A wave with no stuff
Every wave we have met so far needed something to do the wiggling. The rope wave needed a rope. The pond's rings needed water. Sound needs air — which is why, out in space, even the biggest explosion plays out in perfect silence. Take away the stuff, and there is nothing left to wiggle. No stuff, no wave. It seems like a rule you could trust.
Here, at last, is the wave that breaks it. Inside everything around you sit specks far too small to see, each holding a tiny electric charge — they're called electrons. Shake one up and down, and something remarkable spreads out from it: a ripple of electric-and-magnetic push, each part of the ripple nudging the next part into being. And this ripple needs nothing at all to travel through. No rope. No water. No air. Perfectly empty space suits it perfectly.
The proof hangs in the sky. Between the Sun and your face stretch ninety-three million miles with almost nothing in them — no air, no water, no stuff of any kind. Yet sunlight crosses the whole gap (the trip takes about eight minutes) and lands warm on your cheek. Light is one of these waves. And so — here is the heart of this entire book — is radio. Radio waves and light are the same kind of wave; they differ only in how fast they wiggle. Your eyes happen to be built to catch one slice of the family. A radio is built to catch another.
Fast. Really fast.
Chapter 1 made a promise: waves that cross a room a million times faster than a shout. Time to pay up. Through empty space, every member of this family — radio waves, sunlight, all of them — travels at exactly the same speed: about 186,000 miles every second. That is fast enough to go around the entire Earth seven times between one tick of a clock and the next.
Numbers that size slide right off the brain, so try this one instead. Say "hello" to someone across the room. By the time the sound has traveled the few inches from your mouth to your own ears, a radio wave that left your lips at the same instant is already a hundred miles away. Sound, which always seemed quick, turns out to be the slowpoke of the pair — that "million times faster" was barely an exaggeration. From here on, you can treat a radio wave's trip across a room as taking no time at all.
One family, very different personalities
If radio and light really are the same kind of wave, what makes the glow of your lamp so different from the chatter of Wi-Fi? One knob. The second knob from chapter 1: frequency. An AM radio station's wave wiggles about a million times every second. A Wi-Fi signal wiggles a few billion times every second. The light from your lamp wiggles hundreds of trillions of times every second. That is the entire difference.
And because every member of the family travels at the same speed, frequency drags something else along with it. A wave that wiggles faster doesn't reach you any sooner — instead, its ripples crowd closer together. How much room one complete wiggle takes up is called its wavelength: slow wiggle, long lazy waves; fast wiggle, short tightly packed ones. In the picture below, the amber bar measures exactly one complete wave.
A slow wiggle — each complete wave stretches out long
A fast wiggle — same speed, so the waves pack in short
An AM station's waves are each longer than three football fields laid end to end; the waves of lamplight are about a hundred times thinner than one of your hairs. (Tuck wavelength away somewhere safe — when antennas join the story, the size of a wave will matter a great deal.) Lined up from the slowest wiggle to the fastest, the whole family is called the spectrum — and you can go for a walk along it right now.
100 MHz
wiggles 100 million times every second
each wave is 3 m — about a doorway
Why Wi-Fi never bumps into FM
The air around you, right now, is thick with these waves. Every radio station in town is passing through your room this very second — and through you. So is every Wi-Fi network on your street, every phone call in your neighborhood, the quiet chirps of satellites overhead. All of them criss-crossing the same room, at the same moment, all at once. What actually happens where all those waves cross paths is a story strange enough to deserve a chapter of its own — it's next.
It sounds like chaos. It should be chaos. Yet every signal in that crowded air arrives perfectly intact, still wiggling at its own faithful speed. And a receiver can be built to listen for one wiggling-speed and politely ignore all the others — the way you could pick out a friend humming one low, steady note across a room full of high, fast chatter. Choosing which speed to listen for is called tuning — it is what the dial on a car radio has always done — and exactly how a circuit pulls it off is a story for a later chapter.
Different frequencies are different lanes on an invisible highway. Thousands of signals can share the same air at the same moment without ever colliding, as long as each one keeps to its own lane. That is why every country has a keeper of the airwaves who divides the spectrum into lanes and hands them out — one stretch for AM stations, another for FM, others for pilots, ships, phones and Wi-Fi — so that nobody hums on top of anybody else.
The carrier, found
Two chapters of ropes, ponds and sound, and now, at last, radio's own wave: invisible, needs nothing at all to travel through, crosses your room in no time worth counting, and comes in a million separate lanes. Because its job will be to carry a message from one place to another, engineers gave this wave a fitting name: the carrier.
There is just one problem left, and it is a funny one. A perfect, steady sine wave says absolutely nothing. Wiggle, wiggle, wiggle — every cycle identical to the last, forever. Listen to it for an hour and you learn nothing you didn't know in the first second. It doesn't speak; it only hums. To make it carry a voice, we will have to disturb its perfection on purpose. That trick is older than your great-grandparents' radio, and it is coming soon — but not quite yet. First, there is one more thing you need to see: what happens when waves share the same air. There is agreement in it, and argument, and a little magic — and it is waiting in the next chapter.