: a supermassive black hole.

British science writer Nigel Calder has compared the black hole to a beast from Greek mythology. In a maze on Crete there once lived a Minotaur, Calder said, and the Minotaur demanded a diet of young people. "Now the maze is a galaxy, and at the core of that vast congregation of stars lurks a black hole that feeds on gas or dismembered stars."

The black hole at the heart of our galaxy is called Sagittarius A* (Sgr A*). It is only 4 million times more massive than the sun, a baby as far as supermassive black holes go (other galaxies contain far larger specimens), but it is on our doorstep. "The center of our Milky Way is very close," said Genzel, "Only 24,000 light years away."

Because Sgr A* is so close to Earth, we can study it in far more detail.

Genzel's finding, published in Nature last month, is that the black hole is emitting powerful infrared flares from a region just outside its "event horizon." Physicists are often portrayed as geeks and weirdos, but they sure have the knack for giving things cool names. Last week we heard that the Voyager spacecraft, launched 26 years ago, was crossing the "termination shock": the border between the solar system and interstellar space. But the black hole's event horizon is the point of no return, where gravity becomes so intense that not even light can escape.

"Inside the event horizon photons can't get out," said Genzel, "But outside they can."

The flares flicker on a scale of minutes, suggesting that the black hole is rotating.

"This is a major breakthrough," said Genzel. "We know from theory that a black hole can only have mass, spin and electrical charge. Last year, we were able to unambiguously prove the existence and determine the mass of the galactic center black hole."

Genzel explained that there are two important implications if Sgr A* is indeed spinning.

"First, we can test general relativity in a strong gravity regime." Einstein's general relativity has only been tested in the weak gravity regime on Earth. Genzel is no hippie, but he said, "We need cosmic experiments."

The second implication is that the spin may help tell us how the hole came to be. "Suppose a black hole is created by a collapsing gas cloud with random spin," said Genzel. "As it collapses, its spin increases, like a pirouetting ice dancer bringing her arms in to spin faster." Measuring spin might tell us how a black hole is formed.

There is an intimate link between the formation of black holes and galaxies, and even of the universe itself. All involve a singularity, a point of unimaginable pressure at the center of a collapsing star.

What happens at the singularity? Here's Calder again: "Matter would disappear, leaving behind only its intense gravity, like the grin of Lewis Carroll's Cheshire Cat."

With the help of instruments like the VLT in the middle of the Atacama Desert, we are now starting to understand just what that smile means.

Nigel Calder has recently written an encyclopedic book on all aspects of science: "Magic Universe: The Oxford Guide to Modern Science," published by Oxford University Press. Rowan Hooper is a researcher at Trinity College, Dublin. He welcomes comments at rowan.hooper@tcd.ie

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