whatA sheet of heart muscle: every cell an excitable spark that fires, then must rest. A pacer on the left edge drives clean plane waves across it -- sinus rhythm, written line by line on the probe's ECG below.

whyThe claim is the arrhythmia textbook, run live. A beat landed in the refractory TAIL of a passing wave -- the vulnerable window -- blocks on one side only, and the surviving half curls: spiral re-entry, a rotor beating faster than the pacer and seizing the tissue. On patchy tissue the rotors run an electrical storm: roughly twice the sinus rate and irregular -- the beat-interval cv is printed beside the rate -- with ectopic beats shedding and reforming wavelets; past four singularities the label reads fibrillation. The rhythm is CLASSIFIED by measurement, not narration -- phase singularities counted by winding number: zero is sinus, a few is re-entry, many is fibrillation. The vulnerable window itself is measured: too early does nothing, too late makes a harmless ring, only the window makes rotors. Double-click is the paddles: every cell fires at once, everything goes refractory together, silence -- then the pacer resumes.

howFitzHugh-Nagumo with a five-point Laplacian (150 by 104, dt 0.14), patchy recovery rate in seeded blobs; the u = 0 isoline drawn by marching squares is the wavefront. Phase singularities are plaquette winding numbers of the (u, v) phase. An ectopic beat resets the sinus node -- the compensatory pause -- which is what gives a newborn rotor its chance before its faster rhythm takes over honestly. The S1-S2 trials run on a sealed, pacer-silenced sheet.