Birds in flocks, bacteria and cells: In many collective systems, individual elements respond to only part of their surroundings, seemingly defying Newton’s third law of motion—action equals reaction. These exceptions are known as nonreciprocal interactions. A Dresden physics team working with Roderich Moessner, a founding member of the Würzburg–Dresden Cluster of Excellence ctd.qmat, has now developed a theory that makes it possible to describe these interactions efficiently and simulate them far more precisely.