THE English have Silicon Fen and Silicon Roundabout, the Scots have Silicon Glen. Berlin boasts Silicon Allee, New York Silicon Alley. But the brain of the tech world is the ecosystem in and around San Francisco. Silicon Valley’s entrepreneurs and innovators, technologists and moneymen are busy revolutionising nearly every aspect of the global economy.A place named for its skill in making silicon-packed semiconductors is transforming how firms make decisions, people make friends and protesters make a fuss. Startups touch more people, more quickly than ever before. Airbnb, a seven-year-old firm that helps people turn their homes into hotels, operates in 34,000 towns and cities around the world. “On-demand” firms like Uber are changing what it means to be an employee. Just as the big platforms like Google, Facebook and Apple benefit from “network effects”, because each new user makes the service more valuable for all the others, so the Valley’s success as a venue to launch, fund, staff and sell a technology firm is feeding on itself (see article).