You do not really need QED to understand this. ... QED does not say that photons always travel at c. Special Relativity states that photons that are not interacting with anything (in a vacuum) always travel at c. When the absorption and emission by atoms is added then the effective speed of photons is changed. This is where QED comes in.
The attempt to attribute the explanation of the
photon speed in transparent materials by absorption and emission to Special Relativity must be considered deliberate disinformation. Special Relativity is in this respect based on Maxwell's theory and Maxwell's theory explains different propagation speeds of electromagnetic waves by different permittivity and permeability values.
By claiming that "the light has an amplitude to go faster or slower than the speed c, but these amplitudes cancel each other out over long distances" (QED, p89-90), Richard Feynman somehow repeated the error of Niels Bohr (
BKS theory), who had opposed the assumption of orderliness in emission and absorption processes of electromagnetic wave quanta by denying conservation of momentum and energy in single events. Both Bohr and Feynman were scientists in the theological tradition, preferring impressive counter-intuitive explanations to unprejudiced realistic ones.
P.S. "reemitted" suggests that the same photon that was absorbed is emitted. This is not correct.
Whether we consider the
reemitted photon the same or a different photon, is irrelevant. Relevant however is that the
reemitted photon must have exactly the same energy and momentum as the absorbed photon.
It is a different photon or even multiple photons, e.g. an electron absorbs a photon with a certain energy and ends up in an energy state that can decay via intermediate energy states back to the original energy state. Each decay emits a photon.
You confuse the extremely regular behaviour of light in optical lenses with scattering of photons e.g.
in the atmosphere, where momentum and energy of photons actually are changed.
If continuous absorption and reemission were a genuinely physical explanation of the reduced speed in transparent media, then it would be possible to answer such questions as: How often (on average) is a 450-nm-photon absorbed and reemitted when traversing 1 cm of diamond?
By the way, in principle it is possible to label each atom of a given diamond with a unique number. If photons traversing this diamond actually were more than half of the time (n
_diamond > 2) at rest (i.e. transformed into energy and momentum of carbon atoms), then it would in principle be possible to determine the path of a photon by enumerating the label-numbers of the atoms where the photon rested.
Yet on the other hand,
QED makes the
absurd claim that photons somehow use all possible paths.
Cheers, Wolfgang
Science of yesterday degenerates into religion of today and superstition of tomorrow