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useful. But spectral lines turned out to have another crucial property. They’re exquisitely sensitive motion detectors. You can think of light as waves of electromagnetic energy. In the visible spectrum, the more tightly packed the waves are—which is to say, the shorter the distance between them, or the shorter the wavelength—the closer a color is to the violet end of the rainbow. If the waves are more loosely packed, they’re closer to the red end. The same goes when you go beyond the visible part of the spectrum: Ultraviolet light is more tightly packed than violet; gamma rays are more tightly packed than ultraviolet, and X-rays even more. The piano analogy works here too: a note sounds higher pitched because sound waves—which are simplypressure waves in the air—are more tightly packed together. Looser packing makes for a lower note.
    All of this is pretty straightforward when the thing you’re looking at or listening to is sitting still. But now imagine something that’s making a lot of noise while it’s coming toward you—a train speeding toward you with its horn blaring, for example, as you stand close to the track. As the train comes toward you, its motion squeezes the sound waves together, so the pitch sounds higher to you than it really is. When the train passes and starts moving away the squeezing stops abruptly, and suddenly the sound waves are being stretched instead. The pitch drops instantly (in old movies, train whistles are doing this all the time; these days, you mostly hear the effect with police and ambulance sirens).
    This change in pitch works for things that are moving toward or away from you; if they’re moving from side to side, there’s no squeezing and no stretching. So you need to be right next to the railroad track to hear the switch from high pitch to low.
    Exactly the same thing happens with light. If a shining object is moving toward you, its light looks slightly higher-pitched than it really is. That is to say, it looks bluer. If it’s moving away, it looks redder. If a star happens to be moving toward you, it isn’t just the light, but also the dark lines that interrupt it, that shift in the blue direction. This trick of light is what let Edwin Hubble discover the expanding universe back in the 1920s. When he broke apart the light from galaxies beyond the Milky Way, he thought he would see shifts inthe locations of dark spectral absorption lines to show that some of the galaxies were moving away from us and some moving toward us. Instead, the lines showed that they were all speeding away. This meant either that the Milky Way was somehow very repulsive, or, more reasonably, that the whole universe was expanding, with every galaxy speeding away from every other galaxy.
    During graduate school and on into his postdoctoral fellowship, Geoff Marcy had gotten very good at finding and measuring spectral lines. So he went with his strength and decided to look for planets this way. With astrometry, you have to be looking down on an alien solar system from above to see the star moving from side to side (“down” and “above” don’t really have any meaning in space—you could just as easily say “up from below,” and you’d be equally inaccurate, but it’s such an instinctive way to describe it that astronomers talk this way anyway, and nonscientists understand instantly what they’re talking about).
    Marcy wanted to look for that same motion, but from an edge-on perspective. He wanted to catch planets tugging their stars toward Earth (just the tiniest bit), then away. Since the red-shifting and blue-shifting of spectral lines betrays that sort of motion, that’s what he proposed to look for.
    The only problem with this idea, Marcy soon learned, was that it was nearly impossible. When cosmologists use shifting spectral lines to measure the speed of galaxies racing apart as the universe expands,