Let there be light

Steven Prawer is Professor of Physics at the University of Melbourne.  

The first visitor of the day watched as the curator flicked the switch and watched with satisfaction as the exhibits came to life. She put her hand over the light source to play with the shadows and was struck by how bright the light was and yet how cool it was to the touch. She mused that apart from the primordial light that preceded the creation of the sun, for most of human experience light, fire and heat had been inextricably linked. And yet, the streams of light flowing in the exhibits seemed to emerge miraculously from their sources without the need for either heat or fire. She recalled her science teacher describing light as being composed of tiny particles, called photons, travelling through space almost like a stream of water being composed of millions of little droplets. But where did the photons come from and where did they go once they had bounced around the exhibits creating ultimately delight and wonder in those lucky enough to observe them? Unlike a stream of water, the room never got full of photons, at least not in the sense of taking up space that could not be used by other objects. 

In fact, deep inside the crystals in the light sources that were illuminating the exhibits, particles of a different nature were moving towards one another. Driven by the supplied power, electrons and ‘holes’ were getting closer together and when they collided, a photon was born in a burst of creativity. The collisions were a little like a golf ball falling into the cup with a satisfying clunk, but with light, rather than sound being the consequence of the recombination. 

Once free of the confines of the crystal, the photon, joined by billions upon billions of other photons, launched off at the speed of light in many different directions. For some photons, life was short and meaningless, as they hit one of the walls of the gallery and were annihilated in burst of energy that ultimately would warm the walls, if ever so slightly. Other photons survived for longer. Approaching one of the mirror surfaces it was impossible to tell which photons would be absorbed and which would be reflected. At least for the shiny mirror the probability of survival was high, perhaps as high as 99%, but even so, billions of photons met their demise at the mirror surface. For those that did survive, each photon would bounce off the surface at exactly the same angle as they were incident, just like a tennis ball bouncing off a wall. When she wandered over to Michaela Gleave’s Eclipse Machine (Blue/Red), she observed the half silvered mirrors which both let light through and at the same time also reflected some of the light. She mused about what controlled the fate of an individual photon that enabled it to pass through, or be reflected?

Surfaces didn’t need to be solid to reflect light. Wandering over to Rikki-Paul Bunder’s Polypropylene Eclipse, she observed the patterns of light reflected from the very thin plastic sheet. Where had she seen similar patterns before? Perhaps in the swimming pool at home where the light shining through the gentle ripples on the surface of the water created patterns of so called ‘caustics’ on the bottom of the pool, or even sometimes at the bottom of a wine glass. Gently blowing on the plastic sheet, she watched as the caustic danced in patterns that were impossible to follow, but were indeed simply a result of the photons bouncing off the sheet according to very simple rules. 

Then there was the challenge of the coloured surfaces and despite the non-political stance of the photon community, here the chance of survival very much depended on colour. Black surfaces were relatively indiscriminate absorbing photons of all colours. But the red surface was kind only to photons of the red persuasion, absorbing all the other photons  and leaving only red photons present in the reflected beam. White surfaces were once again indiscriminate, with the probability of reflection the same for photons of all colours.

Another visitor of the day sauntered into the gallery taking up a position in front of Sarrita King’s Lightning. Some of the lucky photons which had survived the perilous journey from the light source and reflected off the painting were incident on the outer layer of his eye. Once again many photons did not survive the collision, but for those that did, some lucky photons entered into the transparent cornea and were bent towards a spot at the back of the eye. Indeed, for the first time since their escape from the crystal, all the photons entering the eye were constrained to be focused onto a plane of tissue about 1mm thick at the back of the eye ball, the retina.If one could have looked into his eye, one would have seen a miniature version of Lightingprojected on his retina. The photon passed through the layers of the retina until they collided with a rod cell. Instantly, the photon was annihilated, and its energy completely transferred to the rod cell which responded by sending an electrical pulse that, through a complex pathway, eventually found its way to the back of the brain. All of the subtly of the picture was transformed into a set of clicks of neurons that either were excited or were not in his brain. Somehow, in a process that no one understands, this set of discrete clicks in his brain were integrated into an image that meant something. 

Even more intriguing was staring at Taree Mackenzie’s Pepper’s Ghost, Diamonds, Green and Magenta. Two light sources, green and magenta, combined to look white. How does that happen, he wondered? Clearly, colour didn’t reside in the object. The green photons looked green until combined with the magenta ones. Where did they combine? Actually never. The green photons entered his eye and were absorbed by green cones that generated clicks in the back of his brain. The magenta photons excited both the red and blue cones and also produced clicks in the back of his brain. It was his brain that interpreted the combination of clicks as white light. Colour didn’t really exist except inside his head. It wasn’t even as if he could take comfort in the thought that it was the combination of intensities of the different colours that resulted in his perception of white light. All that his brain received were a series of clicks, every click corresponding to a single photon. His brain really was digital, but unlike a DVD which reduced the most complex of moving images to a stream of 1’s and 0’s, his brain was capable of extracting meaning from the series of clicks. Or as Pittsburgh Professor M. Chirimuuta says in her book Outside Colour, “Colour is not an object of sight but a way of seeing things.”[1]

So what is light? At one level it is just a stream of photons, tiny packets of energy. They can convey information from one place to another only by existing or not existing. We bring them into existence from nothing every time we flick the switch. They deliver their information only by virtue of their annihilation in a detector, most commonly in the sensors at the back of our eyes. Between their creation and their annihilation they can be steered in a different direction, but ultimately they either are or are not. The artist ultimately is just manipulating the pattern of photons that either do or do not arrive at the retina. 

And yet, somehow, he mused,  it is more than that. As Chirimuuta writes, ‘We can appreciate how very weird it is to even expect there to be a connection between the manifest visual world, brought to us by our senses, and the rarefied scientific image of a world made up of physical particles’.[2]Understanding the code by which the clicks are turned into meaningful colours and images may forever be beyond our reach; but even if someday that code is unlocked, and we fully understand the biological model of colour and perception, the mystery of extracting pleasure from the visual image will remain. 

[1]Chirimuuta, M. (2015). Outside Colour: Perceptual Science and the Puzzle of Colour in Philosophy MIT Press, p81

[2]Ibid, p.31