This Explains Everything

by Mr. John Brockman

  Thinking about the body as a machine was a grand advance in the 16th century, when it offered an alternative to vitalism and vague notions of the life force. Now it’s outmoded. It distorts our view of biological systems by fostering a tendency to think of them as simpler and more sensibly “designed” than they are. Experts know better. They recognize that the mechanisms regulating blood clotting are represented only crudely by the neat diagrams medical students memorize; most molecules in the clotting system interact with many others. Experts on the amygdala know that it has many functions, not just one or two, and they are mediated by scores of pathways to other brain loci. Serotonin exists not mainly to regulate mood and anxiety; it is essential to vascular tone, intestinal motility, and bone deposition. Leptin is not mainly a fat hormone; it has many functions, performing different ones at different times, even in the same cell. The reality of organic systems is vastly untidy. If only their parts were all distinct, with specific functions for each! Alas, these systems are not like machines. Our human minds have as little intuitive feeling for organic complexity as they do for quantum physics.

  Recent progress in genetics confronts the problem. Naming genes according to postulated functions is as natural as defining chairs and boats by their functions. If each gene were a box on a blueprint labeled with its specific function, biology would be so much more tractable! However, it is increasingly clear that most traits are influenced by many genes, and most genes influence many traits. For instance, about 80 percent of the variation in human height is accounted for by genetic variation. It would seem straightforward to find the responsible genes. But looking for them has revealed that the 180 loci with the largest effects together account for only about 10 percent of the phenotypic variation. Recent findings in medical genetics are more discouraging. Just a decade ago, hope was high that we would soon find the variations that account for highly heritable diseases, such as schizophrenia and autism. But scanning the entire genome has revealed that there are no common alleles with large effects on these diseases. Some say we should have known. Natural selection would, after all, tend to eliminate alleles that cause disease. But thinking about the body as a machine aroused unrealistic hopes.

  The grand vision for some neuroscientists is to trace every molecule and pathway to characterize all circuits in order to understand how the brain works. Molecules, loci, and pathways do serve differentiated functions; this is real knowledge, with great importance for human health. But understanding how the brain works by drawing a diagram that describes all the components and their connections and functions is a dream that may be unfulfillable. The problem is not merely fitting a million items on a page; the problem is that no such diagram can adequately describe the structure of organic systems. They are products of minuscule changes (from diverse mutations, migration, drift, and selection), which develop into systems with incompletely differentiated parts and incomprehensible interconnections—systems that nonetheless work very well indeed. Trying to reverse-engineer brain systems focuses important attention on functional significance, but it is inherently limited, because brain systems were never engineered in the first place.

  Natural selection shapes systems whose complexity is indescribable in terms satisfying to human minds. Some may feel this is nihilistic. It does discourage hopes for finding simple specific descriptions for all biological systems. However, recognizing a quest as hopeless is often the key to progress. As Haldane put it, “We are thus brought face to face with a conclusion which to the biologist is just as significant and fundamental, and just as true to the facts observed, as the conclusion that mass persists is to the physicists. . . . [T]he structure of a living organism has no real resemblance in its behaviour to that of a machine. . . . In the living organism, . . . the ‘structure’ is only the appearance given by what seems at first to be a constant flow of specific material, beginning and ending in the environment.”*

  If bodies are not like machines, what are they like? They are more like Darwin’s “tangled bank,” with its “elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner.”* Lovely. But can an ecological metaphor replace the metaphor of body as machine? Not likely. Perhaps someday understanding how natural selection shapes organic complexity will be so widely and deeply understood that scientists will be able to say, “A body is like . . . a living body,” and everyone will know exactly what that means.

  HOW TO HAVE A GOOD IDEA

  MARCEL KINSBOURNE

  Professor of psychology, The New School; coauthor (with Paula J. Kaplan), Children’s Learning and Attention Problems

  You don’t have to be human to have a good idea. You can even be a fish.

  There’s a large fish in shallow Micronesian waters that feeds on little fishes. The little fishes dwell in holes in the mud but swarm out to feed. The big fish starts to gobble up the little fishes, one by one, but they immediately retreat back into their holes when his meal has barely begun. What to do?

  I have put this problem to my classes over the years, and I remember only one student who came up with the big fish’s Good Idea. Of course he did it after a little thought, not after millions of years of evolution, but who’s counting?

  Here’s the elegant trick. When the school of little fish appears, instead of gobbling, he swims low so that his belly rubs over the mud and blocks the escape holes. Then he can dine at leisure.

  What do we learn? To have a good idea, stop having a bad one. The trick was to inhibit the easy, obvious, but ineffective attempts, permitting a better solution to come to mind. That worked for the big fish by some mechanism of mutation and natural selection in fish antiquity. Instead of tinkering with the obvious—obsessing about eating faster, taking bigger bites, etc.—junk plan A, and plan B comes swimming up. For humans, if the second solution doesn’t work either, block that too, and wait. A third floats into awareness, and so on, until the insoluble is solved, even if the most intuitively obvious premises have to be overridden in the process.

  To the novice, the Good Idea seems magical, a leap of intellectual lightning. More likely, however, it resulted from an iterative process as outlined above, with enough experience to help reject seductive but misleading premises. Thus the extraordinary actually arises, step by step, out of the ordinary.

  Having a good idea is far from rare in the evolution of nonhuman species. Indeed, many if not most species need to have an idea or trick that works well enough for them to continue to exist. Admittedly, they may not be able to extrapolate its principle from the context in which it emerged and generalize it, as (some) people can, courtesy of their prefrontal cortex.

  When the finest minds fail to resolve a classical problem over decades or centuries of trying, they were probably trapped by a premise that was so culturally “given” that it didn’t even occur to them to challenge it—or they didn’t notice it at all. But cultural context changes, and what seemed totally obvious yesterday becomes dubious, at best, today or tomorrow. Sooner or later, someone who may be no more gifted than his predecessors but is unshackled from some basic and incorrect assumption may hit upon the solution with relative ease.

  Alternatively, one can be a fish, wait a million years or two, and see what comes up.

  OUT OF THE MOUTHS OF BABES

  NICHOLAS A. CHRISTAKIS

  Physician and social scientist, Harvard University; coauthor (with James Fowler), Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives

  My favorite explanation is one that I sought as a boy. Why is the sky blue? It’s a question every toddler asks, but it’s also one that most great scientists since the time of Aristotle, including Leonardo da Vinci, Isaac Newton, Johannes Kepler, René Descartes, Leonhard Euler, and even Albert Einstein, have asked.

  One of the things I like most about this explanation—beyond the artless simplicity of the question itself—is how many centuries of effort it took to arrive at and how many branches of science it
involves.

  Unlike other everyday phenomena, such as the rising and setting sun, the color of the sky did not elicit much myth-making, even by the Greeks or the Chinese. There were few nonscientific explanations for its color. It took a while for the azure sky to be problematized, but, when it was, it kept our (scientific) attention. How could the atmosphere be colored, when the air we breathe is not?

  Aristotle is the first, as far as we know, to ask why the sky is blue. His answer, in the treatise On Colors, is that the air close at hand is clear and the deep air of the sky is blue in the same way that a thin layer of water is clear but a deep well of water looks black. This idea was still being echoed in the 13th century, by Roger Bacon. Kepler, too, reinvented a similar explanation, arguing that the air merely looks colorless because the tint of its color is so faint when in a thin layer. But none of them offered an explanation for the blueness of the atmosphere.

  In the Codex Leicester, Leonardo da Vinci, writing in the early 16th century, noted, “I say that the blue which is seen in the atmosphere is not its own color, but is caused by the heated moisture having evaporated into the most minute, imperceptible particles, which beams of the solar rays attract and cause to seem luminous against the deep, intense darkness of the region of fire that forms a covering above them.” Alas, Leonardo does not actually say why these particles should be blue either.

  Newton contributed, both by asking why the sky was blue and by demonstrating, through his pathbreaking experiments with refraction, that white light could be decomposed into its constituent colors.

  Many now-forgotten and many still-remembered scientists since Newton joined in. What might refract more blue light toward our eyes? In 1760, the mathematician Leonhard Euler speculated that the wave theory of light might help to explain why the sky is blue. The 19th century saw a flurry of experiments and observations of all sorts, from expeditions to mountaintops for observation to elaborate efforts to re-create the blue sky in a bottle—as chronicled in Peter Pesic’s wonderful book, Sky in a Bottle. Countless careful observations of blueness at different locations, altitudes, and times were made, including with bespoke devices known as cyanometers. Horace-Bénédict de Saussure invented the first cyanometer in 1789. His version had fifty-three sections with varying shades of blue arranged in a circle. Saussure reasoned that something suspended in the air must be responsible for the blue color.

  Indeed, for a very long time it had been suspected that something in the air modified the light and made it appear blue. Eventually it was realized that it was the air itself that did this—that the gaseous molecules that compose the air are essential to making it appear blue. And so the blueness of the sky is connected to the discovery of the physical reality of atoms. The color of the sky is deeply connected to atomic theory, and even to Avogadro’s number. This in turn attracted Einstein’s attention in the period from 1905 to 1910.

  So, the sky is blue because the incident light interacts with the gas molecules in the air in such a fashion that more of the light in the blue part of the spectrum is scattered, reaching our eyes on the surface of the planet. All the frequencies of the incident light can be scattered this way, but the high-frequency (short wavelength) blue is scattered more than the lower frequencies in a process known as Rayleigh scattering, described in the 1870s. John William Strutt, Lord Rayleigh, who received the Nobel Prize in physics in 1904 for the discovery of argon, demonstrated that when the wavelength of the light is on the same order as the size of the gas molecules, the intensity of scattered light varies inversely with the fourth power of its wavelength. Shorter wavelengths like blue (and violet) are scattered more than longer ones. It’s as if all the molecules in the air preferentially glowed blue, which is what we then see everywhere around us.

  Yet the sky should appear violet, since violet light is scattered even more than blue light. The sky does not appear violet to us because of the final, biological part of the puzzle, which is the way our eyes are designed: They are more sensitive to blue than violet light.

  The explanation for why the sky is blue involves much of the natural sciences: the colors in the visual spectrum, the wave nature of light, the angle at which sunlight hits the atmosphere, the mathematics of scattering, the size of nitrogen and oxygen molecules, and even the way human eyes perceive color. It’s most of science, in a question a young child can ask.

  THE BEAUTY IN A SUNRISE

  PHILIP CAMPBELL

  Editor-in-chief, Nature

  When I was first excited by physics, the depths of its explanations were compelling to me in very esoteric contexts. For example, the binding of matter, energy, and spacetime in general relativity seemed (and indeed is) an extraordinarily elegant and deep explanation.

  Nowadays I am even more compelled by powerful explanations that lie behind the things we see around us that are too easily taken for granted. And I find myself drawn to a context experienced every day by just about everybody.

  How generous is that himself the sun

  —arriving truly, faithfully who goes

  (never for a moment ceasing to begin

  the mystery of day for someone’s eyes)

  Thus wrote e. e. cummings in the opening of his lyrical celebration of our star. Those words highlight a daily moment—a sunrise—whose associated human sense of (in)significance and mystery may for some be only deepened by appreciating (at least) three great explanations underlying the experience. And each has at least one of the Edge Question’s requested qualities: depth, elegance, and beauty.

  If you care about such things, and (like me) live at a northern middle latitude, you will know the range of the horizon visible from your home, between roughly southeast and northeast, across which the point of sunrise shifts back and forth over the year, with sunrise getting later as it moves northward and the days shorten, and the motion reversing at the winter solstice. And beyond that quite complex behavior is the simple truth of the sun’s fidelity—we can indeed trust it to come up somewhere in the east every morning.

  Like a great work of art, a great scientific explanation loses none of its power to inspire awe afresh whenever one contemplates it. So it is with the explanation that those daily and annual cycles of sunrises are explicable by a tilted rotating Earth orbiting the sun, whose average axial direction can be considered fixed relative to the stars thanks to a still-mysterious conservation law.

  Unlike my two other chosen explanations, this one encountered skepticism from scientists for decades. The heliocentric view of the solar system, articulated by Copernicus in the mid 16th century, was not widely accepted until well into the 17th. For me, that triumph over the combination of scientific skepticism and religious hostility only adds to the explanation’s appeal.

  Another explanation is certainly elegant and lies behind the changing hues of the sky as the sun rises. Lord Rayleigh succeeded James Clerk Maxwell as Cavendish professor of physics at Cambridge. One of his early achievements was to deduce laws of the scattering of light. His first effort reached the right answer on an invalid foundation—the scattering of light in an elastic aether. Although the existence of such an aether wasn’t shown to be a fallacy until some years later, he redid his calculations using Maxwell’s deeply unifying theories of electromagnetism. “Rayleigh scattering” is the expression of those theories in contexts where an electromagnetic wave encounters electrically polarized particles much smaller than its wavelength. The amount of scattering, Rayleigh discovered, is inversely proportional to the fourth power of the wavelength. By 1899, he had shown that air molecules themselves were powerful scatterers.

  There, in one bound, is the essential explanation of why the sky is blue and why sunrises are reddened. Blue light is scattered much more by air molecules than light of longer wavelengths. The sun’s disk is accordingly reddened, and all the more so when seen through the long atmospheric path at sunrise and sunset. (To fully account for the effect, you also need to take into account the sun’s spectrum and the visual responses of hu
man eyes.) The pink clouds that can add so much to the beauty of a sunrise consist of comparatively large droplets that scatter the wavelengths of reddened sunlight more equally than air molecules—colorwise, what you see is what they get.

  The third explanation behind a sunrise is conceptually and cosmologically the deepest. What is happening in the sun to generate its seemingly eternal light and heat? Understanding the nuclear reactions at the sun’s core was just a part of an explanation that, thanks especially to Burbidge, Burbidge, Fowler, and Hoyle in 1957,* simultaneously allowed us to understand not only the light from many kinds of stars but also how almost all the naturally occurring chemical elements are produced throughout the universe: in chains of reactions occurring within stable and cataclysmically unstable cosmic balls of gas in their various stages of stellar evolution, driven by the shifting influences of all the fundamental forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces.

  Edge readers know that scientific understanding enhances rather than destroys nature’s beauty. All of these explanations, for me, contribute to the beauty in a sunrise.

  Ah, but what is the explanation of beauty? Brain scientists grapple with nuclear-magnetic-resonance images; a recent meta-analysis indicated that all of our aesthetic judgments seem to include the use of neural circuits in the right anterior insula, an area of the cerebral cortex typically associated with visceral perception. Perhaps our sense of beauty is a by-product of the evolutionary maintenance of the senses of belonging and of disgust. For what it’s worth, as exoplanets pour out of our telescopes, I believe we will encounter astrochemical evidence for some form of extraterrestrial organism well before we achieve a deep, elegant, or beautiful explanation of human aesthetics.