Some experiments reveal, above all, the limits of what can be known. In July, researchers at the Regensburg Center for Ultrafast Nanoscopy, working with the Max Planck Institute in Hamburg, reported a new kind of tradeoff hidden inside quantum measurement: the more precisely you pin down when an electron moves, the less precisely you can know where it is. It is not a violation of Heisenberg's uncertainty principle, but something that sits quietly beside it, a “space-time limit” for electron motion, observed directly for the first time.