Do Free Electrons Emit or Absorb Photons?

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Discussion Overview

The discussion centers on the behavior of free electrons in relation to photon emission and absorption. Participants explore the conditions under which free electrons can emit or absorb photons, the implications of their motion, and the underlying principles governing these processes. The scope includes theoretical considerations, classical and quantum mechanics, and references to specific phenomena like Compton scattering and free-electron lasers.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that free electrons emit electromagnetic radiation when oscillating, as seen in free-electron lasers.
  • Questions arise about whether free electrons can travel in a straight line in a vacuum and whether they can absorb photons.
  • There is a suggestion that the frequency of photons a free electron can absorb may depend on its interaction with an oscillating electric field.
  • One participant states that free electrons cannot emit photons unless they are under acceleration, implying that a truly free electron not interacting with anything cannot emit.
  • Another participant raises the possibility of spontaneous photon emission by free electrons and questions whether this would result in a loss of energy and affect the frequency of emitted photons.
  • Compton scattering is mentioned as a process where electrons can scatter photons, indicating a complex interaction between free electrons and electromagnetic radiation.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for free electrons to emit or absorb photons, with no consensus reached on whether free electrons can spontaneously emit photons or the implications of their motion in a vacuum.

Contextual Notes

Limitations include the dependence on classical versus quantum mechanical descriptions, as well as the unresolved nature of the conditions under which free electrons interact with photons.

CosmicVoyager
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Greetings,

I know that when an electron in an atom emits or absorbs a photon, what changes is the electrons orbit.

What about free electrons? If an electrons moving freely through space emits or absorbs an electron, what changes about the electron? And what determines what frequency photons correspond to free electron?

Thanks
 
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EM radiation is emitted from oscillating charges. If you have free electrons, they will emit at whatever frequency they are driven to oscillate at. This is the principle behind a free-electron laser.

http://en.wikipedia.org/wiki/Free-electron_laser
 
So electrons can't travel in a straight line out in the vacuum of space?

If they can, can they absorb photons?

If so, what determines the frequency they can absorb?
 
CosmicVoyager said:
So electrons can't travel in a straight line out in the vacuum of space?

If they can, can they absorb photons?

If so, what determines the frequency they can absorb?

Just invert my previous answer ... EM radiation is an oscillating electric field (at least in the classical description) ... classically, the negatively charged free electron would try to follow the motion of the field. Quantum mechanically, we would expect a non-zero probability that a free-electron can absorb EM-radiation at (almost?) any frequency. What's more, electrons can scatter photons inelastically in a process called Compton scattering (http://en.wikipedia.org/wiki/Compton_scattering).

Having said all of that, the correct way to describe this is by using quantized fields. Specifically, quantum electrodynamics (QED) deals with the interactions of EM-radiation and matter on the quantum level. I am less familiar with the details of QED, but I believe it is consistent with what I said above. Certainly one can draw a Feyman diagram representing absorption or emission of a photon from a free-electron.
 
Can an electron that is moving freely in the vacuum of space and not interacting with anything spontaneously emit a photon?

If so, does that cause the electron to slow down because it has lost energy?

Are faster moving electrons capable of emitting higher frequency photons?
 
An electron can only emit when it is under acceleration, therefore if it is a "free" electron, and is not interacting with anything, it cannot be under acceleration, and thus cannot emit.
 
CosmicVoyager said:
Greetings,

I know that when an electron in an atom emits or absorbs a photon, what changes is the electrons orbit.

What about free electrons? If an electrons moving freely through space emits or absorbs an electron, what changes about the electron? And what determines what frequency photons correspond to free electron?

Thanks

Check Compton scattering
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/comptint.html
 

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