Experimental Tests Of The Photon Rest Mass

The foundation of Maxwell classical electromagnetic theory has promoted the great development of physics in 20th century, based on which the theories of special relativity, quantum mechanics, and quantum electrodynamics were established and hence constituted the fundamental frames of modern physics. One of the major contributions of Maxwellian theory was that it set up an essential relation between the electromagnetic field and light in vacuum, and defined the speed of electromagnetic wave as the constant speed of light c, which implies a hypothesis that the rest mass of the photon should be zero exactly. Whether the rest mass of the photon is zero or not hence became one of the fundamental problems confronted by many physicists. Up to now, all experimental results were only obtained the upper constrains on the photon mass. The experimental test of the photon rest mass is equal to verify the constancy of c, and any conclusive evidence of a finite mass for the photon will bring great influence on development of the whole physics.Our group started the experimental test of the photon rest mass in 2000, and presented a scheme of the modulation method with the precise torsion balance. Based on the comprehensive researches on the related fields and two years elaborate preparation, the first modulation experiment of precise torsion balance was completed. Experimental results disclosed that the upper limit on the photon mass was 1.2×10-51g, and this figure, collected by Particle Data Group in 2004, has improved almost 20-fold compared with that of the static torsion balance experiment. The signal is modulated by rotating the torsion balance to ensure the effectiveness of detection for all possible orientations of the vector potential, and the influences of sidereal disturbances of environment are also removed by virtue of this modulation method. In 2005, we have fulfilled the improved experiment, which included a new cave laboratory with more homogeneous gravitational background, a modified pendulum with a quadrupole moment of mass distribution to further quiet the contribution from the local gravity gradients, a high vacuum chamber to suppress the disturbance from the residual air current, the synchronous monitors of the rotating period of the turntable and temperature, the modulation experiment of background magnetic field, and so on. New result gave an more stringent limit of 1.5×10-52g on the photon rest mass, with an improvement of almost one order compared with the result of our previous experiment.This work was partially supported by the National Natural Science Foundation of China (10075021, 10121503) and the National Basic Research Program of China (2003CB716300).