If two black holes merge and no one is around to hear them, do they still make a sound?
Careful—this is a trick question. Despite their reputation as the most fearsome objects in the universe, black holes by themselves aren’t actually very noisy. Any sounds emitted inside the event horizon, the boundary beyond which light itself cannot escape a black hole’s gravitational pull, would never reach the outside universe. So two bare black holes meeting and merging in the cosmic dark would be expected to make essentially no sound at all. What little noise black holes might emit would come from their more mundane meals—the sonic protests of materials being ripped apart and frictionally heated as they funnel into a black hole’s insatiable maw. But these death rattles would be stifled by the near vacuum of space—a place, it turns out, where indeed no one can hear you scream, even if you are being devoured by a black hole.
There are, however, soundless ways to listen to merging black holes, as Janna Levin will explain in a special presentation broadcast live on this page at 7 P.M. Eastern time from the Perimeter Institute for Theoretical Physics in Ontario. Levin is the Tow Professor of physics and astronomy at Barnard College of Columbia University, and an award-winning author of, most recently, Black Hole Blues and Other Songs from Outer Space. Two colliding black holes come together with such violence that they distort the fabric of reality itself, creating ripples in spacetime—gravitational waves—that emanate outward from the merger at the speed of light.
Although predicted more than a century ago by Albert Einstein, gravitational waves were not directly observed until 2015, when the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected some originating from a black hole merger more than a billion light-years away. Converted into audible sounds, the recorded waveforms of the signal register as a distinct chirp—the “sound” of two black holes colliding, heard for the very first time. Since then LIGO has tuned in to several more chirps from other merging black holes, providing a soundtrack to what has previously been a silent-movie view of the universe.
Join Levin this evening as she presents the astounding science of gravitational waves and the remarkable century-spanning story of their discovery. Her talk is part of Perimeter’s public lecture series presented by BMO Financial. Online viewers can join the conversation by tweeting to @Perimeter using the #piLIVE hashtag.