Billions & Billions by Carl Sagan Read Free Book Online

to—the frequency is very high, about 600 trillion (6 × 10 14 ) waves striking your eyeballs every second. Because the speed of light is 30 billion (3 × 10 10 ) centimeters a second (186,000 miles per second), the wavelength of visible light is about 30 billion divided by 600 trillion, or 0.00005 (3 × 10 10 /6 × 10 14 = 0.5 × 10 -4 ) centimeters—much too small for us to see were it possible somehow for the waves themselves to be illuminated.

    As different frequencies of sound are perceived by humans as different musical tones, so different frequencies of light are perceived as different colors. Red light has a frequency of about 460 trillion (4.6 × 10 12 ) waves per second, violet light about 710 trillion (7.1 × 10 12 ) waves per second. Between them are the familiar colors of the rainbow. Every color corresponds to a frequency.
    As with the question of the meaning of a musical tone to a person deaf since birth, there’s the complementary question of the meaning of color to a person blind since birth. Again, the answer is uniquely and unambiguously a wave frequency—which can be measured optically and detected, if we so wish, as a musical tone. A blind person, properly trained and equipped in physics, can distinguish rose red from apple red from blood red. With the right kind of spectrometric library, she might be able to make much better compositional distinctions than the untrained human eye. Yes, there’s a feeling of redness that sighted people sense around 460 trillion hertz. But I don’t think that’s anything more than what it feels like to sense 460 trillion hertz. There’s no magic to it, as beautiful as it may be.
    Just as there are sounds too high-pitched and too low-pitched for us to hear, so there are frequencies of light, or colors, outside our range of vision. They extend to much higher frequencies (around a billion billion * —10 18 —waves per second for gamma rays) and to much lower ones (less than one wave per second for long radio waves). Running through the spectrum of light from high frequency to low are broad swaths called gamma rays, X rays, ultraviolet light, visible light, infrared light, and radio waves. These are all waves that travel through a vacuum. Each is as legitimate a kind of light as ordinary visible light is.
    There is an astronomy for each of these frequency ranges. The sky looks quite different in each regime of light. For example, bright stars are invisible in the light of gamma rays. But the enigmatic gamma ray bursters, detected by orbiting gamma ray observatories, are, so far, almost wholly indetectable in ordinary visible light. If we viewed the Universe in visible light only—as we did for most of our history—we would not know of the existence of gamma ray sources in the sky. The same is true of X-ray, ultraviolet, infrared, and radio sources (as well as the more exotic neutrino and cosmic ray sources, and—perhaps—gravity wave sources).
    We’re prejudiced toward visible light. We’re visible light chauvinists. That’s the only kind of light to which our eyes are sensitive. But if our bodies could transmit and receive radio waves, early humans might have been able to communicate with each other over great distances; if X rays, our ancestors might have peered usefully into the hidden interiors of plants, people, other animals, and minerals. So why didn’t we evolve eyes sensitive to these other frequencies of light?
    Any material you choose likes to absorb light of certain frequencies, but not of others. A different substance has a differentpreference. There is a natural resonance between light and chemistry. Some frequencies, such as gamma rays, are indiscriminately gobbled up by virtually all materials. If you had a gamma ray flashlight, the light would be readily absorbed by the air along its path. Gamma rays from space, traversing a much longer path through the Earth’s atmosphere, would be entirely absorbed before they reached the ground. Down here