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Now, two centuries after Luddites smashed then-newfangled weaving frames in northern England, predictions of permanent technological unemployment are being revived. In a December working paper for the National Bureau of Economic Research, called "Smart Machines and Long-Term Misery," the Columbia economist Jeffrey Sachs and the Boston University economist Laurence Kotlikoff pose the question, "What if machines are getting so smart, thanks to their microprocessor brains, that they no longer need unskilled labor to operate?" After all, they point out, "Smart machines now collect our highway tolls, check us out at stores, take our blood pressure, massage our backs, give us directions, answer our phones, print our documents, transmit our messages, rock our babies, read our books, turn on...