Scitech | Intergalactic music

Physicists use sound ripples to detect black holes

In olden times, astronomers used to say they heard the “music of the spheres,” meaning that they could interpret the beautiful, periodic movements of the planets across the sky as musical tones and harmonic intervals.

Today, thanks to astrophysicists like Janna Levin of Barnard College at Columbia University, that phrase has a whole new meaning and is no longer restricted to lumps of rock or burning balls of gas. The final speaker in McGill’s commemorative lecture series to honour the International Year of Astronomy, Levin is in hot pursuit of some of the strangest music this universe has to offer, produced by the darkest, and perhaps most mysterious, objects humankind has ever known. She’s listening for the songs emitted by black holes.

Levin is not just “another theoretical physicist.” Standing before the audience Thursday evening in red high heels and a yellow leather jacket was a scientist who writes prize-winning popular science books, paints, and writes essays to accompany contemporary art exhibitions that are influenced by the “weirdness” of physics. Levin has even been on The Colbert Report. It’s almost fitting that she listens to the sounds of black holes as her day job.

“A black hole,” Levin explained, “is the death state of a star, a super-massive dense object that stretches space-time into a singularity, creating a gravitational field so strong that, past a certain point – the event horizon – even light cannot escape.”

Levin said that until now, black holes, due to their invisible nature, have only been observed indirectly, often just by studying some visible object orbiting around seemingly empty space. For Levin and many other physicists, this isn’t enough. It might actually be more effective to hear black holes rather than see them – especially those far from any visible neighbouring objects.

Since black holes are so dense, a pair orbiting each other should produce a noticeable sound.

“Just as a big object, such as a whale, can create ripples and waves in water by its movement, so can the orbits of two black holes create waves in the fabric of space-time,” Levin said.

For those who might be worried about being stretched or shrunk by passing black hole waves, rest assured that the magnitude of this phenomenon is tiny, perhaps about the fraction of the radius of an atom. Of course, this also means that black hole waves are almost impossible to detect, and is one of the reasons we haven’t actually heard them yet – though giant experiments such as the Laser Interferometer Gravitational Wave Observatory and the planned Laser Interferometer Space Antenna should soon change that.

Despite the lack of recordings, Levin didn’t disappoint and played a few audio clips of what these waves should sound like, according to mathematical models. “Songs” may be a bit of an overstatement, but the waves produced by a newborn black hole eating up a dying star sounded eerily like whale song. A recording of two black holes spiralling in on each other generated a distinctive techno-like beat which got faster and faster until the beats – which Levin explained could be likened to two black holes acting as “mallets” on the “drum” of the space-time continuum – became indistinguishable and the black holes finally collapsed into one with a high pitch slide and a disturbing “pop” sound.

While new scientific knowledge can be gained by listening for black holes, Levin hopes these sounds will also be inspirational, much like the Hubble Space Telescope’s deep space photographs. They could give us a “soundtrack of the universe,” and although these “songs” may not turn out to be chart-toppers, chances are good they will find their way into at least one sci-fi movie theme song.

So when will these recordings be made? According to Levin, probably not for another 10 years or so. Stay tuned.


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