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Lab Experiment Probes Photon Charge

Photonics Spectra
Mar 2003
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.


GLOSSARY
photon
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
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