Pulsars suggest that ultra–low-frequency gravitational waves are rippling through the cosmos. The signal seen by international pulsar timing array collaborations in 2023 could come from a stochastic gravitational-wave background—the sum of many distant sources—or from a single nearby binary of supermassive black holes. To tell these apart, Hideki Asada, theoretical physicist and Professor at Hirosaki University, and Shun Yamamoto, researcher at the Graduate School of Science and Technology, Hirosaki University, propose a method that exploits beat phenomena between gravitational waves at nearly the same frequency, searching for their imprint in the tiny shifts of pulsars’ radio-pulse arrival times.