Last September researchers using the Chandra X-ray Observatory detected sound waves blaring from a distant black hole. That news prompted a lot of double takes. After all, astronomers always say sound can’t carry in a vacuum, and nothing, including sound waves, should be able to escape from a black hole. Even the makers of the movie Alien knew that “in space, no one can hear you scream.”
To understand how a black hole can hum, let’s review a little basic physics. Unlike light, sound has no independent existence. It is merely a disturbance in a medium. Jostle a bunch of atoms so that they in turn jostle others, and you have created an acoustic wave—just a fancy name for sound. The number of times per second that the atoms shift back and forth defines the wave’s frequency, experienced by the human eardrum and brain as pitch. The power of the jostling determines the wave’s amplitude, which we hear as volume.
Sound can exist anywhere there are atoms to be jostled, but more power is needed to get an acoustic wave going through a thin medium. Solids transmit sound efficiently: The grinding of Europa’s thick ice sheets would make the surface of that Jovian moon far from library-quiet to an astronaut there. Gases and plasmas can keep sound moving as well, just less effectively. Saturn’s fierce 600-mile-per-hour easterlies produce deafening howls. Violent solar flares trigger vast acoustic ripples that spread out for tens of thousands of miles across the surface of the sun. Hurricane-force winds resonate across the rusty deserts of Mars, although they generate only about as much noise as a modest breeze blowing through Earth’s far denser atmosphere.
The concentration of atoms in an intergalactic nebula is fantastically low even compared with the thin air of Mars, but sound can exist there too, provided something can spark a big enough acoustic wave. The Chandra results show that a supermassive black hole in the heart of the Perseus galaxy cluster, 250 million light-years from Earth, generates enough of a sonic wallop to do the job. The sound does not come from the black hole itself but from a disk of hot gas swirling just beyond its edges. Due to mechanisms that are poorly understood, twin jets of electrically charged gas, or plasma, shoot out perpendicular to the disk. The jets create vast, long-lived bubbles whose expansion sends acoustic waves reverberating through the surrounding nebula.
The persistence of those plasma bubbles was a mystery until the recent Chandra observations. Such hot spots ought to dissipate relatively quickly due to gravitational meddling from massive nearby galaxies, which should rob them of energy and cool them. The structures could remain intact only if a force kept heating them up. Sound waves carrying a continuous supply of acoustic energy would neatly explain the bubbles’ long-term existence—exactly what the new X-ray images seem to show. “The sound is roughly a steady note because the acoustic pulses keep happening at regular intervals,” says Peter Edmonds of the Harvard-Smithsonian Center for Astrophysics, who led the Chandra study.
The regularity of the rippling acoustic pulses from the black hole is what justifies the scientists’ use of the word sound to describe the process. Create 262 disturbances per second and you’re playing a middle C. Divide that rate by a factor of two and you get another C one octave lower, and so on. Any regularly recurring acoustic pressure wave therefore has a corresponding musical tone. Chandra researchers analyzed the activity at the center of the Perseus cluster and concluded that it produces a very, very low B-flat.
Alas, we cannot experience this tone directly. The deepest bass note humans can hear has a frequency of about 20 cycles per second. Each pulse from the black hole occurs once every 10 million years. The B-flat from Perseus lies 57 octaves below middle C. Only creatures with ears the size of galaxies could savor that meditative cosmic om.