Storm in a Teacup: The Physics of Everyday Life(48)

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?

Imagine looking down on a busy city. A man walking down the street pulls a phone from his pocket, taps at the touchscreen, and holds the phone to his ear. Now, add a superpower to your sight, the ability to see radio waves of different wavelengths as different colors. Green waves ripple outward in all directions from the man’s phone, brightest and strongest at the phone itself, but dimming as they travel away. There’s a mobile phone base station 100 yards away, and it detects the green waves and decodes the message, identifying the number he wants to contact. Then the base station sends out its own signal back to the man’s phone, another green ripple, but the color of this new signal is fractionally different from the original green. This is the first trick of modern telecommunications. Whereas the Titanic could only send a signal that was a mixture of lots of different wavelengths, our technology is now incredibly precise about which wavelengths are sent and received. The wavelength of the original signal from the phone was 13.412 inches, and the wavelength used to send the return signal was 13.409 inches. The phone and the base station can listen and talk on channels with wavelengths that are different by only a tiny fraction. Our eyes can’t distinguish color with anywhere near the same precision. But like the red and blue ink on my white paper, those waves are separate and don’t interfere with each other. As the man walks down the street, the green waves rippling out from his phone carry a pattern, the message that is being relayed on. A woman across the street is also talking on the phone, using a fractionally different wavelength again. But the base station can distinguish between the two. This is why the government sells bandwidth as a range; if your phone network is using that range, you’re free to make the differences between channels as tiny as you like, as long as the hardware can separate them out. So as we look down on this section of the city, we see lots of bright spots, as phones send out their signals. The signals are bouncing off buildings and slowly being absorbed by the surroundings, but most of them reach a base station before they get too weak.

As the man we were watching walks down the street, away from the base station, we start to see new colors. The streets ahead of him are full of red radio splotches, all centered on the next base station, which is sending out many shades of red to the phones around it. As the strong green signal from the first base station fades, the man’s phone detects the new frequencies and starts communicating with the new base station. He has no idea he’s reaching the edge of the “green” section, but as he does so his phone switches wavelengths so that it’s now sending out shades of red. These aren’t picked up by the original green base station, but they are relayed on by the new red one. If he keeps walking, he might walk into areas where the radio waves look like green or yellow or blue to us, with our superhero radio vision. No two patches of the same color touch; but if he walks even farther, he might walk into a new green area. This is the second trick of our mobile phone networks. By keeping the signal strength very low, we make sure that the signals can only reach the nearest base station. That means that a little way farther on, you can have a new station, using the same green frequencies. But the signals from the two green stations are too weak ever to reach each other, so there’s no problem with interference. Information flows to and from the center of each cell (that’s what the patches around each base station are called),?? but doesn’t interfere with the information from other cells. It doesn’t matter that everyone is talking at once, because they’re all talking using slightly different waves. And the technology can separate out all these conversations by tuning its receivers with incredible precision. If your phone sends signals at a wavelength that’s wrong by a tiny fraction, the message will never get through. But the incredible precision of modern technology means that the tiniest subtleties are enough to tell the waves apart.