For months, Wall Street has been running on an adrenaline rush. Now, the comedown has begun. After racing to record highs in October, U.S. markets have been jolted by a sudden case of vertigo. On Tuesday, the Nasdaq slid 2% - its worst day in nearly a month - as traders finally began to question whether artificial intelligence, the market's favorite miracle cure, might have been seriously overpriced. By Wednesday morning, futures hovered near zero.
