On the Charge of the Photon

The papers setting upper bounds on the value of electric charge of the photon are briefly reviewed. The theoretical framework of these bounds is shown to be incomplete. Hence the bounds seem to be unreliable. There exists about a dozen of papers questioning the neutrality of photon and setting an upper limit on its charge. The title of the first paper [1] was " Experimental limit on the " charge " of the photon ". Please note that the word " charge " was in quotation marks, which, I suspect, reflected the scepticism of the authors. They searched for the effect of electric field V = 20 kv on the energy of 14.4 KeV photons emitted by Fe 57 and investigated with the use of recoilless resonance scattering. Neither an energy shift nor a line broadening has been observed allowing the authors to set the limit e γ /e < 10 −15. The authors of experiment [2] were searching for small charge (< ∼ 0.1e) of a Millikan type spheroid suspended magnetically and exposed to electric field. In this experiment the light used in the suspension system (photons of average energy 2 eV) for duration of at least 8 hours would increase the charge of spheroid if photons were charged. Non-observation of any effect led the authors to an estimate e γ /e < 10 −16. This result was a by-product of experiment looking for free quarks and was briefly described in the last paragraph of the article [2].