About the Cover

Bird flocks, fish schools, and insect swarms have all been studied through the lens of active-matter physics. Studying human crowds, though, presents logistical and ethical challenges. The story on page 8 describes one research team’s solution: observing and analyzing the crowd of revelers at a long-running Spanish festival, a scene from which is shown here. The unique collective phenomena that emerge can be described with a physical model that treats the crowd as a continuous medium. (Photo courtesy of the Bartolo Lab, ENS de Lyon.)