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our societies have not yet caught up.
    We place an overseas call, and we can sense that brief interval between when we finish asking a question and when the person we’re talking to begins to answer. That delay is the time it takes for the sound our voice makes to get into the telephone, run electrically along the wires, reach a transmission station, be beamed up by microwaves to a communications satellite in geosynchronous orbit, be beamed down to a satellite receiving station, run through the wires some more, wiggle a diaphragm in a handset (halfway around the world, it may be), make sound waves in a very short length of air, enter someone’s ear, carry an electrochemical message from ear to brain, and be understood.
    The round-trip light travel time from the Earth to geosynchronous altitude is a quarter of a second. The farther apart the transmitter and receiver are, the longer it takes. In conversations with the
Apollo
astronauts on the Moon, the time delay between question and answer was longer. That was because the round-trip light (or radio) travel time between the Earth and the Moon is 2.6 seconds. It takes 20 minutes to receive a message from a spacecraft favorably situated in Martian orbit. In August 1989, we received pictures, taken by the
Voyager 2
spacecraft, of Neptuneand its moons and ring arcs—data sent to us from the planetary frontiers of the Solar System, taking five hours to reach us at the speed of light. It was one of the longest long-distance calls ever placed by the human species.
    —
    In many contexts, light behaves as a wave. For example, imagine light passing through two parallel slits in a darkened room. What image does it cast on a screen behind the slits? Answer: an image of the slits—more exactly, a series of parallel bright and dark images of the slits—an “interference pattern.” Rather than traveling like a bullet in a straight line, the waves spread from the two slits at various angles. Where crest falls on crest, we have a bright image of the slit: “constructive” interference; and where crest falls on trough, we have darkness: “destructive” interference. This is the signature behavior of a wave. You’d see the same thing with water waves and two holes cut at surface level in the pilings of a pier on a waterfront.
    And yet light
also
behaves as a stream of little bullets, called photons. This is how an ordinary photocell (in a camera, for instance, or a light-powered calculator) works. Each arriving photon ejects an electron from a sensitive surface; many photons generate many electrons, a flow of electric current. How can light simultaneously be a wave and a particle? It might be better to think of it as something else, neither a wave nor a particle, something with no ready counterpart in the everyday world of the palpable, that under some circumstances partakes of the properties of a wave, and, under others, of a particle. This wave-particle dualism is another reminder of a central humbling fact: Nature does not always conform to our predispositions and preferences, to what we deem comfortable and easy to understand.
    And yet for most purposes, light is similar to sound. Light waves are three-dimensional, have a frequency, a wavelength, and a speed (the speed of light). But, astonishingly, they do not require a medium, like water or air, to propagate in. Light reaches us from the Sun and the distant stars, even though the intervening space is a nearly perfect vacuum. In space, astronauts without a radio link cannot hear each other, even if they are a few centimeters apart. There is no air to carry the sound. But they can see one another perfectly well. Have them lean forward so their helmets touch, and they
can
hear one another. Take away all the air in your room and you will be unable to hear an acquaintance complain about it, although you will for a moment have no difficulty seeing him flailing and gasping.
    For ordinary visible light—the kind our eyes are sensitive