<<Moderator note: Split from thread Photon the smallest particle>> @Orodruin - Let me ask a question. This is an aspect of QFT that has always puzzled me. Suppose we have an RF cavity or a laser cavity with a standing E-M wave. I understand that we can view the field in the cavity as a macroscopic number photons in the same quantum state. In this case, why do we say that the photons making up the field have zero spatial extent? How can a macroscopic number of photons with zero spatial extent add up to a measurable E-M field which fills the cavity? Is there a way of thinking of this that I am missing?