American Scientist

To the Editors:

I'm quite taken with Craig Hogan's use of sound as a metaphor for gravitational waves ("The Sounds of Spacetime," November-December). In fact, maybe it is more than mere metaphor. Human hearing is incredibly sensitive: we can perceive vibrations with atomic-scale amplitudes. So if we were close enough to, say, a black hole merger, we could literally hear it. A quick back-of-envelope calculation suggests that an astronaut within 10 astronomical units of a binary stellar-mass black hole could directly hear the gravitational waves—the waves would stretch the bones of the inner ear by a perceptible amount.

George Musser
Glen Ridge, NJ

Dr. Hogan responds:

Like the LISA detector, human hearing can detect subatomic motions over a factor of almost 1,000 in frequency. The final merger of two black holes of, say, 10 times the mass of the sun, emits waves in the easily audible kilohertz range, so a brave astronaut would hear their brief chirp with no extra apparatus. Human hearing also uses extraordinary tricks of signal processing that might help pick out recognizable sound sources from the cacophonous LISA data stream. I for one can hardly wait to listen with my own ears to the sound of real LISA data—sped up by a factor of 200,000 or so to be audible.