Quantum antenna thought experiment

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Discussion Overview

The discussion revolves around the interaction between photons and charged particles in the context of antennas, exploring how classical phenomena, such as momentum transfer, emerge from quantum interactions. It touches on concepts of momentum conservation, both linear and angular, and the implications of these interactions in classical and quantum frameworks.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants describe how classical antennas absorb electromagnetic waves, resulting in momentum transfer to charged particles in specific directions.
  • Others note that while antennas experience a net force in the direction of radiation, the effects on perpendicular acceleration cancel out, leading to a discussion on the microscopic effects in intense laser fields.
  • There is confusion expressed regarding how photons interact with electrons to impart perpendicular momentum, with some participants suggesting a focus on the electromagnetic field rather than individual photons.
  • Participants discuss the conservation of linear and angular momentum during photon absorption, questioning whether electrons gain angular momentum in the process.
  • One participant raises a scenario regarding an isolated electron under acceleration and its ability to emit a photon, linking this to conservation laws.
  • Another participant challenges the feasibility of a single photon interacting with an electron without further interactions, emphasizing conservation principles.

Areas of Agreement / Disagreement

Participants express confusion and uncertainty regarding the interactions between photons and electrons, particularly in terms of momentum transfer and conservation laws. Multiple competing views remain, and the discussion does not reach a consensus on these points.

Contextual Notes

Limitations include assumptions about the nature of interactions between photons and electrons, the dependence on classical versus quantum descriptions, and unresolved questions regarding the conservation of energy and momentum in specific scenarios.

jaydnul
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If you have a classical antenna absorbing an electromagnetic wave, the charged particles inside the antenna will be given momentum in the directions perpendicular to the direction of propagation of the EM wave (because the E and B fields are perpendicular to propagation).

If just a single charged particle is absorbed by a single photon, the conservation of momentum would seem to indicate that the charged particle would gain momentum in the same direction as the photon.

How does this classical phenomenon emerge from the quantum one?
 
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An antenna gets pushed back a bit if it absorbs radiation, while the net effects on perpendicular acceleration cancel.
The overall force is in the direction of radiation in both cases. You just don't see the effect on antennas as it is tiny, and you see collective effects (back and forth of electrons) in the antenna due to the coherent radiation - you can get the same motion on a microscopic scale in intense laser fields.
 
mfb said:
...you see collective effects (back and forth of electrons) in the antenna due to the coherent radiation,,,.

This is what has me confused. How is the photon interacting with the electron to give it perpendicular momentum?
 
The photon has linear and angular momentum. The linear momentum is conserved along the direction of propagation when the photon is absorbed. What about the angular momentum? Does the electron gain the angular momentum?
 
Jd0g33 said:
This is what has me confused. How is the photon interacting with the electron to give it perpendicular momentum?
Well, don't consider photons for classical antennas. Consider the electromagnetic field - in particular the electric component.
Jd0g33 said:
Does the electron gain the angular momentum?
A single photon cannot absorb a photon without any further interaction, that would violate energy/momentum conservation. The overall reaction conserves angular momentum, of course.
 
I assume you mean a single electron cannot absorb a photon without further interaction. So does that mean a single isolated electron under acceleration couldn't emit a photon because it would violate the conservation of energy/momentum?
 
Under acceleration of what? If it accelerates, it interacts with something.

The process photon + electron -> electron is impossible. This is easy to see if you consider the reverse process in the electron rest frame: an electron at rest "decays" to an electron and a photon. Wait, where did the energy come from?
 
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