The Nobel Prize for physics in 2016 was awarded for the theory of topological matter. Topological insulators are new materials with special electronic properties and are of great fundamental and applications-oriented interest. Nevertheless, physicists have wrestled with a ten-year-old puzzle in which the results from the two best methods to probe their electronic states disagree. Researchers from Amsterdam, including two FOM-funded PhD candidates, with collaborators in France, Switzerland and Germany now know exactly why.