5 ms·

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

colechristensen 7y ago
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.
- mirimir 7y ago
  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.
- elcomet 7y ago
  > 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
  - fhars 7y ago
    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.)
    - tsimionescu 7y ago
      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.
      - dTal 7y ago
        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!
        tsimionescu 7y ago
        It would be pretty interesting to imagine what daily life would look like with only the strong and weak forces, true.
        mirimir 7y ago
        Greg Egan gets into that in at least one of his novels, Diaspora.
- amelius 7y ago
  > 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?
  - mirimir 7y ago
    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.
  - Retra 7y ago
    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.
  - colechristensen 7y ago
    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.
Taniwha 7y ago
the electrons in the outer shells of metals are essentially free to move (which is why metals conduct electricity so well)
- mirimir 7y ago
  Yes. And yet, aren't photons still interacting with the metal atoms, not those ~free electrons?
  - Taniwha 7y ago
    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)
    - mirimir 7y ago
      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.
mirimir 7y ago
OK, there are free-electron lasers. But that's a totally different thing, based on synchrotron radiation, in a vacuum, with no atoms involved.

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.

mirimir 7y ago
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.
elcomet 7y ago
> 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
- fhars 7y ago
  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.)
  - tsimionescu 7y ago
    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.
    - dTal 7y ago
      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!
      - tsimionescu 7y ago
        It would be pretty interesting to imagine what daily life would look like with only the strong and weak forces, true.
        mirimir 7y ago
        Greg Egan gets into that in at least one of his novels, Diaspora.
amelius 7y ago
> 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?
- mirimir 7y ago
  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.
- Retra 7y ago
  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.
- colechristensen 7y ago
  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.

Taniwha 7y ago

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

mirimir 7y ago
Yes. And yet, aren't photons still interacting with the metal atoms, not those ~free electrons?
- Taniwha 7y ago
  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)
  - mirimir 7y ago
    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.

mirimir 7y ago

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