Variability time scales in active galaxies Active galactic nuclei contain a supermassive black hole (SMBH) surrounded by an accretion disk. As disk material falls toward the SMBH, it heats up enough to emit optical light. Burke et al . investigated how such optical emission varies over time in a sample of 67 active galaxies (see the Perspective by Lira and Arevalo). They observed a characteristic variability in timing that scaled with the SMBH mass. The results elucidate the physical processes within accretion disks and provide a method to estimate SMBH mass from optical variability observations. —KTS