Suppose you’re looking for a single person, somewhere in the world. (We’ll call him Waldo.) You know who he is, nearly everything about him, but you don’t know where he’s hiding. How do you find him?
The scale is just too great for anything but a computerized scan. The first chance is facial recognition — scan his face against cameras at airports or photos on social media — although you’ll be counting on Waldo walking past a friendly camera and giving it a good view. But his voice could be even better: How long could Waldo go without making a phone call on public lines? And even if he’s careful about phone calls, the world is full of microphones — how long before he gets picked up in the background while his friend talks to her Echo?