Billions & Billions (7 page)

There is an astronomy for each of these frequency ranges. The sky looks quite different in each regime of light. For example, bright stars are invisible in the light of gamma rays. But the enigmatic gamma ray bursters, detected by orbiting gamma ray observatories, are, so far, almost wholly indetectable in ordinary visible light. If we viewed the Universe in visible light only—as we did for most of our history—we would not know of the existence of gamma ray sources in the sky. The same is true of X-ray, ultraviolet, infrared, and radio sources (as well as the more exotic neutrino and cosmic ray sources, and—perhaps—gravity wave sources).

We’re prejudiced toward visible light. We’re visible light chauvinists. That’s the only kind of light to which our eyes are sensitive. But if our bodies could transmit and receive radio waves, early humans might have been able to communicate with each other over great distances; if X rays, our ancestors might have peered usefully into the hidden interiors of plants, people, other animals, and minerals. So why didn’t we evolve eyes sensitive to these other frequencies of light?

Any material you choose likes to absorb light of certain frequencies, but not of others. A different substance has a different
preference. There is a natural resonance between light and chemistry. Some frequencies, such as gamma rays, are indiscriminately gobbled up by virtually all materials. If you had a gamma ray flashlight, the light would be readily absorbed by the air along its path. Gamma rays from space, traversing a much longer path through the Earth’s atmosphere, would be entirely absorbed before they reached the ground. Down here on Earth, it’s very dark in gamma rays—except around such things as nuclear weapons. If you want to see gamma rays from the center of the Galaxy, you must move your instruments into space. Something similar is true for X rays, ultraviolet light, and most infrared frequencies.

On the other hand, most materials are poor absorbers of visible light. Air, for example, is generally transparent to visible light. So one reason we see at visible frequencies is that this is the kind of light that gets through our atmosphere down to where we are. Gamma ray eyes would be of limited use in an atmosphere which makes things pitch black in gamma rays. Natural selection knows better.

The other reason we see in visible light is because that’s where the Sun puts out most of its energy. A very hot star emits much of its light in the ultraviolet. A very cool star emits mostly in the infrared. But the Sun, in some respects an average star, puts out most of its energy in the visible. Indeed, to remarkably high precision, the human eye is most sensitive at the exact frequency in the yellow part of the spectrum at which the Sun is brightest.

Might the beings of some other planet see mainly at very different frequencies? This seems to me not at all likely. Virtually all cosmically abundant gases tend to be transparent in the visible and opaque at nearby frequencies. All but the coolest stars put out much, if not most, of their energy at visible frequencies. It
seems to be only a coincidence that the transparency of matter and the luminosity of stars both prefer the same narrow range of frequencies. That coincidence applies not just to our Solar System, but throughout the Universe. It follows from fundamental laws of radiation, quantum mechanics, and nuclear physics. There might be occasional exceptions, but I think the beings of other worlds, if any, will probably see at very much the same frequencies as we do.
^*

Vegetation absorbs red and blue light, reflects green light, and so appears green to us. We could draw a picture of how much light is reflected at different colors. Something that absorbs blue and reflects red light appears to us red; something that absorbs red light and reflects blue appears to us blue. We see an object as white when it reflects light roughly equally in different colors. But this is also true of gray materials and black materials. The difference between black and white is not a matter of color, but of how much light they reflect. The terms are relative, not absolute.

Perhaps the brightest natural material is freshly fallen snow. But it reflects only about 75 percent of the sunlight falling on it. The darkest material that we ordinarily come into contact with—black velvet, say—reflects only a few percent of the light that falls on it. “As different as black and white” is a conceptual error: Black and white are fundamentally the same thing; the difference is only in the relative amounts of light reflected, not in their color.

Among humans, most “whites” are not as white as freshly fallen snow (or even a white refrigerator); most “blacks” are not
as black as black velvet. The terms are relative, vague, confusing. The fraction of incident light that human skin reflects (the reflectivity) varies widely from individual to individual. Skin pigmentation is produced mainly by an organic molecule called melanin, which the body manufactures from tyrosine, an amino acid common in proteins. Albinos suffer from a hereditary disease in which melanin is not made. Their skin and hair are milky white. The irises of their eyes are pink. Albino animals are rare in Nature because their skins provide little protection against solar radiation, and because they lack protective camouflage. Albinos tend not to last long.

In the United States, almost everyone is brown. Our skins reflect somewhat more light toward the red end of the visible light spectrum than toward the blue. It makes no more sense to describe individuals with high melanin content as “colored” than it does to describe individuals with low melanin content as “bleached.”

Only at visible and immediately adjacent frequencies are any significant differences in skin reflectivity manifest. People of Northern European ancestry and people of Central African ancestry are equally black in the ultraviolet and in the infrared, where nearly all organic molecules, not just melanin, absorb light. Only in the visible, where many molecules are transparent, is the anomaly of white skin even possible. Over most of the spectrum, all humans are black.
^*

Sunlight is composed of a mixture of waves with frequencies corresponding to all the colors of the rainbow. There is slightly more yellow light than red or blue, which is partly why the Sun looks yellow. All of these colors fall on, say, the petal of a rose. So why does the rose look red? Because all colors other than red are preferentially absorbed inside the petal. The mixture of light waves strikes the rose. The waves are bounced around helter-skelter below the petal’s surface. As with a wave in the bathtub, after every bounce the wave is weaker. But blue and yellow waves are absorbed at each reflection more than red waves. The net result after many interior bounces is that more red light is reflected back than light of any other color, and it is for this reason that we perceive the beauty of a red rose. In blue or violet flowers exactly the same thing happens, except now red and yellow light is preferentially absorbed after multiple interior bounces and blue and violet light is preferentially reflected.

There’s a particular organic pigment responsible for the absorption of light in such flowers as roses and violets—flowers so strikingly colored that they’re named after their hues. It’s called anthocyanin. Remarkably, a typical anthocyanin is red when placed in acid, blue in alkali, and violet in water. Thus, roses are red because they contain anthocyanin and are slightly acidic; violets are blue because they contain anthocyanin and are slightly alkaline. (I’ve been trying to use these facts in doggerel, but with no success.)

Blue pigments are hard to come by in Nature. The rarity of blue rocks or blue sands on Earth and other worlds is an illustration of this fact. Blue pigments have to be fairly complicated; the anthocyanins are composed of about 20 atoms, each heavier than hydrogen, arranged in a particular pattern.

Living things have inventively put color to use—to absorb sunlight and, through photosynthesis, to make food out of mere air and water; to remind mother birds where the gullets of their fledglings are; to interest a mate; to attract a pollinating insect; for camouflage and disguise; and, at least in humans, out of delight in beauty. But all this is possible only because of the physics of
stars, the chemistry of air, and the elegant machinery of the evolutionary process, which has brought us into such superb harmony with our physical environment.

And when we’re studying other worlds, when we’re examining the chemical composition of their atmospheres or surfaces—when we’re struggling to understand why the high haze of Saturn’s moon Titan is brown and the cantalouped terrain of Neptune’s moon Triton pink—we’re relying on the properties of light waves not very different from the ripples spreading out in the bathtub. Since all the colors that we see—on Earth and everywhere else—are a matter of which wavelengths of sunlight are best reflected, there is still more than poetic merit to think of the Sun as caressing all within its reach, of sunlight as the gaze of God. But you have a much better shot at understanding what’s happening if you think instead of a dripping faucet.

E
very culture has its creation myth—an attempt to understand where the Universe came from, and all within it. Almost always these myths are little more than stories made up
by story tellers. In our time, we have a creation myth also. But it is based on hard scientific evidence. It goes something like this …

We live in an expanding Universe, vast and ancient beyond ordinary human understanding. The galaxies it contains are rushing away from one another, the remnants of an immense explosion, the Big Bang. Some scientists think the Universe may be one of a vast number—perhaps an infinite number—of other closed-off universes. Some may grow and then collapse, live and die, in an instant. Others may expand forever. Some may be poised delicately and undergo a large number—perhaps an infinite number—of expansions and contractions. Our own Universe is about 15 billion years past its origin, or at least its present incarnation, the Big Bang.

There may be different laws of Nature and different forms of matter in those other universes. In many of them life may be impossible, there being no suns and planets, or even no chemical elements more complicated than hydrogen and helium. Others may have an intricacy, diversity, and richness that dwarfs our own. If those other universes exist, we may never be able to plumb their secrets, much less visit them. But there is plenty to occupy us about our own.

Our Universe is composed of some hundred billion galaxies, one of which is the Milky Way. “Our Galaxy,” we like to call it, although we certainly do not have possession of it. It is composed of gas and dust and about 400 billion suns. One of them, in an obscure spiral arm, is the Sun, the local star—as far as we can tell, drab, humdrum, ordinary. Accompanying the Sun in its 250 million year journey around the center of the Milky Way is a retinue of small worlds. Some are planets, some are moons, some asteroids, some comets. We humans are one of the 50 billion species that have grown up and evolved on a small planet,
third from the Sun, that we call the Earth. We have sent spacecraft to examine seventy of the other worlds in our system, and to enter the atmospheres or land on the surfaces of four of them—the Moon, Venus, Mars, and Jupiter. We have been engaged in a mythic endeavor.

—

Prophecy is a lost art. Despite our “eager desire to pierce the thick darkness of futurity,” in Charles McKay’s words, we’re often not very good at it. In science the most important discoveries are often the most unexpected—not a mere extrapolation from what we currently know, but something completely different. The reason is that Nature is far more inventive, subtle, and elegant than humans are. So in a way it’s foolish to attempt to anticipate what the most significant findings in astronomy might be in the next few decades, the future adumbration of our creation myth. But on the other hand, there are discernible trends in the development of new instrumentation that indicate at least the prospect of goosebump-raising new discoveries.