On September 19, 2007, while driving alone near Seattle on her way to work,Tanya Rider went off the road and crashed into a ravine.* For eight days, shewas trapped upside down in the wreckage of her car. Severely dehydrated andsuffering from injuries to her leg and shoulder, she nearly died of kidney fail-ure. Fortunately, rescuers ultimately found her. She spent months recuperat-ing in a medical facility. Happily, she was able to go home for Christmas. Tanya’s story is not just about a woman, an accident, and a rescue. It is astory about bits—the zeroes and ones that make up all our cell phone conver-sations, bank records, and everything else that gets communicated or storedusing modern electronics.
Tanya was found because cell phone companies keep records of cell phone locations. When you carry your cell phone, it regularly sends out a digital "ping," a few bits conveying a "Here I am!” message. Your phone keeps "ping-ing" as long as it remains turned on. Nearby cell phone towers pick up the pings and send them on to your cellular service provider. Your cell phone company uses the pings to direct your incoming calls to the right cell phone towers. Tanya’s cell phone company, Verizon, still had a record of the last location of her cell phone, even after the phone had gone dead. That is how the police found her.