You’re on the 236th-level Kaiserstrasse moving sidewalk when you see her. You’re leaning on the railing, waiting to ask Derya about a job, watching the glittering stream of mites that arc over half the sky — flying up to rewind their nanosprings in the stratospheric sunlight, flying down to make Frankfurt run. You never get tired of watching them. She’s on the Holbeinsteg bridge. Someone’s hung it up here — a hundred meters of clean gray and green twentieth-century modernism, plucked up from the river Main and suspended in the chilly air 2,360 meters up, between a lump of wooded parkland and a cluster of antique subway cars. She’s wearing a 1950s sundress and a broad-brimmed hat, and it’s like an essay on the last century — the