Sounding off on gravitational waves

At this week’s American Physical Society meeting in Salt Lake City, Utah, physicists were still buzzing over the chirp heard ‘round the world. That famous chirp , of course, represents the first direct detection of gravitational waves, announced in February by scientists with the Laser Interferometer Gravitational-Wave Observatory. Elated scientists have chimed in with their own #chirpforLIGO . Even a New Yorker cartoonist got in on the chirp-fest.

Beyond the chirp, LIGO scientists have gone all-in with comparisons between waves of sound and gravity. “Up till now, we’ve been deaf to gravitational waves,” said LIGO executive director David Reitze at the February press conference. “But today, we are able to hear them.” The media has followed suit. The New York Times’ coverage led with the statement that scientists had “heard and recorded the sound of two black holes colliding.”

As a science journalist, I’m pretty much programmed to love analogies, especially ones that simplify concepts as complex as the curvature of spacetime. Yet gravitational waves are not sound waves, and the general public easily could have been led to that conclusion. Sound waves travel only through a medium such as air; ripples in spacetime don’t need any medium to support them. Sound waves propagate at the speed of sound; gravitational waves move at the speed of light. Even someone with superhuman hearing could never listen in on a black hole collision.

So why the connection between sound and gravitational waves? One reason is simple: LIGO detects gravitational waves with frequencies between several hertz and several kilohertz, the sweet spot for human hearing. When two stellar-mass black holes collide, they happen to jiggle spacetime at the same frequency as that of pressure waves in the air that our ears pick up as sound.

But there’s more to it than that, says Scott Hughes, who studies astrophysical general relativity at MIT. Sound tells a story . Aided by our knowledge of language, we can translate a series of pulsations on the eardrum into words or music. Our brains extract subtleties from sound waves, such as pitch, tone, and volume, to determine who or what is generating the sound. Similarly, LIGO researchers used general relativity to translate the wiggles of the split-second burst of gravitational radiation into information about its source. The spins, orbital speeds, and masses of the responsible black holes all had an influence on the waves’ frequency and amplitude.

Sound also has significance in the history of gravitational-wave detection. In the 1970s, physicist Robert Forward built a rudimentary laser interferometer, a forerunner to LIGO. He reported that the instrument’s output came in the form of “a wideband analog signal in the audio region” that was “recorded directly onto magnetic tape through one channel of a high quality stereo tape recorder” and “analyzed by ear.” Had the interferometer been sensitive enough to detect gravitational waves, Forward could have made the historic discovery by hearing it. Even today engineers at LIGO convert their signal into sound, not for detection but as a means of troubleshooting.

Considering all those parallels, I’m not going to slam science communicators who hammered the sound metaphor in describing gravitational waves. I’m OK with celebrating (and laughing at ) the historic chirp. Nonetheless, scientists and journalists have a responsibility to emphasize that analogies only go so far. People should learn that spacetime is pulsating to the rhythm of black holes, neutron stars, and supernovas—but they should also understand that the beat is inaudible.

Extra Dimensions is a new blog written by Andrew Grant, the online editor at Physics Today. The blog provides a forum for exploring the physics behind events in the news and offering fresh perspectives on recent developments in physics and the related sciences.