Is the black hole sound real? Perseus and 57 octaves

The space between the galaxies of a cluster is not empty: it is filled with a very thin gas at millions of degrees. Thin as it is, it is a medium, and sound, a pressure wave, can travel through a medium. The black hole at the centre of the Perseus cluster blasts out matter from time to time; the outbursts push the gas and leave rings spreading outwards.

In a long observation of Perseus with the Chandra X-ray telescope these rings stand out as differences in brightness in the image. In 2003 Fabian and colleagues interpreted them as sound waves that fade as they travel: the rings are about 11 kiloparsecs apart, so one wave takes about 9.6 million years to pass.

A wave with a period of 9.6 million years has a frequency of one divided by that time: about 3.3 × 10⁻¹⁵ Hz. The human ear roughly stops hearing sound below 20 Hz; the gap is beyond imagination.

One octave doubles the frequency. 57 octaves means 2⁵⁷ times, about 1.44 × 10¹⁷ times: that far up, 3.3 × 10⁻¹⁵ Hz becomes about 476 Hz. The same factor compresses time: the 9.6-million-year period shrinks to about a 476th of a second. You can try the calculation yourself in the Octave Converter.

In NASA's 2022 sonification of Perseus the waves were extracted from the Chandra image outwards from the centre and moved 57 and 58 octaves above their true pitch. Along with the real waves the sound also contains other large-scale density changes, such as cavities.

So what you hear is not a microphone recording but a measured wave moved into the audible range. The wave is real; the mapping that brings it to the ear is a human choice.

The LIGO signal known as 'the sound of two black holes colliding' is something else. A gravitational wave is not pressure travelling through a medium but a tiny stretching of space-time itself. In GW150914 the frequency of this stretching climbs from 35 Hz to 250 Hz in 0.2 seconds: it is already in the hearing range, but because no air vibrates the ear cannot hear it.

In Perseus the wave is a real sound wave but extremely slow; in GW150914 the frequency is audible but it is not sound. In both cases the 'sound' reaches the ear through a mapping.

In A Black Hole's Whisper the Perseus wave is heard on an octave staircase: first a pulse every 17 seconds at 2⁴⁴ times its speed, twice as often at each step; then a tone of about 476 Hz 57 octaves up and a second one 58 octaves up. You hear the moment the same wave turns from rhythm into pitch.