What is sound? How can it be generated by a black hole?

From the Chandra Observatory

How an X-ray Telescope Detected Sound from a Black Hole
What is sound? How can it be generated by a black hole? How can an X-ray telescope detect the sound?


Sound is a type of pressure wave. It requires a medium, such as air, water, or the gas in a galaxy cluster, for this pressure wave, or waves, to travel through. Bulk motion of the material does not take place, only the pressure waves move away from the source of the sound. A good physical analogy is a ripple traveling along a slinky.

Sound is generated when an object makes a disturbance in a medium. Examples include vibrations from our vocal chords, from a tuning fork or from a membrane in a stereo speaker. In the Perseus galaxy cluster, scientists believe that sound is generated in the following way: cavities or bubbles in the cluster gas are blown out by jets from a supermassive black hole (located at the center of the cluster). These cavities eventually push against the cluster gas like a piston, generating a pair of sound waves which travel away from the now slowly traveling cavities. [View the animation]

How is the Chandra X-ray Observatory able to detect these sound waves?


The gas that pervades the Perseus cluster is very hot, with temperatures of millions of degrees, so it glows in X-rays. If the gas is directly visible, then pressure waves, if they are large enough, will also be visible, as "ripples" in the cluster gas. This is what is happening in the Perseus cluster. The scientists are able to see the sound waves, rather than hear them. By contrast, sound waves are not seen in the earth's atmosphere because our eyes are only sensitive to light at optical wavelengths, and air is much too cool to glow at these wavelengths. If air is invisible, the pressure waves (sound) passing through it will also be invisible.

Although the sound waves in the Perseus cluster are moving rapidly, distance to the cluster is so large (250 million light years) that the motion of the waves is undetectable from Earth.

What type of sound is being generated by the black hole?

If sound waves are produced at fixed, regular time intervals, then a tone, or note is created, similar to the production of musical notes with a tuning fork. The frequency (or pitch) of the note depends on the size of the tuning fork: a small tuning fork moves more rapidly, generating a closely spaced set of sound waves, corresponding to a high frequency note. A larger tuning fork moves more slowly generating more widely spaced pressure waves and a lower frequency note.

Note on Octaves

Because an octave corresponds to a factor of 2 in frequency, a frequency of one cycle per 300 million million seconds corresponds to a value 257 times lower than 475 cycles per second, the frequency corresponding to B-flat above middle C.

Because the cavities in the Perseus cluster are so large, the scale of the sound waves generated is colossal. But, a single cavity from the black hole only produces a single sound wave. The regular spacing of sound waves visible in the top right of the cluster shows that cavities have been generated by the black hole at regular intervals. Knowing the speed of sound plus the measured distance between waves allowed the scientists to estimate the interval between waves at just under 10 million years, corresponding to a frequency of about one cycle per 300 million million seconds, about 57 octaves below B-flat above middle C.

The sound waves provide a critical source of energy for keeping the cluster gas from cooling too much, preventing massive amounts of star formation. Scientists have calculated that the pitch and intensity of the sound from the black hole would have had to have been roughly constant for about 2.5 billion years to offset cooling from radiation.