Scientists juggle black holes
Posted: Tuesday, April 08, 2008 6:30 PM by Alan Boyle
Researchers have created a first-ever simulation of three black holes circling and colliding, graphically demonstrating how Einstein’s version of gravity differs from Newton's.
They say their supercomputer cluster can juggle the interactions of as many as 22 black holes – and help other researchers recognize the signatures of such rare phenomena on the cosmic frontier.
"Twenty-two is not going to happen in reality, but three or four can happen," Yosef Zlochower, a mathematician at the Rochester Institute of Technology's Center for Computational Relativity and Gravitation, said today in a news release. "We realized that the code itself didn't really care how many black holes there were. As long as we could specify where they were located - and had enough computer power - we could track them."
The computer power is key. Three years ago, the same RIT team - including Manuela Campanelli and Carlos Lousto as well as Zlochower - produced a simulation of two black holes colliding in accordance with Albert Einstein's general theory of relativity. Since then, other researchers have developed their own models for simulating black hole collisions.
RIT's newHorizons supercomputer cluster packs quite a punch: It consists of 85 interconnected nodes, each with its own dual processor. The cluster can draw upon 1.4 terabytes of memory and 36 terabytes of storage space. Nevertheless, doing a single simulation run with three black holes required a whole week's worth of time on the cluster, Lousto told me today.
"It's a very complicated system with many variables in four dimensions, including time, and we need good resolution to solve the problem," he said.
Triple-play for Einstein
Black holes are among the most unusual objects in the universe's menagerie - concentrations of matter so dense that nothing can escape its core, not even light. They are thought to come into existence when massive stars collapse upon themselves, or when galaxies are formed. On a completely different scale, some physicists think subatomic collisions could create microscopic black holes that wink in and out of existence in an instant - but that's another story.
The result of the triple-black-hole simulation turned out to be different from what Newtonian physics would have predicted.
"General relativity has stronger effects," Lousto explained. "It's stronger when massive objects - black holes, in this case - are closer. ... And so we see these effects that black holes may collapse altogether and form a larger black hole, rather than flying apart, as with Newtonian gravity."
Just for fun, the RIT team went on to simulate a much more complex interaction between black holes.
"We made an initial configuration of black holes that spelled R-I-T, and showed how they all collide," Lousto said. "If you do this with Newtonian gravity, they fly apart."
Hunting for gravity waves
So what's the big deal? The simulation could point scientists to the actual signature of a triple play in the cosmos - a signature that would be written with gravity waves.
Around the world, highly sensitive experiments are looking for evidence of gravity waves, a phenomenon that is predicted by Einstein's theories but has not yet been observed. The U.S. effort in this field - known as the Laser Interferometer Gravitational-wave Observatory, or LIGO - is conducted from multimillion-dollar facilities in Washington state and Louisiana.
"We expect LIGO will be able to detect gravitational waves within a few years from now," Campanelli told me.
Gravity waves could give scientists a new set of guideposts on the frontiers of physics. But first, they'd have to know exactly what they're looking for.
"In order to confirm the detection of gravitational waves, scientists need the modeling of gravitational waves coming from space," Campanelli said in today's release. "They need to know what to look for in the data they acquire - otherwise it will look like just noise. If you know what to look for, you can confirm the existence of gravitational waves. That's why they need all these theoretical predictions."
Gravity-wave signatures, in turn, could help scientists confirm what they think they know about black holes. "The waveforms are particularly interesting and far more complex than one expected," Campanelli said. If those waveforms show up in data from LIGO, or a yet-to-be-launched space probe called LISA, that would serve as a powerful reality check for decades' worth of theorizing about black holes.
In the years ahead, the RIT research may turn out to be more than just theoretical: Last year, astronomers reported the detection of a triple-quasar system, 10.5 billion light-years away in the constellation Virgo. Each of the quasars is thought to be lit up by superheated gas falling into a black hole at the center of a galaxy.
"This presumably represents the first observed supermassive black-hole triplet," Lousto said.
And there's probably more of them out there, waiting to be discovered. Some theoretical research has indicated that triple-black-hole interactions could occur in the observable universe as frequently as a few times per year. If that's the case, supercomputer simulations could reveal exactly how the cosmic juggling act is done.
The RIT team's research paper on the simulation, "Close Encounters of Three Black Holes," is due to appear in the May issue of Physical Review D. The research also will be presented next week at a meeting of the American Physical Society.
