Storm in a Teacup: The Physics of Everyday Life(40)

While the Hawaiians were chanting to urge the flat, windless sea to rise into ready-to-surf swell, the ocean thousands of miles away could have looked very different. The winds in massive storms shove on the ocean surface, dumping energy by forcing the water up into waves. But the waves in storms are confused mixtures of short and long waves traveling in different directions, breaking and rebuilding and clashing. Winter storms are common at a latitude of about 45°, so the storms would be to the north of Hawaii in the northern hemisphere winter, and to the south of Hawaii in the southern hemisphere winter. But waves have to travel. Even as the storm winds were dying down, the patch of ruffled ocean would have been expanding out past the edges of the storm and into undisturbed water. Out here, a sorting process could take place. The true nature of the confused mess would be revealed—not jumbled chaos, but a crowd of different wave types all sitting right on top of each other. Water waves that have a longer wavelength (that’s the distance between peaks) travel faster than those with a shorter wavelength. So the first waves to escape would be the longest, racing outward ahead of their shorter cousins. But there is a price to pay as a water wave travels. Energy will gradually be stolen by the surroundings, and the price per mile is higher for the shortest waves. Not only are they losing the race, they’re losing their power as well, and it doesn’t take too long for them to vanish. Thousands of miles from the storm and days later, all that remains are the longest waves, a smooth regular swell, radiating out across the planet.

So Hawaii’s first advantage is being in a spot far enough away from the massive storms to experience them only in the form of that residual smooth, tidy, long-wavelength swell. Its second advantage is that the Pacific Ocean is very deep and islands’ volcanic sides are steep. Waves travel across the ocean surface undisturbed until they suddenly meet a steep slope. Then all the energy that was spread over a huge depth has to become more concentrated in the shallows, so the height of the waves must increase. And very close in to shore, the Hawaiians were waiting for the last gasp of these slow monsters, as the waves became so steep that they had to break over the perfect beaches of the islands. And as they broke, the kings and queens were ready with their surfboards.

Water waves are probably the first waves that most people are aware of. Something that a duck can bob about on is easy to imagine and to understand. But waves come in lots of different types, and many of the same principles apply to them all. All waves have a wavelength, a measurable distance between one peak and the next. Because they’re moving, all of them also have a frequency, the number of times they go through a cycle (peak to trough and back to peak again) in one second. All waves have a speed, too, but some of them (like the water waves) travel at different speeds depending on their wavelength. The problem with most waves is that we can’t see what’s doing the waving. Sound waves travel through air, and they’re compression waves; instead of a moving shape, what’s passed along is a push. The hardest waves to imagine are the most common of all: light waves, which move through electric and magnetic fields. But even though we can’t see electricity, we can see the effects of light being a wave all around us.§

One of the main reasons that waves are interesting and useful is that the environment they’re passing through often changes them. By the time a wave is seen or heard or detected, it’s a treasure trove of information because it carries the signature of where it’s been. But that signature is only stamped in relatively simple ways. There are three main things that can happen to a wave: It can be reflected, it can be refracted, or it can be absorbed.


IF YOU WANDER past the fish counter at a supermarket and look at what’s on offer, what you see is mostly silver. The exceptions to the rule are tropical fish like red mullet and red snapper, and the bottom-dwelling fish such as sole and flounder. But mostly, you’re looking at fish that swim in the open ocean in big schools, like herring, sardines, and mackerel. Silver is interesting because it isn’t really a color. It’s just our word for something that acts as a trampoline for light, bouncing it back out into the world. All waves can be reflected, and almost all materials reflect some light. What’s special about silver is that it sends everything back indiscriminately. Every color is treated in the same way, no exceptions. Polished metal is really good at this trick, and it’s useful because the angle at which the light arrives is the angle at which it leaves. If you take an image of the world and use a mirror to bounce it in a different direction, the relative angles of all those light rays stay the same. It’s difficult to polish metal perfectly enough to get a perfect image, and mirrors have been very highly prized in human history. And yet we take silver fish for granted. The fish can’t even use metal; in order to be silvery, they’ve got to build structures that do the same job out of organic molecules. That’s complicated, and therefore expensive in evolutionary terms. If you’re a herring, why do you bother?

Herring roam the seas in schools, feeding on small shrimp-like creatures and hoping to avoid the big carnivores: dolphins, tuna, cod, whales, and sea lions. But the oceans are huge, empty places with nowhere to hide. The only solution is invisibility, or the closest that nature can come to it: camouflage. So should fish be blue, to match the watery background? The problem with that is that the exact hue depends on the time of day and what’s in the water, so it changes all the time. But the herring absolutely must look like the water behind them, in order to survive. So they turn themselves into swimming mirrors, because the empty ocean behind them looks exactly like the empty ocean in front of them. They can reflect 90 percent of all the light that falls on them, similar to a high-quality aluminum mirror. By bouncing light waves back out into the eyes of potential predators, a herring can swim about behind a shield made of light.