A New Limit On Photon Mass

A New Limit on Photon Mass A new limit on photon mass, less than 10-51 grams or 7 x 10-19electron volts, has been established by an experiment in which light is aimed at a sensitive torsion balance; if light had mass, the rotating balance would suffer an additional tiny torque. This represents a 20-fold improvement over previous limits on photon mass. Photon mass is expected to be zero by most physicists, but this is an assumption which must be checked experimentally. A nonzero mass would make trouble for special relativity, Maxwell's equations, and for Coulomb's inverse-square law for electrical attraction. The work was carried out by Jun Luo and his colleagues at Huazhong University of Science and Technology in Wuhan, China ([email protected], 86-27-8755-6653). They have also carried out a measurement of the universal gravitational constant G (Luo et al., Physical Review D, 15 February 1999) and are currently measuring the force of gravity at the sub-millimeter range (a departure from Newton's inversesquare law might suggest the existence of extra spatial dimensions) and are studying the Casimir force, a quantum effect in which nearby parallel plates are drawn together. (Luo et al.,Physical Review Letters, 28 February 2003) Jun Luo, Liang-Cheng Tu, Zhong-Kun Hu, and En-Jie Luan Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 24 August 2002; published 26 February 2003 A rotating torsion balance method is used to detect the product of the photon mass squared and the ambient cosmic vector potential Ae. The signal is modulated by rotating the torsion balance to ensure the effectiveness of detection for all possible orientations of the vector potential. The influences of sidereal disturbances of environment are also removed by virtue of this modulation method. The experimental result shows μγ2Ae<1.1×10-11   T m/m2, with μγ-1 as the characteristic length associated with photon mass. If the ambient cosmic vector potential Ae is 1012   T m due to cluster level fields, we obtain a new upper limit on photon mass of 1.2×10-51   g.