Zoom: From Atoms and Galaxies to Blizzards and Bees: How Everything Moves (26 page)

CHAPTER 15: Barriers of Light and Sound

A Thirty-Century Quest That Began with Thunder

As fast as it can go, the speed of light, you know

Twelve million miles a minute…

—ERIC IDLE AND TREVOR JONES, “THE GALAXY SONG” (1983)

The sound barrier. The speed of light.

These classic entities created endless mind torture for Homo bewilderus. We who are utterly dependent on sight and sound learned early on that nature performs its symphonies prestissimo. Even the people most associated with sight and sound gained renown, such as the sound-barrier-breaking Chuck Yeager and light’s maestro Albert Einstein, who symbolized its speed with a lowercase c in his famous E = mc2 equation.

The head-scratching began far before those twentieth-century celebrities captured the limelight. It may have all started with thunderstorms. Here is nature’s only exhibition of simultaneous brilliant light and deafening sound. It always attracts attention. As in the days of Aristotle, a lightning bolt can be temporarily blinding. A thunderclap can rattle dishes. The birds outside grow ominously silent.

These days, at least, we regard thunderstorms scientifically. When the flash comes we think “electricity” and console ourselves with the fact that fewer than one hundred Americans a year are killed by lightning. Odds are it won’t be you. If you are female rather than male (men are struck five times more often), live in any state except Florida (the most lightning-prone state by far), and neither fish nor play golf (the most lightning-attracting activities), you can maybe even let yourself enjoy the violence.1

Back when the Parthenon was built, when there were fewer lightning-prone golf courses, thunderstorms were always exhibitions of—you knew this was coming—godly power. The word thunder comes from the name of the old Norse god Thor, the hammer-wielding deity who also gave us the word Thursday.

Yet he scarcely had a monopoly. The Bible makes many references to lightning being dispensed by Jehovah. The first occurs in Exodus 9:23: “And Moses stretched forth his rod toward heaven: and the Lord sent thunder and hail, and the fire ran along upon the ground.”

This intimidating amusement was also a specialty of a wide pantheon of Roman and Greek deities. The ultimate thunder hurlers were the German god Donar and the Greek god Zeus, whom the Romans called Jupiter. When you traveled east, if you’d managed to evade the wrath of the European gods, you’d instead get smitten by the Slavic god Perkunis and then the Indian god Indra.

The lightning bolt itself was often depicted as a spear. In Roman times, whatever it hit was considered sacred. Sometimes, where melted glassy sand marked a strike, the place was so revered it was fenced off. The authorities didn’t quite charge admission, but people killed by lightning were conveniently buried right in that consecrated place rather than carted off to a burial ground. In African and South American cultures, the mythical giant thunderbird was often fingered as the cause of storms.

During the classical Greek period, when science and observation flourished, the importance of vision and hearing generated widespread speculation about how sounds and images might go from point A to point B. And while the hypothesizing is certainly not over, the basic, baffling, early puzzles have now metamorphosed into a continuing modern fountain of gee-whiz science.2

The century during which much of the Old Testament was penned witnessed the first nonreligious ideas about visual and auditory fury, proposed by the Greek thinker Thales (ca. 620–546 BCE) and his followers Anaximander (ca. 611–547 BCE) and Anaximenes (ca. 585–528 BCE). The three get merit points for stepping away from the Zeus-hurling-spears business, even if their conclusions were wrong. They each wrote that thunder is wind smashing through clouds, a process they believed kindled the flame of lightning. Thus thunder came first, a conclusion embraced, curiously, for the next two thousand years.

Not that there weren’t any dissenters. Anaxagoras (ca. 499–427 BCE) said that fire somehow flashed first, only to be doused by the clouds’ rain. Thunder, he believed, was the sound of the lightning being violently extinguished.

Aristotle, his brain crammed with intricate beliefs about everything, entered the fray in his ca. 334 BCE collection of essays called Meteorologica. There he sided with Thales. He wrote that thunder is the sound of trapped air in clouds slamming into other clouds, and added, “Lightning is produced after the impact and so later than thunder, but it appears to us to precede it because we see the flash before we hear the noise.”

It wasn’t entirely malarkey. Here was the very first known statement that light moves faster than sound. This may seem like a groundbreaking notion, one more bit of evidence that Aristotle belonged in an accelerated classroom. In actuality, determining the relative speeds of sound and light never required a genius IQ. Echoes within great halls and canyons had always suggested that sound was a slowpoke.

The colliding-clouds explanation for thunderstorms remained popular for century after century. Around the middle of the first century BCE, the Roman poet Lucretius, in his On the Nature of Things, wrote of thunder:

The winds are battling. For never a sound there comes

From out the serene regions of the sky;

But wheresoever in a host more dense

The clouds foregather, thence more often comes

A crash with mighty rumbling.

Why didn’t the 100 percent more correct lightning-first idea catch on? Probably because in that pre-gunpowder era it would have been a unique event, totally without precedent.3 A light never caused a sound, especially in the heavens. The sun and moon are of course silent. So are common exploding meteors and fireballs. The aurora is mute as well. Even in the biological realm of fireflies and phosphorescent marine creatures, the glows unfold in silence.

Meanwhile, some of the early Greeks went beyond storms to probe the very nature of sound. Pythagoras (ca. 570–495 BCE) wondered why some combinations of musical notes sounded more lovely than others. He made a startling discovery. Experimenting with vibrating strings of various lengths, he found that when they had whole-number ratios to each other the resultant combination was always pleasant and harmonious. For example, if a plucked string produces the musical note A, a string twice as long will also create an A, but an octave lower, corresponding to a numerical ratio of 2:1. The notes in between are produced by plucking strings that have string-length ratios such as 8:5, 3:2, 4:3, and so on. Later, Aristotle correctly wrote that sound is nothing more than the expansion and contraction of air produced by air’s proximity to pulsating or oscillating objects, such as strings, rustling leaves, vocal cords, and the vibrating bronze of a bell.

And that’s where things stood, with no further acoustic progress as century bells tolled and tolled again. Sound remained a mysterious subject right into the dawn of the scientific revolution. In the early 1600s, Shakespeare had King Lear ask (in act 3, scene 4): “What is the cause of thunder?” and there was no answer. At around that same time, in 1637, René Descartes—who wrote cogently about optics and vision and sound propagation—nonetheless maintained that colliding clouds create thunder, wrongly echoing the Greeks two thousand years earlier.

But things were starting to change. We celebrated Galileo in our exploration of gravity and falling bodies, but the great man’s observations about sound were spot-on, too. Early in the seventeenth century he wrote, “Waves are produced by the vibrations of a sonorous body, which spread through the air, bringing to the tympanum of the ear a stimulus which the mind interprets as sound.”

Notice that this answers the age-old “If a tree falls in the forest” conundrum. Nowadays the great majority of people will opine that a falling tree makes a sound even if no one is around to hear it. Not so, suggested Galileo. Rather, the crashing oak creates complex puffs of pressurized air—actually a consortium of subpuffs, or thousands of very brief individual air pressure changes—that spread outward. These brief tiny breezes have no inherent sound. Instead, these silent puffs set the ear’s tympanic membrane vibrating in a very detailed way, with slow pulsations and fast ones superimposed on each other—which, Galileo observed, “the mind interprets as sound.” Galileo was bang-on correct. He advocated something rarely heard before the advent of quantum mechanics: the importance of the observer. We know now that nature and the conscious observer are correlative. They go together. Sound requires the presence of both.

Galileo thus basically defined sound as pressure waves. Rapid, intricate puffs of wind. A disturbance in air or another substance. Later researchers found that a human can perceive a noise—that is, her eardrum will be stimulated enough to vibrate—if the ambient air pressure momentarily fluctuates by just one part in a billion. And, moreover, she’ll hear a sound only if those air pulses don’t repeat themselves more than twenty thousand times or fewer than fifteen times in a second. Those are the parameters for human aural response, which make nerves in the eardrum send electrical signals to the brain. Outside that range, the quick tiny breezes unfold in silence.

After Galileo the science of sound advanced quickly, the revelations ever more amazing. Thunderstorms surrendered their cacophonous secrets as well. Benjamin Franklin famously discovered lightning’s true nature on June 10, 1752, during a dangerous kite-flying experiment that might easily have killed him. He correctly concluded that lightning produces thunder and not the other way around. After all, he had already created sparks in his laboratory, and each was always accompanied by an audible snap. Franklin wrote, rhetorically, “How loud must be the crack of 10,000 acres of electrified cloud?” (He was no mere dabbler. He’d been obsessed with uncovering the secrets of electricity for more than a decade. It was he who coined the words electrician, conductor, and battery.)

Mammatus clouds contain such violently high winds that all planes, small and large, give them a wide berth. (Jorn C. Olsen)

Joseph-Nicolas Delisle (1688–1768) went even further. This French astronomer, who made a killing building an observatory and then setting up Russia’s astronomy program for Peter the Great, started studying thunderstorms at the age of fifty. He decided that while lightning can be seen at great distances, even from more than one hundred miles away, thunder is generally inaudible if the bolt lies farther than a mere fifteen miles away. Even in our time, people still mistakenly attribute silent flashes to “heat lightning”—a nonexistent phenomenon—unaware that these are just distant thunderstorms whose sound waves have already fully dissipated.

Back then, on the rare occasions when a lightning bolt made ground contact and left a scar, a precise distance from the observer could be determined. Using that distance and the previously timed delay between the flash and the thunderclap, “natural philosophers” had no trouble pegging sound at 768 miles per hour. But sound’s speed became popularly famous because of a single intriguing concept: a sound barrier. This demands attention because it seems like a challenge. There’s no smell barrier or light barrier. Why sound alone?

This idea arose in the modern aviation era, before which no man-made object except for bullwhip tips and bullets had ever gone that fast. The barrier is a problem because air’s pressure waves accumulate on any object approaching sound speed. That’s what produces a sonic boom. These dense air compressions created squirrelly control issues on jet aircraft in the early 1950s as pilots attempted to reach that speed. As for the drawn-out rolling of thunder, nineteenth-century scientists rightly attributed it to the sound from nearer sections of the lightning bolt arriving ahead of the rest. Because electrical strokes can easily exceed a mile in length, the incoming sound from its various segments can maintain the rumbling for more than five seconds.

Yet even into the twentieth century, when thunderclouds started to share the celestial stage with flying machines, no one knew how lightning creates thunder. There were three excellent theories, and all seemed plausible. Go ahead, try your luck. Which of these would you vote for?

The steam theory of 1903 said that a lighting stroke suddenly vaporizes all a cloud’s water along its route. This high-pressure steam violently expands, producing the thunder sound, just as it does inside an exploding locomotive boiler.

Another theory, from 1870—at the frenzied height of chemistry advances—held that the electricity in the lightning bolt, like an electrode in a beaker of water, disassociates the cloud’s water into separate hydrogen and oxygen atoms. When these quickly recombine, the result is a big boom. After all, a mixture of those elements always explodes if a spark is at hand. That’s exactly what happened during the Challenger shuttle disaster of 1986.

The third idea was published in Scientific American in 1888. One M. Hirn advanced the theory that “the sound which is known as thunder is due simply to the fact that the air traversed by an electric spark, that is, a flash of lightning, is suddenly raised to a very high temperature, and has its volume considerably increased. The column of gas thus suddenly heated and expanded is sometimes several miles long and… it follows that the noise bursts forth at once from the whole column, though for an observer in any one place, it commences where lightning is at least distance.”

And this last hypothesis—after decades of debate—was accepted by the scientific community. Thunder is explosively expanding air.

All motion. Grand motion. Of an electrical arc on steroids. And supersonic gas expansion. Then pressure waves racing at the speed of sound at right angles to the bolt.

The details came later. Ah, but what details! A lightning bolt is a 55,000-degree sizzle created in ten milliseconds—far hotter than the sun’s comparatively lukewarm 11,000-degree surface. By comparison, steel, that symbol of strength, turns to liquid at “just” 2,500 degrees Fahrenheit. The crazy heat of the lightning bolt breaks atoms into pieces, leaving a wildly expanding plasma that creates a pressure ten times greater than the surrounding air. No wonder such storms don’t tiptoe quietly.