That’s a point I made in the talk – it’s anyone’s guess what happens if the photon is actually massless but if it has a non-zero but vanishingly small mass it’s calculable.

Of course if you use E=mc^2 then you have a justification but that’s equivalent to just using the time-time component of the field equation, i.e. it ignores the space-space component.

]]>P.S. The photo of Einstein and Edd is from 1930! ]]>