Storm in a Teacup: The Physics of Everyday Life(46)

If you wanted an example of how entirely separate different wavelengths are from one another, it would be hard to come up with a better example than this. I realized that I must have written on that page in red ink earlier in the day. Under white light, it’s easy to see red ink on white paper. But under a red headlamp, red ink is invisible. The white paper reflected the red light back into my eyes. And the red ink also reflected red light back into my eyes. With my red headlamp, the page looked empty because the red light was bouncing back off all of it in exactly the same way. So I wrote new notes on the same page in blue ink. I could see the blue ink because it doesn’t reflect red light, so there was a contrast between ink and paper. If I had looked at the page with a blue headlamp, I would have been able to see the red ink but not the blue. Just as if I were turning a radio dial, I could have chosen what to read by choosing the color of illumination I was using. Red light has a longer wavelength than blue light. By selecting the wavelength to pay attention to, I was choosing the information I’d get.

In fact, this is exactly like tuning into a radio station. Most of the ways that we use to detect light (and other sorts of waves) will detect only a very narrow range of wavelengths. If a wave with a different wavelength goes past, we have no way of knowing that it’s there. My notebook made it obvious that this is true for the visible colors, but it’s just as true for the invisible colors. The world around us is absolutely flooded with different light waves, and they all just sit on top of each other like notes in different colored ink. They don’t interact with each other or change the other colors that are present. Each one is entirely independent. You can choose to detect very long-wavelength radio waves, and listen to a radio station. Or you can press the button on a remote control that sends out infrared signals that can only be seen by your television. Or you can write in red ink on a page. Or you can wait for your phone to see what Wi-Fi networks are available—each network is effectively being broadcast in a different color, but these colors have microwave wavelengths. This cacophony of information is there all the time, each wavelength just sitting on top of all the others. And it’s only if you look for information in the right way that you’d ever know it was there. We paint our picture of the world in a very narrow range of wavelengths, the visible colors of the rainbow. But these visible colors aren’t affected in any way by all the other colors out there.

The fact that waves with different wavelengths don’t affect each other is really useful. We can pluck out the interesting ones and be conveniently deaf to the rest. Each different wavelength is affected by the world around it in a different way. The world is sorting and filtering the waves, depending on their wavelengths. This is why, although I was brought up near gray, cloudy, rainy Manchester, where seeing the night sky was a rare treat, I lived only 14 miles from the biggest telescope in the UK. The Lovell telescope at Jodrell Bank is a huge radio telescope with a dish 250 feet in diameter. And even on the grayest Manchester days, when rainclouds stack up miles thick, this telescope has a perfect view of the sky. For visible light, with a wavelength less than a millionth of a meter entering a cloud is like entering a giant pinball machine. The light gets bounced and diverted, and is eventually absorbed completely. But the massive radio waves, exactly the same except that their wavelength is about 2 inches, sail straight through all those minuscule obstacles, completely unaffected. Next time you’re in Manchester in the rain, bear that in mind. Maybe it will provide some small comfort to think that astronomers can still see the majesty of the cosmos, even if you can’t even see the tops of the trees.** Or maybe it won’t.

Earth is only habitable because different wavelengths of light interact differently with the things they touch. Energy streams out from the hot Sun as a broad symphony of light waves, and our rocky planet intercepts a tiny fraction of the torrent. The energy carried by that tiny fraction is what keeps us warm. But if that was all there was to it, the Earth’s average surface temperature would be a frigid 0°F, rather than its current comfortable 57°F. What saves us from being permanently frozen is the Earth’s “greenhouse” effect. The way it works has to do with different wavelengths of light interacting with the atmosphere in different ways.

Imagine the view from a hillside on one of those cartoon-like days when the sky is mostly blue but there are a few puffy white clouds meandering along to add a bit of variety. If you’re looking out over flatter land, you can see green trees, grass, and dark earth. Sunlight illuminates the scene, apart from the shadows left by the clouds. But what’s reaching the ground in front of you is different from what left the incandescent Sun. The atmosphere has absorbed the long infrared wavelengths, and most of the shorter ultraviolet wavelengths, but the visible light has sailed through unaffected. The atmosphere has already selected the waves that reach the ground. It just so happens that they’re the ones we can see. At the visible wavelengths, the sky behaves as an “atmospheric window,” letting everything through. There’s another window for radio waves (that’s why radio telescopes can see the cosmos), but most of the other waves are blocked by the air.

The darker the land you can see, the more of those visible waves it’s absorbing. And the absorbed energy eventually ends up as heat. If you touch dark ground on a sunny day, you’ll feel that heat. The rest is reflected upward, back out through the atmospheric window. If any aliens are out there looking at us, that’s what they’ll see us with.