Imagine a bank that pays negative interest. In this upside-down world, borrowers get paid and savers penalized. Crazy as it sounds, several of Europe’s central banks cut key interest rates below zero in 2014, and now Japan has followed. By mid-2016, some 500 million people in a quarter of the world economy were living with rates in the red. Unthinkable before the 2008 financial crisis, the idea is to jolt lending, spur inflation and reinvigorate the economy after other options have been exhausted. It’s an unorthodox move that hasdistorted financial markets and triggered complaints that the strategy is backfiring. Negative rates will either mark the start of a new era for the world’s central banks, or finally expose the limit of their powers.