In two critical minutes, Nasdaq’s technology failed the world. One hundred and twenty seconds are under scrutiny in the investigation to find out what caused Nasdaq’s giant freeze-up on Thursday after computer problems forced the exchange to halt trading for hours. Scott Patterson is a Wall Street Journal reporter trying to unravel what went wrong. He’s also author of “Dark Pools: The Rise of the Machine Traders and the Rigging of the U.S. Stock Market.”