Read Mirror Earth for Free Online Page B

they’re looking at objects moving at many thousands of miles per hour. They take a spectrum froma galaxy’s collective starlight, lay it next to a reference spectrum from a motionless object—the Sun, for example, or a laboratory reference lamp that generates an artificial spectrum—and see how far a given line has shifted. There’s some imprecision in the process, but if they’re off by a couple of thousand mph or so, that’s plenty accurate enough. They don’t have any need to improve their precision.
    But the back-and-forth motion Jupiter causes in the Sun is a piddling 28 mph or so. The greatest expert in measuring cosmological redshifts would fail utterly to detect it. So Marcy tried tightening up the procedure in every way he could think of to make it more precise. He succeeded up to a point: He managed to get his accuracy down to about 450 mph. But since he was trying to find distant Jupiters, this wasn’t nearly good enough. No matter how careful he was, the act of moving the telescope from star to reference lamp changed his measurement system enough to make the measurement unreliable. Imagine you wanted to measure the length of two different objects—two bricks, say—with high precision. You’d be smart not to use two different rulers, since one of them might be just a little bit off. But if you wanted to be
really
precise, it’s a problem even if you use just one ruler—moving the ruler from one brick to the other could change things. The second brick could be in a slightly warmer place, so the ruler might expand just an infinitesimal amount. Or you might hold the ruler in a slightly different way, so it would sag under gravity differently, distorting its shape. These are absurdly small changes, but if you really needed absolute accuracy, they could make adifference. The best way to make absolutely sure you’re measuring things exactly the same way is to measure them at exactly the same time.
    Marcy decided to do just that. He’d measure a star’s spectrum and a reference spectrum all at once, so he didn’t have to move anything. He might have figured out a way to do it, but it turned out that he didn’t have to. A Canadian postdoc named Bruce Campbell, at the University of British Columbia, had come up with a solution a half decade or so earlier. Working with a colleague named Gordon Walker, Campbell had realized that you could take a gas whose spectral absorption lines were thoroughly understood, put it in a glass container, and let starlight pass through the gas on its way to the spectrometer. The reference spectrum and the real spectrum would be measured at exactly the same time with exactly the same instrument.
    It worked. Campbell and Walker were able to measure the wobbling of stars to an accuracy of plus or minus 30 mph or so. That still wasn’t precise enough to find an alien Jupiter, however, since the measurement error was as big as the signal you’d be looking for. Beyond that, Campbell and Walker had settled on hydrogen fluoride gas for their reference—well understood, but also corrosive, explosive, and horribly toxic. Marcy needed a better gas, and he also needed a collaborator for this project, which was growing increasingly complicated.
    By now, Marcy was on the faculty at San Francisco State University. After asking around a bit, he learned of a recent San Francisco State graduate who had joint degrees in physics and chemistry, and who also had a strong interest in astronomy.Paul Butler was still at the university, working on a master’s in physics and looking for a research topic. When Marcy approached him, Butler was intrigued. Like Marcy, Butler was drawn to research with long odds but a potentially huge payoff. And he loved the challenge of trying to make measurements more precise than anyone had ever been able to pull off.
    The only downside was that Paul Butler had a somewhat rough personality. He