Weeks after the towers fell, Jennifer Bartlett started painting. She had watched them collapse from her roof that September, and in her studio in the West Village she began depicting what almost no one wanted to depict, in her style of solid dots daubed into a grid of little squares. Toward the edges the dots are that distinctive cloudless blue, but most squares she overlaid with two dots, or three, the gray of the smoke superimposed on the red or saffron of the fireball. The dots became embers of exploded airplanes, or TV screen pixels (we had no smartphones then);