Improved Limit on Photon Rest Mass

Recent observations of Alfv\'en waves in the interplanetary medium provide an improved upper limit on the photon rest mass. We find a reliable upper limit μ<~3.6×10^-11 cm^-1, mₚₕ<~1.3×10^-48 g, and a stronger, but less certain upper limit μ<3.1×10^-12 cm^-1, mₚₕ<1.1×10^-49 g. These represent improvements on the heretofore best reliable estimate by 0.5 and 1.5 orders of magnitude, respectively.