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
(junluo@mail.hust.edu.cn, 86-27-8755-6653). They have also carried out
a
measurement of the universal gravitational constant G (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., Phys. Rev. Lett. 90, 081801

See also:
Article in Science
and a more recent review in RMP 2010 . Check the Table on pg 972.