P.M. Edition for May 17. The Dow officially closed above 40000 today, for the first time ever. Special writer Gregory Zuckerman explains why investors are striking gold—and how long it could last. And Heard on the Street columnist David Wainer has more on why the Medicare bubble may have burst. Plus, workers at a Mercedes-Benz plant in Alabama reject unionization, in a setback for the United Auto Workers union. Annmarie Fertoli hosts. Sign up for the WSJ's free What's News newsletter. Learn more about your ad choices. Visit megaphone.fm/adchoices