At the beginning of the 20th century Einstein changed the way we think about Time. Near the end of the 20th century scientists learned how to cool a gas of atoms to temperatures billions of times lower than anything else in the universe. Now, in the 21st century, Einstein's thinking, and ultracold atoms, are shaping one of the key scientific and technological wonders of contemporary life: atomic clocks, the best timekeepers ever made. Such super-accurate clocks are essential to industry, commerce, and science; they are the heart of the Global Positioning System (GPS), which guides cars,...