July 7, 2023, by Brigitte Nerlich
Gravitational waves, music and metaphors
On Thursday, June 29, 2023, the North American Nanohertz Observatory for Gravitational Waves or NANOGrav announced evidence for gravitational waves emitted by pairs of supermassive black holes across the universe or, in the words of the astrophysicist Katie Mack: “We’re using RADIATION JETS from DEAD STARS to detect RIPPLES IN SPACE from the COLLISIONS OF SUPERMASSIVE BLACK HOLES ACROSS THE ENTIRE COSMOS. Honestly that’s just frickin’ awesome.” (And here is a great article she just published in The Washington Post)
I first heard about this news from my son in an email with the subject line “Measuring gravitational waves using the galaxy as an antenna” and telling me “Very cool stuff, especially if it gives us another way to image the very early universe”. He linked to an article in Ars Technica with a url that showed the words ‘picks up signal of cosmic choir’. I saw ‘cosmic choir’ and my ears pricked up. Metaphor! The actual title of the article is different but the deed was done. I had to follow the trail.
I should say that I am writing this post in a rather haphazard way while listening to the sounds of the waves on the seashore….
The first mainstream article I spotted was an article by Hannah Devlin for The Guardian, which seems to have been one of the first out of the starting blocks in terms of mainstream newspaper coverage. It was entitled, at least in its first incarnation: “Music of the spheres: Astronomers detect ‘cosmic bass note’ of gravitational waves: Sound comes from the merging of supermassive black holes across the universe, according to scientists”. And the first sentence says: “Astronomers have detected a rumbling ‘cosmic bass note’ of gravitational waves”. The article was based mainly on a set of papers published that Thursday in the Astrophysical Journal Letters rather than on the NANOGrav announcement.
What interested me were the musical metaphors in this article: music of the spheres, cosmic bass note, sound coming from, rumbling…. And the fact that the scientists could ‘tune into’ “a far deeper frequency range” then previously, when they could only hear “short ‘chirps’ of gravitational waves”.
After that article, I hunted rather randomly for other musical allusions in the press coverage about this celestial event. There was a lot of talk about a ‘background hum of gravitational waves’ (Smithonian Magazine and many more). Sometimes this hum became a ‘clamour’ (Phys.org) or a ‘thrumming’ (NYT), and, of course a ‘rumbling’ as in the Guardian article..
Scientists and journalists also talked about a ‘choir’, in particular a ‘celestial choir’ (CNN, ABC7News, RepublicWorld) or a chorus (The Independent) sometimes a perpetual chorus (Telegraph India, Phys.org and many more) – in short a chorus of low-frequency gravitational waves (SCIENMAG). But why choir or chorus – why choose that metaphor? One hint can be found here: “‘It’s like a choir, with all these supermassive black hole pairs chiming in at different frequencies,’ says NANOGrav scientist Chiara Mingarelli”.
Basically, in previous research scientists were listening into high frequency waves resulting from single events, such as two neutron stars merging, and that only lasted a relatively short time, whereas now they are listening into low frequency waves emitted by loads of pairs of merging supermassive black holes across the universe over enormous time scales – thus a choir or chorus – as far as I understand things!!
Sometimes things became a bit more technical and we learned that “Exotic stars called millisecond pulsars serve as celestial metronomes.” (Ars Technica) Or: “As pulsars spin rapidly, they emit these radio waves rhythmically, akin to a perfectly timed metronome” (Baguio City Guide; read on to learn more)
Some news outlets went beyond hums and thrums and talked about a ‘cosmic symphony’ (RepublicWorld, The Conversation). “Scientists describe it as being akin to hearing a ‘symphony’ of waves echoing through the universe.” (The Independent)
One should stress that “these waves are not, as are electromagnetic and acoustic waves, vibrations that travel through space-time. Rather, they are warpings in space-time.” Or, more poetically “We can measure the ripples of spacetime as it undulates from black holes dancing around each other”.
Drop the bass
And finally, sometimes science communicators use a musical metaphor that baffles old people like me, demonstrating yet again that metaphors are great for making the unfamiliar (at least seem) familiar, but you have to be familiar with aspects of the ‘source domain’, in this case music, in the first place (and of course also with the ‘target domain’, namely gravitational waves, haha!). Unfortunately, I am pretty illiterate in both music and physics.
So when I came across an explainer of the NANOGrav achievement on astrobites (a journal written by graduate students in astronomy) entitled “Drop the Bass: Evidence for a Gravitational Wave Background from a Galaxy-sized Detector”, I was quite stumped.
I found out that “Drop the Bass” is a “catchphrase associated with the drop, or the climactic point in an electronic music track characterized by a sudden switch of rhythm in bass line after a progressive build-up.” So dropping the bass seems to describe a change from detecting high frequency waves (chirps) to detecting low frequency waves (hum, bass) etc.
As the astrobites authors say: “This achievement opens a brand-new observational band for astrophysicists to observe. For the first time, we hear the bass notes of the cosmos’ gravitational wave symphony.”
Music and cosmology
All these musical metaphors are not entirely new. They have been used for many years in gravitational wave communication, at least since LIGO’s first discovery of such waves in 2016. Even in 2011 the Learned Society of Wales put on a talk entitled “Gravitational Waves: Listening to the True Music of the Spheres!” And, of course the roots of such musical metaphors for the cosmos go much further. They have a long history in philosophy, metaphysics and cosmology.
As Tamara Davies explained at the time of first whispers of gravitational waves: “The Harmony of the Spheres is an idea that goes back to ancient times, at least as far back as Pythagoras, when it was noticed that the pitch of a musical note relates to the length of the string that produces it. The combinations of notes that sound harmonious to our ears can be derived mathematically, as their frequencies make simple ratios. It turns out that the orbits of the planets also happen in simple ratios. ” And from there the idea of the music/harmony of the spheres evolved.
Andrew Hicks, an Assistant Professor of Music and Medieval Studies, published a book in 2017 entitled Composing the World: Harmony in the Medieval Platonic Cosmos in which he explained (as summarised by Linda Glaser) that for “medieval philosophers, cosmological aesthetics based on the ‘music of the spheres’ (the idea that stars and planets make a beautiful harmony) governed the moral, physical and psychic equilibrium of the human and assured the coherence of the universe as a whole”.
And, finally, Isaac Newton reputedly described celestial music as the sound of gravity. And gravity plays an important role in…. gravitational waves!
I bet that Newton would love this new research – and of course Einstein too, who dreamed up gravitational waves in the first place and, apparently, said: “I live my daydreams in music. I see my life in terms of music.”
PS Mark Pössel, who works in astronomy outreach, added a comment on Twitter which is worth repeating here, as it contains a reference to an interesting book and clarifies something about sound and gravitational waves: “(a) from what I can see, a milestone on that metaphorical journey was Marcia Bartusiak‘s book https://en.wikipedia.org/wiki/Einstein%27s_Unfinished_Symphony… (b) the musical metaphor is particularly apt because from a gravitational wave source you get one coherent sound, whereas with the usual electromagnetic observations, you get lots of separate bits of information. Just like in looking at a violin, you see all the details of the instrument separately, while in listening to it, you hear a coherent sound.:
Further reading: This post links up with some older posts
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