The Music of Black Holes
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A CD of black hole music most likely can't compete with Britney Spears or the Soggy Bottom Boys, but a new study shows these venerable gravity instruments produce complex tunes whose underlying principles are remarkably similar to pop, bluegrass, classical or any other style you might think of.
The study's results, which will be announced today, have important scientific significance, too.
The music of a black hole is generated in the region just outside the black hole proper, where incoming matter is accelerated to near-the-speed-of-light just before being swallowed.
The notes and pauses are roughly the same in faraway supermassive black holes, which can weigh more than a billion stars, as they are in comparatively puny stellar black holes, which are typically just a few times as massive as the Sun and are found here in our own galaxy.
A stellar black hole typically feeds off gas and dust provided by a companion star, while a supermassive black hole can consume entire stars in a single gulp.
So the similarity in their music implies that incoming matter's final moments are governed by similar principles of physics, supporting a long-held suspicion.
Once inside a black hole, all matter and even light are trapped.
The inaudible music of black holes cannot be directly compared to banjos, trumpets or any other specific instrument. Yet X-ray output from the objects is the sheet music of science for Phil Uttley and Ian McHardy of the University of Southampton. And this electromagnetic output describes the process that fuels the music.
Uttley explained for SPACE.com what he thinks their new study shows:
"Once the fuels starts to get close to a black hole, so that it is dominated by the black hole's enormous gravity, the turbulent accretion processes become the same, regardless of what supplied the fuel in the first place," Uttley said. "The presence of the black hole is the great leveler: Regardless of where the fuel came from, and what form it was in, it all ends up the same way, as a hot, turbulent plasma, spiraling in towards the black hole."
When black holes are actively consuming matter, their most notable products are X-rays, and lots of them. It is these X-rays, and the variations in their output, that make the music.
Uttley and McHardy have studied these emissions for the past six years, using NASA's Rossi X-ray Timing Explorer. They plan to present their findings today at the National Astronomy Meeting in the UK.
Uttley explains that the X-ray emissions can be compared to single notes and whole key changes. But there are differences.
"If you were to transcribe the X-ray output of these black holes as a series of musical notes, so greater X-ray output means a higher pitch, it would not sound quite like any [particular] sort of music, because the variations in X-ray output are essentially random, so no long sequence of notes will ever repeat," Uttley said. "But the 'tune' will still have a musical quality about it."
The general pattern of note changes -- the relative size of the changes in pitch from one note to the next, or from one bar to the next -- are the same as one hears in all kinds of music, he said.
Black holes and the Greek composer Iannis Xenakis
Uttley and McHardy are not the first scientists to find music, or so-called "flicker noise," in Nature's inaudible rhythms.
Similar patterns have been observed in everything from heartbeats to climate change. Other astronomers have detected flicker noise in X-ray outputs and in interplanetary magnetic fields.
Some scientists say music is ubiquitous in Nature (Earth itself hums a tune) and shows up in the arrangements of the planets, in seascapes, and even in our brainwaves. A few researchers have gone so far as to suggest, without any observational studies done of in-shower singing, that humans are born musical.
And in the 1970s, researchers found flicker noise in jazz, classical and other forms of manufactured music.
Uttley said the music of a black hole could be called improv. Pressed for some comparison to a specific artist or style, he said the late Greek composer Iannis Xenakis used flicker noise to randomly generate pieces called stochastic music. "You could use the variations in the X-ray output of black holes to produce just this sort of music," Uttley said.
Long and short of it
The goal of the study was to compare the two types of black holes, the stellar gravity wells gorging on their binary companions and the supermassive variety fed by the material of entire galaxies.
Uttley and McHardy found that notes from stellar black holes vary rapidly, on time-scales of milliseconds to seconds. Supermassive black holes play more ponderously, though, with variations taking about a million times longer, on average.
"In other words, take the tune played out in X-rays by a black hole X-ray binary and slow down the tape by a factor of a million or so and you get the kind of variations we are seeing in active galaxies" anchored by supermassive black holes, Uttley said.
The study further revealed that at any given moment, various black holes are playing different styles of music; instead of some organized celestial symphony; there is a cacophony of differing styles playing out all over the galaxy.
And every few weeks, a stellar black hole switches musical styles, undergoing a distinct transition from one pattern of variability to another.
"Astronomers think that these 'state transitions,' which last several weeks, may represent changes in the mode of accretion, caused by changes in the rate of fuel supplied to the black hole," Uttley said. "Taking the musical analogy a step further, you could liken the new state to a different musical style."
Further observations will be required to learn if supermassive black holes are capable of playing a similar range of styles.
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