The sound of gravitational waves: LIGO's chirp

When heavy bodies move quickly they create waves that spread outwards through space-time. These waves stretch and squeeze everything in their path by a tiny amount. In GW150914 the change is at most 1.0 × 10⁻²¹: a kilometre-long distance changes by a billionth of a billionth of a metre.

The wave does not make the air vibrate; no sound reaches the ear. The LIGO detectors measure the stretching with laser light.

GW150914 was produced by two black holes of about 36 and 29 solar masses. Circling each other, they lost energy and drew closer; the closer they got, the faster they circled and the higher the wave's frequency rose. At the last moment they merged and the signal died away in a short ring-down.

According to the discovery paper by Abbott and colleagues the climb runs from 35 Hz to 250 Hz and lasts about 0.2 seconds. Because it lies within the frequency range the ear hears, the measurement can be played directly as an audio file.

The open-data centre GWOSC publishes audio files for GW150914. In these files the detector data are whitened (the uneven balance of the noise across frequency is corrected) and band-passed to 20-300 Hz. In some files the signal is shifted up by 200 or 400 Hz, so that the low frequencies are not lost on small speakers.

In the raw detector data the signal sits below the noise; every step taken to make it audible is stated with the file.

A Tremor in Space-Time uses the last cycles measured from the Hanford detector's open data and the theoretical shape that matches them: the frequency an octave up, and time stretched 200, 100, 50 and 25 times, heard four times. The 0.2-second event can thus be followed from a forty-second rise down to a one-second chirp.