The particle data group currently quotes : Photon Mass Bound Destroyed by Vortices E. Adelberger, G. Dvali, A. Gruzinov Phys.Rev.Lett.98:010402,2007
m_\gamma < 10^{-26} eV (assuming Higgs type mass)
"Using plasma physics to weigh the photon" D D Ryutov 2007 Plasma Phys. Control. Fusion 49 B429-B438
m_\gamma < 10^{-18} eV (assuming Yukawa type modification of Maxwell only, or modified dispersion relation)
#3
Bob S
4,662
8
In principle, there is no upper limit to the photon energy, although it has no rest mass. A photon of energy Ephoton can hit an extremely relativistic electron with an energy γm0c2 head on, and the photon energy gets boosted to [STRIKE]γ2 Ephoton[/STRIKE] ~4γ2 Ephoton by Compton scattering.
One way to get an upper limit to the photon rest mass would be to very carefully do energy and momentum transfer in photon absorption:
http://www.aip.org/pnu/2003/split/625-2.html http://pdg.lbl.gov/2009/listings/rpp2009-list-photon.pdf
Bob S