Making Black Holes Go 'Round on the Computer (Forwarded)

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Eberly College of Science
University Park, Pennsylvania

CONTACTS:
Bernd Bruegmann: (+1)814-865-1272,
Barbara K. Kennedy (PIO): (+1)814-863-4682,

24 May 2004

Making Black Holes Go 'Round on the Computer

Scientists at Penn State have reached a new milestone in the effort to
model two orbiting black holes, an event expected to spawn strong
gravitational waves. "We have discovered a way to model numerically, for
the first time, one orbit of two inspiraling black holes," says Bernd
Bruegmann, Associate Professor of Physics and a researcher at Penn
State's Institute for Gravitational Physics and Geometry. Bruegmann's
research is part of a world-wide endeavor to catch the first gravity wave in
the act of rolling over the Earth.

A paper describing these simulations will be published in the 28 May 2004
issue of the journal Physical Review Letters. The paper is authored by
Bruegmann and two postdoctoral scholars in his group at Penn State, Nina
Jansen and Wolfgang Tichy.

Black holes are described by Einstein's theory of general relativity, which
gives a highly accurate description of the gravitational interaction.
However, Einstein's equations are complicated and notoriously hard to
solve even numerically. Furthermore, black holes pose their very own
problems. Inside each black hole lurks what is known as a space-time
singularity. Any object coming too close will be pulled to the center of the
black hole without any chance to escape again, and it will experience
enormous gravitational forces that rip it apart.

"When we model these extreme conditions on the computer, we find that
the black holes want to devour and to tear apart the numerical grid of
points that we use to approximate the black holes," Bruegmann says. "A
single black hole is already difficult to model, but two black holes in the
final stages of their inspiral are vastly more difficult because of the highly
non-linear dynamics of Einstein's theory." Computer simulations of black
hole binaries tend to go unstable and crash after a finite time, which used
to be significantly shorter than the time required for one orbit.

"The technique we have developed is based on a grid that moves along
with the black holes, minimizing their motion and distortion, and buying
us enough time for them to complete one spiraling orbit around each other
before the computer simulation crashes," Bruegmann says. He offers an
analogy to illustrate the "co-moving grid" strategy: "If you are standing
outside a carousel and you want to watch one person, you have to keep
moving your head to keep watching him as he circles. But if you are
standing on the carousel, you have to look in only one direction because
that person no longer moves in relation to you, although you both are
going around in circles."

The construction of a co-moving grid is an important innovation of
Bruegmann's work. While not a new idea to physicists, it is a challenge to
make it work with two black holes. The researchers also added a feedback
mechanism to make adjustments dynamically as the black holes evolve.
The result is an elaborate scheme that actually works for two black holes
for about one orbit of the spiraling motion.

"While modelling black hole interactions and gravitational waves is a very
difficult project, Professor Bruegmann's result gives a good view of how
we may finally succeed in this simulation effort," says Richard Matzner,
Professor at the University of Texas at Austin and principal investigator of
the National Science Foundation's former Binary Black Hole Grand
Challenge Alliance that laid much of the groundwork for numerical
relativity in the 90's.

Abhay Ashtekar, Eberly Professor of Physics and Director of the Institute
for Gravitational Physics and Geometry, adds, "The recent simulation of
Professor Bruegmann's group is a landmark because it opens the door to
performing numerical analysis of a variety of black hole collisions which
are among the most interesting events for gravitational wave astronomy."

This research was funded by grants from the National Science Foundation
including one to the Frontier Center for Gravitational Wave Physics
established by the National Science Foundation in the Penn State Institute
for Gravitational Physics and Geometry.

[ B B / B K K ]

BACKGROUND INFORMATION

To predict the motion of two black holes is one of the fundamental open
problems in general relativity. "Einstein gave us his beautiful theory of
general relativity in 1915, and it remains an almost perfect description of
the gravitational interaction even by today's standards," Bruegmann says.
"But a century later we are still trying to puzzle out how something as
simple as the motion of two bodies works in detail." For everyday life and
even for most observations in our solar system, the theory of general
relativity only adds exceedingly tiny corrections to its precursor, Newton's
theory of gravity. Newton's gravity predicts, for example, the Kepler
ellipses for the motion of a planet around the sun, which is the classical
example for a gravitational two-body problem.

In Einstein's gravity, however, orbital motions produce gravitational
waves that carry away energy from the binary system, and the stable,
elliptical Kepler orbits turn into slowly decaying, inward spiraling orbits.
This basic prediction of general relativity is quite well understood when
the motions of the two bodies are not too relativistic. However, the
challenge is to compute in the regime of extremely strong and highly
dynamic gravitational fields as are present between two black holes
shortly before they collide and merge, when their orbits churn up the
fabric of space and time and huge amounts of gravitational radiation are
produced.

IMAGE CAPTION:
[http://www.science.psu.edu/alert/Ima...pse3dLarge.jpg (214KB)]
Black hole binary after about one orbit. The black surfaces represent the
black holes; whatever falls behind this black hole "horizon" can never
escape again. The colored plane shows a measure for the distortion of
spacetime. The numerical method has succeeded in keeping the data
smooth and undistorted. Credit: Bruegmann, Tichy, Jansen 2004