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 Quote by waterfall I know but when Einstein proposed the photon, it was as classical particle theory of light. He didn't actually say it is the quanta of the electromagnetic field as mentioned in the 1920's. But now we know photons are quanta of the EM field. Therefore must we use Einstein classical definition or QFTs when we mentioned about the photons in general?
Of course nowadays we think of photons in terms of QED, not the way Einstein thought about them. Pretty much all Einstein knew was that electromagnetic energy was quantized.