Fast radio bursts, or FRBs, are extragalactic flashes of light that pack a serious wallop, unleashing in a few milliseconds as much energy as Earth's sun does in a century. More than 100 FRBs have been discovered to date, and most of them are one-offs, flaring up just a single time (as far as we know). In January of this year, astronomers reported that one member of the "repeater" class, called FRB 180916.J0158+65, appears to exhibit a 16-day activity cycle: It fires off bursts for a four-day stretch, goes quiet for 12 days and then starts all over again. FRB...