The most important thing about a wave is that it’s a way of letting energy move, but without also having to move air, water, or “stuff” of any kind. This means that waves can billow through our world very easily, disturbing things enough to be interesting and useful, but not so much that they’re shoving our world about and causing major disruption. A lightning strike liberates a lot of energy, and light and sound waves can carry some of that energy out into the rest of the world, sharing it out. Even though the air doesn’t go anywhere overall as the sound ripples past, huge amounts of energy are transferred onward. Light and sound are different types of wave, but the same basic principles apply to both. For example, both light and sound can be changed by the environment that they pass through. In the case of thunder, we can directly hear what’s happening to the waves.
My favorite place to be is about a mile from the lightning strike. Once the flash has signaled that the sound is on its way, I like to imagine that giant pressure ripple spreading out toward me. As I look out across the landscape, I can see right through the ripple, but it takes a few seconds to reach me with the first whipcrack of thunder. These sound waves are traveling at about 1,100 feet every second or 767 mph, which means they’re taking 4.7 seconds to cover a mile. That sharp crack is similar to the original sound made as the lightning bolt expanded right at the ground. But here’s what makes the sound of thunder so distinctive: What I hear just after the initial crack is the sound from slightly higher up the lightning bolt. It started as the same sound, but it took longer to reach me because it had to travel a sloping, and therefore longer, path. And then as the thunder rumbles on, I’m hearing the sound from higher and higher up that same lightning bolt. If it takes five seconds for the first crack to reach me, it’ll take two more seconds before the sound from one mile up hits me, and another four seconds before the sound from two miles up arrives. All these sound waves started off more or less the same, just in different places. And that means that as I listen, I can hear how the atmosphere is changing these waves. As time goes on, the only difference is that they’ve traveled farther. So the highest-pitched sounds, that first sharp crack, disappear very quickly as the high-frequency waves are absorbed by the atmosphere, but the lower-frequency waves rumble on. As time goes on, and the waves have traveled farther and farther, the overall pitch gets lower and lower, because the highest notes are consumed by the air, but the lowest notes just keep going. If you’re far enough away, the air takes it all and the sound never reaches you. But the lightning has a greater reach—these light waves are different and they don’t depend on the air for assistance as they travel. They don’t get absorbed by air as easily, but they can be altered in other ways as they whoosh through the world.
In a sense, waves are very simple. Once they’ve been made, they are always on their way to somewhere else. And whether they’re sound waves or ocean waves or light waves, they can be reflected or refracted or absorbed by their environment. We live our lives in the middle of this complex flood of waves, sensing the patterns in those that give us clues about our surroundings. Our eyes and ears tune in to the vibrations all around us, and those vibrations carry two very important commodities: energy and information.
ON A GRIM, gray, cold winter day, toast is the perfect comfort food. The only problem is that the gratification is not instant. I usually put the kettle on for tea, then put bread in the toaster, and then prowl the kitchen impatiently while I wait for my treat to be ready. After I’ve washed a mug or two and tidied the work surface, I often find myself staring into the toaster, checking on what it’s up to. The nice thing about toasters is that you can see they’re up to something, because the heating elements glow red. They’re not only heating up the air that touches them, they’re radiating light energy too. And this glow is a built-in thermometer. You can tell how hot the element is just from its color. This bright red tells me that the innards of my toaster have reached 1,800°F. That’s horrifically hot—enough to melt aluminum or silver. But if it’s glowing that bright cherry red, then 1,800°F is how hot it is. It’s a rule that comes from the way our universe works. Everything that is this temperature will glow the same color of red, and other colors indicate other temperatures. If you look into a coal fire and see the innermost coals glowing bright yellow, you know that they are around 4,900°F. Something that is white-hot is 7,200°F or above. But when you think about it, that’s odd. Why should color have anything to do with temperature?
While I’m staring into the toaster, I’m watching energy transform from heat to light. One of the most elegant things about the way the universe works is that anything that has a temperature above absolute zero is constantly converting some of its energy to light waves. And light must travel, so the energy whizzes out into the surroundings. The red-hot heating element is converting some of its energy into red light waves, at the long-wavelength end of the rainbow. But most of the energy it’s emitting has even longer wavelengths than that, and we call these waves infrared. Infrared is just like the light we can see, except that each wave is longer. We can only detect it indirectly, by feeling the warmth where it’s been absorbed. Even though we can’t see them, infrared waves are essential for a toaster—they are what heats the toast up.
Hot objects send out more light at some wavelengths than others. At any temperature, there’s a peak wavelength which accounts for most of the light, and the radiated light dies away on either side of that peak. The toaster is sending out a big bulge in the infrared, and the tail of the bulge is visible red. So I see red. I can’t see the light that’s heating my toast, but I can see the tail of longer wavelengths.
If I had some kind of super-toaster that could get even hotter, perhaps to 4,500°F, the heating elements would look yellow. That’s because the hotter object would send out light with shorter wavelengths, so the visible tail would include more of the rainbow: red, orange, yellow, and a little bit of green. When we see both red and green light together, we interpret that as yellow. Only something that has this temperature would send out this exact segment of the rainbow. And if the temperature increased even more—if I had a hyper-toaster that could get to 7,000°F – the light sent out would include the whole rainbow, all the way to blue. And when we see all the rainbow colors at once, we see white. So something that is white-hot is actually sending out a rainbow, but all the colors are mixed up. The disadvantage of the hyper-toaster is that it would melt pretty much whatever you made it out of. But it would brown your toast very quickly. And possibly your kitchen as well.