A Natural History of Color

The Science Behind What We See and How We See it

Rob DeSalle

• Publisher: Pegasus Books
• Year: 2020
• ISBN:
  9781643134437
• Topic: Scienze biologiche
• Subtopic: Biologia

A star curator at the American Museum of Natural History widens the palette and shows how the physical, natural, and cultural context of color are inextricably tied to what we see right before our eyes. Is color a phenomenon of science or a thing of art? Over the years, color has dazzled, enhanced, and clarified the world we see, embraced through the experimental palettes of painting, the advent of the color photograph, Technicolor pictures, color printing, on and on, a vivid and vibrant celebrated continuum. These turns to represent reality in "living color" echo our evolutionary reliance on and indeed privileging of color as a complex and vital form of consumption, classification, and creation. It's everywhere we look, yet do we really know much of anything about it? Finding color in stars and light, examining the system of classification that determines survival through natural selection, studying the arrival of color in our universe and as a fulcrum for philosophy, DeSalle's brilliant A Natural History of Color establishes that an understanding of color on many different levels is at the heart of learning about nature, neurobiology, individualism, even a philosophy of existence. Color and a fine tuned understanding of it is vital to understanding ourselves and our consciousness.

1 The Color of the Universe

We are literally swimming in information that’s all around us, which our bodies can and need to detect. We have evolved mechanisms to detect this wealth of information, from photons to sound waves to individual atoms and molecules to microbes to complex eukaryotes, all coming into contact with us. Detecting and responding to this information is essential for our survival. We and other organisms have evolved sometimes simple, sometimes elaborate, mechanisms for detecting these external stimuli. One of the most important of these mechanisms that we humans use to detect external stimuli is sight—what might be called our “overused sense.” Compared to a lot of other organisms, we rely on it a great deal for survival. It will be the focus of this book, no pun intended. Color is a key ingredient, but before we get to color, we need to delve into light.

Watching the Big Bang

The grand majority of things we have at hand are what we can presently see, touch, smell, taste, and hear. However, a lot of science involves “time travel.” Science looks backward at how and why these things are around us now. Science also wants to predict what is ahead of us too, as prediction is an important part of the utility of science.

Take evolution, for example. What did Darwin have at hand when he formulated his theory of survival of the fittest by natural selection? He had knowledge of the many organisms he collected and observed from his journey on the Beagle. He was well read and also had the knowledge of science developed before his work. Darwin wanted to understand what had happened in the past and so used these tools to come up with the most reasonable, least refutable idea about how life evolved and continues to evolve. He metaphorically travelled back in time many times to come to this conclusion. Specifically, he was able to go back in time and think about common ancestors of living things. He was also able to go back in time to visualize natural selection and how it might work to influence the evolutionary process he observed on his voyage.

Cosmologists are particularly good at time travel. They ask, What do we know about the origin of all this stuff in the universe? As with every material, beginnings can be tracked back about fourteen billion years or so to the Big Bang. As we will soon see, the only thing we know of before the Big Bang was a single point of matter so tightly compacted that it actually could not be seen with the naked eye, or any instrument.

Cosmologists study things like the expansion of the universe and have tried to tackle the biggest time travel problem of all—the Big Bang and the origin of the universe. Most of this time travel involves a unique imagination and a talent for eliminating the impossible, to detail the limits of what might have happened in the past. To address what happened before and during the Big Bang, cosmologists have developed the ultimate time machine that can not only go back in time but can also discern what happened over incredibly small intervals of time. They have concluded that the Big Bang is the origin of all the things we have around us in the universe.

The famous physicist Stephen Hawking and his colleague James Hartle thought about this in detail and did their own time traveling in order to come up with the “no-boundary proposal,” to detail the state of the universe before the Big Bang. More formally known as the Hartle-Hawking proposal, it details that the universe was a singular point of mass with no initial boundaries with respect to time or space. Nice, and obtuse, right? Well actually it makes great sense, because as one time travels backward from the present, the universe compresses more and more until it shrinks to the singularity mentioned above. It shrinks to smaller than the size of a single atom, with all of the particles and mass contracted into a speck-sized clump of extreme mass and incredible heat. When the singularity state is met, time ceases to exist and definition of what happens before the singularity is, simply put, silly to think about. Everything but the singularity is closed to discussion because we have no way to define things, measure things, or even speculate about anything at that point.

Before he passed away at the age of seventy-six in 2018, Hawking was able to tackle many mind-blowing topics in cosmology. For him to conclude that the origin of time (and, for that matter, the origin of everything) is a “no-go” is quite impressive. On a TV show aired only ten days before his death, Hawking explained that going back in time to the Big Bang is a journey toward, but never reaching, nothingness. Time (and mass) shrinks more and more as it gets closer to the origin, but never makes it. As he put it in this interview, “It was always reaching closer to nothing but didn’t become nothing. There was never a Big Bang that produced something from nothing. It just seemed that way from mankind’s point of perspective.” If one were viewing the “rewinding of the tape” of all of this time travel back to the singularity from a “safe distance,” very little if anything would be visible to the human eye, and nothing would be audible or detectable by any of our senses at the end of the rewind. The Hawking-Hartle proposal, whether right or wrong, places the beginning of the tape at a singularity, an imagined situation.

Wouldn’t it be cool to watch this hypothetical video run from the beginning? All it would take is sound science and a little imagination. This is exactly what cosmologists Christopher Andersen, Charlotte A. Rosenstroem, and Oleg Ruchayskiy did in their 2019 paper entitled “How Bright Was the Big Bang?” Andersen and colleagues did this by “placing a hypothetical human observer in the early Universe, and using this human visual system as a proxy for a ‘light detector.’ ” Their thought experiment took into account the various rapid epochs that are predicted for the first second of the universe’s existence. They determine two important characteristics of light in the early universe with respect to the sight our eyes accomplish: the limit of darkness and the limit of visible light. The limit of darkness is the point where complete darkness gives way to being able to see light, and the limit of light is where light becomes blindingly intense.

The first second of the video replay would be accompanied by a multitude of events, far faster than anything we can experience or measure. In a time shorter than anything we know, the singularity expanded in an event known as “inflation,” when it doubled in size nearly 100 times (that is, 2100 times, or 1030 times), but at this point it was still only the size of a golf ball, and it was also unimaginably hot and energetic. As the singularity expanded, the universe cooled immensely, as energy was rapidly released, but it was still incredibly hot (109°C)—much hotter than the sun is now. At one second after the inflation, protons, electrons, and neutrons were formed. And between three and ten seconds after the Big Bang, photons appeared, spreading out from this singular point.

As expansion continued, the universe stayed so hot that photons—particles of electromagnetic radiation—moved inside the very dense soup of electrons, protons, and neutrons. The soup was so dense that the particles smashed into each other and the photons got stuck, as if in an ultra-dense fog, where light was scattered. For a million years the universe was a continuously expanding, foggy blob of particles. Finally, the universe cooled enough to form hydrogen atoms and also to allow photons to be released. Photons could move about in this transparent hydrogen soup for long distances. We still detect them as radio waves, as they are part of the cosmic microwave background (CMB) or cosmic radiation background (CRB). They are very, very weak waves, coming from all around us in space.

While the CMB is technically made up of photons, we cannot see it with our eyes. Our eyes can only detect photons in a small range of energies, or, as we say, wavelengths. Each photon has energy; if it is too high, like with X-rays, our eyes can’t detect them. They pass through the body with little effect. However, when the number of photons is very high, and a high intensity exists, we can get sunburn or radiation damage. Photons with a lower energy can also not be detected by our eyes. We feel them as heat, like the warmth generated by infrared photons from the sun, or those in a microwave oven.

If we go back to Andersen and colleagues’ thought experiment of the human eye viewing the video, the period of time leading up to decoupling was “blindingly bright” and full of photons. And all mainly of the wrong wavelength for the human eye to see. The human eye would only have been able to see anything once the universe was more than one million years old. As more cooling occurred over the next five million years or so, the universe became less and less bright until it reached pitch blackness to the human eye. That human eye in the thought experiment detected no light for over 150 million years, a period of the early universe called the “Dark Ages.” What happened? Stars started to coalesce at this point, and enough of them formed at 150 million years for Andersen and colleagues’ eye to start to detect a little light. As the universe expanded, more and more stars formed and more light was produced to get us to the current state of the universe, where there is neither too much light that would fry our retinas, nor too little light that we couldn’t see.

The Top Five

Humans have seen rainbows for as long as our species has been on this planet. This includes our extinct close relatives, such as Neanderthals, who had as acute color vision as we do. All cultures use rainbows in religion and mythology. The colors in particularly vivid rainbows are among the purest visual treats ever seen by human eyes. Humans have wondered about those colors to the point of invoking supernatural explanations for them. But it wasn’t until a famous experiment in the 17th century that the physical nature of the colors in rainbows was articulated in what can be called a “beautiful” experiment.

Beautiful experiments embody the essence of science. They are characterized by human cleverness and explain some fundamental phenomena in nature. According to a New York Times article by journalist George Johnson published in 2002, three of the top five most beautiful scientific experiments concern light and its composition. Sir Isaac Newton clocks in with the fourth most beautiful experiment focused on the nature of colors. Until Newton performed this beautiful experiment #4, scientists assumed that color was somehow a gemisch, or mixture of light and dark. Sir Robert Hooke, a famous naturalist of the 17th century, who liked to squabble with Newton, felt that colors were like mixing paints at a paint store. Pure white light could be mixed with varying degrees of darkness. Deep red to Hooke was white light mixed with as little darkness as possible. Deep blue, on the other end of the spectrum, was white light mixed with as much darkness as possible before the color turned black. In 1666, Newton, who never backed away from a good fight, especially with Hooke, took a simple experimental device that was popular at the time—a prism—and devised one of these beautiful experiments. Like a glass full of water in sunshine, the prism was well known to produce colors apparently by separating some special quality of the color’s light. According to some scientists of the time, when light was shown through a prism, the prism itself physically altered white light in different ways to produce the many colors—red, orange, yellow, green, blue, indigo, violet, and the spread of colors in between. To these 17th-century scientists, a prism was a somewhat magical device that would “color” white light as it passed through it.

Newton had a hunch that this was a wrong way to think about colors and white light, so he used a primary prism to first get the distribution of colors normally obtained from a prism. He was then able to take the red light emanating from the prism and send it through a second prism. If the prism was coloring light, it would have an impact on the red light going through the second prism. But the color of light coming from the second prism was the same red that was isolated by the first prism. The prism was not actively coloring the light coming through it but rather was separating it into its natural components—the different colors of the spectrum. To nail down his experiment, Newton took a lens and focused the multiple colors coming from the first prism to a small point and produced white light. Not only could you take white light apart, you could also put it back together again. Newton correctly reasoned that white light was composed of all of the colors, and light was a much more complex concept than previously thought. This experiment was critical not only for the development of the physics of light but also as a guide to how science is accomplished. Many of the principles and steps of reasoning that Newton used in this experiment are still in use today.

Figure 1.1. Newton’s double prism experiment. Light (S) comes into the first prism (A). A small patch of separated light emanating from the prism is focused through a small aperture (X) on a divider (B-C). The light emanating from aperture X is of a single color—say, red. This pure red light then travels to the second divider (D-E) and is focused through a second aperture (Y). The light is then passed through a second prism (F). If the prism is coloring the light, as the prevailing notion went, then passing the light from apertures X and Y through prism (F) should produce a spectrum of light. It doesn’t, though. The only logical explanation is that the prism is separating light, and that white light is a mixture of different hues. Wiki Commons, public domain, https://commons.wikimedia.org/wiki/File:NewtonDualPrismExperiment.webp.

While Newton uncovered the complexity of white light, he also developed some ideas about what light was composed of. He felt strongly that light was particulate; it was his gut feeling, though, and not backed up terribly well by data. Newton was right about a lot of things, he usually provided data or strong theory to shore up his conclusions. Not so with light as a particle, though, and since no one is right all the time, science takes over. Here is where the fifth most beautiful science experiment comes in. There was another side to determining what light is (there is always “another” side in science before experimentation occurs) at Newton’s time. Many scientists felt that light behaved like a wave. And there couldn’t be two more different ideas about the makeup of something.

For a lot of us, tossing stones into a still pond is a pleasing endeavor, mostly because of the waves produced. For a very long time, children and adults have enjoyed this pastime, because, like rainbows, the effect probably enthralled them. Sound was also a big-ticket subject in science around the 17th and 18th centuries. Both sound and water waves were studied and characterized in this period of time, which was important for establishing methods of science. The scientists who studied these things used observation and simple mathematical modelling of what they saw. What they discovered about waves is seminal to understanding light even though, as we will see, the explanation isn’t so simple.

Waves have very specific patterns of behavior, and the characteristics of waves were worked out well before the 19th century. If we look at a wave on the ocean, we can see that it has high points and low points. The former is called a “crest” and the latter is called a “trough.” The distance between the trough and the crest is called the “wave height.” Half of the wave height is called the “amplitude” of the wave. This terminology makes waves seem like they are higher than they really are. A wave with a height of ten feet sounds pretty big, right? But if you are watching this wave in the ocean, you will note that it rises no more than five feet above the surface of a smooth ocean. The trick is that the wave also sinks five feet below the surface of a smooth ocean. The distance between crests is called the “wavelength” and is represented by the Greek letter lambda (λ).

If you are watching those waves on the ocean closely, you will eventually see that some are coming in at angles to others and they crash into each other. When they do hit each other, the simple wave patterns are disrupted and a phenomenon called “interference” occurs. If two waves collide with each other at their crests, the amplitude of the wave is bigger than either of the two waves by themselves. If a trough of one wave hits a crest of another, the result is smoother water than either of the two waves produce on their own. What is happening here? It turns out that the waves are in effect adding up their individual effects, and waves that represent the sum of the two individual waves are produced by the interference.

In 1803, Thomas Young, a British physician-polymath, devised the fifth most beautiful experiment. He would force light through a pinhole and manipulate it with various objects, like mirrors or cards. Scientists create novel devices all the time, and the verb “play” is not too far off from what they do with their invented devices. Sitting in a dark room, Young shone light through the pinhole. He took a small card (as he describes, about one thirtieth of an inch thick) and used it to bisect the thin beam of light along its path of projection. He then placed a screen at the end of this “device,” to visualize the effect of splitting the white light. Because the two beams of light separated by the card were from the same source, Young reasoned that if light was a particle, then once the two streams of light were created, they would produce two separate point streams of light. If light was a wave, then the two secondary beams of light would interfere with each other like waves. Indeed, the result of this beautiful experiment was a pattern showing alternating light and dark bands, behaving just like waves would when interference occurs. Where the two beams overlapped their crests, they reinforced each other and made lighter bands. Where the two beams collided one at its crest and the other at its trough, the light from one beam was cancelled out by the other, producing a dark band. Over the years, scientists learned to use a card with two holes instead of a one-thirtieth-inch card to split the single light beam. These experiments are called “double slit experiments,” and they are how more recent experiments in quantum theory were conducted to establish wave or particle behavior of physical phenomena.

While the work o...