From left-wing 9/11 conspiracy theorists to right-wing Obama-hating "birthers"—a sobering, eyewitness look at how America's marketplace of ideas is fracturing into a multitude of tiny, radicalized boutiques—each peddling its own brand of paranoiaThroughout most of our nation's history, the United States has been bound together by a shared worldview. But the 9/11 terrorist attacks opened a rift in the collective national psyche: Increasingly, Americans are abandoning reality and retreating to Internet-based fantasy worlds conjured into existence out of our own fears and prejudices.The most disturbing symptom of this trend is the 9/11 Truth movement, whose members believe that Bush administration officials engineered the September 11, 2001, terrorist attacks as a pretext to launch wars in Afghanistan and Iraq. But these "Truthers" are merely one segment of a vast conspiracist subculture that includes many other groups: anti-Obama extremists who believe their...