http://philsci-archive.pitt.edu/18564/1/QDAT%20Arxiv.pdf
This is the quantum analog of “absorber response,” which at the relativistic level is a mutual non-unitary interaction between emitter and absorber(s) that gives rise to one or more real, on-shell photons, even though such photons are technically “internal.”
his treatment of the real/virtual distinction in both Davies (1971) and (1972) falters into an ambiguous one alternating between (a) the standard, inadequate QFT characterization of the real vs. virtual distinction as a merely contextual one, i.e. as an “internal” vs “external” photon dependent on our zoom level and (b) the mistaken assumption that a real photon must have an infinite lifetime and therefore can only be truly external.12In particular, he appears to apply the uncertainty principle to the lifetime of real photons. However, a real photon obeys energy conservation, and its lifetime is therefore not limited in that way.13The fact that real photons are both emitted and absorbed,and therefore can be considered a form of “internal line,”is key in understanding the relevant quantum analog of the LTB condition. Indeed, all real photons are “internal” in the QDAT, since real photons can only be created through the participation of both emitters and absorbers.
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