National Geographic : 2001 Oct
galaxylight and quasarlight across cosmic distances. We use the term light-year to express a unit of distance (about six trillion miles). But if you were the light itself-if you could be the photon-you'd experience no time. That long journey would be instantaneous. WHAT WE CALL LIGHT is really the same thing-in a dif ferent set of wavelengths-as the radiation that we call radio waves or gamma rays or x-rays. But in practice scientists often use the term "light" to mean the portion of the electromagnetic spectrum in the vicinity of vis ible light. Visible light is unlike any other fundamental element of the universe: It directly, regularly, and dramatically interacts with our senses. Our eyes each have about 125 million rods and cones-specialized cells so sensitive that some can detect a mere handful of photons. "About one-fifth of your brain does nothing but try to deal with the visual world around you," says Sidney Perkowitz. The position of the eyes, semipro tected in the case of the skull close to the brain, is testament to the importance of visual data. Light offers high-resolution information across great distances (you can't hear or smell the moons of Jupiter or the Crab Nebula). So much information is carried by visible light that almost everything from a fly to an octopus has a way to capture it-an eye, eyes, or something similar. It's worth noting that our eyes are designed to detect the kind of light that is radiated in abundance by the particular star-the sun-that gives life to our planet. Visible light is powerful stuff, moving at relatively short wavelengths, which makes it biologically convenient. To see long, stretched-out radio waves, we'd have to have huge eyes, like satellite dishes. Not worth the trouble! Nor would it make sense for our eyes to detect light in the near infrared (though some deep-sea shrimp near hot vents do see this way). We'd be constantly distracted, because any heat-emitting object glows in those wavelengths. "If we were seeing infrared," physicist Charles Townes told me one day,"all of this room would be glowing. The eye itself is infrared-it's warm. We don't want to detect all of that stuff." There is also darkness in the daytime-shadows. There are many kinds of shadows, more than you probably realize-certainly more than I realized until I consulted the shadow expert. I found him at the end of a long and winding drive through Topanga Canyon, just up the coast from Santa Monica, California. David Lynch is an astronomer. He's also the co-author of a book called Color and Light in Nature, in which I discovered something Moods darken when the sun goes into exile during winter in the northern Russian village of Lovozero. Ten in the morning looks almost like midnight (above). A shortage of light can unleash the winter blues, diagnosed as seasonal affective disorder (SAD). Med icating his blues with alcohol, a young man in nearby Murmansk ends up in the drunk tank (left).