Two hundred ships moved through the stars, leaving an iridescent trail of transmission beacons in their wake. Five billion kilometers long, the beacons stretched all the way to Earth, a desiccated and shaken planet that the passengers once called home. Sometimes simple messages from the ships arrived in the data. After a long time, images came and---after an even longer time---clips of the passengers going about their lives. But the vast distances meant these clips were rare. Normally an image arriving on Earth was cause for celebration, because it meant the crew was still alive, or at leas...