Nothing is more beautiful at night than the sight of a coal-black sky uncontaminated by the lights of major metropolises. But when we look up, do we really see what we think we see? Here are all the dark secrets of the dimmest of hues.
The statement that the night sky is black sounds like a truism. “Of course,” we think, “what color would it be, since night is the absence of light, and the absence of light is blackness?” Yet first, we need to state that in most of Europe and large swaths of the United States—in fact, in much of the world—it is extremely difficult to verify the truth of such a claim. We live in a time when a generation of people is being born who may never experience the blackness of a night sky.
Fear of the dark is one of the original human anxieties, which we have paradoxically turned into our strength. Since prehistoric times, the fear has pushed us to remove darkness at all costs. There were dangers lurking in the dark. It exposed our incapability—human eyes don’t cope well in darkness, so most of our activities happened in the bright part of day. At first with fire and later with electricity, we destroyed the most primeval sources of blackness and darkness. Light has become almost synonymous with the presence of humans: wherever there are humans, there is also light. Huge quantities of it. It oozes from streetlamps and uncovered windows; it splashes in colorful waves from large-scale advertisements. It is emitted by diodes and car controls that signal a myriad of different things. In Europe and in many parts of the US, we are almost always within range of some source of light. Most likely it will be the city: the one we live in, or, if we are lucky enough to live far from a large metropolis, the glow emanating from the nearest urban area. It is a familiar sight: even in the middle of the night the sky is not velvety black, but seems dirty instead. It is either yellowish brown from the glare of streetlights’ orange sodium fluorescent tubes, or washed black and gray from the increasingly popular LED lights that produce a cold, bluish glow.
These days, the view of a night sky free of light pollution is a luxury booked as part of tourist packages, a privilege jealously guarded by those who understood in advance what it would mean to lose the most primeval prototype of blackness our species has known. And it is a fundamental loss. Our brain needs darkness. Only immersion in total blackness guarantees a good night’s rest. Worse still, while swallowing blackness, the light also erases our oldest and most beautiful stories. The stars, which at the dawn of time were the sources of myths and dreams, are lost in the homogeneously bland, eternally gray sky. The sad thing is that there are many people who have never seen—and may never see again—the Milky Way. It is becoming increasingly difficult to understand the power of blackness. The blackness of the night is just the tip of the iceberg.
Beyond the Spectrum
Silence is the absence of sound. To formulate an analogous definition—that blackness is the absence of light—is to make several mistakes at once. The first is to assume that blackness is a “non-color;” something that remains after all other colors have disappeared. After all, color is dependent on the light that carries it. It’s thanks to the interaction of light with matter and with the cells in our eyes, that we can talk about color impressions in the first place. So, the color black must be a substitute—our sense of vision’s gauche response to the absence of light (which is not true). The second is to mistakenly assume that light ends where our ability to see it reaches its limit. Just because we can’t see something doesn’t mean that a different—not necessarily human—eye can’t see anything either. Thus, “light” is a very ambiguous notion. The third mistake concerns the issue of “absence.” Is the absence of light even possible? After all, almost everything that surrounds us and is not at absolute zero temperature (zero degrees Kelvin) emits electromagnetic radiation. In other words: everything glows, just not always in a way that is visible to our eyes. So let’s set the record straight on several issues regarding the color black.
If we assume that color is essentially the final impression that the brain registers, then black would definitely be a color. Black is not, however, part of the visible light spectrum—just like white, brown, or purple. They are all non-spectral colors, and their formation depends on the stimulation of more than one cone in the retina (or, as in the case of black, on the lack of stimulation of any cones). Certainly, the blacks as seen by a human, bird, or insect are probably completely different colors. Where we can no longer see anything but darkness, birds may still be able to see light, for instance beyond the violet spectrum available to humans (birds can also see ultraviolet radiation).
It’s also quite possible that when we think we’re seeing black, we’ve only encountered one of many in the array of colors that are “almost black.” Defining black as a color that is produced when matter absorbs all of the visible radiation it encounters, we are setting the bar for black’s ideal realization very high. Manufacturing a material that perfectly absorbs the light of wavelengths visible to humans is an extremely difficult task. Therefore, most blacks are in fact, more or less dark grays or particularly dense, saturated navy blues, maroons, or browns. Everything depends on how the color is created.
The funniest thing is that “imperfection” is especially true of objects and substances considered archetypically black. A black swan? Of course not—the dark coloration of this variety of mute swan is the effect of eumelanin, a black pigment that is deposited in the feathers. While eumelanin itself absorbs all colors of visible light very efficiently, when dispersed in the feathers it loses some of its darkness. Black swans are therefore pewter at best. What about black tulips? Obtaining this once-exotic variety used to be quite a feat—the tulip comes mainly in various shades of purple, yellow, and violet. Things were not made easier by the fact that plants (unlike animals) cannot produce melanin. Floriculturists had to rely on the limited capabilities of tulip physiology and find a mutant that would be almost black. Of course, they succeeded! With the right combination of deeply saturated purple and yellow, the petals of the tulip so colored suddenly began to perfectly absorb the green, blue, orange, and yellow colors of visible light. As it turned out, however, the illusion was not complete, and what we now know as the black tulip is in fact a combination of deep maroon and almost-black purple. At first glance, a black tulip (but also a black rose or a black hyacinth) looks striking, but on closer inspection its black magic disappears. . .
Perfect Darkness
Is there then such a thing as the perfect black? A substance that is absolutely obscure, ultimately dark, absorbing all visible light? The substance that, for a long time, was held to be a near-perfect black pigment was soot—a mixture of microscopic carbon particles and small amounts of its exotic varieties, including nanotubes or fullerenes—formed during incomplete combustion of substances rich in carbon atoms. Take a metal spoon and hold it in the flame of a candle. Soon, the silvery metal will turn a dull black. This is because the candle’s paraffin consists almost exclusively of carbon and hydrogen, and the paraffin is vaporized and burned in the flame. When cold metal is introduced into the fire, the combustion temperature is drastically lowered, and the carbon that normally oxidizes to carbon dioxide, precipitates in the form of flecks that settle on the metal surface. The blackness of soot is so perfect that for many centuries it was one of the most important sources of black pigments in painting and printing. However, it soon became apparent that even soot reflects trace amounts of visible light.
The path to a more perfect version of black was rather straight. Since soot is such a good approximation of perfect black, researchers looked at its components. “The blackest” ingredient of soot turned out to be carbon nanotubes; today, they are the source of the deepest black that can be industrially obtained. If you cover a surface with carbon nanotubes or, more precisely, grow a dense nanotube “lawn,” the result is a material that absorbs 99.994% to 99.997% of incident light. We no longer perceive spatial objects colored in this way as three-dimensional. This is because the impression of “spatiality” of a solid figure is created by the light the object reflects. Human eyes interpret subtle differences in light intensity as measures of the distance or angle of the object’s various surfaces in relation to the direction of our gaze. If light is not reflected, the eye receives insufficient information about the shape of the object, and the solid figure turns into a “blot” with no spatial characteristics.
(Non-)Black Cosmos
On several occasions I was lucky enough to admire a genuinely black night sky, untainted by urban glare and free of the light waste of human activity. I saw the darkest cosmos on the Pacific island of Lord Howe—a tiny arc of sand and coral reefs some six hundred kilometers east of the Australian coast. I can still remember how it felt to be lying on the cooling beach, under a dome of moonless sky, without a trace of artificial light around. The islanders protect the darkness of their nights to such an extent that, sitting on one of the beaches, I could see the green glow of bioluminescent mushrooms in the ground cover of a forest a few hundred meters away.
Melting into the darkness of that sky, I experienced what was essentially perfect blackness. Even though the cosmos could hardly be considered a “body,” it is worth remembering that in physics a body can be anything, including the rather abstract vastness of the universe. In what sense is the universe perfectly black? When we look into the night sky, away from the lights, we have no doubt that it is black. Incidentally, it is the deepest and, in an extremely appealing way, the most terrifying blackness one can experience. No wonder that the myths of our ancestors began there, and that it is in this darkness that the most terrifying beasts awaited us. However, if we looked at the sky with a more sensitive eye—one that’s fine-tuned to the entire spectrum of radiation—we would see that, behind the bright glow of stars and nebulae, behind the gray streak of the Milky Way, glows a more frail, ancient radiance. The entire cosmos smolders like a dying lump of coal. The radiation is in the microwave range—i.e., these are actually quite short radio waves. It is an ancient glow that reaches us from a time soon after the birth of the universe—it is the light which was emitted in the course of the big bang, and which began its journey toward us when the cosmos went from opaque to transparent and light could shine. This light has exactly the same properties as that emitted by a blackbody at the temperature of the universe today (about two degrees above absolute zero). Its energy is so faint that human eyes (and the eyes of all organisms known to us) are unable to register it. Even though we are surrounded by the light emitted by the nascent universe, we shall always see the cosmos as absolutely black. Cosmic Microwave Background Radiation (CMBR) will only move farther and farther away from the visible part of the spectrum, gradually moving toward increasingly long radio waves as the temperature of the universe approaches absolute zero.
Two cosmic clouds rise over Lord Howe Island every night: the Magellanic Clouds, indifferent to our planet’s weather. They can only be seen against the blackest sky—just a trace of artificial light is enough for their fine structure to be lost in the gray. We treat black as such an obvious (and somehow inferior, easier to obtain) opposite to white that we are no longer bothered that, in most places on the planet, the color that shrouds the cosmos over our heads is imperfect. Sometimes I think that, just as the “blackbody” was once the key to determining how to measure light, so now a perfectly black sky should be the mandatory standard for redefining our perception of blackness and thus, resetting the balance between white and black. Since we already go on school field trips to see the primordial Białowieża forest in Poland or the Grand Canyon in the US, why not add at least one trip to see the real night? There are no deadly beasts lurking in the night’s blackness. Instead, a person can find the forgotten or dormant pieces of oneself in it. I don’t know anyone who, after having experienced a genuinely dark night, wouldn’t want to have another go.
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