Researcher shows the photon may have less mass than previously thought
The photon, the fundamental particle to which so many of us owe our livelihood, is usually assumed to have a mass of zero. In fact, experimentalists have succeeded only in placing an upper limit on the photon mass ml, and now Roderic Lakes of the University of Iowa (Iowa City, IA) has lowered the limit to 7 ¥ 10-17 eV (1 ¥ 10-49 g). Interrogating the magnetic vector potential A arising from large-scale weak magnetic fields on a galactic scale or larger, Lakes used a modified Cavendish balance consisting of a steel toroid wrapped in current-carrying coils. By observing the torque on the toroid due to its dipole moment interaction with A, which depends on the product ml2|A|, and estimating the ambient vector potential, the photon mass limit is obtained.
A nonzero photon mass would imply a wavelength-dependent speed of light, the possibility of longitudinal electromagnetic waves where the electric or magnetic field does not oscillate perpendicularly to the direction of travel, a leakage of static electric signals into conduction enclosures, and a more rapid falloff of magnetic dipole fields with distance than the inverse cube dependence.
