Strange but True: Black Holes Sing
Although sound cannot be heard in space, it can sometimes be seen
In the dark heart of the Perseus galaxy cluster, 300 million light-years from Earth, a supermassive black hole has been singing the same note for 2.5 billion years. Its tone registers 57 octaves below middle C and, according to scientists at NASA’s Chandra X-Ray Center, is a resounding B-flat. Yet, how is this possible in the vacuum of space? Sound requires a medium, such as water or air, to travel. Here on Earth a sound wave moves from its origin by causing the surrounding air molecules to vibrate. The vibrations pass from one molecule to another; when they hit an ear, they are understood as noise. But because neither air nor water nor much of anything else exists in the majority of vast reaches of space, it is difficult for sound to travel there. It takes a supermassive black hole—like a robust opera diva—to sing a resonant note in space. These monstrous celestial objects range from hundreds of thousands to tens of billions times our sun’s mass and are commonly found in the center of active galaxies. For example, Sagittarius A*—a supermassive black hole—sits at the center of our own galaxy, the Milky Way. Black holes are notorious for their gravitational might, which is so strong that nothing can escape, according to conventional wisdom. But this isn’t quite correct—some matter does. A black hole’s gravity pulls a mishmash of matter and energy into its surrounding accretion disk—a ringlike structure formed by gas and dust. But some of this matter is violently expelled from the black hole’s poles as “relativistic jets.” These jets surge into the scorching gas surrounding the hole and generate pockets in the otherwise uniform cloud. “Sound waves are pressure waves. And black holes, or at least their relativistic jets, can generate enormous sound waves, which then propagate through surrounding galactic gas,” explains astronomer Steven Allen, a professor of physics at Stanford University who studies the Perseus galaxy cluster. “When relativistic jets, which contain material moving at close to the speed of light, slam into the hot gas that pervades giant elliptical galaxies and clusters of galaxies, they beat a ‘galactic drum,’ as it were.” The jet acts as the “stick,” whereas the surface of the gas is the “drum.” Although people can’t hear these waves (because sound can’t travel through the vast vacuum separating this “drum” and us), we can “see” them using x-ray observations. As sound waves spread through the scorching gas in galaxies and galaxy clusters, regions of greater pressure (sound wave peaks) tend to appear brighter in x-rays; fainter regions (troughs) are dimmer. Chandra x-ray telescope observations of the Perseus Cluster show roughly concentric ripples of brighter and fainter gas, which indicate sound waves. “We can’t see the waves moving,” Allen says. “The relevant timescales are too long, since the period of the waves is about 10 million years—but we have a clear ‘snapshot’ of them.” Perseus’ black hole is not the universe’s sole galactic vocalist. M87, a galaxy that holds one of the universe’s most massive black holes, is also known to croon. Although its song isn’t as steady as Perseus’, it is more involved, with notes as deep as 59 octaves below middle C. “There’s no reason for black holes to sing the same note,” says Peter Edmonds, an astrophysicist at the Chandra X-Ray Center. Galaxies that have more matter may provide a deeper sound, because this matter could lead to bigger, but less common eruptions from the black hole. There are bound to be other important factors contributing to a black hole’s specific sound, such as the temperature of the gas and its location, but the details aren’t well understood, says Edmonds. Other interstellar objects and events produce sound waves as well, he adds. In fact, the echoes of the big bang have been humming and hissing since shortly after the universe’s birth. According to astronomer Mark Whittle of the University of Virginia, the big bang’s sound waves were created during the universe’s first 380,000 years when space was still foggy with gas containing free electrons. Once the fog cleared, however, the universe fell silent. The big bang’s ballad is still detectable though, and is described by Whittle as “a descending scream, changing into a deepening roar, with subsequent growing hiss.” He adds: “Perhaps most remarkably, within the big bang’s sound there is a fundamental tone and a set of harmonics.” Of course, the big bang itself was mute, because it takes time for pressure to act across distances and generate a sound wave. Only later, as the pressure forces crossed regions of outer space and set up sound waves did the latter establish their presence. Closer to home, the sun has been chanting for billions of years. Convection currents on the solar surface produce pressure waves that travel to the inner corona and back to the surface, causing the surface to broil and vibrate. These deep, three-dimensional sound waves allow scientists to better understand the sun’s internal structure. In fact, the music of the spheres, and even of supermassive black holes, provides insights into the fundamental nature of our universe. Though no living thing on Earth can hear the music of outer space, the cosmos continues its orchestral display. For understanding, scientists watch (and listen) closely—making astronomers the best audience on Earth.