Zoom: From Atoms and Galaxies to Blizzards and Bees: How Everything Moves (27 page)

The frenetically expanding gas generates a wide spectrum of sound. But the high notes dissipate right away. In a matter of feet they’re finished. Treble waves pulse quickly and just can’t be sustained. That’s why, when a teenager’s car with its pounding radio passes by, you only hear the thudding bass. The music’s highs don’t even survive the thirty-foot distance to your curbside ears. It also explains why foghorns are designed to produce only low tones. These travel far. A high pitch would be futile.

The thunder’s sound therefore grows deeper the farther it travels. Thus a lightning bolt’s sound track reveals the flash’s location in three ways: by its loudness, by how sharply defined (as opposed to muddy) the noise is, and by the pitch. If you’ve ever almost been struck—in which case the sound and flash would be simultaneous—you’ve heard a much more balanced musical composition, with lots of sharp, crackly high tones. This actually happened right outside my home office just after this book was completed, in time to be included here. The flash and earsplitting crack were perfectly synchronous, as if nature were saying, “You want to relate the experience firsthand? Okay, here you go!” And indeed, the deafening explosion’s pitch wasn’t in the bass range at all. When thunder’s rumble is low and indistinct, it’s always more than two miles away.

For real precision, however, the old rule still stands. You count the seconds between the flash and the first sound. Each second means the bolt is another 1,100 feet away. Five seconds pegs the lightning at a mile—almost on the nose.

Everyone can readily perceive a light-sound gap of one-eighth of a second, which corresponds to 150 feet. So when the flash and the boom seem truly simultaneous, the bolt is nearer than half a short city block, a literal stone’s throw. A close call indeed.

When we speak of the speed of sound we normally mean the speed at which noise moves through air. But sound travels differently through various substances, even through other gases. We acquire a munchkin voice after inhaling a little helium from a party balloon because the sound of our voices speeds up to a wild 3,200 feet per second through helium, or three times faster than it travels through normal air. Sound zooms even faster through liquids and nonporous solids. It travels 4.3 times more quickly through water and fifteen times more quickly through the steel in railroad tracks than it does through air.4

Meanwhile, the lightning bolt, the cause of all this sound and fury, travels to the observer at the speed of light—a million times faster than sound. Essentially it arrives instantaneously. To be precise, a mile-away bolt is seen 0.000005 seconds after it occurs.

For millennia, light was assumed to enjoy a speed so fast it couldn’t be measured. It was perceived to arrive at distant locations at the same instant of its creation. When the truth finally leaked out in the seventeenth century, light became better understood in some ways but also grew curioser and curioser to those who studied it. Even today, few among us who are not science teachers can come right out and state what light really is. It is easier to rattle off its velocity—186,282.4 miles per second—than what it’s made of. That’s true whether we think of light as a particle or a wave.

A wave may seem like something that moves in a visually obvious fashion, but it never involves the forward movement of anything. No actual substance is advancing. As an ocean wave passes over a particular piece of embedded kelp, the vegetation just bobs up and down. So as we saw in chapter 13, while a wave has a forward motion, the water it’s made of does not.

The same is true of sound. A friend shouts hello from across the atrium of a mall. But nothing has traveled from him to you. He merely created a disturbance in the air in front of his mouth, where one molecule jostled the next and so on until the molecule adjacent to your own eardrum caused that membrane to vibrate. No physical object, not a single atom, traveled even an inch.

A classic demonstration of this involves a long rope hanging from the top of a flagpole or scaffolding. By giving the bottom a snap, we create a beautiful wave pattern that fluidly scampers all the way up. It looks like a sine wave performing a lively vertical motion, but in reality each portion of the rope is merely waving back and forth.

So with light, the question right from the beginning became: What exactly is moving?

The ancient Greeks believed light was a beam that traveled outward from the eye. But other early thinkers thought vision was an interplay between this eye beam and something emitted by light sources such as the sun. The Greek who came closest to the truth was Lucretius. In his On the Nature of Things, he wrote, “The light and heat of the sun are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove.”

Lucretius’s view of light as particles—ultimately supported by Isaac Newton—included that profound “lose no time” clause, which meant simultaneity. In any case, light remained popularly regarded as just an eye phenomenon for centuries to come.

It took a full millennium for anything to change. The next true breakthroughs came from Alhazen, whom we’ve already met for his accurate appraisal of the atmosphere. Around 1020 CE he said that vision results solely from light entering the eye; nothing emanates from the eye itself. His popular pinhole camera lent weight to his arguments. But Alhazen went much further. Elaborating brilliantly, he said that light consists of streams of tiny, straight-moving particles that come from the sun and are reflected by objects. He insisted that light travels at a fast but finite speed. Refraction—the bending of light, as when the setting sun looks distorted—is caused, he said, by light slowing down as it passes through substances of progressively greater densities, such as the thick air near the horizon.

Alhazen was absolutely right. His on-the-nose conclusions were six centuries ahead of anyone else’s. For example, Kepler, in 1604, made astute observations about light but nonetheless believed that it moved infinitely fast, and a generation later Descartes seconded this wrong view. Worse, Descartes kept publishing arguments for infinite speed and announced that he would “stake his reputation on it.”

The day arrived when certainty became an illusion. Yet these great men shouldn’t be mocked in hindsight: infinite speed was a very far-out concept. We can all imagine an extremely fast entity; everyone knew that at the very least, light had to be off-the-charts speedy. But something arriving as soon as it departs? Requiring no time at all? This would make light unique in all of nature. (Turns out quantum phenomena go infinitely fast, as we’ll see in the penultimate chapter.)

Meanwhile the “What is it?” debate raged on. It grew heated, almost taking on the quality of a food fight. In the late seventeenth century Newton joined Kepler in arguing that light is a stream of particles, while the likes of Robert Hooke, Christiaan Huygens, and, soon, Leonhard Euler insisted it’s a wave. Of course, if it is, what is it a wave of? These Renaissance scientists were thus forced to believe that space is filled with a plenum (later called an ether) because there had to be a substance that actually waves.

One obvious fact managed to sway many in favor of Newton’s particle idea. When light from an angularly small or distant source such as the sun passes a sharp edge, such as the wall of a house, it casts a sharp-edged shadow on nearby objects. That’s what particles moving in a straight line should do. If, instead, light were made of waves, it ought to spread out—diffract—as ripples do and as sea waves do when passing a jetty. Sharp-edged shadows, added to Newton’s reputation as a genius, made the wave proponents seem like nut jobs.

Meanwhile the finite-versus-infinite uproar finally ended when the Danish observer Ole Rømer determined light’s speed in 1676. His idea was simple. Anyone with a small telescope could watch Jupiter’s four large moons speed up and slow down as they whirled around the giant planet during their 399-day cycle. Meaning that for about half a year they moved faster than they did during the other half. This made easily observed events, such as each moon passing in front of Jupiter, occur up to fifteen minutes “early” or “late” compared to its average orbital speed. The moons zoomed faster whenever Earth approached Jupiter. Conversely, the moons grew strangely sluggish when Earth chugged away.

Something went boing in Rømer’s mind, and he dropped his pastry in midbite. Each image of Jupiter’s animated environment had to travel farther to reach us whenever Earth was flying away from it! At such times our two worlds grew nineteen miles farther apart each second. Every “frame” in the movie, as we might visualize it today, needed to go farther than the last, and this took a bit of time. Of course the scenes would then seem to run in slow motion. The delay proved that light isn’t infinitely fast.

The great Dane calculated light’s speed as 140,000 miles per second. Since the correct speed of 186,282 miles per second was not determined for another two centuries, Rømer did well to underestimate it by a mere 25 percent. Indeed, there was no way to do better without knowing Jupiter’s true distance from Earth, which wasn’t reasonably revealed until another three generations had come and gone.

This is not the place to recount the full, fascinating story of geniuses such as Augustin-Jean Fresnel, Siméon-Denis Poisson, Michael Faraday, James Clerk Maxwell, Max Planck, and Albert Einstein, who each made brilliant breakthroughs in understanding light. Or the quantum-mechanics gang—Satyendra Nath Bose, Niels Bohr, Louis de Broglie, Werner Heisenberg, and Erwin Schrödinger—who made it clearer but stranger. Our mandate is solely that of speed and motion.

Still, a couple of minutes can clarify what, exactly, is moving.

The particles-versus-waves controversy? As if some wise King Solomon ruled nature, everyone was soon declared right.5 Scottish physicist and mathematician James Clerk Maxwell showed that light is a self-sustaining wave of magnetism and an electric pulse positioned at right angles to it. They go together, creating each other in a mutually nurturing way. Light is thus justifiably called an electromagnetic phenomenon. Unlike sound, light is not a disturbance in some medium. Light exists on its own. It’s quite content to fly through the vacuum of space.

The “electro” part of the word, helpfully, sounds like “electron,” the first subatomic particle discovered, in 1899. That’s no coincidence. Turns out light is born one way and one way only. If an atom gets struck by energy, this excites its electrons, which give a figurative yelp and jump to an orbit farther from the nucleus. They don’t like to be there. So in a fraction of a second they fall back to a closer orbit. As they do so, the atom loses a bit of energy. This is instantly converted into a bit of light, which materializes out of the emptiness like magic and then rushes away at its famous speed.

That’s the only way light is ever born. Out of seeming nothingness, whenever an electron moves closer to its atom’s center. Simple, really. But ask all your friends how light is created and you’ll get blank stares.

So light is a wave of electricity and magnetism. At least, that’s the best way to visualize it as it’s traveling. But when it starts off and also when it collides with something, it instead acts as a tiny bullet, a massless particle, a photon. Nowadays one may call it a photon or a wave and be equally correct. However you slice it, there’s a lot of light in the cosmos—one billion photons of light for every subatomic particle of matter.

Anyway, the quantum guys showed that solid objects such as electrons can behave as waves of energy, too. When an observer uses an experimental apparatus to determine the location of the photons or the electrons in an atom, the photons and electrons will always behave as particles and do things only particles can do, such as pass through one little hole or another but not both at once. But when no one’s measuring where exactly each is situated, they behave as waves that blurrily pass through both holes simultaneously to create an interference pattern on a detector on the other side—which only waves can do.

Thus the observer plays a critical role in what he or she sees. Most physicists now think that a human consciousness is required to make an electron’s “wave function” collapse so that it occupies a particular place as a particle. Otherwise it’s just an indefinite probability item with neither location nor motion.

But does an electron’s wave function collapse and turn into an actual particle if a cat is watching? Would light always be waves and never discrete photons if no one were around? Our best answers are “Who knows?” and “Yes,” respectively, but obviously this whole business is Wonderland-strange.

Light’s speed is counterintuitive, too. In a vacuum a photon always travels 186,282 miles in a second. Its reputation as a constant is well deserved, but only when it’s flying through the emptiness of space. When passing through denser transparent media, such as water or glass, photons seem to slow down. Seem? Well, do they slow or don’t they?

You decide.

Light moves at just two-thirds its regular velocity, at “just” 120,000 miles a second, through glass and at 140,000 miles per second through water. This speed change is not at all subtle or cerebral. It makes fish appear in illusory locations in a fishbowl and causes a spoon to appear bent in a half glass of water. The density of glass allows a bottle of soda pop to seem to hold much more than it really does.

But look more closely: photons are colliding with the material’s atoms, getting absorbed, and then new photons are created to continue on. The images you see through your window are made of different photons from the ones that originally struck the far side of the glass. Since the process of absorption and reemission takes a tiny bit of time, light requires longer to pass through a window than it does through air. But between the glass’s atoms each photon really still zooms at its famous, superfast, constant speed.

Each of light’s colors traverses clear materials at its own speed. This variance makes their paths diverge. Alhazen knew this a millennium ago.

The bending of each color’s path as a result of its separate speed is called refraction. We see this when sunlight hits a prism and its component colors get bent into a gorgeous, spread-out spectrum on a wall. Newton explained this by creating a slam-dunk apparatus that silenced all disbelievers. Before him, everyone thought glass merely introduced a distortion that cooked up the colors. Newton won them over by having the colors hit a second, reversed prism that rebent and recombined them. White light emerged. If glass created color as a result of distortion, then Newton’s double prism would have produced more distortion. Instead he proved that when we see white it’s our eye’s response to the mixture of all colors.