So a toaster is just a way of making waves. The red light waves that you see are just some of the waves that it has made because of its temperature. The infrared waves that you can’t see heat up your toast. This is why toast only browns at the surface in a toaster; it’s only the bits that the light touches that can absorb infrared and heat up. The reason I’m quite happy to stare at the toaster while I’m waiting is that I’m imagining all the light it’s giving out that I can’t see. I know it’s there, because the red glow is a giveaway.
But of course, there’s a catch. The problem with this method of generating light waves is that you always get the same set of waves together. There’s no way to choose some of them but not others. An orange-hot coal and molten steel and anything else that’s 2,700°F must emit the same collection of colors all together. So you can measure the temperature of something by its color, when it’s hot enough for you to see the colors. The surface temperature of the Sun is about 9,900°F—that’s why it gives out white light. In fact, this is the only reason we can see stars in the night sky; they’re so hot that light must pour out from their surface and across the universe, light with a specific color that gives their temperature away.
And we—you and I—we also have a color because of our temperature. It’s not a color that we can see, but it’s visible to special cameras adapted for the right sort of infrared. We’re much cooler than the toaster, but we’re still glowing. We emit light waves with wavelengths that are mostly 10–20 times longer than visible light. Each of us is a lightbulb in the infrared, just because of our body temperature. And so are dogs and cats and kangaroos and hippos—all warm-blooded mammals. Anything and everything that is above absolute zero (the scarily cold temperature of ?459°F) is a light bulb like this, with the color crossing from the infrared to even longer wavelengths (the microwave range) as the temperatures get colder.
So we live our lives bathed in waves, and not just the ones we can see, the ones that might catch our eye if we look in the right direction. The Sun, our own bodies, the world around us, and also the technology we create are constantly making light waves. And the same goes for sound waves—high notes, low notes, the ultrasound that bats use to hunt, and the infrasound that elephants use to follow the weather. The amazing thing is that all of these waves can be traveling through the same room, and none of them will interfere with any of the others. The sound waves are the same whether a room is completely dark or full of disco lights. The light waves aren’t affected by piano concertos or screaming babies. All of this is what we tap into when we open our eyes and use our ears. We’re just siphoning off some of the useful bits from the flood, selecting the waves that send us the most useful information.
But which ones do you choose? The answer will be different for the newest self-driving cars and for an animal that needs to survive in a forest. There’s a huge richness of information out there, and you can pick and choose which of the waves will help you most. That is why blue whales and bottle-nosed dolphins can hardly hear each other, and also why neither of them gives a hoot about the color of your wetsuit.
THE GULF OF CALIFORNIA stretches along the western coast of Mexico, a narrow ocean haven 700 miles long that opens into the Pacific at its southern end. The blue water of the channel is protected by dark, raw mountain peaks that poke into the sky from both shores. Marine species migrate vast distances across the oceans to feed and to rest here. Bobbing about in a small boat in the middle of the channel, a fisherman can appreciate the peace. And what peace means is that the flood of waves that bathe that fisherman is low-key and relatively uncomplicated. Light streams from the Sun during daytime, reflected only by blue water and burnished rock. The lapping waves and the creaking of the boat send out the only sound waves. A lone dolphin leaps out of the water, briefly a part of this calm world, and then splashes back down into a completely different world and one that certainly isn’t calm. Down below is the loud, bustling hubbub of an ecosystem at work and play.
The dolphin sends out a high-pitched whistle as it dives downward, communicating to the rest of the pod following on behind. And as the pod catches up, the water is filled with clicks, short sharp waves sent out from the forehead of each dolphin that bounce off the surroundings. Those that make it back to the dolphin are transmitted through its jawbone to its ear, letting each animal build up a picture in sound of what’s nearby. The whistles and squeaks and clicks make this sound like a busy street. These are the sound waves of a community on the move. After spending a while at the surface, breathing and playing, the pod turns downward toward the deepest, darkest blue, on a mission: the hunt. The light waves that were so common above the surface are much less common down here. Light waves are absorbed by water very quickly, so information from light is scarce. The dolphins have eyes that can cope both above and below water, but the measure of light’s usefulness to them is shown in how that eye has evolved. They have no ability to distinguish color at all—why would you need it to when there’s hardly any variety in the color of your world? Their world is blue, but they will never know that. A dolphin can’t see the color blue, so their watery world looks black. But they can see the bright glints of passing silvery fish, so they can see what they need.
The ocean surface is like an Alice-in-Wonderland mirror, separating two worlds but easy to step through. Waves tend to bounce off the interface, so sound from the air stays in the air, and sound from the ocean stays in the ocean. In air, light travels very easily, and sound travels reasonably well. In the ocean, light waves are absorbed very quickly, but sound waves pass through quickly and efficiently. If you want to learn about your environment in the ocean, you need to detect sound waves. Light waves are often of little use unless you’re looking at something very close to you and near the surface.