Astronomers using the Low-Frequency Array (LOFAR) telescope network have detected a vast ridge of radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401. The results imply that intergalactic magnetic fields connect Abell 0399 and Abell 0401 and challenge theories of particle acceleration in the intergalactic medium. LOFAR image centered on the Abell 0399-Abell 0401 system; color and contours show the radio emission at 140...