But you can only make sense of the flood of waves if you have some way to separate out the many messages being sent. Fortunately, the waves themselves provide the answer, and you don’t need any specialist kit to see it for yourself.
The Great Smoky Mountains in Tennessee are spectacular, a huge stretch of valleys and peaks blanketed by deep green forest. The serenity and untouched feel of the forest were particularly striking because to get there we had to drive through Dolly Parton’s home town. I knew of the great country singer, of course, but I wasn’t prepared for the sight of Dollywood, a huge theme park celebrating Tennessee, country music, fairground rides, and of course Dolly herself. And that’s just the epicenter. Pink cowboy hats, highly decorated guitars, and an all-pervasive background of country music seep into the surrounding towns, along with big blond hair, vintage denim jackets, and a big Southern welcome. Bourbon after dinner seemed to be a cultural imperative, although secretly I’d have preferred a cowboy hat. But it all changed when we got up into the mountains the following day. Crowds flocked in with deck chairs and drink coolers, and quietly set themselves up to watch the forest. Anything other than complete darkness would spoil the show, so all lights were extinguished, and flashlights and phones were forbidden. As dusk fell, the dance of the fireflies began. The forest sparkled with the flashing of millions of tiny broadcasting insects. We were there to make a science documentary, and we had just one evening to capture the whole event. The problem with filming something like this is that you need to move about and to be able to see where you’re going. We had been told that if we really had to, we could use red lights, as apparently they won’t disturb the fireflies as much as white lights. So we crept around the forest, dimly glowing red. By about 1 a.m., the fireflies had mostly stopped, and we were preparing to film the last segment. While the director and cameraman were setting up lights, I parked myself in a pitch-black clearing with my red headlamp, huddled under a piece of blackout cloth because of the cold, and scribbled notes about what I was going to say. When the others were ready, I went to join them, and opened my notebook for a last reminder of the ideas I’d had. But under the director’s white headlamp, I couldn’t read my notes. On the page were two sets of scribbles, one in red pen and one in blue, right on top of each other. It was impossible to read either one.
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.