Lab Experiment Probes Photon Charge

Researchers at Brookhaven National Laboratory in Upton, N.Y., have devised a new apparatus for measuring the upper limit on the charge of the photon. The approach, which was described in the Jan. 1 issue of Physical Review D, has yielded results an order of magnitude better than previous laboratory experiments, and it promises a sensitivity of 10^-21 e with minor improvements.

Variations in the time of flight of radio-frequency emissions from pulsars due to dispersion in the intergalactic magnetic field have produced an upper limit on photon charge of 5 x 10^-30 e, but laboratory tests had done no better than 10^-16 e. The new technique monitors the deflection of a 1-mW HeNe laser beam by a modulated magnetic field and has obtained an upper limit of 8.5 x 10^-17 e with a prototype apparatus. The team suggests improvements in the apparatus to boost sensitivity, such as increasing the optical path length and suppressing sources of noise with better isolation of the magnet.