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Human visual perception blesses us with only a glimpse of the light spectrum – for many animals, the ...
2021-07-07 09:00:00

Parallel Rainbows
On Colour (and Its Limits)

Drawing by Marek Raczkowski
Parallel Rainbows
Parallel Rainbows

Birds ruffle their feathers flickering with ultraviolet, lizards stick to stones glowing with infrared. The colour world of animals goes far beyond the spectrum available to humans.

Read in 9 minutes

The universe basks in photons. It is like a plump, shiny cherry dipped in sweet liqueur – it drips in radiation, shooting motes of light all around. The wildest light: cosmic objects in space, monsters moulded from superheavy matter, spreading every possible flavour of ray in all directions. I am lying underneath this cosmos, on a hard road that car wheels have forgotten about. The nearby Białowieża Forest breathes the night, the dome of warm July air presses me to the ground. I am taking the universe in with my eyes, two hungrily dilated tiny holes pierced in the irises.

It is hard not to feel a sense of wonder about it. Even despite the fact that my eyes only let tiny portions of photons into their photosensitive interiors – not enough to see the glowing nebulae tails and the deadly pale vortexes of ancient galaxies. It is funny we should call this way of looking ‘seeing with the naked eye’. That’s right, my eye is stark naked in its jelly-like plumpness, soft and defenceless under the cosmos’s umbrella, and around it my entire sensual map revolves. I am revelling in this seeing, I drink up the photon cocktail – despite it being a pale echo of what the world really shines with.

You see – even though it can do a lot, the human eye only cuts a very fine slice of the world’s radiation. The narrow patch – a cacophony of light – accommodates the entire rainbow of our reality. The visible spectrum is here, in the space of a mere 300 nanometres of electromagnetic pulse; the light waves that we can see beating at the shore. We can see. And right away we think we know this world, that thanks to our sight we know how the world works. That here are the greens, unavoidably engaged in eating light, producing oxygen and nourishing plants; that there are the blues, of the skies and flowers, both dragged to the surface by naughty light, sprayed on microscopic pieces of forget-me-not or in the air, in such a way that all we have left to do is lick the blue taste from the rays of light. Beyond the limits of visibility lurks the wild storm of high- and low- frequency radiation. Some has the taste of metal, of burning, the tangy taste and strangeness of dying. We don’t meet it often, only when something needs to be annihilated. As for others – languid and as long as the locks of billowing seaweed – we know how to mould and sculpt them so that they carry sound and vision. And now I am feeling genuinely livid.

I am angry that my retinas can only do so little, that all these beautiful photons simply ignore the rivers and oceans of wild unknown light, sipping only the rainbow rill of ‘visibility’. Tomorrow, in the light vibrating from the heat, my body will cut the meadow, moving only along the lines of visible light. Next to me, on their own orbits and routes, others will glide who can break through beyond the microcosm visible to humans. Following the hungry gaze of the other eyes, those others will pass me by, in their parallel worlds full of different light and different colours – not so much different, as boldly impossible.

Pixels

When I press the shutter button of my digital reflex camera, I let a pinch of light in, photons rush through the dark interior, ram themselves into the matrix’s pixels and die there, leaving behind electric spectres that the camera collects, writes down, forms into a picture. The eyes have their pixels, too – rods and cones clinging to the retina’s film in the fundus. The photosensitive receptacles of a purple dye that also annihilate photons which get inside, turning everything into electricity, the waves of impulses travelling from the eyes to the brain on the wires and joints. There are three types of rods and cones, each tuned to decipher different photons: orange, green and blue. Hit by the light in different proportions and combinations, cones produce all the colours of the world. But only so much. None can grasp light from outside the rainbow spectrum – everything that shines at lower frequencies than violet and at higher frequencies than red is simply ignored by the human eye.

All the same, we should probably consider ourselves lucky: two of our cone types (orange and green) are separated by just 70 nanometres of photosensitivity. As a species, we have had them for a short time. They formed in the eyes of our simian ancestors to differentiate between the inedible green of unripe fruit and the nutritious redness of fruit packed with sunshine and sugar. Other mammals are still in a two-colour world without red and orange. If a fox is gazing at you from a forest thicket, you two live in radically different realities. Yours – colourful in a familiar way, where the rainbow stretches in all its glory from red to violet and indigo. The fox’s – a world where the vermilion poppy is just as plain as the golden sunflower and the green of the majestic oak tree. The fox’s rainbow would look like a two-coloured strip of blue and dirty green – that is, of course, if we could apply to it our human colour perception, our names for the different hues and our references to the colourful objects of the world around us.

Drawing by Marek Raczkowski
Drawing by Marek Raczkowski

This is probably the biggest problem of travelling through the colour universes of different species; what’s more, it is also an issue when we wonder how colour blind people see the world. How can we be sure that the chartreuse in our mind – trained since childhood by three kinds of photosensitive receptors – is the same that a person with no orange receptors sees? The sense of colour, its interpretation, is only partly about physics. A significant part of the process happens in your head, in experiences, memories and unconscious reflexes. Therefore I am an ardent disbeliever in gimmicks such as colour blindness simulators or dog vision converters – programs and filters that turn an image vibrating with vivid colours into a plain, dirty-yellow one. The worlds of the fox or a colour blind person are neither dirty nor ugly: they are just as criss-crossed by the traces of our quests and moments of enchantment. Joy and sadness simply have slightly different shades there.

Impossible colours

In a sense, we are all colour blind. Our visual sensitivity is embarrassingly limited. This is most irritating at the place where I am now. I have just reached a meadow, I am sliding into thick brush made of oat grass and meadow grass. The air is full of light, and this is great. Plants compete in searching out colours; there are hundreds of pairs of eyes, flying, crawling, gliding, and each one chooses their favourite from the multitude of photons. This is where I surrender, sensorially. For me, this is only a meadow, the green flesh of the grasses and the spots of a dozen vivid colours I can name. Yet flying in the air there are photons of numerous exotic colours. For one of them, we have a miserably flat name: ultraviolet.

Our body is not completely indifferent to ultraviolet – in a sense it can be ‘seen’ by the skin. When bombed with ultraviolet rays, our cells respond with suntan. Our eyes, however, are completely blind to ultraviolet. We have no idea what ultraviolet could look like as a colour. Almost certainly it is a kaleidoscope of at least several colours, unreal and, from our point of view, impossible. After all, ultraviolet occupies more than 300 nanometres (behind the weakest indigos and violets) – 300 nanometres is the exact space that accommodates the whole ‘human’ rainbow and its innumerable, familiar hues.

These are probably the most frustrating parallel universes: where the familiar visible spectrum seamlessly shades into the unknown ultraviolet, and the world adopts colours that for us, humans, are impossible. It is surreptitiously used by plants, which cover their bodies in tattoos tinged with these non-human shades, hidden messages for the chosen ones. The latter are the lucky owners of the fourth type of photoreceptor cells: cone cells tuned to receive ultraviolet. On my meadow, the butterflies and the birds gliding above the roughness of the grasses live in ultra-technicolour. So different than mine, their eyes go beyond violet to about 100–150 nanometres.

For butterflies, seeing ultraviolet means, among other things, spotting the flowers’ subtle signals, the delicate lines and patterns painted on their petals that guide visitors to the sweet nectar. Birds, in turn, need ultraviolet to prink and preen for the opposite sex. Take, for example, the yellow-blue Eurasian blue tit, which you can see in feeders during winter. The amazing blue of its feathers is a masterpiece of evolution: the colour is made without any dye whatsoever, but solely thanks to tiny protein strands arranged in the feather in regular rows and layers. Just like in the holographic sticker you probably have on your credit card – only more beautiful. In fact, blue tits are not even blue! Their feathers, apart from pure blue, also reflect ultraviolet. Seen by the bird’s eye with its set of four avian photoreceptors, the blue tit must look like a miniature peacock, glistening with a multitude of colours, which we cannot even picture. If you are trying to imagine the bird’s point of view and the world painted ultraviolet, then please stop. No matter what wild and avant-garde colour you paint the ultraviolet reflections with, it will always be our boring, human hue. After all, it is defined by the narrow boundaries of the rainbow spectrum, by a tiny sample squeezed between red and violet, lifted from a vast ocean of radiation. The parallel worlds of birds or butterflies are universes full of colour and non-colour, strange places where plants have ultraviolet faces and the rainbow bleeds over beyond the dark blues and violets into extra arches of as yet unnamed colours.

As if this was not enough, birds can do something else. Their retinas, apart from having four types of cone cells detecting colours, are equipped with so-called double cones (or twin cones). Thanks to them, birds can see not only the colours of light, but also its spin – a property called polarization. You can experience a simulacrum of it by looking at a clear sky through glasses with a polarizing filter: the parts of the horizon far from the sun will be of a completely different blue than those directly around it. That which requires from us the use of a piece of prepared plastic probably produces a much subtler impression in a bird. In its eyes, the dome of the sky is wavering under the impulses of light, changing its polarization as the sun slowly moves over the horizon. It is a clock and a map; directions and times of day are encoded in polarization bands. A multi-coloured astrolabe of the real sky, a compass the birds follow during their annual celestial wanderings.

Through the keyhole

Dissecting animal perceptions can be frustrating. Birds’ and butterflies’ super eyes, fitted with extra photosensitive pixels, lizards’ ‘seeing’ (or rather, sensing) infrared, the radiation on the other side of the rainbow spectrum. There is the peacock mantis shrimp from the Stomatopoda order, a fascinating cousin of shrimp, with eyes mounted on mobile stocks. Some peacock mantis shrimp have as many as 12 types of photoreceptor cells, evenly probing electromagnetic waves from the shortest ultraviolets to the near infrared. How amazing must the world seen through such eyes be? How much of this colour information overflow can the small peacock mantis shrimp’s brain fish out and turn into real colour sensations? Our frustration only deepens when we enter the realm of other senses – our world turns out to be surprisingly limited sonically and olfactorily, not to mention senses as absurd as ‘tasting’ the magnetic field (as sharks do) or ‘hearing’ turbulence and water vortices (as in the lateral line system of fish). A keyhole – this is how we perceive the world. A keyhole shaped by our evolution, all the successes and failures of our species, tailored to our biological needs.

I am watching a bluethroat rocking head down on the branch of a low hawthorn bush. We are standing next to each other – I, with my predictable, rainbow world, and the bluethroat immersed in a cacophony of ultraviolet photons, with the sky a pop art mosaic of the most impossible, polarized colours. I envy the bird a little. But I am also happy that I have not discovered everything yet, that there are things and phenomena I can see that are only waiting for me to see them. Let the bluethroat live in its parallel world of unnamed colours. After all, there is a perfect reason for this: without some of these colours, the bluethroat would not be able to make it back from its winter hideout in Africa.

 

Translated from the Polish by Adam Zdrodowski

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Szymon Drobniak

is an evolutionary biologist, researcher on colour, ornithologist, illustrator and author. A passionate fan of diagrams, cross-sections and pea soup. In his spare time, he collects roots (not the mathematical kind).