A New Limit on Photon Mass

[phys]

A New Limit on Photon Mass
A new limit on photon mass, less than 10-51 grams or 7 x 10-19
electron 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
(jun...@mail.hust.edu.cn, 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 inverse-square 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)