I am optimistic, as is the article, that further research into variations in metal, light, angle of incidence, and time will lead to a better understanding of the parameters of this backwards electron motion effect, of the nature of light and electromagnetic waves, and moreover, the substrate/matrix in which both are active, perhaps even of dark matter and a moving-zero mass.
> The photon-drag effect, the rectified current in a medium induced by conservation of momentum of absorbed or redirected light, is a unique probe of the detailed mechanisms underlying radiation pressure. We revisit this effect in gold, a canonical Drude metal. We discover that the signal for p- polarized illumination in ambient air is affected in both sign and magnitude by adsorbed molecules, opening previous measurements for reinterpretation. Further, we show that the intrinsic sign of the photon-drag effect is contrary to the prevailing intuitive model of direct momentum transfer to free electrons.
Photons are basically packets of momentum that atoms exchange.
What do you mean? On a human scale, experience is not about photons or electrons, it's about macroscopic objects, materials and a few forces (gravity and electromagnetism basically), and luminance
And I would say that astronauts in space live quite normally, compared to how they would fare without electromagnetic interactions!
Is the quantized nature of these packets caused by a limitation in the atoms and their ability to form photons, or is it a fundamental limitation of the photons themselves?
The energy of any photon is the Planck constant multiplied by the frequency. The frequency and energy aren't quantized and can be any value as long as they are proportional.
The energy of an electron in an atom isn't quite so simply expressed but is limited to discrete levels.
This is where emission/absorbsion lines come from or to simplify, color.
Electrons can only accept/emit specific amounts of energy and photons of a specific frequency have an exact energy therefore atoms can only absorb or emit very specific frequencies and all others are ignored.
But energy levels in atoms are quantized, and each sort of atom has a distinct set of possible energy levels. And so can emit/absorb a distinct set of possible energy photons. That's what emission/absorption spectra are all about.
Free-electron lasers, conversely, can be built to produce coherent photons of any energy. Albeit constrained by construction methods.
What you're probably referring to is quantization of the electron energy, but I believe that only happens for bound electrons. If an atom loses or gains electrons, things can work quite differently. This is also different for electrons in a metal's conduction band, which are somewhat free, since they are bound to the metal, but not so much to individual atoms.
I remember Feynman diagrams with electrons and photons, but that was about photons <-> electron, positron pairs.
Free electrons accelerated laterally by magnetic fields emit ynchrotron radiation. But does that work in reverse? I've never heard of that. But again, IANAP.