New York is the most diverse city in the United States. Brooklyn the most diverse neighborhood in New York. And Williamsburg the most varied area of Brooklyn. However, the posh 30-somethings pouring in from Manhattan know only from the news that a segregated world of Hasidic Jews lurks a few blocks from their Michelin-starred restaurants and bars serving overpriced organic wine. From South 9th Street, another universe emerges. The men wear black jackets, hats, long beards, sideburns along their temples and speak Yiddish on flip phones reminiscent of the 1990s. The women wear long dresses and wigs. Almost all walk...