Grace Hopper, who spent her working life programming computers for the US Navy, used to hand out pieces of wire about eleven and three-quarter inches long. Each one was a nanosecond: the distance light travels in a billionth of a second. She started because admirals kept asking why a signal bounced off a satellite took so long, and it is hard to argue with a length of wire you are holding. For a microsecond she kept a coil nearly a thousand feet long, and a picosecond, she said, was about the size of a grain of ground pepper. It is the only way I know of to put a unit of time in someone’s pocket. Some of those wires must still be lying in desk drawers, a billionth of a second each, waiting for someone who will have no idea what they are.