In Search of Elusive (or Illusive?) Black Holes

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Last year was a good year for black holes.

After two centuries of speculation and two decades of inconclusive data, a Hubble Space Telescope research team appeared finally to have nailed one. Now a radio-astronomy team reports that it probably has pinned one down too.

A black hole is a mass that has collapsed to such a density that its gravity is too strong for anything - including light - to escape its immediate vicinity. Einstein's theory of general relativity says that these bizarre things should exist. Many astronomers would like them to exist. They would provide a handy explanation for the massive energy pouring out as radiation and sometimes as jets of matter from so-called active galaxies and other objects. So black-hole enthusiasts feel gratified to finally have what both research teams call compelling evidence of these wonders.

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But wait a minute. As sometimes happens at a ``eureka moment'' in science, skeptics have arisen to dampen the celebration. Last year also brought new challenges to Einstein's theory. These modifications suggest that there's no theoretical basis for black holes after all. And that would please a host of skeptics who don't like the appearance of flimflam in astrophysics.

Black holes have smelled a bit like a ``too-good-to-be-true'' confidence game. Like an investment scam that promises outrageously high returns, a proposed black hole can be shaped theoretically to account for any amount of power an astronomer wants.

It works like this. Stars, dust, and gas caught in a black hole's gravity orbit in a so-called accretion disk. As this orbiting matter eventually falls into the black hole, it releases gravitational energy just as does water flowing over a hydroelectric dam. Moreover, the in-falling matter adds to the black-hole mass and increases its gravity. So, by gobbling up matter, a black hole can be any size needed to account for observed energy outpourings.

But there's a catch. If you look inside this black-hole scheme, you find that, like an investment scam, there is, in a sense, nothing there. A black hole's center is a ``singularity'' where no one knows what is going on. Everything disappears into it and laws of physics do not apply.

Some theorists are uncomfortable with this situation. For example, John Moffat of the University of Toronto and, independently, Carroll Alley of the University of Maryland and Haseyin Yilmaz of Hamamatsu Photonics in Japan, think it's unnecessary. They have modified general relativity theory so that it continues to work well where it has always worked but does not produce black holes.

What, then, of the new ``compelling'' evidence? Astronomers have thought that if they could find an accretion disk orbiting an enormous compact mass, they would have the ``signature'' of a black hole. Last spring, Holland Ford of the Johns Hopkins University and the Space Science Telescope Institute in Baltimore and colleagues released space-telescope images that show such a disk at the core of the galaxy M87. Then, in the Jan. 12 issue of Nature, James Moran and colleagues at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., reported radio observations of an accretion disk in the galaxy M106.

Matter in these disks orbits at speeds consistent with black-hole theory. For M87, this implies that the disk circles a mass of some 3 billion suns concentrated in a volume no larger than our solar system. For M106, it would take a mass of 36 million suns in a similar small volume to hold its disk in orbit.

If the research teams have indeed found true black-hole ``signatures,'' theorists will have to confront the embarrassment of singularities head on. However, if skeptics can show convincingly that there is no theoretical basis for black holes, astronomers will be hard pressed to account for the enormous compact masses the research teams have undoubtedly discovered. @QUOTE = A black hole's center is a `singularity' where no one knows what is going on. Everything disappears into it and laws of physics do not apply.

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