I assume you’re referring to Feynmans demonstration that since photons are neutral with no antiparticle in QED a photon going forward in time and backwards in time are the same photon. But from our frame of reference the photon always originates at the origin and hits our eye. From the photons point of reference it’s not meaningful tho to say they go backwards or forwards because time actually doesn’t elapse for them.
I assume you’re referring to Feynmans demonstration that since photons are neutral with no antiparticle in QED a photon going forward in time and backwards in time are the same photon. But from our frame of reference the photon always originates at the origin and hits our eye. From the photons point of reference it’s not meaningful tho to say they go backwards or forwards because time actually doesn’t elapse for them.