When black holes collide: Their echoes in the universe

man in black suit with hands slightly raised
Melissa Beattie-Moss

Pablo Laguna

"If you could make a black hole out of the mass of the sun, it would be the size of State College," said Pablo Laguna, Penn State professor of astronomy and astrophysics. A PowerPoint slide showed a dark circle superimposed on a map of the region—one measure of a black hole's density.

Laguna presented "Black holes and their echoes in the universe" last Wednesday as the fourth installment of Research Unplugged's spring season of conversations between faculty and the community at the Penn State Downtown Theatre.

"I'm going to give you all a thirty-second course in general relativity," he told the crowd, assuring attendees that "black holes are simple," primarily characterized by their extreme gravitational pull, so strong that not even light can escape it. He also added. "Just as we can identify a person by their hair," said Laguna, "in my field, we like to say that black holes have just three hairs: mass, rotation, and charge.

"So far we only have indirect evidence that black holes exist," he admitted. "But that's a lot in this business.

When discussing evidence for a supermassive black hole at the center of our galaxy, Laguna showed astronomers' observations of "stars orbiting a dark companion." He explained that " the size and very strong gravity of this companion cannot be accounted for by one star or a collection of stars." Such an object suggests the presence of a black hole, and though these bits of evidence may not be convincing to everyone, "if it looks like a duck and talks like a duck, I'll accept that it's a duck," he joked.

Commented a woman in the audience, "When one hears the term 'black hole,' images of Star Trek, Star Wars, and other science fiction movies come to mind." She went on to ask, "Is there a relationship between black holes and wormholes?" (In theoretical physics, a wormhole is essentially a shortcut through space and time.) "We find it difficult to believe in time travel," Laguna acknowledged, but he added that he keeps an open mind.

Laguna's research interest is in the phenomenon of the inspiral and merger of binary black holes. These mergers are one of the strongest sources of gravitational waves in the Universe. The waves are very strong near the merger, but extremely weak when they reach the Earth." Binary black holes are very rare, he added, with "current estimates suggesting that five black hole mergers a year will be detectable by instruments.

"As a theorist," he told the audience," I am always amazed at what instruments can do." With a host of recent technological leaps, he added, "We have moved into an era of multi-messenger astronomy, beyond the traditional photon-based data gathered by instruments like the Hubble Space Telescope." One of the important new messengers is gravitational waves.

"The detection of gravitational waves is a formidable undertaking," said Laguna, "requiring innovative engineering, powerful data analysis tools, and careful theoretical modeling." Today's interferometers, by combining the signals of two or more telescopes, can detect the gravitational waves that black holes emit and characterize their amplitude and frequency.

Explained Laguna, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, a project of the National Science Foundation, consists of two observatories, one in Washington state and the other in Louisiana. At each site, a giant interferometer is set up in a triangle, with mirrors three miles apart, powerful lasers, and a vacuum center. "By detecting changes in the separation of the mirrors, we can reconstruct the wave and find the source that produced it," he said.

Even bigger instruments are also in development, such as the Laser Interferometer Space Antenna (LISA), a joint venture of NASA and the European Space Agency planned for launch in 2017. As a space-based observatory, LISA will use an advanced system of laser interferometry to detect gravitational waves directly—if the immense engineering challenges can be met. "Each of the LISA stations are the size of café tables five million kilometers apart in space. And they would have to find each other, in order to work as a single interferometer" Laguna told an amazed audience.

The data obtained by such extremely sensitive instruments is highly susceptible to noise. At the LIGO observatories in Washington and Louisiana, for example, signals are sometimes compromised by nearby logging and drilling activities—"even the tumbleweeds," Laguna said. To counter these effects, he and his colleagues create waveform models to simulate data from black holes, to compare against actual observations.

"The waveforms are like fingerprints," Laguna explained. "The waves of black holes are different than the waves of neutron stars, which are different than the waves of supernova. By studying waves," he said, "hopefully we can identify their source."

Laguna's works is in the area of numerical relativity, which looks for non-analytical solutions to Einstein's equations, such as those representing binary black holes. "Finding these solutions require sophisticated computers," Laguna said. The simulations take about one week with supercomputers, but "would take millions of years for a single person with an ordinary calculator."

This prompted one audience member to ask, "What is the practical use for society of finding out about this stuff?"

"I must justify my existence here," Laguna joked. "The knowledge of black holes may not solve the issues with our economy," he admitted, but he also said that much new technology has been developed because of this type of undertaking. Although research into black holes is expensive, he added, it is a fundamental human impulse to want to know more about our world. "We are all curious about the neighborhood where we live," he said. "Black holes are part of our neighborhood."

Pablo Laguna, Ph.D., is professor of astronomy and astrophysics in Eberly College of Science. He can be reached at pablo@astro.psu.edu.

Last Updated April 09, 2007