A “cosmological compass” has pushed the possible mass of the photon to new
limits.
Physicists assume photons have no mass. If they did, they would generate a
weak magnetism around magnetic fields in the Galaxy. During months of
monitoring with a sensitive compass, Roderic Lakes of Wisconsin University
failed to detect this. He concludes any photon mass must be less than 2 ×
10-53 kilograms.
