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How to make black holes 'sing': certain mergers would create odd gravitational wave signals.

When black holes collide, astronomers expect to record a gravitational wave "chirp." But rapidly spinning black holes, like the one featured in the 2014 film Interstellar, might prefer to sing.

According to the calculations of Caltech physicist Kip Thorne, who served as a consultant for Interstellar, the movie's black hole, known as Gargantua, must have had a mass 100 million times that of the sun and whirled about its own axis at breakneck speeds.

If such a rapidly spinning black hole merges with a companion, it would produce a unique signal--one that gravitational wave detectors might be able to observe, MIT physicist Niels Warburton reported April 18. "There is a completely different gravitational wave signature," said Warburton, who coauthored a related paper posted online March 3 at

The standard signal of merging black holes is a chirp, named for the increase in frequency and amplitude of the gravitational waves produced as the black holes spiral inward. When converted into sound waves, this pattern sounds like a bird's chirp. Warburton and colleagues performed calculations to determine the gravitational wave signature from a merger with a black hole spinning at nearly full tilt. Instead of a chirp, the gravitational waves would maintain a constant pitch but slowly fade away.

"It was certainly very unexpected to see something that didn't chirp," said MIT physicist Jolyon Bloomfield, who was not involved with the research.

If such black hole mergers occur in nature, next-generation gravitational wave observatories like the Evolved Laser Interferometer Space Antenna might provide proof of their existence. Plans call for eLISA to measure gravitational waves from space beginning in 2034.

The Advanced Laser Interferometer Gravitational-Wave Observatory, which made the first detection of gravitational waves (SN: 3/5/16, p. 6), might be able to observe such mergers if the conditions were just right. Although LIGO can't observe mergers of black holes as massive as Gargantua, smaller spinning black holes would produce a similar effect. Spinning black holes are "really interesting from a fundamental physics point of view," said Samuel Gralla of the University of Arizona in Tucson, a coauthor on the new paper.

Black holes can spin faster and faster as they suck in matter, but there may be a limit to how fast they can go. At a black hole's center is a singularity, or region of infinite density, which is hidden by an event horizon--the surface inside of which nothing can escape the black hole's pull. But if the black hole twirls too fast, the singularity becomes exposed. Such a "naked singularity" is thought to be impossible to reach, because the known laws of physics would break down.

According to the scientists' calculations, black hole mergers sing when the larger black hole is rotating just below the limit, at 99.99 percent of its maximum speed. This makes singing black holes an enticing prospect for understanding physics at its extremes.

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Title Annotation:ATOM & COSMOS
Author:Conover, Emily
Publication:Science News
Date:May 14, 2016
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