What does it sound like when two black holes collide? Stefan Helmreich tunes in to how scientists listen to such cosmic cataclysms
The sounds from space have arrived again. The Laser Interferometer Gravitational-Wave Observatory (LIGO) has, for a third time, delivered audible evidence of a cosmic calamity, an “upward sweeping chirp” that is now the signature sound of distant black holes spiralling into and colliding with one another. On 1 June 2017, the BBC affirmed that scientists can now “‘listen’ to those events as they vibrate the very fabric of the cosmos” and MIT news reported that the latest event “made itself heard … through a cosmic microphone on Earth”. Gravitational wave reverberations have become some of the most arresting sounds in today’s astronomy – swooping noises that might remind the well-tempered listener of a Buchla box bleep, an Autechre bloop, or a Holly Herndon arpeggio. But what are these cosmic sounds – and how have they been brought into audibility?
The first gravitational wave signal was made public last year, and the story of its origin goes like this: in a far corner of the Southern Celestial Hemisphere, beyond the dwarf galaxies that make up the Magellanic Clouds, two black holes circled into one another. Colliding, they generated an undulating train of gravitational waves, oscillations in spacetime that arrived at our planet on 14 September 2015. When scientists at LIGO captured these vibrations, one of the first things they did was translate them into sound – a sound they made public in February 2016, in time for the centennial of Albert Einstein’s theorisation of gravitational waves. They called the sound a chirp, a sine wave speedily swooping up in frequency, indicating the accelerating coalescence of the two black holes.
But what kind of sound was this chirp, exactly? How did such a cataclysmic and gargantuan event as the inspiralling of black holes – which one might imagine as more properly represented by the Sunn O))) sublime – cash out as something halfway between the sound of a Theremin and the voiceprint of a small bird? What should aficionados of space sounds – listeners to Pole Reports From Space, Carole Devine’s Poetics Of (Outer) Space, or, for that matter, Hieroglyphic Being’s “Fuck The Ghetto/Think about Outer Space” – make of this latest addition to the store of sounds of space?
One place to start would be with the observation that the cosmic chirp was in the first instance a scientific repackaging of oscillating phenomena, and not a recording of sound as such. Although gravitational waves do have frequencies that map onto the human auditory range (though at vanishingly low amplitudes, 1/1000th the diameter of a proton), these waves are not acoustic pressure waves, but are rather wiggles in spacetime.
The announcement of the chirp was accompanied by an evocative image, a left to right graph of the wave as it was detected at LIGO’s two sites, in Hanford, Washington, and Livingston, Louisiana. With its black background and spectrographically styled swoops of colour (suggesting nothing so much as nebulae), the image underscores the detection as a scientific and spacy accomplishment
Still and all, the sonic analogy has nonetheless been quite generative for astronomers in their apprehension of the signal. When Caltech’s LIGO Laboratory Executive Director David Reitze announced the first detection on 11 February 2016 at the National Press Club in Washington DC, he offered an explication at once clarifying and poetic:
“What LIGO does is it actually takes these vibrations in spacetime, these ripples in spacetime, and it records them on a photo-detector, and you can actually hear them. So, what LIGO has done, it’s the first time the universe has spoken to us through gravitational waves. And this is remarkable. Up till now we’ve been deaf to gravitational waves, but today we are able to hear them.”
Hearing gravitational waves is the result of a chain of machinic mediations, with LIGO a kind of cosmic cochlear implant, a frequency analyser that foretells and filters meaningful vibration into a cyborgian instrument of reception. The LIGO detector is outfitted with template waveforms, mathematically tuned matches to possible waves that might be generated by cosmic crashes. These templates anticipate significant vibrations, oscillations that might make sense with respect to Einstein’s theories of general relativity. To extend Kodwo Eshun’s concept of sonic fiction, from More Brilliant Than The Sun, these templates operate as sonic science fictions, mathematically precise hypotheses about what it might be possible to ‘hear’ in the cosmos. The chirp of the black hole gravitational wave was sieved out from the background hum of the universe.
How was this background itself audited? The LIGO detector, a massive device distributed across two physical sites — Washington and Louisiana — is constantly vibrating, owing to quantum, seismic and thermal vibration. To detect a signal in this flux, scientists draw up what they call a noise budget, a catalogue of all of the noises that need to be listened through:
A ‘noise budget’ for LIGO. Figure 2 from SJ Waldman, “The Advanced LIGO Gravitational Wave Detector”, submitted 14 March 2011, report number LIGO P0900115-v2, arXic:1103.2728
Once these noises are stabilised, scientists operate with a background against which to detect a signal. Scientists are not listening against silence, but rather against a hum that tells them that the detector is on. As Ragnhild Brøvig-Hanssen and Anne Danielsen suggest in Digital Signatures, each recording medium — wax, tape, vinyl, digital — has its own distinctive silence which is often, also, a kind of noise. It is no different with LIGO. But the rumble and fizz that accompanies any detection also saturates that detection with a contagious reality effect. The chirp – a clean, abstract, mathematically streamlined sound – is juxtaposed, emerges from, and therefore acquires the reality of the companion roar and hiss that LIGO is always capturing and creating. Think of listening to a 1950s sci-fi cybernetic soundtrack mixed in with a 21st century soundscape recording of gently droning office noise.
Why was the signal called a chirp? The term originates in radar research, describing a pulse-compressed signal that shows a sweeping increase (or decrease) in frequency, a sweep radar engineers in the 1950s likened to the chirp of a bird, bat, or insect. The term was coined in 1951 in a Bell Lab memo entitled “Not With A Bang, But A Chirp” a reference to the final line of TS Eliot’s 1925 poem The Hollow Men: “Not with a bang, but a whimper.” By the time LIGO scientists use the term, it has a formalised meaning and measure.
But is also operates as a relay between human and non-human sound. On the day the first chirp was announced, YouTube hosted videos of people making the chirp sound – karaoke-ing this sound as a human-inhuman articulation. At the same time, the inarticulate, animal character of the chirp offered by scientists embodied a sense that they were imitating a phenomenon beyond the human.
Katie Mack, in Gravitational Wave Chirp Practice #chirpforLIGO, from Chirp for LIGO Supercut Trailer, 11 February 2016
The chirping sound of the cosmos today, then, is an effect of audio technologies of noise measure and reduction as well as an articulation of human purposes with cosmic phenomena. It is a sound object for our time. It is the sonic analogue of cleanly visual impressions of warped space (see below) – though rescued from, filtered through, the thrumming and humming of mediating machines.
Cover art for David Bowie’s Blackstar, designed by Jonathan Barnbrook, 2015
Around 1980 the astrophysicist Rudolf Kippenhahn, director of the Max Planck Research Institute for Astrophysics in Munich, recollected a talk he gave in 1960. In Making Noise, Hillel Schwartz quotes Kippenhahn as remembering that he “asked the audience to imagine an instrument capable of transforming all the incoming radiation from space into audible sound. We would hear the constant rushing of the starlight and the radio eruptions of the sun as well as the rushing of the radio waves…” [Now, 20 years later] “We could hear the heterodyne ticking of the pulsars – the low humming of the Cancer pulsar for instance, emitting pulses of very high energy from a spherical star cluster… There is not only rushing to be heard in space, there is ticking and drumming, humming, and cracking.” (Quoted in Schwartz 2001: 827).
And chirping. We have heard the universe, and it turns out to enfold the sounds of 20th and 21st century human technologies of listening, recording and electronic soundmaking. The sound of black holes colliding is the sound of science.