Introduction

I’m standing on a street corner in San Francisco, a city I visit far too infrequently. In my hand I hold an electronic device. The device is receiving status updates about the city’s public transportation system in real time. It is telling me that the F-line rail will arrive at the Market and 5th Street station in seven minutes. It displays a map of the city and, by timing radio waves it receives from outer space, triangulates and displays my exact location on that map. A magnetometer determines which direction I’m holding the device and uses that information to indicate the direction I should walk to meet the rail car in time to board it. My friends call me, wondering when I will arrive. A tiny video camera and microphone share ...

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