Two black holes (black dots in this simulation) spiraled around one another before merging into one. Gravitational waves (white and blue) that plowed outward from that collision helped to confirm physicists’ theories of how black holes behave.
Deborah Ferguson, Derek Davis and Rob Coyne/URI, LIGO, MAYA Collaboration, simulation performed with NSF's TACC Frontera supercomputer
Newly reported gravitational waves rang out as clear as a bell.
Spotted in January, these spacetime ripples are the clearest yet discovered, cutting through the background noise better than any previous detection. The waves were born when two black holes merged, forming a larger black hole that reverberated like a struck bell, emitting gravitational waves as its vibrations gradually faded.
The pealing of this black hole matched predictions of black hole behavior according to physicists’ theory of gravity, general relativity, the team reports September 10 in Physical Review Letters.
“Just by hearing a bell, you understand it’s a big bell rather than a small bell,” says Caltech physicist Katerina Chatziioannou, a coauthor of the study. “The frequencies that an object makes when you strike it are unique to it, and the same thing is true with black holes.”
Like actual bells, black holes ring with a fundamental pitch and other frequencies called overtones that fade away more quickly. For the newly reported merger, both the fundamental and the first overtone were detected by the Laser Interferometer Gravitational-Wave Observatory in Hanford, Wash., and Livingston, La.
The event was comparable to previous black hole mergers seen. The two initial black holes each had masses about 30 times that of the sun, similar to the first black hole merger LIGO detected, in 2015.
Improvements to LIGO’s detectors in the 10 years since allowed this signal to come through particularly loud and clear — about 80 times as prominent as the background noise, compared with a signal-to-noise ratio of 26 for LIGO’s first detection.
The measurements matched the predictions for a Kerr black hole, based on a solution to the equations of general relativity first worked out by New Zealand mathematician Roy Kerr in 1963. Kerr’s solution describes black holes that are spinning, as realistic black holes are expected to. Similar tests have been tried before with gravitational wave data, but this is the first time such a clear overtone has been detected, which is necessary for this type of test, Chatziioannou says.
The team also checked a prediction developed by physicist Stephen Hawking in the 1970s. Called the area theorem, it states that the surface area of black hole event horizons can only grow with time. The newly formed black hole had a surface area larger than the sum of the two original ones, matching Hawking’s rule.
While scientists previously tested Hawking’s area theorem with the first black hole merger LIGO detected, the improved signal-to-noise in this new event allowed for a more definitive check.
