How to make gravitational waves sing

SALT LAKE CITY — 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 singing to chirping.

According to the calculations of Caltech physicist Kip Thorne, who served as scientific 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. These characteristics would explain the extreme time dilation on the world where the film’s intrepid planet hunters landed: In one hour there, seven elapsed on Earth, a phenomenon predicted by Einstein’s general theory of relativity.

If 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, physicist Niels Warburton of MIT reported April 18 at a meeting of the American Physical Society. “There is a completely different gravitational wave signature,” said Warburton, who coauthored a related paper posted online at arXiv.org on March 3.

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, they found the gravitational waves would instead maintain a constant pitch, but slowly fade away.

“It was certainly very unexpected to see something that didn’t chirp,” says physicist Jolyon Bloomfield of MIT, who was not involved with the research. “This is really quite interesting work. It shows that the chirp actually goes away — something else is happening here.”

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. “These are definitely detectable with eLISA,”  Warburton said.

The Advanced Laser Interferometer Gravitational-Wave Observatory, which made the first detection of gravitational waves in 2015 (SN: 3/5/16, p. 6), might be able to observe such mergers if the conditions were just right. Although LIGO can’t observe the mergers of black holes as massive as Gargantua, smaller spinning black holes would produce a similar effect.

Finding black holes like Gargantua would have an impact beyond Hollywood. Spinning black holes are “really interesting from a fundamental physics point of view,” says Samuel Gralla of the University of Arizona in Tucson, a coauthor on the new paper.

Black holes can spin up faster and faster as they suck in matter, but scientists think there’s a limit to how fast they can go. At the center of a black hole is a singularity, or region of infinite density, which is hidden by an event horizon — the surface beyond which nothing can escape the black hole’s greedy pull. But if the black hole twirls too fast, the singularity becomes exposed. Such a “naked singularity,” as it is known, 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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