A whisker can be a wicked thing. In 2005, a Connecticut nuclear power plant shut down after a short-circuited pressure sensor triggered a false alarm. The culprit was a solitary metal whisker thinner than a human hair that sprouted inside the sensor’s electronics. Metal whiskers, such as those pictured above, have incapacitated four satellites and short-circuited more than $10 billion in electronics since their discovery in the 1940s, yet until now the mechanism behind whisker formation remained a mystery. This month in Physical Review Applied, physicist Victor Karpov of the University of Toledo in Ohio presents the first theory that quantitatively explains how metal whiskers form. According to Karpov, imperfections on metal surfaces can form small patches of net positive or negative electric charge. The similar charges repel one another, producing an outward stress. Where the material is weak enough, metal whiskers can grow up to 1 centimeter a year. Karpov suggests that treating metal surfaces with electrolytes, which contain free-moving charges, could neutralize these electrically charged patches, thwarting the growth of any wicked whiskers.