Some very distant galaxies called quasars are lit up dramatically by powerful black holes. Oddly, their radio emissions shimmer and twinkle when detected from Earth, even though radio waves are barely scrambled by Earth's atmosphere the way visible light is. One quasar, called PKS 0405-385, brightens and fades by 50 percent in less than an hour. Astronomers have concocted all manner of exotic theories to explain the behavior of the faraway, compact galaxies. The real answer, it seems, is unexpectedly closer at hand but still as interesting as anything imagined. New observations of PKS 0405-385 reveal that a layer of...