In July 2016, Ethereum endured an early test of faith. The people behind the barely year-old blockchain had taken Bitcoin’s idea of decentralized money and run with it, building a digital landscape where users, based on a mutual trust in code, could interact and create applications. Then hackers emptied $50 million from one of those applications, the DAO. Facing a crisis, a core group of developers swiftly altered Ethereum’s code to return the lost funds. The heist raised a key question for...