From battlefields and factory floors to hospital operating rooms and retail call centers, artificial intelligence is rapidly changing how the world thinks and works. IBM's powerful new computer, Watson, recently beat two “Jeopardy!” game-show champions. Drone aircraft are deployed on dangerous reconnaissance and assault missions, reducing soldiers' exposure to harm. Computers diagnose disease, and robotic devices perform delicate surgical procedures. And “smart” machines are bringing new efficiencies to a variety of professions and industries, such as law offices, banks and manufacturing plants. But analysts warn that artificial intelligence may have devastating downsides. Some argue that if battles are fought with machines instead of humans, wars might be easier to start. And economists worry that as robots and other forms of artificial intelligence replace more and more workers, catastrophic levels of unemployment will be the result.