Answering Einstein

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Einstein's 1916 theory of general relativity predicted that some objects in space produce such intense gravitational fields that nothing —not even light—can escape them. Ergo, the black hole. But while the Hubble Space Telescope and Chandra X-ray Observatory have helped identify more than 30 possible black holes, scientists have yet to provide direct proof of their existence.

Black holes are created when stars larger than seven solar masses (one solar mass is the mass of our own Sun) end their lives in a supernova explosion. In some cases, the star is not completely shattered, leaving behind a neutron star. These are so extremely compacted that one with the mass of the Sun would only be ten miles wide. If the surviving star is more than a few solar masses, the gravity of this compact object is so strong its matter collapses inward: a black hole.

Although black holes are invisible, the collision of two of them will produce gravitational waves, ripples in the curvature of space, which could be recognized by gravitational-wave detectors. But as of yet, no one has detected a gravitational wave. Effects from the emission of gravitational waves have been observed in binary pulsars, an indirect detection. Collaborating with black-hole researchers at the Universities of Pittsburgh and Texas, astronomer Pablo Laguna and physicist Jorge Pullin, with their students and post-docs, have been simulating black-hole collisions, the purpose being that once the computer simulations are perfected, researchers will know what to look for with the detectors.

A single black hole by itself doesn't emit gravitational waves. Says Laguna, "In a binary system, the holes orbit each other and emit gravitational waves as they spiral. The system loses energy. They spiral closer and closer and collide, emitting a burst of gravitational energy. This process takes a long time, millions of years. But the estimate is that there are three to ten collisions a year."

Black holes are in principle very simple geometrical objects. At the center of a black hole is a singularity. Surrounding the singularity is a surface that hides it from the rest of the universe. This surface is called the event horizon. Within this surface, avoiding crashing into the singularity requires speeds faster than that of light. The singularity is the point at which the curvature of space, according to the general theory of relativity, is infinite, and since infinity is impossible to handle with a computer, here lies the challenge of simulating black holes.

To simulate black-hole collisions, numerical relativists, such as Laguna, have excised the singularity—in essence, creating a black hole without a black hole. While this may seem an oxymoron, this "surgical procedure" does not affect the outcome of the simulations as long as it is performed inside the event horizon, in the region that external observers cannot see. Basically, the team is making the math computable. Even so, they rely on a number of interlinked computers running on software they themselves have written.

So far, only two groups—the Penn State-Pittsburgh-Texas team and another at the Albert Einstein Institute in Germany—have figured slightly off-centered grazing collisions, but the simulation of astrophysically relevant black-hole collision has not yet happened. "It's still years away," says Laguna. The computers continue to crash.

Laguna notes that people often ask him why, as an astronomer, he studies black holes and not more common astrophysical objects. He answers, "The beauty of black holes is that they are so simple by themselves, yet so complex when they interact. If we are able to successfully simulate how black holes collide and merge, we will have a better understanding of the nature of gravitational waves and the mathematical theory that governs their behavior and Einstein's theory of general relativity. The ramifications in math, computer science, astrophysics, and gravitational physics will be untold."

Pablo Laguna, Ph.D., is professor in the Eberly College of Science, 525 Davey Laboratory, University Park, PA, 16801; 814-863-8470; Pablo@astro.psu.edu. Other members of the Numerical Relativity Group include faculty Doug Arnold, Sam Finn, and Jorge Pullin; postdoctoral scholars Keith Lockitch, Manuel Tiglio, and Dierdre Shoemaker; and graduate students Gioel Calabrese, David Garrison, Bernard Kelly, and Ken Smith. Their research is funded by the National Science Foundation.

Last Updated May 01, 2001