Occasionally, an irritant finds its way into the wrong bit of the oyster. Since it has no way of expelling the intruder, the oyster coats it in something harmless, the same stuff that it coats the inside of its shell with. It’s the mollusk version of sweeping something under the carpet, except that it makes the carpet to fit rather than using one that’s already there. The coating is made of tiny flat platelets cemented together with organic glue and stacked up on top of each other. Once it has begun the coating process, the oyster just keeps going. It was recently discovered that the pearl turns as it forms, going around perhaps once every five hours. The tides and seasons come and go, sharks and manta rays and turtles pass by overhead, and the oyster quietly sits there, filtering the ocean as the growing pearl pirouettes slowly in the dark.
Serenity reigns for years, until our oyster has a terribly bad day and is yanked out of the ocean by a human and pried open. As sunlight hits the pearl for the first time, waves of light bounce off its shiny white surface. But they don’t just bounce off the platelets at the top; some make it through to the next few layers and bounce off those instead, or maybe bounce a few times inside the layers before they make their way out. So now we have a situation where there’s a single type of wave—let’s consider just the green light from the sun—and it’s overlapping with other waves of exactly the same type. The waves still don’t affect each other, but they do add up. Sometimes, the green light wave that bounces off the top surface lines up exactly with the green light wave that has bounced off the next surface down. The peaks and the troughs of the wave shape match perfectly. So they keep going out into the world together, a strengthened green wave. But perhaps the red light arriving from the same angle, and bouncing off the layers in exactly the same way, doesn’t line up as perfectly. The peaks from one red wave line up with the troughs of the other red wave. Add them together, and there’s nothing left to travel in that direction.
These platelet layers are the reason that an obscure filter-feeding mollusk from the South Pacific can make something that’s sought after by the most glamorous individuals in human society. The layers are so thin and so tiny that they’re just the right size to affect how the light waves line up. The important role they play is to shuffle light about a bit so that waves of the same type overlap. The waves add up (a physicist would say that they interfere with each other), and the result is colored patterns. From some angles, the reflected light waves reinforce themselves, and so we see shimmers of pink and green from the shiny white surface. At other angles, it might be blue that lines up, or no color at all. As the pearls are turned in the sunlight, we see the flashes that come from the added-up waves. This is what we call iridescence—a mysterious-looking shininess that is highly prized by humans because it’s so rare and so beautiful. What’s happening is that the pearls are creating an irregular pattern of light waves, and as you move past them, you see different parts of the pattern. But it looks to us almost as though the pearls are glowing, and we love it. More recently, humans have learned to engineer the world on this scale for themselves. But even these days, we still mostly get the oysters to do the hard work for us.
Pearls show what happens when waves of the same type overlap. Sometimes the crests and the troughs line up and add together, making a stronger wave traveling in a specific direction. Sometimes they cancel out, so there’s no wave in that direction at all. A new wave pattern is going to result whenever there’s anything for waves to reflect from, or when there’s more than one source of waves (think of the overlapping ripples from two identical pebbles dropped into a pond next to each other).
But this raises some questions. What happens when other sorts of identical waves overlap? What about mobile phones? We’ve all seen clusters of people standing close together, all having phone conversations with different people, but using identical phone models. They are connected to the world via waves, the same types of wave as hundreds or thousands of other people in the same city. Radio communication as the Titanic went down was hampered because twenty ships in the whole of the North Atlantic were all using the same technology and the same type of wave to send out signals. But today you could have a hundred people in a single building all having separate conversations on identical mobile phones at the same time. How have we managed to organize this cacophony of waves to make that possible?