Fast magnetic reconnection, the rapid convergence, separation and explosive snapping together of magnetic field lines, gives rise to northern lights, solar flares and geomagnetic storms that can disrupt cell phone service and electric power grids. The phenomenon takes place in plasma, the state of matter composed of free electrons and atomic nuclei, or ions, that makes up 99 percent of the visible universe. But whether fast reconnection can occur in partially ionized plasma—plasma that includes atoms as well as free electrons and ions—is not well understood.