Light Seems to Pull Electrons Backward | Hacker News Reader

Light Seems to Pull Electrons Backward

physics.aps.org

58 pointswwarner22 comments

When an ocean wave hits a beach, the swashing action pushes grains of sand up and pulls them down. The shape of the beach (as observable in any given moment) is steady relative to a number of waves and governed by cumulative averages of their features, but will change as the direction of waves, tide, current, etc. shifts over time. A beach with finer sand but similar wave action and angle will have a different incline from one with coarser sand or pebbles.

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.

When light hits the metal, it imparts some momentum. Is it possible (and this is a stupendously naive mental picture and not what would actually be happening, I realize that) that the metal moves in one direction, kicking free electrons back the other direction? Like if you had a sheet of drywall covered in dust hanging from a rope and you threw a baseball at it. The drywall would move in the direction of the balls flight, but the dust would get kicked off back in the direction the ball came from. Is the amount of current created consistent with this sort of picture, electrons being kicked off as the metal absorbs the momentum and accelerates away?

https://arxiv.org/pdf/1812.01673.pdf

> 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.

Doesn't light basically interact with atoms by changing electron energy levels?

Essentially the entire experience of existence on a human scale is exclusively photons and electrons interacting.

Photons are basically packets of momentum that atoms exchange.

Right. But it's odd to read about photons interacting with "free" electrons in metals. Pushing or pulling them. I would expect that changes is electrical potential are somehow mediated through the atoms.

> Essentially the entire experience of existence on a human scale is exclusively photons and electrons interacting.

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

I think what they are trying to say is that almost everything that happens in everyday experience is the result of the residual electromagnetic interactions between the outer electrons of the electrically neutral atoms that make up the world. (Apart from some almost but not quite negligible gravitational interaction that keeps the earth in an orbit around the sun and things like that.)

Gravity is a major part of the human experience, at least for all humans that live close enough to a celestial body to be forced with their feet to the ground. One look at how astronauts live in space will show you that there is more to the regular human experience than electromagnetism.

Gravity towards the Earth specifically, yes. Gravity between objects in daily life is completely negligible.

And I would say that astronauts in space live quite normally, compared to how they would fare without electromagnetic interactions!

It would be pretty interesting to imagine what daily life would look like with only the strong and weak forces, true.

Greg Egan gets into that in at least one of his novels, Diaspora.

> Photons are basically packets of momentum that atoms exchange.

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?

IANAP, but I don't think that photon energies are quantized. That is, a photon can have any energy. But the sizes of those energy/momentum packets are quantized.

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.

Photons are by definition quantized energy packets. If the energy weren't quantized, it wouldn't be a photon.

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.

Fundamental behavior.

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.

the electrons in the outer shells of metals are essentially free to move (which is why metals conduct electricity so well)

Yes. And yet, aren't photons still interacting with the metal atoms, not those ~free electrons?

depends on which they hit - this article seems to imply that when photons hit atoms their momentum is transferred conventionally, but when they hit electrons they move in the opposite direction (and induce a current on the surface)

That's my question. Can photons actually "hit" electrons?

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.

OK, there are free-electron lasers. But that's a totally different thing, based on synchrotron radiation, in a vacuum, with no atoms involved.

I always thought that electrons and photons could be of the same energy in a way

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