It was March 2016, and more than 200 million people around the world were watching a battle 2,500 years in the making. At the Four Seasons Hotel in Seoul, Lee Sedol — one of the greatest players of Go, the ancient Chinese board game — sat across from AlphaGo , a computer program built by the London-based artificial intelligence lab DeepMind. Chess had fallen to machines nearly twenty years earlier, when IBM’s Deep Blue defeated Garry Kasparov. But Go was different. The number of possible moves