A geometric-phase timeline. Global change without local change—a connecting idea in the physics of optical, quantum and other waves—has a multistranded history spanning two centuries. This photo shows a view near Skreen, in the west of Ireland, through a thin crystal sandwiched between crossed polarizers. The dark band across the interference fringes corresponds to a geometric phase of π. [M. Berry]. Find out more under Celebrations, link in our bio! #PiDay, #πDay, #PiDay2026, #3.14