Storm in a Teacup: The Physics of Everyday Life(43)

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.

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.