Gravitational Waves and the Cosmic ‘Sonativum’

Artistic rendition of the merging black holes that gave rise to the gravitational waves reported in February 2016. Image from LIGO collaboration

Only an Ordered Universe blogpost could deserve a title like that.  We cannot let a discovery of such reach, beauty, conceptual depth and powerful simplicity (yes indeed) as the LIGO team’s announcement this month of the first detection of gravitational radiation go without a celebratory comment from the Robert Grosseteste club here.

Robert did, after all, engage in the magisterial De luce in the work of imagining the entire cosmos, and indeed in the propagation of waves across it in the process of its first formation.  Another centrepiece of his thought world was the connection of the universal with the present and microscopic. The Earth is, for him, the repository of all the radiant lumen from the distant celestial spheres, so is endowed with elements of all their characters.  And, as we have shown, his cosmic waves of compression contain an implicit mathematics of remarkable structure.

There is another connection however.  Several commentators have likened the new science to opening up our ‘ears’ onto the universe.  We have been able to see the sky for centuries, but have been deaf to its vibrations.  Tiny as they are (the LIGO detection arms, 4km long, were displaced but a thousandth part of a proton’s diameter by the passing gravitational waves), we now have the capacity to ‘hear’ the universe vibrate.  It’s not an analogy that stretches mathematical truth so vary far – Einstein’s conception of space and time is just the warp and weft of a four-dimensional fabric whose dimples and clefts form around massive objects.  Unverified until this month was the other property of such a fabric – that shaking it at one point sends ripples out to all others.  It is the violent shaking of space and time over a billion years ago by the merging black holes that the twin arms of the LIGO detectors picked up.  They vibrated in sympathy with the space they occupied.

One of the LIGO detectors from the air

The form of the vibration is a rather precise one   – technically the gravitational waves are ‘quadrupole’ in character.  So as they pass, one of the two right-angled arms of the LIGO detector is extended while the other is compressed (this is detected by a tiny difference in the passage time of light out and back along the now-unequal arms).  Then within a fraction of a second, crests move to troughs, and the first arm compresses as the second extends.

Let us hear Grosseteste explaining the motion internal to an object he calls a sonativum (this he defines carefully as an object capable of generating sound when struck violently).  The translation is from our own edition, by Sigbjorn Sonnesyn.

It is necessary that this vibration in the minute parts [of the sonativum] is followed, in a departure from the natural position, by an extension of these same parts according to the diameter of length and a constriction according to the diameter of breadth; and in the return to the natural position there is on the contrary a shortening of the longitudinal diameter and an increase of the latitudinal diameter. And this movement of the sonativum according to extension and contraction in minute parts, which follows the local movement of the vibration, is sound, or natural speed towards sound.

The remarkable passage is from Grosseteste’s De generatione sonorum (on the generation of sounds), probably one of his very earliest scientific works, written somewhere between 1200 and 1210.  In it he draws on Aristotle’s De anima (On the Soul) for learning on sound and voice production, and on Isidore of Seville’s Etymologies (c.560-636) and the Institutiones grammaticae of Priscian (fl. 500) for phonetics. Grosseteste succeeds in bringing his sources into a remarkable synthesis, adding his uniquely mathematical and physical turn of mind.

It describes exactly the geometry of vibration of the gravitational waves, and the LIGO detectors.  Not, of course, because he pre-conceived this physics 800 years ago, but because such vibration is commonplace to those that have eyes and minds to notice it.  It is, for example, the distortion of the circle of a bell when struck so that it rings.  Perhaps Robert’s case in point of a sonativum.  I wonder what he would have thought, however, of the idea that the cosmos itself can ring in the fashion of one of his fundamental motions.

(I have said more on the discovery in the context of Christian faith on my own Faith and Wisdom in Science blog)

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