The heart’s electrical system keeps all its muscle cells beating in sync. A hard whack to the chest at the wrong moment, however, can set up unruly waves of abnormal electrical excitation that are potentially deadly. The resulting kind of arrhythmia may be what caused the football player Damar Hamlin of the Buffalo Bills to collapse on the field after he took a powerful hit during a 2023 National Football League game. Today, powerful defibrillators are usually used to help resynchronize hearts in distress. But Flavio Fenton, who studies the electrical dynamics of the heart, tells Steve Strogatz about a new method under development for treating arrhythmias by stimulating the heart with mild, precisely timed shocks — or possibly even with light.