Strange but True: Black Holes Sing
Love To Sing
Strange but True: Black Holes Sing
Although sound cannot be heard in space, it can sometimes be seen
By Robynne Boyd
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.