The subtlest change in New York is something people don't speak much about but that is in everyone's mind. The city, for the first time in its long history, is destructible. A single flight of planes no bigger than a wedge of geese can quickly end this island fantasy, burn the towers, crumble the bridges, turn the underground passages into lethal chambers, cremate the millions. The intimation of mortality is part of New York now: in the sounds of jets overhead, in the black headlines of the latest edition.