The controversy surrounding the WTC memorial in New York is indicative of a larger disconnect, writes Edward Rothstein. There was a time when American cities were viewed as great models of social engineering, vast islands of humanity where hard work and a general devotion to the public interest could overcome all sorts of human failings. No more: “The [modern] city’s greatest achievement, it often seems, is the protection of the private realm and competing private interests; about the public realm there is no clear understanding… New forms of urban life have to develop. But in the meantime, the public seems to exist only in the midst of cataclysm.”