The arrow of time——from the beginning of the universe to quantum materials

Time-reversal symmetry is a fundamental property of the basic equations of physics—microscopic laws remain unchanged whether time runs forward or backward. Yet the macroscopic world displays an unmistakable arrow of time. Taking the breaking of time-reversal symmetry as its central thread, this article weaves together physical pictures across three scales, showing that the arrow of time is not written into any fundamental equation but emerges from the collective behavior of vast numbers of particles. Condensed matter physics—the bridge between the microscopic and macroscopic worlds—is opening up new perspectives and new battlegrounds for this question in unexpected ways.